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1 -----------------------------------------------------------------------------
2 This file contains a concatenation of the PCRE man pages, converted to plain
3 text format for ease of searching with a text editor, or for use on systems
4 that do not have a man page processor. The small individual files that give
5 synopses of each function in the library have not been included. Neither has
6 the pcredemo program. There are separate text files for the pcregrep and
7 pcretest commands.
8 -----------------------------------------------------------------------------
11 PCRE(3) PCRE(3)
15 PCRE - Perl-compatible regular expressions
20 The PCRE library is a set of functions that implement regular expres-
21 sion pattern matching using the same syntax and semantics as Perl, with
22 just a few differences. Some features that appeared in Python and PCRE
23 before they appeared in Perl are also available using the Python syn-
24 tax, there is some support for one or two .NET and Oniguruma syntax
25 items, and there is an option for requesting some minor changes that
26 give better JavaScript compatibility.
28 The current implementation of PCRE corresponds approximately with Perl
29 5.10/5.11, including support for UTF-8 encoded strings and Unicode gen-
30 eral category properties. However, UTF-8 and Unicode support has to be
31 explicitly enabled; it is not the default. The Unicode tables corre-
32 spond to Unicode release 5.2.0.
34 In addition to the Perl-compatible matching function, PCRE contains an
35 alternative function that matches the same compiled patterns in a dif-
36 ferent way. In certain circumstances, the alternative function has some
37 advantages. For a discussion of the two matching algorithms, see the
38 pcrematching page.
40 PCRE is written in C and released as a C library. A number of people
41 have written wrappers and interfaces of various kinds. In particular,
42 Google Inc. have provided a comprehensive C++ wrapper. This is now
43 included as part of the PCRE distribution. The pcrecpp page has details
44 of this interface. Other people's contributions can be found in the
45 Contrib directory at the primary FTP site, which is:
47 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
49 Details of exactly which Perl regular expression features are and are
50 not supported by PCRE are given in separate documents. See the pcrepat-
51 tern and pcrecompat pages. There is a syntax summary in the pcresyntax
52 page.
54 Some features of PCRE can be included, excluded, or changed when the
55 library is built. The pcre_config() function makes it possible for a
56 client to discover which features are available. The features them-
57 selves are described in the pcrebuild page. Documentation about build-
58 ing PCRE for various operating systems can be found in the README and
59 NON-UNIX-USE files in the source distribution.
61 The library contains a number of undocumented internal functions and
62 data tables that are used by more than one of the exported external
63 functions, but which are not intended for use by external callers.
64 Their names all begin with "_pcre_", which hopefully will not provoke
65 any name clashes. In some environments, it is possible to control which
66 external symbols are exported when a shared library is built, and in
67 these cases the undocumented symbols are not exported.
72 The user documentation for PCRE comprises a number of different sec-
73 tions. In the "man" format, each of these is a separate "man page". In
74 the HTML format, each is a separate page, linked from the index page.
75 In the plain text format, all the sections, except the pcredemo sec-
76 tion, are concatenated, for ease of searching. The sections are as fol-
77 lows:
79 pcre this document
80 pcre-config show PCRE installation configuration information
81 pcreapi details of PCRE's native C API
82 pcrebuild options for building PCRE
83 pcrecallout details of the callout feature
84 pcrecompat discussion of Perl compatibility
85 pcrecpp details of the C++ wrapper
86 pcredemo a demonstration C program that uses PCRE
87 pcregrep description of the pcregrep command
88 pcrematching discussion of the two matching algorithms
89 pcrepartial details of the partial matching facility
90 pcrepattern syntax and semantics of supported
91 regular expressions
92 pcreperform discussion of performance issues
93 pcreposix the POSIX-compatible C API
94 pcreprecompile details of saving and re-using precompiled patterns
95 pcresample discussion of the pcredemo program
96 pcrestack discussion of stack usage
97 pcresyntax quick syntax reference
98 pcretest description of the pcretest testing command
100 In addition, in the "man" and HTML formats, there is a short page for
101 each C library function, listing its arguments and results.
106 There are some size limitations in PCRE but it is hoped that they will
107 never in practice be relevant.
109 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE
110 is compiled with the default internal linkage size of 2. If you want to
111 process regular expressions that are truly enormous, you can compile
112 PCRE with an internal linkage size of 3 or 4 (see the README file in
113 the source distribution and the pcrebuild documentation for details).
114 In these cases the limit is substantially larger. However, the speed
115 of execution is slower.
117 All values in repeating quantifiers must be less than 65536.
119 There is no limit to the number of parenthesized subpatterns, but there
120 can be no more than 65535 capturing subpatterns.
122 The maximum length of name for a named subpattern is 32 characters, and
123 the maximum number of named subpatterns is 10000.
125 The maximum length of a subject string is the largest positive number
126 that an integer variable can hold. However, when using the traditional
127 matching function, PCRE uses recursion to handle subpatterns and indef-
128 inite repetition. This means that the available stack space may limit
129 the size of a subject string that can be processed by certain patterns.
130 For a discussion of stack issues, see the pcrestack documentation.
135 From release 3.3, PCRE has had some support for character strings
136 encoded in the UTF-8 format. For release 4.0 this was greatly extended
137 to cover most common requirements, and in release 5.0 additional sup-
138 port for Unicode general category properties was added.
140 In order process UTF-8 strings, you must build PCRE to include UTF-8
141 support in the code, and, in addition, you must call pcre_compile()
142 with the PCRE_UTF8 option flag, or the pattern must start with the
143 sequence (*UTF8). When either of these is the case, both the pattern
144 and any subject strings that are matched against it are treated as
145 UTF-8 strings instead of strings of 1-byte characters.
147 If you compile PCRE with UTF-8 support, but do not use it at run time,
148 the library will be a bit bigger, but the additional run time overhead
149 is limited to testing the PCRE_UTF8 flag occasionally, so should not be
150 very big.
152 If PCRE is built with Unicode character property support (which implies
153 UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup-
154 ported. The available properties that can be tested are limited to the
155 general category properties such as Lu for an upper case letter or Nd
156 for a decimal number, the Unicode script names such as Arabic or Han,
157 and the derived properties Any and L&. A full list is given in the
158 pcrepattern documentation. Only the short names for properties are sup-
159 ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
160 ter}, is not supported. Furthermore, in Perl, many properties may
161 optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE
162 does not support this.
164 Validity of UTF-8 strings
166 When you set the PCRE_UTF8 flag, the strings passed as patterns and
167 subjects are (by default) checked for validity on entry to the relevant
168 functions. From release 7.3 of PCRE, the check is according the rules
169 of RFC 3629, which are themselves derived from the Unicode specifica-
170 tion. Earlier releases of PCRE followed the rules of RFC 2279, which
171 allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current
172 check allows only values in the range U+0 to U+10FFFF, excluding U+D800
173 to U+DFFF.
175 The excluded code points are the "Low Surrogate Area" of Unicode, of
176 which the Unicode Standard says this: "The Low Surrogate Area does not
177 contain any character assignments, consequently no character code
178 charts or namelists are provided for this area. Surrogates are reserved
179 for use with UTF-16 and then must be used in pairs." The code points
180 that are encoded by UTF-16 pairs are available as independent code
181 points in the UTF-8 encoding. (In other words, the whole surrogate
182 thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
184 If an invalid UTF-8 string is passed to PCRE, an error return
185 (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know
186 that your strings are valid, and therefore want to skip these checks in
187 order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at
188 compile time or at run time, PCRE assumes that the pattern or subject
189 it is given (respectively) contains only valid UTF-8 codes. In this
190 case, it does not diagnose an invalid UTF-8 string.
192 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set,
193 what happens depends on why the string is invalid. If the string con-
194 forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
195 string of characters in the range 0 to 0x7FFFFFFF. In other words,
196 apart from the initial validity test, PCRE (when in UTF-8 mode) handles
197 strings according to the more liberal rules of RFC 2279. However, if
198 the string does not even conform to RFC 2279, the result is undefined.
199 Your program may crash.
201 If you want to process strings of values in the full range 0 to
202 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can
203 set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
204 this situation, you will have to apply your own validity check.
206 General comments about UTF-8 mode
208 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a
209 two-byte UTF-8 character if the value is greater than 127.
211 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
212 characters for values greater than \177.
214 3. Repeat quantifiers apply to complete UTF-8 characters, not to indi-
215 vidual bytes, for example: \x{100}{3}.
217 4. The dot metacharacter matches one UTF-8 character instead of a sin-
218 gle byte.
220 5. The escape sequence \C can be used to match a single byte in UTF-8
221 mode, but its use can lead to some strange effects. This facility is
222 not available in the alternative matching function, pcre_dfa_exec().
224 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
225 test characters of any code value, but, by default, the characters that
226 PCRE recognizes as digits, spaces, or word characters remain the same
227 set as before, all with values less than 256. This remains true even
228 when PCRE is built to include Unicode property support, because to do
229 otherwise would slow down PCRE in many common cases. Note that this
230 also applies to \b, because it is defined in terms of \w and \W. If you
231 really want to test for a wider sense of, say, "digit", you can use
232 explicit Unicode property tests such as \p{Nd}. Alternatively, if you
233 set the PCRE_UCP option, the way that the character escapes work is
234 changed so that Unicode properties are used to determine which charac-
235 ters match. There are more details in the section on generic character
236 types in the pcrepattern documentation.
238 7. Similarly, characters that match the POSIX named character classes
239 are all low-valued characters, unless the PCRE_UCP option is set.
241 8. However, the Perl 5.10 horizontal and vertical whitespace matching
242 escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
243 acters, whether or not PCRE_UCP is set.
245 9. Case-insensitive matching applies only to characters whose values
246 are less than 128, unless PCRE is built with Unicode property support.
247 Even when Unicode property support is available, PCRE still uses its
248 own character tables when checking the case of low-valued characters,
249 so as not to degrade performance. The Unicode property information is
250 used only for characters with higher values. Even when Unicode property
251 support is available, PCRE supports case-insensitive matching only when
252 there is a one-to-one mapping between a letter's cases. There are a
253 small number of many-to-one mappings in Unicode; these are not sup-
254 ported by PCRE.
259 Philip Hazel
260 University Computing Service
261 Cambridge CB2 3QH, England.
263 Putting an actual email address here seems to have been a spam magnet,
264 so I've taken it away. If you want to email me, use my two initials,
265 followed by the two digits 10, at the domain cam.ac.uk.
270 Last updated: 12 May 2010
271 Copyright (c) 1997-2010 University of Cambridge.
272 ------------------------------------------------------------------------------
278 NAME
279 PCRE - Perl-compatible regular expressions
284 This document describes the optional features of PCRE that can be
285 selected when the library is compiled. It assumes use of the configure
286 script, where the optional features are selected or deselected by pro-
287 viding options to configure before running the make command. However,
288 the same options can be selected in both Unix-like and non-Unix-like
289 environments using the GUI facility of cmake-gui if you are using CMake
290 instead of configure to build PCRE.
292 There is a lot more information about building PCRE in non-Unix-like
293 environments in the file called NON_UNIX_USE, which is part of the PCRE
294 distribution. You should consult this file as well as the README file
295 if you are building in a non-Unix-like environment.
297 The complete list of options for configure (which includes the standard
298 ones such as the selection of the installation directory) can be
299 obtained by running
301 ./configure --help
303 The following sections include descriptions of options whose names
304 begin with --enable or --disable. These settings specify changes to the
305 defaults for the configure command. Because of the way that configure
306 works, --enable and --disable always come in pairs, so the complemen-
307 tary option always exists as well, but as it specifies the default, it
308 is not described.
313 By default, the configure script will search for a C++ compiler and C++
314 header files. If it finds them, it automatically builds the C++ wrapper
315 library for PCRE. You can disable this by adding
317 --disable-cpp
319 to the configure command.
324 To build PCRE with support for UTF-8 Unicode character strings, add
326 --enable-utf8
328 to the configure command. Of itself, this does not make PCRE treat
329 strings as UTF-8. As well as compiling PCRE with this option, you also
330 have have to set the PCRE_UTF8 option when you call the pcre_compile()
331 or pcre_compile2() functions.
333 If you set --enable-utf8 when compiling in an EBCDIC environment, PCRE
334 expects its input to be either ASCII or UTF-8 (depending on the runtime
335 option). It is not possible to support both EBCDIC and UTF-8 codes in
336 the same version of the library. Consequently, --enable-utf8 and
337 --enable-ebcdic are mutually exclusive.
342 UTF-8 support allows PCRE to process character values greater than 255
343 in the strings that it handles. On its own, however, it does not pro-
344 vide any facilities for accessing the properties of such characters. If
345 you want to be able to use the pattern escapes \P, \p, and \X, which
346 refer to Unicode character properties, you must add
348 --enable-unicode-properties
350 to the configure command. This implies UTF-8 support, even if you have
351 not explicitly requested it.
353 Including Unicode property support adds around 30K of tables to the
354 PCRE library. Only the general category properties such as Lu and Nd
355 are supported. Details are given in the pcrepattern documentation.
360 By default, PCRE interprets the linefeed (LF) character as indicating
361 the end of a line. This is the normal newline character on Unix-like
362 systems. You can compile PCRE to use carriage return (CR) instead, by
363 adding
365 --enable-newline-is-cr
367 to the configure command. There is also a --enable-newline-is-lf
368 option, which explicitly specifies linefeed as the newline character.
370 Alternatively, you can specify that line endings are to be indicated by
371 the two character sequence CRLF. If you want this, add
373 --enable-newline-is-crlf
375 to the configure command. There is a fourth option, specified by
377 --enable-newline-is-anycrlf
379 which causes PCRE to recognize any of the three sequences CR, LF, or
380 CRLF as indicating a line ending. Finally, a fifth option, specified by
382 --enable-newline-is-any
384 causes PCRE to recognize any Unicode newline sequence.
386 Whatever line ending convention is selected when PCRE is built can be
387 overridden when the library functions are called. At build time it is
388 conventional to use the standard for your operating system.
393 By default, the sequence \R in a pattern matches any Unicode newline
394 sequence, whatever has been selected as the line ending sequence. If
395 you specify
397 --enable-bsr-anycrlf
399 the default is changed so that \R matches only CR, LF, or CRLF. What-
400 ever is selected when PCRE is built can be overridden when the library
401 functions are called.
406 The PCRE building process uses libtool to build both shared and static
407 Unix libraries by default. You can suppress one of these by adding one
408 of
410 --disable-shared
411 --disable-static
413 to the configure command, as required.
418 When PCRE is called through the POSIX interface (see the pcreposix doc-
419 umentation), additional working storage is required for holding the
420 pointers to capturing substrings, because PCRE requires three integers
421 per substring, whereas the POSIX interface provides only two. If the
422 number of expected substrings is small, the wrapper function uses space
423 on the stack, because this is faster than using malloc() for each call.
424 The default threshold above which the stack is no longer used is 10; it
425 can be changed by adding a setting such as
427 --with-posix-malloc-threshold=20
429 to the configure command.
434 Within a compiled pattern, offset values are used to point from one
435 part to another (for example, from an opening parenthesis to an alter-
436 nation metacharacter). By default, two-byte values are used for these
437 offsets, leading to a maximum size for a compiled pattern of around
438 64K. This is sufficient to handle all but the most gigantic patterns.
439 Nevertheless, some people do want to process truyl enormous patterns,
440 so it is possible to compile PCRE to use three-byte or four-byte off-
441 sets by adding a setting such as
443 --with-link-size=3
445 to the configure command. The value given must be 2, 3, or 4. Using
446 longer offsets slows down the operation of PCRE because it has to load
447 additional bytes when handling them.
452 When matching with the pcre_exec() function, PCRE implements backtrack-
453 ing by making recursive calls to an internal function called match().
454 In environments where the size of the stack is limited, this can se-
455 verely limit PCRE's operation. (The Unix environment does not usually
456 suffer from this problem, but it may sometimes be necessary to increase
457 the maximum stack size. There is a discussion in the pcrestack docu-
458 mentation.) An alternative approach to recursion that uses memory from
459 the heap to remember data, instead of using recursive function calls,
460 has been implemented to work round the problem of limited stack size.
461 If you want to build a version of PCRE that works this way, add
463 --disable-stack-for-recursion
465 to the configure command. With this configuration, PCRE will use the
466 pcre_stack_malloc and pcre_stack_free variables to call memory manage-
467 ment functions. By default these point to malloc() and free(), but you
468 can replace the pointers so that your own functions are used instead.
470 Separate functions are provided rather than using pcre_malloc and
471 pcre_free because the usage is very predictable: the block sizes
472 requested are always the same, and the blocks are always freed in
473 reverse order. A calling program might be able to implement optimized
474 functions that perform better than malloc() and free(). PCRE runs
475 noticeably more slowly when built in this way. This option affects only
476 the pcre_exec() function; it is not relevant for pcre_dfa_exec().
481 Internally, PCRE has a function called match(), which it calls repeat-
482 edly (sometimes recursively) when matching a pattern with the
483 pcre_exec() function. By controlling the maximum number of times this
484 function may be called during a single matching operation, a limit can
485 be placed on the resources used by a single call to pcre_exec(). The
486 limit can be changed at run time, as described in the pcreapi documen-
487 tation. The default is 10 million, but this can be changed by adding a
488 setting such as
490 --with-match-limit=500000
492 to the configure command. This setting has no effect on the
493 pcre_dfa_exec() matching function.
495 In some environments it is desirable to limit the depth of recursive
496 calls of match() more strictly than the total number of calls, in order
497 to restrict the maximum amount of stack (or heap, if --disable-stack-
498 for-recursion is specified) that is used. A second limit controls this;
499 it defaults to the value that is set for --with-match-limit, which
500 imposes no additional constraints. However, you can set a lower limit
501 by adding, for example,
503 --with-match-limit-recursion=10000
505 to the configure command. This value can also be overridden at run
506 time.
511 PCRE uses fixed tables for processing characters whose code values are
512 less than 256. By default, PCRE is built with a set of tables that are
513 distributed in the file pcre_chartables.c.dist. These tables are for
514 ASCII codes only. If you add
516 --enable-rebuild-chartables
518 to the configure command, the distributed tables are no longer used.
519 Instead, a program called dftables is compiled and run. This outputs
520 the source for new set of tables, created in the default locale of your
521 C runtime system. (This method of replacing the tables does not work if
522 you are cross compiling, because dftables is run on the local host. If
523 you need to create alternative tables when cross compiling, you will
524 have to do so "by hand".)
529 PCRE assumes by default that it will run in an environment where the
530 character code is ASCII (or Unicode, which is a superset of ASCII).
531 This is the case for most computer operating systems. PCRE can, how-
532 ever, be compiled to run in an EBCDIC environment by adding
534 --enable-ebcdic
536 to the configure command. This setting implies --enable-rebuild-charta-
537 bles. You should only use it if you know that you are in an EBCDIC
538 environment (for example, an IBM mainframe operating system). The
539 --enable-ebcdic option is incompatible with --enable-utf8.
544 By default, pcregrep reads all files as plain text. You can build it so
545 that it recognizes files whose names end in .gz or .bz2, and reads them
546 with libz or libbz2, respectively, by adding one or both of
548 --enable-pcregrep-libz
549 --enable-pcregrep-libbz2
551 to the configure command. These options naturally require that the rel-
552 evant libraries are installed on your system. Configuration will fail
553 if they are not.
558 If you add
560 --enable-pcretest-libreadline
562 to the configure command, pcretest is linked with the libreadline
563 library, and when its input is from a terminal, it reads it using the
564 readline() function. This provides line-editing and history facilities.
565 Note that libreadline is GPL-licensed, so if you distribute a binary of
566 pcretest linked in this way, there may be licensing issues.
568 Setting this option causes the -lreadline option to be added to the
569 pcretest build. In many operating environments with a sytem-installed
570 libreadline this is sufficient. However, in some environments (e.g. if
571 an unmodified distribution version of readline is in use), some extra
572 configuration may be necessary. The INSTALL file for libreadline says
573 this:
575 "Readline uses the termcap functions, but does not link with the
576 termcap or curses library itself, allowing applications which link
577 with readline the to choose an appropriate library."
579 If your environment has not been set up so that an appropriate library
580 is automatically included, you may need to add something like
582 LIBS="-ncurses"
584 immediately before the configure command.
589 pcreapi(3), pcre_config(3).
594 Philip Hazel
595 University Computing Service
596 Cambridge CB2 3QH, England.
601 Last updated: 29 September 2009
602 Copyright (c) 1997-2009 University of Cambridge.
603 ------------------------------------------------------------------------------
609 NAME
610 PCRE - Perl-compatible regular expressions
615 This document describes the two different algorithms that are available
616 in PCRE for matching a compiled regular expression against a given sub-
617 ject string. The "standard" algorithm is the one provided by the
618 pcre_exec() function. This works in the same was as Perl's matching
619 function, and provides a Perl-compatible matching operation.
621 An alternative algorithm is provided by the pcre_dfa_exec() function;
622 this operates in a different way, and is not Perl-compatible. It has
623 advantages and disadvantages compared with the standard algorithm, and
624 these are described below.
626 When there is only one possible way in which a given subject string can
627 match a pattern, the two algorithms give the same answer. A difference
628 arises, however, when there are multiple possibilities. For example, if
629 the pattern
631 ^<.*>
633 is matched against the string
635 <something> <something else> <something further>
637 there are three possible answers. The standard algorithm finds only one
638 of them, whereas the alternative algorithm finds all three.
643 The set of strings that are matched by a regular expression can be rep-
644 resented as a tree structure. An unlimited repetition in the pattern
645 makes the tree of infinite size, but it is still a tree. Matching the
646 pattern to a given subject string (from a given starting point) can be
647 thought of as a search of the tree. There are two ways to search a
648 tree: depth-first and breadth-first, and these correspond to the two
649 matching algorithms provided by PCRE.
654 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
655 sions", the standard algorithm is an "NFA algorithm". It conducts a
656 depth-first search of the pattern tree. That is, it proceeds along a
657 single path through the tree, checking that the subject matches what is
658 required. When there is a mismatch, the algorithm tries any alterna-
659 tives at the current point, and if they all fail, it backs up to the
660 previous branch point in the tree, and tries the next alternative
661 branch at that level. This often involves backing up (moving to the
662 left) in the subject string as well. The order in which repetition
663 branches are tried is controlled by the greedy or ungreedy nature of
664 the quantifier.
666 If a leaf node is reached, a matching string has been found, and at
667 that point the algorithm stops. Thus, if there is more than one possi-
668 ble match, this algorithm returns the first one that it finds. Whether
669 this is the shortest, the longest, or some intermediate length depends
670 on the way the greedy and ungreedy repetition quantifiers are specified
671 in the pattern.
673 Because it ends up with a single path through the tree, it is rela-
674 tively straightforward for this algorithm to keep track of the sub-
675 strings that are matched by portions of the pattern in parentheses.
676 This provides support for capturing parentheses and back references.
681 This algorithm conducts a breadth-first search of the tree. Starting
682 from the first matching point in the subject, it scans the subject
683 string from left to right, once, character by character, and as it does
684 this, it remembers all the paths through the tree that represent valid
685 matches. In Friedl's terminology, this is a kind of "DFA algorithm",
686 though it is not implemented as a traditional finite state machine (it
687 keeps multiple states active simultaneously).
689 Although the general principle of this matching algorithm is that it
690 scans the subject string only once, without backtracking, there is one
691 exception: when a lookaround assertion is encountered, the characters
692 following or preceding the current point have to be independently
693 inspected.
695 The scan continues until either the end of the subject is reached, or
696 there are no more unterminated paths. At this point, terminated paths
697 represent the different matching possibilities (if there are none, the
698 match has failed). Thus, if there is more than one possible match,
699 this algorithm finds all of them, and in particular, it finds the long-
700 est. There is an option to stop the algorithm after the first match
701 (which is necessarily the shortest) is found.
703 Note that all the matches that are found start at the same point in the
704 subject. If the pattern
706 cat(er(pillar)?)
708 is matched against the string "the caterpillar catchment", the result
709 will be the three strings "cat", "cater", and "caterpillar" that start
710 at the fourth character of the subject. The algorithm does not automat-
711 ically move on to find matches that start at later positions.
713 There are a number of features of PCRE regular expressions that are not
714 supported by the alternative matching algorithm. They are as follows:
716 1. Because the algorithm finds all possible matches, the greedy or
717 ungreedy nature of repetition quantifiers is not relevant. Greedy and
718 ungreedy quantifiers are treated in exactly the same way. However, pos-
719 sessive quantifiers can make a difference when what follows could also
720 match what is quantified, for example in a pattern like this:
722 ^a++\w!
724 This pattern matches "aaab!" but not "aaa!", which would be matched by
725 a non-possessive quantifier. Similarly, if an atomic group is present,
726 it is matched as if it were a standalone pattern at the current point,
727 and the longest match is then "locked in" for the rest of the overall
728 pattern.
730 2. When dealing with multiple paths through the tree simultaneously, it
731 is not straightforward to keep track of captured substrings for the
732 different matching possibilities, and PCRE's implementation of this
733 algorithm does not attempt to do this. This means that no captured sub-
734 strings are available.
736 3. Because no substrings are captured, back references within the pat-
737 tern are not supported, and cause errors if encountered.
739 4. For the same reason, conditional expressions that use a backrefer-
740 ence as the condition or test for a specific group recursion are not
741 supported.
743 5. Because many paths through the tree may be active, the \K escape
744 sequence, which resets the start of the match when encountered (but may
745 be on some paths and not on others), is not supported. It causes an
746 error if encountered.
748 6. Callouts are supported, but the value of the capture_top field is
749 always 1, and the value of the capture_last field is always -1.
751 7. The \C escape sequence, which (in the standard algorithm) matches a
752 single byte, even in UTF-8 mode, is not supported because the alterna-
753 tive algorithm moves through the subject string one character at a
754 time, for all active paths through the tree.
756 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
757 are not supported. (*FAIL) is supported, and behaves like a failing
758 negative assertion.
763 Using the alternative matching algorithm provides the following advan-
764 tages:
766 1. All possible matches (at a single point in the subject) are automat-
767 ically found, and in particular, the longest match is found. To find
768 more than one match using the standard algorithm, you have to do kludgy
769 things with callouts.
771 2. Because the alternative algorithm scans the subject string just
772 once, and never needs to backtrack, it is possible to pass very long
773 subject strings to the matching function in several pieces, checking
774 for partial matching each time. The pcrepartial documentation gives
775 details of partial matching.
780 The alternative algorithm suffers from a number of disadvantages:
782 1. It is substantially slower than the standard algorithm. This is
783 partly because it has to search for all possible matches, but is also
784 because it is less susceptible to optimization.
786 2. Capturing parentheses and back references are not supported.
788 3. Although atomic groups are supported, their use does not provide the
789 performance advantage that it does for the standard algorithm.
