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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 5.12, including support for UTF-8 encoded strings and Unicode general
30 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 6.0.0.
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 pcrejit discussion of the just-in-time optimization support
89 pcrelimits details of size and other limits
90 pcrematching discussion of the two matching algorithms
91 pcrepartial details of the partial matching facility
92 pcrepattern syntax and semantics of supported
93 regular expressions
94 pcreperform discussion of performance issues
95 pcreposix the POSIX-compatible C API
96 pcreprecompile details of saving and re-using precompiled patterns
97 pcresample discussion of the pcredemo program
98 pcrestack discussion of stack usage
99 pcresyntax quick syntax reference
100 pcretest description of the pcretest testing command
101 pcreunicode discussion of Unicode and UTF-8 support
103 In addition, in the "man" and HTML formats, there is a short page for
104 each C library function, listing its arguments and results.
109 Philip Hazel
110 University Computing Service
111 Cambridge CB2 3QH, England.
113 Putting an actual email address here seems to have been a spam magnet,
114 so I've taken it away. If you want to email me, use my two initials,
115 followed by the two digits 10, at the domain cam.ac.uk.
120 Last updated: 24 August 2011
121 Copyright (c) 1997-2011 University of Cambridge.
122 ------------------------------------------------------------------------------
128 NAME
129 PCRE - Perl-compatible regular expressions
134 This document describes the optional features of PCRE that can be
135 selected when the library is compiled. It assumes use of the configure
136 script, where the optional features are selected or deselected by pro-
137 viding options to configure before running the make command. However,
138 the same options can be selected in both Unix-like and non-Unix-like
139 environments using the GUI facility of cmake-gui if you are using CMake
140 instead of configure to build PCRE.
142 There is a lot more information about building PCRE in non-Unix-like
143 environments in the file called NON_UNIX_USE, which is part of the PCRE
144 distribution. You should consult this file as well as the README file
145 if you are building in a non-Unix-like environment.
147 The complete list of options for configure (which includes the standard
148 ones such as the selection of the installation directory) can be
149 obtained by running
151 ./configure --help
153 The following sections include descriptions of options whose names
154 begin with --enable or --disable. These settings specify changes to the
155 defaults for the configure command. Because of the way that configure
156 works, --enable and --disable always come in pairs, so the complemen-
157 tary option always exists as well, but as it specifies the default, it
158 is not described.
163 The PCRE building process uses libtool to build both shared and static
164 Unix libraries by default. You can suppress one of these by adding one
165 of
167 --disable-shared
168 --disable-static
170 to the configure command, as required.
175 By default, the configure script will search for a C++ compiler and C++
176 header files. If it finds them, it automatically builds the C++ wrapper
177 library for PCRE. You can disable this by adding
179 --disable-cpp
181 to the configure command.
186 To build PCRE with support for UTF-8 Unicode character strings, add
188 --enable-utf8
190 to the configure command. Of itself, this does not make PCRE treat
191 strings as UTF-8. As well as compiling PCRE with this option, you also
192 have have to set the PCRE_UTF8 option when you call the pcre_compile()
193 or pcre_compile2() functions.
195 If you set --enable-utf8 when compiling in an EBCDIC environment, PCRE
196 expects its input to be either ASCII or UTF-8 (depending on the runtime
197 option). It is not possible to support both EBCDIC and UTF-8 codes in
198 the same version of the library. Consequently, --enable-utf8 and
199 --enable-ebcdic are mutually exclusive.
204 UTF-8 support allows PCRE to process character values greater than 255
205 in the strings that it handles. On its own, however, it does not pro-
206 vide any facilities for accessing the properties of such characters. If
207 you want to be able to use the pattern escapes \P, \p, and \X, which
208 refer to Unicode character properties, you must add
210 --enable-unicode-properties
212 to the configure command. This implies UTF-8 support, even if you have
213 not explicitly requested it.
215 Including Unicode property support adds around 30K of tables to the
216 PCRE library. Only the general category properties such as Lu and Nd
217 are supported. Details are given in the pcrepattern documentation.
222 Just-in-time compiler support is included in the build by specifying
224 --enable-jit
226 This support is available only for certain hardware architectures. If
227 this option is set for an unsupported architecture, a compile time
228 error occurs. See the pcrejit documentation for a discussion of JIT
229 usage. When JIT support is enabled, pcregrep automatically makes use of
230 it, unless you add
232 --disable-pcregrep-jit
234 to the "configure" command.
239 By default, PCRE interprets the linefeed (LF) character as indicating
240 the end of a line. This is the normal newline character on Unix-like
241 systems. You can compile PCRE to use carriage return (CR) instead, by
242 adding
244 --enable-newline-is-cr
246 to the configure command. There is also a --enable-newline-is-lf
247 option, which explicitly specifies linefeed as the newline character.
249 Alternatively, you can specify that line endings are to be indicated by
250 the two character sequence CRLF. If you want this, add
252 --enable-newline-is-crlf
254 to the configure command. There is a fourth option, specified by
256 --enable-newline-is-anycrlf
258 which causes PCRE to recognize any of the three sequences CR, LF, or
259 CRLF as indicating a line ending. Finally, a fifth option, specified by
261 --enable-newline-is-any
263 causes PCRE to recognize any Unicode newline sequence.
265 Whatever line ending convention is selected when PCRE is built can be
266 overridden when the library functions are called. At build time it is
267 conventional to use the standard for your operating system.
272 By default, the sequence \R in a pattern matches any Unicode newline
273 sequence, whatever has been selected as the line ending sequence. If
274 you specify
276 --enable-bsr-anycrlf
278 the default is changed so that \R matches only CR, LF, or CRLF. What-
279 ever is selected when PCRE is built can be overridden when the library
280 functions are called.
285 When PCRE is called through the POSIX interface (see the pcreposix doc-
286 umentation), additional working storage is required for holding the
287 pointers to capturing substrings, because PCRE requires three integers
288 per substring, whereas the POSIX interface provides only two. If the
289 number of expected substrings is small, the wrapper function uses space
290 on the stack, because this is faster than using malloc() for each call.
291 The default threshold above which the stack is no longer used is 10; it
292 can be changed by adding a setting such as
294 --with-posix-malloc-threshold=20
296 to the configure command.
301 Within a compiled pattern, offset values are used to point from one
302 part to another (for example, from an opening parenthesis to an alter-
303 nation metacharacter). By default, two-byte values are used for these
304 offsets, leading to a maximum size for a compiled pattern of around
305 64K. This is sufficient to handle all but the most gigantic patterns.
306 Nevertheless, some people do want to process truyl enormous patterns,
307 so it is possible to compile PCRE to use three-byte or four-byte off-
308 sets by adding a setting such as
310 --with-link-size=3
312 to the configure command. The value given must be 2, 3, or 4. Using
313 longer offsets slows down the operation of PCRE because it has to load
314 additional bytes when handling them.
319 When matching with the pcre_exec() function, PCRE implements backtrack-
320 ing by making recursive calls to an internal function called match().
321 In environments where the size of the stack is limited, this can se-
322 verely limit PCRE's operation. (The Unix environment does not usually
323 suffer from this problem, but it may sometimes be necessary to increase
324 the maximum stack size. There is a discussion in the pcrestack docu-
325 mentation.) An alternative approach to recursion that uses memory from
326 the heap to remember data, instead of using recursive function calls,
327 has been implemented to work round the problem of limited stack size.
328 If you want to build a version of PCRE that works this way, add
330 --disable-stack-for-recursion
332 to the configure command. With this configuration, PCRE will use the
333 pcre_stack_malloc and pcre_stack_free variables to call memory manage-
334 ment functions. By default these point to malloc() and free(), but you
335 can replace the pointers so that your own functions are used instead.
337 Separate functions are provided rather than using pcre_malloc and
338 pcre_free because the usage is very predictable: the block sizes
339 requested are always the same, and the blocks are always freed in
340 reverse order. A calling program might be able to implement optimized
341 functions that perform better than malloc() and free(). PCRE runs
342 noticeably more slowly when built in this way. This option affects only
343 the pcre_exec() function; it is not relevant for pcre_dfa_exec().
348 Internally, PCRE has a function called match(), which it calls repeat-
349 edly (sometimes recursively) when matching a pattern with the
350 pcre_exec() function. By controlling the maximum number of times this
351 function may be called during a single matching operation, a limit can
352 be placed on the resources used by a single call to pcre_exec(). The
353 limit can be changed at run time, as described in the pcreapi documen-
354 tation. The default is 10 million, but this can be changed by adding a
355 setting such as
357 --with-match-limit=500000
359 to the configure command. This setting has no effect on the
360 pcre_dfa_exec() matching function.
362 In some environments it is desirable to limit the depth of recursive
363 calls of match() more strictly than the total number of calls, in order
364 to restrict the maximum amount of stack (or heap, if --disable-stack-
365 for-recursion is specified) that is used. A second limit controls this;
366 it defaults to the value that is set for --with-match-limit, which
367 imposes no additional constraints. However, you can set a lower limit
368 by adding, for example,
370 --with-match-limit-recursion=10000
372 to the configure command. This value can also be overridden at run
373 time.
378 PCRE uses fixed tables for processing characters whose code values are
379 less than 256. By default, PCRE is built with a set of tables that are
380 distributed in the file pcre_chartables.c.dist. These tables are for
381 ASCII codes only. If you add
383 --enable-rebuild-chartables
385 to the configure command, the distributed tables are no longer used.
386 Instead, a program called dftables is compiled and run. This outputs
387 the source for new set of tables, created in the default locale of your
388 C runtime system. (This method of replacing the tables does not work if
389 you are cross compiling, because dftables is run on the local host. If
390 you need to create alternative tables when cross compiling, you will
391 have to do so "by hand".)
396 PCRE assumes by default that it will run in an environment where the
397 character code is ASCII (or Unicode, which is a superset of ASCII).
398 This is the case for most computer operating systems. PCRE can, how-
399 ever, be compiled to run in an EBCDIC environment by adding
401 --enable-ebcdic
403 to the configure command. This setting implies --enable-rebuild-charta-
404 bles. You should only use it if you know that you are in an EBCDIC
405 environment (for example, an IBM mainframe operating system). The
406 --enable-ebcdic option is incompatible with --enable-utf8.
411 By default, pcregrep reads all files as plain text. You can build it so
412 that it recognizes files whose names end in .gz or .bz2, and reads them
413 with libz or libbz2, respectively, by adding one or both of
415 --enable-pcregrep-libz
416 --enable-pcregrep-libbz2
418 to the configure command. These options naturally require that the rel-
419 evant libraries are installed on your system. Configuration will fail
420 if they are not.
425 pcregrep uses an internal buffer to hold a "window" on the file it is
426 scanning, in order to be able to output "before" and "after" lines when
427 it finds a match. The size of the buffer is controlled by a parameter
428 whose default value is 20K. The buffer itself is three times this size,
429 but because of the way it is used for holding "before" lines, the long-
430 est line that is guaranteed to be processable is the parameter size.
431 You can change the default parameter value by adding, for example,
433 --with-pcregrep-bufsize=50K
435 to the configure command. The caller of pcregrep can, however, override
436 this value by specifying a run-time option.
441 If you add
443 --enable-pcretest-libreadline
445 to the configure command, pcretest is linked with the libreadline
446 library, and when its input is from a terminal, it reads it using the
447 readline() function. This provides line-editing and history facilities.
448 Note that libreadline is GPL-licensed, so if you distribute a binary of
449 pcretest linked in this way, there may be licensing issues.
451 Setting this option causes the -lreadline option to be added to the
452 pcretest build. In many operating environments with a sytem-installed
453 libreadline this is sufficient. However, in some environments (e.g. if
454 an unmodified distribution version of readline is in use), some extra
455 configuration may be necessary. The INSTALL file for libreadline says
456 this:
458 "Readline uses the termcap functions, but does not link with the
459 termcap or curses library itself, allowing applications which link
460 with readline the to choose an appropriate library."
462 If your environment has not been set up so that an appropriate library
463 is automatically included, you may need to add something like
465 LIBS="-ncurses"
467 immediately before the configure command.
472 pcreapi(3), pcre_config(3).
477 Philip Hazel
478 University Computing Service
479 Cambridge CB2 3QH, England.
484 Last updated: 06 September 2011
485 Copyright (c) 1997-2011 University of Cambridge.
486 ------------------------------------------------------------------------------
492 NAME
493 PCRE - Perl-compatible regular expressions
498 This document describes the two different algorithms that are available
499 in PCRE for matching a compiled regular expression against a given sub-
500 ject string. The "standard" algorithm is the one provided by the
501 pcre_exec() function. This works in the same was as Perl's matching
502 function, and provides a Perl-compatible matching operation.
504 An alternative algorithm is provided by the pcre_dfa_exec() function;
505 this operates in a different way, and is not Perl-compatible. It has
506 advantages and disadvantages compared with the standard algorithm, and
507 these are described below.
509 When there is only one possible way in which a given subject string can
510 match a pattern, the two algorithms give the same answer. A difference
511 arises, however, when there are multiple possibilities. For example, if
512 the pattern
514 ^<.*>
516 is matched against the string
518 <something> <something else> <something further>
520 there are three possible answers. The standard algorithm finds only one
521 of them, whereas the alternative algorithm finds all three.
526 The set of strings that are matched by a regular expression can be rep-
527 resented as a tree structure. An unlimited repetition in the pattern
528 makes the tree of infinite size, but it is still a tree. Matching the
529 pattern to a given subject string (from a given starting point) can be
530 thought of as a search of the tree. There are two ways to search a
531 tree: depth-first and breadth-first, and these correspond to the two
532 matching algorithms provided by PCRE.
537 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
538 sions", the standard algorithm is an "NFA algorithm". It conducts a
539 depth-first search of the pattern tree. That is, it proceeds along a
540 single path through the tree, checking that the subject matches what is
541 required. When there is a mismatch, the algorithm tries any alterna-
542 tives at the current point, and if they all fail, it backs up to the
543 previous branch point in the tree, and tries the next alternative
544 branch at that level. This often involves backing up (moving to the
545 left) in the subject string as well. The order in which repetition
546 branches are tried is controlled by the greedy or ungreedy nature of
547 the quantifier.
549 If a leaf node is reached, a matching string has been found, and at
550 that point the algorithm stops. Thus, if there is more than one possi-
551 ble match, this algorithm returns the first one that it finds. Whether
552 this is the shortest, the longest, or some intermediate length depends
553 on the way the greedy and ungreedy repetition quantifiers are specified
554 in the pattern.
556 Because it ends up with a single path through the tree, it is rela-
557 tively straightforward for this algorithm to keep track of the sub-
558 strings that are matched by portions of the pattern in parentheses.
559 This provides support for capturing parentheses and back references.
564 This algorithm conducts a breadth-first search of the tree. Starting
565 from the first matching point in the subject, it scans the subject
566 string from left to right, once, character by character, and as it does
567 this, it remembers all the paths through the tree that represent valid
568 matches. In Friedl's terminology, this is a kind of "DFA algorithm",
569 though it is not implemented as a traditional finite state machine (it
570 keeps multiple states active simultaneously).
572 Although the general principle of this matching algorithm is that it
573 scans the subject string only once, without backtracking, there is one
574 exception: when a lookaround assertion is encountered, the characters
575 following or preceding the current point have to be independently
576 inspected.
578 The scan continues until either the end of the subject is reached, or
579 there are no more unterminated paths. At this point, terminated paths
580 represent the different matching possibilities (if there are none, the
581 match has failed). Thus, if there is more than one possible match,
582 this algorithm finds all of them, and in particular, it finds the long-
583 est. The matches are returned in decreasing order of length. There is
584 an option to stop the algorithm after the first match (which is neces-
585 sarily the shortest) is found.
587 Note that all the matches that are found start at the same point in the
588 subject. If the pattern
590 cat(er(pillar)?)?
592 is matched against the string "the caterpillar catchment", the result
593 will be the three strings "caterpillar", "cater", and "cat" that start
594 at the fifth character of the subject. The algorithm does not automati-
595 cally move on to find matches that start at later positions.
597 There are a number of features of PCRE regular expressions that are not
598 supported by the alternative matching algorithm. They are as follows:
600 1. Because the algorithm finds all possible matches, the greedy or
601 ungreedy nature of repetition quantifiers is not relevant. Greedy and
602 ungreedy quantifiers are treated in exactly the same way. However, pos-
603 sessive quantifiers can make a difference when what follows could also
604 match what is quantified, for example in a pattern like this:
606 ^a++\w!
608 This pattern matches "aaab!" but not "aaa!", which would be matched by
609 a non-possessive quantifier. Similarly, if an atomic group is present,
610 it is matched as if it were a standalone pattern at the current point,
611 and the longest match is then "locked in" for the rest of the overall
612 pattern.
614 2. When dealing with multiple paths through the tree simultaneously, it
615 is not straightforward to keep track of captured substrings for the
616 different matching possibilities, and PCRE's implementation of this
617 algorithm does not attempt to do this. This means that no captured sub-
618 strings are available.
620 3. Because no substrings are captured, back references within the pat-
621 tern are not supported, and cause errors if encountered.
623 4. For the same reason, conditional expressions that use a backrefer-
624 ence as the condition or test for a specific group recursion are not
625 supported.
627 5. Because many paths through the tree may be active, the \K escape
628 sequence, which resets the start of the match when encountered (but may
629 be on some paths and not on others), is not supported. It causes an
630 error if encountered.
632 6. Callouts are supported, but the value of the capture_top field is
633 always 1, and the value of the capture_last field is always -1.
635 7. The \C escape sequence, which (in the standard algorithm) matches a
636 single byte, even in UTF-8 mode, is not supported in UTF-8 mode,
637 because the alternative algorithm moves through the subject string one
638 character at a time, for all active paths through the tree.
640 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
641 are not supported. (*FAIL) is supported, and behaves like a failing
642 negative assertion.
647 Using the alternative matching algorithm provides the following advan-
648 tages:
650 1. All possible matches (at a single point in the subject) are automat-
651 ically found, and in particular, the longest match is found. To find
652 more than one match using the standard algorithm, you have to do kludgy
653 things with callouts.
655 2. Because the alternative algorithm scans the subject string just
656 once, and never needs to backtrack, it is possible to pass very long
657 subject strings to the matching function in several pieces, checking
658 for partial matching each time. Although it is possible to do multi-
659 segment matching using the standard algorithm (pcre_exec()), by retain-
660 ing partially matched substrings, it is more complicated. The pcrepar-
661 tial documentation gives details of partial matching and discusses
662 multi-segment matching.
667 The alternative algorithm suffers from a number of disadvantages:
669 1. It is substantially slower than the standard algorithm. This is
670 partly because it has to search for all possible matches, but is also
671 because it is less susceptible to optimization.
673 2. Capturing parentheses and back references are not supported.
675 3. Although atomic groups are supported, their use does not provide the
676 performance advantage that it does for the standard algorithm.
681 Philip Hazel
682 University Computing Service
683 Cambridge CB2 3QH, England.
688 Last updated: 19 November 2011
689 Copyright (c) 1997-2010 University of Cambridge.
690 ------------------------------------------------------------------------------
696 NAME
697 PCRE - Perl-compatible regular expressions
702 #include <pcre.h>
704 pcre *pcre_compile(const char *pattern, int options,
705 const char **errptr, int *erroffset,
706 const unsigned char *tableptr);
708 pcre *pcre_compile2(const char *pattern, int options,
709 int *errorcodeptr,
710 const char **errptr, int *erroffset,
711 const unsigned char *tableptr);
713 pcre_extra *pcre_study(const pcre *code, int options,
714 const char **errptr);
716 void pcre_free_study(pcre_extra *extra);
718 int pcre_exec(const pcre *code, const pcre_extra *extra,
719 const char *subject, int length, int startoffset,
720 int options, int *ovector, int ovecsize);
725 pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize);
727 void pcre_jit_stack_free(pcre_jit_stack *stack);
729 void pcre_assign_jit_stack(pcre_extra *extra,
730 pcre_jit_callback callback, void *data);
732 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
733 const char *subject, int length, int startoffset,
734 int options, int *ovector, int ovecsize,
735 int *workspace, int wscount);
737 int pcre_copy_named_substring(const pcre *code,
738 const char *subject, int *ovector,
739 int stringcount, const char *stringname,
740 char *buffer, int buffersize);
742 int pcre_copy_substring(const char *subject, int *ovector,
743 int stringcount, int stringnumber, char *buffer,
744 int buffersize);
746 int pcre_get_named_substring(const pcre *code,
747 const char *subject, int *ovector,
748 int stringcount, const char *stringname,
749 const char **stringptr);
751 int pcre_get_stringnumber(const pcre *code,
752 const char *name);
754 int pcre_get_stringtable_entries(const pcre *code,
755 const char *name, char **first, char **last);
757 int pcre_get_substring(const char *subject, int *ovector,
758 int stringcount, int stringnumber,
759 const char **stringptr);
761 int pcre_get_substring_list(const char *subject,
762 int *ovector, int stringcount, const char ***listptr);
764 void pcre_free_substring(const char *stringptr);
766 void pcre_free_substring_list(const char **stringptr);
768 const unsigned char *pcre_maketables(void);
770 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
771 int what, void *where);
773 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
775 int pcre_refcount(pcre *code, int adjust);
777 int pcre_config(int what, void *where);
779 char *pcre_version(void);
784 void *(*pcre_malloc)(size_t);
786 void (*pcre_free)(void *);
788 void *(*pcre_stack_malloc)(size_t);
790 void (*pcre_stack_free)(void *);
792 int (*pcre_callout)(pcre_callout_block *);
797 PCRE has its own native API, which is described in this document. There
798 are also some wrapper functions that correspond to the POSIX regular
799 expression API, but they do not give access to all the functionality.
800 They are described in the pcreposix documentation. Both of these APIs
801 define a set of C function calls. A C++ wrapper is also distributed
802 with PCRE. It is documented in the pcrecpp page.
804 The native API C function prototypes are defined in the header file
805 pcre.h, and on Unix systems the library itself is called libpcre. It
806 can normally be accessed by adding -lpcre to the command for linking an
807 application that uses PCRE. The header file defines the macros
808 PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num-
809 bers for the library. Applications can use these to include support
810 for different releases of PCRE.
812 In a Windows environment, if you want to statically link an application
813 program against a non-dll pcre.a file, you must define PCRE_STATIC
814 before including pcre.h or pcrecpp.h, because otherwise the pcre_mal-
815 loc() and pcre_free() exported functions will be declared
816 __declspec(dllimport), with unwanted results.
818 The functions pcre_compile(), pcre_compile2(), pcre_study(), and
819 pcre_exec() are used for compiling and matching regular expressions in
820 a Perl-compatible manner. A sample program that demonstrates the sim-
821 plest way of using them is provided in the file called pcredemo.c in
822 the PCRE source distribution. A listing of this program is given in the
823 pcredemo documentation, and the pcresample documentation describes how
824 to compile and run it.
826 Just-in-time compiler support is an optional feature of PCRE that can
827 be built in appropriate hardware environments. It greatly speeds up the
828 matching performance of many patterns. Simple programs can easily
829 request that it be used if available, by setting an option that is
830 ignored when it is not relevant. More complicated programs might need
831 to make use of the functions pcre_jit_stack_alloc(),
832 pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to control
833 the JIT code's memory usage. These functions are discussed in the
834 pcrejit documentation.
836 A second matching function, pcre_dfa_exec(), which is not Perl-compati-
837 ble, is also provided. This uses a different algorithm for the match-
838 ing. The alternative algorithm finds all possible matches (at a given
839 point in the subject), and scans the subject just once (unless there
840 are lookbehind assertions). However, this algorithm does not return
841 captured substrings. A description of the two matching algorithms and
842 their advantages and disadvantages is given in the pcrematching docu-
843 mentation.
845 In addition to the main compiling and matching functions, there are
846 convenience functions for extracting captured substrings from a subject
847 string that is matched by pcre_exec(). They are:
849 pcre_copy_substring()
850 pcre_copy_named_substring()
851 pcre_get_substring()
852 pcre_get_named_substring()
853 pcre_get_substring_list()
854 pcre_get_stringnumber()
855 pcre_get_stringtable_entries()
857 pcre_free_substring() and pcre_free_substring_list() are also provided,
858 to free the memory used for extracted strings.
860 The function pcre_maketables() is used to build a set of character
861 tables in the current locale for passing to pcre_compile(),
862 pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
863 provided for specialist use. Most commonly, no special tables are
864 passed, in which case internal tables that are generated when PCRE is
865 built are used.
867 The function pcre_fullinfo() is used to find out information about a
868 compiled pattern; pcre_info() is an obsolete version that returns only
869 some of the available information, but is retained for backwards com-
870 patibility. The function pcre_version() returns a pointer to a string
871 containing the version of PCRE and its date of release.
873 The function pcre_refcount() maintains a reference count in a data
874 block containing a compiled pattern. This is provided for the benefit
875 of object-oriented applications.
877 The global variables pcre_malloc and pcre_free initially contain the
878 entry points of the standard malloc() and free() functions, respec-
879 tively. PCRE calls the memory management functions via these variables,
880 so a calling program can replace them if it wishes to intercept the
881 calls. This should be done before calling any PCRE functions.
883 The global variables pcre_stack_malloc and pcre_stack_free are also
884 indirections to memory management functions. These special functions
885 are used only when PCRE is compiled to use the heap for remembering
886 data, instead of recursive function calls, when running the pcre_exec()
887 function. See the pcrebuild documentation for details of how to do
888 this. It is a non-standard way of building PCRE, for use in environ-
889 ments that have limited stacks. Because of the greater use of memory
890 management, it runs more slowly. Separate functions are provided so
891 that special-purpose external code can be used for this case. When
892 used, these functions are always called in a stack-like manner (last
893 obtained, first freed), and always for memory blocks of the same size.
894 There is a discussion about PCRE's stack usage in the pcrestack docu-
895 mentation.
897 The global variable pcre_callout initially contains NULL. It can be set
898 by the caller to a "callout" function, which PCRE will then call at
899 specified points during a matching operation. Details are given in the
900 pcrecallout documentation.
905 PCRE supports five different conventions for indicating line breaks in
906 strings: a single CR (carriage return) character, a single LF (line-
907 feed) character, the two-character sequence CRLF, any of the three pre-
908 ceding, or any Unicode newline sequence. The Unicode newline sequences
909 are the three just mentioned, plus the single characters VT (vertical
910 tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
911 separator, U+2028), and PS (paragraph separator, U+2029).
913 Each of the first three conventions is used by at least one operating
914 system as its standard newline sequence. When PCRE is built, a default
915 can be specified. The default default is LF, which is the Unix stan-
916 dard. When PCRE is run, the default can be overridden, either when a
917 pattern is compiled, or when it is matched.
919 At compile time, the newline convention can be specified by the options
920 argument of pcre_compile(), or it can be specified by special text at
921 the start of the pattern itself; this overrides any other settings. See
922 the pcrepattern page for details of the special character sequences.
924 In the PCRE documentation the word "newline" is used to mean "the char-
925 acter or pair of characters that indicate a line break". The choice of
926 newline convention affects the handling of the dot, circumflex, and
927 dollar metacharacters, the handling of #-comments in /x mode, and, when
928 CRLF is a recognized line ending sequence, the match position advance-
929 ment for a non-anchored pattern. There is more detail about this in the
930 section on pcre_exec() options below.
932 The choice of newline convention does not affect the interpretation of
933 the \n or \r escape sequences, nor does it affect what \R matches,
934 which is controlled in a similar way, but by separate options.
939 The PCRE functions can be used in multi-threading applications, with
940 the proviso that the memory management functions pointed to by
941 pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
942 callout function pointed to by pcre_callout, are shared by all threads.
