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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. There are
6    separate text files for the pcregrep and pcretest commands.
7    -----------------------------------------------------------------------------
8    
9    
10    PCRE(3)                                                                PCRE(3)
11    
12    
13  NAME  NAME
14       pcre - Perl-compatible regular expressions.         PCRE - Perl-compatible regular expressions
15    
16    
17    INTRODUCTION
18    
19  SYNOPSIS         The  PCRE  library is a set of functions that implement regular expres-
20       #include <pcre.h>         sion pattern matching using the same syntax and semantics as Perl, with
21           just  a  few  differences.  The current implementation of PCRE (release
22           6.x) corresponds approximately with Perl  5.8,  including  support  for
23           UTF-8 encoded strings and Unicode general category properties. However,
24           this support has to be explicitly enabled; it is not the default.
25    
26           In addition to the Perl-compatible matching function,  PCRE  also  con-
27           tains  an  alternative matching function that matches the same compiled
28           patterns in a different way. In certain circumstances, the  alternative
29           function  has  some  advantages.  For  a discussion of the two matching
30           algorithms, see the pcrematching page.
31    
32           PCRE is written in C and released as a C library. A  number  of  people
33           have  written  wrappers and interfaces of various kinds. In particular,
34           Google Inc.  have provided a comprehensive C++  wrapper.  This  is  now
35           included as part of the PCRE distribution. The pcrecpp page has details
36           of this interface. Other people's contributions can  be  found  in  the
37           Contrib directory at the primary FTP site, which is:
38    
39           ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
40    
41           Details  of  exactly which Perl regular expression features are and are
42           not supported by PCRE are given in separate documents. See the pcrepat-
43           tern and pcrecompat pages.
44    
45           Some  features  of  PCRE can be included, excluded, or changed when the
46           library is built. The pcre_config() function makes it  possible  for  a
47           client  to  discover  which  features are available. The features them-
48           selves are described in the pcrebuild page. Documentation about  build-
49           ing  PCRE for various operating systems can be found in the README file
50           in the source distribution.
51    
52           The library contains a number of undocumented  internal  functions  and
53           data  tables  that  are  used by more than one of the exported external
54           functions, but which are not intended  for  use  by  external  callers.
55           Their  names  all begin with "_pcre_", which hopefully will not provoke
56           any name clashes. In some environments, it is possible to control which
57           external  symbols  are  exported when a shared library is built, and in
58           these cases the undocumented symbols are not exported.
59    
60    
61    USER DOCUMENTATION
62    
63           The user documentation for PCRE comprises a number  of  different  sec-
64           tions.  In the "man" format, each of these is a separate "man page". In
65           the HTML format, each is a separate page, linked from the  index  page.
66           In  the  plain text format, all the sections are concatenated, for ease
67           of searching. The sections are as follows:
68    
69             pcre              this document
70             pcreapi           details of PCRE's native C API
71             pcrebuild         options for building PCRE
72             pcrecallout       details of the callout feature
73             pcrecompat        discussion of Perl compatibility
74             pcrecpp           details of the C++ wrapper
75             pcregrep          description of the pcregrep command
76             pcrematching      discussion of the two matching algorithms
77             pcrepartial       details of the partial matching facility
78             pcrepattern       syntax and semantics of supported
79                                 regular expressions
80             pcreperform       discussion of performance issues
81             pcreposix         the POSIX-compatible C API
82             pcreprecompile    details of saving and re-using precompiled patterns
83             pcresample        discussion of the sample program
84             pcretest          description of the pcretest testing command
85    
86       pcre *pcre_compile(const char *pattern, int options,         In  addition,  in the "man" and HTML formats, there is a short page for
87            const char **errptr, int *erroffset,         each C library function, listing its arguments and results.
           const unsigned char *tableptr);  
88    
      pcre_extra *pcre_study(const pcre *code, int options,  
           const char **errptr);  
89    
90       int pcre_exec(const pcre *code, const pcre_extra *extra,  LIMITATIONS
           const char *subject, int length, int startoffset,  
           int options, int *ovector, int ovecsize);  
91    
92       int pcre_copy_substring(const char *subject, int *ovector,         There are some size limitations in PCRE but it is hoped that they  will
93            int stringcount, int stringnumber, char *buffer,         never in practice be relevant.
           int buffersize);  
94    
95       int pcre_get_substring(const char *subject, int *ovector,         The  maximum  length of a compiled pattern is 65539 (sic) bytes if PCRE
96            int stringcount, int stringnumber,         is compiled with the default internal linkage size of 2. If you want to
97            const char **stringptr);         process  regular  expressions  that are truly enormous, you can compile
98           PCRE with an internal linkage size of 3 or 4 (see the  README  file  in
99           the  source  distribution and the pcrebuild documentation for details).
100           In these cases the limit is substantially larger.  However,  the  speed
101           of execution will be slower.
102    
103           All values in repeating quantifiers must be less than 65536.  The maxi-
104           mum number of capturing subpatterns is 65535.
105    
106           There is no limit to the number of non-capturing subpatterns,  but  the
107           maximum  depth  of  nesting  of  all kinds of parenthesized subpattern,
108           including capturing subpatterns, assertions, and other types of subpat-
109           tern, is 200.
110    
111           The  maximum  length of a subject string is the largest positive number
112           that an integer variable can hold. However, when using the  traditional
113           matching function, PCRE uses recursion to handle subpatterns and indef-
114           inite repetition.  This means that the available stack space may  limit
115           the size of a subject string that can be processed by certain patterns.
116    
117    
118    UTF-8 AND UNICODE PROPERTY SUPPORT
119    
120           From release 3.3, PCRE has  had  some  support  for  character  strings
121           encoded  in the UTF-8 format. For release 4.0 this was greatly extended
122           to cover most common requirements, and in release 5.0  additional  sup-
123           port for Unicode general category properties was added.
124    
125           In  order  process  UTF-8 strings, you must build PCRE to include UTF-8
126           support in the code, and, in addition,  you  must  call  pcre_compile()
127           with  the PCRE_UTF8 option flag. When you do this, both the pattern and
128           any subject strings that are matched against it are  treated  as  UTF-8
129           strings instead of just strings of bytes.
130    
131           If  you compile PCRE with UTF-8 support, but do not use it at run time,
132           the library will be a bit bigger, but the additional run time  overhead
133           is  limited  to testing the PCRE_UTF8 flag in several places, so should
134           not be very large.
135    
136           If PCRE is built with Unicode character property support (which implies
137           UTF-8  support),  the  escape sequences \p{..}, \P{..}, and \X are sup-
138           ported.  The available properties that can be tested are limited to the
139           general  category  properties such as Lu for an upper case letter or Nd
140           for a decimal number, the Unicode script names such as Arabic  or  Han,
141           and  the  derived  properties  Any  and L&. A full list is given in the
142           pcrepattern documentation. Only the short names for properties are sup-
143           ported.  For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
144           ter}, is not supported.  Furthermore,  in  Perl,  many  properties  may
145           optionally  be  prefixed by "Is", for compatibility with Perl 5.6. PCRE
146           does not support this.
147    
148           The following comments apply when PCRE is running in UTF-8 mode:
149    
150           1. When you set the PCRE_UTF8 flag, the strings passed as patterns  and
151           subjects  are  checked for validity on entry to the relevant functions.
152           If an invalid UTF-8 string is passed, an error return is given. In some
153           situations,  you  may  already  know  that  your strings are valid, and
154           therefore want to skip these checks in order to improve performance. If
155           you  set  the  PCRE_NO_UTF8_CHECK  flag at compile time or at run time,
156           PCRE assumes that the pattern or subject  it  is  given  (respectively)
157           contains  only valid UTF-8 codes. In this case, it does not diagnose an
158           invalid UTF-8 string. If you pass an invalid UTF-8 string to PCRE  when
159           PCRE_NO_UTF8_CHECK  is set, the results are undefined. Your program may
160           crash.
161    
162           2. An unbraced hexadecimal escape sequence (such  as  \xb3)  matches  a
163           two-byte UTF-8 character if the value is greater than 127.
164    
165           3.  Repeat quantifiers apply to complete UTF-8 characters, not to indi-
166           vidual bytes, for example: \x{100}{3}.
167    
168           4. The dot metacharacter matches one UTF-8 character instead of a  sin-
169           gle byte.
170    
171           5.  The  escape sequence \C can be used to match a single byte in UTF-8
172           mode, but its use can lead to some strange effects.  This  facility  is
173           not available in the alternative matching function, pcre_dfa_exec().
174    
175           6.  The  character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
176           test characters of any code value, but the characters that PCRE  recog-
177           nizes  as  digits,  spaces,  or  word characters remain the same set as
178           before, all with values less than 256. This remains true even when PCRE
179           includes  Unicode  property support, because to do otherwise would slow
180           down PCRE in many common cases. If you really want to test for a  wider
181           sense  of,  say,  "digit",  you must use Unicode property tests such as
182           \p{Nd}.
183    
184           7. Similarly, characters that match the POSIX named  character  classes
185           are all low-valued characters.
186    
187           8.  Case-insensitive  matching  applies only to characters whose values
188           are less than 128, unless PCRE is built with Unicode property  support.
189           Even  when  Unicode  property support is available, PCRE still uses its
190           own character tables when checking the case of  low-valued  characters,
191           so  as not to degrade performance.  The Unicode property information is
192           used only for characters with higher values. Even when Unicode property
193           support is available, PCRE supports case-insensitive matching only when
194           there is a one-to-one mapping between a letter's  cases.  There  are  a
195           small  number  of  many-to-one  mappings in Unicode; these are not sup-
196           ported by PCRE.
197    
      int pcre_get_substring_list(const char *subject,  
           int *ovector, int stringcount, const char ***listptr);  
198    
199       const unsigned char *pcre_maketables(void);  AUTHOR
200    
201       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,         Philip Hazel
202            int what, void *where);         University Computing Service,
203           Cambridge CB2 3QG, England.
204    
205       int pcre_info(const pcre *code, int *optptr, *firstcharptr);         Putting an actual email address here seems to have been a spam  magnet,
206           so I've taken it away. If you want to email me, use my initial and sur-
207           name, separated by a dot, at the domain ucs.cam.ac.uk.
208    
209       char *pcre_version(void);  Last updated: 24 January 2006
210    Copyright (c) 1997-2006 University of Cambridge.
211    ------------------------------------------------------------------------------
212    
      void *(*pcre_malloc)(size_t);  
213    
214       void (*pcre_free)(void *);  PCREBUILD(3)                                                      PCREBUILD(3)
215    
216    
217    NAME
218           PCRE - Perl-compatible regular expressions
219    
220    
221  DESCRIPTION  PCRE BUILD-TIME OPTIONS
222       The PCRE library is a set of functions that implement  regu-  
223       lar  expression  pattern  matching using the same syntax and         This  document  describes  the  optional  features  of PCRE that can be
224       semantics as Perl  5,  with  just  a  few  differences  (see         selected when the library is compiled. They are all selected, or  dese-
225       below).  The  current  implementation  corresponds  to  Perl         lected, by providing options to the configure script that is run before
226       5.005, with some additional features from the Perl  develop-         the make command. The complete list of  options  for  configure  (which
227       ment release.         includes  the  standard  ones such as the selection of the installation
228           directory) can be obtained by running
229       PCRE has its own native API,  which  is  described  in  this  
230       document.  There  is  also  a  set of wrapper functions that           ./configure --help
231       correspond to the POSIX regular expression API.   These  are  
232       described in the pcreposix documentation.         The following sections describe certain options whose names begin  with
233           --enable  or  --disable. These settings specify changes to the defaults
234       The native API function prototypes are defined in the header         for the configure command. Because of the  way  that  configure  works,
235       file  pcre.h,  and  on  Unix  systems  the library itself is         --enable  and  --disable  always  come  in  pairs, so the complementary
236       called libpcre.a, so can be accessed by adding -lpcre to the         option always exists as well, but as it specifies the  default,  it  is
237       command  for  linking  an  application  which  calls it. The         not described.
238       header file defines the macros PCRE_MAJOR and PCRE_MINOR  to  
239       contain the major and minor release numbers for the library.  
240       Applications can use these to include support for  different  C++ SUPPORT
241       releases.  
242           By default, the configure script will search for a C++ compiler and C++
243       The functions pcre_compile(), pcre_study(), and  pcre_exec()         header files. If it finds them, it automatically builds the C++ wrapper
244       are  used  for  compiling  and matching regular expressions,         library for PCRE. You can disable this by adding
245       while   pcre_copy_substring(),   pcre_get_substring(),   and  
246       pcre_get_substring_list()   are  convenience  functions  for           --disable-cpp
247       extracting  captured  substrings  from  a  matched   subject  
248       string.  The function pcre_maketables() is used (optionally)         to the configure command.
249       to build a set of character tables in the current locale for  
250       passing to pcre_compile().  
251    UTF-8 SUPPORT
252       The function pcre_fullinfo() is used to find out information  
253       about a compiled pattern; pcre_info() is an obsolete version         To build PCRE with support for UTF-8 character strings, add
254       which returns only some of the available information, but is  
255       retained   for   backwards   compatibility.    The  function           --enable-utf8
256       pcre_version() returns a pointer to a string containing  the  
257       version of PCRE and its date of release.         to  the  configure  command.  Of  itself, this does not make PCRE treat
258           strings as UTF-8. As well as compiling PCRE with this option, you  also
259       The global variables  pcre_malloc  and  pcre_free  initially         have  have to set the PCRE_UTF8 option when you call the pcre_compile()
260       contain the entry points of the standard malloc() and free()         function.
261       functions respectively. PCRE  calls  the  memory  management  
262       functions  via  these  variables,  so  a calling program can  
263       replace them if it  wishes  to  intercept  the  calls.  This  UNICODE CHARACTER PROPERTY SUPPORT
264       should be done before calling any PCRE functions.  
265           UTF-8 support allows PCRE to process character values greater than  255
266           in  the  strings that it handles. On its own, however, it does not pro-
267           vide any facilities for accessing the properties of such characters. If
268           you  want  to  be able to use the pattern escapes \P, \p, and \X, which
