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----------------------------------------------------------------------------- This file contains a concatenation of the PCRE man pages, converted to plain text format for ease of searching with a text editor, or for use on systems that do not have a man page processor. The small individual files that give synopses of each function in the library have not been included. Neither has the pcredemo program. There are separate text files for the pcregrep and pcretest commands. ----------------------------------------------------------------------------- PCRE(3) PCRE(3) NAME PCRE - Perl-compatible regular expressions INTRODUCTION The PCRE library is a set of functions that implement regular expres- sion pattern matching using the same syntax and semantics as Perl, with just a few differences. Some features that appeared in Python and PCRE before they appeared in Perl are also available using the Python syn- tax, there is some support for one or two .NET and Oniguruma syntax items, and there is an option for requesting some minor changes that give better JavaScript compatibility. The current implementation of PCRE corresponds approximately with Perl 5.10, including support for UTF-8 encoded strings and Unicode general category properties. However, UTF-8 and Unicode support has to be explicitly enabled; it is not the default. The Unicode tables corre- spond to Unicode release 5.2.0. In addition to the Perl-compatible matching function, PCRE contains an alternative function that matches the same compiled patterns in a dif- ferent way. In certain circumstances, the alternative function has some advantages. For a discussion of the two matching algorithms, see the pcrematching page. PCRE is written in C and released as a C library. A number of people have written wrappers and interfaces of various kinds. In particular, Google Inc. have provided a comprehensive C++ wrapper. This is now included as part of the PCRE distribution. The pcrecpp page has details of this interface. Other people's contributions can be found in the Contrib directory at the primary FTP site, which is: ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre Details of exactly which Perl regular expression features are and are not supported by PCRE are given in separate documents. See the pcrepat- tern and pcrecompat pages. There is a syntax summary in the pcresyntax page. Some features of PCRE can be included, excluded, or changed when the library is built. The pcre_config() function makes it possible for a client to discover which features are available. The features them- selves are described in the pcrebuild page. Documentation about build- ing PCRE for various operating systems can be found in the README and NON-UNIX-USE files in the source distribution. The library contains a number of undocumented internal functions and data tables that are used by more than one of the exported external functions, but which are not intended for use by external callers. Their names all begin with "_pcre_", which hopefully will not provoke any name clashes. In some environments, it is possible to control which external symbols are exported when a shared library is built, and in these cases the undocumented symbols are not exported. USER DOCUMENTATION The user documentation for PCRE comprises a number of different sec- tions. In the "man" format, each of these is a separate "man page". In the HTML format, each is a separate page, linked from the index page. In the plain text format, all the sections, except the pcredemo sec- tion, are concatenated, for ease of searching. The sections are as fol- lows: pcre this document pcre-config show PCRE installation configuration information pcreapi details of PCRE's native C API pcrebuild options for building PCRE pcrecallout details of the callout feature pcrecompat discussion of Perl compatibility pcrecpp details of the C++ wrapper pcredemo a demonstration C program that uses PCRE pcregrep description of the pcregrep command pcrematching discussion of the two matching algorithms pcrepartial details of the partial matching facility pcrepattern syntax and semantics of supported regular expressions pcreperform discussion of performance issues pcreposix the POSIX-compatible C API pcreprecompile details of saving and re-using precompiled patterns pcresample discussion of the pcredemo program pcrestack discussion of stack usage pcresyntax quick syntax reference pcretest description of the pcretest testing command In addition, in the "man" and HTML formats, there is a short page for each C library function, listing its arguments and results. LIMITATIONS There are some size limitations in PCRE but it is hoped that they will never in practice be relevant. The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is compiled with the default internal linkage size of 2. If you want to process regular expressions that are truly enormous, you can compile PCRE with an internal linkage size of 3 or 4 (see the README file in the source distribution and the pcrebuild documentation for details). In these cases the limit is substantially larger. However, the speed of execution is slower. All values in repeating quantifiers must be less than 65536. There is no limit to the number of parenthesized subpatterns, but there can be no more than 65535 capturing subpatterns. The maximum length of name for a named subpattern is 32 characters, and the maximum number of named subpatterns is 10000. The maximum length of a subject string is the largest positive number that an integer variable can hold. However, when using the traditional matching function, PCRE uses recursion to handle subpatterns and indef- inite repetition. This means that the available stack space may limit the size of a subject string that can be processed by certain patterns. For a discussion of stack issues, see the pcrestack documentation. UTF-8 AND UNICODE PROPERTY SUPPORT From release 3.3, PCRE has had some support for character strings encoded in the UTF-8 format. For release 4.0 this was greatly extended to cover most common requirements, and in release 5.0 additional sup- port for Unicode general category properties was added. In order process UTF-8 strings, you must build PCRE to include UTF-8 support in the code, and, in addition, you must call pcre_compile() with the PCRE_UTF8 option flag, or the pattern must start with the sequence (*UTF8). When either of these is the case, both the pattern and any subject strings that are matched against it are treated as UTF-8 strings instead of strings of 1-byte characters. If you compile PCRE with UTF-8 support, but do not use it at run time, the library will be a bit bigger, but the additional run time overhead is limited to testing the PCRE_UTF8 flag occasionally, so should not be very big. If PCRE is built with Unicode character property support (which implies UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup- ported. The available properties that can be tested are limited to the general category properties such as Lu for an upper case letter or Nd for a decimal number, the Unicode script names such as Arabic or Han, and the derived properties Any and L&. A full list is given in the pcrepattern documentation. Only the short names for properties are sup- ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let- ter}, is not supported. Furthermore, in Perl, many properties may optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE does not support this. Validity of UTF-8 strings When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects are (by default) checked for validity on entry to the relevant functions. From release 7.3 of PCRE, the check is according the rules of RFC 3629, which are themselves derived from the Unicode specifica- tion. Earlier releases of PCRE followed the rules of RFC 2279, which allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current check allows only values in the range U+0 to U+10FFFF, excluding U+D800 to U+DFFF. The excluded code points are the "Low Surrogate Area" of Unicode, of which the Unicode Standard says this: "The Low Surrogate Area does not contain any character assignments, consequently no character code charts or namelists are provided for this area. Surrogates are reserved for use with UTF-16 and then must be used in pairs." The code points that are encoded by UTF-16 pairs are available as independent code points in the UTF-8 encoding. (In other words, the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up UTF-8.) If an invalid UTF-8 string is passed to PCRE, an error return (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that your strings are valid, and therefore want to skip these checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or at run time, PCRE assumes that the pattern or subject it is given (respectively) contains only valid UTF-8 codes. In this case, it does not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what happens depends on why the string is invalid. If the string con- forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity test, PCRE (when in UTF-8 mode) handles strings according to the more liberal rules of RFC 2279. However, if the string does not even conform to RFC 2279, the result is undefined. Your program may crash. If you want to process strings of values in the full range 0 to 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this situation, you will have to apply your own validity check. General comments about UTF-8 mode 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte UTF-8 character if the value is greater than 127. 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8 characters for values greater than \177. 3. Repeat quantifiers apply to complete UTF-8 characters, not to indi- vidual bytes, for example: \x{100}{3}. 4. The dot metacharacter matches one UTF-8 character instead of a sin- gle byte. 5. The escape sequence \C can be used to match a single byte in UTF-8 mode, but its use can lead to some strange effects. This facility is not available in the alternative matching function, pcre_dfa_exec(). 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly test characters of any code value, but the characters that PCRE recog- nizes as digits, spaces, or word characters remain the same set as before, all with values less than 256. This remains true even when PCRE includes Unicode property support, because to do otherwise would slow down PCRE in many common cases. If you really want to test for a wider sense of, say, "digit", you must use Unicode property tests such as \p{Nd}. Note that this also applies to \b, because it is defined in terms of \w and \W. 7. Similarly, characters that match the POSIX named character classes are all low-valued characters. 8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char- acters. 9. Case-insensitive matching applies only to characters whose values are less than 128, unless PCRE is built with Unicode property support. Even when Unicode property support is available, PCRE still uses its own character tables when checking the case of low-valued characters, so as not to degrade performance. The Unicode property information is used only for characters with higher values. Even when Unicode property support is available, PCRE supports case-insensitive matching only when there is a one-to-one mapping between a letter's cases. There are a small number of many-to-one mappings in Unicode; these are not sup- ported by PCRE. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. Putting an actual email address here seems to have been a spam magnet, so I've taken it away. If you want to email me, use my two initials, followed by the two digits 10, at the domain cam.ac.uk. REVISION Last updated: 01 March 2010 Copyright (c) 1997-2010 University of Cambridge. ------------------------------------------------------------------------------ PCREBUILD(3) PCREBUILD(3) NAME PCRE - Perl-compatible regular expressions PCRE BUILD-TIME OPTIONS This document describes the optional features of PCRE that can be selected when the library is compiled. It assumes use of the configure script, where the optional features are selected or deselected by pro- viding options to configure before running the make command. However, the same options can be selected in both Unix-like and non-Unix-like environments using the GUI facility of cmake-gui if you are using CMake instead of configure to build PCRE. There is a lot more information about building PCRE in non-Unix-like environments in the file called NON_UNIX_USE, which is part of the PCRE distribution. You should consult this file as well as the README file if you are building in a non-Unix-like environment. The complete list of options for configure (which includes the standard ones such as the selection of the installation directory) can be obtained by running ./configure --help The following sections include descriptions of options whose names begin with --enable or --disable. These settings specify changes to the defaults for the configure command. Because of the way that configure works, --enable and --disable always come in pairs, so the complemen- tary option always exists as well, but as it specifies the default, it is not described. C++ SUPPORT By default, the configure script will search for a C++ compiler and C++ header files. If it finds them, it automatically builds the C++ wrapper library for PCRE. You can disable this by adding --disable-cpp to the configure command. UTF-8 SUPPORT To build PCRE with support for UTF-8 Unicode character strings, add --enable-utf8 to the configure command. Of itself, this does not make PCRE treat strings as UTF-8. As well as compiling PCRE with this option, you also have have to set the PCRE_UTF8 option when you call the pcre_compile() or pcre_compile2() functions. If you set --enable-utf8 when compiling in an EBCDIC environment, PCRE expects its input to be either ASCII or UTF-8 (depending on the runtime option). It is not possible to support both EBCDIC and UTF-8 codes in the same version of the library. Consequently, --enable-utf8 and --enable-ebcdic are mutually exclusive. UNICODE CHARACTER PROPERTY SUPPORT UTF-8 support allows PCRE to process character values greater than 255 in the strings that it handles. On its own, however, it does not pro- vide any facilities for accessing the properties of such characters. If you want to be able to use the pattern escapes \P, \p, and \X, which refer to Unicode character properties, you must add --enable-unicode-properties to the configure command. This implies UTF-8 support, even if you have not explicitly requested it. Including Unicode property support adds around 30K of tables to the PCRE library. Only the general category properties such as Lu and Nd are supported. Details are given in the pcrepattern documentation. CODE VALUE OF NEWLINE By default, PCRE interprets the linefeed (LF) character as indicating the end of a line. This is the normal newline character on Unix-like systems. You can compile PCRE to use carriage return (CR) instead, by adding --enable-newline-is-cr to the configure command. There is also a --enable-newline-is-lf option, which explicitly specifies linefeed as the newline character. Alternatively, you can specify that line endings are to be indicated by the two character sequence CRLF. If you want this, add --enable-newline-is-crlf to the configure command. There is a fourth option, specified by --enable-newline-is-anycrlf which causes PCRE to recognize any of the three sequences CR, LF, or CRLF as indicating a line ending. Finally, a fifth option, specified by --enable-newline-is-any causes PCRE to recognize any Unicode newline sequence. Whatever line ending convention is selected when PCRE is built can be overridden when the library functions are called. At build time it is conventional to use the standard for your operating system. WHAT \R MATCHES By default, the sequence \R in a pattern matches any Unicode newline sequence, whatever has been selected as the line ending sequence. If you specify --enable-bsr-anycrlf the default is changed so that \R matches only CR, LF, or CRLF. What- ever is selected when PCRE is built can be overridden when the library functions are called. BUILDING SHARED AND STATIC LIBRARIES The PCRE building process uses libtool to build both shared and static Unix libraries by default. You can suppress one of these by adding one of --disable-shared --disable-static to the configure command, as required. POSIX MALLOC USAGE When PCRE is called through the POSIX interface (see the pcreposix doc- umentation), additional working storage is required for holding the pointers to capturing substrings, because PCRE requires three integers per substring, whereas the POSIX interface provides only two. If the number of expected substrings is small, the wrapper function uses space on the stack, because this is faster than using malloc() for each call. The default threshold above which the stack is no longer used is 10; it can be changed by adding a setting such as --with-posix-malloc-threshold=20 to the configure command. HANDLING VERY LARGE PATTERNS Within a compiled pattern, offset values are used to point from one part to another (for example, from an opening parenthesis to an alter- nation metacharacter). By default, two-byte values are used for these offsets, leading to a maximum size for a compiled pattern of around 64K. This is sufficient to handle all but the most gigantic patterns. Nevertheless, some people do want to process truyl enormous patterns, so it is possible to compile PCRE to use three-byte or four-byte off- sets by adding a setting such as --with-link-size=3 to the configure command. The value given must be 2, 3, or 4. Using longer offsets slows down the operation of PCRE because it has to load additional bytes when handling them. AVOIDING EXCESSIVE STACK USAGE When matching with the pcre_exec() function, PCRE implements backtrack- ing by making recursive calls to an internal function called match(). In environments where the size of the stack is limited, this can se- verely limit PCRE's operation. (The Unix environment does not usually suffer from this problem, but it may sometimes be necessary to increase the maximum stack size. There is a discussion in the pcrestack docu- mentation.) An alternative approach to recursion that uses memory from the heap to remember data, instead of using recursive function calls, has been implemented to work round the problem of limited stack size. If you want to build a version of PCRE that works this way, add --disable-stack-for-recursion to the configure command. With this configuration, PCRE will use the pcre_stack_malloc and pcre_stack_free variables to call memory manage- ment functions. By default these point to malloc() and free(), but you can replace the pointers so that your own functions are used instead. Separate functions are provided rather than using pcre_malloc and pcre_free because the usage is very predictable: the block sizes requested are always the same, and the blocks are always freed in reverse order. A calling program might be able to implement optimized functions that perform better than malloc() and free(). PCRE runs noticeably more slowly when built in this way. This option affects only the pcre_exec() function; it is not relevant for pcre_dfa_exec(). LIMITING PCRE RESOURCE USAGE Internally, PCRE has a function called match(), which it calls repeat- edly (sometimes recursively) when matching a pattern with the pcre_exec() function. By controlling the maximum number of times this function may be called during a single matching operation, a limit can be placed on the resources used by a single call to pcre_exec(). The limit can be changed at run time, as described in the pcreapi documen- tation. The default is 10 million, but this can be changed by adding a setting such as --with-match-limit=500000 to the configure command. This setting has no effect on the pcre_dfa_exec() matching function. In some environments it is desirable to limit the depth of recursive calls of match() more strictly than the total number of calls, in order to restrict the maximum amount of stack (or heap, if --disable-stack- for-recursion is specified) that is used. A second limit controls this; it defaults to the value that is set for --with-match-limit, which imposes no additional constraints. However, you can set a lower limit by adding, for example, --with-match-limit-recursion=10000 to the configure command. This value can also be overridden at run time. CREATING CHARACTER TABLES AT BUILD TIME PCRE uses fixed tables for processing characters whose code values are less than 256. By default, PCRE is built with a set of tables that are distributed in the file pcre_chartables.c.dist. These tables are for ASCII codes only. If you add --enable-rebuild-chartables to the configure command, the distributed tables are no longer used. Instead, a program called dftables is compiled and run. This outputs the source for new set of tables, created in the default locale of your C runtime system. (This method of replacing the tables does not work if you are cross compiling, because dftables is run on the local host. If you need to create alternative tables when cross compiling, you will have to do so "by hand".) USING EBCDIC CODE PCRE assumes by default that it will run in an environment where the character code is ASCII (or Unicode, which is a superset of ASCII). This is the case for most computer operating systems. PCRE can, how- ever, be compiled to run in an EBCDIC environment by adding --enable-ebcdic to the configure command. This setting implies --enable-rebuild-charta- bles. You should only use it if you know that you are in an EBCDIC environment (for example, an IBM mainframe operating system). The --enable-ebcdic option is incompatible with --enable-utf8. PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT By default, pcregrep reads all files as plain text. You can build it so that it recognizes files whose names end in .gz or .bz2, and reads them with libz or libbz2, respectively, by adding one or both of --enable-pcregrep-libz --enable-pcregrep-libbz2 to the configure command. These options naturally require that the rel- evant libraries are installed on your system. Configuration will fail if they are not. PCRETEST OPTION FOR LIBREADLINE SUPPORT If you add --enable-pcretest-libreadline to the configure command, pcretest is linked with the libreadline library, and when its input is from a terminal, it reads it using the readline() function. This provides line-editing and history facilities. Note that libreadline is GPL-licensed, so if you distribute a binary of pcretest linked in this way, there may be licensing issues. Setting this option causes the -lreadline option to be added to the pcretest build. In many operating environments with a sytem-installed libreadline this is sufficient. However, in some environments (e.g. if an unmodified distribution version of readline is in use), some extra configuration may be necessary. The INSTALL file for libreadline says this: "Readline uses the termcap functions, but does not link with the termcap or curses library itself, allowing applications which link with readline the to choose an appropriate library." If your environment has not been set up so that an appropriate library is automatically included, you may need to add something like LIBS="-ncurses" immediately before the configure command. SEE ALSO pcreapi(3), pcre_config(3). AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 29 September 2009 Copyright (c) 1997-2009 University of Cambridge. ------------------------------------------------------------------------------ PCREMATCHING(3) PCREMATCHING(3) NAME PCRE - Perl-compatible regular expressions PCRE MATCHING ALGORITHMS This document describes the two different algorithms that are available in PCRE for matching a compiled regular expression against a given sub- ject string. The "standard" algorithm is the one provided by the pcre_exec() function. This works in the same was as Perl's matching function, and provides a Perl-compatible matching operation. An alternative algorithm is provided by the pcre_dfa_exec() function; this operates in a different way, and is not Perl-compatible. It has advantages and disadvantages compared with the standard algorithm, and these are described below. When there is only one possible way in which a given subject string can match a pattern, the two algorithms give the same answer. A difference arises, however, when there are multiple possibilities. For example, if the pattern ^<.*> is matched against the string <something> <something else> <something further> there are three possible answers. The standard algorithm finds only one of them, whereas the alternative algorithm finds all three. REGULAR EXPRESSIONS AS TREES The set of strings that are matched by a regular expression can be rep- resented as a tree structure. An unlimited repetition in the pattern makes the tree of infinite size, but it is still a tree. Matching the pattern to a given subject string (from a given starting point) can be thought of as a search of the tree. There are two ways to search a tree: depth-first and breadth-first, and these correspond to the two matching algorithms provided by PCRE. THE STANDARD MATCHING ALGORITHM In the terminology of Jeffrey Friedl's book "Mastering Regular Expres- sions", the standard algorithm is an "NFA algorithm". It conducts a depth-first search of the pattern tree. That is, it proceeds along a single path through the tree, checking that the subject matches what is required. When there is a mismatch, the algorithm tries any alterna- tives at the current point, and if they all fail, it backs up to the previous branch point in the tree, and tries the next alternative branch at that level. This often involves backing up (moving to the left) in the subject string as well. The order in which repetition branches are tried is controlled by the greedy or ungreedy nature of the quantifier. If a leaf node is reached, a matching string has been found, and at that point the algorithm stops. Thus, if there is more than one possi- ble match, this algorithm returns the first one that it finds. Whether this is the shortest, the longest, or some intermediate length depends on the way the greedy and ungreedy repetition quantifiers are specified in the pattern. Because it ends up with a single path through the tree, it is rela- tively straightforward for this algorithm to keep track of the sub- strings that are matched by portions of the pattern in parentheses. This provides support for capturing parentheses and back references. THE ALTERNATIVE MATCHING ALGORITHM This algorithm conducts a breadth-first search of the tree. Starting from the first matching point in the subject, it scans the subject string from left to right, once, character by character, and as it does this, it remembers all the paths through the tree that represent valid matches. In Friedl's terminology, this is a kind of "DFA algorithm", though it is not implemented as a traditional finite state machine (it keeps multiple states active simultaneously). Although the general principle of this matching algorithm is that it scans the subject string only once, without backtracking, there is one exception: when a lookaround assertion is encountered, the characters following or preceding the current point have to be independently inspected. The scan continues until either the end of the subject is reached, or there are no more unterminated paths. At this point, terminated paths represent the different matching possibilities (if there are none, the match has failed). Thus, if there is more than one possible match, this algorithm finds all of them, and in particular, it finds the long- est. There is an option to stop the algorithm after the first match (which is necessarily the shortest) is found. Note that all the matches that are found start at the same point in the subject. If the pattern cat(er(pillar)?) is matched against the string "the caterpillar catchment", the result will be the three strings "cat", "cater", and "caterpillar" that start at the fourth character of the subject. The algorithm does not automat- ically move on to find matches that start at later positions. There are a number of features of PCRE regular expressions that are not supported by the alternative matching algorithm. They are as follows: 1. Because the algorithm finds all possible matches, the greedy or ungreedy nature of repetition quantifiers is not relevant. Greedy and ungreedy quantifiers are treated in exactly the same way. However, pos- sessive quantifiers can make a difference when what follows could also match what is quantified, for example in a pattern like this: ^a++\w! This pattern matches "aaab!" but not "aaa!", which would be matched by a non-possessive quantifier. Similarly, if an atomic group is present, it is matched as if it were a standalone pattern at the current point, and the longest match is then "locked in" for the rest of the overall pattern. 2. When dealing with multiple paths through the tree simultaneously, it is not straightforward to keep track of captured substrings for the different matching possibilities, and PCRE's implementation of this algorithm does not attempt to do this. This means that no captured sub- strings are available. 3. Because no substrings are captured, back references within the pat- tern are not supported, and cause errors if encountered. 4. For the same reason, conditional expressions that use a backrefer- ence as the condition or test for a specific group recursion are not supported. 5. Because many paths through the tree may be active, the \K escape sequence, which resets the start of the match when encountered (but may be on some paths and not on others), is not supported. It causes an error if encountered. 6. Callouts are supported, but the value of the capture_top field is always 1, and the value of the capture_last field is always -1. 7. The \C escape sequence, which (in the standard algorithm) matches a single byte, even in UTF-8 mode, is not supported because the alterna- tive algorithm moves through the subject string one character at a time, for all active paths through the tree. 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not supported. (*FAIL) is supported, and behaves like a failing negative assertion. ADVANTAGES OF THE ALTERNATIVE ALGORITHM Using the alternative matching algorithm provides the following advan- tages: 1. All possible matches (at a single point in the subject) are automat- ically found, and in particular, the longest match is found. To find more than one match using the standard algorithm, you have to do kludgy things with callouts. 2. Because the alternative algorithm scans the subject string just once, and never needs to backtrack, it is possible to pass very long subject strings to the matching function in several pieces, checking for partial matching each time. The pcrepartial documentation gives details of partial matching. DISADVANTAGES OF THE ALTERNATIVE ALGORITHM The alternative algorithm suffers from a number of disadvantages: 1. It is substantially slower than the standard algorithm. This is partly because it has to search for all possible matches, but is also because it is less susceptible to optimization. 2. Capturing parentheses and back references are not supported. 3. Although atomic groups are supported, their use does not provide the performance advantage that it does for the standard algorithm. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 29 September 2009 Copyright (c) 1997-2009 University of Cambridge. ------------------------------------------------------------------------------ PCREAPI(3) PCREAPI(3) NAME PCRE - Perl-compatible regular expressions PCRE NATIVE API #include <pcre.h> pcre *pcre_compile(const char *pattern, int options, const char **errptr, int *erroffset, const unsigned char *tableptr); pcre *pcre_compile2(const char *pattern, int options, int *errorcodeptr, const char **errptr, int *erroffset, const unsigned char *tableptr); pcre_extra *pcre_study(const pcre *code, int options, const char **errptr); int pcre_exec(const pcre *code, const pcre_extra *extra, const char *subject, int length, int startoffset, int options, int *ovector, int ovecsize); int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, const char *subject, int length, int startoffset, int options, int *ovector, int ovecsize, int *workspace, int wscount); int pcre_copy_named_substring(const pcre *code, const char *subject, int *ovector, int stringcount, const char *stringname, char *buffer, int buffersize); int pcre_copy_substring(const char *subject, int *ovector, int stringcount, int stringnumber, char *buffer, int buffersize); int pcre_get_named_substring(const pcre *code, const char *subject, int *ovector, int stringcount, const char *stringname, const char **stringptr); int pcre_get_stringnumber(const pcre *code, const char *name); int pcre_get_stringtable_entries(const pcre *code, const char *name, char **first, char **last); int pcre_get_substring(const char *subject, int *ovector, int stringcount, int stringnumber, const char **stringptr); int pcre_get_substring_list(const char *subject, int *ovector, int stringcount, const char ***listptr); void pcre_free_substring(const char *stringptr); void pcre_free_substring_list(const char **stringptr); const unsigned char *pcre_maketables(void); int pcre_fullinfo(const pcre *code, const pcre_extra *extra, int what, void *where); int pcre_info(const pcre *code, int *optptr, int *firstcharptr); int pcre_refcount(pcre *code, int adjust); int pcre_config(int what, void *where); char *pcre_version(void); void *(*pcre_malloc)(size_t); void (*pcre_free)(void *); void *(*pcre_stack_malloc)(size_t); void (*pcre_stack_free)(void *); int (*pcre_callout)(pcre_callout_block *); PCRE API OVERVIEW PCRE has its own native API, which is described in this document. There are also some wrapper functions that correspond to the POSIX regular expression API. These are described in the pcreposix documentation. Both of these APIs define a set of C function calls. A C++ wrapper is distributed with PCRE. It is documented in the pcrecpp page. The native API C function prototypes are defined in the header file pcre.h, and on Unix systems the library itself is called libpcre. It can normally be accessed by adding -lpcre to the command for linking an application that uses PCRE. The header file defines the macros PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num- bers for the library. Applications can use these to include support for different releases of PCRE. The functions pcre_compile(), pcre_compile2(), pcre_study(), and pcre_exec() are used for compiling and matching regular expressions in a Perl-compatible manner. A sample program that demonstrates the sim- plest way of using them is provided in the file called pcredemo.c in the PCRE source distribution. A listing of this program is given in the pcredemo documentation, and the pcresample documentation describes how to compile and run it. A second matching function, pcre_dfa_exec(), which is not Perl-compati- ble, is also provided. This uses a different algorithm for the match- ing. The alternative algorithm finds all possible matches (at a given point in the subject), and scans the subject just once (unless there are lookbehind assertions). However, this algorithm does not return captured substrings. A description of the two matching algorithms and their advantages and disadvantages is given in the pcrematching docu- mentation. In addition to the main compiling and matching functions, there are convenience functions for extracting captured substrings from a subject string that is matched by pcre_exec(). They are: pcre_copy_substring() pcre_copy_named_substring() pcre_get_substring() pcre_get_named_substring() pcre_get_substring_list() pcre_get_stringnumber() pcre_get_stringtable_entries() pcre_free_substring() and pcre_free_substring_list() are also provided, to free the memory used for extracted strings. The function pcre_maketables() is used to build a set of character tables in the current locale for passing to pcre_compile(), pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is provided for specialist use. Most commonly, no special tables are passed, in which case internal tables that are generated when PCRE is built are used. The function pcre_fullinfo() is used to find out information about a compiled pattern; pcre_info() is an obsolete version that returns only some of the available information, but is retained for backwards com- patibility. The function pcre_version() returns a pointer to a string containing the version of PCRE and its date of release. The function pcre_refcount() maintains a reference count in a data block containing a compiled pattern. This is provided for the benefit of object-oriented applications. The global variables pcre_malloc and pcre_free initially contain the entry points of the standard malloc() and free() functions, respec- tively. PCRE calls the memory management functions via these variables, so a calling program can replace them if it wishes to intercept the calls. This should be done before calling any PCRE functions. The global variables pcre_stack_malloc and pcre_stack_free are also indirections to memory management functions. These special functions are used only when PCRE is compiled to use the heap for remembering data, instead of recursive function calls, when running the pcre_exec() function. See the pcrebuild documentation for details of how to do this. It is a non-standard way of building PCRE, for use in environ- ments that have limited stacks. Because of the greater use of memory management, it runs more slowly. Separate functions are provided so that special-purpose external code can be used for this case. When used, these functions are always called in a stack-like manner (last obtained, first freed), and always for memory blocks of the same size. There is a discussion about PCRE's stack usage in the pcrestack docu- mentation. The global variable pcre_callout initially contains NULL. It can be set by the caller to a "callout" function, which PCRE will then call at specified points during a matching operation. Details are given in the pcrecallout documentation. NEWLINES PCRE supports five different conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (line- feed) character, the two-character sequence CRLF, any of the three pre- ceding, or any Unicode newline sequence. The Unicode newline sequences are the three just mentioned, plus the single characters VT (vertical tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS (paragraph separator, U+2029). Each of the first three conventions is used by at least one operating system as its standard newline sequence. When PCRE is built, a default can be specified. The default default is LF, which is the Unix stan- dard. When PCRE is run, the default can be overridden, either when a pattern is compiled, or when it is matched. At compile time, the newline convention can be specified by the options argument of pcre_compile(), or it can be specified by special text at the start of the pattern itself; this overrides any other settings. See the pcrepattern page for details of the special character sequences. In the PCRE documentation the word "newline" is used to mean "the char- acter or pair of characters that indicate a line break". The choice of newline convention affects the handling of the dot, circumflex, and dollar metacharacters, the handling of #-comments in /x mode, and, when CRLF is a recognized line ending sequence, the match position advance- ment for a non-anchored pattern. There is more detail about this in the section on pcre_exec() options below. The choice of newline convention does not affect the interpretation of the \n or \r escape sequences, nor does it affect what \R matches, which is controlled in a similar way, but by separate options. MULTITHREADING The PCRE functions can be used in multi-threading applications, with the proviso that the memory management functions pointed to by pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the callout function pointed to by pcre_callout, are shared by all threads. The compiled form of a regular expression is not altered during match- ing, so the same compiled pattern can safely be used by several threads at once. SAVING PRECOMPILED PATTERNS FOR LATER USE The compiled form of a regular expression can be saved and re-used at a later time, possibly by a different program, and even on a host other than the one on which it was compiled. Details are given in the pcreprecompile documentation. However, compiling a regular expression with one version of PCRE for use with a different version is not guar- anteed to work and may cause crashes. CHECKING BUILD-TIME OPTIONS int pcre_config(int what, void *where); The function pcre_config() makes it possible for a PCRE client to dis- cover which optional features have been compiled into the PCRE library. The pcrebuild documentation has more details about these optional fea- tures. The first argument for pcre_config() is an integer, specifying which information is required; the second argument is a pointer to a variable into which the information is placed. The following information is available: PCRE_CONFIG_UTF8 The output is an integer that is set to one if UTF-8 support is avail- able; otherwise it is set to zero. PCRE_CONFIG_UNICODE_PROPERTIES The output is an integer that is set to one if support for Unicode character properties is available; otherwise it is set to zero. PCRE_CONFIG_NEWLINE The output is an integer whose value specifies the default character sequence that is recognized as meaning "newline". The four values that are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF, and -1 for ANY. Though they are derived from ASCII, the same values are returned in EBCDIC environments. The default should normally corre- spond to the standard sequence for your operating system. PCRE_CONFIG_BSR The output is an integer whose value indicates what character sequences the \R escape sequence matches by default. A value of 0 means that \R matches any Unicode line ending sequence; a value of 1 means that \R matches only CR, LF, or CRLF. The default can be overridden when a pat- tern is compiled or matched. PCRE_CONFIG_LINK_SIZE The output is an integer that contains the number of bytes used for internal linkage in compiled regular expressions. The value is 2, 3, or 4. Larger values allow larger regular expressions to be compiled, at the expense of slower matching. The default value of 2 is sufficient for all but the most massive patterns, since it allows the compiled pattern to be up to 64K in size. PCRE_CONFIG_POSIX_MALLOC_THRESHOLD The output is an integer that contains the threshold above which the POSIX interface uses malloc() for output vectors. Further details are given in the pcreposix documentation. PCRE_CONFIG_MATCH_LIMIT The output is a long integer that gives the default limit for the num- ber of internal matching function calls in a pcre_exec() execution. Further details are given with pcre_exec() below. PCRE_CONFIG_MATCH_LIMIT_RECURSION The output is a long integer that gives the default limit for the depth of recursion when calling the internal matching function in a pcre_exec() execution. Further details are given with pcre_exec() below. PCRE_CONFIG_STACKRECURSE The output is an integer that is set to one if internal recursion when running pcre_exec() is implemented by recursive function calls that use the stack to remember their state. This is the usual way that PCRE is compiled. The output is zero if PCRE was compiled to use blocks of data on the heap instead of recursive function calls. In this case, pcre_stack_malloc and pcre_stack_free are called to manage memory blocks on the heap, thus avoiding the use of the stack. COMPILING A PATTERN pcre *pcre_compile(const char *pattern, int options, const char **errptr, int *erroffset, const unsigned char *tableptr); pcre *pcre_compile2(const char *pattern, int options, int *errorcodeptr, const char **errptr, int *erroffset, const unsigned char *tableptr); Either of the functions pcre_compile() or pcre_compile2() can be called to compile a pattern into an internal form. The only difference between the two interfaces is that pcre_compile2() has an additional argument, errorcodeptr, via which a numerical error code can be returned. To avoid too much repetition, we refer just to pcre_compile() below, but the information applies equally to pcre_compile2(). The pattern is a C string terminated by a binary zero, and is passed in the pattern argument. A pointer to a single block of memory that is obtained via pcre_malloc is returned. This contains the compiled code and related data. The pcre type is defined for the returned block; this is a typedef for a structure whose contents are not externally defined. It is up to the caller to free the memory (via pcre_free) when it is no longer required. Although the compiled code of a PCRE regex is relocatable, that is, it does not depend on memory location, the complete pcre data block is not fully relocatable, because it may contain a copy of the tableptr argu- ment, which is an address (see below). The options argument contains various bit settings that affect the com- pilation. It should be zero if no options are required. The available options are described below. Some of them (in particular, those that are compatible with Perl, but some others as well) can also be set and unset from within the pattern (see the detailed description in the pcrepattern documentation). For those options that can be different in different parts of the pattern, the contents of the options argument specifies their settings at the start of compilation and execution. The PCRE_ANCHORED, PCRE_BSR_xxx, and PCRE_NEWLINE_xxx options can be set at the time of matching as well as at compile time. If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise, if compilation of a pattern fails, pcre_compile() returns NULL, and sets the variable pointed to by errptr to point to a textual error mes- sage. This is a static string that is part of the library. You must not try to free it. The byte offset from the start of the pattern to the character that was being processed when the error was discovered is placed in the variable pointed to by erroffset, which must not be NULL. If it is, an immediate error is given. Some errors are not detected until checks are carried out when the whole pattern has been scanned; in this case the offset is set to the end of the pattern. If pcre_compile2() is used instead of pcre_compile(), and the error- codeptr argument is not NULL, a non-zero error code number is returned via this argument in the event of an error. This is in addition to the textual error message. Error codes and messages are listed below. If the final argument, tableptr, is NULL, PCRE uses a default set of character tables that are built when PCRE is compiled, using the default C locale. Otherwise, tableptr must be an address that is the result of a call to pcre_maketables(). This value is stored with the compiled pattern, and used again by pcre_exec(), unless another table pointer is passed to it. For more discussion, see the section on locale support below. This code fragment shows a typical straightforward call to pcre_com- pile(): pcre *re; const char *error; int erroffset; re = pcre_compile( "^A.*Z", /* the pattern */ 0, /* default options */ &error, /* for error message */ &erroffset, /* for error offset */ NULL); /* use default character tables */ The following names for option bits are defined in the pcre.h header file: PCRE_ANCHORED If this bit is set, the pattern is forced to be "anchored", that is, it is constrained to match only at the first matching point in the string that is being searched (the "subject string"). This effect can also be achieved by appropriate constructs in the pattern itself, which is the only way to do it in Perl. PCRE_AUTO_CALLOUT If this bit is set, pcre_compile() automatically inserts callout items, all with number 255, before each pattern item. For discussion of the callout facility, see the pcrecallout documentation. PCRE_BSR_ANYCRLF PCRE_BSR_UNICODE These options (which are mutually exclusive) control what the \R escape sequence matches. The choice is either to match only CR, LF, or CRLF, or to match any Unicode newline sequence. The default is specified when PCRE is built. It can be overridden from within the pattern, or by set- ting an option when a compiled pattern is matched. PCRE_CASELESS If this bit is set, letters in the pattern match both upper and lower case letters. It is equivalent to Perl's /i option, and it can be changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE always understands the concept of case for characters whose values are less than 128, so caseless matching is always possible. For characters with higher values, the concept of case is supported if PCRE is com- piled with Unicode property support, but not otherwise. If you want to use caseless matching for characters 128 and above, you must ensure that PCRE is compiled with Unicode property support as well as with UTF-8 support. PCRE_DOLLAR_ENDONLY If this bit is set, a dollar metacharacter in the pattern matches only at the end of the subject string. Without this option, a dollar also matches immediately before a newline at the end of the string (but not before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. There is no equivalent to this option in Perl, and no way to set it within a pattern. PCRE_DOTALL If this bit is set, a dot metacharater in the pattern matches all char- acters, including those that indicate newline. Without it, a dot does not match when the current position is at a newline. This option is equivalent to Perl's /s option, and it can be changed within a pattern by a (?s) option setting. A negative class such as [^a] always matches newline characters, independent of the setting of this option. PCRE_DUPNAMES If this bit is set, names used to identify capturing subpatterns need not be unique. This can be helpful for certain types of pattern when it is known that only one instance of the named subpattern can ever be matched. There are more details of named subpatterns below; see also the pcrepattern documentation. PCRE_EXTENDED If this bit is set, whitespace data characters in the pattern are totally ignored except when escaped or inside a character class. White- space does not include the VT character (code 11). In addition, charac- ters between an unescaped # outside a character class and the next new- line, inclusive, are also ignored. This is equivalent to Perl's /x option, and it can be changed within a pattern by a (?x) option set- ting. This option makes it possible to include comments inside complicated patterns. Note, however, that this applies only to data characters. Whitespace characters may never appear within special character sequences in a pattern, for example within the sequence (?( which introduces a conditional subpattern. PCRE_EXTRA This option was invented in order to turn on additional functionality of PCRE that is incompatible with Perl, but it is currently of very little use. When set, any backslash in a pattern that is followed by a letter that has no special meaning causes an error, thus reserving these combinations for future expansion. By default, as in Perl, a backslash followed by a letter with no special meaning is treated as a literal. (Perl can, however, be persuaded to give a warning for this.) There are at present no other features controlled by this option. It can also be set by a (?X) option setting within a pattern. PCRE_FIRSTLINE If this option is set, an unanchored pattern is required to match before or at the first newline in the subject string, though the matched text may continue over the newline. PCRE_JAVASCRIPT_COMPAT If this option is set, PCRE's behaviour is changed in some ways so that it is compatible with JavaScript rather than Perl. The changes are as follows: (1) A lone closing square bracket in a pattern causes a compile-time error, because this is illegal in JavaScript (by default it is treated as a data character). Thus, the pattern AB]CD becomes illegal when this option is set. (2) At run time, a back reference to an unset subpattern group matches an empty string (by default this causes the current matching alterna- tive to fail). A pattern such as (\1)(a) succeeds when this option is set (assuming it can find an "a" in the subject), whereas it fails by default, for Perl compatibility. PCRE_MULTILINE By default, PCRE treats the subject string as consisting of a single line of characters (even if it actually contains newlines). The "start of line" metacharacter (^) matches only at the start of the string, while the "end of line" metacharacter ($) matches only at the end of the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY is set). This is the same as Perl. When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs match immediately following or immediately before internal newlines in the subject string, respectively, as well as at the very start and end. This is equivalent to Perl's /m option, and it can be changed within a pattern by a (?m) option setting. If there are no new- lines in a subject string, or no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no effect. PCRE_NEWLINE_CR PCRE_NEWLINE_LF PCRE_NEWLINE_CRLF PCRE_NEWLINE_ANYCRLF PCRE_NEWLINE_ANY These options override the default newline definition that was chosen when PCRE was built. Setting the first or the second specifies that a newline is indicated by a single character (CR or LF, respectively). Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies that any of the three preceding sequences should be recognized. Setting PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be recognized. The Unicode newline sequences are the three just mentioned, plus the single characters VT (vertical tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS (paragraph separator, U+2029). The last two are recognized only in UTF-8 mode. The newline setting in the options word uses three bits that are treated as a number, giving eight possibilities. Currently only six are used (default plus the five values above). This means that if you set more than one newline option, the combination may or may not be sensi- ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and cause an error. The only time that a line break is specially recognized when compiling a pattern is if PCRE_EXTENDED is set, and an unescaped # outside a character class is encountered. This indicates a comment that lasts until after the next line break sequence. In other circumstances, line break sequences are treated as literal data, except that in PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters and are therefore ignored. The newline option that is set at compile time becomes the default that is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden. PCRE_NO_AUTO_CAPTURE If this option is set, it disables the use of numbered capturing paren- theses in the pattern. Any opening parenthesis that is not followed by ? behaves as if it were followed by ?: but named parentheses can still be used for capturing (and they acquire numbers in the usual way). There is no equivalent of this option in Perl. PCRE_UNGREEDY This option inverts the "greediness" of the quantifiers so that they are not greedy by default, but become greedy if followed by "?". It is not compatible with Perl. It can also be set by a (?U) option setting within the pattern. PCRE_UTF8 This option causes PCRE to regard both the pattern and the subject as strings of UTF-8 characters instead of single-byte character strings. However, it is available only when PCRE is built to include UTF-8 sup- port. If not, the use of this option provokes an error. Details of how this option changes the behaviour of PCRE are given in the section on UTF-8 support in the main pcre page. PCRE_NO_UTF8_CHECK When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is automatically checked. There is a discussion about the validity of UTF-8 strings in the main pcre page. If an invalid UTF-8 sequence of bytes is found, pcre_compile() returns an error. If you already know that your pattern is valid, and you want to skip this check for perfor- mance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is set, the effect of passing an invalid UTF-8 string as a pattern is undefined. It may cause your program to crash. Note that this option can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the UTF-8 validity checking of subject strings. COMPILATION ERROR CODES The following table lists the error codes than may be returned by pcre_compile2(), along with the error messages that may be returned by both compiling functions. As PCRE has developed, some error codes have fallen out of use. To avoid confusion, they have not been re-used. 0 no error 1 \ at end of pattern 2 \c at end of pattern 3 unrecognized character follows \ 4 numbers out of order in {} quantifier 5 number too big in {} quantifier 6 missing terminating ] for character class 7 invalid escape sequence in character class 8 range out of order in character class 9 nothing to repeat 10 [this code is not in use] 11 internal error: unexpected repeat 12 unrecognized character after (? or (?- 13 POSIX named classes are supported only within a class 14 missing ) 15 reference to non-existent subpattern 16 erroffset passed as NULL 17 unknown option bit(s) set 18 missing ) after comment 19 [this code is not in use] 20 regular expression is too large 21 failed to get memory 22 unmatched parentheses 23 internal error: code overflow 24 unrecognized character after (?< 25 lookbehind assertion is not fixed length 26 malformed number or name after (?( 27 conditional group contains more than two branches 28 assertion expected after (?( 29 (?R or (?[+-]digits must be followed by ) 30 unknown POSIX class name 31 POSIX collating elements are not supported 32 this version of PCRE is not compiled with PCRE_UTF8 support 33 [this code is not in use] 34 character value in \x{...} sequence is too large 35 invalid condition (?(0) 36 \C not allowed in lookbehind assertion 37 PCRE does not support \L, \l, \N, \U, or \u 38 number after (?C is > 255 39 closing ) for (?C expected 40 recursive call could loop indefinitely 41 unrecognized character after (?P 42 syntax error in subpattern name (missing terminator) 43 two named subpatterns have the same name 44 invalid UTF-8 string 45 support for \P, \p, and \X has not been compiled 46 malformed \P or \p sequence 47 unknown property name after \P or \p 48 subpattern name is too long (maximum 32 characters) 49 too many named subpatterns (maximum 10000) 50 [this code is not in use] 51 octal value is greater than \377 (not in UTF-8 mode) 52 internal error: overran compiling workspace 53 internal error: previously-checked referenced subpattern not found 54 DEFINE group contains more than one branch 55 repeating a DEFINE group is not allowed 56 inconsistent NEWLINE options 57 \g is not followed by a braced, angle-bracketed, or quoted name/number or by a plain number 58 a numbered reference must not be zero 59 (*VERB) with an argument is not supported 60 (*VERB) not recognized 61 number is too big 62 subpattern name expected 63 digit expected after (?+ 64 ] is an invalid data character in JavaScript compatibility mode The numbers 32 and 10000 in errors 48 and 49 are defaults; different values may be used if the limits were changed when PCRE was built. STUDYING A PATTERN pcre_extra *pcre_study(const pcre *code, int options const char **errptr); If a compiled pattern is going to be used several times, it is worth spending more time analyzing it in order to speed up the time taken for matching. The function pcre_study() takes a pointer to a compiled pat- tern as its first argument. If studying the pattern produces additional information that will help speed up matching, pcre_study() returns a pointer to a pcre_extra block, in which the study_data field points to the results of the study. The returned value from pcre_study() can be passed directly to pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con- tains other fields that can be set by the caller before the block is passed; these are described below in the section on matching a pattern. If studying the pattern does not produce any useful information, pcre_study() returns NULL. In that circumstance, if the calling program wants to pass any of the other fields to pcre_exec() or pcre_dfa_exec(), it must set up its own pcre_extra block. The second argument of pcre_study() contains option bits. At present, no options are defined, and this argument should always be zero. The third argument for pcre_study() is a pointer for an error message. If studying succeeds (even if no data is returned), the variable it points to is set to NULL. Otherwise it is set to point to a textual error message. This is a static string that is part of the library. You must not try to free it. You should test the error pointer for NULL after calling pcre_study(), to be sure that it has run successfully. This is a typical call to pcre_study(): pcre_extra *pe; pe = pcre_study( re, /* result of pcre_compile() */ 0, /* no options exist */ &error); /* set to NULL or points to a message */ Studying a pattern does two things: first, a lower bound for the length of subject string that is needed to match the pattern is computed. This does not mean that there are any strings of that length that match, but it does guarantee that no shorter strings match. The value is used by pcre_exec() and pcre_dfa_exec() to avoid wasting time by trying to match strings that are shorter than the lower bound. You can find out the value in a calling program via the pcre_fullinfo() function. Studying a pattern is also useful for non-anchored patterns that do not have a single fixed starting character. A bitmap of possible starting bytes is created. This speeds up finding a position in the subject at which to start matching. LOCALE SUPPORT PCRE handles caseless matching, and determines whether characters are letters, digits, or whatever, by reference to a set of tables, indexed by character value. When running in UTF-8 mode, this applies only to characters with codes less than 128. Higher-valued codes never match escapes such as \w or \d, but can be tested with \p if PCRE is built with Unicode character property support. The use of locales with Uni- code is discouraged. If you are handling characters with codes greater than 128, you should either use UTF-8 and Unicode, or use locales, but not try to mix the two. PCRE contains an internal set of tables that are used when the final argument of pcre_compile() is NULL. These are sufficient for many applications. Normally, the internal tables recognize only ASCII char- acters. However, when PCRE is built, it is possible to cause the inter- nal tables to be rebuilt in the default "C" locale of the local system, which may cause them to be different. The internal tables can always be overridden by tables supplied by the application that calls PCRE. These may be created in a different locale from the default. As more and more applications change to using Uni- code, the need for this locale support is expected to die away. External tables are built by calling the pcre_maketables() function, which has no arguments, in the relevant locale. The result can then be passed to pcre_compile() or pcre_exec() as often as necessary. For example, to build and use tables that are appropriate for the French locale (where accented characters with values greater than 128 are treated as letters), the following code could be used: setlocale(LC_CTYPE, "fr_FR"); tables = pcre_maketables(); re = pcre_compile(..., tables); The locale name "fr_FR" is used on Linux and other Unix-like systems; if you are using Windows, the name for the French locale is "french". When pcre_maketables() runs, the tables are built in memory that is obtained via pcre_malloc. It is the caller's responsibility to ensure that the memory containing the tables remains available for as long as it is needed. The pointer that is passed to pcre_compile() is saved with the compiled pattern, and the same tables are used via this pointer by pcre_study() and normally also by pcre_exec(). Thus, by default, for any single pat- tern, compilation, studying and matching all happen in the same locale, but different patterns can be compiled in different locales. It is possible to pass a table pointer or NULL (indicating the use of the internal tables) to pcre_exec(). Although not intended for this purpose, this facility could be used to match a pattern in a different locale from the one in which it was compiled. Passing table pointers at run time is discussed below in the section on matching a pattern. INFORMATION ABOUT A PATTERN int pcre_fullinfo(const pcre *code, const pcre_extra *extra, int what, void *where); The pcre_fullinfo() function returns information about a compiled pat- tern. It replaces the obsolete pcre_info() function, which is neverthe- less retained for backwards compability (and is documented below). The first argument for pcre_fullinfo() is a pointer to the compiled pattern. The second argument is the result of pcre_study(), or NULL if the pattern was not studied. The third argument specifies which piece of information is required, and the fourth argument is a pointer to a variable to receive the data. The yield of the function is zero for success, or one of the following negative numbers: PCRE_ERROR_NULL the argument code was NULL the argument where was NULL PCRE_ERROR_BADMAGIC the "magic number" was not found PCRE_ERROR_BADOPTION the value of what was invalid The "magic number" is placed at the start of each compiled pattern as an simple check against passing an arbitrary memory pointer. Here is a typical call of pcre_fullinfo(), to obtain the length of the compiled pattern: int rc; size_t length; rc = pcre_fullinfo( re, /* result of pcre_compile() */ pe, /* result of pcre_study(), or NULL */ PCRE_INFO_SIZE, /* what is required */ &length); /* where to put the data */ The possible values for the third argument are defined in pcre.h, and are as follows: PCRE_INFO_BACKREFMAX Return the number of the highest back reference in the pattern. The fourth argument should point to an int variable. Zero is returned if there are no back references. PCRE_INFO_CAPTURECOUNT Return the number of capturing subpatterns in the pattern. The fourth argument should point to an int variable. PCRE_INFO_DEFAULT_TABLES Return a pointer to the internal default character tables within PCRE. The fourth argument should point to an unsigned char * variable. This information call is provided for internal use by the pcre_study() func- tion. External callers can cause PCRE to use its internal tables by passing a NULL table pointer. PCRE_INFO_FIRSTBYTE Return information about the first byte of any matched string, for a non-anchored pattern. The fourth argument should point to an int vari- able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name is still recognized for backwards compatibility.) If there is a fixed first byte, for example, from a pattern such as (cat|cow|coyote), its value is returned. Otherwise, if either (a) the pattern was compiled with the PCRE_MULTILINE option, and every branch starts with "^", or (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set (if it were set, the pattern would be anchored), -1 is returned, indicating that the pattern matches only at the start of a subject string or after any newline within the string. Otherwise -2 is returned. For anchored patterns, -2 is returned. PCRE_INFO_FIRSTTABLE If the pattern was studied, and this resulted in the construction of a 256-bit table indicating a fixed set of bytes for the first byte in any matching string, a pointer to the table is returned. Otherwise NULL is returned. The fourth argument should point to an unsigned char * vari- able. PCRE_INFO_HASCRORLF Return 1 if the pattern contains any explicit matches for CR or LF characters, otherwise 0. The fourth argument should point to an int variable. An explicit match is either a literal CR or LF character, or \r or \n. PCRE_INFO_JCHANGED Return 1 if the (?J) or (?-J) option setting is used in the pattern, otherwise 0. The fourth argument should point to an int variable. (?J) and (?-J) set and unset the local PCRE_DUPNAMES option, respectively. PCRE_INFO_LASTLITERAL Return the value of the rightmost literal byte that must exist in any matched string, other than at its start, if such a byte has been recorded. The fourth argument should point to an int variable. If there is no such byte, -1 is returned. For anchored patterns, a last literal byte is recorded only if it follows something of variable length. For example, for the pattern /^a\d+z\d+/ the returned value is "z", but for /^a\dz\d/ the returned value is -1. PCRE_INFO_MINLENGTH If the pattern was studied and a minimum length for matching subject strings was computed, its value is returned. Otherwise the returned value is -1. The value is a number of characters, not bytes (this may be relevant in UTF-8 mode). The fourth argument should point to an int variable. A non-negative value is a lower bound to the length of any matching string. There may not be any strings of that length that do actually match, but every string that does match is at least that long. PCRE_INFO_NAMECOUNT PCRE_INFO_NAMEENTRYSIZE PCRE_INFO_NAMETABLE PCRE supports the use of named as well as numbered capturing parenthe- ses. The names are just an additional way of identifying the parenthe- ses, which still acquire numbers. Several convenience functions such as pcre_get_named_substring() are provided for extracting captured sub- strings by name. It is also possible to extract the data directly, by first converting the name to a number in order to access the correct pointers in the output vector (described with pcre_exec() below). To do the conversion, you need to use the name-to-number map, which is described by these three values. The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size of each entry; both of these return an int value. The entry size depends on the length of the longest name. PCRE_INFO_NAMETABLE returns a pointer to the first entry of the table (a pointer to char). The first two bytes of each entry are the number of the capturing parenthe- sis, most significant byte first. The rest of the entry is the corre- sponding name, zero terminated. The names are in alphabetical order. Duplicate names may appear if (?