944 The compiled form of a regular expression is not altered during match-
945 ing, so the same compiled pattern can safely be used by several threads
946 at once.
948 If the just-in-time optimization feature is being used, it needs sepa-
949 rate memory stack areas for each thread. See the pcrejit documentation
950 for more details.
955 The compiled form of a regular expression can be saved and re-used at a
956 later time, possibly by a different program, and even on a host other
957 than the one on which it was compiled. Details are given in the
958 pcreprecompile documentation. However, compiling a regular expression
959 with one version of PCRE for use with a different version is not guar-
960 anteed to work and may cause crashes.
965 int pcre_config(int what, void *where);
967 The function pcre_config() makes it possible for a PCRE client to dis-
968 cover which optional features have been compiled into the PCRE library.
969 The pcrebuild documentation has more details about these optional fea-
970 tures.
972 The first argument for pcre_config() is an integer, specifying which
973 information is required; the second argument is a pointer to a variable
974 into which the information is placed. The following information is
975 available:
979 The output is an integer that is set to one if UTF-8 support is avail-
980 able; otherwise it is set to zero.
984 The output is an integer that is set to one if support for Unicode
985 character properties is available; otherwise it is set to zero.
989 The output is an integer that is set to one if support for just-in-time
990 compiling is available; otherwise it is set to zero.
994 The output is an integer whose value specifies the default character
995 sequence that is recognized as meaning "newline". The four values that
996 are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
997 and -1 for ANY. Though they are derived from ASCII, the same values
998 are returned in EBCDIC environments. The default should normally corre-
999 spond to the standard sequence for your operating system.
1003 The output is an integer whose value indicates what character sequences
1004 the \R escape sequence matches by default. A value of 0 means that \R
1005 matches any Unicode line ending sequence; a value of 1 means that \R
1006 matches only CR, LF, or CRLF. The default can be overridden when a pat-
1007 tern is compiled or matched.
1011 The output is an integer that contains the number of bytes used for
1012 internal linkage in compiled regular expressions. The value is 2, 3, or
1013 4. Larger values allow larger regular expressions to be compiled, at
1014 the expense of slower matching. The default value of 2 is sufficient
1015 for all but the most massive patterns, since it allows the compiled
1016 pattern to be up to 64K in size.
1020 The output is an integer that contains the threshold above which the
1021 POSIX interface uses malloc() for output vectors. Further details are
1022 given in the pcreposix documentation.
1026 The output is a long integer that gives the default limit for the num-
1027 ber of internal matching function calls in a pcre_exec() execution.
1028 Further details are given with pcre_exec() below.
1032 The output is a long integer that gives the default limit for the depth
1033 of recursion when calling the internal matching function in a
1034 pcre_exec() execution. Further details are given with pcre_exec()
1035 below.
1039 The output is an integer that is set to one if internal recursion when
1040 running pcre_exec() is implemented by recursive function calls that use
1041 the stack to remember their state. This is the usual way that PCRE is
1042 compiled. The output is zero if PCRE was compiled to use blocks of data
1043 on the heap instead of recursive function calls. In this case,
1044 pcre_stack_malloc and pcre_stack_free are called to manage memory
1045 blocks on the heap, thus avoiding the use of the stack.
1050 pcre *pcre_compile(const char *pattern, int options,
1051 const char **errptr, int *erroffset,
1052 const unsigned char *tableptr);
1054 pcre *pcre_compile2(const char *pattern, int options,
1055 int *errorcodeptr,
1056 const char **errptr, int *erroffset,
1057 const unsigned char *tableptr);
1059 Either of the functions pcre_compile() or pcre_compile2() can be called
1060 to compile a pattern into an internal form. The only difference between
1061 the two interfaces is that pcre_compile2() has an additional argument,
1062 errorcodeptr, via which a numerical error code can be returned. To
1063 avoid too much repetition, we refer just to pcre_compile() below, but
1064 the information applies equally to pcre_compile2().
1066 The pattern is a C string terminated by a binary zero, and is passed in
1067 the pattern argument. A pointer to a single block of memory that is
1068 obtained via pcre_malloc is returned. This contains the compiled code
1069 and related data. The pcre type is defined for the returned block; this
1070 is a typedef for a structure whose contents are not externally defined.
1071 It is up to the caller to free the memory (via pcre_free) when it is no
1072 longer required.
1074 Although the compiled code of a PCRE regex is relocatable, that is, it
1075 does not depend on memory location, the complete pcre data block is not
1076 fully relocatable, because it may contain a copy of the tableptr argu-
1077 ment, which is an address (see below).
1079 The options argument contains various bit settings that affect the com-
1080 pilation. It should be zero if no options are required. The available
1081 options are described below. Some of them (in particular, those that
1082 are compatible with Perl, but some others as well) can also be set and
1083 unset from within the pattern (see the detailed description in the
1084 pcrepattern documentation). For those options that can be different in
1085 different parts of the pattern, the contents of the options argument
1086 specifies their settings at the start of compilation and execution. The
1088 PCRE_NO_START_OPT options can be set at the time of matching as well as
1089 at compile time.
1091 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
1092 if compilation of a pattern fails, pcre_compile() returns NULL, and
1093 sets the variable pointed to by errptr to point to a textual error mes-
1094 sage. This is a static string that is part of the library. You must not
1095 try to free it. Normally, the offset from the start of the pattern to
1096 the byte that was being processed when the error was discovered is
1097 placed in the variable pointed to by erroffset, which must not be NULL
1098 (if it is, an immediate error is given). However, for an invalid UTF-8
1099 string, the offset is that of the first byte of the failing character.
1100 Also, some errors are not detected until checks are carried out when
1101 the whole pattern has been scanned; in these cases the offset passed
1102 back is the length of the pattern.
1104 Note that the offset is in bytes, not characters, even in UTF-8 mode.
1105 It may sometimes point into the middle of a UTF-8 character.
1107 If pcre_compile2() is used instead of pcre_compile(), and the error-
1108 codeptr argument is not NULL, a non-zero error code number is returned
1109 via this argument in the event of an error. This is in addition to the
1110 textual error message. Error codes and messages are listed below.
1112 If the final argument, tableptr, is NULL, PCRE uses a default set of
1113 character tables that are built when PCRE is compiled, using the
1114 default C locale. Otherwise, tableptr must be an address that is the
1115 result of a call to pcre_maketables(). This value is stored with the
1116 compiled pattern, and used again by pcre_exec(), unless another table
1117 pointer is passed to it. For more discussion, see the section on locale
1118 support below.
1120 This code fragment shows a typical straightforward call to pcre_com-
1121 pile():
1123 pcre *re;
1124 const char *error;
1125 int erroffset;
1126 re = pcre_compile(
1127 "^A.*Z", /* the pattern */
1128 0, /* default options */
1129 &error, /* for error message */
1130 &erroffset, /* for error offset */
1131 NULL); /* use default character tables */
1133 The following names for option bits are defined in the pcre.h header
1134 file:
1138 If this bit is set, the pattern is forced to be "anchored", that is, it
1139 is constrained to match only at the first matching point in the string
1140 that is being searched (the "subject string"). This effect can also be
1141 achieved by appropriate constructs in the pattern itself, which is the
1142 only way to do it in Perl.
1146 If this bit is set, pcre_compile() automatically inserts callout items,
1147 all with number 255, before each pattern item. For discussion of the
1148 callout facility, see the pcrecallout documentation.
1153 These options (which are mutually exclusive) control what the \R escape
1154 sequence matches. The choice is either to match only CR, LF, or CRLF,
1155 or to match any Unicode newline sequence. The default is specified when
1156 PCRE is built. It can be overridden from within the pattern, or by set-
1157 ting an option when a compiled pattern is matched.
1161 If this bit is set, letters in the pattern match both upper and lower
1162 case letters. It is equivalent to Perl's /i option, and it can be
1163 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1164 always understands the concept of case for characters whose values are
1165 less than 128, so caseless matching is always possible. For characters
1166 with higher values, the concept of case is supported if PCRE is com-
1167 piled with Unicode property support, but not otherwise. If you want to
1168 use caseless matching for characters 128 and above, you must ensure
1169 that PCRE is compiled with Unicode property support as well as with
1170 UTF-8 support.
1174 If this bit is set, a dollar metacharacter in the pattern matches only
1175 at the end of the subject string. Without this option, a dollar also
1176 matches immediately before a newline at the end of the string (but not
1177 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1178 if PCRE_MULTILINE is set. There is no equivalent to this option in
1179 Perl, and no way to set it within a pattern.
1183 If this bit is set, a dot metacharacter in the pattern matches a char-
1184 acter of any value, including one that indicates a newline. However, it
1185 only ever matches one character, even if newlines are coded as CRLF.
1186 Without this option, a dot does not match when the current position is
1187 at a newline. This option is equivalent to Perl's /s option, and it can
1188 be changed within a pattern by a (?s) option setting. A negative class
1189 such as [^a] always matches newline characters, independent of the set-
1190 ting of this option.
1194 If this bit is set, names used to identify capturing subpatterns need
1195 not be unique. This can be helpful for certain types of pattern when it
1196 is known that only one instance of the named subpattern can ever be
1197 matched. There are more details of named subpatterns below; see also
1198 the pcrepattern documentation.
1202 If this bit is set, whitespace data characters in the pattern are
1203 totally ignored except when escaped or inside a character class. White-
1204 space does not include the VT character (code 11). In addition, charac-
1205 ters between an unescaped # outside a character class and the next new-
1206 line, inclusive, are also ignored. This is equivalent to Perl's /x
1207 option, and it can be changed within a pattern by a (?x) option set-
1208 ting.
1210 Which characters are interpreted as newlines is controlled by the
1211 options passed to pcre_compile() or by a special sequence at the start
1212 of the pattern, as described in the section entitled "Newline conven-
1213 tions" in the pcrepattern documentation. Note that the end of this type
1214 of comment is a literal newline sequence in the pattern; escape
1215 sequences that happen to represent a newline do not count.
1217 This option makes it possible to include comments inside complicated
1218 patterns. Note, however, that this applies only to data characters.
1219 Whitespace characters may never appear within special character
1220 sequences in a pattern, for example within the sequence (?( that intro-
1221 duces a conditional subpattern.
1225 This option was invented in order to turn on additional functionality
1226 of PCRE that is incompatible with Perl, but it is currently of very
1227 little use. When set, any backslash in a pattern that is followed by a
1228 letter that has no special meaning causes an error, thus reserving
1229 these combinations for future expansion. By default, as in Perl, a
1230 backslash followed by a letter with no special meaning is treated as a
1231 literal. (Perl can, however, be persuaded to give an error for this, by
1232 running it with the -w option.) There are at present no other features
1233 controlled by this option. It can also be set by a (?X) option setting
1234 within a pattern.
1238 If this option is set, an unanchored pattern is required to match
1239 before or at the first newline in the subject string, though the
1240 matched text may continue over the newline.
1244 If this option is set, PCRE's behaviour is changed in some ways so that
1245 it is compatible with JavaScript rather than Perl. The changes are as
1246 follows:
1248 (1) A lone closing square bracket in a pattern causes a compile-time
1249 error, because this is illegal in JavaScript (by default it is treated
1250 as a data character). Thus, the pattern AB]CD becomes illegal when this
1251 option is set.
1253 (2) At run time, a back reference to an unset subpattern group matches
1254 an empty string (by default this causes the current matching alterna-
1255 tive to fail). A pattern such as (\1)(a) succeeds when this option is
1256 set (assuming it can find an "a" in the subject), whereas it fails by
1257 default, for Perl compatibility.
1259 (3) \U matches an upper case "U" character; by default \U causes a com-
1260 pile time error (Perl uses \U to upper case subsequent characters).
1262 (4) \u matches a lower case "u" character unless it is followed by four
1263 hexadecimal digits, in which case the hexadecimal number defines the
1264 code point to match. By default, \u causes a compile time error (Perl
1265 uses it to upper case the following character).
1267 (5) \x matches a lower case "x" character unless it is followed by two
1268 hexadecimal digits, in which case the hexadecimal number defines the
1269 code point to match. By default, as in Perl, a hexadecimal number is
1270 always expected after \x, but it may have zero, one, or two digits (so,
1271 for example, \xz matches a binary zero character followed by z).
1275 By default, PCRE treats the subject string as consisting of a single
1276 line of characters (even if it actually contains newlines). The "start
1277 of line" metacharacter (^) matches only at the start of the string,
1278 while the "end of line" metacharacter ($) matches only at the end of
1279 the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1280 is set). This is the same as Perl.
1282 When PCRE_MULTILINE it is set, the "start of line" and "end of line"
1283 constructs match immediately following or immediately before internal
1284 newlines in the subject string, respectively, as well as at the very
1285 start and end. This is equivalent to Perl's /m option, and it can be
1286 changed within a pattern by a (?m) option setting. If there are no new-
1287 lines in a subject string, or no occurrences of ^ or $ in a pattern,
1288 setting PCRE_MULTILINE has no effect.
1296 These options override the default newline definition that was chosen
1297 when PCRE was built. Setting the first or the second specifies that a
1298 newline is indicated by a single character (CR or LF, respectively).
1299 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1300 two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies
1301 that any of the three preceding sequences should be recognized. Setting
1302 PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be
1303 recognized. The Unicode newline sequences are the three just mentioned,
1304 plus the single characters VT (vertical tab, U+000B), FF (formfeed,
1305 U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
1306 (paragraph separator, U+2029). The last two are recognized only in
1307 UTF-8 mode.
1309 The newline setting in the options word uses three bits that are
1310 treated as a number, giving eight possibilities. Currently only six are
1311 used (default plus the five values above). This means that if you set
1312 more than one newline option, the combination may or may not be sensi-
1313 ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1314 PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and
1315 cause an error.
1317 The only time that a line break in a pattern is specially recognized
1318 when compiling is when PCRE_EXTENDED is set. CR and LF are whitespace
1319 characters, and so are ignored in this mode. Also, an unescaped # out-
1320 side a character class indicates a comment that lasts until after the
1321 next line break sequence. In other circumstances, line break sequences
1322 in patterns are treated as literal data.
1324 The newline option that is set at compile time becomes the default that
1325 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1329 If this option is set, it disables the use of numbered capturing paren-
1330 theses in the pattern. Any opening parenthesis that is not followed by
1331 ? behaves as if it were followed by ?: but named parentheses can still
1332 be used for capturing (and they acquire numbers in the usual way).
1333 There is no equivalent of this option in Perl.
1337 This is an option that acts at matching time; that is, it is really an
1338 option for pcre_exec() or pcre_dfa_exec(). If it is set at compile
1339 time, it is remembered with the compiled pattern and assumed at match-
1340 ing time. For details see the discussion of PCRE_NO_START_OPTIMIZE
1341 below.
1345 This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W,
1346 \w, and some of the POSIX character classes. By default, only ASCII
1347 characters are recognized, but if PCRE_UCP is set, Unicode properties
1348 are used instead to classify characters. More details are given in the
1349 section on generic character types in the pcrepattern page. If you set
1350 PCRE_UCP, matching one of the items it affects takes much longer. The
1351 option is available only if PCRE has been compiled with Unicode prop-
1352 erty support.
1356 This option inverts the "greediness" of the quantifiers so that they
1357 are not greedy by default, but become greedy if followed by "?". It is
1358 not compatible with Perl. It can also be set by a (?U) option setting
1359 within the pattern.
1361 PCRE_UTF8
1363 This option causes PCRE to regard both the pattern and the subject as
1364 strings of UTF-8 characters instead of single-byte character strings.
1365 However, it is available only when PCRE is built to include UTF-8 sup-
1366 port. If not, the use of this option provokes an error. Details of how
1367 this option changes the behaviour of PCRE are given in the pcreunicode
1368 page.
1372 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1373 automatically checked. There is a discussion about the validity of
1374 UTF-8 strings in the main pcre page. If an invalid UTF-8 sequence of
1375 bytes is found, pcre_compile() returns an error. If you already know
1376 that your pattern is valid, and you want to skip this check for perfor-
1377 mance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is
1378 set, the effect of passing an invalid UTF-8 string as a pattern is
1379 undefined. It may cause your program to crash. Note that this option
1380 can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the
1381 UTF-8 validity checking of subject strings.
1386 The following table lists the error codes than may be returned by
1387 pcre_compile2(), along with the error messages that may be returned by
1388 both compiling functions. As PCRE has developed, some error codes have
1389 fallen out of use. To avoid confusion, they have not been re-used.
1391 0 no error
1392 1 \ at end of pattern
1393 2 \c at end of pattern
1394 3 unrecognized character follows \
1395 4 numbers out of order in {} quantifier
1396 5 number too big in {} quantifier
1397 6 missing terminating ] for character class
1398 7 invalid escape sequence in character class
1399 8 range out of order in character class
1400 9 nothing to repeat
1401 10 [this code is not in use]
1402 11 internal error: unexpected repeat
1403 12 unrecognized character after (? or (?-
1404 13 POSIX named classes are supported only within a class
1405 14 missing )
1406 15 reference to non-existent subpattern
1407 16 erroffset passed as NULL
1408 17 unknown option bit(s) set
1409 18 missing ) after comment
1410 19 [this code is not in use]
1411 20 regular expression is too large
1412 21 failed to get memory
1413 22 unmatched parentheses
1414 23 internal error: code overflow
1415 24 unrecognized character after (?<
1416 25 lookbehind assertion is not fixed length
1417 26 malformed number or name after (?(
1418 27 conditional group contains more than two branches
1419 28 assertion expected after (?(
1420 29 (?R or (?[+-]digits must be followed by )
1421 30 unknown POSIX class name
1422 31 POSIX collating elements are not supported
1423 32 this version of PCRE is not compiled with PCRE_UTF8 support
1424 33 [this code is not in use]
1425 34 character value in \x{...} sequence is too large
1426 35 invalid condition (?(0)
1427 36 \C not allowed in lookbehind assertion
1428 37 PCRE does not support \L, \l, \N{name}, \U, or \u
1429 38 number after (?C is > 255
1430 39 closing ) for (?C expected
1431 40 recursive call could loop indefinitely
1432 41 unrecognized character after (?P
1433 42 syntax error in subpattern name (missing terminator)
1434 43 two named subpatterns have the same name
1435 44 invalid UTF-8 string
1436 45 support for \P, \p, and \X has not been compiled
1437 46 malformed \P or \p sequence
1438 47 unknown property name after \P or \p
1439 48 subpattern name is too long (maximum 32 characters)
1440 49 too many named subpatterns (maximum 10000)
1441 50 [this code is not in use]
1442 51 octal value is greater than \377 (not in UTF-8 mode)
1443 52 internal error: overran compiling workspace
1444 53 internal error: previously-checked referenced subpattern
1445 not found
1446 54 DEFINE group contains more than one branch
1447 55 repeating a DEFINE group is not allowed
1448 56 inconsistent NEWLINE options
1449 57 \g is not followed by a braced, angle-bracketed, or quoted
1450 name/number or by a plain number
1451 58 a numbered reference must not be zero
1452 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
1453 60 (*VERB) not recognized
1454 61 number is too big
1455 62 subpattern name expected
1456 63 digit expected after (?+
1457 64 ] is an invalid data character in JavaScript compatibility mode
1458 65 different names for subpatterns of the same number are
1459 not allowed
1460 66 (*MARK) must have an argument
1461 67 this version of PCRE is not compiled with PCRE_UCP support
1462 68 \c must be followed by an ASCII character
1463 69 \k is not followed by a braced, angle-bracketed, or quoted name
1465 The numbers 32 and 10000 in errors 48 and 49 are defaults; different
1466 values may be used if the limits were changed when PCRE was built.
1471 pcre_extra *pcre_study(const pcre *code, int options
1472 const char **errptr);
1474 If a compiled pattern is going to be used several times, it is worth
1475 spending more time analyzing it in order to speed up the time taken for
1476 matching. The function pcre_study() takes a pointer to a compiled pat-
1477 tern as its first argument. If studying the pattern produces additional
1478 information that will help speed up matching, pcre_study() returns a
1479 pointer to a pcre_extra block, in which the study_data field points to
1480 the results of the study.
1482 The returned value from pcre_study() can be passed directly to
1483 pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con-
1484 tains other fields that can be set by the caller before the block is
1485 passed; these are described below in the section on matching a pattern.
1487 If studying the pattern does not produce any useful information,
1488 pcre_study() returns NULL. In that circumstance, if the calling program
1489 wants to pass any of the other fields to pcre_exec() or
1490 pcre_dfa_exec(), it must set up its own pcre_extra block.
1492 The second argument of pcre_study() contains option bits. There is only
1493 one option: PCRE_STUDY_JIT_COMPILE. If this is set, and the just-in-
1494 time compiler is available, the pattern is further compiled into
1495 machine code that executes much faster than the pcre_exec() matching
1496 function. If the just-in-time compiler is not available, this option is
1497 ignored. All other bits in the options argument must be zero.
1499 JIT compilation is a heavyweight optimization. It can take some time
1500 for patterns to be analyzed, and for one-off matches and simple pat-
1501 terns the benefit of faster execution might be offset by a much slower
1502 study time. Not all patterns can be optimized by the JIT compiler. For
1503 those that cannot be handled, matching automatically falls back to the
1504 pcre_exec() interpreter. For more details, see the pcrejit documenta-
1505 tion.
1507 The third argument for pcre_study() is a pointer for an error message.
1508 If studying succeeds (even if no data is returned), the variable it
1509 points to is set to NULL. Otherwise it is set to point to a textual
1510 error message. This is a static string that is part of the library. You
1511 must not try to free it. You should test the error pointer for NULL
1512 after calling pcre_study(), to be sure that it has run successfully.
1514 When you are finished with a pattern, you can free the memory used for
1515 the study data by calling pcre_free_study(). This function was added to
1516 the API for release 8.20. For earlier versions, the memory could be
1517 freed with pcre_free(), just like the pattern itself. This will still
1518 work in cases where PCRE_STUDY_JIT_COMPILE is not used, but it is
1519 advisable to change to the new function when convenient.
1521 This is a typical way in which pcre_study() is used (except that in a
1522 real application there should be tests for errors):
1524 int rc;
1525 pcre *re;
1526 pcre_extra *sd;
1527 re = pcre_compile("pattern", 0, &error, &erroroffset, NULL);
1528 sd = pcre_study(
1529 re, /* result of pcre_compile() */
1530 0, /* no options */
1531 &error); /* set to NULL or points to a message */
1532 rc = pcre_exec( /* see below for details of pcre_exec() options */
1533 re, sd, "subject", 7, 0, 0, ovector, 30);
1534 ...
1535 pcre_free_study(sd);
1536 pcre_free(re);
1538 Studying a pattern does two things: first, a lower bound for the length
1539 of subject string that is needed to match the pattern is computed. This
1540 does not mean that there are any strings of that length that match, but
1541 it does guarantee that no shorter strings match. The value is used by
1542 pcre_exec() and pcre_dfa_exec() to avoid wasting time by trying to
1543 match strings that are shorter than the lower bound. You can find out
1544 the value in a calling program via the pcre_fullinfo() function.
1546 Studying a pattern is also useful for non-anchored patterns that do not
1547 have a single fixed starting character. A bitmap of possible starting
1548 bytes is created. This speeds up finding a position in the subject at
1549 which to start matching.
1551 These two optimizations apply to both pcre_exec() and pcre_dfa_exec().
1552 However, they are not used by pcre_exec() if pcre_study() is called
1553 with the PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling is
1554 successful. The optimizations can be disabled by setting the
1555 PCRE_NO_START_OPTIMIZE option when calling pcre_exec() or
1556 pcre_dfa_exec(). You might want to do this if your pattern contains
1557 callouts or (*MARK) (which cannot be handled by the JIT compiler), and
1558 you want to make use of these facilities in cases where matching fails.
1559 See the discussion of PCRE_NO_START_OPTIMIZE below.
1564 PCRE handles caseless matching, and determines whether characters are
1565 letters, digits, or whatever, by reference to a set of tables, indexed
1566 by character value. When running in UTF-8 mode, this applies only to
1567 characters with codes less than 128. By default, higher-valued codes
1568 never match escapes such as \w or \d, but they can be tested with \p if
1569 PCRE is built with Unicode character property support. Alternatively,
1570 the PCRE_UCP option can be set at compile time; this causes \w and
1571 friends to use Unicode property support instead of built-in tables. The
1572 use of locales with Unicode is discouraged. If you are handling charac-
1573 ters with codes greater than 128, you should either use UTF-8 and Uni-
1574 code, or use locales, but not try to mix the two.
1576 PCRE contains an internal set of tables that are used when the final
1577 argument of pcre_compile() is NULL. These are sufficient for many
1578 applications. Normally, the internal tables recognize only ASCII char-
1579 acters. However, when PCRE is built, it is possible to cause the inter-
1580 nal tables to be rebuilt in the default "C" locale of the local system,
1581 which may cause them to be different.
1583 The internal tables can always be overridden by tables supplied by the
1584 application that calls PCRE. These may be created in a different locale
1585 from the default. As more and more applications change to using Uni-
1586 code, the need for this locale support is expected to die away.
1588 External tables are built by calling the pcre_maketables() function,
1589 which has no arguments, in the relevant locale. The result can then be
1590 passed to pcre_compile() or pcre_exec() as often as necessary. For
1591 example, to build and use tables that are appropriate for the French
1592 locale (where accented characters with values greater than 128 are
1593 treated as letters), the following code could be used:
1595 setlocale(LC_CTYPE, "fr_FR");
1596 tables = pcre_maketables();
1597 re = pcre_compile(..., tables);
1599 The locale name "fr_FR" is used on Linux and other Unix-like systems;
1600 if you are using Windows, the name for the French locale is "french".
1602 When pcre_maketables() runs, the tables are built in memory that is
1603 obtained via pcre_malloc. It is the caller's responsibility to ensure
1604 that the memory containing the tables remains available for as long as
1605 it is needed.
1607 The pointer that is passed to pcre_compile() is saved with the compiled
1608 pattern, and the same tables are used via this pointer by pcre_study()
1609 and normally also by pcre_exec(). Thus, by default, for any single pat-
1610 tern, compilation, studying and matching all happen in the same locale,
1611 but different patterns can be compiled in different locales.
1613 It is possible to pass a table pointer or NULL (indicating the use of
1614 the internal tables) to pcre_exec(). Although not intended for this
1615 purpose, this facility could be used to match a pattern in a different
1616 locale from the one in which it was compiled. Passing table pointers at
1617 run time is discussed below in the section on matching a pattern.
1622 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1623 int what, void *where);
1625 The pcre_fullinfo() function returns information about a compiled pat-
1626 tern. It replaces the obsolete pcre_info() function, which is neverthe-
1627 less retained for backwards compability (and is documented below).
1629 The first argument for pcre_fullinfo() is a pointer to the compiled
1630 pattern. The second argument is the result of pcre_study(), or NULL if
1631 the pattern was not studied. The third argument specifies which piece
1632 of information is required, and the fourth argument is a pointer to a
1633 variable to receive the data. The yield of the function is zero for
1634 success, or one of the following negative numbers:
1636 PCRE_ERROR_NULL the argument code was NULL
1637 the argument where was NULL
1638 PCRE_ERROR_BADMAGIC the "magic number" was not found
1639 PCRE_ERROR_BADOPTION the value of what was invalid
1641 The "magic number" is placed at the start of each compiled pattern as
1642 an simple check against passing an arbitrary memory pointer. Here is a
1643 typical call of pcre_fullinfo(), to obtain the length of the compiled
1644 pattern:
1646 int rc;
1647 size_t length;
1648 rc = pcre_fullinfo(
1649 re, /* result of pcre_compile() */
1650 sd, /* result of pcre_study(), or NULL */
1651 PCRE_INFO_SIZE, /* what is required */
1652 &length); /* where to put the data */
1654 The possible values for the third argument are defined in pcre.h, and
1655 are as follows:
1659 Return the number of the highest back reference in the pattern. The
1660 fourth argument should point to an int variable. Zero is returned if
1661 there are no back references.