269           refer to Unicode character properties, you must add
270    
271             --enable-unicode-properties
272    
273           to the configure command. This implies UTF-8 support, even if you  have
274           not explicitly requested it.
275    
276           Including  Unicode  property  support  adds around 90K of tables to the
277           PCRE library, approximately doubling its size. Only the  general  cate-
278           gory  properties  such as Lu and Nd are supported. Details are given in
279           the pcrepattern documentation.
280    
281    
282    CODE VALUE OF NEWLINE
283    
284           By default, PCRE treats character 10 (linefeed) as the newline  charac-
285           ter. This is the normal newline character on Unix-like systems. You can
286           compile PCRE to use character 13 (carriage return) instead by adding
287    
288             --enable-newline-is-cr
289    
290           to the configure command. For completeness there is  also  a  --enable-
291           newline-is-lf  option,  which explicitly specifies linefeed as the new-
292           line character.
293    
294    
295    BUILDING SHARED AND STATIC LIBRARIES
296    
297           The PCRE building process uses libtool to build both shared and  static
298           Unix  libraries by default. You can suppress one of these by adding one
299           of
300    
301             --disable-shared
302             --disable-static
303    
304           to the configure command, as required.
305    
306    
307    POSIX MALLOC USAGE
308    
309           When PCRE is called through the POSIX interface (see the pcreposix doc-
310           umentation),  additional  working  storage  is required for holding the
311           pointers to capturing substrings, because PCRE requires three  integers
312           per  substring,  whereas  the POSIX interface provides only two. If the
313           number of expected substrings is small, the wrapper function uses space
314           on the stack, because this is faster than using malloc() for each call.
315           The default threshold above which the stack is no longer used is 10; it
316           can be changed by adding a setting such as
317    
318             --with-posix-malloc-threshold=20
319    
320           to the configure command.
321    
322    
323    LIMITING PCRE RESOURCE USAGE
324    
325           Internally,  PCRE has a function called match(), which it calls repeat-
326           edly  (possibly  recursively)  when  matching  a   pattern   with   the
327           pcre_exec()  function.  By controlling the maximum number of times this
328           function may be called during a single matching operation, a limit  can
329           be  placed  on  the resources used by a single call to pcre_exec(). The
330           limit can be changed at run time, as described in the pcreapi  documen-
331           tation.  The default is 10 million, but this can be changed by adding a
332           setting such as
333    
334             --with-match-limit=500000
335    
336           to  the  configure  command.  This  setting  has  no  effect   on   the
337           pcre_dfa_exec() matching function.
338    
339    
340    HANDLING VERY LARGE PATTERNS
341    
342           Within  a  compiled  pattern,  offset values are used to point from one
343           part to another (for example, from an opening parenthesis to an  alter-
344           nation  metacharacter).  By default, two-byte values are used for these
345           offsets, leading to a maximum size for a  compiled  pattern  of  around
346           64K.  This  is sufficient to handle all but the most gigantic patterns.
347           Nevertheless, some people do want to process enormous patterns,  so  it
348           is  possible  to compile PCRE to use three-byte or four-byte offsets by
349           adding a setting such as
350    
351             --with-link-size=3
352    
353           to the configure command. The value given must be 2,  3,  or  4.  Using
354           longer  offsets slows down the operation of PCRE because it has to load
355           additional bytes when handling them.
356    
357           If you build PCRE with an increased link size, test 2 (and  test  5  if
358           you  are using UTF-8) will fail. Part of the output of these tests is a
359           representation of the compiled pattern, and this changes with the  link
360           size.
361    
362    
363    AVOIDING EXCESSIVE STACK USAGE
364    
365           When matching with the pcre_exec() function, PCRE implements backtrack-
366           ing by making recursive calls to an internal function  called  match().
367           In  environments  where  the size of the stack is limited, this can se-
368           verely limit PCRE's operation. (The Unix environment does  not  usually
369           suffer  from  this  problem.)  An alternative approach that uses memory
370           from the heap to remember data, instead  of  using  recursive  function
371           calls,  has been implemented to work round this problem. If you want to
372           build a version of PCRE that works this way, add
373    
374             --disable-stack-for-recursion
375    
376           to the configure command. With this configuration, PCRE  will  use  the
377           pcre_stack_malloc  and pcre_stack_free variables to call memory manage-
378           ment functions. Separate functions are provided because  the  usage  is
379           very  predictable:  the  block sizes requested are always the same, and
380           the blocks are always freed in reverse order. A calling  program  might
381           be  able  to implement optimized functions that perform better than the
382           standard malloc() and  free()  functions.  PCRE  runs  noticeably  more
383           slowly when built in this way. This option affects only the pcre_exec()
384           function; it is not relevant for the the pcre_dfa_exec() function.
385    
386    
387    USING EBCDIC CODE
388    
389           PCRE assumes by default that it will run in an  environment  where  the
390           character  code  is  ASCII  (or Unicode, which is a superset of ASCII).
391           PCRE can, however, be compiled to  run  in  an  EBCDIC  environment  by
392           adding
393    
394             --enable-ebcdic
395    
396           to the configure command.
397    
398    Last updated: 15 August 2005
399    Copyright (c) 1997-2005 University of Cambridge.
400    ------------------------------------------------------------------------------
401    
402    
403    PCREMATCHING(3)                                                PCREMATCHING(3)
404    
405    
406    NAME
407           PCRE - Perl-compatible regular expressions
408    
409    
410    PCRE MATCHING ALGORITHMS
411    
412           This document describes the two different algorithms that are available
413           in PCRE for matching a compiled regular expression against a given sub-
414           ject  string.  The  "standard"  algorithm  is  the  one provided by the
415           pcre_exec() function.  This works in the same was  as  Perl's  matching
416           function, and provides a Perl-compatible matching operation.
417    
418           An  alternative  algorithm is provided by the pcre_dfa_exec() function;
419           this operates in a different way, and is not  Perl-compatible.  It  has
420           advantages  and disadvantages compared with the standard algorithm, and
421           these are described below.
422    
423           When there is only one possible way in which a given subject string can
424           match  a pattern, the two algorithms give the same answer. A difference
425           arises, however, when there are multiple possibilities. For example, if
426           the pattern
427    
428             ^<.*>
429    
430           is matched against the string
431    
432             <something> <something else> <something further>
433    
434           there are three possible answers. The standard algorithm finds only one
435           of them, whereas the DFA algorithm finds all three.
436    
437    
438    REGULAR EXPRESSIONS AS TREES
439    
440           The set of strings that are matched by a regular expression can be rep-
441           resented  as  a  tree structure. An unlimited repetition in the pattern
442           makes the tree of infinite size, but it is still a tree.  Matching  the
443           pattern  to a given subject string (from a given starting point) can be
444           thought of as a search of the tree.  There are  two  standard  ways  to
445           search  a  tree: depth-first and breadth-first, and these correspond to
446           the two matching algorithms provided by PCRE.
447    
448    
449    THE STANDARD MATCHING ALGORITHM
450    
451           In the terminology of Jeffrey Friedl's book Mastering  Regular  Expres-
452           sions,  the  standard  algorithm  is  an "NFA algorithm". It conducts a
453           depth-first search of the pattern tree. That is, it  proceeds  along  a
454           single path through the tree, checking that the subject matches what is
455           required. When there is a mismatch, the algorithm  tries  any  alterna-
456           tives  at  the  current point, and if they all fail, it backs up to the
457           previous branch point in the  tree,  and  tries  the  next  alternative
458           branch  at  that  level.  This often involves backing up (moving to the
459           left) in the subject string as well.  The  order  in  which  repetition
460           branches  are  tried  is controlled by the greedy or ungreedy nature of
461           the quantifier.
462    
463           If a leaf node is reached, a matching string has  been  found,  and  at
464           that  point the algorithm stops. Thus, if there is more than one possi-
465           ble match, this algorithm returns the first one that it finds.  Whether
466           this  is the shortest, the longest, or some intermediate length depends
467           on the way the greedy and ungreedy repetition quantifiers are specified
468           in the pattern.
469    
470           Because  it  ends  up  with a single path through the tree, it is rela-
471           tively straightforward for this algorithm to keep  track  of  the  sub-
472           strings  that  are  matched  by portions of the pattern in parentheses.
473           This provides support for capturing parentheses and back references.
474    
475    
476    THE DFA MATCHING ALGORITHM
477    
478           DFA stands for "deterministic finite automaton", but you do not need to
479           understand the origins of that name. This algorithm conducts a breadth-
480           first search of the tree. Starting from the first matching point in the
481           subject,  it scans the subject string from left to right, once, charac-
482           ter by character, and as it does  this,  it  remembers  all  the  paths
483           through the tree that represent valid matches.
484    
485           The  scan  continues until either the end of the subject is reached, or
486           there are no more unterminated paths. At this point,  terminated  paths
487           represent  the different matching possibilities (if there are none, the
488           match has failed).  Thus, if there is more  than  one  possible  match,
489           this algorithm finds all of them, and in particular, it finds the long-
490           est. In PCRE, there is an option to stop the algorithm after the  first
491           match (which is necessarily the shortest) has been found.
492    
493           Note that all the matches that are found start at the same point in the
494           subject. If the pattern
495    
496             cat(er(pillar)?)
497    
498           is matched against the string "the caterpillar catchment",  the  result
499           will  be the three strings "cat", "cater", and "caterpillar" that start
500           at the fourth character of the subject. The algorithm does not automat-
501           ically move on to find matches that start at later positions.
502    
503           There are a number of features of PCRE regular expressions that are not
504           supported by the DFA matching algorithm. They are as follows:
505    
506           1. Because the algorithm finds all  possible  matches,  the  greedy  or
507           ungreedy  nature  of repetition quantifiers is not relevant. Greedy and
508           ungreedy quantifiers are treated in exactly the same way.
509    
510           2. When dealing with multiple paths through the tree simultaneously, it
511           is  not  straightforward  to  keep track of captured substrings for the
512           different matching possibilities, and  PCRE's  implementation  of  this
513           algorithm does not attempt to do this. This means that no captured sub-
514           strings are available.
515    
516           3. Because no substrings are captured, back references within the  pat-
517           tern are not supported, and cause errors if encountered.
518    
519           4.  For  the same reason, conditional expressions that use a backrefer-
520           ence as the condition are not supported.
521    
522           5. Callouts are supported, but the value of the  capture_top  field  is
523           always 1, and the value of the capture_last field is always -1.
524    
525           6.  The \C escape sequence, which (in the standard algorithm) matches a
526           single byte, even in UTF-8 mode, is not supported because the DFA algo-
527           rithm moves through the subject string one character at a time, for all
528           active paths through the tree.
529    
530    
531    ADVANTAGES OF THE DFA ALGORITHM
532    
533           Using the DFA matching algorithm provides the following advantages:
534    
535           1. All possible matches (at a single point in the subject) are automat-
536           ically  found,  and  in particular, the longest match is found. To find
537           more than one match using the standard algorithm, you have to do kludgy
538           things with callouts.
539    
540           2.  There is much better support for partial matching. The restrictions
541           on the content of the pattern that apply when using the standard  algo-
542           rithm  for partial matching do not apply to the DFA algorithm. For non-
543           anchored patterns, the starting position of a partial match  is  avail-
544           able.
545    
546           3.  Because  the  DFA algorithm scans the subject string just once, and
547           never needs to backtrack, it is possible  to  pass  very  long  subject
548           strings  to  the matching function in several pieces, checking for par-
549           tial matching each time.
550    
551    
552    DISADVANTAGES OF THE DFA ALGORITHM
553    
554           The DFA algorithm suffers from a number of disadvantages:
555    
556           1. It is substantially slower than  the  standard  algorithm.  This  is
557           partly  because  it has to search for all possible matches, but is also
558           because it is less susceptible to optimization.
559    
560           2. Capturing parentheses and back references are not supported.
561    
562           3. The "atomic group" feature of PCRE regular expressions is supported,
563           but  does not provide the advantage that it does for the standard algo-
564           rithm.
565    
566    Last updated: 28 February 2005
567    Copyright (c) 1997-2005 University of Cambridge.
568    ------------------------------------------------------------------------------
569    
570    
571    PCREAPI(3)                                                          PCREAPI(3)
572    
573    
574    NAME
575           PCRE - Perl-compatible regular expressions
576    
577    
578    PCRE NATIVE API
579    
580           #include <pcre.h>
581    
582           pcre *pcre_compile(const char *pattern, int options,
583                const char **errptr, int *erroffset,
584                const unsigned char *tableptr);
585    
586           pcre *pcre_compile2(const char *pattern, int options,
587                int *errorcodeptr,
588                const char **errptr, int *erroffset,
589                const unsigned char *tableptr);
590    
591           pcre_extra *pcre_study(const pcre *code, int options,
592                const char **errptr);
593    
594           int pcre_exec(const pcre *code, const pcre_extra *extra,
595                const char *subject, int length, int startoffset,
596                int options, int *ovector, int ovecsize);
597    
598           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
599                const char *subject, int length, int startoffset,
600                int options, int *ovector, int ovecsize,
601                int *workspace, int wscount);
602    
603           int pcre_copy_named_substring(const pcre *code,
604                const char *subject, int *ovector,
605                int stringcount, const char *stringname,
606                char *buffer, int buffersize);
607    
608           int pcre_copy_substring(const char *subject, int *ovector,
609                int stringcount, int stringnumber, char *buffer,
610                int buffersize);
611    
612           int pcre_get_named_substring(const pcre *code,
613                const char *subject, int *ovector,
614                int stringcount, const char *stringname,
615                const char **stringptr);
616    
617           int pcre_get_stringnumber(const pcre *code,
618                const char *name);
619    
620  MULTI-THREADING         int pcre_get_substring(const char *subject, int *ovector,
621       The PCRE functions can be used in  multi-threading  applica-              int stringcount, int stringnumber,
622       tions, with the proviso that the memory management functions              const char **stringptr);
      pointed to by pcre_malloc and pcre_free are  shared  by  all  
      threads.  
   