| is used to create multiple groups with the same number, as described in the section on duplicate subpattern numbers in the pcrepattern page. Duplicate names for subpatterns with different numbers are permitted only if PCRE_DUPNAMES is set. In all cases of duplicate names, they appear in the table in the order in which they were found in the pat- tern. In the absence of (?| this is the order of increasing number; when (?| is used this is not necessarily the case because later subpat- terns may have lower numbers. As a simple example of the name/number table, consider the following pattern (assume PCRE_EXTENDED is set, so white space - including new- lines - is ignored): (?<date> (?<year>(\d\d)?\d\d) - (?<month>\d\d) - (?<day>\d\d) ) There are four named subpatterns, so the table has four entries, and each entry in the table is eight bytes long. The table is as follows, with non-printing bytes shows in hexadecimal, and undefined bytes shown as ??: 00 01 d a t e 00 ?? 00 05 d a y 00 ?? ?? 00 04 m o n t h 00 00 02 y e a r 00 ?? When writing code to extract data from named subpatterns using the name-to-number map, remember that the length of the entries is likely to be different for each compiled pattern. PCRE_INFO_OKPARTIAL Return 1 if the pattern can be used for partial matching with pcre_exec(), otherwise 0. The fourth argument should point to an int variable. From release 8.00, this always returns 1, because the restrictions that previously applied to partial matching have been lifted. The pcrepartial documentation gives details of partial match- ing. PCRE_INFO_OPTIONS Return a copy of the options with which the pattern was compiled. The fourth argument should point to an unsigned long int variable. These option bits are those specified in the call to pcre_compile(), modified by any top-level option settings at the start of the pattern itself. In other words, they are the options that will be in force when matching starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE, and PCRE_EXTENDED. A pattern is automatically anchored by PCRE if all of its top-level alternatives begin with one of the following: ^ unless PCRE_MULTILINE is set \A always \G always .* if PCRE_DOTALL is set and there are no back references to the subpattern in which .* appears For such patterns, the PCRE_ANCHORED bit is set in the options returned by pcre_fullinfo(). PCRE_INFO_SIZE Return the size of the compiled pattern, that is, the value that was passed as the argument to pcre_malloc() when PCRE was getting memory in which to place the compiled data. The fourth argument should point to a size_t variable. PCRE_INFO_STUDYSIZE Return the size of the data block pointed to by the study_data field in a pcre_extra block. That is, it is the value that was passed to pcre_malloc() when PCRE was getting memory into which to place the data created by pcre_study(). If pcre_extra is NULL, or there is no study data, zero is returned. The fourth argument should point to a size_t variable. OBSOLETE INFO FUNCTION int pcre_info(const pcre *code, int *optptr, int *firstcharptr); The pcre_info() function is now obsolete because its interface is too restrictive to return all the available data about a compiled pattern. New programs should use pcre_fullinfo() instead. The yield of pcre_info() is the number of capturing subpatterns, or one of the fol- lowing negative numbers: PCRE_ERROR_NULL the argument code was NULL PCRE_ERROR_BADMAGIC the "magic number" was not found If the optptr argument is not NULL, a copy of the options with which the pattern was compiled is placed in the integer it points to (see PCRE_INFO_OPTIONS above). If the pattern is not anchored and the firstcharptr argument is not NULL, it is used to pass back information about the first character of any matched string (see PCRE_INFO_FIRSTBYTE above). REFERENCE COUNTS int pcre_refcount(pcre *code, int adjust); The pcre_refcount() function is used to maintain a reference count in the data block that contains a compiled pattern. It is provided for the benefit of applications that operate in an object-oriented manner, where different parts of the application may be using the same compiled pattern, but you want to free the block when they are all done. When a pattern is compiled, the reference count field is initialized to zero. It is changed only by calling this function, whose action is to add the adjust value (which may be positive or negative) to it. The yield of the function is the new value. However, the value of the count is constrained to lie between 0 and 65535, inclusive. If the new value is outside these limits, it is forced to the appropriate limit value. Except when it is zero, the reference count is not correctly preserved if a pattern is compiled on one host and then transferred to a host whose byte-order is different. (This seems a highly unlikely scenario.) MATCHING A PATTERN: THE TRADITIONAL FUNCTION int pcre_exec(const pcre *code, const pcre_extra *extra, const char *subject, int length, int startoffset, int options, int *ovector, int ovecsize); The function pcre_exec() is called to match a subject string against a compiled pattern, which is passed in the code argument. If the pattern was studied, the result of the study should be passed in the extra argument. This function is the main matching facility of the library, and it operates in a Perl-like manner. For specialist use there is also an alternative matching function, which is described below in the sec- tion about the pcre_dfa_exec() function. In most applications, the pattern will have been compiled (and option- ally studied) in the same process that calls pcre_exec(). However, it is possible to save compiled patterns and study data, and then use them later in different processes, possibly even on different hosts. For a discussion about this, see the pcreprecompile documentation. Here is an example of a simple call to pcre_exec(): int rc; int ovector[30]; rc = pcre_exec( re, /* result of pcre_compile() */ NULL, /* we didn't study the pattern */ "some string", /* the subject string */ 11, /* the length of the subject string */ 0, /* start at offset 0 in the subject */ 0, /* default options */ ovector, /* vector of integers for substring information */ 30); /* number of elements (NOT size in bytes) */ Extra data for pcre_exec() If the extra argument is not NULL, it must point to a pcre_extra data block. The pcre_study() function returns such a block (when it doesn't return NULL), but you can also create one for yourself, and pass addi- tional information in it. The pcre_extra block contains the following fields (not necessarily in this order): unsigned long int flags; void *study_data; unsigned long int match_limit; unsigned long int match_limit_recursion; void *callout_data; const unsigned char *tables; The flags field is a bitmap that specifies which of the other fields are set. The flag bits are: PCRE_EXTRA_STUDY_DATA PCRE_EXTRA_MATCH_LIMIT PCRE_EXTRA_MATCH_LIMIT_RECURSION PCRE_EXTRA_CALLOUT_DATA PCRE_EXTRA_TABLES Other flag bits should be set to zero. The study_data field is set in the pcre_extra block that is returned by pcre_study(), together with the appropriate flag bit. You should not set this yourself, but you may add to the block by setting the other fields and their corresponding flag bits. The match_limit field provides a means of preventing PCRE from using up a vast amount of resources when running patterns that are not going to match, but which have a very large number of possibilities in their search trees. The classic example is a pattern that uses nested unlim- ited repeats. Internally, PCRE uses a function called match() which it calls repeat- edly (sometimes recursively). The limit set by match_limit is imposed on the number of times this function is called during a match, which has the effect of limiting the amount of backtracking that can take place. For patterns that are not anchored, the count restarts from zero for each position in the subject string. The default value for the limit can be set when PCRE is built; the default default is 10 million, which handles all but the most extreme cases. You can override the default by suppling pcre_exec() with a pcre_extra block in which match_limit is set, and PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT. The match_limit_recursion field is similar to match_limit, but instead of limiting the total number of times that match() is called, it limits the depth of recursion. The recursion depth is a smaller number than the total number of calls, because not all calls to match() are recur- sive. This limit is of use only if it is set smaller than match_limit. Limiting the recursion depth limits the amount of stack that can be used, or, when PCRE has been compiled to use memory on the heap instead of the stack, the amount of heap memory that can be used. The default value for match_limit_recursion can be set when PCRE is built; the default default is the same value as the default for match_limit. You can override the default by suppling pcre_exec() with a pcre_extra block in which match_limit_recursion is set, and PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT. The callout_data field is used in conjunction with the "callout" fea- ture, and is described in the pcrecallout documentation. The tables field is used to pass a character tables pointer to pcre_exec(); this overrides the value that is stored with the compiled pattern. A non-NULL value is stored with the compiled pattern only if custom tables were supplied to pcre_compile() via its tableptr argu- ment. If NULL is passed to pcre_exec() using this mechanism, it forces PCRE's internal tables to be used. This facility is helpful when re- using patterns that have been saved after compiling with an external set of tables, because the external tables might be at a different address when pcre_exec() is called. See the pcreprecompile documenta- tion for a discussion of saving compiled patterns for later use. Option bits for pcre_exec() The unused bits of the options argument for pcre_exec() must be zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_NO_START_OPTIMIZE, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_SOFT, and PCRE_PARTIAL_HARD. PCRE_ANCHORED The PCRE_ANCHORED option limits pcre_exec() to matching at the first matching position. If a pattern was compiled with PCRE_ANCHORED, or turned out to be anchored by virtue of its contents, it cannot be made unachored at matching time. PCRE_BSR_ANYCRLF PCRE_BSR_UNICODE These options (which are mutually exclusive) control what the \R escape sequence matches. The choice is either to match only CR, LF, or CRLF, or to match any Unicode newline sequence. These options override the choice that was made or defaulted when the pattern was compiled. PCRE_NEWLINE_CR PCRE_NEWLINE_LF PCRE_NEWLINE_CRLF PCRE_NEWLINE_ANYCRLF PCRE_NEWLINE_ANY These options override the newline definition that was chosen or defaulted when the pattern was compiled. For details, see the descrip- tion of pcre_compile() above. During matching, the newline choice affects the behaviour of the dot, circumflex, and dollar metacharac- ters. It may also alter the way the match position is advanced after a match failure for an unanchored pattern. When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is set, and a match attempt for an unanchored pattern fails when the cur- rent position is at a CRLF sequence, and the pattern contains no explicit matches for CR or LF characters, the match position is advanced by two characters instead of one, in other words, to after the CRLF. The above rule is a compromise that makes the most common cases work as expected. For example, if the pattern is .+A (and the PCRE_DOTALL option is not set), it does not match the string "\r\nA" because, after failing at the start, it skips both the CR and the LF before retrying. However, the pattern [\r\n]A does match that string, because it con- tains an explicit CR or LF reference, and so advances only by one char- acter after the first failure. An explicit match for CR of LF is either a literal appearance of one of those characters, or one of the \r or \n escape sequences. Implicit matches such as [^X] do not count, nor does \s (which includes CR and LF in the characters that it matches). Notwithstanding the above, anomalous effects may still occur when CRLF is a valid newline sequence and explicit \r or \n escapes appear in the pattern. PCRE_NOTBOL This option specifies that first character of the subject string is not the beginning of a line, so the circumflex metacharacter should not match before it. Setting this without PCRE_MULTILINE (at compile time) causes circumflex never to match. This option affects only the behav- iour of the circumflex metacharacter. It does not affect \A. PCRE_NOTEOL This option specifies that the end of the subject string is not the end of a line, so the dollar metacharacter should not match it nor (except in multiline mode) a newline immediately before it. Setting this with- out PCRE_MULTILINE (at compile time) causes dollar never to match. This option affects only the behaviour of the dollar metacharacter. It does not affect \Z or \z. PCRE_NOTEMPTY An empty string is not considered to be a valid match if this option is set. If there are alternatives in the pattern, they are tried. If all the alternatives match the empty string, the entire match fails. For example, if the pattern a?b? is applied to a string not beginning with "a" or "b", it matches an empty string at the start of the subject. With PCRE_NOTEMPTY set, this match is not valid, so PCRE searches further into the string for occur- rences of "a" or "b". PCRE_NOTEMPTY_ATSTART This is like PCRE_NOTEMPTY, except that an empty string match that is not at the start of the subject is permitted. If the pattern is anchored, such a match can occur only if the pattern contains \K. Perl has no direct equivalent of PCRE_NOTEMPTY or PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern match of the empty string within its split() function, and when using the /g modifier. It is possible to emulate Perl's behaviour after matching a null string by first trying the match again at the same off- set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that fails, by advancing the starting offset (see below) and trying an ordi- nary match again. There is some code that demonstrates how to do this in the pcredemo sample program. PCRE_NO_START_OPTIMIZE There are a number of optimizations that pcre_exec() uses at the start of a match, in order to speed up the process. For example, if it is known that a match must start with a specific character, it searches the subject for that character, and fails immediately if it cannot find it, without actually running the main matching function. When callouts are in use, these optimizations can cause them to be skipped. This option disables the "start-up" optimizations, causing performance to suffer, but ensuring that the callouts do occur. PCRE_NO_UTF8_CHECK When PCRE_UTF8 is set at compile time, the validity of the subject as a UTF-8 string is automatically checked when pcre_exec() is subsequently called. The value of startoffset is also checked to ensure that it points to the start of a UTF-8 character. There is a discussion about the validity of UTF-8 strings in the section on UTF-8 support in the main pcre page. If an invalid UTF-8 sequence of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If startoffset con- tains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned. If you already know that your subject is valid, and you want to skip these checks for performance reasons, you can set the PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to do this for the second and subsequent calls to pcre_exec() if you are making repeated calls to find all the matches in a single subject string. However, you should be sure that the value of startoffset points to the start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid UTF-8 string as a subject, or a value of startoffset that does not point to the start of a UTF-8 char- acter, is undefined. Your program may crash. PCRE_PARTIAL_HARD PCRE_PARTIAL_SOFT These options turn on the partial matching feature. For backwards com- patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial match occurs if the end of the subject string is reached successfully, but there are not enough subject characters to complete the match. If this happens when PCRE_PARTIAL_HARD is set, pcre_exec() immediately returns PCRE_ERROR_PARTIAL. Otherwise, if PCRE_PARTIAL_SOFT is set, matching continues by testing any other alternatives. Only if they all fail is PCRE_ERROR_PARTIAL returned (instead of PCRE_ERROR_NOMATCH). The portion of the string that was inspected when the partial match was found is set as the first matching string. There is a more detailed discussion in the pcrepartial documentation. The string to be matched by pcre_exec() The subject string is passed to pcre_exec() as a pointer in subject, a length (in bytes) in length, and a starting byte offset in startoffset. In UTF-8 mode, the byte offset must point to the start of a UTF-8 char- acter. Unlike the pattern string, the subject may contain binary zero bytes. When the starting offset is zero, the search for a match starts at the beginning of the subject, and this is by far the most common case. A non-zero starting offset is useful when searching for another match in the same subject by calling pcre_exec() again after a previous suc- cess. Setting startoffset differs from just passing over a shortened string and setting PCRE_NOTBOL in the case of a pattern that begins with any kind of lookbehind. For example, consider the pattern \Biss\B which finds occurrences of "iss" in the middle of words. (\B matches only if the current position in the subject is not a word boundary.) When applied to the string "Mississipi" the first call to pcre_exec() finds the first occurrence. If pcre_exec() is called again with just the remainder of the subject, namely "issipi", it does not match, because \B is always false at the start of the subject, which is deemed to be a word boundary. However, if pcre_exec() is passed the entire string again, but with startoffset set to 4, it finds the second occur- rence of "iss" because it is able to look behind the starting point to discover that it is preceded by a letter. If a non-zero starting offset is passed when the pattern is anchored, one attempt to match at the given offset is made. This can only succeed if the pattern does not require the match to be at the start of the subject. How pcre_exec() returns captured substrings In general, a pattern matches a certain portion of the subject, and in addition, further substrings from the subject may be picked out by parts of the pattern. Following the usage in Jeffrey Friedl's book, this is called "capturing" in what follows, and the phrase "capturing subpattern" is used for a fragment of a pattern that picks out a sub- string. PCRE supports several other kinds of parenthesized subpattern that do not cause substrings to be captured. Captured substrings are returned to the caller via a vector of integers whose address is passed in ovector. The number of elements in the vec- tor is passed in ovecsize, which must be a non-negative number. Note: this argument is NOT the size of ovector in bytes. The first two-thirds of the vector is used to pass back captured sub- strings, each substring using a pair of integers. The remaining third of the vector is used as workspace by pcre_exec() while matching cap- turing subpatterns, and is not available for passing back information. The number passed in ovecsize should always be a multiple of three. If it is not, it is rounded down. When a match is successful, information about captured substrings is returned in pairs of integers, starting at the beginning of ovector, and continuing up to two-thirds of its length at the most. The first element of each pair is set to the byte offset of the first character in a substring, and the second is set to the byte offset of the first character after the end of a substring. Note: these values are always byte offsets, even in UTF-8 mode. They are not character counts. The first pair of integers, ovector[0] and ovector[1], identify the portion of the subject string matched by the entire pattern. The next pair is used for the first capturing subpattern, and so on. The value returned by pcre_exec() is one more than the highest numbered pair that has been set. For example, if two substrings have been captured, the returned value is 3. If there are no capturing subpatterns, the return value from a successful match is 1, indicating that just the first pair of offsets has been set. If a capturing subpattern is matched repeatedly, it is the last portion of the string that it matched that is returned. If the vector is too small to hold all the captured substring offsets, it is used as far as possible (up to two-thirds of its length), and the function returns a value of zero. If the substring offsets are not of interest, pcre_exec() may be called with ovector passed as NULL and ovecsize as zero. However, if the pattern contains back references and the ovector is not big enough to remember the related substrings, PCRE has to get additional memory for use during matching. Thus it is usu- ally advisable to supply an ovector. The pcre_fullinfo() function can be used to find out how many capturing subpatterns there are in a compiled pattern. The smallest size for ovector that will allow for n captured substrings, in addition to the offsets of the substring matched by the whole pattern, is (n+1)*3. It is possible for capturing subpattern number n+1 to match some part of the subject when subpattern n has not been used at all. For example, if the string "abc" is matched against the pattern (a|(z))(bc) the return from the function is 4, and subpatterns 1 and 3 are matched, but 2 is not. When this happens, both values in the offset pairs corre- sponding to unused subpatterns are set to -1. Offset values that correspond to unused subpatterns at the end of the expression are also set to -1. For example, if the string "abc" is matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not matched. The return from the function is 2, because the highest used capturing subpattern number is 1. However, you can refer to the offsets for the second and third capturing subpatterns if you wish (assuming the vector is large enough, of course). Some convenience functions are provided for extracting the captured substrings as separate strings. These are described below. Error return values from pcre_exec() If pcre_exec() fails, it returns a negative number. The following are defined in the header file: PCRE_ERROR_NOMATCH (-1) The subject string did not match the pattern. PCRE_ERROR_NULL (-2) Either code or subject was passed as NULL, or ovector was NULL and ovecsize was not zero. PCRE_ERROR_BADOPTION (-3) An unrecognized bit was set in the options argument. PCRE_ERROR_BADMAGIC (-4) PCRE stores a 4-byte "magic number" at the start of the compiled code, to catch the case when it is passed a junk pointer and to detect when a pattern that was compiled in an environment of one endianness is run in an environment with the other endianness. This is the error that PCRE gives when the magic number is not present. PCRE_ERROR_UNKNOWN_OPCODE (-5) While running the pattern match, an unknown item was encountered in the compiled pattern. This error could be caused by a bug in PCRE or by overwriting of the compiled pattern. PCRE_ERROR_NOMEMORY (-6) If a pattern contains back references, but the ovector that is passed to pcre_exec() is not big enough to remember the referenced substrings, PCRE gets a block of memory at the start of matching to use for this purpose. If the call via pcre_malloc() fails, this error is given. The memory is automatically freed at the end of matching. PCRE_ERROR_NOSUBSTRING (-7) This error is used by the pcre_copy_substring(), pcre_get_substring(), and pcre_get_substring_list() functions (see below). It is never returned by pcre_exec(). PCRE_ERROR_MATCHLIMIT (-8) The backtracking limit, as specified by the match_limit field in a pcre_extra structure (or defaulted) was reached. See the description above. PCRE_ERROR_CALLOUT (-9) This error is never generated by pcre_exec() itself. It is provided for use by callout functions that want to yield a distinctive error code. See the pcrecallout documentation for details. PCRE_ERROR_BADUTF8 (-10) A string that contains an invalid UTF-8 byte sequence was passed as a subject. PCRE_ERROR_BADUTF8_OFFSET (-11) The UTF-8 byte sequence that was passed as a subject was valid, but the value of startoffset did not point to the beginning of a UTF-8 charac- ter. PCRE_ERROR_PARTIAL (-12) The subject string did not match, but it did match partially. See the pcrepartial documentation for details of partial matching. PCRE_ERROR_BADPARTIAL (-13) This code is no longer in use. It was formerly returned when the PCRE_PARTIAL option was used with a compiled pattern containing items that were not supported for partial matching. From release 8.00 onwards, there are no restrictions on partial matching. PCRE_ERROR_INTERNAL (-14) An unexpected internal error has occurred. This error could be caused by a bug in PCRE or by overwriting of the compiled pattern. PCRE_ERROR_BADCOUNT (-15) This error is given if the value of the ovecsize argument is negative. PCRE_ERROR_RECURSIONLIMIT (-21) The internal recursion limit, as specified by the match_limit_recursion field in a pcre_extra structure (or defaulted) was reached. See the description above. PCRE_ERROR_BADNEWLINE (-23) An invalid combination of PCRE_NEWLINE_xxx options was given. Error numbers -16 to -20 and -22 are not used by pcre_exec(). EXTRACTING CAPTURED SUBSTRINGS BY NUMBER int pcre_copy_substring(const char *subject, int *ovector, int stringcount, int stringnumber, char *buffer, int buffersize); int pcre_get_substring(const char *subject, int *ovector, int stringcount, int stringnumber, const char **stringptr); int pcre_get_substring_list(const char *subject, int *ovector, int stringcount, const char ***listptr); Captured substrings can be accessed directly by using the offsets returned by pcre_exec() in ovector. For convenience, the functions pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub- string_list() are provided for extracting captured substrings as new, separate, zero-terminated strings. These functions identify substrings by number. The next section describes functions for extracting named substrings. A substring that contains a binary zero is correctly extracted and has a further zero added on the end, but the result is not, of course, a C string. However, you can process such a string by referring to the length that is returned by pcre_copy_substring() and pcre_get_sub- string(). Unfortunately, the interface to pcre_get_substring_list() is not adequate for handling strings containing binary zeros, because the end of the final string is not independently indicated. The first three arguments are the same for all three of these func- tions: subject is the subject string that has just been successfully matched, ovector is a pointer to the vector of integer offsets that was passed to pcre_exec(), and stringcount is the number of substrings that were captured by the match, including the substring that matched the entire regular expression. This is the value returned by pcre_exec() if it is greater than zero. If pcre_exec() returned zero, indicating that it ran out of space in ovector, the value passed as stringcount should be the number of elements in the vector divided by three. The functions pcre_copy_substring() and pcre_get_substring() extract a single substring, whose number is given as stringnumber. A value of zero extracts the substring that matched the entire pattern, whereas higher values extract the captured substrings. For pcre_copy_sub- string(), the string is placed in buffer, whose length is given by buffersize, while for pcre_get_substring() a new block of memory is obtained via pcre_malloc, and its address is returned via stringptr. The yield of the function is the length of the string, not including the terminating zero, or one of these error codes: PCRE_ERROR_NOMEMORY (-6) The buffer was too small for pcre_copy_substring(), or the attempt to get memory failed for pcre_get_substring(). PCRE_ERROR_NOSUBSTRING (-7) There is no substring whose number is stringnumber. The pcre_get_substring_list() function extracts all available sub- strings and builds a list of pointers to them. All this is done in a single block of memory that is obtained via pcre_malloc. The address of the memory block is returned via listptr, which is also the start of the list of string pointers. The end of the list is marked by a NULL pointer. The yield of the function is zero if all went well, or the error code PCRE_ERROR_NOMEMORY (-6) if the attempt to get the memory block failed. When any of these functions encounter a substring that is unset, which can happen when capturing subpattern number n+1 matches some part of the subject, but subpattern n has not been used at all, they return an empty string. This can be distinguished from a genuine zero-length sub- string by inspecting the appropriate offset in ovector, which is nega- tive for unset substrings. The two convenience functions pcre_free_substring() and pcre_free_sub- string_list() can be used to free the memory returned by a previous call of pcre_get_substring() or pcre_get_substring_list(), respec- tively. They do nothing more than call the function pointed to by pcre_free, which of course could be called directly from a C program. However, PCRE is used in some situations where it is linked via a spe- cial interface to another programming language that cannot use pcre_free directly; it is for these cases that the functions are pro- vided. EXTRACTING CAPTURED SUBSTRINGS BY NAME int pcre_get_stringnumber(const pcre *code, const char *name); int pcre_copy_named_substring(const pcre *code, const char *subject, int *ovector, int stringcount, const char *stringname, char *buffer, int buffersize); int pcre_get_named_substring(const pcre *code, const char *subject, int *ovector, int stringcount, const char *stringname, const char **stringptr); To extract a substring by name, you first have to find associated num- ber. For example, for this pattern (a+)b(?<xxx>\d+)... the number of the subpattern called "xxx" is 2. If the name is known to be unique (PCRE_DUPNAMES was not set), you can find the number from the name by calling pcre_get_stringnumber(). The first argument is the com- piled pattern, and the second is the name. The yield of the function is the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no subpattern of that name. Given the number, you can extract the substring directly, or use one of the functions described in the previous section. For convenience, there are also two functions that do the whole job. Most of the arguments of pcre_copy_named_substring() and pcre_get_named_substring() are the same as those for the similarly named functions that extract by number. As these are described in the previous section, they are not re-described here. There are just two differences: First, instead of a substring number, a substring name is given. Sec- ond, there is an extra argument, given at the start, which is a pointer to the compiled pattern. This is needed in order to gain access to the name-to-number translation table. These functions call pcre_get_stringnumber(), and if it succeeds, they then call pcre_copy_substring() or pcre_get_substring(), as appropri- ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the behaviour may not be what you want (see the next section). Warning: If the pattern uses the (?| feature to set up multiple subpat- terns with the same number, as described in the section on duplicate subpattern numbers in the pcrepattern page, you cannot use names to distinguish the different subpatterns, because names are not included in the compiled code. The matching process uses only numbers. For this reason, the use of different names for subpatterns of the same number causes an error at compile time. DUPLICATE SUBPATTERN NAMES int pcre_get_stringtable_entries(const pcre *code, const char *name, char **first, char **last); When a pattern is compiled with the PCRE_DUPNAMES option, names for subpatterns are not required to be unique. (Duplicate names are always allowed for subpatterns with the same number, created by using the (?