794 Philip Hazel
795 University Computing Service
796 Cambridge CB2 3QH, England.
801 Last updated: 29 September 2009
802 Copyright (c) 1997-2009 University of Cambridge.
803 ------------------------------------------------------------------------------
809 NAME
810 PCRE - Perl-compatible regular expressions
815 #include <pcre.h>
817 pcre *pcre_compile(const char *pattern, int options,
818 const char **errptr, int *erroffset,
819 const unsigned char *tableptr);
821 pcre *pcre_compile2(const char *pattern, int options,
822 int *errorcodeptr,
823 const char **errptr, int *erroffset,
824 const unsigned char *tableptr);
826 pcre_extra *pcre_study(const pcre *code, int options,
827 const char **errptr);
829 int pcre_exec(const pcre *code, const pcre_extra *extra,
830 const char *subject, int length, int startoffset,
831 int options, int *ovector, int ovecsize);
833 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
834 const char *subject, int length, int startoffset,
835 int options, int *ovector, int ovecsize,
836 int *workspace, int wscount);
838 int pcre_copy_named_substring(const pcre *code,
839 const char *subject, int *ovector,
840 int stringcount, const char *stringname,
841 char *buffer, int buffersize);
843 int pcre_copy_substring(const char *subject, int *ovector,
844 int stringcount, int stringnumber, char *buffer,
845 int buffersize);
847 int pcre_get_named_substring(const pcre *code,
848 const char *subject, int *ovector,
849 int stringcount, const char *stringname,
850 const char **stringptr);
852 int pcre_get_stringnumber(const pcre *code,
853 const char *name);
855 int pcre_get_stringtable_entries(const pcre *code,
856 const char *name, char **first, char **last);
858 int pcre_get_substring(const char *subject, int *ovector,
859 int stringcount, int stringnumber,
860 const char **stringptr);
862 int pcre_get_substring_list(const char *subject,
863 int *ovector, int stringcount, const char ***listptr);
865 void pcre_free_substring(const char *stringptr);
867 void pcre_free_substring_list(const char **stringptr);
869 const unsigned char *pcre_maketables(void);
871 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
872 int what, void *where);
874 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
876 int pcre_refcount(pcre *code, int adjust);
878 int pcre_config(int what, void *where);
880 char *pcre_version(void);
882 void *(*pcre_malloc)(size_t);
884 void (*pcre_free)(void *);
886 void *(*pcre_stack_malloc)(size_t);
888 void (*pcre_stack_free)(void *);
890 int (*pcre_callout)(pcre_callout_block *);
895 PCRE has its own native API, which is described in this document. There
896 are also some wrapper functions that correspond to the POSIX regular
897 expression API. These are described in the pcreposix documentation.
898 Both of these APIs define a set of C function calls. A C++ wrapper is
899 distributed with PCRE. It is documented in the pcrecpp page.
901 The native API C function prototypes are defined in the header file
902 pcre.h, and on Unix systems the library itself is called libpcre. It
903 can normally be accessed by adding -lpcre to the command for linking an
904 application that uses PCRE. The header file defines the macros
905 PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num-
906 bers for the library. Applications can use these to include support
907 for different releases of PCRE.
909 In a Windows environment, if you want to statically link an application
910 program against a non-dll pcre.a file, you must define PCRE_STATIC
911 before including pcre.h or pcrecpp.h, because otherwise the pcre_mal-
912 loc() and pcre_free() exported functions will be declared
913 __declspec(dllimport), with unwanted results.
915 The functions pcre_compile(), pcre_compile2(), pcre_study(), and
916 pcre_exec() are used for compiling and matching regular expressions in
917 a Perl-compatible manner. A sample program that demonstrates the sim-
918 plest way of using them is provided in the file called pcredemo.c in
919 the PCRE source distribution. A listing of this program is given in the
920 pcredemo documentation, and the pcresample documentation describes how
921 to compile and run it.
923 A second matching function, pcre_dfa_exec(), which is not Perl-compati-
924 ble, is also provided. This uses a different algorithm for the match-
925 ing. The alternative algorithm finds all possible matches (at a given
926 point in the subject), and scans the subject just once (unless there
927 are lookbehind assertions). However, this algorithm does not return
928 captured substrings. A description of the two matching algorithms and
929 their advantages and disadvantages is given in the pcrematching docu-
930 mentation.
932 In addition to the main compiling and matching functions, there are
933 convenience functions for extracting captured substrings from a subject
934 string that is matched by pcre_exec(). They are:
936 pcre_copy_substring()
937 pcre_copy_named_substring()
938 pcre_get_substring()
939 pcre_get_named_substring()
940 pcre_get_substring_list()
941 pcre_get_stringnumber()
942 pcre_get_stringtable_entries()
944 pcre_free_substring() and pcre_free_substring_list() are also provided,
945 to free the memory used for extracted strings.
947 The function pcre_maketables() is used to build a set of character
948 tables in the current locale for passing to pcre_compile(),
949 pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
950 provided for specialist use. Most commonly, no special tables are
951 passed, in which case internal tables that are generated when PCRE is
952 built are used.
954 The function pcre_fullinfo() is used to find out information about a
955 compiled pattern; pcre_info() is an obsolete version that returns only
956 some of the available information, but is retained for backwards com-
957 patibility. The function pcre_version() returns a pointer to a string
958 containing the version of PCRE and its date of release.
960 The function pcre_refcount() maintains a reference count in a data
961 block containing a compiled pattern. This is provided for the benefit
962 of object-oriented applications.
964 The global variables pcre_malloc and pcre_free initially contain the
965 entry points of the standard malloc() and free() functions, respec-
966 tively. PCRE calls the memory management functions via these variables,
967 so a calling program can replace them if it wishes to intercept the
968 calls. This should be done before calling any PCRE functions.
970 The global variables pcre_stack_malloc and pcre_stack_free are also
971 indirections to memory management functions. These special functions
972 are used only when PCRE is compiled to use the heap for remembering
973 data, instead of recursive function calls, when running the pcre_exec()
974 function. See the pcrebuild documentation for details of how to do
975 this. It is a non-standard way of building PCRE, for use in environ-
976 ments that have limited stacks. Because of the greater use of memory
977 management, it runs more slowly. Separate functions are provided so
978 that special-purpose external code can be used for this case. When
979 used, these functions are always called in a stack-like manner (last
980 obtained, first freed), and always for memory blocks of the same size.
981 There is a discussion about PCRE's stack usage in the pcrestack docu-
982 mentation.
984 The global variable pcre_callout initially contains NULL. It can be set
985 by the caller to a "callout" function, which PCRE will then call at
986 specified points during a matching operation. Details are given in the
987 pcrecallout documentation.
992 PCRE supports five different conventions for indicating line breaks in
993 strings: a single CR (carriage return) character, a single LF (line-
994 feed) character, the two-character sequence CRLF, any of the three pre-
995 ceding, or any Unicode newline sequence. The Unicode newline sequences
996 are the three just mentioned, plus the single characters VT (vertical
997 tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
998 separator, U+2028), and PS (paragraph separator, U+2029).
1000 Each of the first three conventions is used by at least one operating
1001 system as its standard newline sequence. When PCRE is built, a default
1002 can be specified. The default default is LF, which is the Unix stan-
1003 dard. When PCRE is run, the default can be overridden, either when a
1004 pattern is compiled, or when it is matched.
1006 At compile time, the newline convention can be specified by the options
1007 argument of pcre_compile(), or it can be specified by special text at
1008 the start of the pattern itself; this overrides any other settings. See
1009 the pcrepattern page for details of the special character sequences.
1011 In the PCRE documentation the word "newline" is used to mean "the char-
1012 acter or pair of characters that indicate a line break". The choice of
1013 newline convention affects the handling of the dot, circumflex, and
1014 dollar metacharacters, the handling of #-comments in /x mode, and, when
1015 CRLF is a recognized line ending sequence, the match position advance-
1016 ment for a non-anchored pattern. There is more detail about this in the
1017 section on pcre_exec() options below.
1019 The choice of newline convention does not affect the interpretation of
1020 the \n or \r escape sequences, nor does it affect what \R matches,
1021 which is controlled in a similar way, but by separate options.
1026 The PCRE functions can be used in multi-threading applications, with
1027 the proviso that the memory management functions pointed to by
1028 pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
1029 callout function pointed to by pcre_callout, are shared by all threads.
1031 The compiled form of a regular expression is not altered during match-
1032 ing, so the same compiled pattern can safely be used by several threads
1033 at once.
1038 The compiled form of a regular expression can be saved and re-used at a
1039 later time, possibly by a different program, and even on a host other
1040 than the one on which it was compiled. Details are given in the
1041 pcreprecompile documentation. However, compiling a regular expression
1042 with one version of PCRE for use with a different version is not guar-
1043 anteed to work and may cause crashes.
1048 int pcre_config(int what, void *where);
1050 The function pcre_config() makes it possible for a PCRE client to dis-
1051 cover which optional features have been compiled into the PCRE library.
1052 The pcrebuild documentation has more details about these optional fea-
1053 tures.
1055 The first argument for pcre_config() is an integer, specifying which
1056 information is required; the second argument is a pointer to a variable
1057 into which the information is placed. The following information is
1058 available:
1062 The output is an integer that is set to one if UTF-8 support is avail-
1063 able; otherwise it is set to zero.
1067 The output is an integer that is set to one if support for Unicode
1068 character properties is available; otherwise it is set to zero.
1072 The output is an integer whose value specifies the default character
1073 sequence that is recognized as meaning "newline". The four values that
1074 are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
1075 and -1 for ANY. Though they are derived from ASCII, the same values
1076 are returned in EBCDIC environments. The default should normally corre-
1077 spond to the standard sequence for your operating system.
1081 The output is an integer whose value indicates what character sequences
1082 the \R escape sequence matches by default. A value of 0 means that \R
1083 matches any Unicode line ending sequence; a value of 1 means that \R
1084 matches only CR, LF, or CRLF. The default can be overridden when a pat-
1085 tern is compiled or matched.
1089 The output is an integer that contains the number of bytes used for
1090 internal linkage in compiled regular expressions. The value is 2, 3, or
1091 4. Larger values allow larger regular expressions to be compiled, at
1092 the expense of slower matching. The default value of 2 is sufficient
1093 for all but the most massive patterns, since it allows the compiled
1094 pattern to be up to 64K in size.
1098 The output is an integer that contains the threshold above which the
1099 POSIX interface uses malloc() for output vectors. Further details are
1100 given in the pcreposix documentation.
1104 The output is a long integer that gives the default limit for the num-
1105 ber of internal matching function calls in a pcre_exec() execution.
1106 Further details are given with pcre_exec() below.
1110 The output is a long integer that gives the default limit for the depth
1111 of recursion when calling the internal matching function in a
1112 pcre_exec() execution. Further details are given with pcre_exec()
1113 below.
1117 The output is an integer that is set to one if internal recursion when
1118 running pcre_exec() is implemented by recursive function calls that use
1119 the stack to remember their state. This is the usual way that PCRE is
1120 compiled. The output is zero if PCRE was compiled to use blocks of data
1121 on the heap instead of recursive function calls. In this case,
1122 pcre_stack_malloc and pcre_stack_free are called to manage memory
1123 blocks on the heap, thus avoiding the use of the stack.
1128 pcre *pcre_compile(const char *pattern, int options,
1129 const char **errptr, int *erroffset,
1130 const unsigned char *tableptr);
1132 pcre *pcre_compile2(const char *pattern, int options,
1133 int *errorcodeptr,
1134 const char **errptr, int *erroffset,
1135 const unsigned char *tableptr);
1137 Either of the functions pcre_compile() or pcre_compile2() can be called
1138 to compile a pattern into an internal form. The only difference between
1139 the two interfaces is that pcre_compile2() has an additional argument,
1140 errorcodeptr, via which a numerical error code can be returned. To
1141 avoid too much repetition, we refer just to pcre_compile() below, but
1142 the information applies equally to pcre_compile2().
1144 The pattern is a C string terminated by a binary zero, and is passed in
1145 the pattern argument. A pointer to a single block of memory that is
1146 obtained via pcre_malloc is returned. This contains the compiled code
1147 and related data. The pcre type is defined for the returned block; this
1148 is a typedef for a structure whose contents are not externally defined.
1149 It is up to the caller to free the memory (via pcre_free) when it is no
1150 longer required.
1152 Although the compiled code of a PCRE regex is relocatable, that is, it
1153 does not depend on memory location, the complete pcre data block is not
1154 fully relocatable, because it may contain a copy of the tableptr argu-
1155 ment, which is an address (see below).
1157 The options argument contains various bit settings that affect the com-
1158 pilation. It should be zero if no options are required. The available
1159 options are described below. Some of them (in particular, those that
1160 are compatible with Perl, but some others as well) can also be set and
1161 unset from within the pattern (see the detailed description in the
1162 pcrepattern documentation). For those options that can be different in
1163 different parts of the pattern, the contents of the options argument
1164 specifies their settings at the start of compilation and execution. The
1165 PCRE_ANCHORED, PCRE_BSR_xxx, and PCRE_NEWLINE_xxx options can be set at
1166 the time of matching as well as at compile time.
1168 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
1169 if compilation of a pattern fails, pcre_compile() returns NULL, and
1170 sets the variable pointed to by errptr to point to a textual error mes-
1171 sage. This is a static string that is part of the library. You must not
1172 try to free it. The byte offset from the start of the pattern to the
1173 character that was being processed when the error was discovered is
1174 placed in the variable pointed to by erroffset, which must not be NULL.
1175 If it is, an immediate error is given. Some errors are not detected
1176 until checks are carried out when the whole pattern has been scanned;
1177 in this case the offset is set to the end of the pattern.
1179 If pcre_compile2() is used instead of pcre_compile(), and the error-
1180 codeptr argument is not NULL, a non-zero error code number is returned
1181 via this argument in the event of an error. This is in addition to the
1182 textual error message. Error codes and messages are listed below.
1184 If the final argument, tableptr, is NULL, PCRE uses a default set of
1185 character tables that are built when PCRE is compiled, using the
1186 default C locale. Otherwise, tableptr must be an address that is the
1187 result of a call to pcre_maketables(). This value is stored with the
1188 compiled pattern, and used again by pcre_exec(), unless another table
1189 pointer is passed to it. For more discussion, see the section on locale
1190 support below.
1192 This code fragment shows a typical straightforward call to pcre_com-
1193 pile():
1195 pcre *re;
1196 const char *error;
1197 int erroffset;
1198 re = pcre_compile(
1199 "^A.*Z", /* the pattern */
1200 0, /* default options */
1201 &error, /* for error message */
1202 &erroffset, /* for error offset */
1203 NULL); /* use default character tables */
1205 The following names for option bits are defined in the pcre.h header
1206 file:
1210 If this bit is set, the pattern is forced to be "anchored", that is, it
1211 is constrained to match only at the first matching point in the string
1212 that is being searched (the "subject string"). This effect can also be
1213 achieved by appropriate constructs in the pattern itself, which is the
1214 only way to do it in Perl.
1218 If this bit is set, pcre_compile() automatically inserts callout items,
1219 all with number 255, before each pattern item. For discussion of the
1220 callout facility, see the pcrecallout documentation.
1225 These options (which are mutually exclusive) control what the \R escape
1226 sequence matches. The choice is either to match only CR, LF, or CRLF,
1227 or to match any Unicode newline sequence. The default is specified when
1228 PCRE is built. It can be overridden from within the pattern, or by set-
1229 ting an option when a compiled pattern is matched.
1233 If this bit is set, letters in the pattern match both upper and lower
1234 case letters. It is equivalent to Perl's /i option, and it can be
1235 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1236 always understands the concept of case for characters whose values are
1237 less than 128, so caseless matching is always possible. For characters
1238 with higher values, the concept of case is supported if PCRE is com-
1239 piled with Unicode property support, but not otherwise. If you want to
1240 use caseless matching for characters 128 and above, you must ensure
1241 that PCRE is compiled with Unicode property support as well as with
1242 UTF-8 support.
1246 If this bit is set, a dollar metacharacter in the pattern matches only
1247 at the end of the subject string. Without this option, a dollar also
1248 matches immediately before a newline at the end of the string (but not
1249 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1250 if PCRE_MULTILINE is set. There is no equivalent to this option in
1251 Perl, and no way to set it within a pattern.
1255 If this bit is set, a dot metacharater in the pattern matches all char-
1256 acters, including those that indicate newline. Without it, a dot does
1257 not match when the current position is at a newline. This option is
1258 equivalent to Perl's /s option, and it can be changed within a pattern
1259 by a (?s) option setting. A negative class such as [^a] always matches
1260 newline characters, independent of the setting of this option.
1264 If this bit is set, names used to identify capturing subpatterns need
1265 not be unique. This can be helpful for certain types of pattern when it
1266 is known that only one instance of the named subpattern can ever be
1267 matched. There are more details of named subpatterns below; see also
1268 the pcrepattern documentation.
1272 If this bit is set, whitespace data characters in the pattern are
1273 totally ignored except when escaped or inside a character class. White-
1274 space does not include the VT character (code 11). In addition, charac-
1275 ters between an unescaped # outside a character class and the next new-
1276 line, inclusive, are also ignored. This is equivalent to Perl's /x
1277 option, and it can be changed within a pattern by a (?x) option set-
1278 ting.
1280 This option makes it possible to include comments inside complicated
1281 patterns. Note, however, that this applies only to data characters.
1282 Whitespace characters may never appear within special character
1283 sequences in a pattern, for example within the sequence (?( which
1284 introduces a conditional subpattern.
1288 This option was invented in order to turn on additional functionality
1289 of PCRE that is incompatible with Perl, but it is currently of very
1290 little use. When set, any backslash in a pattern that is followed by a
1291 letter that has no special meaning causes an error, thus reserving
1292 these combinations for future expansion. By default, as in Perl, a
1293 backslash followed by a letter with no special meaning is treated as a
1294 literal. (Perl can, however, be persuaded to give an error for this, by
1295 running it with the -w option.) There are at present no other features
1296 controlled by this option. It can also be set by a (?X) option setting
1297 within a pattern.
1301 If this option is set, an unanchored pattern is required to match
1302 before or at the first newline in the subject string, though the
1303 matched text may continue over the newline.
1307 If this option is set, PCRE's behaviour is changed in some ways so that
1308 it is compatible with JavaScript rather than Perl. The changes are as
1309 follows:
1311 (1) A lone closing square bracket in a pattern causes a compile-time
1312 error, because this is illegal in JavaScript (by default it is treated
1313 as a data character). Thus, the pattern AB]CD becomes illegal when this
1314 option is set.
1316 (2) At run time, a back reference to an unset subpattern group matches
1317 an empty string (by default this causes the current matching alterna-
1318 tive to fail). A pattern such as (\1)(a) succeeds when this option is
1319 set (assuming it can find an "a" in the subject), whereas it fails by
1320 default, for Perl compatibility.
1324 By default, PCRE treats the subject string as consisting of a single
1325 line of characters (even if it actually contains newlines). The "start
1326 of line" metacharacter (^) matches only at the start of the string,
1327 while the "end of line" metacharacter ($) matches only at the end of
1328 the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1329 is set). This is the same as Perl.
1331 When PCRE_MULTILINE it is set, the "start of line" and "end of line"
1332 constructs match immediately following or immediately before internal
1333 newlines in the subject string, respectively, as well as at the very
1334 start and end. This is equivalent to Perl's /m option, and it can be
1335 changed within a pattern by a (?m) option setting. If there are no new-
1336 lines in a subject string, or no occurrences of ^ or $ in a pattern,
1337 setting PCRE_MULTILINE has no effect.
1345 These options override the default newline definition that was chosen
1346 when PCRE was built. Setting the first or the second specifies that a
1347 newline is indicated by a single character (CR or LF, respectively).
1348 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1349 two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies
1350 that any of the three preceding sequences should be recognized. Setting
1351 PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be
1352 recognized. The Unicode newline sequences are the three just mentioned,
1353 plus the single characters VT (vertical tab, U+000B), FF (formfeed,
1354 U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
1355 (paragraph separator, U+2029). The last two are recognized only in
1356 UTF-8 mode.
1358 The newline setting in the options word uses three bits that are
1359 treated as a number, giving eight possibilities. Currently only six are
1360 used (default plus the five values above). This means that if you set
1361 more than one newline option, the combination may or may not be sensi-
1362 ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1363 PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and
1364 cause an error.
1366 The only time that a line break is specially recognized when compiling
1367 a pattern is if PCRE_EXTENDED is set, and an unescaped # outside a
1368 character class is encountered. This indicates a comment that lasts
1369 until after the next line break sequence. In other circumstances, line
1370 break sequences are treated as literal data, except that in
1371 PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
1372 and are therefore ignored.
1374 The newline option that is set at compile time becomes the default that
1375 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1379 If this option is set, it disables the use of numbered capturing paren-
1380 theses in the pattern. Any opening parenthesis that is not followed by
1381 ? behaves as if it were followed by ?: but named parentheses can still
1382 be used for capturing (and they acquire numbers in the usual way).
1383 There is no equivalent of this option in Perl.
1387 This option changes the way PCRE processes \b, \d, \s, \w, and some of
1388 the POSIX character classes. By default, only ASCII characters are rec-
1389 ognized, but if PCRE_UCP is set, Unicode properties are used instead to
1390 classify characters. More details are given in the section on generic
1391 character types in the pcrepattern page. If you set PCRE_UCP, matching
1392 one of the items it affects takes much longer. The option is available
1393 only if PCRE has been compiled with Unicode property support.
1397 This option inverts the "greediness" of the quantifiers so that they
1398 are not greedy by default, but become greedy if followed by "?". It is
1399 not compatible with Perl. It can also be set by a (?U) option setting
1400 within the pattern.
1402 PCRE_UTF8
1404 This option causes PCRE to regard both the pattern and the subject as
1405 strings of UTF-8 characters instead of single-byte character strings.
1406 However, it is available only when PCRE is built to include UTF-8 sup-
1407 port. If not, the use of this option provokes an error. Details of how
1408 this option changes the behaviour of PCRE are given in the section on
1409 UTF-8 support in the main pcre page.
1413 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1414 automatically checked. There is a discussion about the validity of
1415 UTF-8 strings in the main pcre page. If an invalid UTF-8 sequence of
1416 bytes is found, pcre_compile() returns an error. If you already know
1417 that your pattern is valid, and you want to skip this check for perfor-
1418 mance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is
1419 set, the effect of passing an invalid UTF-8 string as a pattern is
1420 undefined. It may cause your program to crash. Note that this option
1421 can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the
1422 UTF-8 validity checking of subject strings.
1427 The following table lists the error codes than may be returned by
1428 pcre_compile2(), along with the error messages that may be returned by
1429 both compiling functions. As PCRE has developed, some error codes have
1430 fallen out of use. To avoid confusion, they have not been re-used.
1432 0 no error
1433 1 \ at end of pattern
1434 2 \c at end of pattern
1435 3 unrecognized character follows \
1436 4 numbers out of order in {} quantifier
1437 5 number too big in {} quantifier
1438 6 missing terminating ] for character class
1439 7 invalid escape sequence in character class
1440 8 range out of order in character class
1441 9 nothing to repeat
1442 10 [this code is not in use]
1443 11 internal error: unexpected repeat
1444 12 unrecognized character after (? or (?-
1445 13 POSIX named classes are supported only within a class
1446 14 missing )
1447 15 reference to non-existent subpattern
1448 16 erroffset passed as NULL
1449 17 unknown option bit(s) set
1450 18 missing ) after comment
1451 19 [this code is not in use]
1452 20 regular expression is too large
1453 21 failed to get memory
1454 22 unmatched parentheses
1455 23 internal error: code overflow
1456 24 unrecognized character after (?<
1457 25 lookbehind assertion is not fixed length
1458 26 malformed number or name after (?(
1459 27 conditional group contains more than two branches
1460 28 assertion expected after (?(
1461 29 (?R or (?[+-]digits must be followed by )
1462 30 unknown POSIX class name
1463 31 POSIX collating elements are not supported
1464 32 this version of PCRE is not compiled with PCRE_UTF8 support
1465 33 [this code is not in use]
1466 34 character value in \x{...} sequence is too large
1467 35 invalid condition (?(0)
1468 36 \C not allowed in lookbehind assertion
1469 37 PCRE does not support \L, \l, \N, \U, or \u
1470 38 number after (?C is > 255
1471 39 closing ) for (?C expected
1472 40 recursive call could loop indefinitely
1473 41 unrecognized character after (?P
1474 42 syntax error in subpattern name (missing terminator)
1475 43 two named subpatterns have the same name
1476 44 invalid UTF-8 string
1477 45 support for \P, \p, and \X has not been compiled
1478 46 malformed \P or \p sequence
1479 47 unknown property name after \P or \p
1480 48 subpattern name is too long (maximum 32 characters)
1481 49 too many named subpatterns (maximum 10000)
1482 50 [this code is not in use]
1483 51 octal value is greater than \377 (not in UTF-8 mode)
1484 52 internal error: overran compiling workspace
1485 53 internal error: previously-checked referenced subpattern not
1486 found
1487 54 DEFINE group contains more than one branch
1488 55 repeating a DEFINE group is not allowed
1489 56 inconsistent NEWLINE options
1490 57 \g is not followed by a braced, angle-bracketed, or quoted
1491 name/number or by a plain number
1492 58 a numbered reference must not be zero
1493 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
1494 60 (*VERB) not recognized
1495 61 number is too big
1496 62 subpattern name expected
1497 63 digit expected after (?+
1498 64 ] is an invalid data character in JavaScript compatibility mode
1499 65 different names for subpatterns of the same number are not
1500 allowed
1501 66 (*MARK) must have an argument
1502 67 this version of PCRE is not compiled with PCRE_UCP support
1504 The numbers 32 and 10000 in errors 48 and 49 are defaults; different
1505 values may be used if the limits were changed when PCRE was built.
1510 pcre_extra *pcre_study(const pcre *code, int options
1511 const char **errptr);
1513 If a compiled pattern is going to be used several times, it is worth
1514 spending more time analyzing it in order to speed up the time taken for
1515 matching. The function pcre_study() takes a pointer to a compiled pat-
1516 tern as its first argument. If studying the pattern produces additional
1517 information that will help speed up matching, pcre_study() returns a
1518 pointer to a pcre_extra block, in which the study_data field points to
1519 the results of the study.
1521 The returned value from pcre_study() can be passed directly to
1522 pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con-
1523 tains other fields that can be set by the caller before the block is
1524 passed; these are described below in the section on matching a pattern.
1526 If studying the pattern does not produce any useful information,
1527 pcre_study() returns NULL. In that circumstance, if the calling program
1528 wants to pass any of the other fields to pcre_exec() or
1529 pcre_dfa_exec(), it must set up its own pcre_extra block.
1531 The second argument of pcre_study() contains option bits. At present,
1532 no options are defined, and this argument should always be zero.
1534 The third argument for pcre_study() is a pointer for an error message.
1535 If studying succeeds (even if no data is returned), the variable it
1536 points to is set to NULL. Otherwise it is set to point to a textual
1537 error message. This is a static string that is part of the library. You
1538 must not try to free it. You should test the error pointer for NULL
1539 after calling pcre_study(), to be sure that it has run successfully.
1541 This is a typical call to pcre_study():
1543 pcre_extra *pe;
1544 pe = pcre_study(
1545 re, /* result of pcre_compile() */
1546 0, /* no options exist */
1547 &error); /* set to NULL or points to a message */
1549 Studying a pattern does two things: first, a lower bound for the length
1550 of subject string that is needed to match the pattern is computed. This
1551 does not mean that there are any strings of that length that match, but
1552 it does guarantee that no shorter strings match. The value is used by
1553 pcre_exec() and pcre_dfa_exec() to avoid wasting time by trying to
1554 match strings that are shorter than the lower bound. You can find out
1555 the value in a calling program via the pcre_fullinfo() function.
1557 Studying a pattern is also useful for non-anchored patterns that do not
1558 have a single fixed starting character. A bitmap of possible starting
1559 bytes is created. This speeds up finding a position in the subject at
1560 which to start matching.