1665 Return the number of capturing subpatterns in the pattern. The fourth
1666 argument should point to an int variable.
1670 Return a pointer to the internal default character tables within PCRE.
1671 The fourth argument should point to an unsigned char * variable. This
1672 information call is provided for internal use by the pcre_study() func-
1673 tion. External callers can cause PCRE to use its internal tables by
1674 passing a NULL table pointer.
1678 Return information about the first byte of any matched string, for a
1679 non-anchored pattern. The fourth argument should point to an int vari-
1680 able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1681 is still recognized for backwards compatibility.)
1683 If there is a fixed first byte, for example, from a pattern such as
1684 (cat|cow|coyote), its value is returned. Otherwise, if either
1686 (a) the pattern was compiled with the PCRE_MULTILINE option, and every
1687 branch starts with "^", or
1689 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1690 set (if it were set, the pattern would be anchored),
1692 -1 is returned, indicating that the pattern matches only at the start
1693 of a subject string or after any newline within the string. Otherwise
1694 -2 is returned. For anchored patterns, -2 is returned.
1698 If the pattern was studied, and this resulted in the construction of a
1699 256-bit table indicating a fixed set of bytes for the first byte in any
1700 matching string, a pointer to the table is returned. Otherwise NULL is
1701 returned. The fourth argument should point to an unsigned char * vari-
1702 able.
1706 Return 1 if the pattern contains any explicit matches for CR or LF
1707 characters, otherwise 0. The fourth argument should point to an int
1708 variable. An explicit match is either a literal CR or LF character, or
1709 \r or \n.
1713 Return 1 if the (?J) or (?-J) option setting is used in the pattern,
1714 otherwise 0. The fourth argument should point to an int variable. (?J)
1715 and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.
1719 Return 1 if the pattern was studied with the PCRE_STUDY_JIT_COMPILE
1720 option, and just-in-time compiling was successful. The fourth argument
1721 should point to an int variable. A return value of 0 means that JIT
1722 support is not available in this version of PCRE, or that the pattern
1723 was not studied with the PCRE_STUDY_JIT_COMPILE option, or that the JIT
1724 compiler could not handle this particular pattern. See the pcrejit doc-
1725 umentation for details of what can and cannot be handled.
1729 If the pattern was successfully studied with the PCRE_STUDY_JIT_COMPILE
1730 option, return the size of the JIT compiled code, otherwise return
1731 zero. The fourth argument should point to a size_t variable.
1735 Return the value of the rightmost literal byte that must exist in any
1736 matched string, other than at its start, if such a byte has been
1737 recorded. The fourth argument should point to an int variable. If there
1738 is no such byte, -1 is returned. For anchored patterns, a last literal
1739 byte is recorded only if it follows something of variable length. For
1740 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1741 /^a\dz\d/ the returned value is -1.
1745 If the pattern was studied and a minimum length for matching subject
1746 strings was computed, its value is returned. Otherwise the returned
1747 value is -1. The value is a number of characters, not bytes (this may
1748 be relevant in UTF-8 mode). The fourth argument should point to an int
1749 variable. A non-negative value is a lower bound to the length of any
1750 matching string. There may not be any strings of that length that do
1751 actually match, but every string that does match is at least that long.
1757 PCRE supports the use of named as well as numbered capturing parenthe-
1758 ses. The names are just an additional way of identifying the parenthe-
1759 ses, which still acquire numbers. Several convenience functions such as
1760 pcre_get_named_substring() are provided for extracting captured sub-
1761 strings by name. It is also possible to extract the data directly, by
1762 first converting the name to a number in order to access the correct
1763 pointers in the output vector (described with pcre_exec() below). To do
1764 the conversion, you need to use the name-to-number map, which is
1765 described by these three values.
1767 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1768 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1769 of each entry; both of these return an int value. The entry size
1770 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
1771 a pointer to the first entry of the table (a pointer to char). The
1772 first two bytes of each entry are the number of the capturing parenthe-
1773 sis, most significant byte first. The rest of the entry is the corre-
1774 sponding name, zero terminated.
1776 The names are in alphabetical order. Duplicate names may appear if (?|
1777 is used to create multiple groups with the same number, as described in
1778 the section on duplicate subpattern numbers in the pcrepattern page.
1779 Duplicate names for subpatterns with different numbers are permitted
1780 only if PCRE_DUPNAMES is set. In all cases of duplicate names, they
1781 appear in the table in the order in which they were found in the pat-
1782 tern. In the absence of (?| this is the order of increasing number;
1783 when (?| is used this is not necessarily the case because later subpat-
1784 terns may have lower numbers.
1786 As a simple example of the name/number table, consider the following
1787 pattern (assume PCRE_EXTENDED is set, so white space - including new-
1788 lines - is ignored):
1790 (?<date> (?<year>(\d\d)?\d\d) -
1791 (?<month>\d\d) - (?<day>\d\d) )
1793 There are four named subpatterns, so the table has four entries, and
1794 each entry in the table is eight bytes long. The table is as follows,
1795 with non-printing bytes shows in hexadecimal, and undefined bytes shown
1796 as ??:
1798 00 01 d a t e 00 ??
1799 00 05 d a y 00 ?? ??
1800 00 04 m o n t h 00
1801 00 02 y e a r 00 ??
1803 When writing code to extract data from named subpatterns using the
1804 name-to-number map, remember that the length of the entries is likely
1805 to be different for each compiled pattern.
1809 Return 1 if the pattern can be used for partial matching with
1810 pcre_exec(), otherwise 0. The fourth argument should point to an int
1811 variable. From release 8.00, this always returns 1, because the
1812 restrictions that previously applied to partial matching have been
1813 lifted. The pcrepartial documentation gives details of partial match-
1814 ing.
1818 Return a copy of the options with which the pattern was compiled. The
1819 fourth argument should point to an unsigned long int variable. These
1820 option bits are those specified in the call to pcre_compile(), modified
1821 by any top-level option settings at the start of the pattern itself. In
1822 other words, they are the options that will be in force when matching
1823 starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
1824 the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
1827 A pattern is automatically anchored by PCRE if all of its top-level
1828 alternatives begin with one of the following:
1830 ^ unless PCRE_MULTILINE is set
1831 \A always
1832 \G always
1833 .* if PCRE_DOTALL is set and there are no back
1834 references to the subpattern in which .* appears
1836 For such patterns, the PCRE_ANCHORED bit is set in the options returned
1837 by pcre_fullinfo().
1841 Return the size of the compiled pattern. The fourth argument should
1842 point to a size_t variable. This value does not include the size of the
1843 pcre structure that is returned by pcre_compile(). The value that is
1844 passed as the argument to pcre_malloc() when pcre_compile() is getting
1845 memory in which to place the compiled data is the value returned by
1846 this option plus the size of the pcre structure. Studying a compiled
1847 pattern, with or without JIT, does not alter the value returned by this
1848 option.
1852 Return the size of the data block pointed to by the study_data field in
1853 a pcre_extra block. If pcre_extra is NULL, or there is no study data,
1854 zero is returned. The fourth argument should point to a size_t vari-
1855 able. The study_data field is set by pcre_study() to record informa-
1856 tion that will speed up matching (see the section entitled "Studying a
1857 pattern" above). The format of the study_data block is private, but its
1858 length is made available via this option so that it can be saved and
1859 restored (see the pcreprecompile documentation for details).
1864 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1866 The pcre_info() function is now obsolete because its interface is too
1867 restrictive to return all the available data about a compiled pattern.
1868 New programs should use pcre_fullinfo() instead. The yield of
1869 pcre_info() is the number of capturing subpatterns, or one of the fol-
1870 lowing negative numbers:
1872 PCRE_ERROR_NULL the argument code was NULL
1873 PCRE_ERROR_BADMAGIC the "magic number" was not found
1875 If the optptr argument is not NULL, a copy of the options with which
1876 the pattern was compiled is placed in the integer it points to (see
1877 PCRE_INFO_OPTIONS above).
1879 If the pattern is not anchored and the firstcharptr argument is not
1880 NULL, it is used to pass back information about the first character of
1881 any matched string (see PCRE_INFO_FIRSTBYTE above).
1886 int pcre_refcount(pcre *code, int adjust);
1888 The pcre_refcount() function is used to maintain a reference count in
1889 the data block that contains a compiled pattern. It is provided for the
1890 benefit of applications that operate in an object-oriented manner,
1891 where different parts of the application may be using the same compiled
1892 pattern, but you want to free the block when they are all done.
1894 When a pattern is compiled, the reference count field is initialized to
1895 zero. It is changed only by calling this function, whose action is to
1896 add the adjust value (which may be positive or negative) to it. The
1897 yield of the function is the new value. However, the value of the count
1898 is constrained to lie between 0 and 65535, inclusive. If the new value
1899 is outside these limits, it is forced to the appropriate limit value.
1901 Except when it is zero, the reference count is not correctly preserved
1902 if a pattern is compiled on one host and then transferred to a host
1903 whose byte-order is different. (This seems a highly unlikely scenario.)
1908 int pcre_exec(const pcre *code, const pcre_extra *extra,
1909 const char *subject, int length, int startoffset,
1910 int options, int *ovector, int ovecsize);
1912 The function pcre_exec() is called to match a subject string against a
1913 compiled pattern, which is passed in the code argument. If the pattern
1914 was studied, the result of the study should be passed in the extra
1915 argument. You can call pcre_exec() with the same code and extra argu-
1916 ments as many times as you like, in order to match different subject
1917 strings with the same pattern.
1919 This function is the main matching facility of the library, and it
1920 operates in a Perl-like manner. For specialist use there is also an
1921 alternative matching function, which is described below in the section
1922 about the pcre_dfa_exec() function.
1924 In most applications, the pattern will have been compiled (and option-
1925 ally studied) in the same process that calls pcre_exec(). However, it
1926 is possible to save compiled patterns and study data, and then use them
1927 later in different processes, possibly even on different hosts. For a
1928 discussion about this, see the pcreprecompile documentation.
1930 Here is an example of a simple call to pcre_exec():
1932 int rc;
1933 int ovector[30];
1934 rc = pcre_exec(
1935 re, /* result of pcre_compile() */
1936 NULL, /* we didn't study the pattern */
1937 "some string", /* the subject string */
1938 11, /* the length of the subject string */
1939 0, /* start at offset 0 in the subject */
1940 0, /* default options */
1941 ovector, /* vector of integers for substring information */
1942 30); /* number of elements (NOT size in bytes) */
1944 Extra data for pcre_exec()
1946 If the extra argument is not NULL, it must point to a pcre_extra data
1947 block. The pcre_study() function returns such a block (when it doesn't
1948 return NULL), but you can also create one for yourself, and pass addi-
1949 tional information in it. The pcre_extra block contains the following
1950 fields (not necessarily in this order):
1952 unsigned long int flags;
1953 void *study_data;
1954 void *executable_jit;
1955 unsigned long int match_limit;
1956 unsigned long int match_limit_recursion;
1957 void *callout_data;
1958 const unsigned char *tables;
1959 unsigned char **mark;
1961 The flags field is a bitmap that specifies which of the other fields
1962 are set. The flag bits are:
1972 Other flag bits should be set to zero. The study_data field and some-
1973 times the executable_jit field are set in the pcre_extra block that is
1974 returned by pcre_study(), together with the appropriate flag bits. You
1975 should not set these yourself, but you may add to the block by setting
1976 the other fields and their corresponding flag bits.
1978 The match_limit field provides a means of preventing PCRE from using up
1979 a vast amount of resources when running patterns that are not going to
1980 match, but which have a very large number of possibilities in their
1981 search trees. The classic example is a pattern that uses nested unlim-
1982 ited repeats.
1984 Internally, pcre_exec() uses a function called match(), which it calls
1985 repeatedly (sometimes recursively). The limit set by match_limit is
1986 imposed on the number of times this function is called during a match,
1987 which has the effect of limiting the amount of backtracking that can
1988 take place. For patterns that are not anchored, the count restarts from
1989 zero for each position in the subject string.
1991 When pcre_exec() is called with a pattern that was successfully studied
1992 with the PCRE_STUDY_JIT_COMPILE option, the way that the matching is
1993 executed is entirely different. However, there is still the possibility
1994 of runaway matching that goes on for a very long time, and so the
1995 match_limit value is also used in this case (but in a different way) to
1996 limit how long the matching can continue.
1998 The default value for the limit can be set when PCRE is built; the
1999 default default is 10 million, which handles all but the most extreme
2000 cases. You can override the default by suppling pcre_exec() with a
2001 pcre_extra block in which match_limit is set, and
2002 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
2003 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
2005 The match_limit_recursion field is similar to match_limit, but instead
2006 of limiting the total number of times that match() is called, it limits
2007 the depth of recursion. The recursion depth is a smaller number than
2008 the total number of calls, because not all calls to match() are recur-
2009 sive. This limit is of use only if it is set smaller than match_limit.
2011 Limiting the recursion depth limits the amount of machine stack that
2012 can be used, or, when PCRE has been compiled to use memory on the heap
2013 instead of the stack, the amount of heap memory that can be used. This
2014 limit is not relevant, and is ignored, if the pattern was successfully
2015 studied with PCRE_STUDY_JIT_COMPILE.
2017 The default value for match_limit_recursion can be set when PCRE is
2018 built; the default default is the same value as the default for
2019 match_limit. You can override the default by suppling pcre_exec() with
2020 a pcre_extra block in which match_limit_recursion is set, and
2021 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
2022 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
2024 The callout_data field is used in conjunction with the "callout" fea-
2025 ture, and is described in the pcrecallout documentation.
2027 The tables field is used to pass a character tables pointer to
2028 pcre_exec(); this overrides the value that is stored with the compiled
2029 pattern. A non-NULL value is stored with the compiled pattern only if
2030 custom tables were supplied to pcre_compile() via its tableptr argu-
2031 ment. If NULL is passed to pcre_exec() using this mechanism, it forces
2032 PCRE's internal tables to be used. This facility is helpful when re-
2033 using patterns that have been saved after compiling with an external
2034 set of tables, because the external tables might be at a different
2035 address when pcre_exec() is called. See the pcreprecompile documenta-
2036 tion for a discussion of saving compiled patterns for later use.
2038 If PCRE_EXTRA_MARK is set in the flags field, the mark field must be
2039 set to point to a char * variable. If the pattern contains any back-
2040 tracking control verbs such as (*MARK:NAME), and the execution ends up
2041 with a name to pass back, a pointer to the name string (zero termi-
2042 nated) is placed in the variable pointed to by the mark field. The
2043 names are within the compiled pattern; if you wish to retain such a
2044 name you must copy it before freeing the memory of a compiled pattern.
2045 If there is no name to pass back, the variable pointed to by the mark
2046 field set to NULL. For details of the backtracking control verbs, see
2047 the section entitled "Backtracking control" in the pcrepattern documen-
2048 tation.
2050 Option bits for pcre_exec()
2052 The unused bits of the options argument for pcre_exec() must be zero.
2053 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
2058 If the pattern was successfully studied with the PCRE_STUDY_JIT_COMPILE
2059 option, the only supported options for JIT execution are
2061 PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
2062 supported. If an unsupported option is used, JIT execution is disabled
2063 and the normal interpretive code in pcre_exec() is run.
2067 The PCRE_ANCHORED option limits pcre_exec() to matching at the first
2068 matching position. If a pattern was compiled with PCRE_ANCHORED, or
2069 turned out to be anchored by virtue of its contents, it cannot be made
2070 unachored at matching time.
2075 These options (which are mutually exclusive) control what the \R escape
2076 sequence matches. The choice is either to match only CR, LF, or CRLF,
2077 or to match any Unicode newline sequence. These options override the
2078 choice that was made or defaulted when the pattern was compiled.
2086 These options override the newline definition that was chosen or
2087 defaulted when the pattern was compiled. For details, see the descrip-
2088 tion of pcre_compile() above. During matching, the newline choice
2089 affects the behaviour of the dot, circumflex, and dollar metacharac-
2090 ters. It may also alter the way the match position is advanced after a
2091 match failure for an unanchored pattern.
2094 set, and a match attempt for an unanchored pattern fails when the cur-
2095 rent position is at a CRLF sequence, and the pattern contains no
2096 explicit matches for CR or LF characters, the match position is
2097 advanced by two characters instead of one, in other words, to after the
2098 CRLF.
2100 The above rule is a compromise that makes the most common cases work as
2101 expected. For example, if the pattern is .+A (and the PCRE_DOTALL
2102 option is not set), it does not match the string "\r\nA" because, after
2103 failing at the start, it skips both the CR and the LF before retrying.
2104 However, the pattern [\r\n]A does match that string, because it con-
2105 tains an explicit CR or LF reference, and so advances only by one char-
2106 acter after the first failure.
2108 An explicit match for CR of LF is either a literal appearance of one of
2109 those characters, or one of the \r or \n escape sequences. Implicit
2110 matches such as [^X] do not count, nor does \s (which includes CR and
2111 LF in the characters that it matches).
2113 Notwithstanding the above, anomalous effects may still occur when CRLF
2114 is a valid newline sequence and explicit \r or \n escapes appear in the
2115 pattern.
2119 This option specifies that first character of the subject string is not
2120 the beginning of a line, so the circumflex metacharacter should not
2121 match before it. Setting this without PCRE_MULTILINE (at compile time)
2122 causes circumflex never to match. This option affects only the behav-
2123 iour of the circumflex metacharacter. It does not affect \A.
2127 This option specifies that the end of the subject string is not the end
2128 of a line, so the dollar metacharacter should not match it nor (except
2129 in multiline mode) a newline immediately before it. Setting this with-
2130 out PCRE_MULTILINE (at compile time) causes dollar never to match. This
2131 option affects only the behaviour of the dollar metacharacter. It does
2132 not affect \Z or \z.
2136 An empty string is not considered to be a valid match if this option is
2137 set. If there are alternatives in the pattern, they are tried. If all
2138 the alternatives match the empty string, the entire match fails. For
2139 example, if the pattern
2141 a?b?
2143 is applied to a string not beginning with "a" or "b", it matches an
2144 empty string at the start of the subject. With PCRE_NOTEMPTY set, this
2145 match is not valid, so PCRE searches further into the string for occur-
2146 rences of "a" or "b".
2150 This is like PCRE_NOTEMPTY, except that an empty string match that is
2151 not at the start of the subject is permitted. If the pattern is
2152 anchored, such a match can occur only if the pattern contains \K.
2154 Perl has no direct equivalent of PCRE_NOTEMPTY or
2155 PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern
2156 match of the empty string within its split() function, and when using
2157 the /g modifier. It is possible to emulate Perl's behaviour after
2158 matching a null string by first trying the match again at the same off-
2159 set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that
2160 fails, by advancing the starting offset (see below) and trying an ordi-
2161 nary match again. There is some code that demonstrates how to do this
2162 in the pcredemo sample program. In the most general case, you have to
2163 check to see if the newline convention recognizes CRLF as a newline,
2164 and if so, and the current character is CR followed by LF, advance the
2165 starting offset by two characters instead of one.
2169 There are a number of optimizations that pcre_exec() uses at the start
2170 of a match, in order to speed up the process. For example, if it is
2171 known that an unanchored match must start with a specific character, it
2172 searches the subject for that character, and fails immediately if it
2173 cannot find it, without actually running the main matching function.
2174 This means that a special item such as (*COMMIT) at the start of a pat-
2175 tern is not considered until after a suitable starting point for the
2176 match has been found. When callouts or (*MARK) items are in use, these
2177 "start-up" optimizations can cause them to be skipped if the pattern is
2178 never actually used. The start-up optimizations are in effect a pre-
2179 scan of the subject that takes place before the pattern is run.
2181 The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations,
2182 possibly causing performance to suffer, but ensuring that in cases
2183 where the result is "no match", the callouts do occur, and that items
2184 such as (*COMMIT) and (*MARK) are considered at every possible starting
2185 position in the subject string. If PCRE_NO_START_OPTIMIZE is set at
2186 compile time, it cannot be unset at matching time.
2188 Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching
2189 operation. Consider the pattern
2193 When this is compiled, PCRE records the fact that a match must start
2194 with the character "A". Suppose the subject string is "DEFABC". The
2195 start-up optimization scans along the subject, finds "A" and runs the
2196 first match attempt from there. The (*COMMIT) item means that the pat-
2197 tern must match the current starting position, which in this case, it
2198 does. However, if the same match is run with PCRE_NO_START_OPTIMIZE
2199 set, the initial scan along the subject string does not happen. The
2200 first match attempt is run starting from "D" and when this fails,
2201 (*COMMIT) prevents any further matches being tried, so the overall
2202 result is "no match". If the pattern is studied, more start-up opti-
2203 mizations may be used. For example, a minimum length for the subject
2204 may be recorded. Consider the pattern
2206 (*MARK:A)(X|Y)
2208 The minimum length for a match is one character. If the subject is
2209 "ABC", there will be attempts to match "ABC", "BC", "C", and then
2210 finally an empty string. If the pattern is studied, the final attempt
2211 does not take place, because PCRE knows that the subject is too short,
2212 and so the (*MARK) is never encountered. In this case, studying the
2213 pattern does not affect the overall match result, which is still "no
2214 match", but it does affect the auxiliary information that is returned.
2218 When PCRE_UTF8 is set at compile time, the validity of the subject as a
2219 UTF-8 string is automatically checked when pcre_exec() is subsequently
2220 called. The value of startoffset is also checked to ensure that it
2221 points to the start of a UTF-8 character. There is a discussion about
2222 the validity of UTF-8 strings in the section on UTF-8 support in the
2223 main pcre page. If an invalid UTF-8 sequence of bytes is found,
2224 pcre_exec() returns the error PCRE_ERROR_BADUTF8 or, if PCRE_PAR-
2225 TIAL_HARD is set and the problem is a truncated UTF-8 character at the
2226 end of the subject, PCRE_ERROR_SHORTUTF8. In both cases, information
2227 about the precise nature of the error may also be returned (see the
2228 descriptions of these errors in the section entitled Error return val-
2229 ues from pcre_exec() below). If startoffset contains a value that does
2230 not point to the start of a UTF-8 character (or to the end of the sub-
2231 ject), PCRE_ERROR_BADUTF8_OFFSET is returned.
2233 If you already know that your subject is valid, and you want to skip
2234 these checks for performance reasons, you can set the
2235 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
2236 do this for the second and subsequent calls to pcre_exec() if you are
2237 making repeated calls to find all the matches in a single subject
2238 string. However, you should be sure that the value of startoffset
2239 points to the start of a UTF-8 character (or the end of the subject).
2240 When PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid UTF-8
2241 string as a subject or an invalid value of startoffset is undefined.
2242 Your program may crash.
2247 These options turn on the partial matching feature. For backwards com-
2248 patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial
2249 match occurs if the end of the subject string is reached successfully,
2250 but there are not enough subject characters to complete the match. If
2251 this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set,
2252 matching continues by testing any remaining alternatives. Only if no
2253 complete match can be found is PCRE_ERROR_PARTIAL returned instead of
2254 PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the
2255 caller is prepared to handle a partial match, but only if no complete
2256 match can be found.
2258 If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this
2259 case, if a partial match is found, pcre_exec() immediately returns
2260 PCRE_ERROR_PARTIAL, without considering any other alternatives. In
2261 other words, when PCRE_PARTIAL_HARD is set, a partial match is consid-
2262 ered to be more important that an alternative complete match.
2264 In both cases, the portion of the string that was inspected when the
2265 partial match was found is set as the first matching string. There is a
2266 more detailed discussion of partial and multi-segment matching, with
2267 examples, in the pcrepartial documentation.
2269 The string to be matched by pcre_exec()
2271 The subject string is passed to pcre_exec() as a pointer in subject, a
2272 length (in bytes) in length, and a starting byte offset in startoffset.
2273 If this is negative or greater than the length of the subject,
2274 pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is
2275 zero, the search for a match starts at the beginning of the subject,
2276 and this is by far the most common case. In UTF-8 mode, the byte offset
2277 must point to the start of a UTF-8 character (or the end of the sub-
2278 ject). Unlike the pattern string, the subject may contain binary zero
2279 bytes.
2281 A non-zero starting offset is useful when searching for another match
2282 in the same subject by calling pcre_exec() again after a previous suc-
2283 cess. Setting startoffset differs from just passing over a shortened
2284 string and setting PCRE_NOTBOL in the case of a pattern that begins
2285 with any kind of lookbehind. For example, consider the pattern
2287 \Biss\B
2289 which finds occurrences of "iss" in the middle of words. (\B matches
2290 only if the current position in the subject is not a word boundary.)
2291 When applied to the string "Mississipi" the first call to pcre_exec()
2292 finds the first occurrence. If pcre_exec() is called again with just
2293 the remainder of the subject, namely "issipi", it does not match,
2294 because \B is always false at the start of the subject, which is deemed
2295 to be a word boundary. However, if pcre_exec() is passed the entire
2296 string again, but with startoffset set to 4, it finds the second occur-
2297 rence of "iss" because it is able to look behind the starting point to
2298 discover that it is preceded by a letter.
2300 Finding all the matches in a subject is tricky when the pattern can
2301 match an empty string. It is possible to emulate Perl's /g behaviour by
2302 first trying the match again at the same offset, with the
2303 PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that
2304 fails, advancing the starting offset and trying an ordinary match
2305 again. There is some code that demonstrates how to do this in the pcre-
2306 demo sample program. In the most general case, you have to check to see
2307 if the newline convention recognizes CRLF as a newline, and if so, and
2308 the current character is CR followed by LF, advance the starting offset
2309 by two characters instead of one.
2311 If a non-zero starting offset is passed when the pattern is anchored,
2312 one attempt to match at the given offset is made. This can only succeed
2313 if the pattern does not require the match to be at the start of the
2314 subject.
2316 How pcre_exec() returns captured substrings
2318 In general, a pattern matches a certain portion of the subject, and in
2319 addition, further substrings from the subject may be picked out by
2320 parts of the pattern. Following the usage in Jeffrey Friedl's book,
2321 this is called "capturing" in what follows, and the phrase "capturing
2322 subpattern" is used for a fragment of a pattern that picks out a sub-
2323 string. PCRE supports several other kinds of parenthesized subpattern
2324 that do not cause substrings to be captured.
2326 Captured substrings are returned to the caller via a vector of integers
2327 whose address is passed in ovector. The number of elements in the vec-
2328 tor is passed in ovecsize, which must be a non-negative number. Note:
2329 this argument is NOT the size of ovector in bytes.
2331 The first two-thirds of the vector is used to pass back captured sub-
2332 strings, each substring using a pair of integers. The remaining third
2333 of the vector is used as workspace by pcre_exec() while matching cap-
2334 turing subpatterns, and is not available for passing back information.
2335 The number passed in ovecsize should always be a multiple of three. If
2336 it is not, it is rounded down.
2338 When a match is successful, information about captured substrings is
2339 returned in pairs of integers, starting at the beginning of ovector,
2340 and continuing up to two-thirds of its length at the most. The first
2341 element of each pair is set to the byte offset of the first character
2342 in a substring, and the second is set to the byte offset of the first
2343 character after the end of a substring. Note: these values are always
2344 byte offsets, even in UTF-8 mode. They are not character counts.