| feature. Indeed, if such subpatterns are named, they are required to use the same names.) Normally, patterns with duplicate names are such that in any one match, only one of the named subpatterns participates. An example is shown in the pcrepattern documentation. When duplicates are present, pcre_copy_named_substring() and pcre_get_named_substring() return the first substring corresponding to the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING (-7) is returned; no data is returned. The pcre_get_stringnumber() function returns one of the numbers that are associated with the name, but it is not defined which it is. If you want to get full details of all captured substrings for a given name, you must use the pcre_get_stringtable_entries() function. The first argument is the compiled pattern, and the second is the name. The third and fourth are pointers to variables which are updated by the function. After it has run, they point to the first and last entries in the name-to-number table for the given name. The function itself returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if there are none. The format of the table is described above in the sec- tion entitled Information about a pattern. Given all the relevant entries for the name, you can extract each of their numbers, and hence the captured data, if any. FINDING ALL POSSIBLE MATCHES The traditional matching function uses a similar algorithm to Perl, which stops when it finds the first match, starting at a given point in the subject. If you want to find all possible matches, or the longest possible match, consider using the alternative matching function (see below) instead. If you cannot use the alternative function, but still need to find all possible matches, you can kludge it up by making use of the callout facility, which is described in the pcrecallout documen- tation. What you have to do is to insert a callout right at the end of the pat- tern. When your callout function is called, extract and save the cur- rent matched substring. Then return 1, which forces pcre_exec() to backtrack and try other alternatives. Ultimately, when it runs out of matches, pcre_exec() will yield PCRE_ERROR_NOMATCH. MATCHING A PATTERN: THE ALTERNATIVE FUNCTION int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, const char *subject, int length, int startoffset, int options, int *ovector, int ovecsize, int *workspace, int wscount); The function pcre_dfa_exec() is called to match a subject string against a compiled pattern, using a matching algorithm that scans the subject string just once, and does not backtrack. This has different characteristics to the normal algorithm, and is not compatible with Perl. Some of the features of PCRE patterns are not supported. Never- theless, there are times when this kind of matching can be useful. For a discussion of the two matching algorithms, and a list of features that pcre_dfa_exec() does not support, see the pcrematching documenta- tion. The arguments for the pcre_dfa_exec() function are the same as for pcre_exec(), plus two extras. The ovector argument is used in a differ- ent way, and this is described below. The other common arguments are used in the same way as for pcre_exec(), so their description is not repeated here. The two additional arguments provide workspace for the function. The workspace vector should contain at least 20 elements. It is used for keeping track of multiple paths through the pattern tree. More workspace will be needed for patterns and subjects where there are a lot of potential matches. Here is an example of a simple call to pcre_dfa_exec(): int rc; int ovector[10]; int wspace[20]; rc = pcre_dfa_exec( re, /* result of pcre_compile() */ NULL, /* we didn't study the pattern */ "some string", /* the subject string */ 11, /* the length of the subject string */ 0, /* start at offset 0 in the subject */ 0, /* default options */ ovector, /* vector of integers for substring information */ 10, /* number of elements (NOT size in bytes) */ wspace, /* working space vector */ 20); /* number of elements (NOT size in bytes) */ Option bits for pcre_dfa_exec() The unused bits of the options argument for pcre_dfa_exec() must be zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW- LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_HARD, PCRE_PAR- TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last four of these are exactly the same as for pcre_exec(), so their description is not repeated here. PCRE_PARTIAL_HARD PCRE_PARTIAL_SOFT These have the same general effect as they do for pcre_exec(), but the details are slightly different. When PCRE_PARTIAL_HARD is set for pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub- ject is reached and there is still at least one matching possibility that requires additional characters. This happens even if some complete matches have also been found. When PCRE_PARTIAL_SOFT is set, the return code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of the subject is reached, there have been no complete matches, but there is still at least one matching possibility. The portion of the string that was inspected when the longest partial match was found is set as the first matching string in both cases. PCRE_DFA_SHORTEST Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to stop as soon as it has found one match. Because of the way the alterna- tive algorithm works, this is necessarily the shortest possible match at the first possible matching point in the subject string. PCRE_DFA_RESTART When pcre_dfa_exec() returns a partial match, it is possible to call it again, with additional subject characters, and have it continue with the same match. The PCRE_DFA_RESTART option requests this action; when it is set, the workspace and wscount options must reference the same vector as before because data about the match so far is left in them after a partial match. There is more discussion of this facility in the pcrepartial documentation. Successful returns from pcre_dfa_exec() When pcre_dfa_exec() succeeds, it may have matched more than one sub- string in the subject. Note, however, that all the matches from one run of the function start at the same point in the subject. The shorter matches are all initial substrings of the longer matches. For example, if the pattern <.*> is matched against the string This is <something> <something else> <something further> no more the three matched strings are <something> <something> <something else> <something> <something else> <something further> On success, the yield of the function is a number greater than zero, which is the number of matched substrings. The substrings themselves are returned in ovector. Each string uses two elements; the first is the offset to the start, and the second is the offset to the end. In fact, all the strings have the same start offset. (Space could have been saved by giving this only once, but it was decided to retain some compatibility with the way pcre_exec() returns data, even though the meaning of the strings is different.) The strings are returned in reverse order of length; that is, the long- est matching string is given first. If there were too many matches to fit into ovector, the yield of the function is zero, and the vector is filled with the longest matches. Error returns from pcre_dfa_exec() The pcre_dfa_exec() function returns a negative number when it fails. Many of the errors are the same as for pcre_exec(), and these are described above. There are in addition the following errors that are specific to pcre_dfa_exec(): PCRE_ERROR_DFA_UITEM (-16) This return is given if pcre_dfa_exec() encounters an item in the pat- tern that it does not support, for instance, the use of \C or a back reference. PCRE_ERROR_DFA_UCOND (-17) This return is given if pcre_dfa_exec() encounters a condition item that uses a back reference for the condition, or a test for recursion in a specific group. These are not supported. PCRE_ERROR_DFA_UMLIMIT (-18) This return is given if pcre_dfa_exec() is called with an extra block that contains a setting of the match_limit field. This is not supported (it is meaningless). PCRE_ERROR_DFA_WSSIZE (-19) This return is given if pcre_dfa_exec() runs out of space in the workspace vector. PCRE_ERROR_DFA_RECURSE (-20) When a recursive subpattern is processed, the matching function calls itself recursively, using private vectors for ovector and workspace. This error is given if the output vector is not large enough. This should be extremely rare, as a vector of size 1000 is used. SEE ALSO pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar- tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3). AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 03 October 2009 Copyright (c) 1997-2009 University of Cambridge. ------------------------------------------------------------------------------ PCRECALLOUT(3) PCRECALLOUT(3) NAME PCRE - Perl-compatible regular expressions PCRE CALLOUTS int (*pcre_callout)(pcre_callout_block *); PCRE provides a feature called "callout", which is a means of temporar- ily passing control to the caller of PCRE in the middle of pattern matching. The caller of PCRE provides an external function by putting its entry point in the global variable pcre_callout. By default, this variable contains NULL, which disables all calling out. Within a regular expression, (?C) indicates the points at which the external function is to be called. Different callout points can be identified by putting a number less than 256 after the letter C. The default value is zero. For example, this pattern has two callout points: (?C1)abc(?C2)def If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() or pcre_compile2() is called, PCRE automatically inserts callouts, all with number 255, before each item in the pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern A(\d{2}|--) it is processed as if it were (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255) Notice that there is a callout before and after each parenthesis and alternation bar. Automatic callouts can be used for tracking the progress of pattern matching. The pcretest command has an option that sets automatic callouts; when it is used, the output indicates how the pattern is matched. This is useful information when you are trying to optimize the performance of a particular pattern. MISSING CALLOUTS You should be aware that, because of optimizations in the way PCRE matches patterns by default, callouts sometimes do not happen. For example, if the pattern is ab(?C4)cd PCRE knows that any matching string must contain the letter "d". If the subject string is "abyz", the lack of "d" means that matching doesn't ever start, and the callout is never reached. However, with "abyd", though the result is still no match, the callout is obeyed. If the pattern is studied, PCRE knows the minimum length of a matching string, and will immediately give a "no match" return without actually running a match if the subject is not long enough, or, for unanchored patterns, if it has been scanned far enough. You can disable these optimizations by passing the PCRE_NO_START_OPTI- MIZE option to pcre_exec() or pcre_dfa_exec(). This slows down the matching process, but does ensure that callouts such as the example above are obeyed. THE CALLOUT INTERFACE During matching, when PCRE reaches a callout point, the external func- tion defined by pcre_callout is called (if it is set). This applies to both the pcre_exec() and the pcre_dfa_exec() matching functions. The only argument to the callout function is a pointer to a pcre_callout block. This structure contains the following fields: int version; int callout_number; int *offset_vector; const char *subject; int subject_length; int start_match; int current_position; int capture_top; int capture_last; void *callout_data; int pattern_position; int next_item_length; The version field is an integer containing the version number of the block format. The initial version was 0; the current version is 1. The version number will change again in future if additional fields are added, but the intention is never to remove any of the existing fields. The callout_number field contains the number of the callout, as com- piled into the pattern (that is, the number after ?C for manual call- outs, and 255 for automatically generated callouts). The offset_vector field is a pointer to the vector of offsets that was passed by the caller to pcre_exec() or pcre_dfa_exec(). When pcre_exec() is used, the contents can be inspected in order to extract substrings that have been matched so far, in the same way as for extracting substrings after a match has completed. For pcre_dfa_exec() this field is not useful. The subject and subject_length fields contain copies of the values that were passed to pcre_exec(). The start_match field normally contains the offset within the subject at which the current match attempt started. However, if the escape sequence \K has been encountered, this value is changed to reflect the modified starting point. If the pattern is not anchored, the callout function may be called several times from the same point in the pattern for different starting points in the subject. The current_position field contains the offset within the subject of the current match pointer. When the pcre_exec() function is used, the capture_top field contains one more than the number of the highest numbered captured substring so far. If no substrings have been captured, the value of capture_top is one. This is always the case when pcre_dfa_exec() is used, because it does not support captured substrings. The capture_last field contains the number of the most recently cap- tured substring. If no substrings have been captured, its value is -1. This is always the case when pcre_dfa_exec() is used. The callout_data field contains a value that is passed to pcre_exec() or pcre_dfa_exec() specifically so that it can be passed back in call- outs. It is passed in the pcre_callout field of the pcre_extra data structure. If no such data was passed, the value of callout_data in a pcre_callout block is NULL. There is a description of the pcre_extra structure in the pcreapi documentation. The pattern_position field is present from version 1 of the pcre_call- out structure. It contains the offset to the next item to be matched in the pattern string. The next_item_length field is present from version 1 of the pcre_call- out structure. It contains the length of the next item to be matched in the pattern string. When the callout immediately precedes an alterna- tion bar, a closing parenthesis, or the end of the pattern, the length is zero. When the callout precedes an opening parenthesis, the length is that of the entire subpattern. The pattern_position and next_item_length fields are intended to help in distinguishing between different automatic callouts, which all have the same callout number. However, they are set for all callouts. RETURN VALUES The external callout function returns an integer to PCRE. If the value is zero, matching proceeds as normal. If the value is greater than zero, matching fails at the current point, but the testing of other matching possibilities goes ahead, just as if a lookahead assertion had failed. If the value is less than zero, the match is abandoned, and pcre_exec() or pcre_dfa_exec() returns the negative value. Negative values should normally be chosen from the set of PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan- dard "no match" failure. The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions; it will never be used by PCRE itself. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 29 September 2009 Copyright (c) 1997-2009 University of Cambridge. ------------------------------------------------------------------------------ PCRECOMPAT(3) PCRECOMPAT(3) NAME PCRE - Perl-compatible regular expressions DIFFERENCES BETWEEN PCRE AND PERL This document describes the differences in the ways that PCRE and Perl handle regular expressions. The differences described here are with respect to Perl 5.10. 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what it does have are given in the section on UTF-8 support in the main pcre page. 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits them, but they do not mean what you might think. For example, (?!a){3} does not assert that the next three characters are not "a". It just asserts that the next character is not "a" three times. 3. Capturing subpatterns that occur inside negative lookahead asser- tions are counted, but their entries in the offsets vector are never set. Perl sets its numerical variables from any such patterns that are matched before the assertion fails to match something (thereby succeed- ing), but only if the negative lookahead assertion contains just one branch. 4. Though binary zero characters are supported in the subject string, they are not allowed in a pattern string because it is passed as a nor- mal C string, terminated by zero. The escape sequence \0 can be used in the pattern to represent a binary zero. 5. The following Perl escape sequences are not supported: \l, \u, \L, \U, and \N. In fact these are implemented by Perl's general string-han- dling and are not part of its pattern matching engine. If any of these are encountered by PCRE, an error is generated. 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is built with Unicode character property support. The properties that can be tested with \p and \P are limited to the general category prop- erties such as Lu and Nd, script names such as Greek or Han, and the derived properties Any and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the Perl documentation says "Because Perl hides the need for the user to understand the internal representa- tion of Unicode characters, there is no need to implement the somewhat messy concept of surrogates." 7. PCRE does support the \Q...\E escape for quoting substrings. Charac- ters in between are treated as literals. This is slightly different from Perl in that $ and @ are also handled as literals inside the quotes. In Perl, they cause variable interpolation (but of course PCRE does not have variables). Note the following examples: Pattern PCRE matches Perl matches \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz \Qabc\$xyz\E abc\$xyz abc\$xyz \Qabc\E\$\Qxyz\E abc$xyz abc$xyz The \Q...\E sequence is recognized both inside and outside character classes. 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code}) constructions. However, there is support for recursive patterns. This is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout" feature allows an external function to be called during pat- tern matching. See the pcrecallout documentation for details. 9. Subpatterns that are called recursively or as "subroutines" are always treated as atomic groups in PCRE. This is like Python, but unlike Perl. There is a discussion of an example that explains this in more detail in the section on recursion differences from Perl in the pcrepattern page. 10. There are some differences that are concerned with the settings of captured strings when part of a pattern is repeated. For example, matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b". 11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F), (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an argument. PCRE does not support (*MARK). 12. PCRE's handling of duplicate subpattern numbers and duplicate sub- pattern names is not as general as Perl's. This is a consequence of the fact the PCRE works internally just with numbers, using an external ta- ble to translate between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have the same number but different names, is not supported, and causes an error at compile time. If it were allowed, it would not be possible to distinguish which parentheses matched, because both names map to cap- turing subpattern number 1. To avoid this confusing situation, an error is given at compile time. 13. PCRE provides some extensions to the Perl regular expression facil- ities. Perl 5.10 includes new features that are not in earlier ver- sions of Perl, some of which (such as named parentheses) have been in PCRE for some time. This list is with respect to Perl 5.10: (a) Although lookbehind assertions in PCRE must match fixed length strings, each alternative branch of a lookbehind assertion can match a different length of string. Perl requires them all to have the same length. (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta-character matches only at the very end of the string. (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe- cial meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored. (Perl can be made to issue a warning.) (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti- fiers is inverted, that is, by default they are not greedy, but if fol- lowed by a question mark they are. (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried only at the first matching position in the subject string. (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva- lents. (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF by the PCRE_BSR_ANYCRLF option. (h) The callout facility is PCRE-specific. (i) The partial matching facility is PCRE-specific. (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on different hosts that have the other endianness. (k) The alternative matching function (pcre_dfa_exec()) matches in a different way and is not Perl-compatible. (l) PCRE recognizes some special sequences such as (*CR) at the start of a pattern that set overall options that cannot be changed within the pattern. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 04 October 2009 Copyright (c) 1997-2009 University of Cambridge. ------------------------------------------------------------------------------ PCREPATTERN(3) PCREPATTERN(3) NAME PCRE - Perl-compatible regular expressions PCRE REGULAR EXPRESSION DETAILS The syntax and semantics of the regular expressions that are supported by PCRE are described in detail below. There is a quick-reference syn- tax summary in the pcresyntax page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE also supports some alternative regular expression syntax (which does not conflict with the Perl syn- tax) in order to provide some compatibility with regular expressions in Python, .NET, and Oniguruma. Perl's regular expressions are described in its own documentation, and regular expressions in general are covered in a number of books, some of which have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers regular expressions in great detail. This description of PCRE's regular expressions is intended as reference material. The original operation of PCRE was on strings of one-byte characters. However, there is now also support for UTF-8 character strings. To use this, PCRE must be built to include UTF-8 support, and you must call pcre_compile() or pcre_compile2() with the PCRE_UTF8 option. There is also a special sequence that can be given at the start of a pattern: (*UTF8) Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8 option. This feature is not Perl-compatible. How setting UTF-8 mode affects pattern matching is mentioned in several places below. There is also a summary of UTF-8 features in the section on UTF-8 support in the main pcre page. The remainder of this document discusses the patterns that are sup- ported by PCRE when its main matching function, pcre_exec(), is used. From release 6.0, PCRE offers a second matching function, pcre_dfa_exec(), which matches using a different algorithm that is not Perl-compatible. Some of the features discussed below are not available when pcre_dfa_exec() is used. The advantages and disadvantages of the alternative function, and how it differs from the normal function, are discussed in the pcrematching page. NEWLINE CONVENTIONS PCRE supports five different conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (line- feed) character, the two-character sequence CRLF, any of the three pre- ceding, or any Unicode newline sequence. The pcreapi page has further discussion about newlines, and shows how to set the newline convention in the options arguments for the compiling and matching functions. It is also possible to specify a newline convention by starting a pat- tern string with one of the following five sequences: (*CR) carriage return (*LF) linefeed (*CRLF) carriage return, followed by linefeed (*ANYCRLF) any of the three above (*ANY) all Unicode newline sequences These override the default and the options given to pcre_compile() or pcre_compile2(). For example, on a Unix system where LF is the default newline sequence, the pattern (*CR)a.b changes the convention to CR. That pattern matches "a\nb" because LF is no longer a newline. Note that these special settings, which are not Perl-compatible, are recognized only at the very start of a pattern, and that they must be in upper case. If more than one of them is present, the last one is used. The newline convention does not affect what the \R escape sequence matches. By default, this is any Unicode newline sequence, for Perl compatibility. However, this can be changed; see the description of \R in the section entitled "Newline sequences" below. A change of \R set- ting can be combined with a change of newline convention. CHARACTERS AND METACHARACTERS A regular expression is a pattern that is matched against a subject string from left to right. Most characters stand for themselves in a pattern, and match the corresponding characters in the subject. As a trivial example, the pattern The quick brown fox matches a portion of a subject string that is identical to itself. When caseless matching is specified (the PCRE_CASELESS option), letters are matched independently of case. In UTF-8 mode, PCRE always understands the concept of case for characters whose values are less than 128, so caseless matching is always possible. For characters with higher val- ues, the concept of case is supported if PCRE is compiled with Unicode property support, but not otherwise. If you want to use caseless matching for characters 128 and above, you must ensure that PCRE is compiled with Unicode property support as well as with UTF-8 support. The power of regular expressions comes from the ability to include alternatives and repetitions in the pattern. These are encoded in the pattern by the use of metacharacters, which do not stand for themselves but instead are interpreted in some special way. There are two different sets of metacharacters: those that are recog- nized anywhere in the pattern except within square brackets, and those that are recognized within square brackets. Outside square brackets, the metacharacters are as follows: \ general escape character with several uses ^ assert start of string (or line, in multiline mode) $ assert end of string (or line, in multiline mode) . match any character except newline (by default) [ start character class definition | start of alternative branch ( start subpattern ) end subpattern ? extends the meaning of ( also 0 or 1 quantifier also quantifier minimizer * 0 or more quantifier + 1 or more quantifier also "possessive quantifier" { start min/max quantifier Part of a pattern that is in square brackets is called a "character class". In a character class the only metacharacters are: \ general escape character ^ negate the class, but only if the first character - indicates character range [ POSIX character class (only if followed by POSIX syntax) ] terminates the character class The following sections describe the use of each of the metacharacters. BACKSLASH The backslash character has several uses. Firstly, if it is followed by a non-alphanumeric character, it takes away any special meaning that character may have. This use of backslash as an escape character applies both inside and outside character classes. For example, if you want to match a * character, you write \* in the pattern. This escaping action applies whether or not the following character would otherwise be interpreted as a metacharacter, so it is always safe to precede a non-alphanumeric with backslash to specify that it stands for itself. In particular, if you want to match a back- slash, you write \\. If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the pattern (other than in a character class) and characters between a # outside a character class and the next newline are ignored. An escap- ing backslash can be used to include a whitespace or # character as part of the pattern. If you want to remove the special meaning from a sequence of charac- ters, you can do so by putting them between \Q and \E. This is differ- ent from Perl in that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in Perl, $ and @ cause variable interpola- tion. Note the following examples: Pattern PCRE matches Perl matches \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz \Qabc\$xyz\E abc\$xyz abc\$xyz \Qabc\E\$\Qxyz\E abc$xyz abc$xyz The \Q...\E sequence is recognized both inside and outside character classes. Non-printing characters A second use of backslash provides a way of encoding non-printing char- acters in patterns in a visible manner. There is no restriction on the appearance of non-printing characters, apart from the binary zero that terminates a pattern, but when a pattern is being prepared by text editing, it is often easier to use one of the following escape sequences than the binary character it represents: \a alarm, that is, the BEL character (hex 07) \cx "control-x", where x is any character \e escape (hex 1B) \f formfeed (hex 0C) \n linefeed (hex 0A) \r carriage return (hex 0D) \t tab (hex 09) \ddd character with octal code ddd, or back reference \xhh character with hex code hh \x{hhh..} character with hex code hhh.. The precise effect of \cx is as follows: if x is a lower case letter, it is converted to upper case. Then bit 6 of the character (hex 40) is inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex 7B. After \x, from zero to two hexadecimal digits are read (letters can be in upper or lower case). Any number of hexadecimal digits may appear between \x{ and }, but the value of the character code must be less than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code point, which is 10FFFF. If characters other than hexadecimal digits appear between \x{ and }, or if there is no terminating }, this form of escape is not recognized. Instead, the initial \x will be interpreted as a basic hexadecimal escape, with no following digits, giving a character whose value is zero. Characters whose value is less than 256 can be defined by either of the two syntaxes for \x. There is no difference in the way they are han- dled. For example, \xdc is exactly the same as \x{dc}. After \0 up to two further octal digits are read. If there are fewer than two digits, just those that are present are used. Thus the sequence \0\x\07 specifies two binary zeros followed by a BEL character (code value 7). Make sure you supply two digits after the initial zero if the pattern character that follows is itself an octal digit. The handling of a backslash followed by a digit other than 0 is compli- cated. Outside a character class, PCRE reads it and any following dig- its as a decimal number. If the number is less than 10, or if there have been at least that many previous capturing left parentheses in the expression, the entire sequence is taken as a back reference. A description of how this works is given later, following the discussion of parenthesized subpatterns. Inside a character class, or if the decimal number is greater than 9 and there have not been that many capturing subpatterns, PCRE re-reads up to three octal digits following the backslash, and uses them to gen- erate a data character. Any subsequent digits stand for themselves. In non-UTF-8 mode, the value of a character specified in octal must be less than \400. In UTF-8 mode, values up to \777 are permitted. For example: \040 is another way of writing a space \40 is the same, provided there are fewer than 40 previous capturing subpatterns \7 is always a back reference \11 might be a back reference, or another way of writing a tab \011 is always a tab \0113 is a tab followed by the character "3" \113 might be a back reference, otherwise the character with octal code 113 \377 might be a back reference, otherwise the byte consisting entirely of 1 bits \81 is either a back reference, or a binary zero followed by the two characters "8" and "1" Note that octal values of 100 or greater must not be introduced by a leading zero, because no more than three octal digits are ever read. All the sequences that define a single character value can be used both inside and outside character classes. In addition, inside a character class, the sequence \b is interpreted as the backspace character (hex 08), and the sequences \R and \X are interpreted as the characters "R" and "X", respectively. Outside a character class, these sequences have different meanings (see below). Absolute and relative back references The sequence \g followed by an unsigned or a negative number, option- ally enclosed in braces, is an absolute or relative back reference. A named back reference can be coded as \g{name}. Back references are dis- cussed later, following the discussion of parenthesized subpatterns. Absolute and relative subroutine calls For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number enclosed either in angle brackets or single quotes, is an alternative syntax for referencing a subpattern as a "subroutine". Details are discussed later. Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former is a back reference; the latter is a subroutine call. Generic character types Another use of backslash is for specifying generic character types. The following are always recognized: \d any decimal digit \D any character that is not a decimal digit \h any horizontal whitespace character \H any character that is not a horizontal whitespace character \s any whitespace character \S any character that is not a whitespace character \v any vertical whitespace character \V any character that is not a vertical whitespace character \w any "word" character \W any "non-word" character Each pair of escape sequences partitions the complete set of characters into two disjoint sets. Any given character matches one, and only one, of each pair. These character type sequences can appear both inside and outside char- acter classes. They each match one character of the appropriate type. If the current matching point is at the end of the subject string, all of them fail, since there is no character to match. For compatibility with Perl, \s does not match the VT character (code 11). This makes it different from the the POSIX "space" class. The \s characters are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is included in a Perl script, \s may match the VT charac- ter. In PCRE, it never does. In UTF-8 mode, characters with values greater than 128 never match \d, \s, or \w, and always match \D, \S, and \W. This is true even when Uni- code character property support is available. These sequences retain their original meanings from before UTF-8 support was available, mainly for efficiency reasons. Note that this also affects \b, because it is defined in terms of \w and \W. The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to the other sequences, these do match certain high-valued codepoints in UTF-8 mode. The horizontal space characters are: U+0009 Horizontal tab U+0020 Space U+00A0 Non-break space U+1680 Ogham space mark U+180E Mongolian vowel separator U+2000 En quad U+2001 Em quad U+2002 En space U+2003 Em space U+2004 Three-per-em space U+2005 Four-per-em space U+2006 Six-per-em space U+2007 Figure space U+2008 Punctuation space U+2009 Thin space U+200A Hair space U+202F Narrow no-break space U+205F Medium mathematical space U+3000 Ideographic space The vertical space characters are: U+000A Linefeed U+000B Vertical tab U+000C Formfeed U+000D Carriage return U+0085 Next line U+2028 Line separator U+2029 Paragraph separator A "word" character is an underscore or any character less than 256 that is a letter or digit. The definition of letters and digits is con- trolled by PCRE's low-valued character tables, and may vary if locale- specific matching is taking place (see "Locale support" in the pcreapi page). For example, in a French locale such as "fr_FR" in Unix-like systems, or "french" in Windows, some character codes greater than 128 are used for accented letters, and these are matched by \w. The use of locales with Unicode is discouraged. Newline sequences Outside a character class, by default, the escape sequence \R matches any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is equivalent to the following: (?