      The compiled form of a regular  expression  is  not  altered  
      during  matching, so the same compiled pattern can safely be  
      used by several threads at once.  
623    
624           int pcre_get_substring_list(const char *subject,
625                int *ovector, int stringcount, const char ***listptr);
626    
627           void pcre_free_substring(const char *stringptr);
628    
629           void pcre_free_substring_list(const char **stringptr);
630    
631           const unsigned char *pcre_maketables(void);
632    
633           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
634                int what, void *where);
635    
636           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
637    
638           int pcre_refcount(pcre *code, int adjust);
639    
640           int pcre_config(int what, void *where);
641    
642           char *pcre_version(void);
643    
644           void *(*pcre_malloc)(size_t);
645    
646           void (*pcre_free)(void *);
647    
648           void *(*pcre_stack_malloc)(size_t);
649    
650           void (*pcre_stack_free)(void *);
651    
652           int (*pcre_callout)(pcre_callout_block *);
653    
654    
655    PCRE API OVERVIEW
656    
657           PCRE has its own native API, which is described in this document. There
658           is also a set of wrapper functions that correspond to the POSIX regular
659           expression  API.  These  are  described in the pcreposix documentation.
660           Both of these APIs define a set of C function calls. A C++  wrapper  is
661           distributed with PCRE. It is documented in the pcrecpp page.
662    
663           The  native  API  C  function prototypes are defined in the header file
664           pcre.h, and on Unix systems the library itself is called  libpcre.   It
665           can normally be accessed by adding -lpcre to the command for linking an
666           application  that  uses  PCRE.  The  header  file  defines  the  macros
667           PCRE_MAJOR  and  PCRE_MINOR to contain the major and minor release num-
668           bers for the library.  Applications can use these  to  include  support
669           for different releases of PCRE.
670    
671           The   functions   pcre_compile(),  pcre_compile2(),  pcre_study(),  and
672           pcre_exec() are used for compiling and matching regular expressions  in
673           a  Perl-compatible  manner. A sample program that demonstrates the sim-
674           plest way of using them is provided in the file  called  pcredemo.c  in
675           the  source distribution. The pcresample documentation describes how to
676           run it.
677    
678           A second matching function, pcre_dfa_exec(), which is not Perl-compati-
679           ble,  is  also provided. This uses a different algorithm for the match-
680           ing. This allows it to find all possible matches (at a given  point  in
681           the  subject),  not  just  one. However, this algorithm does not return
682           captured substrings. A description of the two matching  algorithms  and
683           their  advantages  and disadvantages is given in the pcrematching docu-
684           mentation.
685    
686           In addition to the main compiling and  matching  functions,  there  are
687           convenience functions for extracting captured substrings from a subject
688           string that is matched by pcre_exec(). They are:
689    
690             pcre_copy_substring()
691             pcre_copy_named_substring()
692             pcre_get_substring()
693             pcre_get_named_substring()
694             pcre_get_substring_list()
695             pcre_get_stringnumber()
696    
697           pcre_free_substring() and pcre_free_substring_list() are also provided,
698           to free the memory used for extracted strings.
699    
700           The  function  pcre_maketables()  is  used  to build a set of character
701           tables  in  the  current  locale   for   passing   to   pcre_compile(),
702           pcre_exec(),  or  pcre_dfa_exec(). This is an optional facility that is
703           provided for specialist use.  Most  commonly,  no  special  tables  are
704           passed,  in  which case internal tables that are generated when PCRE is
705           built are used.
706    
707           The function pcre_fullinfo() is used to find out  information  about  a
708           compiled  pattern; pcre_info() is an obsolete version that returns only
709           some of the available information, but is retained for  backwards  com-
710           patibility.   The function pcre_version() returns a pointer to a string
711           containing the version of PCRE and its date of release.
712    
713           The function pcre_refcount() maintains a  reference  count  in  a  data
714           block  containing  a compiled pattern. This is provided for the benefit
715           of object-oriented applications.
716    
717           The global variables pcre_malloc and pcre_free  initially  contain  the
718           entry  points  of  the  standard malloc() and free() functions, respec-
719           tively. PCRE calls the memory management functions via these variables,
720           so  a  calling  program  can replace them if it wishes to intercept the
721           calls. This should be done before calling any PCRE functions.
722    
723           The global variables pcre_stack_malloc  and  pcre_stack_free  are  also
724           indirections  to  memory  management functions. These special functions
725           are used only when PCRE is compiled to use  the  heap  for  remembering
726           data, instead of recursive function calls, when running the pcre_exec()
727           function. This is a non-standard way of building PCRE, for use in envi-
728           ronments that have limited stacks. Because of the greater use of memory
729           management, it runs more slowly.  Separate functions  are  provided  so
730           that  special-purpose  external  code  can  be used for this case. When
731           used, these functions are always called in a  stack-like  manner  (last
732           obtained,  first freed), and always for memory blocks of the same size.
733    
734           The global variable pcre_callout initially contains NULL. It can be set
735           by  the  caller  to  a "callout" function, which PCRE will then call at
736           specified points during a matching operation. Details are given in  the
737           pcrecallout documentation.
738    
739    
740    MULTITHREADING
741    
742           The  PCRE  functions  can be used in multi-threading applications, with
743           the  proviso  that  the  memory  management  functions  pointed  to  by
744           pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
745           callout function pointed to by pcre_callout, are shared by all threads.
746    
747           The  compiled form of a regular expression is not altered during match-
748           ing, so the same compiled pattern can safely be used by several threads
749           at once.
750    
751    
752    SAVING PRECOMPILED PATTERNS FOR LATER USE
753    
754           The compiled form of a regular expression can be saved and re-used at a
755           later time, possibly by a different program, and even on a  host  other
756           than  the  one  on  which  it  was  compiled.  Details are given in the
757           pcreprecompile documentation.
758    
759    
760    CHECKING BUILD-TIME OPTIONS
761    
762           int pcre_config(int what, void *where);
763    
764           The function pcre_config() makes it possible for a PCRE client to  dis-
765           cover which optional features have been compiled into the PCRE library.
766           The pcrebuild documentation has more details about these optional  fea-
767           tures.
768    
769           The  first  argument  for pcre_config() is an integer, specifying which
770           information is required; the second argument is a pointer to a variable
771           into  which  the  information  is  placed. The following information is
772           available:
773    
774             PCRE_CONFIG_UTF8
775    
776           The output is an integer that is set to one if UTF-8 support is  avail-
777           able; otherwise it is set to zero.
778    
779             PCRE_CONFIG_UNICODE_PROPERTIES
780    
781           The  output  is  an  integer  that is set to one if support for Unicode
782           character properties is available; otherwise it is set to zero.
783    
784             PCRE_CONFIG_NEWLINE
785    
786           The output is an integer that is set to the value of the code  that  is
787           used  for the newline character. It is either linefeed (10) or carriage
788           return (13), and should normally be the  standard  character  for  your
789           operating system.
790    
791             PCRE_CONFIG_LINK_SIZE
792    
793           The  output  is  an  integer that contains the number of bytes used for
794           internal linkage in compiled regular expressions. The value is 2, 3, or
795           4.  Larger  values  allow larger regular expressions to be compiled, at
796           the expense of slower matching. The default value of  2  is  sufficient
797           for  all  but  the  most massive patterns, since it allows the compiled
798           pattern to be up to 64K in size.
799    
800             PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
801    
802           The output is an integer that contains the threshold  above  which  the
803           POSIX  interface  uses malloc() for output vectors. Further details are
804           given in the pcreposix documentation.
805    
806             PCRE_CONFIG_MATCH_LIMIT
807    
808           The output is an integer that gives the default limit for the number of
809           internal  matching  function  calls in a pcre_exec() execution. Further
810           details are given with pcre_exec() below.
811    
812             PCRE_CONFIG_MATCH_LIMIT_RECURSION
813    
814           The output is an integer that gives the default limit for the depth  of
815           recursion  when calling the internal matching function in a pcre_exec()
816           execution. Further details are given with pcre_exec() below.
817    
818             PCRE_CONFIG_STACKRECURSE
819    
820           The output is an integer that is set to one if internal recursion  when
821           running pcre_exec() is implemented by recursive function calls that use
822           the stack to remember their state. This is the usual way that  PCRE  is
823           compiled. The output is zero if PCRE was compiled to use blocks of data
824           on the  heap  instead  of  recursive  function  calls.  In  this  case,
825           pcre_stack_malloc  and  pcre_stack_free  are  called  to  manage memory
826           blocks on the heap, thus avoiding the use of the stack.
827    
828    
829  COMPILING A PATTERN  COMPILING A PATTERN
      The function pcre_compile() is called to compile  a  pattern  
      into  an internal form. The pattern is a C string terminated  
      by a binary zero, and is passed in the argument  pattern.  A  
      pointer  to  a  single  block of memory that is obtained via  
      pcre_malloc is returned. This contains the compiled code and  
      related data. The pcre type is defined for this for conveni-  
      ence, but in fact pcre is just a typedef for void, since the  
      contents  of  the block are not externally defined. It is up  
      to the caller to free  the  memory  when  it  is  no  longer  
      required.  
   