2346 The first pair of integers, ovector[0] and ovector[1], identify the
2347 portion of the subject string matched by the entire pattern. The next
2348 pair is used for the first capturing subpattern, and so on. The value
2349 returned by pcre_exec() is one more than the highest numbered pair that
2350 has been set. For example, if two substrings have been captured, the
2351 returned value is 3. If there are no capturing subpatterns, the return
2352 value from a successful match is 1, indicating that just the first pair
2353 of offsets has been set.
2355 If a capturing subpattern is matched repeatedly, it is the last portion
2356 of the string that it matched that is returned.
2358 If the vector is too small to hold all the captured substring offsets,
2359 it is used as far as possible (up to two-thirds of its length), and the
2360 function returns a value of zero. If neither the actual string matched
2361 not any captured substrings are of interest, pcre_exec() may be called
2362 with ovector passed as NULL and ovecsize as zero. However, if the pat-
2363 tern contains back references and the ovector is not big enough to
2364 remember the related substrings, PCRE has to get additional memory for
2365 use during matching. Thus it is usually advisable to supply an ovector
2366 of reasonable size.
2368 There are some cases where zero is returned (indicating vector over-
2369 flow) when in fact the vector is exactly the right size for the final
2370 match. For example, consider the pattern
2372 (a)(?:(b)c|bd)
2374 If a vector of 6 elements (allowing for only 1 captured substring) is
2375 given with subject string "abd", pcre_exec() will try to set the second
2376 captured string, thereby recording a vector overflow, before failing to
2377 match "c" and backing up to try the second alternative. The zero
2378 return, however, does correctly indicate that the maximum number of
2379 slots (namely 2) have been filled. In similar cases where there is tem-
2380 porary overflow, but the final number of used slots is actually less
2381 than the maximum, a non-zero value is returned.
2383 The pcre_fullinfo() function can be used to find out how many capturing
2384 subpatterns there are in a compiled pattern. The smallest size for
2385 ovector that will allow for n captured substrings, in addition to the
2386 offsets of the substring matched by the whole pattern, is (n+1)*3.
2388 It is possible for capturing subpattern number n+1 to match some part
2389 of the subject when subpattern n has not been used at all. For example,
2390 if the string "abc" is matched against the pattern (a|(z))(bc) the
2391 return from the function is 4, and subpatterns 1 and 3 are matched, but
2392 2 is not. When this happens, both values in the offset pairs corre-
2393 sponding to unused subpatterns are set to -1.
2395 Offset values that correspond to unused subpatterns at the end of the
2396 expression are also set to -1. For example, if the string "abc" is
2397 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
2398 matched. The return from the function is 2, because the highest used
2399 capturing subpattern number is 1, and the offsets for for the second
2400 and third capturing subpatterns (assuming the vector is large enough,
2401 of course) are set to -1.
2403 Note: Elements in the first two-thirds of ovector that do not corre-
2404 spond to capturing parentheses in the pattern are never changed. That
2405 is, if a pattern contains n capturing parentheses, no more than ovec-
2406 tor[0] to ovector[2n+1] are set by pcre_exec(). The other elements (in
2407 the first two-thirds) retain whatever values they previously had.
2409 Some convenience functions are provided for extracting the captured
2410 substrings as separate strings. These are described below.
2412 Error return values from pcre_exec()
2414 If pcre_exec() fails, it returns a negative number. The following are
2415 defined in the header file:
2419 The subject string did not match the pattern.
2423 Either code or subject was passed as NULL, or ovector was NULL and
2424 ovecsize was not zero.
2428 An unrecognized bit was set in the options argument.
2432 PCRE stores a 4-byte "magic number" at the start of the compiled code,
2433 to catch the case when it is passed a junk pointer and to detect when a
2434 pattern that was compiled in an environment of one endianness is run in
2435 an environment with the other endianness. This is the error that PCRE
2436 gives when the magic number is not present.
2440 While running the pattern match, an unknown item was encountered in the
2441 compiled pattern. This error could be caused by a bug in PCRE or by
2442 overwriting of the compiled pattern.
2446 If a pattern contains back references, but the ovector that is passed
2447 to pcre_exec() is not big enough to remember the referenced substrings,
2448 PCRE gets a block of memory at the start of matching to use for this
2449 purpose. If the call via pcre_malloc() fails, this error is given. The
2450 memory is automatically freed at the end of matching.
2452 This error is also given if pcre_stack_malloc() fails in pcre_exec().
2453 This can happen only when PCRE has been compiled with --disable-stack-
2454 for-recursion.
2458 This error is used by the pcre_copy_substring(), pcre_get_substring(),
2459 and pcre_get_substring_list() functions (see below). It is never
2460 returned by pcre_exec().
2464 The backtracking limit, as specified by the match_limit field in a
2465 pcre_extra structure (or defaulted) was reached. See the description
2466 above.
2470 This error is never generated by pcre_exec() itself. It is provided for
2471 use by callout functions that want to yield a distinctive error code.
2472 See the pcrecallout documentation for details.
2476 A string that contains an invalid UTF-8 byte sequence was passed as a
2477 subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size of
2478 the output vector (ovecsize) is at least 2, the byte offset to the
2479 start of the the invalid UTF-8 character is placed in the first ele-
2480 ment, and a reason code is placed in the second element. The reason
2481 codes are listed in the following section. For backward compatibility,
2482 if PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char-
2483 acter at the end of the subject (reason codes 1 to 5),
2484 PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8.
2488 The UTF-8 byte sequence that was passed as a subject was checked and
2489 found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the
2490 value of startoffset did not point to the beginning of a UTF-8 charac-
2491 ter or the end of the subject.
2495 The subject string did not match, but it did match partially. See the
2496 pcrepartial documentation for details of partial matching.
2500 This code is no longer in use. It was formerly returned when the
2501 PCRE_PARTIAL option was used with a compiled pattern containing items
2502 that were not supported for partial matching. From release 8.00
2503 onwards, there are no restrictions on partial matching.
2507 An unexpected internal error has occurred. This error could be caused
2508 by a bug in PCRE or by overwriting of the compiled pattern.
2512 This error is given if the value of the ovecsize argument is negative.
2516 The internal recursion limit, as specified by the match_limit_recursion
2517 field in a pcre_extra structure (or defaulted) was reached. See the
2518 description above.
2522 An invalid combination of PCRE_NEWLINE_xxx options was given.
2526 The value of startoffset was negative or greater than the length of the
2527 subject, that is, the value in length.
2531 This error is returned instead of PCRE_ERROR_BADUTF8 when the subject
2532 string ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD
2533 option is set. Information about the failure is returned as for
2534 PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this case, but
2535 this special error code for PCRE_PARTIAL_HARD precedes the implementa-
2536 tion of returned information; it is retained for backwards compatibil-
2537 ity.
2541 This error is returned when pcre_exec() detects a recursion loop within
2542 the pattern. Specifically, it means that either the whole pattern or a
2543 subpattern has been called recursively for the second time at the same
2544 position in the subject string. Some simple patterns that might do this
2545 are detected and faulted at compile time, but more complicated cases,
2546 in particular mutual recursions between two different subpatterns, can-
2547 not be detected until run time.
2551 This error is returned when a pattern that was successfully studied
2552 using the PCRE_STUDY_JIT_COMPILE option is being matched, but the mem-
2553 ory available for the just-in-time processing stack is not large
2554 enough. See the pcrejit documentation for more details.
2556 Error numbers -16 to -20 and -22 are not used by pcre_exec().
2558 Reason codes for invalid UTF-8 strings
2560 When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT-
2561 UTF8, and the size of the output vector (ovecsize) is at least 2, the
2562 offset of the start of the invalid UTF-8 character is placed in the
2563 first output vector element (ovector[0]) and a reason code is placed in
2564 the second element (ovector[1]). The reason codes are given names in
2565 the pcre.h header file:
2573 The string ends with a truncated UTF-8 character; the code specifies
2574 how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8
2575 characters to be no longer than 4 bytes, the encoding scheme (origi-
2576 nally defined by RFC 2279) allows for up to 6 bytes, and this is
2577 checked first; hence the possibility of 4 or 5 missing bytes.
2583 PCRE_UTF8_ERR10
2585 The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
2586 the character do not have the binary value 0b10 (that is, either the
2587 most significant bit is 0, or the next bit is 1).
2589 PCRE_UTF8_ERR11
2590 PCRE_UTF8_ERR12
2592 A character that is valid by the RFC 2279 rules is either 5 or 6 bytes
2593 long; these code points are excluded by RFC 3629.
2595 PCRE_UTF8_ERR13
2597 A 4-byte character has a value greater than 0x10fff; these code points
2598 are excluded by RFC 3629.
2600 PCRE_UTF8_ERR14
2602 A 3-byte character has a value in the range 0xd800 to 0xdfff; this
2603 range of code points are reserved by RFC 3629 for use with UTF-16, and
2604 so are excluded from UTF-8.
2606 PCRE_UTF8_ERR15
2607 PCRE_UTF8_ERR16
2608 PCRE_UTF8_ERR17
2609 PCRE_UTF8_ERR18
2610 PCRE_UTF8_ERR19
2612 A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes
2613 for a value that can be represented by fewer bytes, which is invalid.
2614 For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor-
2615 rect coding uses just one byte.
2617 PCRE_UTF8_ERR20
2619 The two most significant bits of the first byte of a character have the
2620 binary value 0b10 (that is, the most significant bit is 1 and the sec-
2621 ond is 0). Such a byte can only validly occur as the second or subse-
2622 quent byte of a multi-byte character.
2624 PCRE_UTF8_ERR21
2626 The first byte of a character has the value 0xfe or 0xff. These values
2627 can never occur in a valid UTF-8 string.
2632 int pcre_copy_substring(const char *subject, int *ovector,
2633 int stringcount, int stringnumber, char *buffer,
2634 int buffersize);
2636 int pcre_get_substring(const char *subject, int *ovector,
2637 int stringcount, int stringnumber,
2638 const char **stringptr);
2640 int pcre_get_substring_list(const char *subject,
2641 int *ovector, int stringcount, const char ***listptr);
2643 Captured substrings can be accessed directly by using the offsets
2644 returned by pcre_exec() in ovector. For convenience, the functions
2645 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
2646 string_list() are provided for extracting captured substrings as new,
2647 separate, zero-terminated strings. These functions identify substrings
2648 by number. The next section describes functions for extracting named
2649 substrings.
2651 A substring that contains a binary zero is correctly extracted and has
2652 a further zero added on the end, but the result is not, of course, a C
2653 string. However, you can process such a string by referring to the
2654 length that is returned by pcre_copy_substring() and pcre_get_sub-
2655 string(). Unfortunately, the interface to pcre_get_substring_list() is
2656 not adequate for handling strings containing binary zeros, because the
2657 end of the final string is not independently indicated.
2659 The first three arguments are the same for all three of these func-
2660 tions: subject is the subject string that has just been successfully
2661 matched, ovector is a pointer to the vector of integer offsets that was
2662 passed to pcre_exec(), and stringcount is the number of substrings that
2663 were captured by the match, including the substring that matched the
2664 entire regular expression. This is the value returned by pcre_exec() if
2665 it is greater than zero. If pcre_exec() returned zero, indicating that
2666 it ran out of space in ovector, the value passed as stringcount should
2667 be the number of elements in the vector divided by three.
2669 The functions pcre_copy_substring() and pcre_get_substring() extract a
2670 single substring, whose number is given as stringnumber. A value of
2671 zero extracts the substring that matched the entire pattern, whereas
2672 higher values extract the captured substrings. For pcre_copy_sub-
2673 string(), the string is placed in buffer, whose length is given by
2674 buffersize, while for pcre_get_substring() a new block of memory is
2675 obtained via pcre_malloc, and its address is returned via stringptr.
2676 The yield of the function is the length of the string, not including
2677 the terminating zero, or one of these error codes:
2681 The buffer was too small for pcre_copy_substring(), or the attempt to
2682 get memory failed for pcre_get_substring().
2686 There is no substring whose number is stringnumber.
2688 The pcre_get_substring_list() function extracts all available sub-
2689 strings and builds a list of pointers to them. All this is done in a
2690 single block of memory that is obtained via pcre_malloc. The address of
2691 the memory block is returned via listptr, which is also the start of
2692 the list of string pointers. The end of the list is marked by a NULL
2693 pointer. The yield of the function is zero if all went well, or the
2694 error code
2698 if the attempt to get the memory block failed.
2700 When any of these functions encounter a substring that is unset, which
2701 can happen when capturing subpattern number n+1 matches some part of
2702 the subject, but subpattern n has not been used at all, they return an
2703 empty string. This can be distinguished from a genuine zero-length sub-
2704 string by inspecting the appropriate offset in ovector, which is nega-
2705 tive for unset substrings.
2707 The two convenience functions pcre_free_substring() and pcre_free_sub-
2708 string_list() can be used to free the memory returned by a previous
2709 call of pcre_get_substring() or pcre_get_substring_list(), respec-
2710 tively. They do nothing more than call the function pointed to by
2711 pcre_free, which of course could be called directly from a C program.
2712 However, PCRE is used in some situations where it is linked via a spe-
2713 cial interface to another programming language that cannot use
2714 pcre_free directly; it is for these cases that the functions are pro-
2715 vided.
2720 int pcre_get_stringnumber(const pcre *code,
2721 const char *name);
2723 int pcre_copy_named_substring(const pcre *code,
2724 const char *subject, int *ovector,
2725 int stringcount, const char *stringname,
2726 char *buffer, int buffersize);
2728 int pcre_get_named_substring(const pcre *code,
2729 const char *subject, int *ovector,
2730 int stringcount, const char *stringname,
2731 const char **stringptr);
2733 To extract a substring by name, you first have to find associated num-
2734 ber. For example, for this pattern
2736 (a+)b(?<xxx>\d+)...
2738 the number of the subpattern called "xxx" is 2. If the name is known to
2739 be unique (PCRE_DUPNAMES was not set), you can find the number from the
2740 name by calling pcre_get_stringnumber(). The first argument is the com-
2741 piled pattern, and the second is the name. The yield of the function is
2742 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2743 subpattern of that name.
2745 Given the number, you can extract the substring directly, or use one of
2746 the functions described in the previous section. For convenience, there
2747 are also two functions that do the whole job.
2749 Most of the arguments of pcre_copy_named_substring() and
2750 pcre_get_named_substring() are the same as those for the similarly
2751 named functions that extract by number. As these are described in the
2752 previous section, they are not re-described here. There are just two
2753 differences:
2755 First, instead of a substring number, a substring name is given. Sec-
2756 ond, there is an extra argument, given at the start, which is a pointer
2757 to the compiled pattern. This is needed in order to gain access to the
2758 name-to-number translation table.
2760 These functions call pcre_get_stringnumber(), and if it succeeds, they
2761 then call pcre_copy_substring() or pcre_get_substring(), as appropri-
2762 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
2763 behaviour may not be what you want (see the next section).
2765 Warning: If the pattern uses the (?| feature to set up multiple subpat-
2766 terns with the same number, as described in the section on duplicate
2767 subpattern numbers in the pcrepattern page, you cannot use names to
2768 distinguish the different subpatterns, because names are not included
2769 in the compiled code. The matching process uses only numbers. For this
2770 reason, the use of different names for subpatterns of the same number
2771 causes an error at compile time.
2776 int pcre_get_stringtable_entries(const pcre *code,
2777 const char *name, char **first, char **last);
2779 When a pattern is compiled with the PCRE_DUPNAMES option, names for
2780 subpatterns are not required to be unique. (Duplicate names are always
2781 allowed for subpatterns with the same number, created by using the (?|
2782 feature. Indeed, if such subpatterns are named, they are required to
2783 use the same names.)
2785 Normally, patterns with duplicate names are such that in any one match,
2786 only one of the named subpatterns participates. An example is shown in
2787 the pcrepattern documentation.
2789 When duplicates are present, pcre_copy_named_substring() and
2790 pcre_get_named_substring() return the first substring corresponding to
2791 the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
2792 (-7) is returned; no data is returned. The pcre_get_stringnumber()
2793 function returns one of the numbers that are associated with the name,
2794 but it is not defined which it is.
2796 If you want to get full details of all captured substrings for a given
2797 name, you must use the pcre_get_stringtable_entries() function. The
2798 first argument is the compiled pattern, and the second is the name. The
2799 third and fourth are pointers to variables which are updated by the
2800 function. After it has run, they point to the first and last entries in
2801 the name-to-number table for the given name. The function itself
2802 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
2803 there are none. The format of the table is described above in the sec-
2804 tion entitled Information about a pattern above. Given all the rele-
2805 vant entries for the name, you can extract each of their numbers, and
2806 hence the captured data, if any.
2811 The traditional matching function uses a similar algorithm to Perl,
2812 which stops when it finds the first match, starting at a given point in
2813 the subject. If you want to find all possible matches, or the longest
2814 possible match, consider using the alternative matching function (see
2815 below) instead. If you cannot use the alternative function, but still
2816 need to find all possible matches, you can kludge it up by making use
2817 of the callout facility, which is described in the pcrecallout documen-
2818 tation.
2820 What you have to do is to insert a callout right at the end of the pat-
2821 tern. When your callout function is called, extract and save the cur-
2822 rent matched substring. Then return 1, which forces pcre_exec() to
2823 backtrack and try other alternatives. Ultimately, when it runs out of
2824 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2829 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2830 const char *subject, int length, int startoffset,
2831 int options, int *ovector, int ovecsize,
2832 int *workspace, int wscount);
2834 The function pcre_dfa_exec() is called to match a subject string
2835 against a compiled pattern, using a matching algorithm that scans the
2836 subject string just once, and does not backtrack. This has different
2837 characteristics to the normal algorithm, and is not compatible with
2838 Perl. Some of the features of PCRE patterns are not supported. Never-
2839 theless, there are times when this kind of matching can be useful. For
2840 a discussion of the two matching algorithms, and a list of features
2841 that pcre_dfa_exec() does not support, see the pcrematching documenta-
2842 tion.
2844 The arguments for the pcre_dfa_exec() function are the same as for
2845 pcre_exec(), plus two extras. The ovector argument is used in a differ-
2846 ent way, and this is described below. The other common arguments are
2847 used in the same way as for pcre_exec(), so their description is not
2848 repeated here.
2850 The two additional arguments provide workspace for the function. The
2851 workspace vector should contain at least 20 elements. It is used for
2852 keeping track of multiple paths through the pattern tree. More
2853 workspace will be needed for patterns and subjects where there are a
2854 lot of potential matches.
2856 Here is an example of a simple call to pcre_dfa_exec():
2858 int rc;
2859 int ovector[10];
2860 int wspace[20];
2861 rc = pcre_dfa_exec(
2862 re, /* result of pcre_compile() */
2863 NULL, /* we didn't study the pattern */
2864 "some string", /* the subject string */
2865 11, /* the length of the subject string */
2866 0, /* start at offset 0 in the subject */
2867 0, /* default options */
2868 ovector, /* vector of integers for substring information */
2869 10, /* number of elements (NOT size in bytes) */
2870 wspace, /* working space vector */
2871 20); /* number of elements (NOT size in bytes) */
2873 Option bits for pcre_dfa_exec()
2875 The unused bits of the options argument for pcre_dfa_exec() must be
2876 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
2881 four of these are exactly the same as for pcre_exec(), so their
2882 description is not repeated here.
2887 These have the same general effect as they do for pcre_exec(), but the
2888 details are slightly different. When PCRE_PARTIAL_HARD is set for
2889 pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub-
2890 ject is reached and there is still at least one matching possibility
2891 that requires additional characters. This happens even if some complete
2892 matches have also been found. When PCRE_PARTIAL_SOFT is set, the return
2893 code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end
2894 of the subject is reached, there have been no complete matches, but
2895 there is still at least one matching possibility. The portion of the
2896 string that was inspected when the longest partial match was found is
2897 set as the first matching string in both cases. There is a more
2898 detailed discussion of partial and multi-segment matching, with exam-
2899 ples, in the pcrepartial documentation.
2903 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
2904 stop as soon as it has found one match. Because of the way the alterna-
2905 tive algorithm works, this is necessarily the shortest possible match
2906 at the first possible matching point in the subject string.
2910 When pcre_dfa_exec() returns a partial match, it is possible to call it
2911 again, with additional subject characters, and have it continue with
2912 the same match. The PCRE_DFA_RESTART option requests this action; when
2913 it is set, the workspace and wscount options must reference the same
2914 vector as before because data about the match so far is left in them
2915 after a partial match. There is more discussion of this facility in the
2916 pcrepartial documentation.
2918 Successful returns from pcre_dfa_exec()
2920 When pcre_dfa_exec() succeeds, it may have matched more than one sub-
2921 string in the subject. Note, however, that all the matches from one run
2922 of the function start at the same point in the subject. The shorter
2923 matches are all initial substrings of the longer matches. For example,
2924 if the pattern
2926 <.*>
2928 is matched against the string
2930 This is <something> <something else> <something further> no more
2932 the three matched strings are
2934 <something>
2935 <something> <something else>
2936 <something> <something else> <something further>
2938 On success, the yield of the function is a number greater than zero,
2939 which is the number of matched substrings. The substrings themselves
2940 are returned in ovector. Each string uses two elements; the first is
2941 the offset to the start, and the second is the offset to the end. In
2942 fact, all the strings have the same start offset. (Space could have
2943 been saved by giving this only once, but it was decided to retain some
2944 compatibility with the way pcre_exec() returns data, even though the
2945 meaning of the strings is different.)
2947 The strings are returned in reverse order of length; that is, the long-
2948 est matching string is given first. If there were too many matches to
2949 fit into ovector, the yield of the function is zero, and the vector is
2950 filled with the longest matches. Unlike pcre_exec(), pcre_dfa_exec()
2951 can use the entire ovector for returning matched strings.
2953 Error returns from pcre_dfa_exec()
2955 The pcre_dfa_exec() function returns a negative number when it fails.
2956 Many of the errors are the same as for pcre_exec(), and these are
2957 described above. There are in addition the following errors that are
2958 specific to pcre_dfa_exec():
2962 This return is given if pcre_dfa_exec() encounters an item in the pat-
2963 tern that it does not support, for instance, the use of \C or a back
2964 reference.
2968 This return is given if pcre_dfa_exec() encounters a condition item
2969 that uses a back reference for the condition, or a test for recursion
2970 in a specific group. These are not supported.
2974 This return is given if pcre_dfa_exec() is called with an extra block
2975 that contains a setting of the match_limit or match_limit_recursion
2976 fields. This is not supported (these fields are meaningless for DFA
2977 matching).
2981 This return is given if pcre_dfa_exec() runs out of space in the
2982 workspace vector.
2986 When a recursive subpattern is processed, the matching function calls
2987 itself recursively, using private vectors for ovector and workspace.
2988 This error is given if the output vector is not large enough. This
2989 should be extremely rare, as a vector of size 1000 is used.
2994 pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
2995 tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
3000 Philip Hazel
3001 University Computing Service
3002 Cambridge CB2 3QH, England.
3007 Last updated: 02 December 2011
3008 Copyright (c) 1997-2011 University of Cambridge.
3009 ------------------------------------------------------------------------------
3015 NAME
3016 PCRE - Perl-compatible regular expressions
3021 int (*pcre_callout)(pcre_callout_block *);
3023 PCRE provides a feature called "callout", which is a means of temporar-
3024 ily passing control to the caller of PCRE in the middle of pattern
3025 matching. The caller of PCRE provides an external function by putting
3026 its entry point in the global variable pcre_callout. By default, this
3027 variable contains NULL, which disables all calling out.
3029 Within a regular expression, (?C) indicates the points at which the
3030 external function is to be called. Different callout points can be
3031 identified by putting a number less than 256 after the letter C. The
3032 default value is zero. For example, this pattern has two callout
3033 points:
3035 (?C1)abc(?C2)def
3037 If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() or
3038 pcre_compile2() is called, PCRE automatically inserts callouts, all
3039 with number 255, before each item in the pattern. For example, if
3040 PCRE_AUTO_CALLOUT is used with the pattern
3042 A(\d{2}|--)
3044 it is processed as if it were
3046 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
3048 Notice that there is a callout before and after each parenthesis and
3049 alternation bar. Automatic callouts can be used for tracking the
3050 progress of pattern matching. The pcretest command has an option that
3051 sets automatic callouts; when it is used, the output indicates how the
3052 pattern is matched. This is useful information when you are trying to
3053 optimize the performance of a particular pattern.
3055 The use of callouts in a pattern makes it ineligible for optimization
3056 by the just-in-time compiler. Studying such a pattern with the
3057 PCRE_STUDY_JIT_COMPILE option always fails.
3062 You should be aware that, because of optimizations in the way PCRE
3063 matches patterns by default, callouts sometimes do not happen. For
3064 example, if the pattern is
3066 ab(?C4)cd
3068 PCRE knows that any matching string must contain the letter "d". If the
3069 subject string is "abyz", the lack of "d" means that matching doesn't
3070 ever start, and the callout is never reached. However, with "abyd",
3071 though the result is still no match, the callout is obeyed.
3073 If the pattern is studied, PCRE knows the minimum length of a matching
3074 string, and will immediately give a "no match" return without actually
3075 running a match if the subject is not long enough, or, for unanchored
3076 patterns, if it has been scanned far enough.
3078 You can disable these optimizations by passing the PCRE_NO_START_OPTI-
3079 MIZE option to pcre_compile(), pcre_exec(), or pcre_dfa_exec(), or by
3080 starting the pattern with (*NO_START_OPT). This slows down the matching
3081 process, but does ensure that callouts such as the example above are
3082 obeyed.
3087 During matching, when PCRE reaches a callout point, the external func-
3088 tion defined by pcre_callout is called (if it is set). This applies to
3089 both the pcre_exec() and the pcre_dfa_exec() matching functions. The
3090 only argument to the callout function is a pointer to a pcre_callout
3091 block. This structure contains the following fields:
3093 int version;
3094 int callout_number;
3095 int *offset_vector;
3096 const char *subject;
3097 int subject_length;
3098 int start_match;
3099 int current_position;
3100 int capture_top;
3101 int capture_last;
3102 void *callout_data;
3103 int pattern_position;
3104 int next_item_length;
3105 const unsigned char *mark;
3107 The version field is an integer containing the version number of the
3108 block format. The initial version was 0; the current version is 2. The
3109 version number will change again in future if additional fields are
3110 added, but the intention is never to remove any of the existing fields.
3112 The callout_number field contains the number of the callout, as com-
3113 piled into the pattern (that is, the number after ?C for manual call-
3114 outs, and 255 for automatically generated callouts).
3116 The offset_vector field is a pointer to the vector of offsets that was
3117 passed by the caller to pcre_exec() or pcre_dfa_exec(). When
3118 pcre_exec() is used, the contents can be inspected in order to extract
3119 substrings that have been matched so far, in the same way as for
3120 extracting substrings after a match has completed. For pcre_dfa_exec()
3121 this field is not useful.
3123 The subject and subject_length fields contain copies of the values that
3124 were passed to pcre_exec().
3126 The start_match field normally contains the offset within the subject
3127 at which the current match attempt started. However, if the escape
3128 sequence \K has been encountered, this value is changed to reflect the
3129 modified starting point. If the pattern is not anchored, the callout
3130 function may be called several times from the same point in the pattern
3131 for different starting points in the subject.
3133 The current_position field contains the offset within the subject of
3134 the current match pointer.
3136 When the pcre_exec() function is used, the capture_top field contains
3137 one more than the number of the highest numbered captured substring so
3138 far. If no substrings have been captured, the value of capture_top is
3139 one. This is always the case when pcre_dfa_exec() is used, because it
3140 does not support captured substrings.
3142 The capture_last field contains the number of the most recently cap-
3143 tured substring. If no substrings have been captured, its value is -1.