>\r\n|\n|\x0b|\f|\r|\x85) This is an example of an "atomic group", details of which are given below. This particular group matches either the two-character sequence CR followed by LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next line, U+0085). The two-character sequence is treated as a single unit that cannot be split. In UTF-8 mode, two additional characters whose codepoints are greater than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa- rator, U+2029). Unicode character property support is not needed for these characters to be recognized. It is possible to restrict \R to match only CR, LF, or CRLF (instead of the complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched. (BSR is an abbrevation for "backslash R".) This can be made the default when PCRE is built; if this is the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option. It is also possible to specify these settings by starting a pattern string with one of the following sequences: (*BSR_ANYCRLF) CR, LF, or CRLF only (*BSR_UNICODE) any Unicode newline sequence These override the default and the options given to pcre_compile() or pcre_compile2(), but they can be overridden by options given to pcre_exec() or pcre_dfa_exec(). Note that these special settings, which are not Perl-compatible, are recognized only at the very start of a pattern, and that they must be in upper case. If more than one of them is present, the last one is used. They can be combined with a change of newline convention, for example, a pattern can start with: (*ANY)(*BSR_ANYCRLF) Inside a character class, \R matches the letter "R". Unicode character properties When PCRE is built with Unicode character property support, three addi- tional escape sequences that match characters with specific properties are available. When not in UTF-8 mode, these sequences are of course limited to testing characters whose codepoints are less than 256, but they do work in this mode. The extra escape sequences are: \p{xx} a character with the xx property \P{xx} a character without the xx property \X an extended Unicode sequence The property names represented by xx above are limited to the Unicode script names, the general category properties, and "Any", which matches any character (including newline). Other properties such as "InMusical- Symbols" are not currently supported by PCRE. Note that \P{Any} does not match any characters, so always causes a match failure. Sets of Unicode characters are defined as belonging to certain scripts. A character from one of these sets can be matched using a script name. For example: \p{Greek} \P{Han} Those that are not part of an identified script are lumped together as "Common". The current list of scripts is: Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp- tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe- rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam, Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic, Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian, Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai, Yi. Each character has exactly one general category property, specified by a two-letter abbreviation. For compatibility with Perl, negation can be specified by including a circumflex between the opening brace and the property name. For example, \p{^Lu} is the same as \P{Lu}. If only one letter is specified with \p or \P, it includes all the gen- eral category properties that start with that letter. In this case, in the absence of negation, the curly brackets in the escape sequence are optional; these two examples have the same effect: \p{L} \pL The following general category property codes are supported: C Other Cc Control Cf Format Cn Unassigned Co Private use Cs Surrogate L Letter Ll Lower case letter Lm Modifier letter Lo Other letter Lt Title case letter Lu Upper case letter M Mark Mc Spacing mark Me Enclosing mark Mn Non-spacing mark N Number Nd Decimal number Nl Letter number No Other number P Punctuation Pc Connector punctuation Pd Dash punctuation Pe Close punctuation Pf Final punctuation Pi Initial punctuation Po Other punctuation Ps Open punctuation S Symbol Sc Currency symbol Sk Modifier symbol Sm Mathematical symbol So Other symbol Z Separator Zl Line separator Zp Paragraph separator Zs Space separator The special property L& is also supported: it matches a character that has the Lu, Ll, or Lt property, in other words, a letter that is not classified as a modifier or "other". The Cs (Surrogate) property applies only to characters in the range U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check- ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in the pcreapi page). Perl does not support the Cs property. The long synonyms for property names that Perl supports (such as \p{Letter}) are not supported by PCRE, nor is it permitted to prefix any of these properties with "Is". No character that is in the Unicode table has the Cn (unassigned) prop- erty. Instead, this property is assumed for any code point that is not in the Unicode table. Specifying caseless matching does not affect these escape sequences. For example, \p{Lu} always matches only upper case letters. The \X escape matches any number of Unicode characters that form an extended Unicode sequence. \X is equivalent to (?>\PM\pM*) That is, it matches a character without the "mark" property, followed by zero or more characters with the "mark" property, and treats the sequence as an atomic group (see below). Characters with the "mark" property are typically accents that affect the preceding character. None of them have codepoints less than 256, so in non-UTF-8 mode \X matches any one character. Matching characters by Unicode property is not fast, because PCRE has to search a structure that contains data for over fifteen thousand characters. That is why the traditional escape sequences such as \d and \w do not use Unicode properties in PCRE. Resetting the match start The escape sequence \K, which is a Perl 5.10 feature, causes any previ- ously matched characters not to be included in the final matched sequence. For example, the pattern: foo\Kbar matches "foobar", but reports that it has matched "bar". This feature is similar to a lookbehind assertion (described below). However, in this case, the part of the subject before the real match does not have to be of fixed length, as lookbehind assertions do. The use of \K does not interfere with the setting of captured substrings. For example, when the pattern (foo)\Kbar matches "foobar", the first substring is still set to "foo". Perl documents that the use of \K within assertions is "not well defined". In PCRE, \K is acted upon when it occurs inside positive assertions, but is ignored in negative assertions. Simple assertions The final use of backslash is for certain simple assertions. An asser- tion specifies a condition that has to be met at a particular point in a match, without consuming any characters from the subject string. The use of subpatterns for more complicated assertions is described below. The backslashed assertions are: \b matches at a word boundary \B matches when not at a word boundary \A matches at the start of the subject \Z matches at the end of the subject also matches before a newline at the end of the subject \z matches only at the end of the subject \G matches at the first matching position in the subject These assertions may not appear in character classes (but note that \b has a different meaning, namely the backspace character, inside a char- acter class). A word boundary is a position in the subject string where the current character and the previous character do not both match \w or \W (i.e. one matches \w and the other matches \W), or the start or end of the string if the first or last character matches \w, respectively. Neither PCRE nor Perl has a separte "start of word" or "end of word" metase- quence. However, whatever follows \b normally determines which it is. For example, the fragment \ba matches "a" at the start of a word. The \A, \Z, and \z assertions differ from the traditional circumflex and dollar (described in the next section) in that they only ever match at the very start and end of the subject string, whatever options are set. Thus, they are independent of multiline mode. These three asser- tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the circumflex and dollar metacharacters. However, if the startoffset argument of pcre_exec() is non-zero, indi- cating that matching is to start at a point other than the beginning of the subject, \A can never match. The difference between \Z and \z is that \Z matches before a newline at the end of the string as well as at the very end, whereas \z matches only at the end. The \G assertion is true only when the current matching position is at the start point of the match, as specified by the startoffset argument of pcre_exec(). It differs from \A when the value of startoffset is non-zero. By calling pcre_exec() multiple times with appropriate argu- ments, you can mimic Perl's /g option, and it is in this kind of imple- mentation where \G can be useful. Note, however, that PCRE's interpretation of \G, as the start of the current match, is subtly different from Perl's, which defines it as the end of the previous match. In Perl, these can be different when the previously matched string was empty. Because PCRE does just one match at a time, it cannot reproduce this behaviour. If all the alternatives of a pattern begin with \G, the expression is anchored to the starting match position, and the "anchored" flag is set in the compiled regular expression. CIRCUMFLEX AND DOLLAR Outside a character class, in the default matching mode, the circumflex character is an assertion that is true only if the current matching point is at the start of the subject string. If the startoffset argu- ment of pcre_exec() is non-zero, circumflex can never match if the PCRE_MULTILINE option is unset. Inside a character class, circumflex has an entirely different meaning (see below). Circumflex need not be the first character of the pattern if a number of alternatives are involved, but it should be the first thing in each alternative in which it appears if the pattern is ever to match that branch. If all possible alternatives start with a circumflex, that is, if the pattern is constrained to match only at the start of the sub- ject, it is said to be an "anchored" pattern. (There are also other constructs that can cause a pattern to be anchored.) A dollar character is an assertion that is true only if the current matching point is at the end of the subject string, or immediately before a newline at the end of the string (by default). Dollar need not be the last character of the pattern if a number of alternatives are involved, but it should be the last item in any branch in which it appears. Dollar has no special meaning in a character class. The meaning of dollar can be changed so that it matches only at the very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This does not affect the \Z assertion. The meanings of the circumflex and dollar characters are changed if the PCRE_MULTILINE option is set. When this is the case, a circumflex matches immediately after internal newlines as well as at the start of the subject string. It does not match after a newline that ends the string. A dollar matches before any newlines in the string, as well as at the very end, when PCRE_MULTILINE is set. When newline is specified as the two-character sequence CRLF, isolated CR and LF characters do not indicate newlines. For example, the pattern /^abc$/ matches the subject string "def\nabc" (where \n represents a newline) in multiline mode, but not otherwise. Consequently, patterns that are anchored in single line mode because all branches start with ^ are not anchored in multiline mode, and a match for circumflex is possible when the startoffset argument of pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. Note that the sequences \A, \Z, and \z can be used to match the start and end of the subject in both modes, and if all branches of a pattern start with \A it is always anchored, whether or not PCRE_MULTILINE is set. FULL STOP (PERIOD, DOT) Outside a character class, a dot in the pattern matches any one charac- ter in the subject string except (by default) a character that signi- fies the end of a line. In UTF-8 mode, the matched character may be more than one byte long. When a line ending is defined as a single character, dot never matches that character; when the two-character sequence CRLF is used, dot does not match CR if it is immediately followed by LF, but otherwise it matches all characters (including isolated CRs and LFs). When any Uni- code line endings are being recognized, dot does not match CR or LF or any of the other line ending characters. The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL option is set, a dot matches any one character, without exception. If the two-character sequence CRLF is present in the subject string, it takes two dots to match it. The handling of dot is entirely independent of the handling of circum- flex and dollar, the only relationship being that they both involve newlines. Dot has no special meaning in a character class. MATCHING A SINGLE BYTE Outside a character class, the escape sequence \C matches any one byte, both in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending characters. The feature is provided in Perl in order to match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char- acters into individual bytes, what remains in the string may be a mal- formed UTF-8 string. For this reason, the \C escape sequence is best avoided. PCRE does not allow \C to appear in lookbehind assertions (described below), because in UTF-8 mode this would make it impossible to calcu- late the length of the lookbehind. SQUARE BRACKETS AND CHARACTER CLASSES An opening square bracket introduces a character class, terminated by a closing square bracket. A closing square bracket on its own is not spe- cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square bracket causes a compile-time error. If a closing square bracket is required as a member of the class, it should be the first data character in the class (after an initial circumflex, if present) or escaped with a backslash. A character class matches a single character in the subject. In UTF-8 mode, the character may be more than one byte long. A matched character must be in the set of characters defined by the class, unless the first character in the class definition is a circumflex, in which case the subject character must not be in the set defined by the class. If a circumflex is actually required as a member of the class, ensure it is not the first character, or escape it with a backslash. For example, the character class [aeiou] matches any lower case vowel, while [^aeiou] matches any character that is not a lower case vowel. Note that a circumflex is just a convenient notation for specifying the characters that are in the class by enumerating those that are not. A class that starts with a circumflex is not an assertion; it still con- sumes a character from the subject string, and therefore it fails if the current pointer is at the end of the string. In UTF-8 mode, characters with values greater than 255 can be included in a class as a literal string of bytes, or by using the \x{ escaping mechanism. When caseless matching is set, any letters in a class represent both their upper case and lower case versions, so for example, a caseless [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a caseful version would. In UTF-8 mode, PCRE always understands the concept of case for characters whose values are less than 128, so caseless matching is always possible. For characters with higher values, the concept of case is supported if PCRE is compiled with Unicode property support, but not otherwise. If you want to use caseless matching in UTF8-mode for characters 128 and above, you must ensure that PCRE is compiled with Unicode property support as well as with UTF-8 support. Characters that might indicate line breaks are never treated in any special way when matching character classes, whatever line-ending sequence is in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class such as [^a] always matches one of these characters. The minus (hyphen) character can be used to specify a range of charac- ters in a character class. For example, [d-m] matches any letter between d and m, inclusive. If a minus character is required in a class, it must be escaped with a backslash or appear in a position where it cannot be interpreted as indicating a range, typically as the first or last character in the class. It is not possible to have the literal character "]" as the end charac- ter of a range. A pattern such as [W-]46] is interpreted as a class of two characters ("W" and "-") followed by a literal string "46]", so it would match "W46]" or "-46]". However, if the "]" is escaped with a backslash it is interpreted as the end of range, so [W-\]46] is inter- preted as a class containing a range followed by two other characters. The octal or hexadecimal representation of "]" can also be used to end a range. Ranges operate in the collating sequence of character values. They can also be used for characters specified numerically, for example [\000-\037]. In UTF-8 mode, ranges can include characters whose values are greater than 255, for example [\x{100}-\x{2ff}]. If a range that includes letters is used when caseless matching is set, it matches the letters in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character tables for a French locale are in use, [\xc8-\xcb] matches accented E characters in both cases. In UTF-8 mode, PCRE supports the concept of case for characters with values greater than 128 only when it is compiled with Unicode property support. The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear in a character class, and add the characters that they match to the class. For example, [\dABCDEF] matches any hexadecimal digit. A circum- flex can conveniently be used with the upper case character types to specify a more restricted set of characters than the matching lower case type. For example, the class [^\W_] matches any letter or digit, but not underscore. The only metacharacters that are recognized in character classes are backslash, hyphen (only where it can be interpreted as specifying a range), circumflex (only at the start), opening square bracket (only when it can be interpreted as introducing a POSIX class name - see the next section), and the terminating closing square bracket. However, escaping other non-alphanumeric characters does no harm. POSIX CHARACTER CLASSES Perl supports the POSIX notation for character classes. This uses names enclosed by [: and :] within the enclosing square brackets. PCRE also supports this notation. For example, [01[:alpha:]%] matches "0", "1", any alphabetic character, or "%". The supported class names are alnum letters and digits alpha letters ascii character codes 0 - 127 blank space or tab only cntrl control characters digit decimal digits (same as \d) graph printing characters, excluding space lower lower case letters print printing characters, including space punct printing characters, excluding letters and digits space white space (not quite the same as \s) upper upper case letters word "word" characters (same as \w) xdigit hexadecimal digits The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and space (32). Notice that this list includes the VT character (code 11). This makes "space" different to \s, which does not include VT (for Perl compatibility). The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 5.8. Another Perl extension is negation, which is indicated by a ^ character after the colon. For example, [12[:^digit:]] matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not supported, and an error is given if they are encountered. In UTF-8 mode, characters with values greater than 128 do not match any of the POSIX character classes. VERTICAL BAR Vertical bar characters are used to separate alternative patterns. For example, the pattern gilbert|sullivan matches either "gilbert" or "sullivan". Any number of alternatives may appear, and an empty alternative is permitted (matching the empty string). The matching process tries each alternative in turn, from left to right, and the first one that succeeds is used. If the alternatives are within a subpattern (defined below), "succeeds" means matching the rest of the main pattern as well as the alternative in the subpattern. INTERNAL OPTION SETTING The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED options (which are Perl-compatible) can be changed from within the pattern by a sequence of Perl option letters enclosed between "(?" and ")". The option letters are i for PCRE_CASELESS m for PCRE_MULTILINE s for PCRE_DOTALL x for PCRE_EXTENDED For example, (?im) sets caseless, multiline matching. It is also possi- ble to unset these options by preceding the letter with a hyphen, and a combined setting and unsetting such as (?im-sx), which sets PCRE_CASE- LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also permitted. If a letter appears both before and after the hyphen, the option is unset. The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be changed in the same way as the Perl-compatible options by using the characters J, U and X respectively. When one of these option changes occurs at top level (that is, not inside subpattern parentheses), the change applies to the remainder of the pattern that follows. If the change is placed right at the start of a pattern, PCRE extracts it into the global options (and it will there- fore show up in data extracted by the pcre_fullinfo() function). An option change within a subpattern (see below for a description of subpatterns) affects only that part of the current pattern that follows it, so (a(?i)b)c matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). By this means, options can be made to have different settings in different parts of the pattern. Any changes made in one alternative do carry on into subsequent branches within the same subpattern. For example, (a(?i)b|c) matches "ab", "aB", "c", and "C", even though when matching "C" the first branch is abandoned before the option setting. This is because the effects of option settings happen at compile time. There would be some very weird behaviour otherwise. Note: There are other PCRE-specific options that can be set by the application when the compile or match functions are called. In some cases the pattern can contain special leading sequences such as (*CRLF) to override what the application has set or what has been defaulted. Details are given in the section entitled "Newline sequences" above. There is also the (*UTF8) leading sequence that can be used to set UTF-8 mode; this is equivalent to setting the PCRE_UTF8 option. SUBPATTERNS Subpatterns are delimited by parentheses (round brackets), which can be nested. Turning part of a pattern into a subpattern does two things: 1. It localizes a set of alternatives. For example, the pattern cat(aract|erpillar|) matches one of the words "cat", "cataract", or "caterpillar". Without the parentheses, it would match "cataract", "erpillar" or an empty string. 2. It sets up the subpattern as a capturing subpattern. This means that, when the whole pattern matches, that portion of the subject string that matched the subpattern is passed back to the caller via the ovector argument of pcre_exec(). Opening parentheses are counted from left to right (starting from 1) to obtain numbers for the capturing subpatterns. For example, if the string "the red king" is matched against the pat- tern the ((red|white) (king|queen)) the captured substrings are "red king", "red", and "king", and are num- bered 1, 2, and 3, respectively. The fact that plain parentheses fulfil two functions is not always helpful. There are often times when a grouping subpattern is required without a capturing requirement. If an opening parenthesis is followed by a question mark and a colon, the subpattern does not do any captur- ing, and is not counted when computing the number of any subsequent capturing subpatterns. For example, if the string "the white queen" is matched against the pattern the ((?:red|white) (king|queen)) the captured substrings are "white queen" and "queen", and are numbered 1 and 2. The maximum number of capturing subpatterns is 65535. As a convenient shorthand, if any option settings are required at the start of a non-capturing subpattern, the option letters may appear between the "?" and the ":". Thus the two patterns (?i:saturday|sunday) (?:(?i)saturday|sunday) match exactly the same set of strings. Because alternative branches are tried from left to right, and options are not reset until the end of the subpattern is reached, an option setting in one branch does affect subsequent branches, so the above patterns match "SUNDAY" as well as "Saturday". DUPLICATE SUBPATTERN NUMBERS Perl 5.10 introduced a feature whereby each alternative in a subpattern uses the same numbers for its capturing parentheses. Such a subpattern starts with (?| and is itself a non-capturing subpattern. For example, consider this pattern: (?|(Sat)ur|(Sun))day Because the two alternatives are inside a (?| group, both sets of cap- turing parentheses are numbered one. Thus, when the pattern matches, you can look at captured substring number one, whichever alternative matched. This construct is useful when you want to capture part, but not all, of one of a number of alternatives. Inside a (?| group, paren- theses are numbered as usual, but the number is reset at the start of each branch. The numbers of any capturing buffers that follow the sub- pattern start after the highest number used in any branch. The follow- ing example is taken from the Perl documentation. The numbers under- neath show in which buffer the captured content will be stored. # before ---------------branch-reset----------- after / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x # 1 2 2 3 2 3 4 A back reference to a numbered subpattern uses the most recent value that is set for that number by any subpattern. The following pattern matches "abcabc" or "defdef": /(?|(abc)|(def))\1/ In contrast, a recursive or "subroutine" call to a numbered subpattern always refers to the first one in the pattern with the given number. The following pattern matches "abcabc" or "defabc": /(?|(abc)|(def))(?1)/ If a condition test for a subpattern's having matched refers to a non- unique number, the test is true if any of the subpatterns of that num- ber have matched. An alternative approach to using this "branch reset" feature is to use duplicate named subpatterns, as described in the next section. NAMED SUBPATTERNS Identifying capturing parentheses by number is simple, but it can be very hard to keep track of the numbers in complicated regular expres- sions. Furthermore, if an expression is modified, the numbers may change. To help with this difficulty, PCRE supports the naming of sub- patterns. This feature was not added to Perl until release 5.10. Python had the feature earlier, and PCRE introduced it at release 4.0, using the Python syntax. PCRE now supports both the Perl and the Python syn- tax. Perl allows identically numbered subpatterns to have different names, but PCRE does not. In PCRE, a subpattern can be named in one of three ways: (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing parentheses from other parts of the pattern, such as back references, recursion, and conditions, can be made by name as well as by number. Names consist of up to 32 alphanumeric characters and underscores. Named capturing parentheses are still allocated numbers as well as names, exactly as if the names were not present. The PCRE API provides function calls for extracting the name-to-number translation table from a compiled pattern. There is also a convenience function for extracting a captured substring by name. By default, a name must be unique within a pattern, but it is possible to relax this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate names are also always permitted for subpatterns with the same number, set up as described in the previous section.) Dupli- cate names can be useful for patterns where only one instance of the named parentheses can match. Suppose you want to match the name of a weekday, either as a 3-letter abbreviation or as the full name, and in both cases you want to extract the abbreviation. This pattern (ignoring the line breaks) does the job: (?<DN>Mon|Fri|Sun)(?:day)?| (?<DN>Tue)(?:sday)?| (?<DN>Wed)(?:nesday)?| (?<DN>Thu)(?:rsday)?| (?<DN>Sat)(?:urday)? There are five capturing substrings, but only one is ever set after a match. (An alternative way of solving this problem is to use a "branch reset" subpattern, as described in the previous section.) The convenience function for extracting the data by name returns the substring for the first (and in this example, the only) subpattern of that name that matched. This saves searching to find which numbered subpattern it was. If you make a back reference to a non-unique named subpattern from elsewhere in the pattern, the one that corresponds to the first occur- rence of the name is used. In the absence of duplicate numbers (see the previous section) this is the one with the lowest number. If you use a named reference in a condition test (see the section about conditions below), either to check whether a subpattern has matched, or to check for recursion, all subpatterns with the same name are tested. If the condition is true for any one of them, the overall condition is true. This is the same behaviour as testing by number. For further details of the interfaces for handling named subpatterns, see the pcreapi documen- tation. Warning: You cannot use different names to distinguish between two sub- patterns with the same number because PCRE uses only the numbers when matching. For this reason, an error is given at compile time if differ- ent names are given to subpatterns with the same number. However, you can give the same name to subpatterns with the same number, even when PCRE_DUPNAMES is not set. REPETITION Repetition is specified by quantifiers, which can follow any of the following items: a literal data character the dot metacharacter the \C escape sequence the \X escape sequence (in UTF-8 mode with Unicode properties) the \R escape sequence an escape such as \d that matches a single character a character class a back reference (see next section) a parenthesized subpattern (unless it is an assertion) a recursive or "subroutine" call to a subpattern The general repetition quantifier specifies a minimum and maximum num- ber of permitted matches, by giving the two numbers in curly brackets (braces), separated by a comma. The numbers must be less than 65536, and the first must be less than or equal to the second. For example: z{2,4} matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special character. If the second number is omitted, but the comma is present, there is no upper limit; if the second number and the comma are both omitted, the quantifier specifies an exact number of required matches. Thus [aeiou]{3,} matches at least 3 successive vowels, but may match many more, while \d{8} matches exactly 8 digits. An opening curly bracket that appears in a position where a quantifier is not allowed, or one that does not match the syntax of a quantifier, is taken as a literal character. For exam- ple, {,6} is not a quantifier, but a literal string of four characters. In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char- acters, each of which is represented by a two-byte sequence. Similarly, when Unicode property support is available, \X{3} matches three Unicode extended sequences, each of which may be several bytes long (and they may be of different lengths). The quantifier {0} is permitted, causing the expression to behave as if the previous item and the quantifier were not present. This may be use- ful for subpatterns that are referenced as subroutines from elsewhere in the pattern. Items other than subpatterns that have a {0} quantifier are omitted from the compiled pattern. For convenience, the three most common quantifiers have single-charac- ter abbreviations: * is equivalent to {0,} + is equivalent to {1,} ? is equivalent to {0,1} It is possible to construct infinite loops by following a subpattern that can match no characters with a quantifier that has no upper limit, for example: (a?)* Earlier versions of Perl and PCRE used to give an error at compile time for such patterns. However, because there are cases where this can be useful, such patterns are now accepted, but if any repetition of the subpattern does in fact match no characters, the loop is forcibly bro- ken. By default, the quantifiers are "greedy", that is, they match as much as possible (up to the maximum number of permitted times), without causing the rest of the pattern to fail. The classic example of where this gives problems is in trying to match comments in C programs. These appear between /* and */ and within the comment, individual * and / characters may appear. An attempt to match C comments by applying the pattern /\*.*\*/ to the string /* first comment */ not comment /* second comment */ fails, because it matches the entire string owing to the greediness of the .* item. However, if a quantifier is followed by a question mark, it ceases to be greedy, and instead matches the minimum number of times possible, so the pattern /\*.*?\*/ does the right thing with the C comments. The meaning of the various quantifiers is not otherwise changed, just the preferred number of matches. Do not confuse this use of question mark with its use as a quantifier in its own right. Because it has two uses, it can sometimes appear doubled, as in \d??