      The size of a compiled pattern is  roughly  proportional  to  
      the length of the pattern string, except that each character  
      class (other than those containing just a single  character,  
      negated  or  not)  requires 33 bytes, and repeat quantifiers  
      with a minimum greater than one or a bounded  maximum  cause  
      the  relevant  portions of the compiled pattern to be repli-  
      cated.  
   
      The options argument contains independent bits  that  affect  
      the  compilation.  It  should  be  zero  if  no  options are  
      required. Some of the options, in particular, those that are  
      compatible  with Perl, can also be set and unset from within  
      the pattern (see the detailed description of regular expres-  
      sions below). For these options, the contents of the options  
      argument specifies their initial settings at  the  start  of  
      compilation  and  execution. The PCRE_ANCHORED option can be  
      set at the time of matching as well as at compile time.  
   
      If errptr is NULL, pcre_compile() returns NULL  immediately.  
      Otherwise, if compilation of a pattern fails, pcre_compile()  
      returns NULL, and sets the variable pointed to by errptr  to  
      point  to a textual error message. The offset from the start  
      of  the  pattern  to  the  character  where  the  error  was  
      discovered   is   placed  in  the  variable  pointed  to  by  
      erroffset, which must not be NULL. If it  is,  an  immediate  
      error is given.  
   