3144 This is always the case when pcre_dfa_exec() is used.
3146 The callout_data field contains a value that is passed to pcre_exec()
3147 or pcre_dfa_exec() specifically so that it can be passed back in call-
3148 outs. It is passed in the pcre_callout field of the pcre_extra data
3149 structure. If no such data was passed, the value of callout_data in a
3150 pcre_callout block is NULL. There is a description of the pcre_extra
3151 structure in the pcreapi documentation.
3153 The pattern_position field is present from version 1 of the pcre_call-
3154 out structure. It contains the offset to the next item to be matched in
3155 the pattern string.
3157 The next_item_length field is present from version 1 of the pcre_call-
3158 out structure. It contains the length of the next item to be matched in
3159 the pattern string. When the callout immediately precedes an alterna-
3160 tion bar, a closing parenthesis, or the end of the pattern, the length
3161 is zero. When the callout precedes an opening parenthesis, the length
3162 is that of the entire subpattern.
3164 The pattern_position and next_item_length fields are intended to help
3165 in distinguishing between different automatic callouts, which all have
3166 the same callout number. However, they are set for all callouts.
3168 The mark field is present from version 2 of the pcre_callout structure.
3169 In callouts from pcre_exec() it contains a pointer to the zero-termi-
3170 nated name of the most recently passed (*MARK), (*PRUNE), or (*THEN)
3171 item in the match, or NULL if no such items have been passed. Instances
3172 of (*PRUNE) or (*THEN) without a name do not obliterate a previous
3173 (*MARK). In callouts from pcre_dfa_exec() this field always contains
3174 NULL.
3179 The external callout function returns an integer to PCRE. If the value
3180 is zero, matching proceeds as normal. If the value is greater than
3181 zero, matching fails at the current point, but the testing of other
3182 matching possibilities goes ahead, just as if a lookahead assertion had
3183 failed. If the value is less than zero, the match is abandoned, and
3184 pcre_exec() or pcre_dfa_exec() returns the negative value.
3186 Negative values should normally be chosen from the set of
3187 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
3188 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
3189 reserved for use by callout functions; it will never be used by PCRE
3190 itself.
3195 Philip Hazel
3196 University Computing Service
3197 Cambridge CB2 3QH, England.
3202 Last updated: 30 November 2011
3203 Copyright (c) 1997-2011 University of Cambridge.
3204 ------------------------------------------------------------------------------
3210 NAME
3211 PCRE - Perl-compatible regular expressions
3216 This document describes the differences in the ways that PCRE and Perl
3217 handle regular expressions. The differences described here are with
3218 respect to Perl versions 5.10 and above.
3220 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
3221 of what it does have are given in the pcreunicode page.
3223 2. PCRE allows repeat quantifiers only on parenthesized assertions, but
3224 they do not mean what you might think. For example, (?!a){3} does not
3225 assert that the next three characters are not "a". It just asserts that
3226 the next character is not "a" three times (in principle: PCRE optimizes
3227 this to run the assertion just once). Perl allows repeat quantifiers on
3228 other assertions such as \b, but these do not seem to have any use.
3230 3. Capturing subpatterns that occur inside negative lookahead asser-
3231 tions are counted, but their entries in the offsets vector are never
3232 set. Perl sets its numerical variables from any such patterns that are
3233 matched before the assertion fails to match something (thereby succeed-
3234 ing), but only if the negative lookahead assertion contains just one
3235 branch.
3237 4. Though binary zero characters are supported in the subject string,
3238 they are not allowed in a pattern string because it is passed as a nor-
3239 mal C string, terminated by zero. The escape sequence \0 can be used in
3240 the pattern to represent a binary zero.
3242 5. The following Perl escape sequences are not supported: \l, \u, \L,
3243 \U, and \N when followed by a character name or Unicode value. (\N on
3244 its own, matching a non-newline character, is supported.) In fact these
3245 are implemented by Perl's general string-handling and are not part of
3246 its pattern matching engine. If any of these are encountered by PCRE,
3247 an error is generated by default. However, if the PCRE_JAVASCRIPT_COM-
3248 PAT option is set, \U and \u are interpreted as JavaScript interprets
3249 them.
3251 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
3252 is built with Unicode character property support. The properties that
3253 can be tested with \p and \P are limited to the general category prop-
3254 erties such as Lu and Nd, script names such as Greek or Han, and the
3255 derived properties Any and L&. PCRE does support the Cs (surrogate)
3256 property, which Perl does not; the Perl documentation says "Because
3257 Perl hides the need for the user to understand the internal representa-
3258 tion of Unicode characters, there is no need to implement the somewhat
3259 messy concept of surrogates."
3261 7. PCRE implements a simpler version of \X than Perl, which changed to
3262 make \X match what Unicode calls an "extended grapheme cluster". This
3263 is more complicated than an extended Unicode sequence, which is what
3264 PCRE matches.
3266 8. PCRE does support the \Q...\E escape for quoting substrings. Charac-
3267 ters in between are treated as literals. This is slightly different
3268 from Perl in that $ and @ are also handled as literals inside the
3269 quotes. In Perl, they cause variable interpolation (but of course PCRE
3270 does not have variables). Note the following examples:
3272 Pattern PCRE matches Perl matches
3274 \Qabc$xyz\E abc$xyz abc followed by the
3275 contents of $xyz
3276 \Qabc\$xyz\E abc\$xyz abc\$xyz
3277 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3279 The \Q...\E sequence is recognized both inside and outside character
3280 classes.
3282 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
3283 constructions. However, there is support for recursive patterns. This
3284 is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE
3285 "callout" feature allows an external function to be called during pat-
3286 tern matching. See the pcrecallout documentation for details.
3288 10. Subpatterns that are called as subroutines (whether or not recur-
3289 sively) are always treated as atomic groups in PCRE. This is like
3290 Python, but unlike Perl. Captured values that are set outside a sub-
3291 routine call can be reference from inside in PCRE, but not in Perl.
3292 There is a discussion that explains these differences in more detail in
3293 the section on recursion differences from Perl in the pcrepattern page.
3295 11. If (*THEN) is present in a group that is called as a subroutine,
3296 its action is limited to that group, even if the group does not contain
3297 any | characters.
3299 12. There are some differences that are concerned with the settings of
3300 captured strings when part of a pattern is repeated. For example,
3301 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
3302 unset, but in PCRE it is set to "b".
3304 13. PCRE's handling of duplicate subpattern numbers and duplicate sub-
3305 pattern names is not as general as Perl's. This is a consequence of the
3306 fact the PCRE works internally just with numbers, using an external ta-
3307 ble to translate between numbers and names. In particular, a pattern
3308 such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have
3309 the same number but different names, is not supported, and causes an
3310 error at compile time. If it were allowed, it would not be possible to
3311 distinguish which parentheses matched, because both names map to cap-
3312 turing subpattern number 1. To avoid this confusing situation, an error
3313 is given at compile time.
3315 14. Perl recognizes comments in some places that PCRE does not, for
3316 example, between the ( and ? at the start of a subpattern. If the /x
3317 modifier is set, Perl allows whitespace between ( and ? but PCRE never
3318 does, even if the PCRE_EXTENDED option is set.
3320 15. PCRE provides some extensions to the Perl regular expression facil-
3321 ities. Perl 5.10 includes new features that are not in earlier ver-
3322 sions of Perl, some of which (such as named parentheses) have been in
3323 PCRE for some time. This list is with respect to Perl 5.10:
3325 (a) Although lookbehind assertions in PCRE must match fixed length
3326 strings, each alternative branch of a lookbehind assertion can match a
3327 different length of string. Perl requires them all to have the same
3328 length.
3330 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
3331 meta-character matches only at the very end of the string.
3333 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
3334 cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
3335 ignored. (Perl can be made to issue a warning.)
3337 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
3338 fiers is inverted, that is, by default they are not greedy, but if fol-
3339 lowed by a question mark they are.
3341 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
3342 tried only at the first matching position in the subject string.
3345 and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva-
3346 lents.
3348 (g) The \R escape sequence can be restricted to match only CR, LF, or
3349 CRLF by the PCRE_BSR_ANYCRLF option.
3351 (h) The callout facility is PCRE-specific.
3353 (i) The partial matching facility is PCRE-specific.
3355 (j) Patterns compiled by PCRE can be saved and re-used at a later time,
3356 even on different hosts that have the other endianness. However, this
3357 does not apply to optimized data created by the just-in-time compiler.
3359 (k) The alternative matching function (pcre_dfa_exec()) matches in a
3360 different way and is not Perl-compatible.
3362 (l) PCRE recognizes some special sequences such as (*CR) at the start
3363 of a pattern that set overall options that cannot be changed within the
3364 pattern.
3369 Philip Hazel
3370 University Computing Service
3371 Cambridge CB2 3QH, England.
3376 Last updated: 14 November 2011
3377 Copyright (c) 1997-2011 University of Cambridge.
3378 ------------------------------------------------------------------------------
3384 NAME
3385 PCRE - Perl-compatible regular expressions
3390 The syntax and semantics of the regular expressions that are supported
3391 by PCRE are described in detail below. There is a quick-reference syn-
3392 tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
3393 semantics as closely as it can. PCRE also supports some alternative
3394 regular expression syntax (which does not conflict with the Perl syn-
3395 tax) in order to provide some compatibility with regular expressions in
3396 Python, .NET, and Oniguruma.
3398 Perl's regular expressions are described in its own documentation, and
3399 regular expressions in general are covered in a number of books, some
3400 of which have copious examples. Jeffrey Friedl's "Mastering Regular
3401 Expressions", published by O'Reilly, covers regular expressions in
3402 great detail. This description of PCRE's regular expressions is
3403 intended as reference material.
3405 The original operation of PCRE was on strings of one-byte characters.
3406 However, there is now also support for UTF-8 character strings. To use
3407 this, PCRE must be built to include UTF-8 support, and you must call
3408 pcre_compile() or pcre_compile2() with the PCRE_UTF8 option. There is
3409 also a special sequence that can be given at the start of a pattern:
3411 (*UTF8)
3413 Starting a pattern with this sequence is equivalent to setting the
3414 PCRE_UTF8 option. This feature is not Perl-compatible. How setting
3415 UTF-8 mode affects pattern matching is mentioned in several places
3416 below. There is also a summary of UTF-8 features in the pcreunicode
3417 page.
3419 Another special sequence that may appear at the start of a pattern or
3420 in combination with (*UTF8) is:
3422 (*UCP)
3424 This has the same effect as setting the PCRE_UCP option: it causes
3425 sequences such as \d and \w to use Unicode properties to determine
3426 character types, instead of recognizing only characters with codes less
3427 than 128 via a lookup table.
3429 If a pattern starts with (*NO_START_OPT), it has the same effect as
3430 setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
3431 time. There are also some more of these special sequences that are con-
3432 cerned with the handling of newlines; they are described below.
3434 The remainder of this document discusses the patterns that are sup-
3435 ported by PCRE when its main matching function, pcre_exec(), is used.
3436 From release 6.0, PCRE offers a second matching function,
3437 pcre_dfa_exec(), which matches using a different algorithm that is not
3438 Perl-compatible. Some of the features discussed below are not available
3439 when pcre_dfa_exec() is used. The advantages and disadvantages of the
3440 alternative function, and how it differs from the normal function, are
3441 discussed in the pcrematching page.
3446 PCRE supports five different conventions for indicating line breaks in
3447 strings: a single CR (carriage return) character, a single LF (line-
3448 feed) character, the two-character sequence CRLF, any of the three pre-
3449 ceding, or any Unicode newline sequence. The pcreapi page has further
3450 discussion about newlines, and shows how to set the newline convention
3451 in the options arguments for the compiling and matching functions.
3453 It is also possible to specify a newline convention by starting a pat-
3454 tern string with one of the following five sequences:
3456 (*CR) carriage return
3457 (*LF) linefeed
3458 (*CRLF) carriage return, followed by linefeed
3459 (*ANYCRLF) any of the three above
3460 (*ANY) all Unicode newline sequences
3462 These override the default and the options given to pcre_compile() or
3463 pcre_compile2(). For example, on a Unix system where LF is the default
3464 newline sequence, the pattern
3466 (*CR)a.b
3468 changes the convention to CR. That pattern matches "a\nb" because LF is
3469 no longer a newline. Note that these special settings, which are not
3470 Perl-compatible, are recognized only at the very start of a pattern,
3471 and that they must be in upper case. If more than one of them is
3472 present, the last one is used.
3474 The newline convention affects the interpretation of the dot metachar-
3475 acter when PCRE_DOTALL is not set, and also the behaviour of \N. How-
3476 ever, it does not affect what the \R escape sequence matches. By
3477 default, this is any Unicode newline sequence, for Perl compatibility.
3478 However, this can be changed; see the description of \R in the section
3479 entitled "Newline sequences" below. A change of \R setting can be com-
3480 bined with a change of newline convention.
3485 A regular expression is a pattern that is matched against a subject
3486 string from left to right. Most characters stand for themselves in a
3487 pattern, and match the corresponding characters in the subject. As a
3488 trivial example, the pattern
3490 The quick brown fox
3492 matches a portion of a subject string that is identical to itself. When
3493 caseless matching is specified (the PCRE_CASELESS option), letters are
3494 matched independently of case. In UTF-8 mode, PCRE always understands
3495 the concept of case for characters whose values are less than 128, so
3496 caseless matching is always possible. For characters with higher val-
3497 ues, the concept of case is supported if PCRE is compiled with Unicode
3498 property support, but not otherwise. If you want to use caseless
3499 matching for characters 128 and above, you must ensure that PCRE is
3500 compiled with Unicode property support as well as with UTF-8 support.
3502 The power of regular expressions comes from the ability to include
3503 alternatives and repetitions in the pattern. These are encoded in the
3504 pattern by the use of metacharacters, which do not stand for themselves
3505 but instead are interpreted in some special way.
3507 There are two different sets of metacharacters: those that are recog-
3508 nized anywhere in the pattern except within square brackets, and those
3509 that are recognized within square brackets. Outside square brackets,
3510 the metacharacters are as follows:
3512 \ general escape character with several uses
3513 ^ assert start of string (or line, in multiline mode)
3514 $ assert end of string (or line, in multiline mode)
3515 . match any character except newline (by default)
3516 [ start character class definition
3517 | start of alternative branch
3518 ( start subpattern
3519 ) end subpattern
3520 ? extends the meaning of (
3521 also 0 or 1 quantifier
3522 also quantifier minimizer
3523 * 0 or more quantifier
3524 + 1 or more quantifier
3525 also "possessive quantifier"
3526 { start min/max quantifier
3528 Part of a pattern that is in square brackets is called a "character
3529 class". In a character class the only metacharacters are:
3531 \ general escape character
3532 ^ negate the class, but only if the first character
3533 - indicates character range
3534 [ POSIX character class (only if followed by POSIX
3535 syntax)
3536 ] terminates the character class
3538 The following sections describe the use of each of the metacharacters.
3543 The backslash character has several uses. Firstly, if it is followed by
3544 a character that is not a number or a letter, it takes away any special
3545 meaning that character may have. This use of backslash as an escape
3546 character applies both inside and outside character classes.
3548 For example, if you want to match a * character, you write \* in the
3549 pattern. This escaping action applies whether or not the following
3550 character would otherwise be interpreted as a metacharacter, so it is
3551 always safe to precede a non-alphanumeric with backslash to specify
3552 that it stands for itself. In particular, if you want to match a back-
3553 slash, you write \\.
3555 In UTF-8 mode, only ASCII numbers and letters have any special meaning
3556 after a backslash. All other characters (in particular, those whose
3557 codepoints are greater than 127) are treated as literals.
3559 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
3560 the pattern (other than in a character class) and characters between a
3561 # outside a character class and the next newline are ignored. An escap-
3562 ing backslash can be used to include a whitespace or # character as
3563 part of the pattern.
3565 If you want to remove the special meaning from a sequence of charac-
3566 ters, you can do so by putting them between \Q and \E. This is differ-
3567 ent from Perl in that $ and @ are handled as literals in \Q...\E
3568 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
3569 tion. Note the following examples:
3571 Pattern PCRE matches Perl matches
3573 \Qabc$xyz\E abc$xyz abc followed by the
3574 contents of $xyz
3575 \Qabc\$xyz\E abc\$xyz abc\$xyz
3576 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3578 The \Q...\E sequence is recognized both inside and outside character
3579 classes. An isolated \E that is not preceded by \Q is ignored. If \Q
3580 is not followed by \E later in the pattern, the literal interpretation
3581 continues to the end of the pattern (that is, \E is assumed at the
3582 end). If the isolated \Q is inside a character class, this causes an
3583 error, because the character class is not terminated.
3585 Non-printing characters
3587 A second use of backslash provides a way of encoding non-printing char-
3588 acters in patterns in a visible manner. There is no restriction on the
3589 appearance of non-printing characters, apart from the binary zero that
3590 terminates a pattern, but when a pattern is being prepared by text
3591 editing, it is often easier to use one of the following escape
3592 sequences than the binary character it represents:
3594 \a alarm, that is, the BEL character (hex 07)
3595 \cx "control-x", where x is any ASCII character
3596 \e escape (hex 1B)
3597 \f formfeed (hex 0C)
3598 \n linefeed (hex 0A)
3599 \r carriage return (hex 0D)
3600 \t tab (hex 09)
3601 \ddd character with octal code ddd, or back reference
3602 \xhh character with hex code hh
3603 \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
3604 \uhhhh character with hex code hhhh (JavaScript mode only)
3606 The precise effect of \cx is as follows: if x is a lower case letter,
3607 it is converted to upper case. Then bit 6 of the character (hex 40) is
3608 inverted. Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({
3609 is 7B), while \c; becomes hex 7B (; is 3B). If the byte following \c
3610 has a value greater than 127, a compile-time error occurs. This locks
3611 out non-ASCII characters in both byte mode and UTF-8 mode. (When PCRE
3612 is compiled in EBCDIC mode, all byte values are valid. A lower case
3613 letter is converted to upper case, and then the 0xc0 bits are flipped.)
3615 By default, after \x, from zero to two hexadecimal digits are read
3616 (letters can be in upper or lower case). Any number of hexadecimal dig-
3617 its may appear between \x{ and }, but the value of the character code
3618 must be less than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8
3619 mode. That is, the maximum value in hexadecimal is 7FFFFFFF. Note that
3620 this is bigger than the largest Unicode code point, which is 10FFFF.
3622 If characters other than hexadecimal digits appear between \x{ and },
3623 or if there is no terminating }, this form of escape is not recognized.
3624 Instead, the initial \x will be interpreted as a basic hexadecimal
3625 escape, with no following digits, giving a character whose value is
3626 zero.
3628 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x
3629 is as just described only when it is followed by two hexadecimal dig-
3630 its. Otherwise, it matches a literal "x" character. In JavaScript
3631 mode, support for code points greater than 256 is provided by \u, which
3632 must be followed by four hexadecimal digits; otherwise it matches a
3633 literal "u" character.
3635 Characters whose value is less than 256 can be defined by either of the
3636 two syntaxes for \x (or by \u in JavaScript mode). There is no differ-
3637 ence in the way they are handled. For example, \xdc is exactly the same
3638 as \x{dc} (or \u00dc in JavaScript mode).
3640 After \0 up to two further octal digits are read. If there are fewer
3641 than two digits, just those that are present are used. Thus the
3642 sequence \0\x\07 specifies two binary zeros followed by a BEL character
3643 (code value 7). Make sure you supply two digits after the initial zero
3644 if the pattern character that follows is itself an octal digit.
3646 The handling of a backslash followed by a digit other than 0 is compli-
3647 cated. Outside a character class, PCRE reads it and any following dig-
3648 its as a decimal number. If the number is less than 10, or if there
3649 have been at least that many previous capturing left parentheses in the
3650 expression, the entire sequence is taken as a back reference. A
3651 description of how this works is given later, following the discussion
3652 of parenthesized subpatterns.
3654 Inside a character class, or if the decimal number is greater than 9
3655 and there have not been that many capturing subpatterns, PCRE re-reads
3656 up to three octal digits following the backslash, and uses them to gen-
3657 erate a data character. Any subsequent digits stand for themselves. In
3658 non-UTF-8 mode, the value of a character specified in octal must be
3659 less than \400. In UTF-8 mode, values up to \777 are permitted. For
3660 example:
3662 \040 is another way of writing a space
3663 \40 is the same, provided there are fewer than 40
3664 previous capturing subpatterns
3665 \7 is always a back reference
3666 \11 might be a back reference, or another way of
3667 writing a tab
3668 \011 is always a tab
3669 \0113 is a tab followed by the character "3"
3670 \113 might be a back reference, otherwise the
3671 character with octal code 113
3672 \377 might be a back reference, otherwise
3673 the byte consisting entirely of 1 bits
3674 \81 is either a back reference, or a binary zero
3675 followed by the two characters "8" and "1"
3677 Note that octal values of 100 or greater must not be introduced by a
3678 leading zero, because no more than three octal digits are ever read.
3680 All the sequences that define a single character value can be used both
3681 inside and outside character classes. In addition, inside a character
3682 class, \b is interpreted as the backspace character (hex 08).
3684 \N is not allowed in a character class. \B, \R, and \X are not special
3685 inside a character class. Like other unrecognized escape sequences,
3686 they are treated as the literal characters "B", "R", and "X" by
3687 default, but cause an error if the PCRE_EXTRA option is set. Outside a
3688 character class, these sequences have different meanings.
3690 Unsupported escape sequences
3692 In Perl, the sequences \l, \L, \u, and \U are recognized by its string
3693 handler and used to modify the case of following characters. By
3694 default, PCRE does not support these escape sequences. However, if the
3695 PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and
3696 \u can be used to define a character by code point, as described in the
3697 previous section.
3699 Absolute and relative back references
3701 The sequence \g followed by an unsigned or a negative number, option-
3702 ally enclosed in braces, is an absolute or relative back reference. A
3703 named back reference can be coded as \g{name}. Back references are dis-
3704 cussed later, following the discussion of parenthesized subpatterns.
3706 Absolute and relative subroutine calls
3708 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
3709 name or a number enclosed either in angle brackets or single quotes, is
3710 an alternative syntax for referencing a subpattern as a "subroutine".
3711 Details are discussed later. Note that \g{...} (Perl syntax) and
3712 \g<...> (Oniguruma syntax) are not synonymous. The former is a back
3713 reference; the latter is a subroutine call.
3715 Generic character types
3717 Another use of backslash is for specifying generic character types:
3719 \d any decimal digit
3720 \D any character that is not a decimal digit
3721 \h any horizontal whitespace character
3722 \H any character that is not a horizontal whitespace character
3723 \s any whitespace character
3724 \S any character that is not a whitespace character
3725 \v any vertical whitespace character
3726 \V any character that is not a vertical whitespace character
3727 \w any "word" character
3728 \W any "non-word" character
3730 There is also the single sequence \N, which matches a non-newline char-
3731 acter. This is the same as the "." metacharacter when PCRE_DOTALL is
3732 not set. Perl also uses \N to match characters by name; PCRE does not
3733 support this.
3735 Each pair of lower and upper case escape sequences partitions the com-
3736 plete set of characters into two disjoint sets. Any given character
3737 matches one, and only one, of each pair. The sequences can appear both
3738 inside and outside character classes. They each match one character of
3739 the appropriate type. If the current matching point is at the end of
3740 the subject string, all of them fail, because there is no character to
3741 match.
3743 For compatibility with Perl, \s does not match the VT character (code
3744 11). This makes it different from the the POSIX "space" class. The \s
3745 characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
3746 "use locale;" is included in a Perl script, \s may match the VT charac-
3747 ter. In PCRE, it never does.
3749 A "word" character is an underscore or any character that is a letter
3750 or digit. By default, the definition of letters and digits is con-
3751 trolled by PCRE's low-valued character tables, and may vary if locale-
3752 specific matching is taking place (see "Locale support" in the pcreapi
3753 page). For example, in a French locale such as "fr_FR" in Unix-like
3754 systems, or "french" in Windows, some character codes greater than 128
3755 are used for accented letters, and these are then matched by \w. The
3756 use of locales with Unicode is discouraged.
3758 By default, in UTF-8 mode, characters with values greater than 128
3759 never match \d, \s, or \w, and always match \D, \S, and \W. These
3760 sequences retain their original meanings from before UTF-8 support was
3761 available, mainly for efficiency reasons. However, if PCRE is compiled
3762 with Unicode property support, and the PCRE_UCP option is set, the be-
3763 haviour is changed so that Unicode properties are used to determine
3764 character types, as follows:
3766 \d any character that \p{Nd} matches (decimal digit)
3767 \s any character that \p{Z} matches, plus HT, LF, FF, CR
3768 \w any character that \p{L} or \p{N} matches, plus underscore
3770 The upper case escapes match the inverse sets of characters. Note that
3771 \d matches only decimal digits, whereas \w matches any Unicode digit,
3772 as well as any Unicode letter, and underscore. Note also that PCRE_UCP
3773 affects \b, and \B because they are defined in terms of \w and \W.
3774 Matching these sequences is noticeably slower when PCRE_UCP is set.
3776 The sequences \h, \H, \v, and \V are features that were added to Perl
3777 at release 5.10. In contrast to the other sequences, which match only
3778 ASCII characters by default, these always match certain high-valued
3779 codepoints in UTF-8 mode, whether or not PCRE_UCP is set. The horizon-
3780 tal space characters are:
3782 U+0009 Horizontal tab
3783 U+0020 Space
3784 U+00A0 Non-break space
3785 U+1680 Ogham space mark
3786 U+180E Mongolian vowel separator
3787 U+2000 En quad
3788 U+2001 Em quad
3789 U+2002 En space
3790 U+2003 Em space
3791 U+2004 Three-per-em space
3792 U+2005 Four-per-em space
3793 U+2006 Six-per-em space
3794 U+2007 Figure space
3795 U+2008 Punctuation space
3796 U+2009 Thin space
3797 U+200A Hair space
3798 U+202F Narrow no-break space
3799 U+205F Medium mathematical space
3800 U+3000 Ideographic space
3802 The vertical space characters are:
3804 U+000A Linefeed
3805 U+000B Vertical tab
3806 U+000C Formfeed
3807 U+000D Carriage return
3808 U+0085 Next line
3809 U+2028 Line separator
3810 U+2029 Paragraph separator
3812 Newline sequences
3814 Outside a character class, by default, the escape sequence \R matches
3815 any Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the
3816 following:
3818 (?>\r\n|\n|\x0b|\f|\r|\x85)
3820 This is an example of an "atomic group", details of which are given
3821 below. This particular group matches either the two-character sequence
3822 CR followed by LF, or one of the single characters LF (linefeed,
3823 U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
3824 return, U+000D), or NEL (next line, U+0085). The two-character sequence
3825 is treated as a single unit that cannot be split.
3827 In UTF-8 mode, two additional characters whose codepoints are greater
3828 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
3829 rator, U+2029). Unicode character property support is not needed for
3830 these characters to be recognized.
3832 It is possible to restrict \R to match only CR, LF, or CRLF (instead of
3833 the complete set of Unicode line endings) by setting the option
3834 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
3835 (BSR is an abbrevation for "backslash R".) This can be made the default
3836 when PCRE is built; if this is the case, the other behaviour can be
3837 requested via the PCRE_BSR_UNICODE option. It is also possible to
3838 specify these settings by starting a pattern string with one of the
3839 following sequences:
3841 (*BSR_ANYCRLF) CR, LF, or CRLF only
3842 (*BSR_UNICODE) any Unicode newline sequence
3844 These override the default and the options given to pcre_compile() or
3845 pcre_compile2(), but they can be overridden by options given to
3846 pcre_exec() or pcre_dfa_exec(). Note that these special settings, which
3847 are not Perl-compatible, are recognized only at the very start of a
3848 pattern, and that they must be in upper case. If more than one of them
3849 is present, the last one is used. They can be combined with a change of
3850 newline convention; for example, a pattern can start with:
3854 They can also be combined with the (*UTF8) or (*UCP) special sequences.
3855 Inside a character class, \R is treated as an unrecognized escape
3856 sequence, and so matches the letter "R" by default, but causes an error
3857 if PCRE_EXTRA is set.