\d which matches one digit by preference, but can match two if that is the only way the rest of the pattern matches. If the PCRE_UNGREEDY option is set (an option that is not available in Perl), the quantifiers are not greedy by default, but individual ones can be made greedy by following them with a question mark. In other words, it inverts the default behaviour. When a parenthesized subpattern is quantified with a minimum repeat count that is greater than 1 or with a limited maximum, more memory is required for the compiled pattern, in proportion to the size of the minimum or maximum. If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv- alent to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is implicitly anchored, because whatever follows will be tried against every character position in the subject string, so there is no point in retrying the overall match at any position after the first. PCRE normally treats such a pattern as though it were preceded by \A. In cases where it is known that the subject string contains no new- lines, it is worth setting PCRE_DOTALL in order to obtain this opti- mization, or alternatively using ^ to indicate anchoring explicitly. However, there is one situation where the optimization cannot be used. When .* is inside capturing parentheses that are the subject of a back reference elsewhere in the pattern, a match at the start may fail where a later one succeeds. Consider, for example: (.*)abc\1 If the subject is "xyz123abc123" the match point is the fourth charac- ter. For this reason, such a pattern is not implicitly anchored. When a capturing subpattern is repeated, the value captured is the sub- string that matched the final iteration. For example, after (tweedle[dume]{3}\s*)+ has matched "tweedledum tweedledee" the value of the captured substring is "tweedledee". However, if there are nested capturing subpatterns, the corresponding captured values may have been set in previous itera- tions. For example, after /(a|(b))+/ matches "aba" the value of the second captured substring is "b". ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") repetition, failure of what follows normally causes the repeated item to be re-evaluated to see if a different number of repeats allows the rest of the pattern to match. Sometimes it is useful to prevent this, either to change the nature of the match, or to cause it fail earlier than it otherwise might, when the author of the pattern knows there is no point in carrying on. Consider, for example, the pattern \d+foo when applied to the subject line 123456bar After matching all 6 digits and then failing to match "foo", the normal action of the matcher is to try again with only 5 digits matching the \d+ item, and then with 4, and so on, before ultimately failing. "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides the means for specifying that once a subpattern has matched, it is not to be re-evaluated in this way. If we use atomic grouping for the previous example, the matcher gives up immediately on failing to match "foo" the first time. The notation is a kind of special parenthesis, starting with (?> as in this example: (?>\d+)foo This kind of parenthesis "locks up" the part of the pattern it con- tains once it has matched, and a failure further into the pattern is prevented from backtracking into it. Backtracking past it to previous items, however, works as normal. An alternative description is that a subpattern of this type matches the string of characters that an identical standalone pattern would match, if anchored at the current point in the subject string. Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as the above example can be thought of as a maximizing repeat that must swallow everything it can. So, while both \d+ and \d+? are pre- pared to adjust the number of digits they match in order to make the rest of the pattern match, (?>\d+) can only match an entire sequence of digits. Atomic groups in general can of course contain arbitrarily complicated subpatterns, and can be nested. However, when the subpattern for an atomic group is just a single repeated item, as in the example above, a simpler notation, called a "possessive quantifier" can be used. This consists of an additional + character following a quantifier. Using this notation, the previous example can be rewritten as \d++foo Note that a possessive quantifier can be used with an entire group, for example: (abc|xyz){2,3}+ Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY option is ignored. They are a convenient notation for the simpler forms of atomic group. However, there is no difference in the meaning of a possessive quantifier and the equivalent atomic group, though there may be a performance difference; possessive quantifiers should be slightly faster. The possessive quantifier syntax is an extension to the Perl 5.8 syn- tax. Jeffrey Friedl originated the idea (and the name) in the first edition of his book. Mike McCloskey liked it, so implemented it when he built Sun's Java package, and PCRE copied it from there. It ultimately found its way into Perl at release 5.10. PCRE has an optimization that automatically "possessifies" certain sim- ple pattern constructs. For example, the sequence A+B is treated as A++B because there is no point in backtracking into a sequence of A's when B must follow. When a pattern contains an unlimited repeat inside a subpattern that can itself be repeated an unlimited number of times, the use of an atomic group is the only way to avoid some failing matches taking a very long time indeed. The pattern (\D+|<\d+>)*[!?] matches an unlimited number of substrings that either consist of non- digits, or digits enclosed in <>, followed by either ! or ?. When it matches, it runs quickly. However, if it is applied to aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa it takes a long time before reporting failure. This is because the string can be divided between the internal \D+ repeat and the external * repeat in a large number of ways, and all have to be tried. (The example uses [!?] rather than a single character at the end, because both PCRE and Perl have an optimization that allows for fast failure when a single character is used. They remember the last single charac- ter that is required for a match, and fail early if it is not present in the string.) If the pattern is changed so that it uses an atomic group, like this: ((?>\D+)|<\d+>)*[!?] sequences of non-digits cannot be broken, and failure happens quickly. BACK REFERENCES Outside a character class, a backslash followed by a digit greater than 0 (and possibly further digits) is a back reference to a capturing sub- pattern earlier (that is, to its left) in the pattern, provided there have been that many previous capturing left parentheses. However, if the decimal number following the backslash is less than 10, it is always taken as a back reference, and causes an error only if there are not that many capturing left parentheses in the entire pat- tern. In other words, the parentheses that are referenced need not be to the left of the reference for numbers less than 10. A "forward back reference" of this type can make sense when a repetition is involved and the subpattern to the right has participated in an earlier itera- tion. It is not possible to have a numerical "forward back reference" to a subpattern whose number is 10 or more using this syntax because a sequence such as \50 is interpreted as a character defined in octal. See the subsection entitled "Non-printing characters" above for further details of the handling of digits following a backslash. There is no such problem when named parentheses are used. A back reference to any subpattern is possible using named parentheses (see below). Another way of avoiding the ambiguity inherent in the use of digits following a backslash is to use the \g escape sequence, which is a fea- ture introduced in Perl 5.10. This escape must be followed by an unsigned number or a negative number, optionally enclosed in braces. These examples are all identical: (ring), \1 (ring), \g1 (ring), \g{1} An unsigned number specifies an absolute reference without the ambigu- ity that is present in the older syntax. It is also useful when literal digits follow the reference. A negative number is a relative reference. Consider this example: (abc(def)ghi)\g{-1} The sequence \g{-1} is a reference to the most recently started captur- ing subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2} would be equivalent to \1. The use of relative references can be helpful in long patterns, and also in patterns that are created by joining together fragments that contain references within themselves. A back reference matches whatever actually matched the capturing sub- pattern in the current subject string, rather than anything matching the subpattern itself (see "Subpatterns as subroutines" below for a way of doing that). So the pattern (sens|respons)e and \1ibility matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility". If caseful matching is in force at the time of the back reference, the case of letters is relevant. For exam- ple, ((?i)rah)\s+\1 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original capturing subpattern is matched caselessly. There are several different ways of writing back references to named subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified back reference syntax, in which \g can be used for both numeric and named references, is also supported. We could rewrite the above example in any of the following ways: (?<p1>(?i)rah)\s+\k<p1> (?'p1'(?i)rah)\s+\k{p1} (?P<p1>(?i)rah)\s+(?P=p1) (?<p1>(?i)rah)\s+\g{p1} A subpattern that is referenced by name may appear in the pattern before or after the reference. There may be more than one back reference to the same subpattern. If a subpattern has not actually been used in a particular match, any back references to it always fail by default. For example, the pattern (a|(bc))\2 always fails if it starts to match "a" rather than "bc". However, if the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer- ence to an unset value matches an empty string. Because there may be many capturing parentheses in a pattern, all dig- its following a backslash are taken as part of a potential back refer- ence number. If the pattern continues with a digit character, some delimiter must be used to terminate the back reference. If the PCRE_EXTENDED option is set, this can be whitespace. Otherwise, the \g{ syntax or an empty comment (see "Comments" below) can be used. Recursive back references A back reference that occurs inside the parentheses to which it refers fails when the subpattern is first used, so, for example, (a\1) never matches. However, such references can be useful inside repeated sub- patterns. For example, the pattern (a|b\1)+ matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- ation of the subpattern, the back reference matches the character string corresponding to the previous iteration. In order for this to work, the pattern must be such that the first iteration does not need to match the back reference. This can be done using alternation, as in the example above, or by a quantifier with a minimum of zero. Back references of this type cause the group that they reference to be treated as an atomic group. Once the whole group has been matched, a subsequent matching failure cannot cause backtracking into the middle of the group. ASSERTIONS An assertion is a test on the characters following or preceding the current matching point that does not actually consume any characters. The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described above. More complicated assertions are coded as subpatterns. There are two kinds: those that look ahead of the current position in the subject string, and those that look behind it. An assertion subpattern is matched in the normal way, except that it does not cause the current matching position to be changed. Assertion subpatterns are not capturing subpatterns, and may not be repeated, because it makes no sense to assert the same thing several times. If any kind of assertion contains capturing subpatterns within it, these are counted for the purposes of numbering the capturing sub- patterns in the whole pattern. However, substring capturing is carried out only for positive assertions, because it does not make sense for negative assertions. Lookahead assertions Lookahead assertions start with (?= for positive assertions and (?! for negative assertions. For example, \w+(?=;) matches a word followed by a semicolon, but does not include the semi- colon in the match, and foo(?!bar) matches any occurrence of "foo" that is not followed by "bar". Note that the apparently similar pattern (?!foo)bar does not find an occurrence of "bar" that is preceded by something other than "foo"; it finds any occurrence of "bar" whatsoever, because the assertion (?!foo) is always true when the next three characters are "bar". A lookbehind assertion is needed to achieve the other effect. If you want to force a matching failure at some point in a pattern, the most convenient way to do it is with (?!) because an empty string always matches, so an assertion that requires there not to be an empty string must always fail. The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a synonym for (?!). Lookbehind assertions Lookbehind assertions start with (?<= for positive assertions and (?<! for negative assertions. For example, (?<!foo)bar does find an occurrence of "bar" that is not preceded by "foo". The contents of a lookbehind assertion are restricted such that all the strings it matches must have a fixed length. However, if there are sev- eral top-level alternatives, they do not all have to have the same fixed length. Thus (?<=bullock|donkey) is permitted, but (?<!dogs?|cats?) causes an error at compile time. Branches that match different length strings are permitted only at the top level of a lookbehind assertion. This is an extension compared with Perl (5.8 and 5.10), which requires all branches to match the same length of string. An assertion such as (?<=ab(c|de)) is not permitted, because its single top-level branch can match two different lengths, but it is acceptable to PCRE if rewritten to use two top-level branches: (?<=abc|abde) In some cases, the Perl 5.10 escape sequence \K (see above) can be used instead of a lookbehind assertion to get round the fixed-length restriction. The implementation of lookbehind assertions is, for each alternative, to temporarily move the current position back by the fixed length and then try to match. If there are insufficient characters before the cur- rent position, the assertion fails. PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode) to appear in lookbehind assertions, because it makes it impossi- ble to calculate the length of the lookbehind. The \X and \R escapes, which can match different numbers of bytes, are also not permitted. "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long as the subpattern matches a fixed-length string. Recursion, however, is not supported. Possessive quantifiers can be used in conjunction with lookbehind assertions to specify efficient matching of fixed-length strings at the end of subject strings. Consider a simple pattern such as abcd$ when applied to a long string that does not match. Because matching proceeds from left to right, PCRE will look for each "a" in the subject and then see if what follows matches the rest of the pattern. If the pattern is specified as ^.*abcd$ the initial .* matches the entire string at first, but when this fails (because there is no following "a"), it backtracks to match all but the last character, then all but the last two characters, and so on. Once again the search for "a" covers the entire string, from right to left, so we are no better off. However, if the pattern is written as ^.*+(?<=abcd) there can be no backtracking for the .*+ item; it can match only the entire string. The subsequent lookbehind assertion does a single test on the last four characters. If it fails, the match fails immediately. For long strings, this approach makes a significant difference to the processing time. Using multiple assertions Several assertions (of any sort) may occur in succession. For example, (?<=\d{3})(?<!999)foo matches "foo" preceded by three digits that are not "999". Notice that each of the assertions is applied independently at the same point in the subject string. First there is a check that the previous three characters are all digits, and then there is a check that the same three characters are not "999". This pattern does not match "foo" pre- ceded by six characters, the first of which are digits and the last three of which are not "999". For example, it doesn't match "123abc- foo". A pattern to do that is (?<=\d{3}...)(?<!999)foo This time the first assertion looks at the preceding six characters, checking that the first three are digits, and then the second assertion checks that the preceding three characters are not "999". Assertions can be nested in any combination. For example, (?<=(?<!foo)bar)baz matches an occurrence of "baz" that is preceded by "bar" which in turn is not preceded by "foo", while (?<=\d{3}(?!999)...)foo is another pattern that matches "foo" preceded by three digits and any three characters that are not "999". CONDITIONAL SUBPATTERNS It is possible to cause the matching process to obey a subpattern con- ditionally or to choose between two alternative subpatterns, depending on the result of an assertion, or whether a specific capturing subpat- tern has already been matched. The two possible forms of conditional subpattern are: (?(condition)yes-pattern) (?(condition)yes-pattern|no-pattern) If the condition is satisfied, the yes-pattern is used; otherwise the no-pattern (if present) is used. If there are more than two alterna- tives in the subpattern, a compile-time error occurs. There are four kinds of condition: references to subpatterns, refer- ences to recursion, a pseudo-condition called DEFINE, and assertions. Checking for a used subpattern by number If the text between the parentheses consists of a sequence of digits, the condition is true if a capturing subpattern of that number has pre- viously matched. If there is more than one capturing subpattern with the same number (see the earlier section about duplicate subpattern numbers), the condition is true if any of them have been set. An alter- native notation is to precede the digits with a plus or minus sign. In this case, the subpattern number is relative rather than absolute. The most recently opened parentheses can be referenced by (?(-1), the next most recent by (?(-2), and so on. In looping constructs it can also make sense to refer to subsequent groups with constructs such as (?(+2). Consider the following pattern, which contains non-significant white space to make it more readable (assume the PCRE_EXTENDED option) and to divide it into three parts for ease of discussion: ( \( )? [^()]+ (?(1) \) ) The first part matches an optional opening parenthesis, and if that character is present, sets it as the first captured substring. The sec- ond part matches one or more characters that are not parentheses. The third part is a conditional subpattern that tests whether the first set of parentheses matched or not. If they did, that is, if subject started with an opening parenthesis, the condition is true, and so the yes-pat- tern is executed and a closing parenthesis is required. Otherwise, since no-pattern is not present, the subpattern matches nothing. In other words, this pattern matches a sequence of non-parentheses, optionally enclosed in parentheses. If you were embedding this pattern in a larger one, you could use a relative reference: ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... This makes the fragment independent of the parentheses in the larger pattern. Checking for a used subpattern by name Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used subpattern by name. For compatibility with earlier versions of PCRE, which had this facility before Perl, the syntax (?(name)...) is also recognized. However, there is a possible ambiguity with this syn- tax, because subpattern names may consist entirely of digits. PCRE looks first for a named subpattern; if it cannot find one and the name consists entirely of digits, PCRE looks for a subpattern of that num- ber, which must be greater than zero. Using subpattern names that con- sist entirely of digits is not recommended. Rewriting the above example to use a named subpattern gives this: (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) If the name used in a condition of this kind is a duplicate, the test is applied to all subpatterns of the same name, and is true if any one of them has matched. Checking for pattern recursion If the condition is the string (R), and there is no subpattern with the name R, the condition is true if a recursive call to the whole pattern or any subpattern has been made. If digits or a name preceded by amper- sand follow the letter R, for example: (?(R3)...) or (?(R&name)...) the condition is true if the most recent recursion is into a subpattern whose number or name is given. This condition does not check the entire recursion stack. If the name used in a condition of this kind is a duplicate, the test is applied to all subpatterns of the same name, and is true if any one of them is the most recent recursion. At "top level", all these recursion test conditions are false. The syntax for recursive patterns is described below. Defining subpatterns for use by reference only If the condition is the string (DEFINE), and there is no subpattern with the name DEFINE, the condition is always false. In this case, there may be only one alternative in the subpattern. It is always skipped if control reaches this point in the pattern; the idea of DEFINE is that it can be used to define "subroutines" that can be ref- erenced from elsewhere. (The use of "subroutines" is described below.) For example, a pattern to match an IPv4 address could be written like this (ignore whitespace and line breaks): (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) \b (?&byte) (\.(?&byte)){3} \b The first part of the pattern is a DEFINE group inside which a another group named "byte" is defined. This matches an individual component of an IPv4 address (a number less than 256). When matching takes place, this part of the pattern is skipped because DEFINE acts like a false condition. The rest of the pattern uses references to the named group to match the four dot-separated components of an IPv4 address, insist- ing on a word boundary at each end. Assertion conditions If the condition is not in any of the above formats, it must be an assertion. This may be a positive or negative lookahead or lookbehind assertion. Consider this pattern, again containing non-significant white space, and with the two alternatives on the second line: (?(?=[^a-z]*[a-z]) \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) The condition is a positive lookahead assertion that matches an optional sequence of non-letters followed by a letter. In other words, it tests for the presence of at least one letter in the subject. If a letter is found, the subject is matched against the first alternative; otherwise it is matched against the second. This pattern matches strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. COMMENTS The sequence (?# marks the start of a comment that continues up to the next closing parenthesis. Nested parentheses are not permitted. The characters that make up a comment play no part in the pattern matching at all. If the PCRE_EXTENDED option is set, an unescaped # character outside a character class introduces a comment that continues to immediately after the next newline in the pattern. RECURSIVE PATTERNS Consider the problem of matching a string in parentheses, allowing for unlimited nested parentheses. Without the use of recursion, the best that can be done is to use a pattern that matches up to some fixed depth of nesting. It is not possible to handle an arbitrary nesting depth. For some time, Perl has provided a facility that allows regular expres- sions to recurse (amongst other things). It does this by interpolating Perl code in the expression at run time, and the code can refer to the expression itself. A Perl pattern using code interpolation to solve the parentheses problem can be created like this: $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; The (?p{...}) item interpolates Perl code at run time, and in this case refers recursively to the pattern in which it appears. Obviously, PCRE cannot support the interpolation of Perl code. Instead, it supports special syntax for recursion of the entire pattern, and also for individual subpattern recursion. After its introduction in PCRE and Python, this kind of recursion was subsequently introduced into Perl at release 5.10. A special item that consists of (? followed by a number greater than zero and a closing parenthesis is a recursive call of the subpattern of the given number, provided that it occurs inside that subpattern. (If not, it is a "subroutine" call, which is described in the next sec- tion.) The special item (?R) or (?0) is a recursive call of the entire regular expression. This PCRE pattern solves the nested parentheses problem (assume the PCRE_EXTENDED option is set so that white space is ignored): \( ( [^()]++ | (?R) )* \) First it matches an opening parenthesis. Then it matches any number of substrings which can either be a sequence of non-parentheses, or a recursive match of the pattern itself (that is, a correctly parenthe- sized substring). Finally there is a closing parenthesis. Note the use of a possessive quantifier to avoid backtracking into sequences of non- parentheses. If this were part of a larger pattern, you would not want to recurse the entire pattern, so instead you could use this: ( \( ( [^()]++ | (?1) )* \) ) We have put the pattern into parentheses, and caused the recursion to refer to them instead of the whole pattern. In a larger pattern, keeping track of parenthesis numbers can be tricky. This is made easier by the use of relative references (a Perl 5.10 feature). Instead of (?1) in the pattern above you can write (?-2) to refer to the second most recently opened parentheses preceding the recursion. In other words, a negative number counts capturing parentheses leftwards from the point at which it is encountered. It is also possible to refer to subsequently opened parentheses, by writing references such as (?+2). However, these cannot be recursive because the reference is not inside the parentheses that are refer- enced. They are always "subroutine" calls, as described in the next section. An alternative approach is to use named parentheses instead. The Perl syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We could rewrite the above example as follows: (?<pn> \( ( [^()]++ | (?&pn) )* \) ) If there is more than one subpattern with the same name, the earliest one is used. This particular example pattern that we have been looking at contains nested unlimited repeats, and so the use of a possessive quantifier for matching strings of non-parentheses is important when applying the pat- tern to strings that do not match. For example, when this pattern is applied to (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() it yields "no match" quickly. However, if a possessive quantifier is not used, the match runs for a very long time indeed because there are so many different ways the + and * repeats can carve up the subject, and all have to be tested before failure can be reported. At the end of a match, the values of capturing parentheses are those from the outermost level. If you want to obtain intermediate values, a callout function can be used (see below and the pcrecallout documenta- tion). If the pattern above is matched against (ab(cd)ef) the value for the inner capturing parentheses (numbered 2) is "ef", which is the last value taken on at the top level. If a capturing sub- pattern is not matched at the top level, its final value is unset, even if it is (temporarily) set at a deeper level. If there are more than 15 capturing parentheses in a pattern, PCRE has to obtain extra memory to store data during a recursion, which it does by using pcre_malloc, freeing it via pcre_free afterwards. If no memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. Do not confuse the (?R) item with the condition (R), which tests for recursion. Consider this pattern, which matches text in angle brack- ets, allowing for arbitrary nesting. Only digits are allowed in nested brackets (that is, when recursing), whereas any characters are permit- ted at the outer level. < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > In this pattern, (?(R) is the start of a conditional subpattern, with two different alternatives for the recursive and non-recursive cases. The (?R) item is the actual recursive call. Recursion difference from Perl In PCRE (like Python, but unlike Perl), a recursive subpattern call is always treated as an atomic group. That is, once it has matched some of the subject string, it is never re-entered, even if it contains untried alternatives and there is a subsequent matching failure. This can be illustrated by the following pattern, which purports to match a palin- dromic string that contains an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"): ^(.|(.)(?1)\2)$ The idea is that it either matches a single character, or two identical characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE it does not if the pattern is longer than three characters. Consider the subject string "abcba": At the top level, the first character is matched, but as it is not at the end of the string, the first alternative fails; the second alterna- tive is taken and the recursion kicks in. The recursive call to subpat- tern 1 successfully matches the next character ("b"). (Note that the beginning and end of line tests are not part of the recursion). Back at the top level, the next character ("c") is compared with what subpattern 2 matched, which was "a". This fails. Because the recursion is treated as an atomic group, there are now no backtracking points, and so the entire match fails. (Perl is able, at this point, to re- enter the recursion and try the second alternative.) However, if the pattern is written with the alternatives in the other order, things are different: ^((.)(?1)\2|.)$ This time, the recursing alternative is tried first, and continues to recurse until it runs out of characters, at which point the recursion fails. But this time we do have another alternative to try at the higher level. That is the big difference: in the previous case the remaining alternative is at a deeper recursion level, which PCRE cannot use. To change the pattern so that matches all palindromic strings, not just those with an odd number of characters, it is tempting to change the pattern to this: ^((.)(?1)\2|.?)$ Again, this works in Perl, but not in PCRE, and for the same reason. When a deeper recursion has matched a single character, it cannot be entered again in order to match an empty string. The solution is to separate the two cases, and write out the odd and even cases as alter- natives at the higher level: ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) If you want to match typical palindromic phrases, the pattern has to ignore all non-word characters, which can be done like this: ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ If run with the PCRE_CASELESS option, this pattern matches phrases such as "A man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note the use of the possessive quantifier *+ to avoid backtrack- ing into sequences of non-word characters. Without this, PCRE takes a great deal longer (ten times or more) to match typical phrases, and Perl takes so long that you think it has gone into a loop. WARNING: The palindrome-matching patterns above work only if the sub- ject string does not start with a palindrome that is shorter than the entire string. For example, although "abcba" is correctly matched, if the subject is "ababa", PCRE finds the palindrome "aba" at the start, then fails at top level because the end of the string does not follow. Once again, it cannot jump back into the recursion to try other alter- natives, so the entire match fails. SUBPATTERNS AS SUBROUTINES If the syntax for a recursive subpattern reference (either by number or by name) is used outside the parentheses to which it refers, it oper- ates like a subroutine in a programming language. The "called" subpat- tern may be defined before or after the reference. A numbered reference can be absolute or relative, as in these examples: (...(absolute)...)...(?2)... (...(relative)...)...(?-1)... (...(?+1)...(relative)... An earlier example pointed out that the pattern (sens|respons)e and \1ibility matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility". If instead the pattern (sens|respons)e and (?1)ibility is used, it does match "sense and responsibility" as well as the other two strings. Another example is given in the discussion of DEFINE above. Like recursive subpatterns, a subroutine call is always treated as an atomic group. That is, once it has matched some of the subject string, it is never re-entered, even if it contains untried alternatives and there is a subsequent matching failure. Any capturing parentheses that are set during the subroutine call revert to their previous values afterwards. When a subpattern is used as a subroutine, processing options such as case-independence are fixed when the subpattern is defined. They cannot be changed for different calls. For example, consider this pattern: (abc)(?i:(?