      If the final  argument,  tableptr,  is  NULL,  PCRE  uses  a  
      default  set  of character tables which are built when it is  
      compiled, using the default C  locale.  Otherwise,  tableptr  
      must  be  the result of a call to pcre_maketables(). See the  
      section on locale support below.  
   
      The following option bits are defined in the header file:  
   
        PCRE_ANCHORED  
   
      If this bit is set, the pattern is forced to be  "anchored",  
      that is, it is constrained to match only at the start of the  
      string which is being searched (the "subject string").  This  
      effect can also be achieved by appropriate constructs in the  
      pattern itself, which is the only way to do it in Perl.  
   
        PCRE_CASELESS  
   
      If this bit is set, letters in the pattern match both  upper  
      and  lower  case  letters.  It  is  equivalent  to Perl's /i  
      option.  
   
        PCRE_DOLLAR_ENDONLY  
   
      If this bit is set, a dollar metacharacter  in  the  pattern  
      matches  only at the end of the subject string. Without this  
      option, a dollar also matches immediately before  the  final  
      character  if it is a newline (but not before any other new-  
      lines).  The  PCRE_DOLLAR_ENDONLY  option  is   ignored   if  
      PCRE_MULTILINE is set. There is no equivalent to this option  
      in Perl.  
   
        PCRE_DOTALL  
   
      If this bit is  set,  a  dot  metacharater  in  the  pattern  
      matches all characters, including newlines. Without it, new-  
      lines are excluded. This option is equivalent to  Perl's  /s  
      option.  A negative class such as [^a] always matches a new-  
      line character, independent of the setting of this option.  
   
        PCRE_EXTENDED  
   
      If this bit is set, whitespace data characters in  the  pat-  
      tern  are  totally  ignored  except when escaped or inside a  
      character class, and characters between an unescaped #  out-  
      side  a  character  class  and  the  next newline character,  
      inclusive, are also ignored. This is equivalent to Perl's /x  
      option,  and  makes  it  possible to include comments inside  
      complicated patterns. Note, however, that this applies  only  
      to  data  characters. Whitespace characters may never appear  
      within special character sequences in a pattern, for example  
      within  the sequence (?( which introduces a conditional sub-  
      pattern.  
   
        PCRE_EXTRA  
   
      This option was invented in  order  to  turn  on  additional  
      functionality of PCRE that is incompatible with Perl, but it  
      is currently of very little use. When set, any backslash  in  
      a  pattern  that is followed by a letter that has no special  
      meaning causes an error, thus reserving  these  combinations  
      for  future  expansion.  By default, as in Perl, a backslash  
      followed by a letter with no special meaning is treated as a  
      literal.  There  are at present no other features controlled  
      by this option. It can also be set by a (?X) option  setting  
      within a pattern.  
   
        PCRE_MULTILINE  
   
      By default, PCRE treats the subject string as consisting  of  
      a  single "line" of characters (even if it actually contains  
      several newlines). The "start  of  line"  metacharacter  (^)  
      matches  only  at the start of the string, while the "end of  
      line" metacharacter ($) matches  only  at  the  end  of  the  
      string,    or   before   a   terminating   newline   (unless  
      PCRE_DOLLAR_ENDONLY is set). This is the same as Perl.  
   
      When PCRE_MULTILINE it is set, the "start of line" and  "end  
      of  line"  constructs match immediately following or immedi-  
      ately before any newline  in  the  subject  string,  respec-  
      tively,  as  well  as  at  the  very  start and end. This is  
      equivalent to Perl's /m option. If there are no "\n" charac-  
      ters  in  a subject string, or no occurrences of ^ or $ in a  
      pattern, setting PCRE_MULTILINE has no effect.  
   