3859 Unicode character properties
3861 When PCRE is built with Unicode character property support, three addi-
3862 tional escape sequences that match characters with specific properties
3863 are available. When not in UTF-8 mode, these sequences are of course
3864 limited to testing characters whose codepoints are less than 256, but
3865 they do work in this mode. The extra escape sequences are:
3867 \p{xx} a character with the xx property
3868 \P{xx} a character without the xx property
3869 \X an extended Unicode sequence
3871 The property names represented by xx above are limited to the Unicode
3872 script names, the general category properties, "Any", which matches any
3873 character (including newline), and some special PCRE properties
3874 (described in the next section). Other Perl properties such as "InMu-
3875 sicalSymbols" are not currently supported by PCRE. Note that \P{Any}
3876 does not match any characters, so always causes a match failure.
3878 Sets of Unicode characters are defined as belonging to certain scripts.
3879 A character from one of these sets can be matched using a script name.
3880 For example:
3882 \p{Greek}
3883 \P{Han}
3885 Those that are not part of an identified script are lumped together as
3886 "Common". The current list of scripts is:
3888 Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
3889 Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common,
3890 Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp-
3891 tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek,
3892 Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe-
3893 rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
3894 Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
3895 Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam,
3896 Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
3897 Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya,
3898 Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian,
3899 Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le,
3900 Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh,
3901 Ugaritic, Vai, Yi.
3903 Each character has exactly one Unicode general category property, spec-
3904 ified by a two-letter abbreviation. For compatibility with Perl, nega-
3905 tion can be specified by including a circumflex between the opening
3906 brace and the property name. For example, \p{^Lu} is the same as
3907 \P{Lu}.
3909 If only one letter is specified with \p or \P, it includes all the gen-
3910 eral category properties that start with that letter. In this case, in
3911 the absence of negation, the curly brackets in the escape sequence are
3912 optional; these two examples have the same effect:
3914 \p{L}
3915 \pL
3917 The following general category property codes are supported:
3919 C Other
3920 Cc Control
3921 Cf Format
3922 Cn Unassigned
3923 Co Private use
3924 Cs Surrogate
3926 L Letter
3927 Ll Lower case letter
3928 Lm Modifier letter
3929 Lo Other letter
3930 Lt Title case letter
3931 Lu Upper case letter
3933 M Mark
3934 Mc Spacing mark
3935 Me Enclosing mark
3936 Mn Non-spacing mark
3938 N Number
3939 Nd Decimal number
3940 Nl Letter number
3941 No Other number
3943 P Punctuation
3944 Pc Connector punctuation
3945 Pd Dash punctuation
3946 Pe Close punctuation
3947 Pf Final punctuation
3948 Pi Initial punctuation
3949 Po Other punctuation
3950 Ps Open punctuation
3952 S Symbol
3953 Sc Currency symbol
3954 Sk Modifier symbol
3955 Sm Mathematical symbol
3956 So Other symbol
3958 Z Separator
3959 Zl Line separator
3960 Zp Paragraph separator
3961 Zs Space separator
3963 The special property L& is also supported: it matches a character that
3964 has the Lu, Ll, or Lt property, in other words, a letter that is not
3965 classified as a modifier or "other".
3967 The Cs (Surrogate) property applies only to characters in the range
3968 U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see
3969 RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
3970 ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in
3971 the pcreapi page). Perl does not support the Cs property.
3973 The long synonyms for property names that Perl supports (such as
3974 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
3975 any of these properties with "Is".
3977 No character that is in the Unicode table has the Cn (unassigned) prop-
3978 erty. Instead, this property is assumed for any code point that is not
3979 in the Unicode table.
3981 Specifying caseless matching does not affect these escape sequences.
3982 For example, \p{Lu} always matches only upper case letters.
3984 The \X escape matches any number of Unicode characters that form an
3985 extended Unicode sequence. \X is equivalent to
3987 (?>\PM\pM*)
3989 That is, it matches a character without the "mark" property, followed
3990 by zero or more characters with the "mark" property, and treats the
3991 sequence as an atomic group (see below). Characters with the "mark"
3992 property are typically accents that affect the preceding character.
3993 None of them have codepoints less than 256, so in non-UTF-8 mode \X
3994 matches any one character.
3996 Note that recent versions of Perl have changed \X to match what Unicode
3997 calls an "extended grapheme cluster", which has a more complicated def-
3998 inition.
4000 Matching characters by Unicode property is not fast, because PCRE has
4001 to search a structure that contains data for over fifteen thousand
4002 characters. That is why the traditional escape sequences such as \d and
4003 \w do not use Unicode properties in PCRE by default, though you can
4004 make them do so by setting the PCRE_UCP option for pcre_compile() or by
4005 starting the pattern with (*UCP).
4007 PCRE's additional properties
4009 As well as the standard Unicode properties described in the previous
4010 section, PCRE supports four more that make it possible to convert tra-
4011 ditional escape sequences such as \w and \s and POSIX character classes
4012 to use Unicode properties. PCRE uses these non-standard, non-Perl prop-
4013 erties internally when PCRE_UCP is set. They are:
4015 Xan Any alphanumeric character
4016 Xps Any POSIX space character
4017 Xsp Any Perl space character
4018 Xwd Any Perl "word" character
4020 Xan matches characters that have either the L (letter) or the N (num-
4021 ber) property. Xps matches the characters tab, linefeed, vertical tab,
4022 formfeed, or carriage return, and any other character that has the Z
4023 (separator) property. Xsp is the same as Xps, except that vertical tab
4024 is excluded. Xwd matches the same characters as Xan, plus underscore.
4026 Resetting the match start
4028 The escape sequence \K causes any previously matched characters not to
4029 be included in the final matched sequence. For example, the pattern:
4031 foo\Kbar
4033 matches "foobar", but reports that it has matched "bar". This feature
4034 is similar to a lookbehind assertion (described below). However, in
4035 this case, the part of the subject before the real match does not have
4036 to be of fixed length, as lookbehind assertions do. The use of \K does
4037 not interfere with the setting of captured substrings. For example,
4038 when the pattern
4040 (foo)\Kbar
4042 matches "foobar", the first substring is still set to "foo".
4044 Perl documents that the use of \K within assertions is "not well
4045 defined". In PCRE, \K is acted upon when it occurs inside positive
4046 assertions, but is ignored in negative assertions.
4048 Simple assertions
4050 The final use of backslash is for certain simple assertions. An asser-
4051 tion specifies a condition that has to be met at a particular point in
4052 a match, without consuming any characters from the subject string. The
4053 use of subpatterns for more complicated assertions is described below.
4054 The backslashed assertions are:
4056 \b matches at a word boundary
4057 \B matches when not at a word boundary
4058 \A matches at the start of the subject
4059 \Z matches at the end of the subject
4060 also matches before a newline at the end of the subject
4061 \z matches only at the end of the subject
4062 \G matches at the first matching position in the subject
4064 Inside a character class, \b has a different meaning; it matches the
4065 backspace character. If any other of these assertions appears in a
4066 character class, by default it matches the corresponding literal char-
4067 acter (for example, \B matches the letter B). However, if the
4068 PCRE_EXTRA option is set, an "invalid escape sequence" error is gener-
4069 ated instead.
4071 A word boundary is a position in the subject string where the current
4072 character and the previous character do not both match \w or \W (i.e.
4073 one matches \w and the other matches \W), or the start or end of the
4074 string if the first or last character matches \w, respectively. In
4075 UTF-8 mode, the meanings of \w and \W can be changed by setting the
4076 PCRE_UCP option. When this is done, it also affects \b and \B. Neither
4077 PCRE nor Perl has a separate "start of word" or "end of word" metase-
4078 quence. However, whatever follows \b normally determines which it is.
4079 For example, the fragment \ba matches "a" at the start of a word.
4081 The \A, \Z, and \z assertions differ from the traditional circumflex
4082 and dollar (described in the next section) in that they only ever match
4083 at the very start and end of the subject string, whatever options are
4084 set. Thus, they are independent of multiline mode. These three asser-
4085 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
4086 affect only the behaviour of the circumflex and dollar metacharacters.
4087 However, if the startoffset argument of pcre_exec() is non-zero, indi-
4088 cating that matching is to start at a point other than the beginning of
4089 the subject, \A can never match. The difference between \Z and \z is
4090 that \Z matches before a newline at the end of the string as well as at
4091 the very end, whereas \z matches only at the end.
4093 The \G assertion is true only when the current matching position is at
4094 the start point of the match, as specified by the startoffset argument
4095 of pcre_exec(). It differs from \A when the value of startoffset is
4096 non-zero. By calling pcre_exec() multiple times with appropriate argu-
4097 ments, you can mimic Perl's /g option, and it is in this kind of imple-
4098 mentation where \G can be useful.
4100 Note, however, that PCRE's interpretation of \G, as the start of the
4101 current match, is subtly different from Perl's, which defines it as the
4102 end of the previous match. In Perl, these can be different when the
4103 previously matched string was empty. Because PCRE does just one match
4104 at a time, it cannot reproduce this behaviour.
4106 If all the alternatives of a pattern begin with \G, the expression is
4107 anchored to the starting match position, and the "anchored" flag is set
4108 in the compiled regular expression.
4113 Outside a character class, in the default matching mode, the circumflex
4114 character is an assertion that is true only if the current matching
4115 point is at the start of the subject string. If the startoffset argu-
4116 ment of pcre_exec() is non-zero, circumflex can never match if the
4117 PCRE_MULTILINE option is unset. Inside a character class, circumflex
4118 has an entirely different meaning (see below).
4120 Circumflex need not be the first character of the pattern if a number
4121 of alternatives are involved, but it should be the first thing in each
4122 alternative in which it appears if the pattern is ever to match that
4123 branch. If all possible alternatives start with a circumflex, that is,
4124 if the pattern is constrained to match only at the start of the sub-
4125 ject, it is said to be an "anchored" pattern. (There are also other
4126 constructs that can cause a pattern to be anchored.)
4128 A dollar character is an assertion that is true only if the current
4129 matching point is at the end of the subject string, or immediately
4130 before a newline at the end of the string (by default). Dollar need not
4131 be the last character of the pattern if a number of alternatives are
4132 involved, but it should be the last item in any branch in which it
4133 appears. Dollar has no special meaning in a character class.
4135 The meaning of dollar can be changed so that it matches only at the
4136 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
4137 compile time. This does not affect the \Z assertion.
4139 The meanings of the circumflex and dollar characters are changed if the
4140 PCRE_MULTILINE option is set. When this is the case, a circumflex
4141 matches immediately after internal newlines as well as at the start of
4142 the subject string. It does not match after a newline that ends the
4143 string. A dollar matches before any newlines in the string, as well as
4144 at the very end, when PCRE_MULTILINE is set. When newline is specified
4145 as the two-character sequence CRLF, isolated CR and LF characters do
4146 not indicate newlines.
4148 For example, the pattern /^abc$/ matches the subject string "def\nabc"
4149 (where \n represents a newline) in multiline mode, but not otherwise.
4150 Consequently, patterns that are anchored in single line mode because
4151 all branches start with ^ are not anchored in multiline mode, and a
4152 match for circumflex is possible when the startoffset argument of
4153 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
4154 PCRE_MULTILINE is set.
4156 Note that the sequences \A, \Z, and \z can be used to match the start
4157 and end of the subject in both modes, and if all branches of a pattern
4158 start with \A it is always anchored, whether or not PCRE_MULTILINE is
4159 set.
4164 Outside a character class, a dot in the pattern matches any one charac-
4165 ter in the subject string except (by default) a character that signi-
4166 fies the end of a line. In UTF-8 mode, the matched character may be
4167 more than one byte long.
4169 When a line ending is defined as a single character, dot never matches
4170 that character; when the two-character sequence CRLF is used, dot does
4171 not match CR if it is immediately followed by LF, but otherwise it
4172 matches all characters (including isolated CRs and LFs). When any Uni-
4173 code line endings are being recognized, dot does not match CR or LF or
4174 any of the other line ending characters.
4176 The behaviour of dot with regard to newlines can be changed. If the
4177 PCRE_DOTALL option is set, a dot matches any one character, without
4178 exception. If the two-character sequence CRLF is present in the subject
4179 string, it takes two dots to match it.
4181 The handling of dot is entirely independent of the handling of circum-
4182 flex and dollar, the only relationship being that they both involve
4183 newlines. Dot has no special meaning in a character class.
4185 The escape sequence \N behaves like a dot, except that it is not
4186 affected by the PCRE_DOTALL option. In other words, it matches any
4187 character except one that signifies the end of a line. Perl also uses
4188 \N to match characters by name; PCRE does not support this.
4193 Outside a character class, the escape sequence \C matches any one byte,
4194 both in and out of UTF-8 mode. Unlike a dot, it always matches line-
4195 ending characters. The feature is provided in Perl in order to match
4196 individual bytes in UTF-8 mode, but it is unclear how it can usefully
4197 be used. Because \C breaks up characters into individual bytes, match-
4198 ing one byte with \C in UTF-8 mode means that the rest of the string
4199 may start with a malformed UTF-8 character. This has undefined results,
4200 because PCRE assumes that it is dealing with valid UTF-8 strings (and
4201 by default it checks this at the start of processing unless the
4202 PCRE_NO_UTF8_CHECK option is used).
4204 PCRE does not allow \C to appear in lookbehind assertions (described
4205 below) in UTF-8 mode, because this would make it impossible to calcu-
4206 late the length of the lookbehind.
4208 In general, the \C escape sequence is best avoided in UTF-8 mode. How-
4209 ever, one way of using it that avoids the problem of malformed UTF-8
4210 characters is to use a lookahead to check the length of the next char-
4211 acter, as in this pattern (ignore white space and line breaks):
4213 (?| (?=[\x00-\x7f])(\C) |
4214 (?=[\x80-\x{7ff}])(\C)(\C) |
4215 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
4216 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
4218 A group that starts with (?| resets the capturing parentheses numbers
4219 in each alternative (see "Duplicate Subpattern Numbers" below). The
4220 assertions at the start of each branch check the next UTF-8 character
4221 for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
4222 character's individual bytes are then captured by the appropriate num-
4223 ber of groups.
4228 An opening square bracket introduces a character class, terminated by a
4229 closing square bracket. A closing square bracket on its own is not spe-
4230 cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
4231 a lone closing square bracket causes a compile-time error. If a closing
4232 square bracket is required as a member of the class, it should be the
4233 first data character in the class (after an initial circumflex, if
4234 present) or escaped with a backslash.
4236 A character class matches a single character in the subject. In UTF-8
4237 mode, the character may be more than one byte long. A matched character
4238 must be in the set of characters defined by the class, unless the first
4239 character in the class definition is a circumflex, in which case the
4240 subject character must not be in the set defined by the class. If a
4241 circumflex is actually required as a member of the class, ensure it is
4242 not the first character, or escape it with a backslash.
4244 For example, the character class [aeiou] matches any lower case vowel,
4245 while [^aeiou] matches any character that is not a lower case vowel.
4246 Note that a circumflex is just a convenient notation for specifying the
4247 characters that are in the class by enumerating those that are not. A
4248 class that starts with a circumflex is not an assertion; it still con-
4249 sumes a character from the subject string, and therefore it fails if
4250 the current pointer is at the end of the string.
4252 In UTF-8 mode, characters with values greater than 255 can be included
4253 in a class as a literal string of bytes, or by using the \x{ escaping
4254 mechanism.
4256 When caseless matching is set, any letters in a class represent both
4257 their upper case and lower case versions, so for example, a caseless
4258 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
4259 match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
4260 understands the concept of case for characters whose values are less
4261 than 128, so caseless matching is always possible. For characters with
4262 higher values, the concept of case is supported if PCRE is compiled
4263 with Unicode property support, but not otherwise. If you want to use
4264 caseless matching in UTF8-mode for characters 128 and above, you must
4265 ensure that PCRE is compiled with Unicode property support as well as
4266 with UTF-8 support.
4268 Characters that might indicate line breaks are never treated in any
4269 special way when matching character classes, whatever line-ending
4270 sequence is in use, and whatever setting of the PCRE_DOTALL and
4271 PCRE_MULTILINE options is used. A class such as [^a] always matches one
4272 of these characters.
4274 The minus (hyphen) character can be used to specify a range of charac-
4275 ters in a character class. For example, [d-m] matches any letter
4276 between d and m, inclusive. If a minus character is required in a
4277 class, it must be escaped with a backslash or appear in a position
4278 where it cannot be interpreted as indicating a range, typically as the
4279 first or last character in the class.
4281 It is not possible to have the literal character "]" as the end charac-
4282 ter of a range. A pattern such as [W-]46] is interpreted as a class of
4283 two characters ("W" and "-") followed by a literal string "46]", so it
4284 would match "W46]" or "-46]". However, if the "]" is escaped with a
4285 backslash it is interpreted as the end of range, so [W-\]46] is inter-
4286 preted as a class containing a range followed by two other characters.
4287 The octal or hexadecimal representation of "]" can also be used to end
4288 a range.
4290 Ranges operate in the collating sequence of character values. They can
4291 also be used for characters specified numerically, for example
4292 [\000-\037]. In UTF-8 mode, ranges can include characters whose values
4293 are greater than 255, for example [\x{100}-\x{2ff}].
4295 If a range that includes letters is used when caseless matching is set,
4296 it matches the letters in either case. For example, [W-c] is equivalent
4297 to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
4298 character tables for a French locale are in use, [\xc8-\xcb] matches
4299 accented E characters in both cases. In UTF-8 mode, PCRE supports the
4300 concept of case for characters with values greater than 128 only when
4301 it is compiled with Unicode property support.
4303 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
4304 \w, and \W may appear in a character class, and add the characters that
4305 they match to the class. For example, [\dABCDEF] matches any hexadeci-
4306 mal digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of
4307 \d, \s, \w and their upper case partners, just as it does when they
4308 appear outside a character class, as described in the section entitled
4309 "Generic character types" above. The escape sequence \b has a different
4310 meaning inside a character class; it matches the backspace character.
4311 The sequences \B, \N, \R, and \X are not special inside a character
4312 class. Like any other unrecognized escape sequences, they are treated
4313 as the literal characters "B", "N", "R", and "X" by default, but cause
4314 an error if the PCRE_EXTRA option is set.
4316 A circumflex can conveniently be used with the upper case character
4317 types to specify a more restricted set of characters than the matching
4318 lower case type. For example, the class [^\W_] matches any letter or
4319 digit, but not underscore, whereas [\w] includes underscore. A positive
4320 character class should be read as "something OR something OR ..." and a
4321 negative class as "NOT something AND NOT something AND NOT ...".
4323 The only metacharacters that are recognized in character classes are
4324 backslash, hyphen (only where it can be interpreted as specifying a
4325 range), circumflex (only at the start), opening square bracket (only
4326 when it can be interpreted as introducing a POSIX class name - see the
4327 next section), and the terminating closing square bracket. However,
4328 escaping other non-alphanumeric characters does no harm.
4333 Perl supports the POSIX notation for character classes. This uses names
4334 enclosed by [: and :] within the enclosing square brackets. PCRE also
4335 supports this notation. For example,
4337 [01[:alpha:]%]
4339 matches "0", "1", any alphabetic character, or "%". The supported class
4340 names are:
4342 alnum letters and digits
4343 alpha letters
4344 ascii character codes 0 - 127
4345 blank space or tab only
4346 cntrl control characters
4347 digit decimal digits (same as \d)
4348 graph printing characters, excluding space
4349 lower lower case letters
4350 print printing characters, including space
4351 punct printing characters, excluding letters and digits and space
4352 space white space (not quite the same as \s)
4353 upper upper case letters
4354 word "word" characters (same as \w)
4355 xdigit hexadecimal digits
4357 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
4358 and space (32). Notice that this list includes the VT character (code
4359 11). This makes "space" different to \s, which does not include VT (for
4360 Perl compatibility).
4362 The name "word" is a Perl extension, and "blank" is a GNU extension
4363 from Perl 5.8. Another Perl extension is negation, which is indicated
4364 by a ^ character after the colon. For example,
4366 [12[:^digit:]]
4368 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
4369 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
4370 these are not supported, and an error is given if they are encountered.
4372 By default, in UTF-8 mode, characters with values greater than 128 do
4373 not match any of the POSIX character classes. However, if the PCRE_UCP
4374 option is passed to pcre_compile(), some of the classes are changed so
4375 that Unicode character properties are used. This is achieved by replac-
4376 ing the POSIX classes by other sequences, as follows:
4378 [:alnum:] becomes \p{Xan}
4379 [:alpha:] becomes \p{L}
4380 [:blank:] becomes \h
4381 [:digit:] becomes \p{Nd}
4382 [:lower:] becomes \p{Ll}
4383 [:space:] becomes \p{Xps}
4384 [:upper:] becomes \p{Lu}
4385 [:word:] becomes \p{Xwd}
4387 Negated versions, such as [:^alpha:] use \P instead of \p. The other
4388 POSIX classes are unchanged, and match only characters with code points
4389 less than 128.
4394 Vertical bar characters are used to separate alternative patterns. For
4395 example, the pattern
4397 gilbert|sullivan
4399 matches either "gilbert" or "sullivan". Any number of alternatives may
4400 appear, and an empty alternative is permitted (matching the empty
4401 string). The matching process tries each alternative in turn, from left
4402 to right, and the first one that succeeds is used. If the alternatives
4403 are within a subpattern (defined below), "succeeds" means matching the
4404 rest of the main pattern as well as the alternative in the subpattern.
4409 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
4410 PCRE_EXTENDED options (which are Perl-compatible) can be changed from
4411 within the pattern by a sequence of Perl option letters enclosed
4412 between "(?" and ")". The option letters are
4414 i for PCRE_CASELESS
4416 s for PCRE_DOTALL
4417 x for PCRE_EXTENDED
4419 For example, (?im) sets caseless, multiline matching. It is also possi-
4420 ble to unset these options by preceding the letter with a hyphen, and a
4421 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
4423 is also permitted. If a letter appears both before and after the
4424 hyphen, the option is unset.
4427 can be changed in the same way as the Perl-compatible options by using
4428 the characters J, U and X respectively.
4430 When one of these option changes occurs at top level (that is, not
4431 inside subpattern parentheses), the change applies to the remainder of
4432 the pattern that follows. If the change is placed right at the start of
4433 a pattern, PCRE extracts it into the global options (and it will there-
4434 fore show up in data extracted by the pcre_fullinfo() function).
4436 An option change within a subpattern (see below for a description of
4437 subpatterns) affects only that part of the subpattern that follows it,
4438 so
4440 (a(?i)b)c
4442 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
4443 used). By this means, options can be made to have different settings
4444 in different parts of the pattern. Any changes made in one alternative
4445 do carry on into subsequent branches within the same subpattern. For
4446 example,
4448 (a(?i)b|c)
4450 matches "ab", "aB", "c", and "C", even though when matching "C" the
4451 first branch is abandoned before the option setting. This is because
4452 the effects of option settings happen at compile time. There would be
4453 some very weird behaviour otherwise.
4455 Note: There are other PCRE-specific options that can be set by the
4456 application when the compile or match functions are called. In some
4457 cases the pattern can contain special leading sequences such as (*CRLF)
4458 to override what the application has set or what has been defaulted.
4459 Details are given in the section entitled "Newline sequences" above.
4460 There are also the (*UTF8) and (*UCP) leading sequences that can be
4461 used to set UTF-8 and Unicode property modes; they are equivalent to
4462 setting the PCRE_UTF8 and the PCRE_UCP options, respectively.
4467 Subpatterns are delimited by parentheses (round brackets), which can be
4468 nested. Turning part of a pattern into a subpattern does two things:
4470 1. It localizes a set of alternatives. For example, the pattern
4472 cat(aract|erpillar|)
4474 matches "cataract", "caterpillar", or "cat". Without the parentheses,
4475 it would match "cataract", "erpillar" or an empty string.
4477 2. It sets up the subpattern as a capturing subpattern. This means
4478 that, when the whole pattern matches, that portion of the subject
4479 string that matched the subpattern is passed back to the caller via the
4480 ovector argument of pcre_exec(). Opening parentheses are counted from
4481 left to right (starting from 1) to obtain numbers for the capturing
4482 subpatterns. For example, if the string "the red king" is matched
4483 against the pattern
4485 the ((red|white) (king|queen))
4487 the captured substrings are "red king", "red", and "king", and are num-
4488 bered 1, 2, and 3, respectively.
4490 The fact that plain parentheses fulfil two functions is not always
4491 helpful. There are often times when a grouping subpattern is required
4492 without a capturing requirement. If an opening parenthesis is followed
4493 by a question mark and a colon, the subpattern does not do any captur-
4494 ing, and is not counted when computing the number of any subsequent
4495 capturing subpatterns. For example, if the string "the white queen" is
4496 matched against the pattern
4498 the ((?:red|white) (king|queen))
4500 the captured substrings are "white queen" and "queen", and are numbered
4501 1 and 2. The maximum number of capturing subpatterns is 65535.
4503 As a convenient shorthand, if any option settings are required at the
4504 start of a non-capturing subpattern, the option letters may appear
4505 between the "?" and the ":". Thus the two patterns
4507 (?i:saturday|sunday)
4508 (?:(?i)saturday|sunday)
4510 match exactly the same set of strings. Because alternative branches are
4511 tried from left to right, and options are not reset until the end of
4512 the subpattern is reached, an option setting in one branch does affect
4513 subsequent branches, so the above patterns match "SUNDAY" as well as
4514 "Saturday".
4519 Perl 5.10 introduced a feature whereby each alternative in a subpattern
4520 uses the same numbers for its capturing parentheses. Such a subpattern
4521 starts with (?| and is itself a non-capturing subpattern. For example,
4522 consider this pattern:
4524 (?|(Sat)ur|(Sun))day
4526 Because the two alternatives are inside a (?| group, both sets of cap-
4527 turing parentheses are numbered one. Thus, when the pattern matches,
4528 you can look at captured substring number one, whichever alternative
4529 matched. This construct is useful when you want to capture part, but
4530 not all, of one of a number of alternatives. Inside a (?| group, paren-
4531 theses are numbered as usual, but the number is reset at the start of
4532 each branch. The numbers of any capturing parentheses that follow the
4533 subpattern start after the highest number used in any branch. The fol-
4534 lowing example is taken from the Perl documentation. The numbers under-
4535 neath show in which buffer the captured content will be stored.
4537 # before ---------------branch-reset----------- after
4538 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
4539 # 1 2 2 3 2 3 4
4541 A back reference to a numbered subpattern uses the most recent value
4542 that is set for that number by any subpattern. The following pattern
4543 matches "abcabc" or "defdef":
4545 /(?|(abc)|(def))\1/
4547 In contrast, a subroutine call to a numbered subpattern always refers
4548 to the first one in the pattern with the given number. The following
4549 pattern matches "abcabc" or "defabc":
4551 /(?|(abc)|(def))(?1)/
4553 If a condition test for a subpattern's having matched refers to a non-
4554 unique number, the test is true if any of the subpatterns of that num-
4555 ber have matched.
4557 An alternative approach to using this "branch reset" feature is to use
4558 duplicate named subpatterns, as described in the next section.