-1)) It matches "abcabc". It does not match "abcABC" because the change of processing option does not affect the called subpattern. ONIGURUMA SUBROUTINE SYNTAX For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number enclosed either in angle brackets or single quotes, is an alternative syntax for referencing a subpattern as a subroutine, possibly recursively. Here are two of the examples used above, rewrit- ten using this syntax: (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) (sens|respons)e and \g'1'ibility PCRE supports an extension to Oniguruma: if a number is preceded by a plus or a minus sign it is taken as a relative reference. For example: (abc)(?i:\g<-1>) Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former is a back reference; the latter is a subroutine call. CALLOUTS Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl code to be obeyed in the middle of matching a regular expression. This makes it possible, amongst other things, to extract different sub- strings that match the same pair of parentheses when there is a repeti- tion. PCRE provides a similar feature, but of course it cannot obey arbitrary Perl code. The feature is called "callout". The caller of PCRE provides an external function by putting its entry point in the global variable pcre_callout. By default, this variable contains NULL, which disables all calling out. Within a regular expression, (?C) indicates the points at which the external function is to be called. If you want to identify different callout points, you can put a number less than 256 after the letter C. The default value is zero. For example, this pattern has two callout points: (?C1)abc(?C2)def If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are automatically installed before each item in the pattern. They are all numbered 255. During matching, when PCRE reaches a callout point (and pcre_callout is set), the external function is called. It is provided with the number of the callout, the position in the pattern, and, optionally, one item of data originally supplied by the caller of pcre_exec(). The callout function may cause matching to proceed, to backtrack, or to fail alto- gether. A complete description of the interface to the callout function is given in the pcrecallout documentation. BACKTRACKING CONTROL Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which are described in the Perl documentation as "experimental and sub- ject to change or removal in a future version of Perl". It goes on to say: "Their usage in production code should be noted to avoid problems during upgrades." The same remarks apply to the PCRE features described in this section. Since these verbs are specifically related to backtracking, most of them can be used only when the pattern is to be matched using pcre_exec(), which uses a backtracking algorithm. With the exception of (*FAIL), which behaves like a failing negative assertion, they cause an error if encountered by pcre_dfa_exec(). If any of these verbs are used in an assertion or subroutine subpattern (including recursive subpatterns), their effect is confined to that subpattern; it does not extend to the surrounding pattern. Note that such subpatterns are processed as anchored at the point where they are tested. The new verbs make use of what was previously invalid syntax: an open- ing parenthesis followed by an asterisk. In Perl, they are generally of the form (*VERB:ARG) but PCRE does not support the use of arguments, so its general form is just (*VERB). Any number of these verbs may occur in a pattern. There are two kinds: Verbs that act immediately The following verbs act as soon as they are encountered: (*ACCEPT) This verb causes the match to end successfully, skipping the remainder of the pattern. When inside a recursion, only the innermost pattern is ended immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is captured. (This feature was added to PCRE at release 8.00.) For example: A((?:A|B(*ACCEPT)|C)D) This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap- tured by the outer parentheses. (*FAIL) or (*F) This verb causes the match to fail, forcing backtracking to occur. It is equivalent to (?!) but easier to read. The Perl documentation notes that it is probably useful only when combined with (?{}) or (??{}). Those are, of course, Perl features that are not present in PCRE. The nearest equivalent is the callout feature, as for example in this pat- tern: a+(?C)(*FAIL) A match with the string "aaaa" always fails, but the callout is taken before each backtrack happens (in this example, 10 times). Verbs that act after backtracking The following verbs do nothing when they are encountered. Matching con- tinues with what follows, but if there is no subsequent match, a fail- ure is forced. The verbs differ in exactly what kind of failure occurs. (*COMMIT) This verb causes the whole match to fail outright if the rest of the pattern does not match. Even if the pattern is unanchored, no further attempts to find a match by advancing the starting point take place. Once (*COMMIT) has been passed, pcre_exec() is committed to finding a match at the current starting point, or not at all. For example: a+(*COMMIT)b This matches "xxaab" but not "aacaab". It can be thought of as a kind of dynamic anchor, or "I've started, so I must finish." (*PRUNE) This verb causes the match to fail at the current position if the rest of the pattern does not match. If the pattern is unanchored, the normal "bumpalong" advance to the next starting character then happens. Back- tracking can occur as usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but if there is no match to the right, back- tracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alternative to an atomic group or possessive quantifier, but there are some uses of (*PRUNE) that cannot be expressed in any other way. (*SKIP) This verb is like (*PRUNE), except that if the pattern is unanchored, the "bumpalong" advance is not to the next character, but to the posi- tion in the subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text was matched leading up to it cannot be part of a successful match. Consider: a+(*SKIP)b If the subject is "aaaac...", after the first match attempt fails (starting at the first character in the string), the starting point skips on to start the next attempt at "c". Note that a possessive quan- tifer does not have the same effect as this example; although it would suppress backtracking during the first match attempt, the second attempt would start at the second character instead of skipping on to "c". (*THEN) This verb causes a skip to the next alternation if the rest of the pat- tern does not match. That is, it cancels pending backtracking, but only within the current alternation. Its name comes from the observation that it can be used for a pattern-based if-then-else block: ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... If the COND1 pattern matches, FOO is tried (and possibly further items after the end of the group if FOO succeeds); on failure the matcher skips to the second alternative and tries COND2, without backtracking into COND1. If (*THEN) is used outside of any alternation, it acts exactly like (*PRUNE). SEE ALSO pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3). AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 06 March 2010 Copyright (c) 1997-2010 University of Cambridge. ------------------------------------------------------------------------------ PCRESYNTAX(3) PCRESYNTAX(3) NAME PCRE - Perl-compatible regular expressions PCRE REGULAR EXPRESSION SYNTAX SUMMARY The full syntax and semantics of the regular expressions that are sup- ported by PCRE are described in the pcrepattern documentation. This document contains just a quick-reference summary of the syntax. QUOTING \x where x is non-alphanumeric is a literal x \Q...\E treat enclosed characters as literal CHARACTERS \a alarm, that is, the BEL character (hex 07) \cx "control-x", where x is any character \e escape (hex 1B) \f formfeed (hex 0C) \n newline (hex 0A) \r carriage return (hex 0D) \t tab (hex 09) \ddd character with octal code ddd, or backreference \xhh character with hex code hh \x{hhh..} character with hex code hhh.. CHARACTER TYPES . any character except newline; in dotall mode, any character whatsoever \C one byte, even in UTF-8 mode (best avoided) \d a decimal digit \D a character that is not a decimal digit \h a horizontal whitespace character \H a character that is not a horizontal whitespace character \p{xx} a character with the xx property \P{xx} a character without the xx property \R a newline sequence \s a whitespace character \S a character that is not a whitespace character \v a vertical whitespace character \V a character that is not a vertical whitespace character \w a "word" character \W a "non-word" character \X an extended Unicode sequence In PCRE, \d, \D, \s, \S, \w, and \W recognize only ASCII characters. GENERAL CATEGORY PROPERTY CODES FOR \p and \P C Other Cc Control Cf Format Cn Unassigned Co Private use Cs Surrogate L Letter Ll Lower case letter Lm Modifier letter Lo Other letter Lt Title case letter Lu Upper case letter L& Ll, Lu, or Lt M Mark Mc Spacing mark Me Enclosing mark Mn Non-spacing mark N Number Nd Decimal number Nl Letter number No Other number P Punctuation Pc Connector punctuation Pd Dash punctuation Pe Close punctuation Pf Final punctuation Pi Initial punctuation Po Other punctuation Ps Open punctuation S Symbol Sc Currency symbol Sk Modifier symbol Sm Mathematical symbol So Other symbol Z Separator Zl Line separator Zp Paragraph separator Zs Space separator SCRIPT NAMES FOR \p AND \P Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp- tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe- rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam, Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic, Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian, Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai, Yi. CHARACTER CLASSES [...] positive character class [^...] negative character class [x-y] range (can be used for hex characters) [[:xxx:]] positive POSIX named set [[:^xxx:]] negative POSIX named set alnum alphanumeric alpha alphabetic ascii 0-127 blank space or tab cntrl control character digit decimal digit graph printing, excluding space lower lower case letter print printing, including space punct printing, excluding alphanumeric space whitespace upper upper case letter word same as \w xdigit hexadecimal digit In PCRE, POSIX character set names recognize only ASCII characters. You can use \Q...\E inside a character class. QUANTIFIERS ? 0 or 1, greedy ?+ 0 or 1, possessive ?? 0 or 1, lazy * 0 or more, greedy *+ 0 or more, possessive *? 0 or more, lazy + 1 or more, greedy ++ 1 or more, possessive +? 1 or more, lazy {n} exactly n {n,m} at least n, no more than m, greedy {n,m}+ at least n, no more than m, possessive {n,m}? at least n, no more than m, lazy {n,} n or more, greedy {n,}+ n or more, possessive {n,}? n or more, lazy ANCHORS AND SIMPLE ASSERTIONS \b word boundary (only ASCII letters recognized) \B not a word boundary ^ start of subject also after internal newline in multiline mode \A start of subject $ end of subject also before newline at end of subject also before internal newline in multiline mode \Z end of subject also before newline at end of subject \z end of subject \G first matching position in subject MATCH POINT RESET \K reset start of match ALTERNATION expr|expr|expr... CAPTURING (...) capturing group (?<name>...) named capturing group (Perl) (?'name'...) named capturing group (Perl) (?P<name>...) named capturing group (Python) (?:...) non-capturing group (?|...) non-capturing group; reset group numbers for capturing groups in each alternative ATOMIC GROUPS (?>...) atomic, non-capturing group COMMENT (?#....) comment (not nestable) OPTION SETTING (?i) caseless (?J) allow duplicate names (?m) multiline (?s) single line (dotall) (?U) default ungreedy (lazy) (?x) extended (ignore white space) (?-...) unset option(s) The following is recognized only at the start of a pattern or after one of the newline-setting options with similar syntax: (*UTF8) set UTF-8 mode LOOKAHEAD AND LOOKBEHIND ASSERTIONS (?=...) positive look ahead (?!...) negative look ahead (?<=...) positive look behind (?<!...) negative look behind Each top-level branch of a look behind must be of a fixed length. BACKREFERENCES \n reference by number (can be ambiguous) \gn reference by number \g{n} reference by number \g{-n} relative reference by number \k<name> reference by name (Perl) \k'name' reference by name (Perl) \g{name} reference by name (Perl) \k{name} reference by name (.NET) (?P=name) reference by name (Python) SUBROUTINE REFERENCES (POSSIBLY RECURSIVE) (?R) recurse whole pattern (?n) call subpattern by absolute number (?+n) call subpattern by relative number (?-n) call subpattern by relative number (?&name) call subpattern by name (Perl) (?P>name) call subpattern by name (Python) \g<name> call subpattern by name (Oniguruma) \g'name' call subpattern by name (Oniguruma) \g<n> call subpattern by absolute number (Oniguruma) \g'n' call subpattern by absolute number (Oniguruma) \g<+n> call subpattern by relative number (PCRE extension) \g'+n' call subpattern by relative number (PCRE extension) \g<-n> call subpattern by relative number (PCRE extension) \g'-n' call subpattern by relative number (PCRE extension) CONDITIONAL PATTERNS (?(condition)yes-pattern) (?(condition)yes-pattern|no-pattern) (?(n)... absolute reference condition (?(+n)... relative reference condition (?(-n)... relative reference condition (?(<name>)... named reference condition (Perl) (?('name')... named reference condition (Perl) (?(name)... named reference condition (PCRE) (?(R)... overall recursion condition (?(Rn)... specific group recursion condition (?(R&name)... specific recursion condition (?(DEFINE)... define subpattern for reference (?(assert)... assertion condition BACKTRACKING CONTROL The following act immediately they are reached: (*ACCEPT) force successful match (*FAIL) force backtrack; synonym (*F) The following act only when a subsequent match failure causes a back- track to reach them. They all force a match failure, but they differ in what happens afterwards. Those that advance the start-of-match point do so only if the pattern is not anchored. (*COMMIT) overall failure, no advance of starting point (*PRUNE) advance to next starting character (*SKIP) advance start to current matching position (*THEN) local failure, backtrack to next alternation NEWLINE CONVENTIONS These are recognized only at the very start of the pattern or after a (*BSR_...) or (*UTF8) option. (*CR) carriage return only (*LF) linefeed only (*CRLF) carriage return followed by linefeed (*ANYCRLF) all three of the above (*ANY) any Unicode newline sequence WHAT \R MATCHES These are recognized only at the very start of the pattern or after a (*...) option that sets the newline convention or UTF-8 mode. (*BSR_ANYCRLF) CR, LF, or CRLF (*BSR_UNICODE) any Unicode newline sequence CALLOUTS (?C) callout (?Cn) callout with data n SEE ALSO pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3). AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 01 March 2010 Copyright (c) 1997-2010 University of Cambridge. ------------------------------------------------------------------------------ PCREPARTIAL(3) PCREPARTIAL(3) NAME PCRE - Perl-compatible regular expressions PARTIAL MATCHING IN PCRE In normal use of PCRE, if the subject string that is passed to pcre_exec() or pcre_dfa_exec() matches as far as it goes, but is too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances where it might be helpful to distinguish this case from other cases in which there is no match. Consider, for example, an application where a human is required to type in data for a field with specific formatting requirements. An example might be a date in the form ddmmmyy, defined by this pattern: ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$ If the application sees the user's keystrokes one by one, and can check that what has been typed so far is potentially valid, it is able to raise an error as soon as a mistake is made, by beeping and not reflecting the character that has been typed, for example. This immedi- ate feedback is likely to be a better user interface than a check that is delayed until the entire string has been entered. Partial matching can also sometimes be useful when the subject string is very long and is not all available at once. PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and PCRE_PARTIAL_HARD options, which can be set when calling pcre_exec() or pcre_dfa_exec(). For backwards compatibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. The essential difference between the two options is whether or not a partial match is preferred to an alternative com- plete match, though the details differ between the two matching func- tions. If both options are set, PCRE_PARTIAL_HARD takes precedence. Setting a partial matching option disables two of PCRE's optimizations. PCRE remembers the last literal byte in a pattern, and abandons match- ing immediately if such a byte is not present in the subject string. This optimization cannot be used for a subject string that might match only partially. If the pattern was studied, PCRE knows the minimum length of a matching string, and does not bother to run the matching function on shorter strings. This optimization is also disabled for partial matching. PARTIAL MATCHING USING pcre_exec() A partial match occurs during a call to pcre_exec() whenever the end of the subject string is reached successfully, but matching cannot con- tinue because more characters are needed. However, at least one charac- ter must have been matched. (In other words, a partial match can never be an empty string.) If PCRE_PARTIAL_SOFT is set, the partial match is remembered, but matching continues as normal, and other alternatives in the pattern are tried. If no complete match can be found, pcre_exec() returns PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. If there are at least two slots in the offsets vector, the first of them is set to the offset of the earliest character that was inspected when the partial match was found. For convenience, the second offset points to the end of the string so that a substring can easily be identified. For the majority of patterns, the first offset identifies the start of the partially matched string. However, for patterns that contain look- behind assertions, or \K, or begin with \b or \B, earlier characters have been inspected while carrying out the match. For example: /(?<=abc)123/ This pattern matches "123", but only if it is preceded by "abc". If the subject string is "xyzabc12", the offsets after a partial match are for the substring "abc12", because all these characters are needed if another match is tried with extra characters added. If there is more than one partial match, the first one that was found provides the data that is returned. Consider this pattern: /123\w+X|dogY/ If this is matched against the subject string "abc123dog", both alter- natives fail to match, but the end of the subject is reached during matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The offsets are set to 3 and 9, identifying "123dog" as the first partial match that was found. (In this example, there are two partial matches, because "dog" on its own partially matches the second alternative.) If PCRE_PARTIAL_HARD is set for pcre_exec(), it returns PCRE_ERROR_PAR- TIAL as soon as a partial match is found, without continuing to search for possible complete matches. The difference between the two options can be illustrated by a pattern such as: /dog(sbody)?/ This matches either "dog" or "dogsbody", greedily (that is, it prefers the longer string if possible). If it is matched against the string "dog" with PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand, if the pattern is made ungreedy the result is dif- ferent: /dog(sbody)??/ In this case the result is always a complete match because pcre_exec() finds that first, and it never continues after finding a match. It might be easier to follow this explanation by thinking of the two pat- terns like this: /dog(sbody)?/ is the same as /dogsbody|dog/ /dog(sbody)??/ is the same as /dog|dogsbody/ The second pattern will never match "dogsbody" when pcre_exec() is used, because it will always find the shorter match first. PARTIAL MATCHING USING pcre_dfa_exec() The pcre_dfa_exec() function moves along the subject string character by character, without backtracking, searching for all possible matches simultaneously. If the end of the subject is reached before the end of the pattern, there is the possibility of a partial match, again pro- vided that at least one character has matched. When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there have been no complete matches. Otherwise, the complete matches are returned. However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any complete matches. The portion of the string that was inspected when the longest partial match was found is set as the first matching string, provided there are at least two slots in the offsets vector. Because pcre_dfa_exec() always searches for all possible matches, and there is no difference between greedy and ungreedy repetition, its be- haviour is different from pcre_exec when PCRE_PARTIAL_HARD is set. Con- sider the string "dog" matched against the ungreedy pattern shown above: /dog(sbody)??/ Whereas pcre_exec() stops as soon as it finds the complete match for "dog", pcre_dfa_exec() also finds the partial match for "dogsbody", and so returns that when PCRE_PARTIAL_HARD is set. PARTIAL MATCHING AND WORD BOUNDARIES If a pattern ends with one of sequences \b or \B, which test for word boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter- intuitive results. Consider this pattern: /\bcat\b/ This matches "cat", provided there is a word boundary at either end. If the subject string is "the cat", the comparison of the final "t" with a following character cannot take place, so a partial match is found. However, pcre_exec() carries on with normal matching, which matches \b at the end of the subject when the last character is a letter, thus finding a complete match. The result, therefore, is not PCRE_ERROR_PAR- TIAL. The same thing happens with pcre_dfa_exec(), because it also finds the complete match. Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because then the partial match takes precedence. FORMERLY RESTRICTED PATTERNS For releases of PCRE prior to 8.00, because of the way certain internal optimizations were implemented in the pcre_exec() function, the PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with all patterns. From release 8.00 onwards, the restrictions no longer apply, and partial matching with pcre_exec() can be requested for any pattern. Items that were formerly restricted were repeated single characters and repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not conform to the restrictions, pcre_exec() returned the error code PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to find out if a compiled pattern can be used for partial matching now always returns 1. EXAMPLE OF PARTIAL MATCHING USING PCRETEST If the escape sequence \P is present in a pcretest data line, the PCRE_PARTIAL_SOFT option is used for the match. Here is a run of pcretest that uses the date example quoted above: re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ data> 25jun04\P 0: 25jun04 1: jun data> 25dec3\P Partial match: 23dec3 data> 3ju\P Partial match: 3ju data> 3juj\P No match data> j\P No match The first data string is matched completely, so pcretest shows the matched substrings. The remaining four strings do not match the com- plete pattern, but the first two are partial matches. Similar output is obtained when pcre_dfa_exec() is used. If the escape sequence \P is present more than once in a pcretest data line, the PCRE_PARTIAL_HARD option is set for the match. MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() When a partial match has been found using pcre_dfa_exec(), it is possi- ble to continue the match by providing additional subject data and calling pcre_dfa_exec() again with the same compiled regular expres- sion, this time setting the PCRE_DFA_RESTART option. You must pass the same working space as before, because this is where details of the pre- vious partial match are stored. Here is an example using pcretest, using the \R escape sequence to set the PCRE_DFA_RESTART option (\D specifies the use of pcre_dfa_exec()): re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ data> 23ja\P\D Partial match: 23ja data> n05\R\D 0: n05 The first call has "23ja" as the subject, and requests partial match- ing; the second call has "n05" as the subject for the continued (restarted) match. Notice that when the match is complete, only the last part is shown; PCRE does not retain the previously partially- matched string. It is up to the calling program to do that if it needs to. You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with PCRE_DFA_RESTART to continue partial matching over multiple segments. This facility can be used to pass very long subject strings to pcre_dfa_exec(). MULTI-SEGMENT MATCHING WITH pcre_exec() From release 8.00, pcre_exec() can also be used to do multi-segment matching. Unlike pcre_dfa_exec(), it is not possible to restart the previous match with a new segment of data. Instead, new data must be added to the previous subject string, and the entire match re-run, starting from the point where the partial match occurred. Earlier data can be discarded. Consider an unanchored pattern that matches dates: re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/ data> The date is 23ja\P Partial match: 23ja At this stage, an application could discard the text preceding "23ja", add on text from the next segment, and call pcre_exec() again. Unlike pcre_dfa_exec(), the entire matching string must always be available, and the complete matching process occurs for each call, so more memory and more processing time is needed. Note: If the pattern contains lookbehind assertions, or \K, or starts with \b or \B, the string that is returned for a partial match will include characters that precede the partially matched string itself, because these must be retained when adding on more characters for a subsequent matching attempt. ISSUES WITH MULTI-SEGMENT MATCHING Certain types of pattern may give problems with multi-segment matching, whichever matching function is used. 1. If the pattern contains tests for the beginning or end of a line, you need to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropri- ate, when the subject string for any call does not contain the begin- ning or end of a line. 2. Lookbehind assertions at the start of a pattern are catered for in the offsets that are returned for a partial match. However, in theory, a lookbehind assertion later in the pattern could require even earlier characters to be inspected, and it might not have been reached when a partial match occurs. This is probably an extremely unlikely case; you could guard against it to a certain extent by always including extra characters at the start. 3. Matching a subject string that is split into multiple segments may not always produce exactly the same result as matching over one single long string, especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and Word Boundaries" above describes an issue that arises if the pattern ends with \b or \B. Another kind of difference may occur when there are multiple matching possibilities, because a partial match result is given only when there are no completed matches. This means that as soon as the shortest match has been found, continua- tion to a new subject segment is no longer possible. Consider again this pcretest example: re> /dog(sbody)?/ data> dogsb\P 0: dog data> do\P\D Partial match: do data> gsb\R\P\D 0: g data> dogsbody\D 0: dogsbody 1: dog The first data line passes the string "dogsb" to pcre_exec(), setting the PCRE_PARTIAL_SOFT option. Although the string is a partial match for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string "dog" is a complete match. Similarly, when the subject is presented to pcre_dfa_exec() in several parts ("do" and "gsb" being the first two) the match stops when "dog" has been found, and it is not possible to continue. On the other hand, if "dogsbody" is presented as a single string, pcre_dfa_exec() finds both matches. Because of these problems, it is probably best to use PCRE_PARTIAL_HARD when matching multi-segment data. The example above then behaves dif- ferently: re> /dog(sbody)?/ data> dogsb\P\P Partial match: dogsb data> do\P\D Partial match: do data> gsb\R\P\P\D Partial match: gsb 4. Patterns that contain alternatives at the top level which do not all start with the same pattern item may not work as expected when PCRE_DFA_RESTART is used with pcre_dfa_exec(). For example, consider this pattern: 1234|3789 If the first part of the subject is "ABC123", a partial match of the first alternative is found at offset 3. There is no partial match for the second alternative, because such a match does not start at the same point in the subject string. Attempting to continue with the string "7890" does not yield a match because only those alternatives that match at one point in the subject are remembered. The problem arises because the start of the second alternative matches within the first alternative. There is no problem with anchored patterns or patterns such as: 1234|ABCD where no string can be a partial match for both alternatives. This is not a problem if pcre_exec() is used, because the entire match has to be rerun each time: re> /1234|3789/ data> ABC123\P Partial match: 123 data> 1237890 0: 3789 Of course, instead of using PCRE_DFA_PARTIAL, the same technique of re- running the entire match can also be used with pcre_dfa_exec(). Another possibility is to work with two buffers. If a partial match at offset n in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on the second buffer, you can then try a new match starting at offset n+1 in the first buffer. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 19 October 2009 Copyright (c) 1997-2009 University of Cambridge. ------------------------------------------------------------------------------ PCREPRECOMPILE(3) PCREPRECOMPILE(3) NAME PCRE - Perl-compatible regular expressions SAVING AND RE-USING PRECOMPILED PCRE PATTERNS If you are running an application that uses a large number of regular expression patterns, it may be useful to store them in a precompiled form instead of having to compile them every time the application is run. If you are not using any private character tables (see the pcre_maketables() documentation), this is relatively straightforward. If you are using private tables, it is a little bit more complicated. If you save compiled patterns to a file, you can copy them to a differ- ent host and run them there. This works even if the new host has the opposite endianness to the one on which the patterns were compiled. There may be a small performance penalty, but it should be insignifi- cant. However, compiling regular expressions with one version of PCRE for use with a different version is not guaranteed to work and may cause crashes. SAVING A COMPILED PATTERN The value returned by pcre_compile() points to a single block of memory that holds the compiled pattern and associated data. You can find the length of this block in bytes by calling pcre_fullinfo() with an argu- ment of PCRE_INFO_SIZE. You can then save the data in any appropriate manner. Here is sample code that compiles a pattern and writes it to a file. It assumes that the variable fd refers to a file that is open for output: int erroroffset, rc, size; char *error; pcre *re; re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL); if (re == NULL) { ... handle errors ... } rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size); if (rc < 0) { ... handle errors ... } rc = fwrite(re, 1, size, fd); if (rc != size) { ... handle errors ... } In this example, the bytes that comprise the compiled pattern are copied exactly. Note that this is binary data that may contain any of the 256 possible byte values. On systems that make a distinction between binary and non-binary data, be sure that the file is opened for binary output. If you want to write more than one pattern to a file, you will have to devise a way of separating them. For binary data, preceding each pat- tern with its length is probably the most straightforward approach. Another possibility is to write out the data in hexadecimal instead of binary, one pattern to a line. Saving compiled patterns in a file is only one possible way of storing them for later use. They could equally well be saved in a database, or in the memory of some daemon process that passes them via sockets to the processes that want them. If the pattern has been studied, it is also possible to save the study data in a similar way to the compiled pattern itself. When studying generates additional information, pcre_study() returns a pointer to a pcre_extra data block. Its format is defined in the section on matching a pattern in the pcreapi documentation. The study_data field points to the binary study data, and this is what you must save (not the pcre_extra block itself). The length of the study data can be obtained by calling pcre_fullinfo() with an argument of PCRE_INFO_STUDYSIZE. Remember to check that pcre_study() did return a non-NULL value before trying to save the study data. RE-USING A PRECOMPILED PATTERN Re-using a precompiled pattern is straightforward. Having reloaded it into main memory, you pass its pointer to pcre_exec() or pcre_dfa_exec() in the usual way. This should work even on another host, and even if that host has the opposite endianness to the one where the pattern was compiled. However, if you passed a pointer to custom character tables when the pattern was compiled (the tableptr argument of pcre_compile()), you must now pass a similar pointer to pcre_exec() or pcre_dfa_exec(), because the value saved with the compiled pattern will obviously be nonsense. A field in a pcre_extra() block is used to pass this data, as described in the section on matching a pattern in the pcreapi documen- tation. If you did not provide custom character tables when the pattern was compiled, the pointer in the compiled pattern is NULL, which causes pcre_exec() to use PCRE's internal tables. Thus, you do not need to take any special action at run time in this case. If you saved study data with the compiled pattern, you need to create your own pcre_extra data block and set the study_data field to point to the reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA bit in the flags field to indicate that study data is present. Then pass the pcre_extra block to pcre_exec() or pcre_dfa_exec() in the usual way. COMPATIBILITY WITH DIFFERENT PCRE RELEASES In general, it is safest to recompile all saved patterns when you update to a new PCRE release, though not all updates actually require this. Recompiling is definitely needed for release 7.2. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 13 June 2007 Copyright (c) 1997-2007 University of Cambridge. ------------------------------------------------------------------------------ PCREPERFORM(3) PCREPERFORM(3) NAME PCRE - Perl-compatible regular expressions PCRE PERFORMANCE Two aspects of performance are discussed below: memory usage and pro- cessing time. The way you express your pattern as a regular expression can affect both of them. COMPILED PATTERN MEMORY USAGE Patterns are compiled by PCRE into a reasonably efficient byte code, so that most simple patterns do not use much memory. However, there is one case where the memory usage of a compiled pattern can be unexpectedly large. If a parenthesized subpattern has a quantifier with a minimum greater than 1 and/or a limited maximum, the whole subpattern is repeated in the compiled code. For example, the pattern (abc|def){2,4} is compiled as if it were (abc|def)(abc|def)((abc|def)(abc|def)?)