        PCRE_UNGREEDY  
   
      This option inverts the "greediness" of the  quantifiers  so  
      that  they  are  not greedy by default, but become greedy if  
      followed by "?". It is not compatible with Perl. It can also  
      be set by a (?U) option setting within the pattern.  
830    
831           pcre *pcre_compile(const char *pattern, int options,
832                const char **errptr, int *erroffset,
833                const unsigned char *tableptr);
834    
835           pcre *pcre_compile2(const char *pattern, int options,
836                int *errorcodeptr,
837                const char **errptr, int *erroffset,
838                const unsigned char *tableptr);
839    
840           Either of the functions pcre_compile() or pcre_compile2() can be called
841           to compile a pattern into an internal form. The only difference between
842           the two interfaces is that pcre_compile2() has an additional  argument,
843           errorcodeptr, via which a numerical error code can be returned.
844    
845           The pattern is a C string terminated by a binary zero, and is passed in
846           the pattern argument. A pointer to a single block  of  memory  that  is
847           obtained  via  pcre_malloc is returned. This contains the compiled code
848           and related data. The pcre type is defined for the returned block; this
849           is a typedef for a structure whose contents are not externally defined.
850           It is up to the caller  to  free  the  memory  when  it  is  no  longer
851           required.
852    
853           Although  the compiled code of a PCRE regex is relocatable, that is, it
854           does not depend on memory location, the complete pcre data block is not
855           fully  relocatable, because it may contain a copy of the tableptr argu-
856           ment, which is an address (see below).
857    
858           The options argument contains independent bits that affect the compila-
859           tion.  It  should  be  zero  if  no options are required. The available
860           options are described below. Some of them, in  particular,  those  that
861           are  compatible  with  Perl,  can also be set and unset from within the
862           pattern (see the detailed description  in  the  pcrepattern  documenta-
863           tion).  For  these options, the contents of the options argument speci-
864           fies their initial settings at the start of compilation and  execution.
865           The  PCRE_ANCHORED option can be set at the time of matching as well as
866           at compile time.
867    
868           If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
869           if  compilation  of  a  pattern fails, pcre_compile() returns NULL, and
870           sets the variable pointed to by errptr to point to a textual error mes-
871           sage. This is a static string that is part of the library. You must not
872           try to free it. The offset from the start of the pattern to the charac-
873           ter where the error was discovered is placed in the variable pointed to
874           by erroffset, which must not be NULL. If it is, an immediate  error  is
875           given.
876    
877           If  pcre_compile2()  is  used instead of pcre_compile(), and the error-
878           codeptr argument is not NULL, a non-zero error code number is  returned
879           via  this argument in the event of an error. This is in addition to the
880           textual error message. Error codes and messages are listed below.
881    
882           If the final argument, tableptr, is NULL, PCRE uses a  default  set  of
883           character  tables  that  are  built  when  PCRE  is compiled, using the
884           default C locale. Otherwise, tableptr must be an address  that  is  the
885           result  of  a  call to pcre_maketables(). This value is stored with the
886           compiled pattern, and used again by pcre_exec(), unless  another  table
887           pointer is passed to it. For more discussion, see the section on locale
888           support below.
889    
890           This code fragment shows a typical straightforward  call  to  pcre_com-
891           pile():
892    
893             pcre *re;
894             const char *error;
895             int erroffset;
896             re = pcre_compile(
897               "^A.*Z",          /* the pattern */
898               0,                /* default options */
899               &error,           /* for error message */
900               &erroffset,       /* for error offset */
901               NULL);            /* use default character tables */
902    
903           The  following  names  for option bits are defined in the pcre.h header
904           file:
905    
906             PCRE_ANCHORED
907    
908           If this bit is set, the pattern is forced to be "anchored", that is, it
909           is  constrained to match only at the first matching point in the string
910           that is being searched (the "subject string"). This effect can also  be
911           achieved  by appropriate constructs in the pattern itself, which is the
912           only way to do it in Perl.
913    
914             PCRE_AUTO_CALLOUT
915    
916           If this bit is set, pcre_compile() automatically inserts callout items,
917           all  with  number  255, before each pattern item. For discussion of the
918           callout facility, see the pcrecallout documentation.
919    
920             PCRE_CASELESS
921    
922           If this bit is set, letters in the pattern match both upper  and  lower
923           case  letters.  It  is  equivalent  to  Perl's /i option, and it can be
924           changed within a pattern by a (?i) option setting. In UTF-8 mode,  PCRE
925           always  understands the concept of case for characters whose values are
926           less than 128, so caseless matching is always possible. For  characters
927           with  higher  values,  the concept of case is supported if PCRE is com-
928           piled with Unicode property support, but not otherwise. If you want  to
929           use  caseless  matching  for  characters 128 and above, you must ensure
930           that PCRE is compiled with Unicode property support  as  well  as  with
931           UTF-8 support.
932    
933             PCRE_DOLLAR_ENDONLY
934    
935           If  this bit is set, a dollar metacharacter in the pattern matches only
936           at the end of the subject string. Without this option,  a  dollar  also
937           matches  immediately before the final character if it is a newline (but
938           not before any  other  newlines).  The  PCRE_DOLLAR_ENDONLY  option  is
939           ignored if PCRE_MULTILINE is set. There is no equivalent to this option
940           in Perl, and no way to set it within a pattern.
941    
942             PCRE_DOTALL
943    
944           If this bit is set, a dot metacharater in the pattern matches all char-
945           acters,  including  newlines.  Without  it, newlines are excluded. This
946           option is equivalent to Perl's /s option, and it can be changed  within
947           a  pattern  by  a  (?s)  option  setting. A negative class such as [^a]
948           always matches a newline character, independent of the setting of  this
949           option.
950    
951             PCRE_EXTENDED
952    
953           If  this  bit  is  set,  whitespace  data characters in the pattern are
954           totally ignored except when escaped or inside a character class. White-
955           space does not include the VT character (code 11). In addition, charac-
956           ters between an unescaped # outside a character class and the next new-
957           line  character,  inclusive,  are  also  ignored. This is equivalent to
958           Perl's /x option, and it can be changed within  a  pattern  by  a  (?x)
959           option setting.
960    
961           This  option  makes  it possible to include comments inside complicated
962           patterns.  Note, however, that this applies only  to  data  characters.
963           Whitespace   characters  may  never  appear  within  special  character
964           sequences in a pattern, for  example  within  the  sequence  (?(  which
965           introduces a conditional subpattern.
966    
967             PCRE_EXTRA
968    
969           This  option  was invented in order to turn on additional functionality
970           of PCRE that is incompatible with Perl, but it  is  currently  of  very
971           little  use. When set, any backslash in a pattern that is followed by a
972           letter that has no special meaning  causes  an  error,  thus  reserving
973           these  combinations  for  future  expansion.  By default, as in Perl, a
974           backslash followed by a letter with no special meaning is treated as  a
975           literal.  There  are  at  present  no other features controlled by this
976           option. It can also be set by a (?X) option setting within a pattern.
977    
978             PCRE_FIRSTLINE
979    
980           If this option is set, an  unanchored  pattern  is  required  to  match
981           before  or at the first newline character in the subject string, though
982           the matched text may continue over the newline.
983    
984             PCRE_MULTILINE
985    
986           By default, PCRE treats the subject string as consisting  of  a  single
987           line  of characters (even if it actually contains newlines). The "start
988           of line" metacharacter (^) matches only at the  start  of  the  string,
989           while  the  "end  of line" metacharacter ($) matches only at the end of
990           the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
991           is set). This is the same as Perl.
992    
993           When  PCRE_MULTILINE  it  is set, the "start of line" and "end of line"
994           constructs match immediately following or immediately before  any  new-
995           line  in the subject string, respectively, as well as at the very start
996           and end. This is equivalent to Perl's /m option, and it can be  changed
997           within a pattern by a (?m) option setting. If there are no "\n" charac-
998           ters in a subject string, or no occurrences of ^ or  $  in  a  pattern,
999           setting PCRE_MULTILINE has no effect.
1000    
1001             PCRE_NO_AUTO_CAPTURE
1002    
1003           If this option is set, it disables the use of numbered capturing paren-
1004           theses in the pattern. Any opening parenthesis that is not followed  by
1005           ?  behaves as if it were followed by ?: but named parentheses can still
1006           be used for capturing (and they acquire  numbers  in  the  usual  way).
1007           There is no equivalent of this option in Perl.
1008    
1009             PCRE_UNGREEDY
1010    
1011           This  option  inverts  the "greediness" of the quantifiers so that they
1012           are not greedy by default, but become greedy if followed by "?". It  is
1013           not  compatible  with Perl. It can also be set by a (?U) option setting
1014           within the pattern.
1015    
1016             PCRE_UTF8
1017    
1018           This option causes PCRE to regard both the pattern and the  subject  as
1019           strings  of  UTF-8 characters instead of single-byte character strings.
1020           However, it is available only when PCRE is built to include UTF-8  sup-
1021           port.  If not, the use of this option provokes an error. Details of how
1022           this option changes the behaviour of PCRE are given in the  section  on
1023           UTF-8 support in the main pcre page.
1024    
1025             PCRE_NO_UTF8_CHECK
1026    
1027           When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1028           automatically checked. If an invalid UTF-8 sequence of bytes is  found,
1029           pcre_compile()  returns an error. If you already know that your pattern
1030           is valid, and you want to skip this check for performance reasons,  you
1031           can  set  the  PCRE_NO_UTF8_CHECK option. When it is set, the effect of
1032           passing an invalid UTF-8 string as a pattern is undefined. It may cause
1033           your  program  to  crash.   Note that this option can also be passed to
1034           pcre_exec() and pcre_dfa_exec(), to suppress the UTF-8 validity  check-
1035           ing of subject strings.
1036    
1037    
1038    COMPILATION ERROR CODES
1039    
1040           The  following  table  lists  the  error  codes than may be returned by
1041           pcre_compile2(), along with the error messages that may be returned  by
1042           both compiling functions.
1043    
1044              0  no error
1045              1  \ at end of pattern
1046              2  \c at end of pattern
1047              3  unrecognized character follows \
1048              4  numbers out of order in {} quantifier
1049              5  number too big in {} quantifier
1050              6  missing terminating ] for character class
1051              7  invalid escape sequence in character class
1052              8  range out of order in character class
1053              9  nothing to repeat
1054             10  operand of unlimited repeat could match the empty string
1055             11  internal error: unexpected repeat
1056             12  unrecognized character after (?
1057             13  POSIX named classes are supported only within a class
1058             14  missing )
1059             15  reference to non-existent subpattern
1060             16  erroffset passed as NULL
1061             17  unknown option bit(s) set
1062             18  missing ) after comment
1063             19  parentheses nested too deeply
1064             20  regular expression too large
1065             21  failed to get memory
1066             22  unmatched parentheses
1067             23  internal error: code overflow
1068             24  unrecognized character after (?<
1069             25  lookbehind assertion is not fixed length
1070             26  malformed number after (?(
1071             27  conditional group contains more than two branches
1072             28  assertion expected after (?(
1073             29  (?R or (?digits must be followed by )
1074             30  unknown POSIX class name
1075             31  POSIX collating elements are not supported
1076             32  this version of PCRE is not compiled with PCRE_UTF8 support
1077             33  spare error
1078             34  character value in \x{...} sequence is too large
1079             35  invalid condition (?(0)
1080             36  \C not allowed in lookbehind assertion
1081             37  PCRE does not support \L, \l, \N, \U, or \u
1082             38  number after (?C is > 255
1083             39  closing ) for (?C expected
1084             40  recursive call could loop indefinitely
1085             41  unrecognized character after (?P
1086             42  syntax error after (?P
1087             43  two named groups have the same name
1088             44  invalid UTF-8 string
1089             45  support for \P, \p, and \X has not been compiled
1090             46  malformed \P or \p sequence
1091             47  unknown property name after \P or \p
1092    
1093    
1094  STUDYING A PATTERN  STUDYING A PATTERN
      When a pattern is going to be  used  several  times,  it  is  
      worth  spending  more time analyzing it in order to speed up  
      the time taken for matching. The function pcre_study() takes  
      a  pointer  to a compiled pattern as its first argument, and  
      returns a  pointer  to  a  pcre_extra  block  (another  void  
      typedef)  containing  additional  information about the pat-  
      tern; this can be passed to pcre_exec().  If  no  additional  
      information is available, NULL is returned.  
   
      The second argument contains option  bits.  At  present,  no  
      options  are  defined  for  pcre_study(),  and this argument  
      should always be zero.  
   
      The third argument for pcre_study() is a pointer to an error  
      message. If studying succeeds (even if no data is returned),  
      the variable it points to  is  set  to  NULL.  Otherwise  it  
      points to a textual error message.  
   
      At present, studying a  pattern  is  useful  only  for  non-  
      anchored  patterns  that do not have a single fixed starting  
      character. A  bitmap  of  possible  starting  characters  is  
      created.  
1095    
1096           pcre_extra *pcre_study(const pcre *code, int options
1097                const char **errptr);
1098    
1099           If  a  compiled  pattern is going to be used several times, it is worth
1100           spending more time analyzing it in order to speed up the time taken for
1101           matching.  The function pcre_study() takes a pointer to a compiled pat-
1102           tern as its first argument. If studying the pattern produces additional
1103           information  that  will  help speed up matching, pcre_study() returns a
1104           pointer to a pcre_extra block, in which the study_data field points  to
1105           the results of the study.
1106    
1107           The  returned  value  from  pcre_study()  can  be  passed  directly  to
1108           pcre_exec(). However, a pcre_extra block  also  contains  other  fields
1109           that  can  be  set  by the caller before the block is passed; these are
1110           described below in the section on matching a pattern.
1111    
1112           If studying the pattern does not  produce  any  additional  information
1113           pcre_study() returns NULL. In that circumstance, if the calling program
1114           wants to pass any of the other fields to pcre_exec(), it  must  set  up
1115           its own pcre_extra block.
1116    
1117           The  second  argument of pcre_study() contains option bits. At present,
1118           no options are defined, and this argument should always be zero.
1119    
1120           The third argument for pcre_study() is a pointer for an error  message.
1121           If  studying  succeeds  (even  if no data is returned), the variable it
1122           points to is set to NULL. Otherwise it is set to  point  to  a  textual
1123           error message. This is a static string that is part of the library. You
1124           must not try to free it. You should test the  error  pointer  for  NULL
1125           after calling pcre_study(), to be sure that it has run successfully.
1126    
1127           This is a typical call to pcre_study():
1128    
1129             pcre_extra *pe;
1130             pe = pcre_study(
1131               re,             /* result of pcre_compile() */
1132               0,              /* no options exist */
1133               &error);        /* set to NULL or points to a message */
1134    
1135           At present, studying a pattern is useful only for non-anchored patterns
1136           that do not have a single fixed starting character. A bitmap of  possi-
1137           ble starting bytes is created.
1138    
1139    
1140  LOCALE SUPPORT  LOCALE SUPPORT
      PCRE handles caseless matching, and determines whether char-  
      acters  are  letters, digits, or whatever, by reference to a  
      set of tables. The library contains a default set of  tables  
      which  is  created in the default C locale when PCRE is com-  
      piled.  This  is   used   when   the   final   argument   of  
      pcre_compile()  is NULL, and is sufficient for many applica-  
      tions.  
   