4563 Identifying capturing parentheses by number is simple, but it can be
4564 very hard to keep track of the numbers in complicated regular expres-
4565 sions. Furthermore, if an expression is modified, the numbers may
4566 change. To help with this difficulty, PCRE supports the naming of sub-
4567 patterns. This feature was not added to Perl until release 5.10. Python
4568 had the feature earlier, and PCRE introduced it at release 4.0, using
4569 the Python syntax. PCRE now supports both the Perl and the Python syn-
4570 tax. Perl allows identically numbered subpatterns to have different
4571 names, but PCRE does not.
4573 In PCRE, a subpattern can be named in one of three ways: (?<name>...)
4574 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
4575 to capturing parentheses from other parts of the pattern, such as back
4576 references, recursion, and conditions, can be made by name as well as
4577 by number.
4579 Names consist of up to 32 alphanumeric characters and underscores.
4580 Named capturing parentheses are still allocated numbers as well as
4581 names, exactly as if the names were not present. The PCRE API provides
4582 function calls for extracting the name-to-number translation table from
4583 a compiled pattern. There is also a convenience function for extracting
4584 a captured substring by name.
4586 By default, a name must be unique within a pattern, but it is possible
4587 to relax this constraint by setting the PCRE_DUPNAMES option at compile
4588 time. (Duplicate names are also always permitted for subpatterns with
4589 the same number, set up as described in the previous section.) Dupli-
4590 cate names can be useful for patterns where only one instance of the
4591 named parentheses can match. Suppose you want to match the name of a
4592 weekday, either as a 3-letter abbreviation or as the full name, and in
4593 both cases you want to extract the abbreviation. This pattern (ignoring
4594 the line breaks) does the job:
4596 (?<DN>Mon|Fri|Sun)(?:day)?|
4597 (?<DN>Tue)(?:sday)?|
4598 (?<DN>Wed)(?:nesday)?|
4599 (?<DN>Thu)(?:rsday)?|
4600 (?<DN>Sat)(?:urday)?
4602 There are five capturing substrings, but only one is ever set after a
4603 match. (An alternative way of solving this problem is to use a "branch
4604 reset" subpattern, as described in the previous section.)
4606 The convenience function for extracting the data by name returns the
4607 substring for the first (and in this example, the only) subpattern of
4608 that name that matched. This saves searching to find which numbered
4609 subpattern it was.
4611 If you make a back reference to a non-unique named subpattern from
4612 elsewhere in the pattern, the one that corresponds to the first occur-
4613 rence of the name is used. In the absence of duplicate numbers (see the
4614 previous section) this is the one with the lowest number. If you use a
4615 named reference in a condition test (see the section about conditions
4616 below), either to check whether a subpattern has matched, or to check
4617 for recursion, all subpatterns with the same name are tested. If the
4618 condition is true for any one of them, the overall condition is true.
4619 This is the same behaviour as testing by number. For further details of
4620 the interfaces for handling named subpatterns, see the pcreapi documen-
4621 tation.
4623 Warning: You cannot use different names to distinguish between two sub-
4624 patterns with the same number because PCRE uses only the numbers when
4625 matching. For this reason, an error is given at compile time if differ-
4626 ent names are given to subpatterns with the same number. However, you
4627 can give the same name to subpatterns with the same number, even when
4628 PCRE_DUPNAMES is not set.
4633 Repetition is specified by quantifiers, which can follow any of the
4634 following items:
4636 a literal data character
4637 the dot metacharacter
4638 the \C escape sequence
4639 the \X escape sequence (in UTF-8 mode with Unicode properties)
4640 the \R escape sequence
4641 an escape such as \d or \pL that matches a single character
4642 a character class
4643 a back reference (see next section)
4644 a parenthesized subpattern (including assertions)
4645 a subroutine call to a subpattern (recursive or otherwise)
4647 The general repetition quantifier specifies a minimum and maximum num-
4648 ber of permitted matches, by giving the two numbers in curly brackets
4649 (braces), separated by a comma. The numbers must be less than 65536,
4650 and the first must be less than or equal to the second. For example:
4652 z{2,4}
4654 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
4655 special character. If the second number is omitted, but the comma is
4656 present, there is no upper limit; if the second number and the comma
4657 are both omitted, the quantifier specifies an exact number of required
4658 matches. Thus
4660 [aeiou]{3,}
4662 matches at least 3 successive vowels, but may match many more, while
4664 \d{8}
4666 matches exactly 8 digits. An opening curly bracket that appears in a
4667 position where a quantifier is not allowed, or one that does not match
4668 the syntax of a quantifier, is taken as a literal character. For exam-
4669 ple, {,6} is not a quantifier, but a literal string of four characters.
4671 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
4672 individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
4673 acters, each of which is represented by a two-byte sequence. Similarly,
4674 when Unicode property support is available, \X{3} matches three Unicode
4675 extended sequences, each of which may be several bytes long (and they
4676 may be of different lengths).
4678 The quantifier {0} is permitted, causing the expression to behave as if
4679 the previous item and the quantifier were not present. This may be use-
4680 ful for subpatterns that are referenced as subroutines from elsewhere
4681 in the pattern (but see also the section entitled "Defining subpatterns
4682 for use by reference only" below). Items other than subpatterns that
4683 have a {0} quantifier are omitted from the compiled pattern.
4685 For convenience, the three most common quantifiers have single-charac-
4686 ter abbreviations:
4688 * is equivalent to {0,}
4689 + is equivalent to {1,}
4690 ? is equivalent to {0,1}
4692 It is possible to construct infinite loops by following a subpattern
4693 that can match no characters with a quantifier that has no upper limit,
4694 for example:
4696 (a?)*
4698 Earlier versions of Perl and PCRE used to give an error at compile time
4699 for such patterns. However, because there are cases where this can be
4700 useful, such patterns are now accepted, but if any repetition of the
4701 subpattern does in fact match no characters, the loop is forcibly bro-
4702 ken.
4704 By default, the quantifiers are "greedy", that is, they match as much
4705 as possible (up to the maximum number of permitted times), without
4706 causing the rest of the pattern to fail. The classic example of where
4707 this gives problems is in trying to match comments in C programs. These
4708 appear between /* and */ and within the comment, individual * and /
4709 characters may appear. An attempt to match C comments by applying the
4710 pattern
4712 /\*.*\*/
4714 to the string
4716 /* first comment */ not comment /* second comment */
4718 fails, because it matches the entire string owing to the greediness of
4719 the .* item.
4721 However, if a quantifier is followed by a question mark, it ceases to
4722 be greedy, and instead matches the minimum number of times possible, so
4723 the pattern
4725 /\*.*?\*/
4727 does the right thing with the C comments. The meaning of the various
4728 quantifiers is not otherwise changed, just the preferred number of
4729 matches. Do not confuse this use of question mark with its use as a
4730 quantifier in its own right. Because it has two uses, it can sometimes
4731 appear doubled, as in
4733 \d??\d
4735 which matches one digit by preference, but can match two if that is the
4736 only way the rest of the pattern matches.
4738 If the PCRE_UNGREEDY option is set (an option that is not available in
4739 Perl), the quantifiers are not greedy by default, but individual ones
4740 can be made greedy by following them with a question mark. In other
4741 words, it inverts the default behaviour.
4743 When a parenthesized subpattern is quantified with a minimum repeat
4744 count that is greater than 1 or with a limited maximum, more memory is
4745 required for the compiled pattern, in proportion to the size of the
4746 minimum or maximum.
4748 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
4749 alent to Perl's /s) is set, thus allowing the dot to match newlines,
4750 the pattern is implicitly anchored, because whatever follows will be
4751 tried against every character position in the subject string, so there
4752 is no point in retrying the overall match at any position after the
4753 first. PCRE normally treats such a pattern as though it were preceded
4754 by \A.
4756 In cases where it is known that the subject string contains no new-
4757 lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
4758 mization, or alternatively using ^ to indicate anchoring explicitly.
4760 However, there is one situation where the optimization cannot be used.
4761 When .* is inside capturing parentheses that are the subject of a back
4762 reference elsewhere in the pattern, a match at the start may fail where
4763 a later one succeeds. Consider, for example:
4765 (.*)abc\1
4767 If the subject is "xyz123abc123" the match point is the fourth charac-
4768 ter. For this reason, such a pattern is not implicitly anchored.
4770 When a capturing subpattern is repeated, the value captured is the sub-
4771 string that matched the final iteration. For example, after
4773 (tweedle[dume]{3}\s*)+
4775 has matched "tweedledum tweedledee" the value of the captured substring
4776 is "tweedledee". However, if there are nested capturing subpatterns,
4777 the corresponding captured values may have been set in previous itera-
4778 tions. For example, after
4780 /(a|(b))+/
4782 matches "aba" the value of the second captured substring is "b".
4787 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
4788 repetition, failure of what follows normally causes the repeated item
4789 to be re-evaluated to see if a different number of repeats allows the
4790 rest of the pattern to match. Sometimes it is useful to prevent this,
4791 either to change the nature of the match, or to cause it fail earlier
4792 than it otherwise might, when the author of the pattern knows there is
4793 no point in carrying on.
4795 Consider, for example, the pattern \d+foo when applied to the subject
4796 line
4798 123456bar
4800 After matching all 6 digits and then failing to match "foo", the normal
4801 action of the matcher is to try again with only 5 digits matching the
4802 \d+ item, and then with 4, and so on, before ultimately failing.
4803 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
4804 the means for specifying that once a subpattern has matched, it is not
4805 to be re-evaluated in this way.
4807 If we use atomic grouping for the previous example, the matcher gives
4808 up immediately on failing to match "foo" the first time. The notation
4809 is a kind of special parenthesis, starting with (?> as in this example:
4811 (?>\d+)foo
4813 This kind of parenthesis "locks up" the part of the pattern it con-
4814 tains once it has matched, and a failure further into the pattern is
4815 prevented from backtracking into it. Backtracking past it to previous
4816 items, however, works as normal.
4818 An alternative description is that a subpattern of this type matches
4819 the string of characters that an identical standalone pattern would
4820 match, if anchored at the current point in the subject string.
4822 Atomic grouping subpatterns are not capturing subpatterns. Simple cases
4823 such as the above example can be thought of as a maximizing repeat that
4824 must swallow everything it can. So, while both \d+ and \d+? are pre-
4825 pared to adjust the number of digits they match in order to make the
4826 rest of the pattern match, (?>\d+) can only match an entire sequence of
4827 digits.
4829 Atomic groups in general can of course contain arbitrarily complicated
4830 subpatterns, and can be nested. However, when the subpattern for an
4831 atomic group is just a single repeated item, as in the example above, a
4832 simpler notation, called a "possessive quantifier" can be used. This
4833 consists of an additional + character following a quantifier. Using
4834 this notation, the previous example can be rewritten as
4836 \d++foo
4838 Note that a possessive quantifier can be used with an entire group, for
4839 example:
4841 (abc|xyz){2,3}+
4843 Possessive quantifiers are always greedy; the setting of the
4844 PCRE_UNGREEDY option is ignored. They are a convenient notation for the
4845 simpler forms of atomic group. However, there is no difference in the
4846 meaning of a possessive quantifier and the equivalent atomic group,
4847 though there may be a performance difference; possessive quantifiers
4848 should be slightly faster.
4850 The possessive quantifier syntax is an extension to the Perl 5.8 syn-
4851 tax. Jeffrey Friedl originated the idea (and the name) in the first
4852 edition of his book. Mike McCloskey liked it, so implemented it when he
4853 built Sun's Java package, and PCRE copied it from there. It ultimately
4854 found its way into Perl at release 5.10.
4856 PCRE has an optimization that automatically "possessifies" certain sim-
4857 ple pattern constructs. For example, the sequence A+B is treated as
4858 A++B because there is no point in backtracking into a sequence of A's
4859 when B must follow.
4861 When a pattern contains an unlimited repeat inside a subpattern that
4862 can itself be repeated an unlimited number of times, the use of an
4863 atomic group is the only way to avoid some failing matches taking a
4864 very long time indeed. The pattern
4866 (\D+|<\d+>)*[!?]
4868 matches an unlimited number of substrings that either consist of non-
4869 digits, or digits enclosed in <>, followed by either ! or ?. When it
4870 matches, it runs quickly. However, if it is applied to
4872 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
4874 it takes a long time before reporting failure. This is because the
4875 string can be divided between the internal \D+ repeat and the external
4876 * repeat in a large number of ways, and all have to be tried. (The
4877 example uses [!?] rather than a single character at the end, because
4878 both PCRE and Perl have an optimization that allows for fast failure
4879 when a single character is used. They remember the last single charac-
4880 ter that is required for a match, and fail early if it is not present
4881 in the string.) If the pattern is changed so that it uses an atomic
4882 group, like this:
4884 ((?>\D+)|<\d+>)*[!?]
4886 sequences of non-digits cannot be broken, and failure happens quickly.
4891 Outside a character class, a backslash followed by a digit greater than
4892 0 (and possibly further digits) is a back reference to a capturing sub-
4893 pattern earlier (that is, to its left) in the pattern, provided there
4894 have been that many previous capturing left parentheses.
4896 However, if the decimal number following the backslash is less than 10,
4897 it is always taken as a back reference, and causes an error only if
4898 there are not that many capturing left parentheses in the entire pat-
4899 tern. In other words, the parentheses that are referenced need not be
4900 to the left of the reference for numbers less than 10. A "forward back
4901 reference" of this type can make sense when a repetition is involved
4902 and the subpattern to the right has participated in an earlier itera-
4903 tion.
4905 It is not possible to have a numerical "forward back reference" to a
4906 subpattern whose number is 10 or more using this syntax because a
4907 sequence such as \50 is interpreted as a character defined in octal.
4908 See the subsection entitled "Non-printing characters" above for further
4909 details of the handling of digits following a backslash. There is no
4910 such problem when named parentheses are used. A back reference to any
4911 subpattern is possible using named parentheses (see below).
4913 Another way of avoiding the ambiguity inherent in the use of digits
4914 following a backslash is to use the \g escape sequence. This escape
4915 must be followed by an unsigned number or a negative number, optionally
4916 enclosed in braces. These examples are all identical:
4918 (ring), \1
4919 (ring), \g1
4920 (ring), \g{1}
4922 An unsigned number specifies an absolute reference without the ambigu-
4923 ity that is present in the older syntax. It is also useful when literal
4924 digits follow the reference. A negative number is a relative reference.
4925 Consider this example:
4927 (abc(def)ghi)\g{-1}
4929 The sequence \g{-1} is a reference to the most recently started captur-
4930 ing subpattern before \g, that is, is it equivalent to \2 in this exam-
4931 ple. Similarly, \g{-2} would be equivalent to \1. The use of relative
4932 references can be helpful in long patterns, and also in patterns that
4933 are created by joining together fragments that contain references
4934 within themselves.
4936 A back reference matches whatever actually matched the capturing sub-
4937 pattern in the current subject string, rather than anything matching
4938 the subpattern itself (see "Subpatterns as subroutines" below for a way
4939 of doing that). So the pattern
4941 (sens|respons)e and \1ibility
4943 matches "sense and sensibility" and "response and responsibility", but
4944 not "sense and responsibility". If caseful matching is in force at the
4945 time of the back reference, the case of letters is relevant. For exam-
4946 ple,
4948 ((?i)rah)\s+\1
4950 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
4951 original capturing subpattern is matched caselessly.
4953 There are several different ways of writing back references to named
4954 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
4955 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
4956 unified back reference syntax, in which \g can be used for both numeric
4957 and named references, is also supported. We could rewrite the above
4958 example in any of the following ways:
4960 (?<p1>(?i)rah)\s+\k<p1>
4961 (?'p1'(?i)rah)\s+\k{p1}
4962 (?P<p1>(?i)rah)\s+(?P=p1)
4963 (?<p1>(?i)rah)\s+\g{p1}
4965 A subpattern that is referenced by name may appear in the pattern
4966 before or after the reference.
4968 There may be more than one back reference to the same subpattern. If a
4969 subpattern has not actually been used in a particular match, any back
4970 references to it always fail by default. For example, the pattern
4972 (a|(bc))\2
4974 always fails if it starts to match "a" rather than "bc". However, if
4975 the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer-
4976 ence to an unset value matches an empty string.
4978 Because there may be many capturing parentheses in a pattern, all dig-
4979 its following a backslash are taken as part of a potential back refer-
4980 ence number. If the pattern continues with a digit character, some
4981 delimiter must be used to terminate the back reference. If the
4982 PCRE_EXTENDED option is set, this can be whitespace. Otherwise, the \g{
4983 syntax or an empty comment (see "Comments" below) can be used.
4985 Recursive back references
4987 A back reference that occurs inside the parentheses to which it refers
4988 fails when the subpattern is first used, so, for example, (a\1) never
4989 matches. However, such references can be useful inside repeated sub-
4990 patterns. For example, the pattern
4992 (a|b\1)+
4994 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
4995 ation of the subpattern, the back reference matches the character
4996 string corresponding to the previous iteration. In order for this to
4997 work, the pattern must be such that the first iteration does not need
4998 to match the back reference. This can be done using alternation, as in
4999 the example above, or by a quantifier with a minimum of zero.
5001 Back references of this type cause the group that they reference to be
5002 treated as an atomic group. Once the whole group has been matched, a
5003 subsequent matching failure cannot cause backtracking into the middle
5004 of the group.
5009 An assertion is a test on the characters following or preceding the
5010 current matching point that does not actually consume any characters.
5011 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
5012 described above.
5014 More complicated assertions are coded as subpatterns. There are two
5015 kinds: those that look ahead of the current position in the subject
5016 string, and those that look behind it. An assertion subpattern is
5017 matched in the normal way, except that it does not cause the current
5018 matching position to be changed.
5020 Assertion subpatterns are not capturing subpatterns. If such an asser-
5021 tion contains capturing subpatterns within it, these are counted for
5022 the purposes of numbering the capturing subpatterns in the whole pat-
5023 tern. However, substring capturing is carried out only for positive
5024 assertions, because it does not make sense for negative assertions.
5026 For compatibility with Perl, assertion subpatterns may be repeated;
5027 though it makes no sense to assert the same thing several times, the
5028 side effect of capturing parentheses may occasionally be useful. In
5029 practice, there only three cases:
5031 (1) If the quantifier is {0}, the assertion is never obeyed during
5032 matching. However, it may contain internal capturing parenthesized
5033 groups that are called from elsewhere via the subroutine mechanism.
5035 (2) If quantifier is {0,n} where n is greater than zero, it is treated
5036 as if it were {0,1}. At run time, the rest of the pattern match is
5037 tried with and without the assertion, the order depending on the greed-
5038 iness of the quantifier.
5040 (3) If the minimum repetition is greater than zero, the quantifier is
5041 ignored. The assertion is obeyed just once when encountered during
5042 matching.
5044 Lookahead assertions
5046 Lookahead assertions start with (?= for positive assertions and (?! for
5047 negative assertions. For example,
5049 \w+(?=;)
5051 matches a word followed by a semicolon, but does not include the semi-
5052 colon in the match, and
5054 foo(?!bar)
5056 matches any occurrence of "foo" that is not followed by "bar". Note
5057 that the apparently similar pattern
5059 (?!foo)bar
5061 does not find an occurrence of "bar" that is preceded by something
5062 other than "foo"; it finds any occurrence of "bar" whatsoever, because
5063 the assertion (?!foo) is always true when the next three characters are
5064 "bar". A lookbehind assertion is needed to achieve the other effect.
5066 If you want to force a matching failure at some point in a pattern, the
5067 most convenient way to do it is with (?!) because an empty string
5068 always matches, so an assertion that requires there not to be an empty
5069 string must always fail. The backtracking control verb (*FAIL) or (*F)
5070 is a synonym for (?!).
5072 Lookbehind assertions
5074 Lookbehind assertions start with (?<= for positive assertions and (?<!
5075 for negative assertions. For example,
5077 (?<!foo)bar
5079 does find an occurrence of "bar" that is not preceded by "foo". The
5080 contents of a lookbehind assertion are restricted such that all the
5081 strings it matches must have a fixed length. However, if there are sev-
5082 eral top-level alternatives, they do not all have to have the same
5083 fixed length. Thus
5085 (?<=bullock|donkey)
5087 is permitted, but
5089 (?<!dogs?|cats?)
5091 causes an error at compile time. Branches that match different length
5092 strings are permitted only at the top level of a lookbehind assertion.
5093 This is an extension compared with Perl, which requires all branches to
5094 match the same length of string. An assertion such as
5096 (?<=ab(c|de))
5098 is not permitted, because its single top-level branch can match two
5099 different lengths, but it is acceptable to PCRE if rewritten to use two
5100 top-level branches:
5102 (?<=abc|abde)
5104 In some cases, the escape sequence \K (see above) can be used instead
5105 of a lookbehind assertion to get round the fixed-length restriction.
5107 The implementation of lookbehind assertions is, for each alternative,
5108 to temporarily move the current position back by the fixed length and
5109 then try to match. If there are insufficient characters before the cur-
5110 rent position, the assertion fails.
5112 In UTF-8 mode, PCRE does not allow the \C escape (which matches a sin-
5113 gle byte, even in UTF-8 mode) to appear in lookbehind assertions,
5114 because it makes it impossible to calculate the length of the lookbe-
5115 hind. The \X and \R escapes, which can match different numbers of
5116 bytes, are also not permitted.
5118 "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
5119 lookbehinds, as long as the subpattern matches a fixed-length string.
5120 Recursion, however, is not supported.
5122 Possessive quantifiers can be used in conjunction with lookbehind
5123 assertions to specify efficient matching of fixed-length strings at the
5124 end of subject strings. Consider a simple pattern such as
5126 abcd$
5128 when applied to a long string that does not match. Because matching
5129 proceeds from left to right, PCRE will look for each "a" in the subject
5130 and then see if what follows matches the rest of the pattern. If the
5131 pattern is specified as
5133 ^.*abcd$
5135 the initial .* matches the entire string at first, but when this fails
5136 (because there is no following "a"), it backtracks to match all but the
5137 last character, then all but the last two characters, and so on. Once
5138 again the search for "a" covers the entire string, from right to left,
5139 so we are no better off. However, if the pattern is written as
5141 ^.*+(?<=abcd)
5143 there can be no backtracking for the .*+ item; it can match only the
5144 entire string. The subsequent lookbehind assertion does a single test
5145 on the last four characters. If it fails, the match fails immediately.
5146 For long strings, this approach makes a significant difference to the
5147 processing time.
5149 Using multiple assertions
5151 Several assertions (of any sort) may occur in succession. For example,
5153 (?<=\d{3})(?<!999)foo
5155 matches "foo" preceded by three digits that are not "999". Notice that
5156 each of the assertions is applied independently at the same point in
5157 the subject string. First there is a check that the previous three
5158 characters are all digits, and then there is a check that the same
5159 three characters are not "999". This pattern does not match "foo" pre-
5160 ceded by six characters, the first of which are digits and the last
5161 three of which are not "999". For example, it doesn't match "123abc-
5162 foo". A pattern to do that is
5164 (?<=\d{3}...)(?<!999)foo
5166 This time the first assertion looks at the preceding six characters,
5167 checking that the first three are digits, and then the second assertion
5168 checks that the preceding three characters are not "999".
5170 Assertions can be nested in any combination. For example,
5172 (?<=(?<!foo)bar)baz
5174 matches an occurrence of "baz" that is preceded by "bar" which in turn
5175 is not preceded by "foo", while
5177 (?<=\d{3}(?!999)...)foo
5179 is another pattern that matches "foo" preceded by three digits and any
5180 three characters that are not "999".
5185 It is possible to cause the matching process to obey a subpattern con-
5186 ditionally or to choose between two alternative subpatterns, depending
5187 on the result of an assertion, or whether a specific capturing subpat-
5188 tern has already been matched. The two possible forms of conditional
5189 subpattern are:
5191 (?(condition)yes-pattern)
5192 (?(condition)yes-pattern|no-pattern)
5194 If the condition is satisfied, the yes-pattern is used; otherwise the
5195 no-pattern (if present) is used. If there are more than two alterna-
5196 tives in the subpattern, a compile-time error occurs. Each of the two
5197 alternatives may itself contain nested subpatterns of any form, includ-
5198 ing conditional subpatterns; the restriction to two alternatives
5199 applies only at the level of the condition. This pattern fragment is an
5200 example where the alternatives are complex:
5202 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
5205 There are four kinds of condition: references to subpatterns, refer-
5206 ences to recursion, a pseudo-condition called DEFINE, and assertions.
5208 Checking for a used subpattern by number
5210 If the text between the parentheses consists of a sequence of digits,
5211 the condition is true if a capturing subpattern of that number has pre-
5212 viously matched. If there is more than one capturing subpattern with
5213 the same number (see the earlier section about duplicate subpattern
5214 numbers), the condition is true if any of them have matched. An alter-
5215 native notation is to precede the digits with a plus or minus sign. In
5216 this case, the subpattern number is relative rather than absolute. The
5217 most recently opened parentheses can be referenced by (?(-1), the next
5218 most recent by (?(-2), and so on. Inside loops it can also make sense
5219 to refer to subsequent groups. The next parentheses to be opened can be
5220 referenced as (?(+1), and so on. (The value zero in any of these forms
5221 is not used; it provokes a compile-time error.)
5223 Consider the following pattern, which contains non-significant white
5224 space to make it more readable (assume the PCRE_EXTENDED option) and to
5225 divide it into three parts for ease of discussion:
5227 ( \( )? [^()]+ (?(1) \) )
5229 The first part matches an optional opening parenthesis, and if that
5230 character is present, sets it as the first captured substring. The sec-
5231 ond part matches one or more characters that are not parentheses. The
5232 third part is a conditional subpattern that tests whether or not the
5233 first set of parentheses matched. If they did, that is, if subject
5234 started with an opening parenthesis, the condition is true, and so the
5235 yes-pattern is executed and a closing parenthesis is required. Other-
5236 wise, since no-pattern is not present, the subpattern matches nothing.
5237 In other words, this pattern matches a sequence of non-parentheses,
5238 optionally enclosed in parentheses.
5240 If you were embedding this pattern in a larger one, you could use a
5241 relative reference:
5243 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
5245 This makes the fragment independent of the parentheses in the larger
5246 pattern.
5248 Checking for a used subpattern by name
5250 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
5251 used subpattern by name. For compatibility with earlier versions of
5252 PCRE, which had this facility before Perl, the syntax (?(name)...) is
5253 also recognized. However, there is a possible ambiguity with this syn-
5254 tax, because subpattern names may consist entirely of digits. PCRE
5255 looks first for a named subpattern; if it cannot find one and the name
5256 consists entirely of digits, PCRE looks for a subpattern of that num-
5257 ber, which must be greater than zero. Using subpattern names that con-
5258 sist entirely of digits is not recommended.
5260 Rewriting the above example to use a named subpattern gives this:
5262 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
5264 If the name used in a condition of this kind is a duplicate, the test
5265 is applied to all subpatterns of the same name, and is true if any one
5266 of them has matched.
5268 Checking for pattern recursion
5270 If the condition is the string (R), and there is no subpattern with the
5271 name R, the condition is true if a recursive call to the whole pattern
5272 or any subpattern has been made. If digits or a name preceded by amper-
5273 sand follow the letter R, for example:
5275 (?(R3)...) or (?(R&name)...)
5277 the condition is true if the most recent recursion is into a subpattern
5278 whose number or name is given. This condition does not check the entire
5279 recursion stack. If the name used in a condition of this kind is a
5280 duplicate, the test is applied to all subpatterns of the same name, and
5281 is true if any one of them is the most recent recursion.
5283 At "top level", all these recursion test conditions are false. The
5284 syntax for recursive patterns is described below.