? (Technical aside: It is done this way so that backtrack points within each of the repetitions can be independently maintained.) For regular expressions whose quantifiers use only small numbers, this is not usually a problem. However, if the numbers are large, and par- ticularly if such repetitions are nested, the memory usage can become an embarrassment. For example, the very simple pattern ((ab){1,1000}c){1,3} uses 51K bytes when compiled. When PCRE is compiled with its default internal pointer size of two bytes, the size limit on a compiled pat- tern is 64K, and this is reached with the above pattern if the outer repetition is increased from 3 to 4. PCRE can be compiled to use larger internal pointers and thus handle larger compiled patterns, but it is better to try to rewrite your pattern to use less memory if you can. One way of reducing the memory usage for such patterns is to make use of PCRE's "subroutine" facility. Re-writing the above pattern as ((ab)(?2){0,999}c)(?1){0,2} reduces the memory requirements to 18K, and indeed it remains under 20K even with the outer repetition increased to 100. However, this pattern is not exactly equivalent, because the "subroutine" calls are treated as atomic groups into which there can be no backtracking if there is a subsequent matching failure. Therefore, PCRE cannot do this kind of rewriting automatically. Furthermore, there is a noticeable loss of speed when executing the modified pattern. Nevertheless, if the atomic grouping is not a problem and the loss of speed is acceptable, this kind of rewriting will allow you to process patterns that PCRE cannot otherwise handle. STACK USAGE AT RUN TIME When pcre_exec() is used for matching, certain kinds of pattern can cause it to use large amounts of the process stack. In some environ- ments the default process stack is quite small, and if it runs out the result is often SIGSEGV. This issue is probably the most frequently raised problem with PCRE. Rewriting your pattern can often help. The pcrestack documentation discusses this issue in detail. PROCESSING TIME Certain items in regular expression patterns are processed more effi- ciently than others. It is more efficient to use a character class like [aeiou] than a set of single-character alternatives such as (a|e|i|o|u). In general, the simplest construction that provides the required behaviour is usually the most efficient. Jeffrey Friedl's book contains a lot of useful general discussion about optimizing regular expressions for efficient performance. This document contains a few observations about PCRE. Using Unicode character properties (the \p, \P, and \X escapes) is slow, because PCRE has to scan a structure that contains data for over fifteen thousand characters whenever it needs a character's property. If you can find an alternative pattern that does not use character properties, it will probably be faster. When a pattern begins with .* not in parentheses, or in parentheses that are not the subject of a backreference, and the PCRE_DOTALL option is set, the pattern is implicitly anchored by PCRE, since it can match only at the start of a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this optimization, because the . metacharacter does not then match a newline, and if the subject string contains new- lines, the pattern may match from the character immediately following one of them instead of from the very start. For example, the pattern .*second matches the subject "first\nand second" (where \n stands for a newline character), with the match starting at the seventh character. In order to do this, PCRE has to retry the match starting after every newline in the subject. If you are using such a pattern with subject strings that do not con- tain newlines, the best performance is obtained by setting PCRE_DOTALL, or starting the pattern with ^.* or ^.*? to indicate explicit anchor- ing. That saves PCRE from having to scan along the subject looking for a newline to restart at. Beware of patterns that contain nested indefinite repeats. These can take a long time to run when applied to a string that does not match. Consider the pattern fragment ^(a+)* This can match "aaaa" in 16 different ways, and this number increases very rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4 times, and for each of those cases other than 0 or 4, the + repeats can match different numbers of times.) When the remainder of the pattern is such that the entire match is going to fail, PCRE has in principle to try every possible variation, and this can take an extremely long time, even for relatively short strings. An optimization catches some of the more simple cases such as (a+)*b where a literal character follows. Before embarking on the standard matching procedure, PCRE checks that there is a "b" later in the sub- ject string, and if there is not, it fails the match immediately. How- ever, when there is no following literal this optimization cannot be used. You can see the difference by comparing the behaviour of (a+)*\d with the pattern above. The former gives a failure almost instantly when applied to a whole line of "a" characters, whereas the latter takes an appreciable time with strings longer than about 20 characters. In many cases, the solution to this kind of performance issue is to use an atomic group or a possessive quantifier. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 07 March 2010 Copyright (c) 1997-2010 University of Cambridge. ------------------------------------------------------------------------------ PCREPOSIX(3) PCREPOSIX(3) NAME PCRE - Perl-compatible regular expressions. SYNOPSIS OF POSIX API #include <pcreposix.h> int regcomp(regex_t *preg, const char *pattern, int cflags); int regexec(regex_t *preg, const char *string, size_t nmatch, regmatch_t pmatch[], int eflags); size_t regerror(int errcode, const regex_t *preg, char *errbuf, size_t errbuf_size); void regfree(regex_t *preg); DESCRIPTION This set of functions provides a POSIX-style API to the PCRE regular expression package. See the pcreapi documentation for a description of PCRE's native API, which contains much additional functionality. The functions described here are just wrapper functions that ultimately call the PCRE native API. Their prototypes are defined in the pcreposix.h header file, and on Unix systems the library itself is called pcreposix.a, so can be accessed by adding -lpcreposix to the command for linking an application that uses them. Because the POSIX functions call the native ones, it is also necessary to add -lpcre. I have implemented only those POSIX option bits that can be reasonably mapped to PCRE native options. In addition, the option REG_EXTENDED is defined with the value zero. This has no effect, but since programs that are written to the POSIX interface often use it, this makes it easier to slot in PCRE as a replacement library. Other POSIX options are not even defined. There are also some other options that are not defined by POSIX. These have been added at the request of users who want to make use of certain PCRE-specific features via the POSIX calling interface. When PCRE is called via these functions, it is only the API that is POSIX-like in style. The syntax and semantics of the regular expres- sions themselves are still those of Perl, subject to the setting of various PCRE options, as described below. "POSIX-like in style" means that the API approximates to the POSIX definition; it is not fully POSIX-compatible, and in multi-byte encoding domains it is probably even less compatible. The header for these functions is supplied as pcreposix.h to avoid any potential clash with other POSIX libraries. It can, of course, be renamed or aliased as regex.h, which is the "correct" name. It provides two structure types, regex_t for compiled internal forms, and reg- match_t for returning captured substrings. It also defines some con- stants whose names start with "REG_"; these are used for setting options and identifying error codes. COMPILING A PATTERN The function regcomp() is called to compile a pattern into an internal form. The pattern is a C string terminated by a binary zero, and is passed in the argument pattern. The preg argument is a pointer to a regex_t structure that is used as a base for storing information about the compiled regular expression. The argument cflags is either zero, or contains one or more of the bits defined by the following macros: REG_DOTALL The PCRE_DOTALL option is set when the regular expression is passed for compilation to the native function. Note that REG_DOTALL is not part of the POSIX standard. REG_ICASE The PCRE_CASELESS option is set when the regular expression is passed for compilation to the native function. REG_NEWLINE The PCRE_MULTILINE option is set when the regular expression is passed for compilation to the native function. Note that this does not mimic the defined POSIX behaviour for REG_NEWLINE (see the following sec- tion). REG_NOSUB The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is passed for compilation to the native function. In addition, when a pat- tern that is compiled with this flag is passed to regexec() for match- ing, the nmatch and pmatch arguments are ignored, and no captured strings are returned. REG_UNGREEDY The PCRE_UNGREEDY option is set when the regular expression is passed for compilation to the native function. Note that REG_UNGREEDY is not part of the POSIX standard. REG_UTF8 The PCRE_UTF8 option is set when the regular expression is passed for compilation to the native function. This causes the pattern itself and all data strings used for matching it to be treated as UTF-8 strings. Note that REG_UTF8 is not part of the POSIX standard. In the absence of these flags, no options are passed to the native function. This means the the regex is compiled with PCRE default semantics. In particular, the way it handles newline characters in the subject string is the Perl way, not the POSIX way. Note that setting PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE. It does not affect the way newlines are matched by . (they are not) or by a negative class such as [^a] (they are). The yield of regcomp() is zero on success, and non-zero otherwise. The preg structure is filled in on success, and one member of the structure is public: re_nsub contains the number of capturing subpatterns in the regular expression. Various error codes are defined in the header file. NOTE: If the yield of regcomp() is non-zero, you must not attempt to use the contents of the preg structure. If, for example, you pass it to regexec(), the result is undefined and your program is likely to crash. MATCHING NEWLINE CHARACTERS This area is not simple, because POSIX and Perl take different views of things. It is not possible to get PCRE to obey POSIX semantics, but then PCRE was never intended to be a POSIX engine. The following table lists the different possibilities for matching newline characters in PCRE: Default Change with . matches newline no PCRE_DOTALL newline matches [^a] yes not changeable $ matches \n at end yes PCRE_DOLLARENDONLY $ matches \n in middle no PCRE_MULTILINE ^ matches \n in middle no PCRE_MULTILINE This is the equivalent table for POSIX: Default Change with . matches newline yes REG_NEWLINE newline matches [^a] yes REG_NEWLINE $ matches \n at end no REG_NEWLINE $ matches \n in middle no REG_NEWLINE ^ matches \n in middle no REG_NEWLINE PCRE's behaviour is the same as Perl's, except that there is no equiva- lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is no way to stop newline from matching [^a]. The default POSIX newline handling can be obtained by setting PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE behave exactly as for the REG_NEWLINE action. MATCHING A PATTERN The function regexec() is called to match a compiled pattern preg against a given string, which is by default terminated by a zero byte (but see REG_STARTEND below), subject to the options in eflags. These can be: REG_NOTBOL The PCRE_NOTBOL option is set when calling the underlying PCRE matching function. REG_NOTEMPTY The PCRE_NOTEMPTY option is set when calling the underlying PCRE match- ing function. Note that REG_NOTEMPTY is not part of the POSIX standard. However, setting this option can give more POSIX-like behaviour in some situations. REG_NOTEOL The PCRE_NOTEOL option is set when calling the underlying PCRE matching function. REG_STARTEND The string is considered to start at string + pmatch[0].rm_so and to have a terminating NUL located at string + pmatch[0].rm_eo (there need not actually be a NUL at that location), regardless of the value of nmatch. This is a BSD extension, compatible with but not specified by IEEE Standard 1003.2 (POSIX.2), and should be used with caution in software intended to be portable to other systems. Note that a non-zero rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location of the string, not how it is matched. If the pattern was compiled with the REG_NOSUB flag, no data about any matched strings is returned. The nmatch and pmatch arguments of regexec() are ignored. If the value of nmatch is zero, or if the value pmatch is NULL, no data about any matched strings is returned. Otherwise,the portion of the string that was matched, and also any cap- tured substrings, are returned via the pmatch argument, which points to an array of nmatch structures of type regmatch_t, containing the mem- bers rm_so and rm_eo. These contain the offset to the first character of each substring and the offset to the first character after the end of each substring, respectively. The 0th element of the vector relates to the entire portion of string that was matched; subsequent elements relate to the capturing subpatterns of the regular expression. Unused entries in the array have both structure members set to -1. A successful match yields a zero return; various error codes are defined in the header file, of which REG_NOMATCH is the "expected" failure code. ERROR MESSAGES The regerror() function maps a non-zero errorcode from either regcomp() or regexec() to a printable message. If preg is not NULL, the error should have arisen from the use of that structure. A message terminated by a binary zero is placed in errbuf. The length of the message, including the zero, is limited to errbuf_size. The yield of the func- tion is the size of buffer needed to hold the whole message. MEMORY USAGE Compiling a regular expression causes memory to be allocated and asso- ciated with the preg structure. The function regfree() frees all such memory, after which preg may no longer be used as a compiled expres- sion. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 02 September 2009 Copyright (c) 1997-2009 University of Cambridge. ------------------------------------------------------------------------------ PCRECPP(3) PCRECPP(3) NAME PCRE - Perl-compatible regular expressions. SYNOPSIS OF C++ WRAPPER #include <pcrecpp.h> DESCRIPTION The C++ wrapper for PCRE was provided by Google Inc. Some additional functionality was added by Giuseppe Maxia. This brief man page was con- structed from the notes in the pcrecpp.h file, which should be con- sulted for further details. MATCHING INTERFACE The "FullMatch" operation checks that supplied text matches a supplied pattern exactly. If pointer arguments are supplied, it copies matched sub-strings that match sub-patterns into them. Example: successful match pcrecpp::RE re("h.*o"); re.FullMatch("hello"); Example: unsuccessful match (requires full match): pcrecpp::RE re("e"); !re.FullMatch("hello"); Example: creating a temporary RE object: pcrecpp::RE("h.*o").FullMatch("hello"); You can pass in a "const char*" or a "string" for "text". The examples below tend to use a const char*. You can, as in the different examples above, store the RE object explicitly in a variable or use a temporary RE object. The examples below use one mode or the other arbitrarily. Either could correctly be used for any of these examples. You must supply extra pointer arguments to extract matched subpieces. Example: extracts "ruby" into "s" and 1234 into "i" int i; string s; pcrecpp::RE re("(\\w+):(\\d+)"); re.FullMatch("ruby:1234", &s, &i); Example: does not try to extract any extra sub-patterns re.FullMatch("ruby:1234", &s); Example: does not try to extract into NULL re.FullMatch("ruby:1234", NULL, &i); Example: integer overflow causes failure !re.FullMatch("ruby:1234567891234", NULL, &i); Example: fails because there aren't enough sub-patterns: !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s); Example: fails because string cannot be stored in integer !pcrecpp::RE("(.*)").FullMatch("ruby", &i); The provided pointer arguments can be pointers to any scalar numeric type, or one of: string (matched piece is copied to string) StringPiece (StringPiece is mutated to point to matched piece) T (where "bool T::ParseFrom(const char*, int)" exists) NULL (the corresponding matched sub-pattern is not copied) The function returns true iff all of the following conditions are sat- isfied: a. "text" matches "pattern" exactly; b. The number of matched sub-patterns is >= number of supplied pointers; c. The "i"th argument has a suitable type for holding the string captured as the "i"th sub-pattern. If you pass in void * NULL for the "i"th argument, or a non-void * NULL of the correct type, or pass fewer arguments than the number of sub-patterns, "i"th captured sub-pattern is ignored. CAVEAT: An optional sub-pattern that does not exist in the matched string is assigned the empty string. Therefore, the following will return false (because the empty string is not a valid number): int number; pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number); The matching interface supports at most 16 arguments per call. If you need more, consider using the more general interface pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch. NOTE: Do not use no_arg, which is used internally to mark the end of a list of optional arguments, as a placeholder for missing arguments, as this can lead to segfaults. QUOTING METACHARACTERS You can use the "QuoteMeta" operation to insert backslashes before all potentially meaningful characters in a string. The returned string, used as a regular expression, will exactly match the original string. Example: string quoted = RE::QuoteMeta(unquoted); Note that it's legal to escape a character even if it has no special meaning in a regular expression -- so this function does that. (This also makes it identical to the perl function of the same name; see "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes "1\.5\-2\.0\?". PARTIAL MATCHES You can use the "PartialMatch" operation when you want the pattern to match any substring of the text. Example: simple search for a string: pcrecpp::RE("ell").PartialMatch("hello"); Example: find first number in a string: int number; pcrecpp::RE re("(\\d+)"); re.PartialMatch("x*100 + 20", &number); assert(number == 100); UTF-8 AND THE MATCHING INTERFACE By default, pattern and text are plain text, one byte per character. The UTF8 flag, passed to the constructor, causes both pattern and string to be treated as UTF-8 text, still a byte stream but potentially multiple bytes per character. In practice, the text is likelier to be UTF-8 than the pattern, but the match returned may depend on the UTF8 flag, so always use it when matching UTF8 text. For example, "." will match one byte normally but with UTF8 set may match up to three bytes of a multi-byte character. Example: pcrecpp::RE_Options options; options.set_utf8(); pcrecpp::RE re(utf8_pattern, options); re.FullMatch(utf8_string); Example: using the convenience function UTF8(): pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8()); re.FullMatch(utf8_string); NOTE: The UTF8 flag is ignored if pcre was not configured with the --enable-utf8 flag. PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE PCRE defines some modifiers to change the behavior of the regular expression engine. The C++ wrapper defines an auxiliary class, RE_Options, as a vehicle to pass such modifiers to a RE class. Cur- rently, the following modifiers are supported: modifier description Perl corresponding PCRE_CASELESS case insensitive match /i PCRE_MULTILINE multiple lines match /m PCRE_DOTALL dot matches newlines /s PCRE_DOLLAR_ENDONLY $ matches only at end N/A PCRE_EXTRA strict escape parsing N/A PCRE_EXTENDED ignore whitespaces /x PCRE_UTF8 handles UTF8 chars built-in PCRE_UNGREEDY reverses * and *? N/A PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*) (*) Both Perl and PCRE allow non capturing parentheses by means of the "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap- ture, while (ab|cd) does. For a full account on how each modifier works, please check the PCRE API reference page. For each modifier, there are two member functions whose name is made out of the modifier in lowercase, without the "PCRE_" prefix. For instance, PCRE_CASELESS is handled by bool caseless() which returns true if the modifier is set, and RE_Options & set_caseless(bool) which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can be accessed through the set_match_limit() and match_limit() member functions. Setting match_limit to a non-zero value will limit the exe- cution of pcre to keep it from doing bad things like blowing the stack or taking an eternity to return a result. A value of 5000 is good enough to stop stack blowup in a 2MB thread stack. Setting match_limit to zero disables match limiting. Alternatively, you can call match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to limit how much PCRE recurses. match_limit() limits the number of matches PCRE does; match_limit_recursion() limits the depth of internal recursion, and therefore the amount of stack that is used. Normally, to pass one or more modifiers to a RE class, you declare a RE_Options object, set the appropriate options, and pass this object to a RE constructor. Example: RE_options opt; opt.set_caseless(true); if (RE("HELLO", opt).PartialMatch("hello world")) ... RE_options has two constructors. The default constructor takes no argu- ments and creates a set of flags that are off by default. The optional parameter option_flags is to facilitate transfer of legacy code from C programs. This lets you do RE(pattern, RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str); However, new code is better off doing RE(pattern, RE_Options().set_caseless(true).set_multiline(true)) .PartialMatch(str); If you are going to pass one of the most used modifiers, there are some convenience functions that return a RE_Options class with the appropri- ate modifier already set: CASELESS(), UTF8(), MULTILINE(), DOTALL(), and EXTENDED(). If you need to set several options at once, and you don't want to go through the pains of declaring a RE_Options object and setting several options, there is a parallel method that give you such ability on the fly. You can concatenate several set_xxxxx() member functions, since each of them returns a reference to its class object. For example, to pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one statement, you may write: RE(" ^ xyz \\s+ .* blah$", RE_Options() .set_caseless(true) .set_extended(true) .set_multiline(true)).PartialMatch(sometext); SCANNING TEXT INCREMENTALLY The "Consume" operation may be useful if you want to repeatedly match regular expressions at the front of a string and skip over them as they match. This requires use of the "StringPiece" type, which represents a sub-range of a real string. Like RE, StringPiece is defined in the pcrecpp namespace. Example: read lines of the form "var = value" from a string. string contents = ...; // Fill string somehow pcrecpp::StringPiece input(contents); // Wrap in a StringPiece string var; int value; pcrecpp::RE re("(\\w+) = (\\d+)\n"); while (re.Consume(&input, &var, &value)) { ...; } Each successful call to "Consume" will set "var/value", and also advance "input" so it points past the matched text. The "FindAndConsume" operation is similar to "Consume" but does not anchor your match at the beginning of the string. For example, you could extract all words from a string by repeatedly calling pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word) PARSING HEX/OCTAL/C-RADIX NUMBERS By default, if you pass a pointer to a numeric value, the corresponding text is interpreted as a base-10 number. You can instead wrap the pointer with a call to one of the operators Hex(), Octal(), or CRadix() to interpret the text in another base. The CRadix operator interprets C-style "0" (base-8) and "0x" (base-16) prefixes, but defaults to base-10. Example: int a, b, c, d; pcrecpp::RE re("(.*) (.*) (.*) (.*)"); re.FullMatch("100 40 0100 0x40", pcrecpp::Octal(&a), pcrecpp::Hex(&b), pcrecpp::CRadix(&c), pcrecpp::CRadix(&d)); will leave 64 in a, b, c, and d. REPLACING PARTS OF STRINGS You can replace the first match of "pattern" in "str" with "rewrite". Within "rewrite", backslash-escaped digits (\1 to \9) can be used to insert text matching corresponding parenthesized group from the pat- tern. \0 in "rewrite" refers to the entire matching text. For example: string s = "yabba dabba doo"; pcrecpp::RE("b+").Replace("d", &s); will leave "s" containing "yada dabba doo". The result is true if the pattern matches and a replacement occurs, false otherwise. GlobalReplace is like Replace except that it replaces all occurrences of the pattern in the string with the rewrite. Replacements are not subject to re-matching. For example: string s = "yabba dabba doo"; pcrecpp::RE("b+").GlobalReplace("d", &s); will leave "s" containing "yada dada doo". It returns the number of replacements made. Extract is like Replace, except that if the pattern matches, "rewrite" is copied into "out" (an additional argument) with substitutions. The non-matching portions of "text" are ignored. Returns true iff a match occurred and the extraction happened successfully; if no match occurs, the string is left unaffected. AUTHOR The C++ wrapper was contributed by Google Inc. Copyright (c) 2007 Google Inc. REVISION Last updated: 17 March 2009 ------------------------------------------------------------------------------ PCRESAMPLE(3) PCRESAMPLE(3) NAME PCRE - Perl-compatible regular expressions PCRE SAMPLE PROGRAM A simple, complete demonstration program, to get you started with using PCRE, is supplied in the file pcredemo.c in the PCRE distribution. A listing of this program is given in the pcredemo documentation. If you do not have a copy of the PCRE distribution, you can save this listing to re-create pcredemo.c. The program compiles the regular expression that is its first argument, and matches it against the subject string in its second argument. No PCRE options are set, and default character tables are used. If match- ing succeeds, the program outputs the portion of the subject that matched, together with the contents of any captured substrings. If the -g option is given on the command line, the program then goes on to check for further matches of the same regular expression in the same subject string. The logic is a little bit tricky because of the possi- bility of matching an empty string. Comments in the code explain what is going on. If PCRE is installed in the standard include and library directories for your operating system, you should be able to compile the demonstra- tion program using this command: gcc -o pcredemo pcredemo.c -lpcre If PCRE is installed elsewhere, you may need to add additional options to the command line. For example, on a Unix-like system that has PCRE installed in /usr/local, you can compile the demonstration program using a command like this: gcc -o pcredemo -I/usr/local/include pcredemo.c \ -L/usr/local/lib -lpcre Once you have compiled the demonstration program, you can run simple tests like this: ./pcredemo 'cat|dog' 'the cat sat on the mat' ./pcredemo -g 'cat|dog' 'the dog sat on the cat' Note that there is a much more comprehensive test program, called pcretest, which supports many more facilities for testing regular expressions and the PCRE library. The pcredemo program is provided as a simple coding example. When you try to run pcredemo when PCRE is not installed in the standard library directory, you may get an error like this on some operating systems (e.g. Solaris): ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or directory This is caused by the way shared library support works on those sys- tems. You need to add -R/usr/local/lib (for example) to the compile command to get round this problem. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 30 September 2009 Copyright (c) 1997-2009 University of Cambridge. ------------------------------------------------------------------------------ PCRESTACK(3) PCRESTACK(3) NAME PCRE - Perl-compatible regular expressions PCRE DISCUSSION OF STACK USAGE When you call pcre_exec(), it makes use of an internal function called match(). This calls itself recursively at branch points in the pattern, in order to remember the state of the match so that it can back up and try a different alternative if the first one fails. As matching pro- ceeds deeper and deeper into the tree of possibilities, the recursion depth increases. Not all calls of match() increase the recursion depth; for an item such as a* it may be called several times at the same level, after matching different numbers of a's. Furthermore, in a number of cases where the result of the recursive call would immediately be passed back as the result of the current call (a "tail recursion"), the function is just restarted instead. The pcre_dfa_exec() function operates in an entirely different way, and uses recursion only when there is a regular expression recursion or subroutine call in the pattern. This includes the processing of asser- tion and "once-only" subpatterns, which are handled like subroutine calls. Normally, these are never very deep, and the limit on the com- plexity of pcre_dfa_exec() is controlled by the amount of workspace it is given. However, it is possible to write patterns with runaway infi- nite recursions; such patterns will cause pcre_dfa_exec() to run out of stack. At present, there is no protection against this. The comments that follow do NOT apply to pcre_dfa_exec(); they are rel- evant only for pcre_exec(). Reducing pcre_exec()'s stack usage Each time that match() is actually called recursively, it uses memory from the process stack. For certain kinds of pattern and data, very large amounts of stack may be needed, despite the recognition of "tail recursion". You can often reduce the amount of recursion, and there- fore the amount of stack used, by modifying the pattern that is being matched. Consider, for example, this pattern: ([^<]|<(?!inet))+ It matches from wherever it starts until it encounters "<inet" or the end of the data, and is the kind of pattern that might be used when processing an XML file. Each iteration of the outer parentheses matches either one character that is not "<" or a "<" that is not followed by "inet". However, each time a parenthesis is processed, a recursion occurs, so this formulation uses a stack frame for each matched charac- ter. For a long string, a lot of stack is required. Consider now this rewritten pattern, which matches exactly the same strings: ([^<]++|<(?!inet))+ This uses very much less stack, because runs of characters that do not contain "<" are "swallowed" in one item inside the parentheses. Recur- sion happens only when a "<" character that is not followed by "inet" is encountered (and we assume this is relatively rare). A possessive quantifier is used to stop any backtracking into the runs of non-"<" characters, but that is not related to stack usage. This example shows that one way of avoiding stack problems when match- ing long subject strings is to write repeated parenthesized subpatterns to match more than one character whenever possible. Compiling PCRE to use heap instead of stack for pcre_exec() In environments where stack memory is constrained, you might want to compile PCRE to use heap memory instead of stack for remembering back- up points when pcre_exec() is running. This makes it run a lot more slowly, however. Details of how to do this are given in the pcrebuild documentation. When built in this way, instead of using the stack, PCRE obtains and frees memory by calling the functions that are pointed to by the pcre_stack_malloc and pcre_stack_free variables. By default, these point to malloc() and free(), but you can replace the pointers to cause PCRE to use your own functions. Since the block sizes are always the same, and are always freed in reverse order, it may be possible to implement customized memory handlers that are more efficient than the standard functions. Limiting pcre_exec()'s stack usage You can set limits on the number of times that match() is called, both in total and recursively. If a limit is exceeded, pcre_exec() returns an error code. Setting suitable limits should prevent it from running out of stack. The default values of the limits are very large, and unlikely ever to operate. They can be changed when PCRE is built, and they can also be set when pcre_exec() is called. For details of these interfaces, see the pcrebuild documentation and the section on extra data for pcre_exec() in the pcreapi documentation. As a very rough rule of thumb, you should reckon on about 500 bytes per recursion. Thus, if you want to limit your stack usage to 8Mb, you should set the limit at 16000 recursions. A 64Mb stack, on the other hand, can support around 128000 recursions. In Unix-like environments, the pcretest test program has a command line option (-S) that can be used to increase the size of its stack. As long as the stack is large enough, another option (-M) can be used to find the smallest limits that allow a particular pattern to match a given subject string. This is done by calling pcre_exec() repeatedly with different limits. Changing stack size in Unix-like systems In Unix-like environments, there is not often a problem with the stack unless very long strings are involved, though the default limit on stack size varies from system to system. Values from 8Mb to 64Mb are common. You can find your default limit by running the command: ulimit -s Unfortunately, the effect of running out of stack is often SIGSEGV, though sometimes a more explicit error message is given. You can nor- mally increase the limit on stack size by code such as this: struct rlimit rlim; getrlimit(RLIMIT_STACK, &rlim); rlim.rlim_cur = 100*1024*1024; setrlimit(RLIMIT_STACK, &rlim); This reads the current limits (soft and hard) using getrlimit(), then attempts to increase the soft limit to 100Mb using setrlimit(). You must do this before calling pcre_exec(). Changing stack size in Mac OS X Using setrlimit(), as described above, should also work on Mac OS X. It is also possible to set a stack size when linking a program. There is a discussion about stack sizes in Mac OS X at this web site: http://developer.apple.com/qa/qa2005/qa1419.html. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 03 January 2010 Copyright (c) 1997-2010 University of Cambridge. ------------------------------------------------------------------------------
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