      An alternative set of tables can, however, be supplied. Such  
      tables  are built by calling the pcre_maketables() function,  
      which has no arguments, in the relevant locale.  The  result  
      can  then be passed to pcre_compile() as often as necessary.  
      For example, to build and use tables  that  are  appropriate  
      for  the French locale (where accented characters with codes  
      greater than 128 are treated as letters), the following code  
      could be used:  
   
        setlocale(LC_CTYPE, "fr");  
        tables = pcre_maketables();  
        re = pcre_compile(..., tables);  
   
      The  tables  are  built  in  memory  that  is  obtained  via  
      pcre_malloc.  The  pointer that is passed to pcre_compile is  
      saved with the compiled pattern, and  the  same  tables  are  
      used  via this pointer by pcre_study() and pcre_exec(). Thus  
      for any single pattern, compilation, studying  and  matching  
      all happen in the same locale, but different patterns can be  
      compiled in different locales. It is the caller's  responsi-  
      bility  to  ensure  that  the  memory  containing the tables  
      remains available for as long as it is needed.  
1141    
1142           PCRE  handles  caseless matching, and determines whether characters are
1143           letters digits, or whatever, by reference to a set of  tables,  indexed
1144           by  character  value.  When running in UTF-8 mode, this applies only to
1145           characters with codes less than 128. Higher-valued  codes  never  match
1146           escapes  such  as  \w or \d, but can be tested with \p if PCRE is built
1147           with Unicode character property support. The use of locales  with  Uni-
1148           code is discouraged.
1149    
1150           An  internal set of tables is created in the default C locale when PCRE
1151           is built. This is used when the final  argument  of  pcre_compile()  is
1152           NULL,  and  is  sufficient for many applications. An alternative set of
1153           tables can, however, be supplied. These may be created in  a  different
1154           locale  from the default. As more and more applications change to using
1155           Unicode, the need for this locale support is expected to die away.
1156    
1157           External tables are built by calling  the  pcre_maketables()  function,
1158           which  has no arguments, in the relevant locale. The result can then be
1159           passed to pcre_compile() or pcre_exec()  as  often  as  necessary.  For
1160           example,  to  build  and use tables that are appropriate for the French
1161           locale (where accented characters with  values  greater  than  128  are
1162           treated as letters), the following code could be used:
1163    
1164             setlocale(LC_CTYPE, "fr_FR");
1165             tables = pcre_maketables();
1166             re = pcre_compile(..., tables);
1167    
1168           When  pcre_maketables()  runs,  the  tables are built in memory that is
1169           obtained via pcre_malloc. It is the caller's responsibility  to  ensure
1170           that  the memory containing the tables remains available for as long as
1171           it is needed.
1172    
1173           The pointer that is passed to pcre_compile() is saved with the compiled
1174           pattern,  and the same tables are used via this pointer by pcre_study()
1175           and normally also by pcre_exec(). Thus, by default, for any single pat-
1176           tern, compilation, studying and matching all happen in the same locale,
1177           but different patterns can be compiled in different locales.
1178    
1179           It is possible to pass a table pointer or NULL (indicating the  use  of
1180           the  internal  tables)  to  pcre_exec(). Although not intended for this
1181           purpose, this facility could be used to match a pattern in a  different
1182           locale from the one in which it was compiled. Passing table pointers at
1183           run time is discussed below in the section on matching a pattern.
1184    
1185    
1186  INFORMATION ABOUT A PATTERN  INFORMATION ABOUT A PATTERN
      The pcre_fullinfo() function  returns  information  about  a  
      compiled pattern. It replaces the obsolete pcre_info() func-  
      tion, which is nevertheless retained for backwards compabil-  
      ity (and is documented below).  
   
      The first argument for pcre_fullinfo() is a pointer  to  the  
      compiled  pattern.  The  second  argument  is  the result of  
      pcre_study(), or NULL if the pattern was  not  studied.  The  
      third  argument  specifies  which  piece  of  information is  
      required, while the fourth argument is a pointer to a  vari-  
      able  to receive the data. The yield of the function is zero  
      for success, or one of the following negative numbers:  
   
        PCRE_ERROR_NULL       the argument code was NULL  
                              the argument where was NULL  
        PCRE_ERROR_BADMAGIC   the "magic number" was not found  
        PCRE_ERROR_BADOPTION  the value of what was invalid  
   
      The possible values for the third argument  are  defined  in  
      pcre.h, and are as follows:  
   
        PCRE_INFO_OPTIONS  
   
      Return a copy of the options with which the pattern was com-  
      piled.  The fourth argument should point to au unsigned long  
      int variable. These option bits are those specified  in  the  
      call  to  pcre_compile(),  modified  by any top-level option  
      settings  within  the   pattern   itself,   and   with   the  
      PCRE_ANCHORED  bit  forcibly  set if the form of the pattern  
      implies that it can match only at the  start  of  a  subject  
      string.  
   
        PCRE_INFO_SIZE  
   
      Return the size of the compiled pattern, that is, the  value  
      that  was  passed as the argument to pcre_malloc() when PCRE  
      was getting memory in which to place the compiled data.  The  
      fourth argument should point to a size_t variable.  
   
        PCRE_INFO_CAPTURECOUNT  
   
      Return the number of capturing subpatterns in  the  pattern.  
      The fourth argument should point to an int variable.  
   
        PCRE_INFO_BACKREFMAX  
   
      Return the number of the highest back reference in the  pat-  
      tern.  The  fourth argument should point to an int variable.  
      Zero is returned if there are no back references.  
   
        PCRE_INFO_FIRSTCHAR  
   
      Return information about the first character of any  matched  
      string,  for  a  non-anchored  pattern.  If there is a fixed  
      first   character,   e.g.   from   a   pattern    such    as  
      (cat|cow|coyote),  it  is returned in the integer pointed to  
      by where. Otherwise, if either  
   
      (a) the pattern was compiled with the PCRE_MULTILINE option,  
      and every branch starts with "^", or  
   
      (b) every  branch  of  the  pattern  starts  with  ".*"  and  
      PCRE_DOTALL is not set (if it were set, the pattern would be  
      anchored),  
   
      -1 is returned, indicating that the pattern matches only  at  
      the  start  of a subject string or after any "\n" within the  
      string. Otherwise -2 is returned.  For anchored patterns, -2  
      is returned.  
   
        PCRE_INFO_FIRSTTABLE  
   
      If the pattern was studied, and this resulted  in  the  con-  
      struction of a 256-bit table indicating a fixed set of char-  
      acters for the first character in  any  matching  string,  a  
      pointer   to  the  table  is  returned.  Otherwise  NULL  is  
      returned. The fourth argument should point  to  an  unsigned  
      char * variable.  
   
        PCRE_INFO_LASTLITERAL  
   
      For a non-anchored pattern, return the value of  the  right-  
      most  literal  character  which  must  exist  in any matched  
      string, other than at its start. The fourth argument  should  
      point  to an int variable. If there is no such character, or  
      if the pattern is anchored, -1 is returned. For example, for  
      the pattern /a\d+z\d+/ the returned value is 'z'.  
   
      The pcre_info() function is now obsolete because its  inter-  
      face  is  too  restrictive  to return all the available data  
      about  a  compiled  pattern.   New   programs   should   use  
      pcre_fullinfo()  instead.  The  yield  of pcre_info() is the  
      number of capturing subpatterns, or  one  of  the  following  
      negative numbers:  
   
        PCRE_ERROR_NULL       the argument code was NULL  
        PCRE_ERROR_BADMAGIC   the "magic number" was not found  
   
      If the optptr argument is not NULL, a copy  of  the  options  
      with which the pattern was compiled is placed in the integer  
      it points to (see PCRE_INFO_OPTIONS above).  
   