1565 PCRE handles caseless matching, and determines whether characters are
1566 letters, digits, or whatever, by reference to a set of tables, indexed
1567 by character value. When running in UTF-8 mode, this applies only to
1568 characters with codes less than 128. By default, higher-valued codes
1569 never match escapes such as \w or \d, but they can be tested with \p if
1570 PCRE is built with Unicode character property support. Alternatively,
1571 the PCRE_UCP option can be set at compile time; this causes \w and
1572 friends to use Unicode property support instead of built-in tables. The
1573 use of locales with Unicode is discouraged. If you are handling charac-
1574 ters with codes greater than 128, you should either use UTF-8 and Uni-
1575 code, or use locales, but not try to mix the two.
1577 PCRE contains an internal set of tables that are used when the final
1578 argument of pcre_compile() is NULL. These are sufficient for many
1579 applications. Normally, the internal tables recognize only ASCII char-
1580 acters. However, when PCRE is built, it is possible to cause the inter-
1581 nal tables to be rebuilt in the default "C" locale of the local system,
1582 which may cause them to be different.
1584 The internal tables can always be overridden by tables supplied by the
1585 application that calls PCRE. These may be created in a different locale
1586 from the default. As more and more applications change to using Uni-
1587 code, the need for this locale support is expected to die away.
1589 External tables are built by calling the pcre_maketables() function,
1590 which has no arguments, in the relevant locale. The result can then be
1591 passed to pcre_compile() or pcre_exec() as often as necessary. For
1592 example, to build and use tables that are appropriate for the French
1593 locale (where accented characters with values greater than 128 are
1594 treated as letters), the following code could be used:
1596 setlocale(LC_CTYPE, "fr_FR");
1597 tables = pcre_maketables();
1598 re = pcre_compile(..., tables);
1600 The locale name "fr_FR" is used on Linux and other Unix-like systems;
1601 if you are using Windows, the name for the French locale is "french".
1603 When pcre_maketables() runs, the tables are built in memory that is
1604 obtained via pcre_malloc. It is the caller's responsibility to ensure
1605 that the memory containing the tables remains available for as long as
1606 it is needed.
1608 The pointer that is passed to pcre_compile() is saved with the compiled
1609 pattern, and the same tables are used via this pointer by pcre_study()
1610 and normally also by pcre_exec(). Thus, by default, for any single pat-
1611 tern, compilation, studying and matching all happen in the same locale,
1612 but different patterns can be compiled in different locales.
1614 It is possible to pass a table pointer or NULL (indicating the use of
1615 the internal tables) to pcre_exec(). Although not intended for this
1616 purpose, this facility could be used to match a pattern in a different
1617 locale from the one in which it was compiled. Passing table pointers at
1618 run time is discussed below in the section on matching a pattern.
1623 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1624 int what, void *where);
1626 The pcre_fullinfo() function returns information about a compiled pat-
1627 tern. It replaces the obsolete pcre_info() function, which is neverthe-
1628 less retained for backwards compability (and is documented below).
1630 The first argument for pcre_fullinfo() is a pointer to the compiled
1631 pattern. The second argument is the result of pcre_study(), or NULL if
1632 the pattern was not studied. The third argument specifies which piece
1633 of information is required, and the fourth argument is a pointer to a
1634 variable to receive the data. The yield of the function is zero for
1635 success, or one of the following negative numbers:
1637 PCRE_ERROR_NULL the argument code was NULL
1638 the argument where was NULL
1639 PCRE_ERROR_BADMAGIC the "magic number" was not found
1640 PCRE_ERROR_BADOPTION the value of what was invalid
1642 The "magic number" is placed at the start of each compiled pattern as
1643 an simple check against passing an arbitrary memory pointer. Here is a
1644 typical call of pcre_fullinfo(), to obtain the length of the compiled
1645 pattern:
1647 int rc;
1648 size_t length;
1649 rc = pcre_fullinfo(
1650 re, /* result of pcre_compile() */
1651 pe, /* result of pcre_study(), or NULL */
1652 PCRE_INFO_SIZE, /* what is required */
1653 &length); /* where to put the data */
1655 The possible values for the third argument are defined in pcre.h, and
1656 are as follows:
1660 Return the number of the highest back reference in the pattern. The
1661 fourth argument should point to an int variable. Zero is returned if
1662 there are no back references.
1666 Return the number of capturing subpatterns in the pattern. The fourth
1667 argument should point to an int variable.
1671 Return a pointer to the internal default character tables within PCRE.
1672 The fourth argument should point to an unsigned char * variable. This
1673 information call is provided for internal use by the pcre_study() func-
1674 tion. External callers can cause PCRE to use its internal tables by
1675 passing a NULL table pointer.
1679 Return information about the first byte of any matched string, for a
1680 non-anchored pattern. The fourth argument should point to an int vari-
1681 able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1682 is still recognized for backwards compatibility.)
1684 If there is a fixed first byte, for example, from a pattern such as
1685 (cat|cow|coyote), its value is returned. Otherwise, if either
1687 (a) the pattern was compiled with the PCRE_MULTILINE option, and every
1688 branch starts with "^", or
1690 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1691 set (if it were set, the pattern would be anchored),
1693 -1 is returned, indicating that the pattern matches only at the start
1694 of a subject string or after any newline within the string. Otherwise
1695 -2 is returned. For anchored patterns, -2 is returned.
1699 If the pattern was studied, and this resulted in the construction of a
1700 256-bit table indicating a fixed set of bytes for the first byte in any
1701 matching string, a pointer to the table is returned. Otherwise NULL is
1702 returned. The fourth argument should point to an unsigned char * vari-
1703 able.
1707 Return 1 if the pattern contains any explicit matches for CR or LF
1708 characters, otherwise 0. The fourth argument should point to an int
1709 variable. An explicit match is either a literal CR or LF character, or
1710 \r or \n.
1714 Return 1 if the (?J) or (?-J) option setting is used in the pattern,
1715 otherwise 0. The fourth argument should point to an int variable. (?J)
1716 and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.
1720 Return the value of the rightmost literal byte that must exist in any
1721 matched string, other than at its start, if such a byte has been
1722 recorded. The fourth argument should point to an int variable. If there
1723 is no such byte, -1 is returned. For anchored patterns, a last literal
1724 byte is recorded only if it follows something of variable length. For
1725 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1726 /^a\dz\d/ the returned value is -1.
1730 If the pattern was studied and a minimum length for matching subject
1731 strings was computed, its value is returned. Otherwise the returned
1732 value is -1. The value is a number of characters, not bytes (this may
1733 be relevant in UTF-8 mode). The fourth argument should point to an int
1734 variable. A non-negative value is a lower bound to the length of any
1735 matching string. There may not be any strings of that length that do
1736 actually match, but every string that does match is at least that long.
1742 PCRE supports the use of named as well as numbered capturing parenthe-
1743 ses. The names are just an additional way of identifying the parenthe-
1744 ses, which still acquire numbers. Several convenience functions such as
1745 pcre_get_named_substring() are provided for extracting captured sub-
1746 strings by name. It is also possible to extract the data directly, by
1747 first converting the name to a number in order to access the correct
1748 pointers in the output vector (described with pcre_exec() below). To do
1749 the conversion, you need to use the name-to-number map, which is
1750 described by these three values.
1752 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1753 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1754 of each entry; both of these return an int value. The entry size
1755 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
1756 a pointer to the first entry of the table (a pointer to char). The
1757 first two bytes of each entry are the number of the capturing parenthe-
1758 sis, most significant byte first. The rest of the entry is the corre-
1759 sponding name, zero terminated.
1761 The names are in alphabetical order. Duplicate names may appear if (?|
1762 is used to create multiple groups with the same number, as described in
1763 the section on duplicate subpattern numbers in the pcrepattern page.
1764 Duplicate names for subpatterns with different numbers are permitted
1765 only if PCRE_DUPNAMES is set. In all cases of duplicate names, they
1766 appear in the table in the order in which they were found in the pat-
1767 tern. In the absence of (?| this is the order of increasing number;
1768 when (?| is used this is not necessarily the case because later subpat-
1769 terns may have lower numbers.
1771 As a simple example of the name/number table, consider the following
1772 pattern (assume PCRE_EXTENDED is set, so white space - including new-
1773 lines - is ignored):
1775 (?<date> (?<year>(\d\d)?\d\d) -
1776 (?<month>\d\d) - (?<day>\d\d) )
1778 There are four named subpatterns, so the table has four entries, and
1779 each entry in the table is eight bytes long. The table is as follows,
1780 with non-printing bytes shows in hexadecimal, and undefined bytes shown
1781 as ??:
1783 00 01 d a t e 00 ??
1784 00 05 d a y 00 ?? ??
1785 00 04 m o n t h 00
1786 00 02 y e a r 00 ??
1788 When writing code to extract data from named subpatterns using the
1789 name-to-number map, remember that the length of the entries is likely
1790 to be different for each compiled pattern.
1794 Return 1 if the pattern can be used for partial matching with
1795 pcre_exec(), otherwise 0. The fourth argument should point to an int
1796 variable. From release 8.00, this always returns 1, because the
1797 restrictions that previously applied to partial matching have been
1798 lifted. The pcrepartial documentation gives details of partial match-
1799 ing.
1803 Return a copy of the options with which the pattern was compiled. The
1804 fourth argument should point to an unsigned long int variable. These
1805 option bits are those specified in the call to pcre_compile(), modified
1806 by any top-level option settings at the start of the pattern itself. In
1807 other words, they are the options that will be in force when matching
1808 starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
1809 the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
1812 A pattern is automatically anchored by PCRE if all of its top-level
1813 alternatives begin with one of the following:
1815 ^ unless PCRE_MULTILINE is set
1816 \A always
1817 \G always
1818 .* if PCRE_DOTALL is set and there are no back
1819 references to the subpattern in which .* appears
1821 For such patterns, the PCRE_ANCHORED bit is set in the options returned
1822 by pcre_fullinfo().
1826 Return the size of the compiled pattern, that is, the value that was
1827 passed as the argument to pcre_malloc() when PCRE was getting memory in
1828 which to place the compiled data. The fourth argument should point to a
1829 size_t variable.
1833 Return the size of the data block pointed to by the study_data field in
1834 a pcre_extra block. That is, it is the value that was passed to
1835 pcre_malloc() when PCRE was getting memory into which to place the data
1836 created by pcre_study(). If pcre_extra is NULL, or there is no study
1837 data, zero is returned. The fourth argument should point to a size_t
1838 variable.
1843 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1845 The pcre_info() function is now obsolete because its interface is too
1846 restrictive to return all the available data about a compiled pattern.
1847 New programs should use pcre_fullinfo() instead. The yield of
1848 pcre_info() is the number of capturing subpatterns, or one of the fol-
1849 lowing negative numbers:
1851 PCRE_ERROR_NULL the argument code was NULL
1852 PCRE_ERROR_BADMAGIC the "magic number" was not found
1854 If the optptr argument is not NULL, a copy of the options with which
1855 the pattern was compiled is placed in the integer it points to (see
1856 PCRE_INFO_OPTIONS above).
1858 If the pattern is not anchored and the firstcharptr argument is not
1859 NULL, it is used to pass back information about the first character of
1860 any matched string (see PCRE_INFO_FIRSTBYTE above).
1865 int pcre_refcount(pcre *code, int adjust);
1867 The pcre_refcount() function is used to maintain a reference count in
1868 the data block that contains a compiled pattern. It is provided for the
1869 benefit of applications that operate in an object-oriented manner,
1870 where different parts of the application may be using the same compiled
1871 pattern, but you want to free the block when they are all done.
1873 When a pattern is compiled, the reference count field is initialized to
1874 zero. It is changed only by calling this function, whose action is to
1875 add the adjust value (which may be positive or negative) to it. The
1876 yield of the function is the new value. However, the value of the count
1877 is constrained to lie between 0 and 65535, inclusive. If the new value
1878 is outside these limits, it is forced to the appropriate limit value.
1880 Except when it is zero, the reference count is not correctly preserved
1881 if a pattern is compiled on one host and then transferred to a host
1882 whose byte-order is different. (This seems a highly unlikely scenario.)
1887 int pcre_exec(const pcre *code, const pcre_extra *extra,
1888 const char *subject, int length, int startoffset,
1889 int options, int *ovector, int ovecsize);
1891 The function pcre_exec() is called to match a subject string against a
1892 compiled pattern, which is passed in the code argument. If the pattern
1893 was studied, the result of the study should be passed in the extra
1894 argument. This function is the main matching facility of the library,
1895 and it operates in a Perl-like manner. For specialist use there is also
1896 an alternative matching function, which is described below in the sec-
1897 tion about the pcre_dfa_exec() function.
1899 In most applications, the pattern will have been compiled (and option-
1900 ally studied) in the same process that calls pcre_exec(). However, it
1901 is possible to save compiled patterns and study data, and then use them
1902 later in different processes, possibly even on different hosts. For a
1903 discussion about this, see the pcreprecompile documentation.
1905 Here is an example of a simple call to pcre_exec():
1907 int rc;
1908 int ovector[30];
1909 rc = pcre_exec(
1910 re, /* result of pcre_compile() */
1911 NULL, /* we didn't study the pattern */
1912 "some string", /* the subject string */
1913 11, /* the length of the subject string */
1914 0, /* start at offset 0 in the subject */
1915 0, /* default options */
1916 ovector, /* vector of integers for substring information */
1917 30); /* number of elements (NOT size in bytes) */
1919 Extra data for pcre_exec()
1921 If the extra argument is not NULL, it must point to a pcre_extra data
1922 block. The pcre_study() function returns such a block (when it doesn't
1923 return NULL), but you can also create one for yourself, and pass addi-
1924 tional information in it. The pcre_extra block contains the following
1925 fields (not necessarily in this order):
1927 unsigned long int flags;
1928 void *study_data;
1929 unsigned long int match_limit;
1930 unsigned long int match_limit_recursion;
1931 void *callout_data;
1932 const unsigned char *tables;
1933 unsigned char **mark;
1935 The flags field is a bitmap that specifies which of the other fields
1936 are set. The flag bits are:
1945 Other flag bits should be set to zero. The study_data field is set in
1946 the pcre_extra block that is returned by pcre_study(), together with
1947 the appropriate flag bit. You should not set this yourself, but you may
1948 add to the block by setting the other fields and their corresponding
1949 flag bits.
1951 The match_limit field provides a means of preventing PCRE from using up
1952 a vast amount of resources when running patterns that are not going to
1953 match, but which have a very large number of possibilities in their
1954 search trees. The classic example is a pattern that uses nested unlim-
1955 ited repeats.
1957 Internally, PCRE uses a function called match() which it calls repeat-
1958 edly (sometimes recursively). The limit set by match_limit is imposed
1959 on the number of times this function is called during a match, which
1960 has the effect of limiting the amount of backtracking that can take
1961 place. For patterns that are not anchored, the count restarts from zero
1962 for each position in the subject string.
1964 The default value for the limit can be set when PCRE is built; the
1965 default default is 10 million, which handles all but the most extreme
1966 cases. You can override the default by suppling pcre_exec() with a
1967 pcre_extra block in which match_limit is set, and
1968 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
1969 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1971 The match_limit_recursion field is similar to match_limit, but instead
1972 of limiting the total number of times that match() is called, it limits
1973 the depth of recursion. The recursion depth is a smaller number than
1974 the total number of calls, because not all calls to match() are recur-
1975 sive. This limit is of use only if it is set smaller than match_limit.
1977 Limiting the recursion depth limits the amount of stack that can be
1978 used, or, when PCRE has been compiled to use memory on the heap instead
1979 of the stack, the amount of heap memory that can be used.
1981 The default value for match_limit_recursion can be set when PCRE is
1982 built; the default default is the same value as the default for
1983 match_limit. You can override the default by suppling pcre_exec() with
1984 a pcre_extra block in which match_limit_recursion is set, and
1985 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
1986 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1988 The callout_data field is used in conjunction with the "callout" fea-
1989 ture, and is described in the pcrecallout documentation.
1991 The tables field is used to pass a character tables pointer to
1992 pcre_exec(); this overrides the value that is stored with the compiled
1993 pattern. A non-NULL value is stored with the compiled pattern only if
1994 custom tables were supplied to pcre_compile() via its tableptr argu-
1995 ment. If NULL is passed to pcre_exec() using this mechanism, it forces
1996 PCRE's internal tables to be used. This facility is helpful when re-
1997 using patterns that have been saved after compiling with an external
1998 set of tables, because the external tables might be at a different
1999 address when pcre_exec() is called. See the pcreprecompile documenta-
2000 tion for a discussion of saving compiled patterns for later use.
2002 If PCRE_EXTRA_MARK is set in the flags field, the mark field must be
2003 set to point to a char * variable. If the pattern contains any back-
2004 tracking control verbs such as (*MARK:NAME), and the execution ends up
2005 with a name to pass back, a pointer to the name string (zero termi-
2006 nated) is placed in the variable pointed to by the mark field. The
2007 names are within the compiled pattern; if you wish to retain such a
2008 name you must copy it before freeing the memory of a compiled pattern.
2009 If there is no name to pass back, the variable pointed to by the mark
2010 field set to NULL. For details of the backtracking control verbs, see
2011 the section entitled "Backtracking control" in the pcrepattern documen-
2012 tation.
2014 Option bits for pcre_exec()
2016 The unused bits of the options argument for pcre_exec() must be zero.
2017 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
2024 The PCRE_ANCHORED option limits pcre_exec() to matching at the first
2025 matching position. If a pattern was compiled with PCRE_ANCHORED, or
2026 turned out to be anchored by virtue of its contents, it cannot be made
2027 unachored at matching time.
2032 These options (which are mutually exclusive) control what the \R escape
2033 sequence matches. The choice is either to match only CR, LF, or CRLF,
2034 or to match any Unicode newline sequence. These options override the
2035 choice that was made or defaulted when the pattern was compiled.
2043 These options override the newline definition that was chosen or
2044 defaulted when the pattern was compiled. For details, see the descrip-
2045 tion of pcre_compile() above. During matching, the newline choice
2046 affects the behaviour of the dot, circumflex, and dollar metacharac-
2047 ters. It may also alter the way the match position is advanced after a
2048 match failure for an unanchored pattern.
2051 set, and a match attempt for an unanchored pattern fails when the cur-
2052 rent position is at a CRLF sequence, and the pattern contains no
2053 explicit matches for CR or LF characters, the match position is
2054 advanced by two characters instead of one, in other words, to after the
2055 CRLF.
2057 The above rule is a compromise that makes the most common cases work as
2058 expected. For example, if the pattern is .+A (and the PCRE_DOTALL
2059 option is not set), it does not match the string "\r\nA" because, after
2060 failing at the start, it skips both the CR and the LF before retrying.
2061 However, the pattern [\r\n]A does match that string, because it con-
2062 tains an explicit CR or LF reference, and so advances only by one char-
2063 acter after the first failure.
2065 An explicit match for CR of LF is either a literal appearance of one of
2066 those characters, or one of the \r or \n escape sequences. Implicit
2067 matches such as [^X] do not count, nor does \s (which includes CR and
2068 LF in the characters that it matches).
2070 Notwithstanding the above, anomalous effects may still occur when CRLF
2071 is a valid newline sequence and explicit \r or \n escapes appear in the
2072 pattern.
2076 This option specifies that first character of the subject string is not
2077 the beginning of a line, so the circumflex metacharacter should not
2078 match before it. Setting this without PCRE_MULTILINE (at compile time)
2079 causes circumflex never to match. This option affects only the behav-
2080 iour of the circumflex metacharacter. It does not affect \A.
2084 This option specifies that the end of the subject string is not the end
2085 of a line, so the dollar metacharacter should not match it nor (except
2086 in multiline mode) a newline immediately before it. Setting this with-
2087 out PCRE_MULTILINE (at compile time) causes dollar never to match. This
2088 option affects only the behaviour of the dollar metacharacter. It does
2089 not affect \Z or \z.
2093 An empty string is not considered to be a valid match if this option is
2094 set. If there are alternatives in the pattern, they are tried. If all
2095 the alternatives match the empty string, the entire match fails. For
2096 example, if the pattern
2098 a?b?
2100 is applied to a string not beginning with "a" or "b", it matches an
2101 empty string at the start of the subject. With PCRE_NOTEMPTY set, this
2102 match is not valid, so PCRE searches further into the string for occur-
2103 rences of "a" or "b".
2107 This is like PCRE_NOTEMPTY, except that an empty string match that is
2108 not at the start of the subject is permitted. If the pattern is
2109 anchored, such a match can occur only if the pattern contains \K.
2111 Perl has no direct equivalent of PCRE_NOTEMPTY or
2112 PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern
2113 match of the empty string within its split() function, and when using
2114 the /g modifier. It is possible to emulate Perl's behaviour after
2115 matching a null string by first trying the match again at the same off-
2116 set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that
2117 fails, by advancing the starting offset (see below) and trying an ordi-
2118 nary match again. There is some code that demonstrates how to do this
2119 in the pcredemo sample program.
2123 There are a number of optimizations that pcre_exec() uses at the start
2124 of a match, in order to speed up the process. For example, if it is
2125 known that an unanchored match must start with a specific character, it
2126 searches the subject for that character, and fails immediately if it
2127 cannot find it, without actually running the main matching function.
2128 This means that a special item such as (*COMMIT) at the start of a pat-
2129 tern is not considered until after a suitable starting point for the
2130 match has been found. When callouts are in use, these "start-up" opti-
2131 mizations can cause them to be skipped if the pattern is never actually
2132 used. The PCRE_NO_START_OPTIMIZE option disables the start-up optimiza-
2133 tions, causing performance to suffer, but ensuring that the callouts do
2134 occur, and that items such as (*COMMIT) are considered at every possi-
2135 ble starting position in the subject string.
2139 When PCRE_UTF8 is set at compile time, the validity of the subject as a
2140 UTF-8 string is automatically checked when pcre_exec() is subsequently
2141 called. The value of startoffset is also checked to ensure that it
2142 points to the start of a UTF-8 character. There is a discussion about
2143 the validity of UTF-8 strings in the section on UTF-8 support in the
2144 main pcre page. If an invalid UTF-8 sequence of bytes is found,
2145 pcre_exec() returns the error PCRE_ERROR_BADUTF8. If startoffset con-
2146 tains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.
2148 If you already know that your subject is valid, and you want to skip
2149 these checks for performance reasons, you can set the
2150 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
2151 do this for the second and subsequent calls to pcre_exec() if you are
2152 making repeated calls to find all the matches in a single subject
2153 string. However, you should be sure that the value of startoffset
2154 points to the start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
2155 set, the effect of passing an invalid UTF-8 string as a subject, or a
2156 value of startoffset that does not point to the start of a UTF-8 char-
2157 acter, is undefined. Your program may crash.
2162 These options turn on the partial matching feature. For backwards com-
2163 patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial
2164 match occurs if the end of the subject string is reached successfully,
2165 but there are not enough subject characters to complete the match. If
2166 this happens when PCRE_PARTIAL_HARD is set, pcre_exec() immediately
2167 returns PCRE_ERROR_PARTIAL. Otherwise, if PCRE_PARTIAL_SOFT is set,
2168 matching continues by testing any other alternatives. Only if they all
2169 fail is PCRE_ERROR_PARTIAL returned (instead of PCRE_ERROR_NOMATCH).
2170 The portion of the string that was inspected when the partial match was
2171 found is set as the first matching string. There is a more detailed
2172 discussion in the pcrepartial documentation.
2174 The string to be matched by pcre_exec()
2176 The subject string is passed to pcre_exec() as a pointer in subject, a
2177 length (in bytes) in length, and a starting byte offset in startoffset.
2178 In UTF-8 mode, the byte offset must point to the start of a UTF-8 char-
2179 acter. Unlike the pattern string, the subject may contain binary zero
2180 bytes. When the starting offset is zero, the search for a match starts
2181 at the beginning of the subject, and this is by far the most common
2182 case.
2184 A non-zero starting offset is useful when searching for another match
2185 in the same subject by calling pcre_exec() again after a previous suc-
2186 cess. Setting startoffset differs from just passing over a shortened
2187 string and setting PCRE_NOTBOL in the case of a pattern that begins
2188 with any kind of lookbehind. For example, consider the pattern
2190 \Biss\B
2192 which finds occurrences of "iss" in the middle of words. (\B matches
2193 only if the current position in the subject is not a word boundary.)
2194 When applied to the string "Mississipi" the first call to pcre_exec()
2195 finds the first occurrence. If pcre_exec() is called again with just
2196 the remainder of the subject, namely "issipi", it does not match,
2197 because \B is always false at the start of the subject, which is deemed
2198 to be a word boundary. However, if pcre_exec() is passed the entire
2199 string again, but with startoffset set to 4, it finds the second occur-
2200 rence of "iss" because it is able to look behind the starting point to
2201 discover that it is preceded by a letter.
2203 If a non-zero starting offset is passed when the pattern is anchored,
2204 one attempt to match at the given offset is made. This can only succeed
2205 if the pattern does not require the match to be at the start of the
2206 subject.
2208 How pcre_exec() returns captured substrings
2210 In general, a pattern matches a certain portion of the subject, and in
2211 addition, further substrings from the subject may be picked out by
2212 parts of the pattern. Following the usage in Jeffrey Friedl's book,
2213 this is called "capturing" in what follows, and the phrase "capturing
2214 subpattern" is used for a fragment of a pattern that picks out a sub-
2215 string. PCRE supports several other kinds of parenthesized subpattern
2216 that do not cause substrings to be captured.
2218 Captured substrings are returned to the caller via a vector of integers
2219 whose address is passed in ovector. The number of elements in the vec-
2220 tor is passed in ovecsize, which must be a non-negative number. Note:
2221 this argument is NOT the size of ovector in bytes.
2223 The first two-thirds of the vector is used to pass back captured sub-
2224 strings, each substring using a pair of integers. The remaining third
2225 of the vector is used as workspace by pcre_exec() while matching cap-
2226 turing subpatterns, and is not available for passing back information.
2227 The number passed in ovecsize should always be a multiple of three. If
2228 it is not, it is rounded down.
2230 When a match is successful, information about captured substrings is
2231 returned in pairs of integers, starting at the beginning of ovector,
2232 and continuing up to two-thirds of its length at the most. The first
2233 element of each pair is set to the byte offset of the first character
2234 in a substring, and the second is set to the byte offset of the first
2235 character after the end of a substring. Note: these values are always
2236 byte offsets, even in UTF-8 mode. They are not character counts.
2238 The first pair of integers, ovector[0] and ovector[1], identify the
2239 portion of the subject string matched by the entire pattern. The next
2240 pair is used for the first capturing subpattern, and so on. The value
2241 returned by pcre_exec() is one more than the highest numbered pair that
2242 has been set. For example, if two substrings have been captured, the
2243 returned value is 3. If there are no capturing subpatterns, the return
2244 value from a successful match is 1, indicating that just the first pair
2245 of offsets has been set.
2247 If a capturing subpattern is matched repeatedly, it is the last portion
2248 of the string that it matched that is returned.
2250 If the vector is too small to hold all the captured substring offsets,
2251 it is used as far as possible (up to two-thirds of its length), and the
2252 function returns a value of zero. If the substring offsets are not of
2253 interest, pcre_exec() may be called with ovector passed as NULL and
2254 ovecsize as zero. However, if the pattern contains back references and
2255 the ovector is not big enough to remember the related substrings, PCRE
2256 has to get additional memory for use during matching. Thus it is usu-
2257 ally advisable to supply an ovector.