5286 Defining subpatterns for use by reference only
5288 If the condition is the string (DEFINE), and there is no subpattern
5289 with the name DEFINE, the condition is always false. In this case,
5290 there may be only one alternative in the subpattern. It is always
5291 skipped if control reaches this point in the pattern; the idea of
5292 DEFINE is that it can be used to define subroutines that can be refer-
5293 enced from elsewhere. (The use of subroutines is described below.) For
5294 example, a pattern to match an IPv4 address such as ""
5295 could be written like this (ignore whitespace and line breaks):
5297 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
5298 \b (?&byte) (\.(?&byte)){3} \b
5300 The first part of the pattern is a DEFINE group inside which a another
5301 group named "byte" is defined. This matches an individual component of
5302 an IPv4 address (a number less than 256). When matching takes place,
5303 this part of the pattern is skipped because DEFINE acts like a false
5304 condition. The rest of the pattern uses references to the named group
5305 to match the four dot-separated components of an IPv4 address, insist-
5306 ing on a word boundary at each end.
5308 Assertion conditions
5310 If the condition is not in any of the above formats, it must be an
5311 assertion. This may be a positive or negative lookahead or lookbehind
5312 assertion. Consider this pattern, again containing non-significant
5313 white space, and with the two alternatives on the second line:
5315 (?(?=[^a-z]*[a-z])
5316 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
5318 The condition is a positive lookahead assertion that matches an
5319 optional sequence of non-letters followed by a letter. In other words,
5320 it tests for the presence of at least one letter in the subject. If a
5321 letter is found, the subject is matched against the first alternative;
5322 otherwise it is matched against the second. This pattern matches
5323 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
5324 letters and dd are digits.
5329 There are two ways of including comments in patterns that are processed
5330 by PCRE. In both cases, the start of the comment must not be in a char-
5331 acter class, nor in the middle of any other sequence of related charac-
5332 ters such as (?: or a subpattern name or number. The characters that
5333 make up a comment play no part in the pattern matching.
5335 The sequence (?# marks the start of a comment that continues up to the
5336 next closing parenthesis. Nested parentheses are not permitted. If the
5337 PCRE_EXTENDED option is set, an unescaped # character also introduces a
5338 comment, which in this case continues to immediately after the next
5339 newline character or character sequence in the pattern. Which charac-
5340 ters are interpreted as newlines is controlled by the options passed to
5341 pcre_compile() or by a special sequence at the start of the pattern, as
5342 described in the section entitled "Newline conventions" above. Note
5343 that the end of this type of comment is a literal newline sequence in
5344 the pattern; escape sequences that happen to represent a newline do not
5345 count. For example, consider this pattern when PCRE_EXTENDED is set,
5346 and the default newline convention is in force:
5348 abc #comment \n still comment
5350 On encountering the # character, pcre_compile() skips along, looking
5351 for a newline in the pattern. The sequence \n is still literal at this
5352 stage, so it does not terminate the comment. Only an actual character
5353 with the code value 0x0a (the default newline) does so.
5358 Consider the problem of matching a string in parentheses, allowing for
5359 unlimited nested parentheses. Without the use of recursion, the best
5360 that can be done is to use a pattern that matches up to some fixed
5361 depth of nesting. It is not possible to handle an arbitrary nesting
5362 depth.
5364 For some time, Perl has provided a facility that allows regular expres-
5365 sions to recurse (amongst other things). It does this by interpolating
5366 Perl code in the expression at run time, and the code can refer to the
5367 expression itself. A Perl pattern using code interpolation to solve the
5368 parentheses problem can be created like this:
5370 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
5372 The (?p{...}) item interpolates Perl code at run time, and in this case
5373 refers recursively to the pattern in which it appears.
5375 Obviously, PCRE cannot support the interpolation of Perl code. Instead,
5376 it supports special syntax for recursion of the entire pattern, and
5377 also for individual subpattern recursion. After its introduction in
5378 PCRE and Python, this kind of recursion was subsequently introduced
5379 into Perl at release 5.10.
5381 A special item that consists of (? followed by a number greater than
5382 zero and a closing parenthesis is a recursive subroutine call of the
5383 subpattern of the given number, provided that it occurs inside that
5384 subpattern. (If not, it is a non-recursive subroutine call, which is
5385 described in the next section.) The special item (?R) or (?0) is a
5386 recursive call of the entire regular expression.
5388 This PCRE pattern solves the nested parentheses problem (assume the
5389 PCRE_EXTENDED option is set so that white space is ignored):
5391 \( ( [^()]++ | (?R) )* \)
5393 First it matches an opening parenthesis. Then it matches any number of
5394 substrings which can either be a sequence of non-parentheses, or a
5395 recursive match of the pattern itself (that is, a correctly parenthe-
5396 sized substring). Finally there is a closing parenthesis. Note the use
5397 of a possessive quantifier to avoid backtracking into sequences of non-
5398 parentheses.
5400 If this were part of a larger pattern, you would not want to recurse
5401 the entire pattern, so instead you could use this:
5403 ( \( ( [^()]++ | (?1) )* \) )
5405 We have put the pattern into parentheses, and caused the recursion to
5406 refer to them instead of the whole pattern.
5408 In a larger pattern, keeping track of parenthesis numbers can be
5409 tricky. This is made easier by the use of relative references. Instead
5410 of (?1) in the pattern above you can write (?-2) to refer to the second
5411 most recently opened parentheses preceding the recursion. In other
5412 words, a negative number counts capturing parentheses leftwards from
5413 the point at which it is encountered.
5415 It is also possible to refer to subsequently opened parentheses, by
5416 writing references such as (?+2). However, these cannot be recursive
5417 because the reference is not inside the parentheses that are refer-
5418 enced. They are always non-recursive subroutine calls, as described in
5419 the next section.
5421 An alternative approach is to use named parentheses instead. The Perl
5422 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
5423 supported. We could rewrite the above example as follows:
5425 (?<pn> \( ( [^()]++ | (?&pn) )* \) )
5427 If there is more than one subpattern with the same name, the earliest
5428 one is used.
5430 This particular example pattern that we have been looking at contains
5431 nested unlimited repeats, and so the use of a possessive quantifier for
5432 matching strings of non-parentheses is important when applying the pat-
5433 tern to strings that do not match. For example, when this pattern is
5434 applied to
5436 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
5438 it yields "no match" quickly. However, if a possessive quantifier is
5439 not used, the match runs for a very long time indeed because there are
5440 so many different ways the + and * repeats can carve up the subject,
5441 and all have to be tested before failure can be reported.
5443 At the end of a match, the values of capturing parentheses are those
5444 from the outermost level. If you want to obtain intermediate values, a
5445 callout function can be used (see below and the pcrecallout documenta-
5446 tion). If the pattern above is matched against
5448 (ab(cd)ef)
5450 the value for the inner capturing parentheses (numbered 2) is "ef",
5451 which is the last value taken on at the top level. If a capturing sub-
5452 pattern is not matched at the top level, its final captured value is
5453 unset, even if it was (temporarily) set at a deeper level during the
5454 matching process.
5456 If there are more than 15 capturing parentheses in a pattern, PCRE has
5457 to obtain extra memory to store data during a recursion, which it does
5458 by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
5459 can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
5461 Do not confuse the (?R) item with the condition (R), which tests for
5462 recursion. Consider this pattern, which matches text in angle brack-
5463 ets, allowing for arbitrary nesting. Only digits are allowed in nested
5464 brackets (that is, when recursing), whereas any characters are permit-
5465 ted at the outer level.
5467 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
5469 In this pattern, (?(R) is the start of a conditional subpattern, with
5470 two different alternatives for the recursive and non-recursive cases.
5471 The (?R) item is the actual recursive call.
5473 Differences in recursion processing between PCRE and Perl
5475 Recursion processing in PCRE differs from Perl in two important ways.
5476 In PCRE (like Python, but unlike Perl), a recursive subpattern call is
5477 always treated as an atomic group. That is, once it has matched some of
5478 the subject string, it is never re-entered, even if it contains untried
5479 alternatives and there is a subsequent matching failure. This can be
5480 illustrated by the following pattern, which purports to match a palin-
5481 dromic string that contains an odd number of characters (for example,
5482 "a", "aba", "abcba", "abcdcba"):
5484 ^(.|(.)(?1)\2)$
5486 The idea is that it either matches a single character, or two identical
5487 characters surrounding a sub-palindrome. In Perl, this pattern works;
5488 in PCRE it does not if the pattern is longer than three characters.
5489 Consider the subject string "abcba":
5491 At the top level, the first character is matched, but as it is not at
5492 the end of the string, the first alternative fails; the second alterna-
5493 tive is taken and the recursion kicks in. The recursive call to subpat-
5494 tern 1 successfully matches the next character ("b"). (Note that the
5495 beginning and end of line tests are not part of the recursion).
5497 Back at the top level, the next character ("c") is compared with what
5498 subpattern 2 matched, which was "a". This fails. Because the recursion
5499 is treated as an atomic group, there are now no backtracking points,
5500 and so the entire match fails. (Perl is able, at this point, to re-
5501 enter the recursion and try the second alternative.) However, if the
5502 pattern is written with the alternatives in the other order, things are
5503 different:
5505 ^((.)(?1)\2|.)$
5507 This time, the recursing alternative is tried first, and continues to
5508 recurse until it runs out of characters, at which point the recursion
5509 fails. But this time we do have another alternative to try at the
5510 higher level. That is the big difference: in the previous case the
5511 remaining alternative is at a deeper recursion level, which PCRE cannot
5512 use.
5514 To change the pattern so that it matches all palindromic strings, not
5515 just those with an odd number of characters, it is tempting to change
5516 the pattern to this:
5518 ^((.)(?1)\2|.?)$
5520 Again, this works in Perl, but not in PCRE, and for the same reason.
5521 When a deeper recursion has matched a single character, it cannot be
5522 entered again in order to match an empty string. The solution is to
5523 separate the two cases, and write out the odd and even cases as alter-
5524 natives at the higher level:
5526 ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
5528 If you want to match typical palindromic phrases, the pattern has to
5529 ignore all non-word characters, which can be done like this:
5531 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
5533 If run with the PCRE_CASELESS option, this pattern matches phrases such
5534 as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
5535 Perl. Note the use of the possessive quantifier *+ to avoid backtrack-
5536 ing into sequences of non-word characters. Without this, PCRE takes a
5537 great deal longer (ten times or more) to match typical phrases, and
5538 Perl takes so long that you think it has gone into a loop.
5540 WARNING: The palindrome-matching patterns above work only if the sub-
5541 ject string does not start with a palindrome that is shorter than the
5542 entire string. For example, although "abcba" is correctly matched, if
5543 the subject is "ababa", PCRE finds the palindrome "aba" at the start,
5544 then fails at top level because the end of the string does not follow.
5545 Once again, it cannot jump back into the recursion to try other alter-
5546 natives, so the entire match fails.
5548 The second way in which PCRE and Perl differ in their recursion pro-
5549 cessing is in the handling of captured values. In Perl, when a subpat-
5550 tern is called recursively or as a subpattern (see the next section),
5551 it has no access to any values that were captured outside the recur-
5552 sion, whereas in PCRE these values can be referenced. Consider this
5553 pattern:
5555 ^(.)(\1|a(?2))
5557 In PCRE, this pattern matches "bab". The first capturing parentheses
5558 match "b", then in the second group, when the back reference \1 fails
5559 to match "b", the second alternative matches "a" and then recurses. In
5560 the recursion, \1 does now match "b" and so the whole match succeeds.
5561 In Perl, the pattern fails to match because inside the recursive call
5562 \1 cannot access the externally set value.
5567 If the syntax for a recursive subpattern call (either by number or by
5568 name) is used outside the parentheses to which it refers, it operates
5569 like a subroutine in a programming language. The called subpattern may
5570 be defined before or after the reference. A numbered reference can be
5571 absolute or relative, as in these examples:
5573 (...(absolute)...)...(?2)...
5574 (...(relative)...)...(?-1)...
5575 (...(?+1)...(relative)...
5577 An earlier example pointed out that the pattern
5579 (sens|respons)e and \1ibility
5581 matches "sense and sensibility" and "response and responsibility", but
5582 not "sense and responsibility". If instead the pattern
5584 (sens|respons)e and (?1)ibility
5586 is used, it does match "sense and responsibility" as well as the other
5587 two strings. Another example is given in the discussion of DEFINE
5588 above.
5590 All subroutine calls, whether recursive or not, are always treated as
5591 atomic groups. That is, once a subroutine has matched some of the sub-
5592 ject string, it is never re-entered, even if it contains untried alter-
5593 natives and there is a subsequent matching failure. Any capturing
5594 parentheses that are set during the subroutine call revert to their
5595 previous values afterwards.
5597 Processing options such as case-independence are fixed when a subpat-
5598 tern is defined, so if it is used as a subroutine, such options cannot
5599 be changed for different calls. For example, consider this pattern:
5601 (abc)(?i:(?-1))
5603 It matches "abcabc". It does not match "abcABC" because the change of
5604 processing option does not affect the called subpattern.
5609 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
5610 name or a number enclosed either in angle brackets or single quotes, is
5611 an alternative syntax for referencing a subpattern as a subroutine,
5612 possibly recursively. Here are two of the examples used above, rewrit-
5613 ten using this syntax:
5615 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
5616 (sens|respons)e and \g'1'ibility
5618 PCRE supports an extension to Oniguruma: if a number is preceded by a
5619 plus or a minus sign it is taken as a relative reference. For example:
5621 (abc)(?i:\g<-1>)
5623 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
5624 synonymous. The former is a back reference; the latter is a subroutine
5625 call.
5630 Perl has a feature whereby using the sequence (?{...}) causes arbitrary
5631 Perl code to be obeyed in the middle of matching a regular expression.
5632 This makes it possible, amongst other things, to extract different sub-
5633 strings that match the same pair of parentheses when there is a repeti-
5634 tion.
5636 PCRE provides a similar feature, but of course it cannot obey arbitrary
5637 Perl code. The feature is called "callout". The caller of PCRE provides
5638 an external function by putting its entry point in the global variable
5639 pcre_callout. By default, this variable contains NULL, which disables
5640 all calling out.
5642 Within a regular expression, (?C) indicates the points at which the
5643 external function is to be called. If you want to identify different
5644 callout points, you can put a number less than 256 after the letter C.
5645 The default value is zero. For example, this pattern has two callout
5646 points:
5648 (?C1)abc(?C2)def
5650 If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
5651 automatically installed before each item in the pattern. They are all
5652 numbered 255.
5654 During matching, when PCRE reaches a callout point (and pcre_callout is
5655 set), the external function is called. It is provided with the number
5656 of the callout, the position in the pattern, and, optionally, one item
5657 of data originally supplied by the caller of pcre_exec(). The callout
5658 function may cause matching to proceed, to backtrack, or to fail alto-
5659 gether. A complete description of the interface to the callout function
5660 is given in the pcrecallout documentation.
5665 Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
5666 which are described in the Perl documentation as "experimental and sub-
5667 ject to change or removal in a future version of Perl". It goes on to
5668 say: "Their usage in production code should be noted to avoid problems
5669 during upgrades." The same remarks apply to the PCRE features described
5670 in this section.
5672 Since these verbs are specifically related to backtracking, most of
5673 them can be used only when the pattern is to be matched using
5674 pcre_exec(), which uses a backtracking algorithm. With the exception of
5675 (*FAIL), which behaves like a failing negative assertion, they cause an
5676 error if encountered by pcre_dfa_exec().
5678 If any of these verbs are used in an assertion or in a subpattern that
5679 is called as a subroutine (whether or not recursively), their effect is
5680 confined to that subpattern; it does not extend to the surrounding pat-
5681 tern, with one exception: the name from a *(MARK), (*PRUNE), or (*THEN)
5682 that is encountered in a successful positive assertion is passed back
5683 when a match succeeds (compare capturing parentheses in assertions).
5684 Note that such subpatterns are processed as anchored at the point where
5685 they are tested. Note also that Perl's treatment of subroutines is dif-
5686 ferent in some cases.
5688 The new verbs make use of what was previously invalid syntax: an open-
5689 ing parenthesis followed by an asterisk. They are generally of the form
5690 (*VERB) or (*VERB:NAME). Some may take either form, with differing be-
5691 haviour, depending on whether or not an argument is present. A name is
5692 any sequence of characters that does not include a closing parenthesis.
5693 If the name is empty, that is, if the closing parenthesis immediately
5694 follows the colon, the effect is as if the colon were not there. Any
5695 number of these verbs may occur in a pattern.
5697 PCRE contains some optimizations that are used to speed up matching by
5698 running some checks at the start of each match attempt. For example, it
5699 may know the minimum length of matching subject, or that a particular
5700 character must be present. When one of these optimizations suppresses
5701 the running of a match, any included backtracking verbs will not, of
5702 course, be processed. You can suppress the start-of-match optimizations
5703 by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com-
5704 pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT).
5706 Experiments with Perl suggest that it too has similar optimizations,
5707 sometimes leading to anomalous results.
5709 Verbs that act immediately
5711 The following verbs act as soon as they are encountered. They may not
5712 be followed by a name.
5714 (*ACCEPT)
5716 This verb causes the match to end successfully, skipping the remainder
5717 of the pattern. However, when it is inside a subpattern that is called
5718 as a subroutine, only that subpattern is ended successfully. Matching
5719 then continues at the outer level. If (*ACCEPT) is inside capturing
5720 parentheses, the data so far is captured. For example:
5722 A((?:A|B(*ACCEPT)|C)D)
5724 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap-
5725 tured by the outer parentheses.
5727 (*FAIL) or (*F)
5729 This verb causes a matching failure, forcing backtracking to occur. It
5730 is equivalent to (?!) but easier to read. The Perl documentation notes
5731 that it is probably useful only when combined with (?{}) or (??{}).
5732 Those are, of course, Perl features that are not present in PCRE. The
5733 nearest equivalent is the callout feature, as for example in this pat-
5734 tern:
5736 a+(?C)(*FAIL)
5738 A match with the string "aaaa" always fails, but the callout is taken
5739 before each backtrack happens (in this example, 10 times).
5741 Recording which path was taken
5743 There is one verb whose main purpose is to track how a match was
5744 arrived at, though it also has a secondary use in conjunction with
5745 advancing the match starting point (see (*SKIP) below).
5747 (*MARK:NAME) or (*:NAME)
5749 A name is always required with this verb. There may be as many
5750 instances of (*MARK) as you like in a pattern, and their names do not
5751 have to be unique.
5753 When a match succeeds, the name of the last-encountered (*MARK) on the
5754 matching path is passed back to the caller via the pcre_extra data
5755 structure, as described in the section on pcre_extra in the pcreapi
5756 documentation. Here is an example of pcretest output, where the /K mod-
5757 ifier requests the retrieval and outputting of (*MARK) data:
5759 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
5760 data> XY
5761 0: XY
5762 MK: A
5763 XZ
5764 0: XZ
5765 MK: B
5767 The (*MARK) name is tagged with "MK:" in this output, and in this exam-
5768 ple it indicates which of the two alternatives matched. This is a more
5769 efficient way of obtaining this information than putting each alterna-
5770 tive in its own capturing parentheses.
5772 If (*MARK) is encountered in a positive assertion, its name is recorded
5773 and passed back if it is the last-encountered. This does not happen for
5774 negative assertions.
5776 After a partial match or a failed match, the name of the last encoun-
5777 tered (*MARK) in the entire match process is returned. For example:
5779 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
5780 data> XP
5781 No match, mark = B
5783 Note that in this unanchored example the mark is retained from the
5784 match attempt that started at the letter "X". Subsequent match attempts
5785 starting at "P" and then with an empty string do not get as far as the
5786 (*MARK) item, but nevertheless do not reset it.
5788 Verbs that act after backtracking
5790 The following verbs do nothing when they are encountered. Matching con-
5791 tinues with what follows, but if there is no subsequent match, causing
5792 a backtrack to the verb, a failure is forced. That is, backtracking
5793 cannot pass to the left of the verb. However, when one of these verbs
5794 appears inside an atomic group, its effect is confined to that group,
5795 because once the group has been matched, there is never any backtrack-
5796 ing into it. In this situation, backtracking can "jump back" to the
5797 left of the entire atomic group. (Remember also, as stated above, that
5798 this localization also applies in subroutine calls and assertions.)
5800 These verbs differ in exactly what kind of failure occurs when back-
5801 tracking reaches them.
5803 (*COMMIT)
5805 This verb, which may not be followed by a name, causes the whole match
5806 to fail outright if the rest of the pattern does not match. Even if the
5807 pattern is unanchored, no further attempts to find a match by advancing
5808 the starting point take place. Once (*COMMIT) has been passed,
5809 pcre_exec() is committed to finding a match at the current starting
5810 point, or not at all. For example:
5812 a+(*COMMIT)b
5814 This matches "xxaab" but not "aacaab". It can be thought of as a kind
5815 of dynamic anchor, or "I've started, so I must finish." The name of the
5816 most recently passed (*MARK) in the path is passed back when (*COMMIT)
5817 forces a match failure.
5819 Note that (*COMMIT) at the start of a pattern is not the same as an
5820 anchor, unless PCRE's start-of-match optimizations are turned off, as
5821 shown in this pcretest example:
5823 re> /(*COMMIT)abc/
5824 data> xyzabc
5825 0: abc
5826 xyzabc\Y
5827 No match
5829 PCRE knows that any match must start with "a", so the optimization
5830 skips along the subject to "a" before running the first match attempt,
5831 which succeeds. When the optimization is disabled by the \Y escape in
5832 the second subject, the match starts at "x" and so the (*COMMIT) causes
5833 it to fail without trying any other starting points.
5835 (*PRUNE) or (*PRUNE:NAME)
5837 This verb causes the match to fail at the current starting position in
5838 the subject if the rest of the pattern does not match. If the pattern
5839 is unanchored, the normal "bumpalong" advance to the next starting
5840 character then happens. Backtracking can occur as usual to the left of
5841 (*PRUNE), before it is reached, or when matching to the right of
5842 (*PRUNE), but if there is no match to the right, backtracking cannot
5843 cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alter-
5844 native to an atomic group or possessive quantifier, but there are some
5845 uses of (*PRUNE) that cannot be expressed in any other way. The behav-
5846 iour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
5847 anchored pattern (*PRUNE) has the same effect as (*COMMIT).
5849 (*SKIP)
5851 This verb, when given without a name, is like (*PRUNE), except that if
5852 the pattern is unanchored, the "bumpalong" advance is not to the next
5853 character, but to the position in the subject where (*SKIP) was encoun-
5854 tered. (*SKIP) signifies that whatever text was matched leading up to
5855 it cannot be part of a successful match. Consider:
5857 a+(*SKIP)b
5859 If the subject is "aaaac...", after the first match attempt fails
5860 (starting at the first character in the string), the starting point
5861 skips on to start the next attempt at "c". Note that a possessive quan-
5862 tifer does not have the same effect as this example; although it would
5863 suppress backtracking during the first match attempt, the second
5864 attempt would start at the second character instead of skipping on to
5865 "c".
5867 (*SKIP:NAME)
5869 When (*SKIP) has an associated name, its behaviour is modified. If the
5870 following pattern fails to match, the previous path through the pattern
5871 is searched for the most recent (*MARK) that has the same name. If one
5872 is found, the "bumpalong" advance is to the subject position that cor-
5873 responds to that (*MARK) instead of to where (*SKIP) was encountered.
5874 If no (*MARK) with a matching name is found, the (*SKIP) is ignored.
5876 (*THEN) or (*THEN:NAME)
5878 This verb causes a skip to the next innermost alternative if the rest
5879 of the pattern does not match. That is, it cancels pending backtrack-
5880 ing, but only within the current alternative. Its name comes from the
5881 observation that it can be used for a pattern-based if-then-else block:
5883 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
5885 If the COND1 pattern matches, FOO is tried (and possibly further items
5886 after the end of the group if FOO succeeds); on failure, the matcher
5887 skips to the second alternative and tries COND2, without backtracking
5888 into COND1. The behaviour of (*THEN:NAME) is exactly the same as
5889 (*MARK:NAME)(*THEN). If (*THEN) is not inside an alternation, it acts
5890 like (*PRUNE).
5892 Note that a subpattern that does not contain a | character is just a
5893 part of the enclosing alternative; it is not a nested alternation with
5894 only one alternative. The effect of (*THEN) extends beyond such a sub-
5895 pattern to the enclosing alternative. Consider this pattern, where A,
5896 B, etc. are complex pattern fragments that do not contain any | charac-
5897 ters at this level:
5899 A (B(*THEN)C) | D
5901 If A and B are matched, but there is a failure in C, matching does not
5902 backtrack into A; instead it moves to the next alternative, that is, D.
5903 However, if the subpattern containing (*THEN) is given an alternative,
5904 it behaves differently:
5906 A (B(*THEN)C | (*FAIL)) | D
5908 The effect of (*THEN) is now confined to the inner subpattern. After a
5909 failure in C, matching moves to (*FAIL), which causes the whole subpat-
5910 tern to fail because there are no more alternatives to try. In this
5911 case, matching does now backtrack into A.
5913 Note also that a conditional subpattern is not considered as having two
5914 alternatives, because only one is ever used. In other words, the |
5915 character in a conditional subpattern has a different meaning. Ignoring
5916 white space, consider:
5918 ^.*? (?(?=a) a | b(*THEN)c )
5920 If the subject is "ba", this pattern does not match. Because .*? is
5921 ungreedy, it initially matches zero characters. The condition (?=a)
5922 then fails, the character "b" is matched, but "c" is not. At this
5923 point, matching does not backtrack to .*? as might perhaps be expected
5924 from the presence of the | character. The conditional subpattern is
5925 part of the single alternative that comprises the whole pattern, and so
5926 the match fails. (If there was a backtrack into .*?, allowing it to
5927 match "b", the match would succeed.)
5929 The verbs just described provide four different "strengths" of control
5930 when subsequent matching fails. (*THEN) is the weakest, carrying on the
5931 match at the next alternative. (*PRUNE) comes next, failing the match
5932 at the current starting position, but allowing an advance to the next
5933 character (for an unanchored pattern). (*SKIP) is similar, except that
5934 the advance may be more than one character. (*COMMIT) is the strongest,
5935 causing the entire match to fail.
5937 If more than one such verb is present in a pattern, the "strongest" one
5938 wins. For example, consider this pattern, where A, B, etc. are complex
5939 pattern fragments:
5941 (A(*COMMIT)B(*THEN)C|D)
5943 Once A has matched, PCRE is committed to this match, at the current
5944 starting position. If subsequently B matches, but C does not, the nor-
5945 mal (*THEN) action of trying the next alternative (that is, D) does not
5946 happen because (*COMMIT) overrides.
5951 pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3).
5956 Philip Hazel
5957 University Computing Service
5958 Cambridge CB2 3QH, England.
5963 Last updated: 29 November 2011
5964 Copyright (c) 1997-2011 University of Cambridge.
5965 ------------------------------------------------------------------------------
5971 NAME
5972 PCRE - Perl-compatible regular expressions
5977 The full syntax and semantics of the regular expressions that are sup-
5978 ported by PCRE are described in the pcrepattern documentation. This
5979 document contains just a quick-reference summary of the syntax.
5984 \x where x is non-alphanumeric is a literal x
5985 \Q...\E treat enclosed characters as literal
5990 \a alarm, that is, the BEL character (hex 07)
5991 \cx "control-x", where x is any ASCII character
5992 \e escape (hex 1B)
5993 \f formfeed (hex 0C)
5994 \n newline (hex 0A)
5995 \r carriage return (hex 0D)
5996 \t tab (hex 09)
5997 \ddd character with octal code ddd, or backreference
5998 \xhh character with hex code hh
5999 \x{hhh..} character with hex code hhh..
6004 . any character except newline;
6005 in dotall mode, any character whatsoever
6006 \C one byte, even i