2259 The pcre_fullinfo() function can be used to find out how many capturing
2260 subpatterns there are in a compiled pattern. The smallest size for
2261 ovector that will allow for n captured substrings, in addition to the
2262 offsets of the substring matched by the whole pattern, is (n+1)*3.
2264 It is possible for capturing subpattern number n+1 to match some part
2265 of the subject when subpattern n has not been used at all. For example,
2266 if the string "abc" is matched against the pattern (a|(z))(bc) the
2267 return from the function is 4, and subpatterns 1 and 3 are matched, but
2268 2 is not. When this happens, both values in the offset pairs corre-
2269 sponding to unused subpatterns are set to -1.
2271 Offset values that correspond to unused subpatterns at the end of the
2272 expression are also set to -1. For example, if the string "abc" is
2273 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
2274 matched. The return from the function is 2, because the highest used
2275 capturing subpattern number is 1. However, you can refer to the offsets
2276 for the second and third capturing subpatterns if you wish (assuming
2277 the vector is large enough, of course).
2279 Some convenience functions are provided for extracting the captured
2280 substrings as separate strings. These are described below.
2282 Error return values from pcre_exec()
2284 If pcre_exec() fails, it returns a negative number. The following are
2285 defined in the header file:
2289 The subject string did not match the pattern.
2293 Either code or subject was passed as NULL, or ovector was NULL and
2294 ovecsize was not zero.
2298 An unrecognized bit was set in the options argument.
2302 PCRE stores a 4-byte "magic number" at the start of the compiled code,
2303 to catch the case when it is passed a junk pointer and to detect when a
2304 pattern that was compiled in an environment of one endianness is run in
2305 an environment with the other endianness. This is the error that PCRE
2306 gives when the magic number is not present.
2310 While running the pattern match, an unknown item was encountered in the
2311 compiled pattern. This error could be caused by a bug in PCRE or by
2312 overwriting of the compiled pattern.
2316 If a pattern contains back references, but the ovector that is passed
2317 to pcre_exec() is not big enough to remember the referenced substrings,
2318 PCRE gets a block of memory at the start of matching to use for this
2319 purpose. If the call via pcre_malloc() fails, this error is given. The
2320 memory is automatically freed at the end of matching.
2322 This error is also given if pcre_stack_malloc() fails in pcre_exec().
2323 This can happen only when PCRE has been compiled with --disable-stack-
2324 for-recursion.
2328 This error is used by the pcre_copy_substring(), pcre_get_substring(),
2329 and pcre_get_substring_list() functions (see below). It is never
2330 returned by pcre_exec().
2334 The backtracking limit, as specified by the match_limit field in a
2335 pcre_extra structure (or defaulted) was reached. See the description
2336 above.
2340 This error is never generated by pcre_exec() itself. It is provided for
2341 use by callout functions that want to yield a distinctive error code.
2342 See the pcrecallout documentation for details.
2346 A string that contains an invalid UTF-8 byte sequence was passed as a
2347 subject.
2351 The UTF-8 byte sequence that was passed as a subject was valid, but the
2352 value of startoffset did not point to the beginning of a UTF-8 charac-
2353 ter.
2357 The subject string did not match, but it did match partially. See the
2358 pcrepartial documentation for details of partial matching.
2362 This code is no longer in use. It was formerly returned when the
2363 PCRE_PARTIAL option was used with a compiled pattern containing items
2364 that were not supported for partial matching. From release 8.00
2365 onwards, there are no restrictions on partial matching.
2369 An unexpected internal error has occurred. This error could be caused
2370 by a bug in PCRE or by overwriting of the compiled pattern.
2374 This error is given if the value of the ovecsize argument is negative.
2378 The internal recursion limit, as specified by the match_limit_recursion
2379 field in a pcre_extra structure (or defaulted) was reached. See the
2380 description above.
2384 An invalid combination of PCRE_NEWLINE_xxx options was given.
2386 Error numbers -16 to -20 and -22 are not used by pcre_exec().
2391 int pcre_copy_substring(const char *subject, int *ovector,
2392 int stringcount, int stringnumber, char *buffer,
2393 int buffersize);
2395 int pcre_get_substring(const char *subject, int *ovector,
2396 int stringcount, int stringnumber,
2397 const char **stringptr);
2399 int pcre_get_substring_list(const char *subject,
2400 int *ovector, int stringcount, const char ***listptr);
2402 Captured substrings can be accessed directly by using the offsets
2403 returned by pcre_exec() in ovector. For convenience, the functions
2404 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
2405 string_list() are provided for extracting captured substrings as new,
2406 separate, zero-terminated strings. These functions identify substrings
2407 by number. The next section describes functions for extracting named
2408 substrings.
2410 A substring that contains a binary zero is correctly extracted and has
2411 a further zero added on the end, but the result is not, of course, a C
2412 string. However, you can process such a string by referring to the
2413 length that is returned by pcre_copy_substring() and pcre_get_sub-
2414 string(). Unfortunately, the interface to pcre_get_substring_list() is
2415 not adequate for handling strings containing binary zeros, because the
2416 end of the final string is not independently indicated.
2418 The first three arguments are the same for all three of these func-
2419 tions: subject is the subject string that has just been successfully
2420 matched, ovector is a pointer to the vector of integer offsets that was
2421 passed to pcre_exec(), and stringcount is the number of substrings that
2422 were captured by the match, including the substring that matched the
2423 entire regular expression. This is the value returned by pcre_exec() if
2424 it is greater than zero. If pcre_exec() returned zero, indicating that
2425 it ran out of space in ovector, the value passed as stringcount should
2426 be the number of elements in the vector divided by three.
2428 The functions pcre_copy_substring() and pcre_get_substring() extract a
2429 single substring, whose number is given as stringnumber. A value of
2430 zero extracts the substring that matched the entire pattern, whereas
2431 higher values extract the captured substrings. For pcre_copy_sub-
2432 string(), the string is placed in buffer, whose length is given by
2433 buffersize, while for pcre_get_substring() a new block of memory is
2434 obtained via pcre_malloc, and its address is returned via stringptr.
2435 The yield of the function is the length of the string, not including
2436 the terminating zero, or one of these error codes:
2440 The buffer was too small for pcre_copy_substring(), or the attempt to
2441 get memory failed for pcre_get_substring().
2445 There is no substring whose number is stringnumber.
2447 The pcre_get_substring_list() function extracts all available sub-
2448 strings and builds a list of pointers to them. All this is done in a
2449 single block of memory that is obtained via pcre_malloc. The address of
2450 the memory block is returned via listptr, which is also the start of
2451 the list of string pointers. The end of the list is marked by a NULL
2452 pointer. The yield of the function is zero if all went well, or the
2453 error code
2457 if the attempt to get the memory block failed.
2459 When any of these functions encounter a substring that is unset, which
2460 can happen when capturing subpattern number n+1 matches some part of
2461 the subject, but subpattern n has not been used at all, they return an
2462 empty string. This can be distinguished from a genuine zero-length sub-
2463 string by inspecting the appropriate offset in ovector, which is nega-
2464 tive for unset substrings.
2466 The two convenience functions pcre_free_substring() and pcre_free_sub-
2467 string_list() can be used to free the memory returned by a previous
2468 call of pcre_get_substring() or pcre_get_substring_list(), respec-
2469 tively. They do nothing more than call the function pointed to by
2470 pcre_free, which of course could be called directly from a C program.
2471 However, PCRE is used in some situations where it is linked via a spe-
2472 cial interface to another programming language that cannot use
2473 pcre_free directly; it is for these cases that the functions are pro-
2474 vided.
2479 int pcre_get_stringnumber(const pcre *code,
2480 const char *name);
2482 int pcre_copy_named_substring(const pcre *code,
2483 const char *subject, int *ovector,
2484 int stringcount, const char *stringname,
2485 char *buffer, int buffersize);
2487 int pcre_get_named_substring(const pcre *code,
2488 const char *subject, int *ovector,
2489 int stringcount, const char *stringname,
2490 const char **stringptr);
2492 To extract a substring by name, you first have to find associated num-
2493 ber. For example, for this pattern
2495 (a+)b(?<xxx>\d+)...
2497 the number of the subpattern called "xxx" is 2. If the name is known to
2498 be unique (PCRE_DUPNAMES was not set), you can find the number from the
2499 name by calling pcre_get_stringnumber(). The first argument is the com-
2500 piled pattern, and the second is the name. The yield of the function is
2501 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2502 subpattern of that name.
2504 Given the number, you can extract the substring directly, or use one of
2505 the functions described in the previous section. For convenience, there
2506 are also two functions that do the whole job.
2508 Most of the arguments of pcre_copy_named_substring() and
2509 pcre_get_named_substring() are the same as those for the similarly
2510 named functions that extract by number. As these are described in the
2511 previous section, they are not re-described here. There are just two
2512 differences:
2514 First, instead of a substring number, a substring name is given. Sec-
2515 ond, there is an extra argument, given at the start, which is a pointer
2516 to the compiled pattern. This is needed in order to gain access to the
2517 name-to-number translation table.
2519 These functions call pcre_get_stringnumber(), and if it succeeds, they
2520 then call pcre_copy_substring() or pcre_get_substring(), as appropri-
2521 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
2522 behaviour may not be what you want (see the next section).
2524 Warning: If the pattern uses the (?| feature to set up multiple subpat-
2525 terns with the same number, as described in the section on duplicate
2526 subpattern numbers in the pcrepattern page, you cannot use names to
2527 distinguish the different subpatterns, because names are not included
2528 in the compiled code. The matching process uses only numbers. For this
2529 reason, the use of different names for subpatterns of the same number
2530 causes an error at compile time.
2535 int pcre_get_stringtable_entries(const pcre *code,
2536 const char *name, char **first, char **last);
2538 When a pattern is compiled with the PCRE_DUPNAMES option, names for
2539 subpatterns are not required to be unique. (Duplicate names are always
2540 allowed for subpatterns with the same number, created by using the (?|
2541 feature. Indeed, if such subpatterns are named, they are required to
2542 use the same names.)
2544 Normally, patterns with duplicate names are such that in any one match,
2545 only one of the named subpatterns participates. An example is shown in
2546 the pcrepattern documentation.
2548 When duplicates are present, pcre_copy_named_substring() and
2549 pcre_get_named_substring() return the first substring corresponding to
2550 the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
2551 (-7) is returned; no data is returned. The pcre_get_stringnumber()
2552 function returns one of the numbers that are associated with the name,
2553 but it is not defined which it is.
2555 If you want to get full details of all captured substrings for a given
2556 name, you must use the pcre_get_stringtable_entries() function. The
2557 first argument is the compiled pattern, and the second is the name. The
2558 third and fourth are pointers to variables which are updated by the
2559 function. After it has run, they point to the first and last entries in
2560 the name-to-number table for the given name. The function itself
2561 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
2562 there are none. The format of the table is described above in the sec-
2563 tion entitled Information about a pattern. Given all the relevant
2564 entries for the name, you can extract each of their numbers, and hence
2565 the captured data, if any.
2570 The traditional matching function uses a similar algorithm to Perl,
2571 which stops when it finds the first match, starting at a given point in
2572 the subject. If you want to find all possible matches, or the longest
2573 possible match, consider using the alternative matching function (see
2574 below) instead. If you cannot use the alternative function, but still
2575 need to find all possible matches, you can kludge it up by making use
2576 of the callout facility, which is described in the pcrecallout documen-
2577 tation.
2579 What you have to do is to insert a callout right at the end of the pat-
2580 tern. When your callout function is called, extract and save the cur-
2581 rent matched substring. Then return 1, which forces pcre_exec() to
2582 backtrack and try other alternatives. Ultimately, when it runs out of
2583 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2588 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2589 const char *subject, int length, int startoffset,
2590 int options, int *ovector, int ovecsize,
2591 int *workspace, int wscount);
2593 The function pcre_dfa_exec() is called to match a subject string
2594 against a compiled pattern, using a matching algorithm that scans the
2595 subject string just once, and does not backtrack. This has different
2596 characteristics to the normal algorithm, and is not compatible with
2597 Perl. Some of the features of PCRE patterns are not supported. Never-
2598 theless, there are times when this kind of matching can be useful. For
2599 a discussion of the two matching algorithms, and a list of features
2600 that pcre_dfa_exec() does not support, see the pcrematching documenta-
2601 tion.
2603 The arguments for the pcre_dfa_exec() function are the same as for
2604 pcre_exec(), plus two extras. The ovector argument is used in a differ-
2605 ent way, and this is described below. The other common arguments are
2606 used in the same way as for pcre_exec(), so their description is not
2607 repeated here.
2609 The two additional arguments provide workspace for the function. The
2610 workspace vector should contain at least 20 elements. It is used for
2611 keeping track of multiple paths through the pattern tree. More
2612 workspace will be needed for patterns and subjects where there are a
2613 lot of potential matches.
2615 Here is an example of a simple call to pcre_dfa_exec():
2617 int rc;
2618 int ovector[10];
2619 int wspace[20];
2620 rc = pcre_dfa_exec(
2621 re, /* result of pcre_compile() */
2622 NULL, /* we didn't study the pattern */
2623 "some string", /* the subject string */
2624 11, /* the length of the subject string */
2625 0, /* start at offset 0 in the subject */
2626 0, /* default options */
2627 ovector, /* vector of integers for substring information */
2628 10, /* number of elements (NOT size in bytes) */
2629 wspace, /* working space vector */
2630 20); /* number of elements (NOT size in bytes) */
2632 Option bits for pcre_dfa_exec()
2634 The unused bits of the options argument for pcre_dfa_exec() must be
2635 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
2640 four of these are exactly the same as for pcre_exec(), so their
2641 description is not repeated here.
2646 These have the same general effect as they do for pcre_exec(), but the
2647 details are slightly different. When PCRE_PARTIAL_HARD is set for
2648 pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub-
2649 ject is reached and there is still at least one matching possibility
2650 that requires additional characters. This happens even if some complete
2651 matches have also been found. When PCRE_PARTIAL_SOFT is set, the return
2652 code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end
2653 of the subject is reached, there have been no complete matches, but
2654 there is still at least one matching possibility. The portion of the
2655 string that was inspected when the longest partial match was found is
2656 set as the first matching string in both cases.
2660 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
2661 stop as soon as it has found one match. Because of the way the alterna-
2662 tive algorithm works, this is necessarily the shortest possible match
2663 at the first possible matching point in the subject string.
2667 When pcre_dfa_exec() returns a partial match, it is possible to call it
2668 again, with additional subject characters, and have it continue with
2669 the same match. The PCRE_DFA_RESTART option requests this action; when
2670 it is set, the workspace and wscount options must reference the same
2671 vector as before because data about the match so far is left in them
2672 after a partial match. There is more discussion of this facility in the
2673 pcrepartial documentation.
2675 Successful returns from pcre_dfa_exec()
2677 When pcre_dfa_exec() succeeds, it may have matched more than one sub-
2678 string in the subject. Note, however, that all the matches from one run
2679 of the function start at the same point in the subject. The shorter
2680 matches are all initial substrings of the longer matches. For example,
2681 if the pattern
2683 <.*>
2685 is matched against the string
2687 This is <something> <something else> <something further> no more
2689 the three matched strings are
2691 <something>
2692 <something> <something else>
2693 <something> <something else> <something further>
2695 On success, the yield of the function is a number greater than zero,
2696 which is the number of matched substrings. The substrings themselves
2697 are returned in ovector. Each string uses two elements; the first is
2698 the offset to the start, and the second is the offset to the end. In
2699 fact, all the strings have the same start offset. (Space could have
2700 been saved by giving this only once, but it was decided to retain some
2701 compatibility with the way pcre_exec() returns data, even though the
2702 meaning of the strings is different.)
2704 The strings are returned in reverse order of length; that is, the long-
2705 est matching string is given first. If there were too many matches to
2706 fit into ovector, the yield of the function is zero, and the vector is
2707 filled with the longest matches.
2709 Error returns from pcre_dfa_exec()
2711 The pcre_dfa_exec() function returns a negative number when it fails.
2712 Many of the errors are the same as for pcre_exec(), and these are
2713 described above. There are in addition the following errors that are
2714 specific to pcre_dfa_exec():
2718 This return is given if pcre_dfa_exec() encounters an item in the pat-
2719 tern that it does not support, for instance, the use of \C or a back
2720 reference.
2724 This return is given if pcre_dfa_exec() encounters a condition item
2725 that uses a back reference for the condition, or a test for recursion
2726 in a specific group. These are not supported.
2730 This return is given if pcre_dfa_exec() is called with an extra block
2731 that contains a setting of the match_limit field. This is not supported
2732 (it is meaningless).
2736 This return is given if pcre_dfa_exec() runs out of space in the
2737 workspace vector.
2741 When a recursive subpattern is processed, the matching function calls
2742 itself recursively, using private vectors for ovector and workspace.
2743 This error is given if the output vector is not large enough. This
2744 should be extremely rare, as a vector of size 1000 is used.
2749 pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
2750 tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2755 Philip Hazel
2756 University Computing Service
2757 Cambridge CB2 3QH, England.
2762 Last updated: 15 June 2010
2763 Copyright (c) 1997-2010 University of Cambridge.
2764 ------------------------------------------------------------------------------
2770 NAME
2771 PCRE - Perl-compatible regular expressions
2776 int (*pcre_callout)(pcre_callout_block *);
2778 PCRE provides a feature called "callout", which is a means of temporar-
2779 ily passing control to the caller of PCRE in the middle of pattern
2780 matching. The caller of PCRE provides an external function by putting
2781 its entry point in the global variable pcre_callout. By default, this
2782 variable contains NULL, which disables all calling out.
2784 Within a regular expression, (?C) indicates the points at which the
2785 external function is to be called. Different callout points can be
2786 identified by putting a number less than 256 after the letter C. The
2787 default value is zero. For example, this pattern has two callout
2788 points:
2790 (?C1)abc(?C2)def
2792 If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() or
2793 pcre_compile2() is called, PCRE automatically inserts callouts, all
2794 with number 255, before each item in the pattern. For example, if
2795 PCRE_AUTO_CALLOUT is used with the pattern
2797 A(\d{2}|--)
2799 it is processed as if it were
2801 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2803 Notice that there is a callout before and after each parenthesis and
2804 alternation bar. Automatic callouts can be used for tracking the
2805 progress of pattern matching. The pcretest command has an option that
2806 sets automatic callouts; when it is used, the output indicates how the
2807 pattern is matched. This is useful information when you are trying to
2808 optimize the performance of a particular pattern.
2813 You should be aware that, because of optimizations in the way PCRE
2814 matches patterns by default, callouts sometimes do not happen. For
2815 example, if the pattern is
2817 ab(?C4)cd
2819 PCRE knows that any matching string must contain the letter "d". If the
2820 subject string is "abyz", the lack of "d" means that matching doesn't
2821 ever start, and the callout is never reached. However, with "abyd",
2822 though the result is still no match, the callout is obeyed.
2824 If the pattern is studied, PCRE knows the minimum length of a matching
2825 string, and will immediately give a "no match" return without actually
2826 running a match if the subject is not long enough, or, for unanchored
2827 patterns, if it has been scanned far enough.
2829 You can disable these optimizations by passing the PCRE_NO_START_OPTI-
2830 MIZE option to pcre_exec() or pcre_dfa_exec(). This slows down the
2831 matching process, but does ensure that callouts such as the example
2832 above are obeyed.
2837 During matching, when PCRE reaches a callout point, the external func-
2838 tion defined by pcre_callout is called (if it is set). This applies to
2839 both the pcre_exec() and the pcre_dfa_exec() matching functions. The
2840 only argument to the callout function is a pointer to a pcre_callout
2841 block. This structure contains the following fields:
2843 int version;
2844 int callout_number;
2845 int *offset_vector;
2846 const char *subject;
2847 int subject_length;
2848 int start_match;
2849 int current_position;
2850 int capture_top;
2851 int capture_last;
2852 void *callout_data;
2853 int pattern_position;
2854 int next_item_length;
2856 The version field is an integer containing the version number of the
2857 block format. The initial version was 0; the current version is 1. The
2858 version number will change again in future if additional fields are
2859 added, but the intention is never to remove any of the existing fields.
2861 The callout_number field contains the number of the callout, as com-
2862 piled into the pattern (that is, the number after ?C for manual call-
2863 outs, and 255 for automatically generated callouts).
2865 The offset_vector field is a pointer to the vector of offsets that was
2866 passed by the caller to pcre_exec() or pcre_dfa_exec(). When
2867 pcre_exec() is used, the contents can be inspected in order to extract
2868 substrings that have been matched so far, in the same way as for
2869 extracting substrings after a match has completed. For pcre_dfa_exec()
2870 this field is not useful.
2872 The subject and subject_length fields contain copies of the values that
2873 were passed to pcre_exec().
2875 The start_match field normally contains the offset within the subject
2876 at which the current match attempt started. However, if the escape
2877 sequence \K has been encountered, this value is changed to reflect the
2878 modified starting point. If the pattern is not anchored, the callout
2879 function may be called several times from the same point in the pattern
2880 for different starting points in the subject.
2882 The current_position field contains the offset within the subject of
2883 the current match pointer.
2885 When the pcre_exec() function is used, the capture_top field contains
2886 one more than the number of the highest numbered captured substring so
2887 far. If no substrings have been captured, the value of capture_top is
2888 one. This is always the case when pcre_dfa_exec() is used, because it
2889 does not support captured substrings.
2891 The capture_last field contains the number of the most recently cap-
2892 tured substring. If no substrings have been captured, its value is -1.
2893 This is always the case when pcre_dfa_exec() is used.
2895 The callout_data field contains a value that is passed to pcre_exec()
2896 or pcre_dfa_exec() specifically so that it can be passed back in call-
2897 outs. It is passed in the pcre_callout field of the pcre_extra data
2898 structure. If no such data was passed, the value of callout_data in a
2899 pcre_callout block is NULL. There is a description of the pcre_extra
2900 structure in the pcreapi documentation.
2902 The pattern_position field is present from version 1 of the pcre_call-
2903 out structure. It contains the offset to the next item to be matched in
2904 the pattern string.
2906 The next_item_length field is present from version 1 of the pcre_call-
2907 out structure. It contains the length of the next item to be matched in
2908 the pattern string. When the callout immediately precedes an alterna-
2909 tion bar, a closing parenthesis, or the end of the pattern, the length
2910 is zero. When the callout precedes an opening parenthesis, the length
2911 is that of the entire subpattern.
2913 The pattern_position and next_item_length fields are intended to help
2914 in distinguishing between different automatic callouts, which all have
2915 the same callout number. However, they are set for all callouts.
2920 The external callout function returns an integer to PCRE. If the value
2921 is zero, matching proceeds as normal. If the value is greater than
2922 zero, matching fails at the current point, but the testing of other
2923 matching possibilities goes ahead, just as if a lookahead assertion had
2924 failed. If the value is less than zero, the match is abandoned, and
2925 pcre_exec() or pcre_dfa_exec() returns the negative value.
2927 Negative values should normally be chosen from the set of
2928 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2929 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
2930 reserved for use by callout functions; it will never be used by PCRE
2931 itself.
2936 Philip Hazel
2937 University Computing Service
2938 Cambridge CB2 3QH, England.
2943 Last updated: 29 September 2009
2944 Copyright (c) 1997-2009 University of Cambridge.
2945 ------------------------------------------------------------------------------
2951 NAME
2952 PCRE - Perl-compatible regular expressions
2957 This document describes the differences in the ways that PCRE and Perl
2958 handle regular expressions. The differences described here are with
2959 respect to Perl 5.10/5.11.
2961 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
2962 of what it does have are given in the section on UTF-8 support in the
2963 main pcre page.
2965 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2966 permits them, but they do not mean what you might think. For example,
2967 (?!a){3} does not assert that the next three characters are not "a". It
2968 just asserts that the next character is not "a" three times.
2970 3. Capturing subpatterns that occur inside negative lookahead asser-
2971 tions are counted, but their entries in the offsets vector are never
2972 set. Perl sets its numerical variables from any such patterns that are
2973 matched before the assertion fails to match something (thereby succeed-
2974 ing), but only if the negative lookahead assertion contains just one
2975 branch.
2977 4. Though binary zero characters are supported in the subject string,
2978 they are not allowed in a pattern string because it is passed as a nor-
2979 mal C string, terminated by zero. The escape sequence \0 can be used in
2980 the pattern to represent a binary zero.
2982 5. The following Perl escape sequences are not supported: \l, \u, \L,
2983 \U, and \N. In fact these are implemented by Perl's general string-han-
2984 dling and are not part of its pattern matching engine. If any of these
2985 are encountered by PCRE, an error is generated.
2987 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
2988 is built with Unicode character property support. The properties that
2989 can be tested with \p and \P are limited to the general category prop-
2990 erties such as Lu and Nd, script names such as Greek or Han, and the
2991 derived properties Any and L&. PCRE does support the Cs (surrogate)
2992 property, which Perl does not; the Perl documentation says "Because
2993 Perl hides the need for the user to understand the internal representa-
2994 tion of Unicode characters, there is no need to implement the somewhat
2995 messy concept of surrogates."
2997 7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2998 ters in between are treated as literals. This is slightly different
2999 from Perl in that $ and @ are also handled as literals inside the
3000 quotes. In Perl, they cause variable interpolation (but of course PCRE
3001 does not have variables). Note the following examples:
3003 Pattern PCRE matches Perl matches
3005 \Qabc$xyz\E abc$xyz abc followed by the
3006 contents of $xyz
3007 \Qabc\$xyz\E abc\$xyz abc\$xyz
3008 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3010 The \Q...\E sequence is recognized both inside and outside character
3011 classes.
3013 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
3014 constructions. However, there is support for recursive patterns. This
3015 is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE
3016 "callout" feature allows an external function to be called during pat-
3017 tern matching. See the pcrecallout documentation for details.
3019 9. Subpatterns that are called recursively or as "subroutines" are
3020 always treated as atomic groups in PCRE. This is like Python, but
3021 unlike Perl. There is a discussion of an example that explains this in
3022 more detail in the section on recursion differences from Perl in the
3023 pcrepattern page.
3025 10. There are some differences that are concerned with the settings of
3026 captured strings when part of a pattern is repeated. For example,
3027 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
3028 unset, but in PCRE it is set to "b".
3030 11. PCRE's handling of duplicate subpattern numbers and duplicate sub-
3031 pattern names is not as general as Perl's. This is a consequence of the
3032 fact the PCRE works internally just with numbers, using an external ta-
3033 ble to translate between numbers and names. In particular, a pattern
3034 such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have
3035 the same number but different names, is not supported, and causes an
3036 error at compile time. If it were allowed, it would not be possible to
3037 distinguish which parentheses matched, because both names map to cap-
3038 turing subpattern number 1. To avoid this confusing situation, an error
3039 is given at compile time.
3041 12. PCRE provides some extensions to the Perl regular expression facil-
3042 ities. Perl 5.10 includes new features that are not in earlier ver-
3043 sions of Perl, some of which (such as named parentheses) have been in
3044 PCRE for some time. This list is with respect to Perl 5.10:
3046 (a) Although lookbehind assertions in PCRE must match fixed length
3047 strings, each alternative branch of a lookbehind assertion can match a
3048 different length of string. Perl requires them all to have the same
3049 length.
3051 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
3052 meta-character matches only at the very end of the string.
3054 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
3055 cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
3056 ignored. (Perl can be made to issue a warning.)
3058 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
3059 fiers is inverted, that is, by default they are not greedy, but if fol-
3060 lowed by a question mark they are.
3062 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
3063 tried only at the first matching position in the subject string.
3066 and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva-
3067 lents.
3069 (g) The \R escape sequence can be restricted to match only CR, LF, or
3070 CRLF by the PCRE_BSR_ANYCRLF option.
3072 (h) The callout facility is PCRE-specific.
3074 (i) The partial matching facility is PCRE-specific.
3076 (j) Patterns compiled by PCRE can be saved and re-used at a later time,
3077 even on different hosts that have the other endianness.
3079 (k) The alternative matching function (pcre_dfa_exec()) matches in a
3080 different way and is not Perl-compatible.
3082 (l) PCRE recognizes some special sequences such as (*CR) at the start
3083 of a pattern that set overall options that cannot be changed within the
3084 pattern.
3089 Philip Hazel
3090 University Computing Service
3091 Cambridge CB2 3QH, England.
3096 Last updated: 12 May 2010
3097 Copyright (c) 1997-2010 University of Cambridge.
3098 ------------------------------------------------------------------------------
3104 NAME
3105 PCRE - Perl-compatible regular expressions
3110 The syntax and semantics of the regular expressions that are supported
3111 by PCRE are described in detail below. There is a quick-reference syn-
3112 tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
3113 semantics as closely as it can. PCRE also supports some alternative
3114 regular expression syntax (which does not conflict with the Perl syn-
3115 tax) in order to provide some compatibility with regular expressions in
3116 Python, .NET, and Oniguruma.
3118 Perl's regular expressions are described in its own documentation, and
3119 regular expressions in general are covered in a number of books, some
3120 of which have copious examples. Jeffrey Friedl's "Mastering Regular
3121 Expressions", published by O'Reilly, covers regular expressions in
3122 great detail. This description of PCRE's regular expressions is
3123 intended as reference material.
3125 The original operation of PCRE was on strings of one-byte characters.
3126 However, there is now also support for UTF-8 character strings. To use
3127 this, PCRE must be built to include UTF-8 support, and you must call
3128 pcre_compile() or pcre_compile2() with the PCRE_UTF8 option. There is
3129 also a special sequence that can be given at the start of a pattern:
3131 (*UTF8)
3133 Starting a pattern with this sequence is equivalent to setting the
3134 PCRE_UTF8 option. This feature is not Perl-compatible. How setting
3135 UTF-8 mode affects pattern matching is mentioned in several places
3136 below. There is also a summary of UTF-8 features in the section on
3137 UTF-8 support in the main pcre page.
3139 Another special sequence that may appear at the start of a pattern or
3140 in combination with (*UTF8) is:
3142 (*UCP)
3144 This has the same effect as setting the PCRE_UCP option: it causes
3145 sequences such as \d and \w to use Unicode properties to determine
3146 character types, instead of recognizing only characters with codes less
3147 than 128 via a lookup table.
3149 The remainder of this document discusses the patterns that are sup-
3150 ported by PCRE when its main matching function, pcre_exec(), is used.
3151 From release 6.0, PCRE offers a second matching function,
3152 pcre_dfa_exec(), which matches using a different algorithm that is not
3153 Perl-compatible. Some of the features discussed below are not available
3154 when pcre_dfa_exec() is used. The advantages and disadvantages of the
3155 alternative function, and how it differs from the normal function, are
3156 discussed in the pcrematching page.
3161 PCRE supports five different conventions for indicating line breaks in
3162 strings: a single CR (carriage return) character, a single LF (line-
3163 feed) character, the two-character sequence CRLF, any of the three pre-
3164 ceding, or any Unicode newline sequence. The pcreapi page has further
3165 discussion about newlines, and shows how to set the newline convention
3166 in the options arguments for the compiling and matching functions.
3168 It is also possible to specify a newline convention by starting a pat-
3169 tern string with one of the following five sequences:
3171 (*CR) carriage return
3172 (*LF) linefeed
3173 (*CRLF) carriage return, followed by linefeed
3174 (*ANYCRLF) any of the three above
3175 (*ANY) all Unicode newline sequences
3177 These override the default and the options given to pcre_compile() or
3178 pcre_compile2(). For example, on a Unix system where LF is the default
3179 newline sequence, the pattern
3181 (*CR)a.b
3183 changes the convention to CR. That pattern matches "a\nb" because LF is
3184 no longer a newline. Note that these special settings, which are not
3185 Perl-compatible, are recognized only at the very start of a pattern,
3186 and that they must be in upper case. If more than one of them is
3187 present, the last one is used.
3189 The newline convention affects the interpretation of the dot metachar-
3190 acter when PCRE_DOTALL is not set, and also the behaviour of \N. How-
3191 ever, it does not affect what the \R escape sequence matches. By
3192 default, this is any Unicode newline sequence, for Perl compatibility.
3193 However, this can be changed; see the description of \R in the section
3194 entitled "Newline sequences" below. A change of \R setting can be com-
3195 bined with a change of newline convention.
3200 A regular expression is a pattern that is matched against a subject
3201 string from left to right. Most characters stand for themselves in a
3202 pattern, and match the corresponding characters in the subject. As a
3203 trivial example, the pattern
3205 The quick brown fox
3207 matches a portion of a subject string that is identical to itself. When
3208 caseless matching is specified (the PCRE_CASELESS option), letters are
3209 matched independently of case. In UTF-8 mode, PCRE always understands
3210 the concept of case for characters whose values are less than 128, so
3211 caseless matching is always possible. For characters with higher val-
3212 ues, the concept of case is supported if PCRE is compiled with Unicode
3213 property support, but not otherwise. If you want to use caseless
3214 matching for characters 128 and above, you must ensure that PCRE is
3215 compiled with Unicode property support as well as with UTF-8 support.
3217 The power of regular expressions comes from the ability to include
3218 alternatives and repetitions in the pattern. These are encoded in the
3219 pattern by the use of metacharacters, which do not stand for themselves
3220 but instead are interpreted in some special way.
3222 There are two different sets of metacharacters: those that are recog-
3223 nized anywhere in the pattern except within square brackets, and those
3224 that are recognized within square brackets. Outside square brackets,
3225 the metacharacters are as follows:
3227 \ general escape character with several uses
3228 ^ assert start of string (or line, in multiline mode)
3229 $ assert end of string (or line, in multiline mode)
3230 . match any character except newline (by default)
3231 [ start character class definition
3232 | start of alternative branch
3233 ( start subpattern
3234 ) end subpattern
3235 ? extends the meaning of (
3236 also 0 or 1 quantifier
3237 also quantifier minimizer
3238 * 0 or more quantifier
3239 + 1 or more quantifier
3240 also "possessive quantifier"
3241 { start min/max quantifier
3243 Part of a pattern that is in square brackets is called a "character
3244 class". In a character class the only metacharacters are:
3246 \ general escape character
3247 ^ negate the class, but only if the first character
3248 - indicates character range
3249 [ POSIX character class (only if followed by POSIX
3250 syntax)
3251 ] terminates the character class
3253 The following sections describe the use of each of the metacharacters.
3258 The backslash character has several uses. Firstly, if it is followed by
3259 a non-alphanumeric character, it takes away any special meaning that
3260 character may have. This use of backslash as an escape character
3261 applies both inside and outside character classes.
3263 For example, if you want to match a * character, you write \* in the
3264 pattern. This escaping action applies whether or not the following
3265 character would otherwise be interpreted as a metacharacter, so it is
3266 always safe to precede a non-alphanumeric with backslash to specify
3267 that it stands for itself. In particular, if you want to match a back-
3268 slash, you write \\.
3270 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
3271 the pattern (other than in a character class) and characters between a
3272 # outside a character class and the next newline are ignored. An escap-
3273 ing backslash can be used to include a whitespace or # character as
3274 part of the pattern.
3276 If you want to remove the special meaning from a sequence of charac-
3277 ters, you can do so by putting them between \Q and \E. This is differ-
3278 ent from Perl in that $ and @ are handled as literals in \Q...\E
3279 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
3280 tion. Note the following examples:
3282 Pattern PCRE matches Perl matches
3284 \Qabc$xyz\E abc$xyz abc followed by the
3285 contents of $xyz
3286 \Qabc\$xyz\E abc\$xyz abc\$xyz
3287 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3289 The \Q...\E sequence is recognized both inside and outside character
3290 classes.
3292 Non-printing characters
3294 A second use of backslash provides a way of encoding non-printing char-
3295 acters in patterns in a visible manner. There is no restriction on the
3296 appearance of non-printing characters, apart from the binary zero that
3297 terminates a pattern, but when a pattern is being prepared by text
3298 editing, it is often easier to use one of the following escape
3299 sequences than the binary character it represents:
3301 \a alarm, that is, the BEL character (hex 07)
3302 \cx "control-x", where x is any character
3303 \e escape (hex 1B)
3304 \f formfeed (hex 0C)
3305 \n linefeed (hex 0A)
3306 \r carriage return (hex 0D)
3307 \t tab (hex 09)
3308 \ddd character with octal code ddd, or back reference
3309 \xhh character with hex code hh
3310 \x{hhh..} character with hex code hhh..
3312 The precise effect of \cx is as follows: if x is a lower case letter,
3313 it is converted to upper case. Then bit 6 of the character (hex 40) is
3314 inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
3315 becomes hex 7B.
3317 After \x, from zero to two hexadecimal digits are read (letters can be
3318 in upper or lower case). Any number of hexadecimal digits may appear
3319 between \x{ and }, but the value of the character code must be less
3320 than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
3321 the maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger
3322 than the largest Unicode code point, which is 10FFFF.
3324 If characters other than hexadecimal digits appear between \x{ and },
3325 or if there is no terminating }, this form of escape is not recognized.
3326 Instead, the initial \x will be interpreted as a basic hexadecimal
3327 escape, with no following digits, giving a character whose value is
3328 zero.
3330 Characters whose value is less than 256 can be defined by either of the
3331 two syntaxes for \x. There is no difference in the way they are han-
3332 dled. For example, \xdc is exactly the same as \x{dc}.
3334 After \0 up to two further octal digits are read. If there are fewer
3335 than two digits, just those that are present are used. Thus the
3336 sequence \0\x\07 specifies two binary zeros followed by a BEL character
3337 (code value 7). Make sure you supply two digits after the initial zero
3338 if the pattern character that follows is itself an octal digit.
3340 The handling of a backslash followed by a digit other than 0 is compli-
3341 cated. Outside a character class, PCRE reads it and any following dig-
3342 its as a decimal number. If the number is less than 10, or if there
3343 have been at least that many previous capturing left parentheses in the
3344 expression, the entire sequence is taken as a back reference. A
3345 description of how this works is given later, following the discussion
3346 of parenthesized subpatterns.
3348 Inside a character class, or if the decimal number is greater than 9
3349 and there have not been that many capturing subpatterns, PCRE re-reads
3350 up to three octal digits following the backslash, and uses them to gen-
3351 erate a data character. Any subsequent digits stand for themselves. In
3352 non-UTF-8 mode, the value of a character specified in octal must be
3353 less than \400. In UTF-8 mode, values up to \777 are permitted. For
3354 example:
3356 \040 is another way of writing a space
3357 \40 is the same, provided there are fewer than 40
3358 previous capturing subpatterns
3359 \7 is always a back reference
3360 \11 might be a back reference, or another way of
3361 writing a tab
3362 \011 is always a tab
3363 \0113 is a tab followed by the character "3"
3364 \113 might be a back reference, otherwise the
3365 character with octal code 113
3366 \377 might be a back reference, otherwise
3367 the byte consisting entirely of 1 bits
3368 \81 is either a back reference, or a binary zero
3369 followed by the two characters "8" and "1"
3371 Note that octal values of 100 or greater must not be introduced by a
3372 leading zero, because no more than three octal digits are ever read.
3374 All the sequences that define a single character value can be used both
3375 inside and outside character classes. In addition, inside a character
3376 class, the sequence \b is interpreted as the backspace character (hex
3377 08). The sequences \B, \N, \R, and \X are not special inside a charac-
3378 ter class. Like any other unrecognized escape sequences, they are
3379 treated as the literal characters "B", "N", "R", and "X" by default,
3380 but cause an error if the PCRE_EXTRA option is set. Outside a character
3381 class, these sequences have different meanings.
3383 Absolute and relative back references
3385 The sequence \g followed by an unsigned or a negative number, option-
3386 ally enclosed in braces, is an absolute or relative back reference. A
3387 named back reference can be coded as \g{name}. Back references are dis-
3388 cussed later, following the discussion of parenthesized subpatterns.
3390 Absolute and relative subroutine calls
3392 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
3393 name or a number enclosed either in angle brackets or single quotes, is
3394 an alternative syntax for referencing a subpattern as a "subroutine".
3395 Details are discussed later. Note that \g{...} (Perl syntax) and
3396 \g<...> (Oniguruma syntax) are not synonymous. The former is a back
3397 reference; the latter is a subroutine call.
3399 Generic character types
3401 Another use of backslash is for specifying generic character types:
3403 \d any decimal digit
3404 \D any character that is not a decimal digit
3405 \h any horizontal whitespace character
3406 \H any character that is not a horizontal whitespace character
3407 \s any whitespace character
3408 \S any character that is not a whitespace character
3409 \v any vertical whitespace character
3410 \V any character that is not a vertical whitespace character
3411 \w any "word" character
3412 \W any "non-word" character
3414 There is also the single sequence \N, which matches a non-newline char-
3415 acter. This is the same as the "." metacharacter when PCRE_DOTALL is
3416 not set.
3418 Each pair of lower and upper case escape sequences partitions the com-
3419 plete set of characters into two disjoint sets. Any given character
3420 matches one, and only one, of each pair. The sequences can appear both
3421 inside and outside character classes. They each match one character of
3422 the appropriate type. If the current matching point is at the end of
3423 the subject string, all of them fail, because there is no character to
3424 match.
3426 For compatibility with Perl, \s does not match the VT character (code
3427 11). This makes it different from the the POSIX "space" class. The \s
3428 characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
3429 "use locale;" is included in a Perl script, \s may match the VT charac-
3430 ter. In PCRE, it never does.
3432 A "word" character is an underscore or any character that is a letter
3433 or digit. By default, the definition of letters and digits is con-
3434 trolled by PCRE's low-valued character tables, and may vary if locale-
3435 specific matching is taking place (see "Locale support" in the pcreapi
3436 page). For example, in a French locale such as "fr_FR" in Unix-like
3437 systems, or "french" in Windows, some character codes greater than 128
3438 are used for accented letters, and these are then matched by \w. The
3439 use of locales with Unicode is discouraged.
3441 By default, in UTF-8 mode, characters with values greater than 128
3442 never match \d, \s, or \w, and always match \D, \S, and \W. These
3443 sequences retain their original meanings from before UTF-8 support was
3444 available, mainly for efficiency reasons. However, if PCRE is compiled
3445 with Unicode property support, and the PCRE_UCP option is set, the be-
3446 haviour is changed so that Unicode properties are used to determine
3447 character types, as follows:
3449 \d any character that \p{Nd} matches (decimal digit)
3450 \s any character that \p{Z} matches, plus HT, LF, FF, CR
3451 \w any character that \p{L} or \p{N} matches, plus underscore
3453 The upper case escapes match the inverse sets of characters. Note that
3454 \d matches only decimal digits, whereas \w matches any Unicode digit,
3455 as well as any Unicode letter, and underscore. Note also that PCRE_UCP
3456 affects \b, and \B because they are defined in terms of \w and \W.
3457 Matching these sequences is noticeably slower when PCRE_UCP is set.
3459 The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
3460 the other sequences, which match only ASCII characters by default,
3461 these always match certain high-valued codepoints in UTF-8 mode,
3462 whether or not PCRE_UCP is set. The horizontal space characters are:
3464 U+0009 Horizontal tab
3465 U+0020 Space
3466 U+00A0 Non-break space
3467 U+1680 Ogham space mark
3468 U+180E Mongolian vowel separator
3469 U+2000 En quad
3470 U+2001 Em quad
3471 U+2002 En space
3472 U+2003 Em space
3473 U+2004 Three-per-em space
3474 U+2005 Four-per-em space
3475 U+2006 Six-per-em space
3476 U+2007 Figure space
3477 U+2008 Punctuation space
3478 U+2009 Thin space
3479 U+200A Hair space
3480 U+202F Narrow no-break space
3481 U+205F Medium mathematical space
3482 U+3000 Ideographic space
3484 The vertical space characters are:
3486 U+000A Linefeed
3487 U+000B Vertical tab
3488 U+000C Formfeed
3489 U+000D Carriage return
3490 U+0085 Next line
3491 U+2028 Line separator
3492 U+2029 Paragraph separator
3494 Newline sequences
3496 Outside a character class, by default, the escape sequence \R matches
3497 any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8
3498 mode \R is equivalent to the following:
3500 (?>\r\n|\n|\x0b|\f|\r|\x85)
3502 This is an example of an "atomic group", details of which are given
3503 below. This particular group matches either the two-character sequence
3504 CR followed by LF, or one of the single characters LF (linefeed,
3505 U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
3506 return, U+000D), or NEL (next line, U+0085). The two-character sequence
3507 is treated as a single unit that cannot be split.
3509 In UTF-8 mode, two additional characters whose codepoints are greater
3510 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
3511 rator, U+2029). Unicode character property support is not needed for
3512 these characters to be recognized.
3514 It is possible to restrict \R to match only CR, LF, or CRLF (instead of
3515 the complete set of Unicode line endings) by setting the option
3516 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
3517 (BSR is an abbrevation for "backslash R".) This can be made the default
3518 when PCRE is built; if this is the case, the other behaviour can be
3519 requested via the PCRE_BSR_UNICODE option. It is also possible to
3520 specify these settings by starting a pattern string with one of the
3521 following sequences:
3523 (*BSR_ANYCRLF) CR, LF, or CRLF only
3524 (*BSR_UNICODE) any Unicode newline sequence
3526 These override the default and the options given to pcre_compile() or
3527 pcre_compile2(), but they can be overridden by options given to
3528 pcre_exec() or pcre_dfa_exec(). Note that these special settings, which
3529 are not Perl-compatible, are recognized only at the very start of a
3530 pattern, and that they must be in upper case. If more than one of them
3531 is present, the last one is used. They can be combined with a change of
3532 newline convention; for example, a pattern can start with:
3536 They can also be combined with the (*UTF8) or (*UCP) special sequences.
3537 Inside a character class, \R is treated as an unrecognized escape
3538 sequence, and so matches the letter "R" by default, but causes an error
3539 if PCRE_EXTRA is set.
3541 Unicode character properties
3543 When PCRE is built with Unicode character property support, three addi-
3544 tional escape sequences that match characters with specific properties
3545 are available. When not in UTF-8 mode, these sequences are of course
3546 limited to testing characters whose codepoints are less than 256, but
3547 they do work in this mode. The extra escape sequences are:
3549 \p{xx} a character with the xx property
3550 \P{xx} a character without the xx property
3551 \X an extended Unicode sequence
3553 The property names represented by xx above are limited to the Unicode
3554 script names, the general category properties, "Any", which matches any
3555 character (including newline), and some special PCRE properties
3556 (described in the next section). Other Perl properties such as "InMu-
3557 sicalSymbols" are not currently supported by PCRE. Note that \P{Any}
3558 does not match any characters, so always causes a match failure.
3560 Sets of Unicode characters are defined as belonging to certain scripts.
3561 A character from one of these sets can be matched using a script name.
3562 For example:
3564 \p{Greek}
3565 \P{Han}
3567 Those that are not part of an identified script are lumped together as
3568 "Common". The current list of scripts is:
3570 Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
3571 Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common,
3572 Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp-
3573 tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek,
3574 Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe-
3575 rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
3576 Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
3577 Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam,
3578 Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
3579 Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya,
3580 Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian,
3581 Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le,
3582 Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh,
3583 Ugaritic, Vai, Yi.
3585 Each character has exactly one Unicode general category property, spec-
3586 ified by a two-letter abbreviation. For compatibility with Perl, nega-
3587 tion can be specified by including a circumflex between the opening
3588 brace and the property name. For example, \p{^Lu} is the same as
3589 \P{Lu}.
3591 If only one letter is specified with \p or \P, it includes all the gen-
3592 eral category properties that start with that letter. In this case, in
3593 the absence of negation, the curly brackets in the escape sequence are
3594 optional; these two examples have the same effect:
3596 \p{L}
3597 \pL
3599 The following general category property codes are supported:
3601 C Other
3602 Cc Control
3603 Cf Format
3604 Cn Unassigned
3605 Co Private use
3606 Cs Surrogate
3608 L Letter
3609 Ll Lower case letter
3610 Lm Modifier letter
3611 Lo Other letter
3612 Lt Title case letter
3613 Lu Upper case letter
3615 M Mark
3616 Mc Spacing mark
3617 Me Enclosing mark
3618 Mn Non-spacing mark
3620 N Number
3621 Nd Decimal number
3622 Nl Letter number
3623 No Other number
3625 P Punctuation
3626 Pc Connector punctuation
3627 Pd Dash punctuation
3628 Pe Close punctuation
3629 Pf Final punctuation
3630 Pi Initial punctuation
3631 Po Other punctuation
3632 Ps Open punctuation
3634 S Symbol
3635 Sc Currency symbol
3636 Sk Modifier symbol
3637 Sm Mathematical symbol
3638 So Other symbol
3640 Z Separator
3641 Zl Line separator
3642 Zp Paragraph separator
3643 Zs Space separator
3645 The special property L& is also supported: it matches a character that
3646 has the Lu, Ll, or Lt property, in other words, a letter that is not
3647 classified as a modifier or "other".
3649 The Cs (Surrogate) property applies only to characters in the range
3650 U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see
3651 RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
3652 ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in
3653 the pcreapi page). Perl does not support the Cs property.
3655 The long synonyms for property names that Perl supports (such as
3656 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
3657 any of these properties with "Is".
3659 No character that is in the Unicode table has the Cn (unassigned) prop-
3660 erty. Instead, this property is assumed for any code point that is not
3661 in the Unicode table.
3663 Specifying caseless matching does not affect these escape sequences.
3664 For example, \p{Lu} always matches only upper case letters.
3666 The \X escape matches any number of Unicode characters that form an
3667 extended Unicode sequence. \X is equivalent to
3669 (?>\PM\pM*)
3671 That is, it matches a character without the "mark" property, followed
3672 by zero or more characters with the "mark" property, and treats the
3673 sequence as an atomic group (see below). Characters with the "mark"
3674 property are typically accents that affect the preceding character.
3675 None of them have codepoints less than 256, so in non-UTF-8 mode \X
3676 matches any one character.
3678 Matching characters by Unicode property is not fast, because PCRE has
3679 to search a structure that contains data for over fifteen thousand
3680 characters. That is why the traditional escape sequences such as \d and
3681 \w do not use Unicode properties in PCRE by default, though you can
3682 make them do so by setting the PCRE_UCP option for pcre_compile() or by
3683 starting the pattern with (*UCP).
3685 PCRE's additional properties
3687 As well as the standard Unicode properties described in the previous
3688 section, PCRE supports four more that make it possible to convert tra-
3689 ditional escape sequences such as \w and \s and POSIX character classes
3690 to use Unicode properties. PCRE uses these non-standard, non-Perl prop-
3691 erties internally when PCRE_UCP is set. They are:
3693 Xan Any alphanumeric character
3694 Xps Any POSIX space character
3695 Xsp Any Perl space character
3696 Xwd Any Perl "word" character
3698 Xan matches characters that have either the L (letter) or the N (num-
3699 ber) property. Xps matches the characters tab, linefeed, vertical tab,
3700 formfeed, or carriage return, and any other character that has the Z
3701 (separator) property. Xsp is the same as Xps, except that vertical tab
3702 is excluded. Xwd matches the same characters as Xan, plus underscore.
3704 Resetting the match start
3706 The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
3707 ously matched characters not to be included in the final matched
3708 sequence. For example, the pattern:
3710 foo\Kbar
3712 matches "foobar", but reports that it has matched "bar". This feature
3713 is similar to a lookbehind assertion (described below). However, in
3714 this case, the part of the subject before the real match does not have
3715 to be of fixed length, as lookbehind assertions do. The use of \K does
3716 not interfere with the setting of captured substrings. For example,
3717 when the pattern
3719 (foo)\Kbar
3721 matches "foobar", the first substring is still set to "foo".
3723 Perl documents that the use of \K within assertions is "not well
3724 defined". In PCRE, \K is acted upon when it occurs inside positive
3725 assertions, but is ignored in negative assertions.
3727 Simple assertions
3729 The final use of backslash is for certain simple assertions. An asser-
3730 tion specifies a condition that has to be met at a particular point in
3731 a match, without consuming any characters from the subject string. The
3732 use of subpatterns for more complicated assertions is described below.
3733 The backslashed assertions are:
3735 \b matches at a word boundary
3736 \B matches when not at a word boundary
3737 \A matches at the start of the subject
3738 \Z matches at the end of the subject
3739 also matches before a newline at the end of the subject
3740 \z matches only at the end of the subject
3741 \G matches at the first matching position in the subject
3743 Inside a character class, \b has a different meaning; it matches the
3744 backspace character. If any other of these assertions appears in a
3745 character class, by default it matches the corresponding literal char-
3746 acter (for example, \B matches the letter B). However, if the
3747 PCRE_EXTRA option is set, an "invalid escape sequence" error is gener-
3748 ated instead.
3750 A word boundary is a position in the subject string where the current
3751 character and the previous character do not both match \w or \W (i.e.
3752 one matches \w and the other matches \W), or the start or end of the
3753 string if the first or last character matches \w, respectively. In
3754 UTF-8 mode, the meanings of \w and \W can be changed by setting the
3755 PCRE_UCP option. When this is done, it also affects \b and \B. Neither
3756 PCRE nor Perl has a separate "start of word" or "end of word" metase-
3757 quence. However, whatever follows \b normally determines which it is.
3758 For example, the fragment \ba matches "a" at the start of a word.
3760 The \A, \Z, and \z assertions differ from the traditional circumflex
3761 and dollar (described in the next section) in that they only ever match
3762 at the very start and end of the subject string, whatever options are
3763 set. Thus, they are independent of multiline mode. These three asser-
3764 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
3765 affect only the behaviour of the circumflex and dollar metacharacters.
3766 However, if the startoffset argument of pcre_exec() is non-zero, indi-
3767 cating that matching is to start at a point other than the beginning of
3768 the subject, \A can never match. The difference between \Z and \z is
3769 that \Z matches before a newline at the end of the string as well as at
3770 the very end, whereas \z matches only at the end.
3772 The \G assertion is true only when the current matching position is at
3773 the start point of the match, as specified by the startoffset argument
3774 of pcre_exec(). It differs from \A when the value of startoffset is
3775 non-zero. By calling pcre_exec() multiple times with appropriate argu-
3776 ments, you can mimic Perl's /g option, and it is in this kind of imple-
3777 mentation where \G can be useful.
3779 Note, however, that PCRE's interpretation of \G, as the start of the
3780 current match, is subtly different from Perl's, which defines it as the
3781 end of the previous match. In Perl, these can be different when the
3782 previously matched string was empty. Because PCRE does just one match
3783 at a time, it cannot reproduce this behaviour.
3785 If all the alternatives of a pattern begin with \G, the expression is
3786 anchored to the starting match position, and the "anchored" flag is set
3787 in the compiled regular expression.
3792 Outside a character class, in the default matching mode, the circumflex
3793 character is an assertion that is true only if the current matching
3794 point is at the start of the subject string. If the startoffset argu-
3795 ment of pcre_exec() is non-zero, circumflex can never match if the
3796 PCRE_MULTILINE option is unset. Inside a character class, circumflex
3797 has an entirely different meaning (see below).
3799 Circumflex need not be the first character of the pattern if a number
3800 of alternatives are involved, but it should be the first thing in each
3801 alternative in which it appears if the pattern is ever to match that
3802 branch. If all possible alternatives start with a circumflex, that is,
3803 if the pattern is constrained to match only at the start of the sub-
3804 ject, it is said to be an "anchored" pattern. (There are also other
3805 constructs that can cause a pattern to be anchored.)
3807 A dollar character is an assertion that is true only if the current
3808 matching point is at the end of the subject string, or immediately
3809 before a newline at the end of the string (by default). Dollar need not
3810 be the last character of the pattern if a number of alternatives are
3811 involved, but it should be the last item in any branch in which it
3812 appears. Dollar has no special meaning in a character class.
3814 The meaning of dollar can be changed so that it matches only at the
3815 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
3816 compile time. This does not affect the \Z assertion.
3818 The meanings of the circumflex and dollar characters are changed if the
3819 PCRE_MULTILINE option is set. When this is the case, a circumflex
3820 matches immediately after internal newlines as well as at the start of
3821 the subject string. It does not match after a newline that ends the
3822 string. A dollar matches before any newlines in the string, as well as
3823 at the very end, when PCRE_MULTILINE is set. When newline is specified
3824 as the two-character sequence CRLF, isolated CR and LF characters do
3825 not indicate newlines.
3827 For example, the pattern /^abc$/ matches the subject string "def\nabc"
3828 (where \n represents a newline) in multiline mode, but not otherwise.
3829 Consequently, patterns that are anchored in single line mode because
3830 all branches start with ^ are not anchored in multiline mode, and a
3831 match for circumflex is possible when the startoffset argument of
3832 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
3833 PCRE_MULTILINE is set.
3835 Note that the sequences \A, \Z, and \z can be used to match the start
3836 and end of the subject in both modes, and if all branches of a pattern
3837 start with \A it is always anchored, whether or not PCRE_MULTILINE is
3838 set.
3843 Outside a character class, a dot in the pattern matches any one charac-
3844 ter in the subject string except (by default) a character that signi-
3845 fies the end of a line. In UTF-8 mode, the matched character may be
3846 more than one byte long.
3848 When a line ending is defined as a single character, dot never matches
3849 that character; when the two-character sequence CRLF is used, dot does
3850 not match CR if it is immediately followed by LF, but otherwise it
3851 matches all characters (including isolated CRs and LFs). When any Uni-
3852 code line endings are being recognized, dot does not match CR or LF or
3853 any of the other line ending characters.
3855 The behaviour of dot with regard to newlines can be changed. If the
3856 PCRE_DOTALL option is set, a dot matches any one character, without
3857 exception. If the two-character sequence CRLF is present in the subject
3858 string, it takes two dots to match it.
3860 The handling of dot is entirely independent of the handling of circum-
3861 flex and dollar, the only relationship being that they both involve
3862 newlines. Dot has no special meaning in a character class.
3864 The escape sequence \N always behaves as a dot does when PCRE_DOTALL is
3865 not set. In other words, it matches any one character except one that
3866 signifies the end of a line.
3871 Outside a character class, the escape sequence \C matches any one byte,
3872 both in and out of UTF-8 mode. Unlike a dot, it always matches any
3873 line-ending characters. The feature is provided in Perl in order to
3874 match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
3875 acters into individual bytes, what remains in the string may be a mal-
3876 formed UTF-8 string. For this reason, the \C escape sequence is best
3877 avoided.
3879 PCRE does not allow \C to appear in lookbehind assertions (described
3880 below), because in UTF-8 mode this would make it impossible to calcu-
3881 late the length of the lookbehind.
3886 An opening square bracket introduces a character class, terminated by a
3887 closing square bracket. A closing square bracket on its own is not spe-
3888 cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
3889 a lone closing square bracket causes a compile-time error. If a closing
3890 square bracket is required as a member of the class, it should be the
3891 first data character in the class (after an initial circumflex, if
3892 present) or escaped with a backslash.
3894 A character class matches a single character in the subject. In UTF-8
3895 mode, the character may be more than one byte long. A matched character
3896 must be in the set of characters defined by the class, unless the first
3897 character in the class definition is a circumflex, in which case the
3898 subject character must not be in the set defined by the class. If a
3899 circumflex is actually required as a member of the class, ensure it is
3900 not the first character, or escape it with a backslash.
3902 For example, the character class [aeiou] matches any lower case vowel,
3903 while [^aeiou] matches any character that is not a lower case vowel.
3904 Note that a circumflex is just a convenient notation for specifying the
3905 characters that are in the class by enumerating those that are not. A
3906 class that starts with a circumflex is not an assertion; it still con-
3907 sumes a character from the subject string, and therefore it fails if
3908 the current pointer is at the end of the string.
3910 In UTF-8 mode, characters with values greater than 255 can be included
3911 in a class as a literal string of bytes, or by using the \x{ escaping
3912 mechanism.
3914 When caseless matching is set, any letters in a class represent both
3915 their upper case and lower case versions, so for example, a caseless
3916 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
3917 match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
3918 understands the concept of case for characters whose values are less
3919 than 128, so caseless matching is always possible. For characters with
3920 higher values, the concept of case is supported if PCRE is compiled
3921 with Unicode property support, but not otherwise. If you want to use
3922 caseless matching in UTF8-mode for characters 128 and above, you must
3923 ensure that PCRE is compiled with Unicode property support as well as
3924 with UTF-8 support.
3926 Characters that might indicate line breaks are never treated in any
3927 special way when matching character classes, whatever line-ending
3928 sequence is in use, and whatever setting of the PCRE_DOTALL and
3929 PCRE_MULTILINE options is used. A class such as [^a] always matches one
3930 of these characters.
3932 The minus (hyphen) character can be used to specify a range of charac-
3933 ters in a character class. For example, [d-m] matches any letter
3934 between d and m, inclusive. If a minus character is required in a
3935 class, it must be escaped with a backslash or appear in a position
3936 where it cannot be interpreted as indicating a range, typically as the
3937 first or last character in the class.
3939 It is not possible to have the literal character "]" as the end charac-
3940 ter of a range. A pattern such as [W-]46] is interpreted as a class of
3941 two characters ("W" and "-") followed by a literal string "46]", so it
3942 would match "W46]" or "-46]". However, if the "]" is escaped with a
3943 backslash it is interpreted as the end of range, so [W-\]46] is inter-
3944 preted as a class containing a range followed by two other characters.
3945 The octal or hexadecimal representation of "]" can also be used to end
3946 a range.
3948 Ranges operate in the collating sequence of character values. They can
3949 also be used for characters specified numerically, for example
3950 [\000-\037]. In UTF-8 mode, ranges can include characters whose values
3951 are greater than 255, for example [\x{100}-\x{2ff}].
3953 If a range that includes letters is used when caseless matching is set,
3954 it matches the letters in either case. For example, [W-c] is equivalent
3955 to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
3956 character tables for a French locale are in use, [\xc8-\xcb] matches
3957 accented E characters in both cases. In UTF-8 mode, PCRE supports the
3958 concept of case for characters with values greater than 128 only when
3959 it is compiled with Unicode property support.
3961 The character types \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, \w, and \W
3962 may also appear in a character class, and add the characters that they
3963 match to the class. For example, [\dABCDEF] matches any hexadecimal
3964 digit. A circumflex can conveniently be used with the upper case char-
3965 acter types to specify a more restricted set of characters than the
3966 matching lower case type. For example, the class [^\W_] matches any
3967 letter or digit, but not underscore.
3969 The only metacharacters that are recognized in character classes are
3970 backslash, hyphen (only where it can be interpreted as specifying a
3971 range), circumflex (only at the start), opening square bracket (only
3972 when it can be interpreted as introducing a POSIX class name - see the
3973 next section), and the terminating closing square bracket. However,
3974 escaping other non-alphanumeric characters does no harm.
3979 Perl supports the POSIX notation for character classes. This uses names
3980 enclosed by [: and :] within the enclosing square brackets. PCRE also
3981 supports this notation. For example,
3983 [01[:alpha:]%]
3985 matches "0", "1", any alphabetic character, or "%". The supported class
3986 names are:
3988 alnum letters and digits
3989 alpha letters
3990 ascii character codes 0 - 127
3991 blank space or tab only
3992 cntrl control characters
3993 digit decimal digits (same as \d)
3994 graph printing characters, excluding space
3995 lower lower case letters
3996 print printing characters, including space
3997 punct printing characters, excluding letters and digits and space
3998 space white space (not quite the same as \s)
3999 upper upper case letters
4000 word "word" characters (same as \w)
4001 xdigit hexadecimal digits
4003 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
4004 and space (32). Notice that this list includes the VT character (code
4005 11). This makes "space" different to \s, which does not include VT (for
4006 Perl compatibility).
4008 The name "word" is a Perl extension, and "blank" is a GNU extension
4009 from Perl 5.8. Another Perl extension is negation, which is indicated
4010 by a ^ character after the colon. For example,
4012 [12[:^digit:]]
4014 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
4015 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
4016 these are not supported, and an error is given if they are encountered.
4018 By default, in UTF-8 mode, characters with values greater than 128 do
4019 not match any of the POSIX character classes. However, if the PCRE_UCP
4020 option is passed to pcre_compile(), some of the classes are changed so
4021 that Unicode character properties are used. This is achieved by replac-
4022 ing the POSIX classes by other sequences, as follows:
4024 [:alnum:] becomes \p{Xan}
4025 [:alpha:] becomes \p{L}
4026 [:blank:] becomes \h
4027 [:digit:] becomes \p{Nd}
4028 [:lower:] becomes \p{Ll}
4029 [:space:] becomes \p{Xps}
4030 [:upper:] becomes \p{Lu}
4031 [:word:] becomes \p{Xwd}
4033 Negated versions, such as [:^alpha:] use \P instead of \p. The other
4034 POSIX classes are unchanged, and match only characters with code points
4035 less than 128.
4040 Vertical bar characters are used to separate alternative patterns. For
4041 example, the pattern
4043 gilbert|sullivan
4045 matches either "gilbert" or "sullivan". Any number of alternatives may
4046 appear, and an empty alternative is permitted (matching the empty
4047 string). The matching process tries each alternative in turn, from left
4048 to right, and the first one that succeeds is used. If the alternatives
4049 are within a subpattern (defined below), "succeeds" means matching the
4050 rest of the main pattern as well as the alternative in the subpattern.
4055 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
4056 PCRE_EXTENDED options (which are Perl-compatible) can be changed from
4057 within the pattern by a sequence of Perl option letters enclosed
4058 between "(?" and ")". The option letters are
4060 i for PCRE_CASELESS
4062 s for PCRE_DOTALL
4063 x for PCRE_EXTENDED
4065 For example, (?im) sets caseless, multiline matching. It is also possi-
4066 ble to unset these options by preceding the letter with a hyphen, and a
4067 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
4069 is also permitted. If a letter appears both before and after the
4070 hyphen, the option is unset.
4073 can be changed in the same way as the Perl-compatible options by using
4074 the characters J, U and X respectively.
4076 When one of these option changes occurs at top level (that is, not
4077 inside subpattern parentheses), the change applies to the remainder of
4078 the pattern that follows. If the change is placed right at the start of
4079 a pattern, PCRE extracts it into the global options (and it will there-
4080 fore show up in data extracted by the pcre_fullinfo() function).
4082 An option change within a subpattern (see below for a description of
4083 subpatterns) affects only that part of the current pattern that follows
4084 it, so
4086 (a(?i)b)c
4088 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
4089 used). By this means, options can be made to have different settings
4090 in different parts of the pattern. Any changes made in one alternative
4091 do carry on into subsequent branches within the same subpattern. For
4092 example,
4094 (a(?i)b|c)
4096 matches "ab", "aB", "c", and "C", even though when matching "C" the
4097 first branch is abandoned before the option setting. This is because
4098 the effects of option settings happen at compile time. There would be
4099 some very weird behaviour otherwise.
4101 Note: There are other PCRE-specific options that can be set by the
4102 application when the compile or match functions are called. In some
4103 cases the pattern can contain special leading sequences such as (*CRLF)
4104 to override what the application has set or what has been defaulted.
4105 Details are given in the section entitled "Newline sequences" above.
4106 There are also the (*UTF8) and (*UCP) leading sequences that can be
4107 used to set UTF-8 and Unicode property modes; they are equivalent to
4108 setting the PCRE_UTF8 and the PCRE_UCP options, respectively.
4113 Subpatterns are delimited by parentheses (round brackets), which can be
4114 nested. Turning part of a pattern into a subpattern does two things:
4116 1. It localizes a set of alternatives. For example, the pattern
4118 cat(aract|erpillar|)
4120 matches one of the words "cat", "cataract", or "caterpillar". Without
4121 the parentheses, it would match "cataract", "erpillar" or an empty
4122 string.
4124 2. It sets up the subpattern as a capturing subpattern. This means
4125 that, when the whole pattern matches, that portion of the subject
4126 string that matched the subpattern is passed back to the caller via the
4127 ovector argument of pcre_exec(). Opening parentheses are counted from
4128 left to right (starting from 1) to obtain numbers for the capturing
4129 subpatterns.
4131 For example, if the string "the red king" is matched against the pat-
4132 tern
4134 the ((red|white) (king|queen))
4136 the captured substrings are "red king", "red", and "king", and are num-
4137 bered 1, 2, and 3, respectively.
4139 The fact that plain parentheses fulfil two functions is not always
4140 helpful. There are often times when a grouping subpattern is required
4141 without a capturing requirement. If an opening parenthesis is followed
4142 by a question mark and a colon, the subpattern does not do any captur-
4143 ing, and is not counted when computing the number of any subsequent
4144 capturing subpatterns. For example, if the string "the white queen" is
4145 matched against the pattern
4147 the ((?:red|white) (king|queen))
4149 the captured substrings are "white queen" and "queen", and are numbered
4150 1 and 2. The maximum number of capturing subpatterns is 65535.
4152 As a convenient shorthand, if any option settings are required at the
4153 start of a non-capturing subpattern, the option letters may appear
4154 between the "?" and the ":". Thus the two patterns
4156 (?i:saturday|sunday)
4157 (?:(?i)saturday|sunday)
4159 match exactly the same set of strings. Because alternative branches are
4160 tried from left to right, and options are not reset until the end of
4161 the subpattern is reached, an option setting in one branch does affect
4162 subsequent branches, so the above patterns match "SUNDAY" as well as
4163 "Saturday".
4168 Perl 5.10 introduced a feature whereby each alternative in a subpattern
4169 uses the same numbers for its capturing parentheses. Such a subpattern
4170 starts with (?| and is itself a non-capturing subpattern. For example,
4171 consider this pattern:
4173 (?|(Sat)ur|(Sun))day
4175 Because the two alternatives are inside a (?| group, both sets of cap-
4176 turing parentheses are numbered one. Thus, when the pattern matches,
4177 you can look at captured substring number one, whichever alternative
4178 matched. This construct is useful when you want to capture part, but
4179 not all, of one of a number of alternatives. Inside a (?| group, paren-
4180 theses are numbered as usual, but the number is reset at the start of
4181 each branch. The numbers of any capturing buffers that follow the sub-
4182 pattern start after the highest number used in any branch. The follow-
4183 ing example is taken from the Perl documentation. The numbers under-
4184 neath show in which buffer the captured content will be stored.
4186 # before ---------------branch-reset----------- after
4187 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
4188 # 1 2 2 3 2 3 4
4190 A back reference to a numbered subpattern uses the most recent value
4191 that is set for that number by any subpattern. The following pattern
4192 matches "abcabc" or "defdef":
4194 /(?|(abc)|(def))\1/
4196 In contrast, a recursive or "subroutine" call to a numbered subpattern
4197 always refers to the first one in the pattern with the given number.
4198 The following pattern matches "abcabc" or "defabc":
4200 /(?|(abc)|(def))(?1)/
4202 If a condition test for a subpattern's having matched refers to a non-
4203 unique number, the test is true if any of the subpatterns of that num-
4204 ber have matched.
4206 An alternative approach to using this "branch reset" feature is to use
4207 duplicate named subpatterns, as described in the next section.
4212 Identifying capturing parentheses by number is simple, but it can be
4213 very hard to keep track of the numbers in complicated regular expres-
4214 sions. Furthermore, if an expression is modified, the numbers may
4215 change. To help with this difficulty, PCRE supports the naming of sub-
4216 patterns. This feature was not added to Perl until release 5.10. Python
4217 had the feature earlier, and PCRE introduced it at release 4.0, using
4218 the Python syntax. PCRE now supports both the Perl and the Python syn-
4219 tax. Perl allows identically numbered subpatterns to have different
4220 names, but PCRE does not.
4222 In PCRE, a subpattern can be named in one of three ways: (?<name>...)
4223 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
4224 to capturing parentheses from other parts of the pattern, such as back
4225 references, recursion, and conditions, can be made by name as well as
4226 by number.
4228 Names consist of up to 32 alphanumeric characters and underscores.
4229 Named capturing parentheses are still allocated numbers as well as
4230 names, exactly as if the names were not present. The PCRE API provides
4231 function calls for extracting the name-to-number translation table from
4232 a compiled pattern. There is also a convenience function for extracting
4233 a captured substring by name.
4235 By default, a name must be unique within a pattern, but it is possible
4236 to relax this constraint by setting the PCRE_DUPNAMES option at compile
4237 time. (Duplicate names are also always permitted for subpatterns with
4238 the same number, set up as described in the previous section.) Dupli-
4239 cate names can be useful for patterns where only one instance of the
4240 named parentheses can match. Suppose you want to match the name of a
4241 weekday, either as a 3-letter abbreviation or as the full name, and in
4242 both cases you want to extract the abbreviation. This pattern (ignoring
4243 the line breaks) does the job:
4245 (?<DN>Mon|Fri|Sun)(?:day)?|
4246 (?<DN>Tue)(?:sday)?|
4247 (?<DN>Wed)(?:nesday)?|
4248 (?<DN>Thu)(?:rsday)?|
4249 (?<DN>Sat)(?:urday)?
4251 There are five capturing substrings, but only one is ever set after a
4252 match. (An alternative way of solving this problem is to use a "branch
4253 reset" subpattern, as described in the previous section.)
4255 The convenience function for extracting the data by name returns the
4256 substring for the first (and in this example, the only) subpattern of
4257 that name that matched. This saves searching to find which numbered
4258 subpattern it was.
4260 If you make a back reference to a non-unique named subpattern from
4261 elsewhere in the pattern, the one that corresponds to the first occur-
4262 rence of the name is used. In the absence of duplicate numbers (see the
4263 previous section) this is the one with the lowest number. If you use a
4264 named reference in a condition test (see the section about conditions
4265 below), either to check whether a subpattern has matched, or to check
4266 for recursion, all subpatterns with the same name are tested. If the
4267 condition is true for any one of them, the overall condition is true.
4268 This is the same behaviour as testing by number. For further details of
4269 the interfaces for handling named subpatterns, see the pcreapi documen-
4270 tation.
4272 Warning: You cannot use different names to distinguish between two sub-
4273 patterns with the same number because PCRE uses only the numbers when
4274 matching. For this reason, an error is given at compile time if differ-
4275 ent names are given to subpatterns with the same number. However, you
4276 can give the same name to subpatterns with the same number, even when
4277 PCRE_DUPNAMES is not set.
4282 Repetition is specified by quantifiers, which can follow any of the
4283 following items:
4285 a literal data character
4286 the dot metacharacter
4287 the \C escape sequence
4288 the \X escape sequence (in UTF-8 mode with Unicode properties)
4289 the \R escape sequence
4290 an escape such as \d that matches a single character
4291 a character class
4292 a back reference (see next section)
4293 a parenthesized subpattern (unless it is an assertion)
4294 a recursive or "subroutine" call to a subpattern
4296 The general repetition quantifier specifies a minimum and maximum num-
4297 ber of permitted matches, by giving the two numbers in curly brackets
4298 (braces), separated by a comma. The numbers must be less than 65536,
4299 and the first must be less than or equal to the second. For example:
4301 z{2,4}
4303 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
4304 special character. If the second number is omitted, but the comma is
4305 present, there is no upper limit; if the second number and the comma
4306 are both omitted, the quantifier specifies an exact number of required
4307 matches. Thus
4309 [aeiou]{3,}
4311 matches at least 3 successive vowels, but may match many more, while
4313 \d{8}
4315 matches exactly 8 digits. An opening curly bracket that appears in a
4316 position where a quantifier is not allowed, or one that does not match
4317 the syntax of a quantifier, is taken as a literal character. For exam-
4318 ple, {,6} is not a quantifier, but a literal string of four characters.
4320 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
4321 individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
4322 acters, each of which is represented by a two-byte sequence. Similarly,
4323 when Unicode property support is available, \X{3} matches three Unicode
4324 extended sequences, each of which may be several bytes long (and they
4325 may be of different lengths).
4327 The quantifier {0} is permitted, causing the expression to behave as if
4328 the previous item and the quantifier were not present. This may be use-
4329 ful for subpatterns that are referenced as subroutines from elsewhere
4330 in the pattern. Items other than subpatterns that have a {0} quantifier
4331 are omitted from the compiled pattern.
4333 For convenience, the three most common quantifiers have single-charac-
4334 ter abbreviations:
4336 * is equivalent to {0,}
4337 + is equivalent to {1,}
4338 ? is equivalent to {0,1}
4340 It is possible to construct infinite loops by following a subpattern
4341 that can match no characters with a quantifier that has no upper limit,
4342 for example:
4344 (a?)*
4346 Earlier versions of Perl and PCRE used to give an error at compile time
4347 for such patterns. However, because there are cases where this can be
4348 useful, such patterns are now accepted, but if any repetition of the
4349 subpattern does in fact match no characters, the loop is forcibly bro-
4350 ken.
4352 By default, the quantifiers are "greedy", that is, they match as much
4353 as possible (up to the maximum number of permitted times), without
4354 causing the rest of the pattern to fail. The classic example of where
4355 this gives problems is in trying to match comments in C programs. These
4356 appear between /* and */ and within the comment, individual * and /
4357 characters may appear. An attempt to match C comments by applying the
4358 pattern
4360 /\*.*\*/
4362 to the string
4364 /* first comment */ not comment /* second comment */
4366 fails, because it matches the entire string owing to the greediness of
4367 the .* item.
4369 However, if a quantifier is followed by a question mark, it ceases to
4370 be greedy, and instead matches the minimum number of times possible, so
4371 the pattern
4373 /\*.*?\*/
4375 does the right thing with the C comments. The meaning of the various
4376 quantifiers is not otherwise changed, just the preferred number of
4377 matches. Do not confuse this use of question mark with its use as a
4378 quantifier in its own right. Because it has two uses, it can sometimes
4379 appear doubled, as in
4381 \d??\d
4383 which matches one digit by preference, but can match two if that is the
4384 only way the rest of the pattern matches.
4386 If the PCRE_UNGREEDY option is set (an option that is not available in
4387 Perl), the quantifiers are not greedy by default, but individual ones
4388 can be made greedy by following them with a question mark. In other
4389 words, it inverts the default behaviour.
4391 When a parenthesized subpattern is quantified with a minimum repeat
4392 count that is greater than 1 or with a limited maximum, more memory is
4393 required for the compiled pattern, in proportion to the size of the
4394 minimum or maximum.
4396 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
4397 alent to Perl's /s) is set, thus allowing the dot to match newlines,
4398 the pattern is implicitly anchored, because whatever follows will be
4399 tried against every character position in the subject string, so there
4400 is no point in retrying the overall match at any position after the
4401 first. PCRE normally treats such a pattern as though it were preceded
4402 by \A.
4404 In cases where it is known that the subject string contains no new-
4405 lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
4406 mization, or alternatively using ^ to indicate anchoring explicitly.
4408 However, there is one situation where the optimization cannot be used.
4409 When .* is inside capturing parentheses that are the subject of a back
4410 reference elsewhere in the pattern, a match at the start may fail where
4411 a later one succeeds. Consider, for example:
4413 (.*)abc\1
4415 If the subject is "xyz123abc123" the match point is the fourth charac-
4416 ter. For this reason, such a pattern is not implicitly anchored.
4418 When a capturing subpattern is repeated, the value captured is the sub-
4419 string that matched the final iteration. For example, after
4421 (tweedle[dume]{3}\s*)+
4423 has matched "tweedledum tweedledee" the value of the captured substring
4424 is "tweedledee". However, if there are nested capturing subpatterns,
4425 the corresponding captured values may have been set in previous itera-
4426 tions. For example, after
4428 /(a|(b))+/
4430 matches "aba" the value of the second captured substring is "b".
4435 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
4436 repetition, failure of what follows normally causes the repeated item
4437 to be re-evaluated to see if a different number of repeats allows the
4438 rest of the pattern to match. Sometimes it is useful to prevent this,
4439 either to change the nature of the match, or to cause it fail earlier
4440 than it otherwise might, when the author of the pattern knows there is
4441 no point in carrying on.
4443 Consider, for example, the pattern \d+foo when applied to the subject
4444 line
4446 123456bar
4448 After matching all 6 digits and then failing to match "foo", the normal
4449 action of the matcher is to try again with only 5 digits matching the
4450 \d+ item, and then with 4, and so on, before ultimately failing.
4451 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
4452 the means for specifying that once a subpattern has matched, it is not
4453 to be re-evaluated in this way.
4455 If we use atomic grouping for the previous example, the matcher gives
4456 up immediately on failing to match "foo" the first time. The notation
4457 is a kind of special parenthesis, starting with (?> as in this example:
4459 (?>\d+)foo
4461 This kind of parenthesis "locks up" the part of the pattern it con-
4462 tains once it has matched, and a failure further into the pattern is
4463 prevented from backtracking into it. Backtracking past it to previous
4464 items, however, works as normal.
4466 An alternative description is that a subpattern of this type matches
4467 the string of characters that an identical standalone pattern would
4468 match, if anchored at the current point in the subject string.
4470 Atomic grouping subpatterns are not capturing subpatterns. Simple cases
4471 such as the above example can be thought of as a maximizing repeat that
4472 must swallow everything it can. So, while both \d+ and \d+? are pre-
4473 pared to adjust the number of digits they match in order to make the
4474 rest of the pattern match, (?>\d+) can only match an entire sequence of
4475 digits.
4477 Atomic groups in general can of course contain arbitrarily complicated
4478 subpatterns, and can be nested. However, when the subpattern for an
4479 atomic group is just a single repeated item, as in the example above, a
4480 simpler notation, called a "possessive quantifier" can be used. This
4481 consists of an additional + character following a quantifier. Using
4482 this notation, the previous example can be rewritten as
4484 \d++foo
4486 Note that a possessive quantifier can be used with an entire group, for
4487 example:
4489 (abc|xyz){2,3}+
4491 Possessive quantifiers are always greedy; the setting of the
4492 PCRE_UNGREEDY option is ignored. They are a convenient notation for the
4493 simpler forms of atomic group. However, there is no difference in the
4494 meaning of a possessive quantifier and the equivalent atomic group,
4495 though there may be a performance difference; possessive quantifiers
4496 should be slightly faster.
4498 The possessive quantifier syntax is an extension to the Perl 5.8 syn-
4499 tax. Jeffrey Friedl originated the idea (and the name) in the first
4500 edition of his book. Mike McCloskey liked it, so implemented it when he
4501 built Sun's Java package, and PCRE copied it from there. It ultimately
4502 found its way into Perl at release 5.10.
4504 PCRE has an optimization that automatically "possessifies" certain sim-
4505 ple pattern constructs. For example, the sequence A+B is treated as
4506 A++B because there is no point in backtracking into a sequence of A's
4507 when B must follow.
4509 When a pattern contains an unlimited repeat inside a subpattern that
4510 can itself be repeated an unlimited number of times, the use of an
4511 atomic group is the only way to avoid some failing matches taking a
4512 very long time indeed. The pattern
4514 (\D+|<\d+>)*[!?]
4516 matches an unlimited number of substrings that either consist of non-
4517 digits, or digits enclosed in <>, followed by either ! or ?. When it
4518 matches, it runs quickly. However, if it is applied to
4520 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
4522 it takes a long time before reporting failure. This is because the
4523 string can be divided between the internal \D+ repeat and the external
4524 * repeat in a large number of ways, and all have to be tried. (The
4525 example uses [!?] rather than a single character at the end, because
4526 both PCRE and Perl have an optimization that allows for fast failure
4527 when a single character is used. They remember the last single charac-
4528 ter that is required for a match, and fail early if it is not present
4529 in the string.) If the pattern is changed so that it uses an atomic
4530 group, like this:
4532 ((?>\D+)|<\d+>)*[!?]
4534 sequences of non-digits cannot be broken, and failure happens quickly.
4539 Outside a character class, a backslash followed by a digit greater than
4540 0 (and possibly further digits) is a back reference to a capturing sub-
4541 pattern earlier (that is, to its left) in the pattern, provided there
4542 have been that many previous capturing left parentheses.
4544 However, if the decimal number following the backslash is less than 10,
4545 it is always taken as a back reference, and causes an error only if
4546 there are not that many capturing left parentheses in the entire pat-
4547 tern. In other words, the parentheses that are referenced need not be
4548 to the left of the reference for numbers less than 10. A "forward back
4549 reference" of this type can make sense when a repetition is involved
4550 and the subpattern to the right has participated in an earlier itera-
4551 tion.
4553 It is not possible to have a numerical "forward back reference" to a
4554 subpattern whose number is 10 or more using this syntax because a
4555 sequence such as \50 is interpreted as a character defined in octal.
4556 See the subsection entitled "Non-printing characters" above for further
4557 details of the handling of digits following a backslash. There is no
4558 such problem when named parentheses are used. A back reference to any
4559 subpattern is possible using named parentheses (see below).
4561 Another way of avoiding the ambiguity inherent in the use of digits
4562 following a backslash is to use the \g escape sequence, which is a fea-
4563 ture introduced in Perl 5.10. This escape must be followed by an
4564 unsigned number or a negative number, optionally enclosed in braces.
4565 These examples are all identical:
4567 (ring), \1
4568 (ring), \g1
4569 (ring), \g{1}
4571 An unsigned number specifies an absolute reference without the ambigu-
4572 ity that is present in the older syntax. It is also useful when literal
4573 digits follow the reference. A negative number is a relative reference.
4574 Consider this example:
4576 (abc(def)ghi)\g{-1}
4578 The sequence \g{-1} is a reference to the most recently started captur-
4579 ing subpattern before \g, that is, is it equivalent to \2. Similarly,
4580 \g{-2} would be equivalent to \1. The use of relative references can be
4581 helpful in long patterns, and also in patterns that are created by
4582 joining together fragments that contain references within themselves.
4584 A back reference matches whatever actually matched the capturing sub-
4585 pattern in the current subject string, rather than anything matching
4586 the subpattern itself (see "Subpatterns as subroutines" below for a way
4587 of doing that). So the pattern
4589 (sens|respons)e and \1ibility
4591 matches "sense and sensibility" and "response and responsibility", but
4592 not "sense and responsibility". If caseful matching is in force at the
4593 time of the back reference, the case of letters is relevant. For exam-
4594 ple,
4596 ((?i)rah)\s+\1
4598 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
4599 original capturing subpattern is matched caselessly.
4601 There are several different ways of writing back references to named
4602 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
4603 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
4604 unified back reference syntax, in which \g can be used for both numeric
4605 and named references, is also supported. We could rewrite the above
4606 example in any of the following ways:
4608 (?<p1>(?i)rah)\s+\k<p1>
4609 (?'p1'(?i)rah)\s+\k{p1}
4610 (?P<p1>(?i)rah)\s+(?P=p1)
4611 (?<p1>(?i)rah)\s+\g{p1}
4613 A subpattern that is referenced by name may appear in the pattern
4614 before or after the reference.
4616 There may be more than one back reference to the same subpattern. If a
4617 subpattern has not actually been used in a particular match, any back
4618 references to it always fail by default. For example, the pattern
4620 (a|(bc))\2
4622 always fails if it starts to match "a" rather than "bc". However, if
4623 the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer-
4624 ence to an unset value matches an empty string.
4626 Because there may be many capturing parentheses in a pattern, all dig-
4627 its following a backslash are taken as part of a potential back refer-
4628 ence number. If the pattern continues with a digit character, some
4629 delimiter must be used to terminate the back reference. If the
4630 PCRE_EXTENDED option is set, this can be whitespace. Otherwise, the \g{
4631 syntax or an empty comment (see "Comments" below) can be used.
4633 Recursive back references
4635 A back reference that occurs inside the parentheses to which it refers
4636 fails when the subpattern is first used, so, for example, (a\1) never
4637 matches. However, such references can be useful inside repeated sub-
4638 patterns. For example, the pattern
4640 (a|b\1)+
4642 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
4643 ation of the subpattern, the back reference matches the character
4644 string corresponding to the previous iteration. In order for this to
4645 work, the pattern must be such that the first iteration does not need
4646 to match the back reference. This can be done using alternation, as in
4647 the example above, or by a quantifier with a minimum of zero.
4649 Back references of this type cause the group that they reference to be
4650 treated as an atomic group. Once the whole group has been matched, a
4651 subsequent matching failure cannot cause backtracking into the middle
4652 of the group.
4657 An assertion is a test on the characters following or preceding the
4658 current matching point that does not actually consume any characters.
4659 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
4660 described above.
4662 More complicated assertions are coded as subpatterns. There are two
4663 kinds: those that look ahead of the current position in the subject
4664 string, and those that look behind it. An assertion subpattern is
4665 matched in the normal way, except that it does not cause the current
4666 matching position to be changed.
4668 Assertion subpatterns are not capturing subpatterns, and may not be
4669 repeated, because it makes no sense to assert the same thing several
4670 times. If any kind of assertion contains capturing subpatterns within
4671 it, these are counted for the purposes of numbering the capturing sub-
4672 patterns in the whole pattern. However, substring capturing is carried
4673 out only for positive assertions, because it does not make sense for
4674 negative assertions.
4676 Lookahead assertions
4678 Lookahead assertions start with (?= for positive assertions and (?! for
4679 negative assertions. For example,
4681 \w+(?=;)
4683 matches a word followed by a semicolon, but does not include the semi-
4684 colon in the match, and
4686 foo(?!bar)
4688 matches any occurrence of "foo" that is not followed by "bar". Note
4689 that the apparently similar pattern
4691 (?!foo)bar
4693 does not find an occurrence of "bar" that is preceded by something
4694 other than "foo"; it finds any occurrence of "bar" whatsoever, because
4695 the assertion (?!foo) is always true when the next three characters are
4696 "bar". A lookbehind assertion is needed to achieve the other effect.
4698 If you want to force a matching failure at some point in a pattern, the
4699 most convenient way to do it is with (?!) because an empty string
4700 always matches, so an assertion that requires there not to be an empty
4701 string must always fail. The Perl 5.10 backtracking control verb
4702 (*FAIL) or (*F) is essentially a synonym for (?!).
4704 Lookbehind assertions
4706 Lookbehind assertions start with (?<= for positive assertions and (?<!
4707 for negative assertions. For example,
4709 (?<!foo)bar
4711 does find an occurrence of "bar" that is not preceded by "foo". The
4712 contents of a lookbehind assertion are restricted such that all the
4713 strings it matches must have a fixed length. However, if there are sev-
4714 eral top-level alternatives, they do not all have to have the same
4715 fixed length. Thus
4717 (?<=bullock|donkey)
4719 is permitted, but
4721 (?<!dogs?|cats?)
4723 causes an error at compile time. Branches that match different length
4724 strings are permitted only at the top level of a lookbehind assertion.
4725 This is an extension compared with Perl (5.8 and 5.10), which requires
4726 all branches to match the same length of string. An assertion such as
4728 (?<=ab(c|de))
4730 is not permitted, because its single top-level branch can match two
4731 different lengths, but it is acceptable to PCRE if rewritten to use two
4732 top-level branches:
4734 (?<=abc|abde)
4736 In some cases, the Perl 5.10 escape sequence \K (see above) can be used
4737 instead of a lookbehind assertion to get round the fixed-length
4738 restriction.
4740 The implementation of lookbehind assertions is, for each alternative,
4741 to temporarily move the current position back by the fixed length and
4742 then try to match. If there are insufficient characters before the cur-
4743 rent position, the assertion fails.
4745 PCRE does not allow the \C escape (which matches a single byte in UTF-8
4746 mode) to appear in lookbehind assertions, because it makes it impossi-
4747 ble to calculate the length of the lookbehind. The \X and \R escapes,
4748 which can match different numbers of bytes, are also not permitted.
4750 "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
4751 lookbehinds, as long as the subpattern matches a fixed-length string.
4752 Recursion, however, is not supported.
4754 Possessive quantifiers can be used in conjunction with lookbehind
4755 assertions to specify efficient matching of fixed-length strings at the
4756 end of subject strings. Consider a simple pattern such as
4758 abcd$
4760 when applied to a long string that does not match. Because matching
4761 proceeds from left to right, PCRE will look for each "a" in the subject
4762 and then see if what follows matches the rest of the pattern. If the
4763 pattern is specified as
4765 ^.*abcd$
4767 the initial .* matches the entire string at first, but when this fails
4768 (because there is no following "a"), it backtracks to match all but the
4769 last character, then all but the last two characters, and so on. Once
4770 again the search for "a" covers the entire string, from right to left,
4771 so we are no better off. However, if the pattern is written as
4773 ^.*+(?<=abcd)
4775 there can be no backtracking for the .*+ item; it can match only the
4776 entire string. The subsequent lookbehind assertion does a single test
4777 on the last four characters. If it fails, the match fails immediately.
4778 For long strings, this approach makes a significant difference to the
4779 processing time.
4781 Using multiple assertions
4783 Several assertions (of any sort) may occur in succession. For example,
4785 (?<=\d{3})(?<!999)foo
4787 matches "foo" preceded by three digits that are not "999". Notice that
4788 each of the assertions is applied independently at the same point in
4789 the subject string. First there is a check that the previous three
4790 characters are all digits, and then there is a check that the same
4791 three characters are not "999". This pattern does not match "foo" pre-
4792 ceded by six characters, the first of which are digits and the last
4793 three of which are not "999". For example, it doesn't match "123abc-
4794 foo". A pattern to do that is
4796 (?<=\d{3}...)(?<!999)foo
4798 This time the first assertion looks at the preceding six characters,
4799 checking that the first three are digits, and then the second assertion
4800 checks that the preceding three characters are not "999".
4802 Assertions can be nested in any combination. For example,
4804 (?<=(?<!foo)bar)baz
4806 matches an occurrence of "baz" that is preceded by "bar" which in turn
4807 is not preceded by "foo", while
4809 (?<=\d{3}(?!999)...)foo
4811 is another pattern that matches "foo" preceded by three digits and any
4812 three characters that are not "999".
4817 It is possible to cause the matching process to obey a subpattern con-
4818 ditionally or to choose between two alternative subpatterns, depending
4819 on the result of an assertion, or whether a specific capturing subpat-
4820 tern has already been matched. The two possible forms of conditional
4821 subpattern are:
4823 (?(condition)yes-pattern)
4824 (?(condition)yes-pattern|no-pattern)
4826 If the condition is satisfied, the yes-pattern is used; otherwise the
4827 no-pattern (if present) is used. If there are more than two alterna-
4828 tives in the subpattern, a compile-time error occurs.
4830 There are four kinds of condition: references to subpatterns, refer-
4831 ences to recursion, a pseudo-condition called DEFINE, and assertions.
4833 Checking for a used subpattern by number
4835 If the text between the parentheses consists of a sequence of digits,
4836 the condition is true if a capturing subpattern of that number has pre-
4837 viously matched. If there is more than one capturing subpattern with
4838 the same number (see the earlier section about duplicate subpattern
4839 numbers), the condition is true if any of them have been set. An alter-
4840 native notation is to precede the digits with a plus or minus sign. In
4841 this case, the subpattern number is relative rather than absolute. The
4842 most recently opened parentheses can be referenced by (?(-1), the next
4843 most recent by (?(-2), and so on. In looping constructs it can also
4844 make sense to refer to subsequent groups with constructs such as
4845 (?(+2).
4847 Consider the following pattern, which contains non-significant white
4848 space to make it more readable (assume the PCRE_EXTENDED option) and to
4849 divide it into three parts for ease of discussion:
4851 ( \( )? [^()]+ (?(1) \) )
4853 The first part matches an optional opening parenthesis, and if that
4854 character is present, sets it as the first captured substring. The sec-
4855 ond part matches one or more characters that are not parentheses. The
4856 third part is a conditional subpattern that tests whether the first set
4857 of parentheses matched or not. If they did, that is, if subject started
4858 with an opening parenthesis, the condition is true, and so the yes-pat-
4859 tern is executed and a closing parenthesis is required. Otherwise,
4860 since no-pattern is not present, the subpattern matches nothing. In
4861 other words, this pattern matches a sequence of non-parentheses,
4862 optionally enclosed in parentheses.
4864 If you were embedding this pattern in a larger one, you could use a
4865 relative reference:
4867 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
4869 This makes the fragment independent of the parentheses in the larger
4870 pattern.
4872 Checking for a used subpattern by name
4874 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
4875 used subpattern by name. For compatibility with earlier versions of
4876 PCRE, which had this facility before Perl, the syntax (?(name)...) is
4877 also recognized. However, there is a possible ambiguity with this syn-
4878 tax, because subpattern names may consist entirely of digits. PCRE
4879 looks first for a named subpattern; if it cannot find one and the name
4880 consists entirely of digits, PCRE looks for a subpattern of that num-
4881 ber, which must be greater than zero. Using subpattern names that con-
4882 sist entirely of digits is not recommended.
4884 Rewriting the above example to use a named subpattern gives this:
4886 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
4888 If the name used in a condition of this kind is a duplicate, the test
4889 is applied to all subpatterns of the same name, and is true if any one
4890 of them has matched.
4892 Checking for pattern recursion
4894 If the condition is the string (R), and there is no subpattern with the
4895 name R, the condition is true if a recursive call to the whole pattern
4896 or any subpattern has been made. If digits or a name preceded by amper-
4897 sand follow the letter R, for example:
4899 (?(R3)...) or (?(R&name)...)
4901 the condition is true if the most recent recursion is into a subpattern
4902 whose number or name is given. This condition does not check the entire
4903 recursion stack. If the name used in a condition of this kind is a
4904 duplicate, the test is applied to all subpatterns of the same name, and
4905 is true if any one of them is the most recent recursion.
4907 At "top level", all these recursion test conditions are false. The
4908 syntax for recursive patterns is described below.
4910 Defining subpatterns for use by reference only
4912 If the condition is the string (DEFINE), and there is no subpattern
4913 with the name DEFINE, the condition is always false. In this case,
4914 there may be only one alternative in the subpattern. It is always
4915 skipped if control reaches this point in the pattern; the idea of
4916 DEFINE is that it can be used to define "subroutines" that can be ref-
4917 erenced from elsewhere. (The use of "subroutines" is described below.)
4918 For example, a pattern to match an IPv4 address could be written like
4919 this (ignore whitespace and line breaks):
4921 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
4922 \b (?&byte) (\.(?&byte)){3} \b
4924 The first part of the pattern is a DEFINE group inside which a another
4925 group named "byte" is defined. This matches an individual component of
4926 an IPv4 address (a number less than 256). When matching takes place,
4927 this part of the pattern is skipped because DEFINE acts like a false
4928 condition. The rest of the pattern uses references to the named group
4929 to match the four dot-separated components of an IPv4 address, insist-
4930 ing on a word boundary at each end.
4932 Assertion conditions
4934 If the condition is not in any of the above formats, it must be an
4935 assertion. This may be a positive or negative lookahead or lookbehind
4936 assertion. Consider this pattern, again containing non-significant
4937 white space, and with the two alternatives on the second line:
4939 (?(?=[^a-z]*[a-z])
4940 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
4942 The condition is a positive lookahead assertion that matches an
4943 optional sequence of non-letters followed by a letter. In other words,
4944 it tests for the presence of at least one letter in the subject. If a
4945 letter is found, the subject is matched against the first alternative;
4946 otherwise it is matched against the second. This pattern matches
4947 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
4948 letters and dd are digits.
4953 The sequence (?# marks the start of a comment that continues up to the
4954 next closing parenthesis. Nested parentheses are not permitted. The
4955 characters that make up a comment play no part in the pattern matching
4956 at all.
4958 If the PCRE_EXTENDED option is set, an unescaped # character outside a
4959 character class introduces a comment that continues to immediately
4960 after the next newline in the pattern.
4965 Consider the problem of matching a string in parentheses, allowing for
4966 unlimited nested parentheses. Without the use of recursion, the best
4967 that can be done is to use a pattern that matches up to some fixed
4968 depth of nesting. It is not possible to handle an arbitrary nesting
4969 depth.
4971 For some time, Perl has provided a facility that allows regular expres-
4972 sions to recurse (amongst other things). It does this by interpolating
4973 Perl code in the expression at run time, and the code can refer to the
4974 expression itself. A Perl pattern using code interpolation to solve the
4975 parentheses problem can be created like this:
4977 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
4979 The (?p{...}) item interpolates Perl code at run time, and in this case
4980 refers recursively to the pattern in which it appears.
4982 Obviously, PCRE cannot support the interpolation of Perl code. Instead,
4983 it supports special syntax for recursion of the entire pattern, and
4984 also for individual subpattern recursion. After its introduction in
4985 PCRE and Python, this kind of recursion was subsequently introduced
4986 into Perl at release 5.10.
4988 A special item that consists of (? followed by a number greater than
4989 zero and a closing parenthesis is a recursive call of the subpattern of
4990 the given number, provided that it occurs inside that subpattern. (If
4991 not, it is a "subroutine" call, which is described in the next sec-
4992 tion.) The special item (?R) or (?0) is a recursive call of the entire
4993 regular expression.
4995 This PCRE pattern solves the nested parentheses problem (assume the
4996 PCRE_EXTENDED option is set so that white space is ignored):
4998 \( ( [^()]++ | (?R) )* \)
5000 First it matches an opening parenthesis. Then it matches any number of
5001 substrings which can either be a sequence of non-parentheses, or a
5002 recursive match of the pattern itself (that is, a correctly parenthe-
5003 sized substring). Finally there is a closing parenthesis. Note the use
5004 of a possessive quantifier to avoid backtracking into sequences of non-
5005 parentheses.
5007 If this were part of a larger pattern, you would not want to recurse
5008 the entire pattern, so instead you could use this:
5010 ( \( ( [^()]++ | (?1) )* \) )
5012 We have put the pattern into parentheses, and caused the recursion to
5013 refer to them instead of the whole pattern.
5015 In a larger pattern, keeping track of parenthesis numbers can be
5016 tricky. This is made easier by the use of relative references (a Perl
5017 5.10 feature). Instead of (?1) in the pattern above you can write
5018 (?-2) to refer to the second most recently opened parentheses preceding
5019 the recursion. In other words, a negative number counts capturing
5020 parentheses leftwards from the point at which it is encountered.
5022 It is also possible to refer to subsequently opened parentheses, by
5023 writing references such as (?+2). However, these cannot be recursive
5024 because the reference is not inside the parentheses that are refer-
5025 enced. They are always "subroutine" calls, as described in the next
5026 section.
5028 An alternative approach is to use named parentheses instead. The Perl
5029 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
5030 supported. We could rewrite the above example as follows:
5032 (?<pn> \( ( [^()]++ | (?&pn) )* \) )
5034 If there is more than one subpattern with the same name, the earliest
5035 one is used.
5037 This particular example pattern that we have been looking at contains
5038 nested unlimited repeats, and so the use of a possessive quantifier for
5039 matching strings of non-parentheses is important when applying the pat-
5040 tern to strings that do not match. For example, when this pattern is
5041 applied to
5043 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
5045 it yields "no match" quickly. However, if a possessive quantifier is
5046 not used, the match runs for a very long time indeed because there are
5047 so many different ways the + and * repeats can carve up the subject,
5048 and all have to be tested before failure can be reported.
5050 At the end of a match, the values of capturing parentheses are those
5051 from the outermost level. If you want to obtain intermediate values, a
5052 callout function can be used (see below and the pcrecallout documenta-
5053 tion). If the pattern above is matched against
5055 (ab(cd)ef)
5057 the value for the inner capturing parentheses (numbered 2) is "ef",
5058 which is the last value taken on at the top level. If a capturing sub-
5059 pattern is not matched at the top level, its final value is unset, even
5060 if it is (temporarily) set at a deeper level.
5062 If there are more than 15 capturing parentheses in a pattern, PCRE has
5063 to obtain extra memory to store data during a recursion, which it does
5064 by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
5065 can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
5067 Do not confuse the (?R) item with the condition (R), which tests for
5068 recursion. Consider this pattern, which matches text in angle brack-
5069 ets, allowing for arbitrary nesting. Only digits are allowed in nested
5070 brackets (that is, when recursing), whereas any characters are permit-
5071 ted at the outer level.
5073 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
5075 In this pattern, (?(R) is the start of a conditional subpattern, with
5076 two different alternatives for the recursive and non-recursive cases.
5077 The (?R) item is the actual recursive call.
5079 Recursion difference from Perl
5081 In PCRE (like Python, but unlike Perl), a recursive subpattern call is
5082 always treated as an atomic group. That is, once it has matched some of
5083 the subject string, it is never re-entered, even if it contains untried
5084 alternatives and there is a subsequent matching failure. This can be
5085 illustrated by the following pattern, which purports to match a palin-
5086 dromic string that contains an odd number of characters (for example,
5087 "a", "aba", "abcba", "abcdcba"):
5089 ^(.|(.)(?1)\2)$
5091 The idea is that it either matches a single character, or two identical
5092 characters surrounding a sub-palindrome. In Perl, this pattern works;
5093 in PCRE it does not if the pattern is longer than three characters.
5094 Consider the subject string "abcba":
5096 At the top level, the first character is matched, but as it is not at
5097 the end of the string, the first alternative fails; the second alterna-
5098 tive is taken and the recursion kicks in. The recursive call to subpat-
5099 tern 1 successfully matches the next character ("b"). (Note that the
5100 beginning and end of line tests are not part of the recursion).
5102 Back at the top level, the next character ("c") is compared with what
5103 subpattern 2 matched, which was "a". This fails. Because the recursion
5104 is treated as an atomic group, there are now no backtracking points,
5105 and so the entire match fails. (Perl is able, at this point, to re-
5106 enter the recursion and try the second alternative.) However, if the
5107 pattern is written with the alternatives in the other order, things are
5108 different:
5110 ^((.)(?1)\2|.)$
5112 This time, the recursing alternative is tried first, and continues to
5113 recurse until it runs out of characters, at which point the recursion
5114 fails. But this time we do have another alternative to try at the
5115 higher level. That is the big difference: in the previous case the
5116 remaining alternative is at a deeper recursion level, which PCRE cannot
5117 use.
5119 To change the pattern so that matches all palindromic strings, not just
5120 those with an odd number of characters, it is tempting to change the
5121 pattern to this:
5123 ^((.)(?1)\2|.?)$
5125 Again, this works in Perl, but not in PCRE, and for the same reason.
5126 When a deeper recursion has matched a single character, it cannot be
5127 entered again in order to match an empty string. The solution is to
5128 separate the two cases, and write out the odd and even cases as alter-
5129 natives at the higher level:
5131 ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
5133 If you want to match typical palindromic phrases, the pattern has to
5134 ignore all non-word characters, which can be done like this:
5136 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
5138 If run with the PCRE_CASELESS option, this pattern matches phrases such
5139 as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
5140 Perl. Note the use of the possessive quantifier *+ to avoid backtrack-
5141 ing into sequences of non-word characters. Without this, PCRE takes a
5142 great deal longer (ten times or more) to match typical phrases, and
5143 Perl takes so long that you think it has gone into a loop.
5145 WARNING: The palindrome-matching patterns above work only if the sub-
5146 ject string does not start with a palindrome that is shorter than the
5147 entire string. For example, although "abcba" is correctly matched, if
5148 the subject is "ababa", PCRE finds the palindrome "aba" at the start,
5149 then fails at top level because the end of the string does not follow.
5150 Once again, it cannot jump back into the recursion to try other alter-
5151 natives, so the entire match fails.
5156 If the syntax for a recursive subpattern reference (either by number or
5157 by name) is used outside the parentheses to which it refers, it oper-
5158 ates like a subroutine in a programming language. The "called" subpat-
5159 tern may be defined before or after the reference. A numbered reference
5160 can be absolute or relative, as in these examples:
5162 (...(absolute)...)...(?2)...
5163 (...(relative)...)...(?-1)...
5164 (...(?+1)...(relative)...
5166 An earlier example pointed out that the pattern
5168 (sens|respons)e and \1ibility
5170 matches "sense and sensibility" and "response and responsibility", but
5171 not "sense and responsibility". If instead the pattern
5173 (sens|respons)e and (?1)ibility
5175 is used, it does match "sense and responsibility" as well as the other
5176 two strings. Another example is given in the discussion of DEFINE
5177 above.
5179 Like recursive subpatterns, a subroutine call is always treated as an
5180 atomic group. That is, once it has matched some of the subject string,
5181 it is never re-entered, even if it contains untried alternatives and
5182 there is a subsequent matching failure. Any capturing parentheses that
5183 are set during the subroutine call revert to their previous values
5184 afterwards.
5186 When a subpattern is used as a subroutine, processing options such as
5187 case-independence are fixed when the subpattern is defined. They cannot
5188 be changed for different calls. For example, consider this pattern:
5190 (abc)(?i:(?-1))
5192 It matches "abcabc". It does not match "abcABC" because the change of
5193 processing option does not affect the called subpattern.
5198 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
5199 name or a number enclosed either in angle brackets or single quotes, is
5200 an alternative syntax for referencing a subpattern as a subroutine,
5201 possibly recursively. Here are two of the examples used above, rewrit-
5202 ten using this syntax:
5204 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
5205 (sens|respons)e and \g'1'ibility
5207 PCRE supports an extension to Oniguruma: if a number is preceded by a
5208 plus or a minus sign it is taken as a relative reference. For example:
5210 (abc)(?i:\g<-1>)
5212 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
5213 synonymous. The former is a back reference; the latter is a subroutine
5214 call.
5219 Perl has a feature whereby using the sequence (?{...}) causes arbitrary
5220 Perl code to be obeyed in the middle of matching a regular expression.
5221 This makes it possible, amongst other things, to extract different sub-
5222 strings that match the same pair of parentheses when there is a repeti-
5223 tion.
5225 PCRE provides a similar feature, but of course it cannot obey arbitrary
5226 Perl code. The feature is called "callout". The caller of PCRE provides
5227 an external function by putting its entry point in the global variable
5228 pcre_callout. By default, this variable contains NULL, which disables
5229 all calling out.
5231 Within a regular expression, (?C) indicates the points at which the
5232 external function is to be called. If you want to identify different
5233 callout points, you can put a number less than 256 after the letter C.
5234 The default value is zero. For example, this pattern has two callout
5235 points:
5237 (?C1)abc(?C2)def
5239 If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
5240 automatically installed before each item in the pattern. They are all
5241 numbered 255.
5243 During matching, when PCRE reaches a callout point (and pcre_callout is
5244 set), the external function is called. It is provided with the number
5245 of the callout, the position in the pattern, and, optionally, one item
5246 of data originally supplied by the caller of pcre_exec(). The callout
5247 function may cause matching to proceed, to backtrack, or to fail alto-
5248 gether. A complete description of the interface to the callout function
5249 is given in the pcrecallout documentation.
5254 Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
5255 which are described in the Perl documentation as "experimental and sub-
5256 ject to change or removal in a future version of Perl". It goes on to
5257 say: "Their usage in production code should be noted to avoid problems
5258 during upgrades." The same remarks apply to the PCRE features described
5259 in this section.
5261 Since these verbs are specifically related to backtracking, most of
5262 them can be used only when the pattern is to be matched using
5263 pcre_exec(), which uses a backtracking algorithm. With the exception of
5264 (*FAIL), which behaves like a failing negative assertion, they cause an
5265 error if encountered by pcre_dfa_exec().
5267 If any of these verbs are used in an assertion or subroutine subpattern
5268 (including recursive subpatterns), their effect is confined to that
5269 subpattern; it does not extend to the surrounding pattern. Note that
5270 such subpatterns are processed as anchored at the point where they are
5271 tested.
5273 The new verbs make use of what was previously invalid syntax: an open-
5274 ing parenthesis followed by an asterisk. They are generally of the form
5275 (*VERB) or (*VERB:NAME). Some may take either form, with differing be-
5276 haviour, depending on whether or not an argument is present. An name is
5277 a sequence of letters, digits, and underscores. If the name is empty,
5278 that is, if the closing parenthesis immediately follows the colon, the
5279 effect is as if the colon were not there. Any number of these verbs may
5280 occur in a pattern.
5282 PCRE contains some optimizations that are used to speed up matching by
5283 running some checks at the start of each match attempt. For example, it
5284 may know the minimum length of matching subject, or that a particular
5285 character must be present. When one of these optimizations suppresses
5286 the running of a match, any included backtracking verbs will not, of
5287 course, be processed. You can suppress the start-of-match optimizations
5288 by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_exec().
5290 Verbs that act immediately
5292 The following verbs act as soon as they are encountered. They may not
5293 be followed by a name.
5295 (*ACCEPT)
5297 This verb causes the match to end successfully, skipping the remainder
5298 of the pattern. When inside a recursion, only the innermost pattern is
5299 ended immediately. If (*ACCEPT) is inside capturing parentheses, the
5300 data so far is captured. (This feature was added to PCRE at release
5301 8.00.) For example:
5303 A((?:A|B(*ACCEPT)|C)D)
5305 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap-
5306 tured by the outer parentheses.
5308 (*FAIL) or (*F)
5310 This verb causes the match to fail, forcing backtracking to occur. It
5311 is equivalent to (?!) but easier to read. The Perl documentation notes
5312 that it is probably useful only when combined with (?{}) or (??{}).
5313 Those are, of course, Perl features that are not present in PCRE. The
5314 nearest equivalent is the callout feature, as for example in this pat-
5315 tern:
5317 a+(?C)(*FAIL)
5319 A match with the string "aaaa" always fails, but the callout is taken
5320 before each backtrack happens (in this example, 10 times).
5322 Recording which path was taken
5324 There is one verb whose main purpose is to track how a match was
5325 arrived at, though it also has a secondary use in conjunction with
5326 advancing the match starting point (see (*SKIP) below).
5328 (*MARK:NAME) or (*:NAME)
5330 A name is always required with this verb. There may be as many
5331 instances of (*MARK) as you like in a pattern, and their names do not
5332 have to be unique.
5334 When a match succeeds, the name of the last-encountered (*MARK) is
5335 passed back to the caller via the pcre_extra data structure, as
5336 described in the section on pcre_extra in the pcreapi documentation. No
5337 data is returned for a partial match. Here is an example of pcretest
5338 output, where the /K modifier requests the retrieval and outputting of
5339 (*MARK) data:
5341 /X(*MARK:A)Y|X(*MARK:B)Z/K
5342 XY
5343 0: XY
5344 MK: A
5345 XZ
5346 0: XZ
5347 MK: B
5349 The (*MARK) name is tagged with "MK:" in this output, and in this exam-
5350 ple it indicates which of the two alternatives matched. This is a more
5351 efficient way of obtaining this information than putting each alterna-
5352 tive in its own capturing parentheses.
5354 A name may also be returned after a failed match if the final path
5355 through the pattern involves (*MARK). However, unless (*MARK) used in
5356 conjunction with (*COMMIT), this is unlikely to happen for an unan-
5357 chored pattern because, as the starting point for matching is advanced,
5358 the final check is often with an empty string, causing a failure before
5359 (*MARK) is reached. For example:
5361 /X(*MARK:A)Y|X(*MARK:B)Z/K
5362 XP
5363 No match
5365 There are three potential starting points for this match (starting with
5366 X, starting with P, and with an empty string). If the pattern is
5367 anchored, the result is different:
5369 /^X(*MARK:A)Y|^X(*MARK:B)Z/K
5370 XP
5371 No match, mark = B
5373 PCRE's start-of-match optimizations can also interfere with this. For
5374 example, if, as a result of a call to pcre_study(), it knows the mini-
5375 mum subject length for a match, a shorter subject will not be scanned
5376 at all.
5378 Note that similar anomalies (though different in detail) exist in Perl,
5379 no doubt for the same reasons. The use of (*MARK) data after a failed
5380 match of an unanchored pattern is not recommended, unless (*COMMIT) is
5381 involved.
5383 Verbs that act after backtracking
5385 The following verbs do nothing when they are encountered. Matching con-
5386 tinues with what follows, but if there is no subsequent match, causing
5387 a backtrack to the verb, a failure is forced. That is, backtracking
5388 cannot pass to the left of the verb. However, when one of these verbs
5389 appears inside an atomic group, its effect is confined to that group,
5390 because once the group has been matched, there is never any backtrack-
5391 ing into it. In this situation, backtracking can "jump back" to the
5392 left of the entire atomic group. (Remember also, as stated above, that
5393 this localization also applies in subroutine calls and assertions.)
5395 These verbs differ in exactly what kind of failure occurs when back-
5396 tracking reaches them.
5398 (*COMMIT)
5400 This verb, which may not be followed by a name, causes the whole match
5401 to fail outright if the rest of the pattern does not match. Even if the
5402 pattern is unanchored, no further attempts to find a match by advancing
5403 the starting point take place. Once (*COMMIT) has been passed,
5404 pcre_exec() is committed to finding a match at the current starting
5405 point, or not at all. For example:
5407 a+(*COMMIT)b
5409 This matches "xxaab" but not "aacaab". It can be thought of as a kind
5410 of dynamic anchor, or "I've started, so I must finish." The name of the
5411 most recently passed (*MARK) in the path is passed back when (*COMMIT)
5412 forces a match failure.
5414 Note that (*COMMIT) at the start of a pattern is not the same as an
5415 anchor, unless PCRE's start-of-match optimizations are turned off, as
5416 shown in this pcretest example:
5418 /(*COMMIT)abc/
5419 xyzabc
5420 0: abc
5421 xyzabc\Y
5422 No match
5424 PCRE knows that any match must start with "a", so the optimization
5425 skips along the subject to "a" before running the first match attempt,
5426 which succeeds. When the optimization is disabled by the \Y escape in
5427 the second subject, the match starts at "x" and so the (*COMMIT) causes
5428 it to fail without trying any other starting points.
5430 (*PRUNE) or (*PRUNE:NAME)
5432 This verb causes the match to fail at the current starting position in
5433 the subject if the rest of the pattern does not match. If the pattern
5434 is unanchored, the normal "bumpalong" advance to the next starting
5435 character then happens. Backtracking can occur as usual to the left of
5436 (*PRUNE), before it is reached, or when matching to the right of
5437 (*PRUNE), but if there is no match to the right, backtracking cannot
5438 cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alter-
5439 native to an atomic group or possessive quantifier, but there are some
5440 uses of (*PRUNE) that cannot be expressed in any other way. The behav-
5441 iour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
5442 match fails completely; the name is passed back if this is the final
5443 attempt. (*PRUNE:NAME) does not pass back a name if the match suc-
5444 ceeds. In an anchored pattern (*PRUNE) has the same effect as (*COM-
5445 MIT).
5447 (*SKIP)
5449 This verb, when given without a name, is like (*PRUNE), except that if
5450 the pattern is unanchored, the "bumpalong" advance is not to the next
5451 character, but to the position in the subject where (*SKIP) was encoun-
5452 tered. (*SKIP) signifies that whatever text was matched leading up to
5453 it cannot be part of a successful match. Consider:
5455 a+(*SKIP)b
5457 If the subject is "aaaac...", after the first match attempt fails
5458 (starting at the first character in the string), the starting point
5459 skips on to start the next attempt at "c". Note that a possessive quan-
5460 tifer does not have the same effect as this example; although it would
5461 suppress backtracking during the first match attempt, the second
5462 attempt would start at the second character instead of skipping on to
5463 "c".
5465 (*SKIP:NAME)
5467 When (*SKIP) has an associated name, its behaviour is modified. If the
5468 following pattern fails to match, the previous path through the pattern
5469 is searched for the most recent (*MARK) that has the same name. If one
5470 is found, the "bumpalong" advance is to the subject position that cor-
5471 responds to that (*MARK) instead of to where (*SKIP) was encountered.
5472 If no (*MARK) with a matching name is found, normal "bumpalong" of one
5473 character happens (the (*SKIP) is ignored).
5475 (*THEN) or (*THEN:NAME)
5477 This verb causes a skip to the next alternation if the rest of the pat-
5478 tern does not match. That is, it cancels pending backtracking, but only
5479 within the current alternation. Its name comes from the observation
5480 that it can be used for a pattern-based if-then-else block:
5482 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
5484 If the COND1 pattern matches, FOO is tried (and possibly further items
5485 after the end of the group if FOO succeeds); on failure the matcher
5486 skips to the second alternative and tries COND2, without backtracking
5487 into COND1. The behaviour of (*THEN:NAME) is exactly the same as
5488 (*MARK:NAME)(*THEN) if the overall match fails. If (*THEN) is not
5489 directly inside an alternation, it acts like (*PRUNE).
5494 pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3).
5499 Philip Hazel
5500 University Computing Service
5501 Cambridge CB2 3QH, England.
5506 Last updated: 18 May 2010
5507 Copyright (c) 1997-2010 University of Cambridge.
5508 ------------------------------------------------------------------------------
5514 NAME
5515 PCRE - Perl-compatible regular expressions
5520 The full syntax and semantics of the regular expressions that are sup-
5521 ported by PCRE are described in the pcrepattern documentation. This
5522 document contains just a quick-reference summary of the syntax.
5527 \x where x is non-alphanumeric is a literal x
5528 \Q...\E treat enclosed characters as literal
5533 \a alarm, that is, the BEL character (hex 07)
5534 \cx "control-x", where x is any character
5535 \e escape (hex 1B)
5536 \f formfeed (hex 0C)
5537 \n newline (hex 0A)
5538 \r carriage return (hex 0D)
5539 \t tab (hex 09)
5540 \ddd character with octal code ddd, or backreference
5541 \xhh character with hex code hh
5542 \x{hhh..} character with hex code hhh..
5547 . any character except newline;
5548 in dotall mode, any character whatsoever
5549 \C one byte, even in UTF-8 mode (best avoided)
5550 \d a decimal digit
5551 \D a character that is not a decimal digit
5552 \h a horizontal whitespace character
5553 \H a character that is not a horizontal whitespace character
5554 \N a character that is not a newline
5555 \p{xx} a character with the xx property
5556 \P{xx} a character without the xx property
5557 \R a newline sequence
5558 \s a whitespace character
5559 \S a character that is not a whitespace character
5560 \v a vertical whitespace character
5561 \V a character that is not a vertical whitespace character
5562 \w a "word" character
5563 \W a "non-word" character
5564 \X an extended Unicode sequence
5566 In PCRE, by default, \d, \D, \s, \S, \w, and \W recognize only ASCII
5567 characters, even in UTF-8 mode. However, this can be changed by setting
5568 the PCRE_UCP option.
5573 C Other
5574 Cc Control
5575 Cf Format
5576 Cn Unassigned
5577 Co Private use
5578 Cs Surrogate
5580 L Letter
5581 Ll Lower case letter
5582 Lm Modifier letter
5583 Lo Other letter
5584 Lt Title case letter
5585 Lu Upper case letter
5586 L& Ll, Lu, or Lt
5588 M Mark
5589 Mc Spacing mark
5590 Me Enclosing mark
5591 Mn Non-spacing mark
5593 N Number
5594 Nd Decimal number
5595 Nl Letter number
5596 No Other number
5598 P Punctuation
5599 Pc Connector punctuation
5600 Pd Dash punctuation
5601 Pe Close punctuation
5602 Pf Final punctuation
5603 Pi Initial punctuation
5604 Po Other punctuation
5605 Ps Open punctuation
5607 S Symbol
5608 Sc Currency symbol
5609 Sk Modifier symbol
5610 Sm Mathematical symbol
5611 So Other symbol
5613 Z Separator
5614 Zl Line separator
5615 Zp Paragraph separator
5616 Zs Space separator
5621 Xan Alphanumeric: union of properties L and N
5622 Xps POSIX space: property Z or tab, NL, VT, FF, CR
5623 Xsp Perl space: property Z or tab, NL, FF, CR
5624 Xwd Perl word: property Xan or underscore
5629 Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
5630 Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common,
5631 Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp-
5632 tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek,
5633 Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe-
5634 rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
5635 Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
5636 Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam,
5637 Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
5638 Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya,
5639 Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian,
5640 Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le,
5641 Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh,
5642 Ugaritic, Vai, Yi.
5647 [...] positive character class
5648 [^...] negative character class
5649 [x-y] range (can be used for hex characters)
5650 [[:xxx:]] positive POSIX named set
5651 [[:^xxx:]] negative POSIX named set
5653 alnum alphanumeric
5654 alpha alphabetic
5655 ascii 0-127
5656 blank space or tab
5657 cntrl control character
5658 digit decimal digit
5659 graph printing, excluding space
5660 lower lower case letter
5661 print printing, including space
5662 punct printing, excluding alphanumeric
5663 space whitespace
5664 upper upper case letter
5665 word same as \w
5666 xdigit hexadecimal digit
5668 In PCRE, POSIX character set names recognize only ASCII characters by
5669 default, but some of them use Unicode properties if PCRE_UCP is set.
5670 You can use \Q...\E inside a character class.
5675 ? 0 or 1, greedy
5676 ?+ 0 or 1, possessive
5677 ?? 0 or 1, lazy
5678 * 0 or more, greedy
5679 *+ 0 or more, possessive
5680 *? 0 or more, lazy
5681 + 1 or more, greedy
5682 ++ 1 or more, possessive
5683 +? 1 or more, lazy
5684 {n} exactly n
5685 {n,m} at least n, no more than m, greedy
5686 {n,m}+ at least n, no more than m, possessive
5687 {n,m}? at least n, no more than m, lazy
5688 {n,} n or more, greedy
5689 {n,}+ n or more, possessive
5690 {n,}? n or more, lazy
5695 \b word boundary
5696 \B not a word boundary
5697 ^ start of subject
5698 also after internal newline in multiline mode
5699 \A start of subject
5700 $ end of subject
5701 also before newline at end of subject
5702 also before internal newline in multiline mode
5703 \Z end of subject
5704 also before newline at end of subject
5705 \z end of subject
5706 \G first matching position in subject
5711 \K reset start of match
5716 expr|expr|expr...
5721 (...) capturing group
5722 (?<name>...) named capturing group (Perl)
5723 (?'name'...) named capturing group (Perl)
5724 (?P<name>...) named capturing group (Python)
5725 (?:...) non-capturing group
5726 (?|...) non-capturing group; reset group numbers for
5727 capturing groups in each alternative
5732 (?>...) atomic, non-capturing group
5737 (?#....) comment (not nestable)
5742 (?i) caseless
5743 (?J) allow duplicate names
5744 (?m) multiline
5745 (?s) single line (dotall)
5746 (?U) default ungreedy (lazy)
5747 (?x) extended (ignore white space)
5748 (?-...) unset option(s)
5750 The following are recognized only at the start of a pattern or after
5751 one of the newline-setting options with similar syntax:
5753 (*UTF8) set UTF-8 mode (PCRE_UTF8)
5754 (*UCP) set PCRE_UCP (use Unicode properties for \d etc)
5759 (?=...) positive look ahead
5760 (?!...) negative look ahead
5761 (?<=...) positive look behind
5762 (?<!...) negative look behind
5764 Each top-level branch of a look behind must be of a fixed length.
5769 \n reference by number (can be ambiguous)
5770 \gn reference by number
5771 \g{n} reference by number
5772 \g{-n} relative reference by number
5773 \k<name> reference by name (Perl)
5774 \k'name' reference by name (Perl)
5775 \g{name} reference by name (Perl)
5776 \k{name} reference by name (.NET)
5777 (?P=name) reference by name (Python)
5782 (?R) recurse whole pattern
5783 (?n) call subpattern by absolute number
5784 (?+n) call subpattern by relative number
5785 (?-n) call subpattern by relative number
5786 (?&name) call subpattern by name (Perl)
5787 (?P>name) call subpattern by name (Python)
5788 \g<name> call subpattern by name (Oniguruma)
5789 \g'name' call subpattern by name (Oniguruma)
5790 \g<n> call subpattern by absolute number (Oniguruma)
5791 \g'n' call subpattern by absolute number (Oniguruma)
5792 \g<+n> call subpattern by relative number (PCRE extension)
5793 \g'+n' call subpattern by relative number (PCRE extension)
5794 \g<-n> call subpattern by relative number (PCRE extension)
5795 \g'-n' call subpattern by relative number (PCRE extension)
5800 (?(condition)yes-pattern)
5801 (?(condition)yes-pattern|no-pattern)
5803 (?(n)... absolute reference condition
5804 (?(+n)... relative reference condition
5805 (?(-n)... relative reference condition
5806 (?(<name>)... named reference condition (Perl)
5807 (?('name')... named reference condition (Perl)
5808 (?(name)... named reference condition (PCRE)
5809 (?(R)... overall recursion condition
5810 (?(Rn)... specific group recursion condition
5811 (?(R&name)... specific recursion condition
5812 (?(DEFINE)... define subpattern for reference
5813 (?(assert)... assertion condition
5818 The following act immediately they are reached:
5820 (*ACCEPT) force successful match
5821 (*FAIL) force backtrack; synonym (*F)
5823 The following act only when a subsequent match failure causes a back-
5824 track to reach them. They all force a match failure, but they differ in
5825 what happens afterwards. Those that advance the start-of-match point do
5826 so only if the pattern is not anchored.
5828 (*COMMIT) overall failure, no advance of starting point
5829 (*PRUNE) advance to next starting character
5830 (*SKIP) advance start to current matching position
5831 (*THEN) local failure, backtrack to next alternation
5836 These are recognized only at the very start of the pattern or after a
5837 (*BSR_...) or (*UTF8) or (*UCP) option.
5839 (*CR) carriage return only
5840 (*LF) linefeed only
5841 (*CRLF) carriage return followed by linefeed
5842 (*ANYCRLF) all three of the above
5843 (*ANY) any Unicode newline sequence
5848 These are recognized only at the very start of the pattern or after a
5849 (*...) option that sets the newline convention or UTF-8 or UCP mode.
5852 (*BSR_UNICODE) any Unicode newline sequence
5857 (?C) callout
5858 (?Cn) callout with data n
5863 pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
5868 Philip Hazel
5869 University Computing Service
5870 Cambridge CB2 3QH, England.
5875 Last updated: 12 May 2010
5876 Copyright (c) 1997-2010 University of Cambridge.
5877 ------------------------------------------------------------------------------
5883 NAME
5884 PCRE - Perl-compatible regular expressions
5889 In normal use of PCRE, if the subject string that is passed to
5890 pcre_exec() or pcre_dfa_exec() matches as far as it goes, but is too
5891 short to match the entire pattern, PCRE_ERROR_NOMATCH is returned.
5892 There are circumstances where it might be helpful to distinguish this
5893 case from other cases in which there is no match.
5895 Consider, for example, an application where a human is required to type
5896 in data for a field with specific formatting requirements. An example
5897 might be a date in the form ddmmmyy, defined by this pattern:
5899 ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
5901 If the application sees the user's keystrokes one by one, and can check
5902 that what has been typed so far is potentially valid, it is able to
5903 raise an error as soon as a mistake is made, by beeping and not
5904 reflecting the character that has been typed, for example. This immedi-
5905 ate feedback is likely to be a better user interface than a check that
5906 is delayed until the entire string has been entered. Partial matching
5907 can also sometimes be useful when the subject string is very long and
5908 is not all available at once.
5910 PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
5911 PCRE_PARTIAL_HARD options, which can be set when calling pcre_exec() or
5912 pcre_dfa_exec(). For backwards compatibility, PCRE_PARTIAL is a synonym
5913 for PCRE_PARTIAL_SOFT. The essential difference between the two options
5914 is whether or not a partial match is preferred to an alternative com-
5915 plete match, though the details differ between the two matching func-
5916 tions. If both options are set, PCRE_PARTIAL_HARD takes precedence.
5918 Setting a partial matching option disables two of PCRE's optimizations.
5919 PCRE remembers the last literal byte in a pattern, and abandons match-
5920 ing immediately if such a byte is not present in the subject string.
5921 This optimization cannot be used for a subject string that might match
5922 only partially. If the pattern was studied, PCRE knows the minimum
5923 length of a matching string, and does not bother to run the matching
5924 function on shorter strings. This optimization is also disabled for
5925 partial matching.
5930 A partial match occurs during a call to pcre_exec() whenever the end of
5931 the subject string is reached successfully, but matching cannot con-
5932 tinue because more characters are needed. However, at least one charac-
5933 ter must have been matched. (In other words, a partial match can never
5934 be an empty string.)
5936 If PCRE_PARTIAL_SOFT is set, the partial match is remembered, but
5937 matching continues as normal, and other alternatives in the pattern are
5938 tried. If no complete match can be found, pcre_exec() returns
5939 PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. If there are at least
5940 two slots in the offsets vector, the first of them is set to the offset
5941 of the earliest character that was inspected when the partial match was
5942 found. For convenience, the second offset points to the end of the
5943 string so that a substring can easily be identified.
5945 For the majority of patterns, the first offset identifies the start of
5946 the partially matched string. However, for patterns that contain look-
5947 behind assertions, or \K, or begin with \b or \B, earlier characters
5948 have been inspected while carrying out the match. For example:
5950 /(?<=abc)123/
5952 This pattern matches "123", but only if it is preceded by "abc". If the
5953 subject string is "xyzabc12", the offsets after a partial match are for
5954 the substring "abc12", because all these characters are needed if
5955 another match is tried with extra characters added.
5957 If there is more than one partial match, the first one that was found
5958 provides the data that is returned. Consider this pattern:
5960 /123\w+X|dogY/
5962 If this is matched against the subject string "abc123dog", both alter-
5963 natives fail to match, but the end of the subject is reached during
5964 matching, so PCRE_ERROR_PARTIAL is returned instead of
5965 PCRE_ERROR_NOMATCH. The offsets are set to 3 and 9, identifying
5966 "123dog" as the first partial match that was found. (In this example,
5967 there are two partial matches, because "dog" on its own partially
5968 matches the second alternative.)
5970 If PCRE_PARTIAL_HARD is set for pcre_exec(), it returns PCRE_ERROR_PAR-
5971 TIAL as soon as a partial match is found, without continuing to search
5972 for possible complete matches. The difference between the two options
5973 can be illustrated by a pattern such as:
5975 /dog(sbody)?/
5977 This matches either "dog" or "dogsbody", greedily (that is, it prefers
5978 the longer string if possible). If it is matched against the string
5979 "dog" with PCRE_PARTIAL_SOFT, it yields a complete match for "dog".
5980 However, if PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL.
5981 On the other hand, if the pattern is made ungreedy the result is dif-
5982 ferent:
5984 /dog(sbody)??/
5986 In this case the result is always a complete match because pcre_exec()
5987 finds that first, and it never continues after finding a match. It
5988 might be easier to follow this explanation by thinking of the two pat-
5989 terns like this:
5991 /dog(sbody)?/ is the same as /dogsbody|dog/
5992 /dog(sbody)??/ is the same as /dog|dogsbody/
5994 The second pattern will never match "dogsbody" when pcre_exec() is
5995 used, because it will always find the shorter match first.
5998 PARTIAL MATCHING USING pcre_dfa_exec()
6000 The pcre_dfa_exec() function moves along the subject string character
6001 by character, without backtracking, searching for all possible matches
6002 simultaneously. If the end of the subject is reached before the end of
6003 the pattern, there is the possibility of a partial match, again pro-
6004 vided that at least one character has matched.
6006 When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if
6007 there have been no complete matches. Otherwise, the complete matches
6008 are returned. However, if PCRE_PARTIAL_HARD is set, a partial match
6009 takes precedence over any complete matches. The portion of the string
6010 that was inspected when the longest partial match was found is set as
6011 the first matching string, provided there are at least two slots in the
6012 offsets vector.
6014 Because pcre_dfa_exec() always searches for all possible matches, and
6015 there is no difference between greedy and ungreedy repetition, its be-
6016 haviour is different from pcre_exec when PCRE_PARTIAL_HARD is set. Con-
6017 sider the string "dog" matched against the ungreedy pattern shown
6018 above:
6020 /dog(sbody)??/
6022 Whereas pcre_exec() stops as soon as it finds the complete match for
6023 "dog", pcre_dfa_exec() also finds the partial match for "dogsbody", and
6024 so returns that when PCRE_PARTIAL_HARD is set.
6029 If a pattern ends with one of sequences \b or \B, which test for word
6030 boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-
6031 intuitive results. Consider this pattern:
6033 /\bcat\b/
6035 This matches "cat", provided there is a word boundary at either end. If
6036 the subject string is "the cat", the comparison of the final "t" with a
6037 following character cannot take place, so a partial match is found.
6038 However, pcre_exec() carries on with normal matching, which matches \b
6039 at the end of the subject when the last character is a letter, thus
6040 finding a complete match. The result, therefore, is not PCRE_ERROR_PAR-
6041 TIAL. The same thing happens with pcre_dfa_exec(), because it also
6042 finds the complete match.
6044 Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL,
6045 because then the partial match takes precedence.
6050 For releases of PCRE prior to 8.00, because of the way certain internal