      If the pattern is not anchored and the firstcharptr argument  
      is  not  NULL, it is used to pass back information about the  
      first    character    of    any    matched    string    (see  
      PCRE_INFO_FIRSTCHAR above).  
1187    
1188           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1189                int what, void *where);
1190    
1191           The pcre_fullinfo() function returns information about a compiled  pat-
1192           tern. It replaces the obsolete pcre_info() function, which is neverthe-
1193           less retained for backwards compability (and is documented below).
1194    
1195           The first argument for pcre_fullinfo() is a  pointer  to  the  compiled
1196           pattern.  The second argument is the result of pcre_study(), or NULL if
1197           the pattern was not studied. The third argument specifies  which  piece
1198           of  information  is required, and the fourth argument is a pointer to a
1199           variable to receive the data. The yield of the  function  is  zero  for
1200           success, or one of the following negative numbers:
1201    
1202             PCRE_ERROR_NULL       the argument code was NULL
1203                                   the argument where was NULL
1204             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1205             PCRE_ERROR_BADOPTION  the value of what was invalid
1206    
1207           The  "magic  number" is placed at the start of each compiled pattern as
1208           an simple check against passing an arbitrary memory pointer. Here is  a
1209           typical  call  of pcre_fullinfo(), to obtain the length of the compiled
1210           pattern:
1211    
1212             int rc;
1213             unsigned long int length;
1214             rc = pcre_fullinfo(
1215               re,               /* result of pcre_compile() */
1216               pe,               /* result of pcre_study(), or NULL */
1217               PCRE_INFO_SIZE,   /* what is required */
1218               &length);         /* where to put the data */
1219    
1220           The possible values for the third argument are defined in  pcre.h,  and
1221           are as follows:
1222    
1223             PCRE_INFO_BACKREFMAX
1224    
1225           Return  the  number  of  the highest back reference in the pattern. The
1226           fourth argument should point to an int variable. Zero  is  returned  if
1227           there are no back references.
1228    
1229             PCRE_INFO_CAPTURECOUNT
1230    
1231           Return  the  number of capturing subpatterns in the pattern. The fourth
1232           argument should point to an int variable.
1233    
1234             PCRE_INFO_DEFAULT_TABLES
1235    
1236           Return a pointer to the internal default character tables within  PCRE.
1237           The  fourth  argument should point to an unsigned char * variable. This
1238           information call is provided for internal use by the pcre_study() func-
1239           tion.  External  callers  can  cause PCRE to use its internal tables by
1240           passing a NULL table pointer.
1241    
1242             PCRE_INFO_FIRSTBYTE
1243    
1244           Return information about the first byte of any matched  string,  for  a
1245           non-anchored    pattern.    (This    option    used    to   be   called
1246           PCRE_INFO_FIRSTCHAR; the old name is  still  recognized  for  backwards
1247           compatibility.)
1248    
1249           If  there  is  a  fixed first byte, for example, from a pattern such as
1250           (cat|cow|coyote), it is returned in the integer pointed  to  by  where.
1251           Otherwise, if either
1252    
1253           (a)  the pattern was compiled with the PCRE_MULTILINE option, and every
1254           branch starts with "^", or
1255    
1256           (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1257           set (if it were set, the pattern would be anchored),
1258    
1259           -1  is  returned, indicating that the pattern matches only at the start
1260           of a subject string or after any newline within the  string.  Otherwise
1261           -2 is returned. For anchored patterns, -2 is returned.
1262    
1263             PCRE_INFO_FIRSTTABLE
1264    
1265           If  the pattern was studied, and this resulted in the construction of a
1266           256-bit table indicating a fixed set of bytes for the first byte in any
1267           matching  string, a pointer to the table is returned. Otherwise NULL is
1268           returned. The fourth argument should point to an unsigned char *  vari-
1269           able.
1270    
1271             PCRE_INFO_LASTLITERAL
1272    
1273           Return  the  value of the rightmost literal byte that must exist in any
1274           matched string, other than at its  start,  if  such  a  byte  has  been
1275           recorded. The fourth argument should point to an int variable. If there
1276           is no such byte, -1 is returned. For anchored patterns, a last  literal
1277           byte  is  recorded only if it follows something of variable length. For
1278           example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1279           /^a\dz\d/ the returned value is -1.
1280    
1281             PCRE_INFO_NAMECOUNT
1282             PCRE_INFO_NAMEENTRYSIZE
1283             PCRE_INFO_NAMETABLE
1284    
1285           PCRE  supports the use of named as well as numbered capturing parenthe-
1286           ses. The names are just an additional way of identifying the  parenthe-
1287           ses,  which  still  acquire  numbers.  A  convenience  function  called
1288           pcre_get_named_substring() is provided  for  extracting  an  individual
1289           captured  substring  by  name.  It is also possible to extract the data
1290           directly, by first converting the name to a number in order  to  access
1291           the  correct  pointers in the output vector (described with pcre_exec()
1292           below). To do the conversion, you need to use the  name-to-number  map,
1293           which is described by these three values.
1294    
1295           The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1296           gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1297           of  each  entry;  both  of  these  return  an int value. The entry size
1298           depends on the length of the longest name. PCRE_INFO_NAMETABLE  returns
1299           a  pointer  to  the  first  entry of the table (a pointer to char). The
1300           first two bytes of each entry are the number of the capturing parenthe-
1301           sis,  most  significant byte first. The rest of the entry is the corre-
1302           sponding name, zero terminated. The names are  in  alphabetical  order.
1303           For  example,  consider  the following pattern (assume PCRE_EXTENDED is
1304           set, so white space - including newlines - is ignored):
1305    
1306             (?P<date> (?P<year>(\d\d)?\d\d) -
1307             (?P<month>\d\d) - (?P<day>\d\d) )
1308    
1309           There are four named subpatterns, so the table has  four  entries,  and
1310           each  entry  in the table is eight bytes long. The table is as follows,
1311           with non-printing bytes shows in hexadecimal, and undefined bytes shown
1312           as ??:
1313    
1314             00 01 d  a  t  e  00 ??
1315             00 05 d  a  y  00 ?? ??
1316             00 04 m  o  n  t  h  00
1317             00 02 y  e  a  r  00 ??
1318    
1319           When  writing  code  to  extract  data from named subpatterns using the
1320           name-to-number map, remember that the length of each entry is likely to
1321           be different for each compiled pattern.
1322    
1323             PCRE_INFO_OPTIONS
1324    
1325           Return  a  copy of the options with which the pattern was compiled. The
1326           fourth argument should point to an unsigned long  int  variable.  These
1327           option bits are those specified in the call to pcre_compile(), modified
1328           by any top-level option settings within the pattern itself.
1329    
1330           A pattern is automatically anchored by PCRE if  all  of  its  top-level
1331           alternatives begin with one of the following:
1332    
1333             ^     unless PCRE_MULTILINE is set
1334             \A    always
1335             \G    always
1336             .*    if PCRE_DOTALL is set and there are no back
1337                     references to the subpattern in which .* appears
1338    
1339           For such patterns, the PCRE_ANCHORED bit is set in the options returned
1340           by pcre_fullinfo().
1341    
1342             PCRE_INFO_SIZE
1343    
1344           Return the size of the compiled pattern, that is, the  value  that  was
1345           passed as the argument to pcre_malloc() when PCRE was getting memory in
1346           which to place the compiled data. The fourth argument should point to a
1347           size_t variable.
1348    
1349             PCRE_INFO_STUDYSIZE
1350    
1351           Return the size of the data block pointed to by the study_data field in
1352           a pcre_extra block. That is,  it  is  the  value  that  was  passed  to
1353           pcre_malloc() when PCRE was getting memory into which to place the data
1354           created by pcre_study(). The fourth argument should point to  a  size_t
1355           variable.
1356    
1357    
1358    OBSOLETE INFO FUNCTION
1359    
1360           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1361    
1362           The  pcre_info()  function is now obsolete because its interface is too
1363           restrictive to return all the available data about a compiled  pattern.
1364           New   programs   should  use  pcre_fullinfo()  instead.  The  yield  of
1365           pcre_info() is the number of capturing subpatterns, or one of the  fol-
1366           lowing negative numbers:
1367    
1368             PCRE_ERROR_NULL       the argument code was NULL
1369             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1370    
1371           If  the  optptr  argument is not NULL, a copy of the options with which
1372           the pattern was compiled is placed in the integer  it  points  to  (see
1373           PCRE_INFO_OPTIONS above).
1374    
1375           If  the  pattern  is  not anchored and the firstcharptr argument is not
1376           NULL, it is used to pass back information about the first character  of
1377           any matched string (see PCRE_INFO_FIRSTBYTE above).
1378    
1379    
1380    REFERENCE COUNTS
1381    
1382           int pcre_refcount(pcre *code, int adjust);
1383    
1384           The  pcre_refcount()  function is used to maintain a reference count in
1385           the data block that contains a compiled pattern. It is provided for the
1386           benefit  of  applications  that  operate  in an object-oriented manner,
1387           where different parts of the application may be using the same compiled
1388           pattern, but you want to free the block when they are all done.
1389    
1390           When a pattern is compiled, the reference count field is initialized to
1391           zero.  It is changed only by calling this function, whose action is  to
1392           add  the  adjust  value  (which may be positive or negative) to it. The
1393           yield of the function is the new value. However, the value of the count
1394           is  constrained to lie between 0 and 65535, inclusive. If the new value
1395           is outside these limits, it is forced to the appropriate limit value.
1396    
1397           Except when it is zero, the reference count is not correctly  preserved
1398           if  a  pattern  is  compiled on one host and then transferred to a host
1399           whose byte-order is different. (This seems a highly unlikely scenario.)
1400    
1401    
1402    MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1403    
1404           int pcre_exec(const pcre *code, const pcre_extra *extra,
1405                const char *subject, int length, int startoffset,
1406                int options, int *ovector, int ovecsize);
1407    
1408           The  function pcre_exec() is called to match a subject string against a
1409           compiled pattern, which is passed in the code argument. If the  pattern
1410           has been studied, the result of the study should be passed in the extra
1411           argument. This function is the main matching facility of  the  library,
1412           and it operates in a Perl-like manner. For specialist use there is also
1413           an alternative matching function, which is described below in the  sec-
1414           tion about the pcre_dfa_exec() function.
1415    
1416           In  most applications, the pattern will have been compiled (and option-
1417           ally studied) in the same process that calls pcre_exec().  However,  it
1418           is possible to save compiled patterns and study data, and then use them
1419           later in different processes, possibly even on different hosts.  For  a
1420           discussion about this, see the pcreprecompile documentation.
1421    
1422           Here is an example of a simple call to pcre_exec():
1423    
1424             int rc;
1425             int ovector[30];
1426             rc = pcre_exec(
1427               re,             /* result of pcre_compile() */
1428               NULL,           /* we didn't study the pattern */
1429               "some string",  /* the subject string */
1430               11,             /* the length of the subject string */
1431               0,              /* start at offset 0 in the subject */
1432               0,              /* default options */
1433               ovector,        /* vector of integers for substring information */
1434               30);            /* number of elements (NOT size in bytes) */
1435    
1436       Extra data for pcre_exec()
1437    
1438           If  the  extra argument is not NULL, it must point to a pcre_extra data
1439           block. The pcre_study() function returns such a block (when it  doesn't
1440           return  NULL), but you can also create one for yourself, and pass addi-
1441           tional information in it. The pcre_extra block contains  the  following
1442           fields (not necessarily in this order):
1443    
1444             unsigned long int flags;
1445             void *study_data;
1446             unsigned long int match_limit;
1447             unsigned long int match_limit_recursion;
1448             void *callout_data;
1449             const unsigned char *tables;
1450    
1451           The  flags  field  is a bitmap that specifies which of the other fields
1452           are set. The flag bits are:
1453    
1454             PCRE_EXTRA_STUDY_DATA
1455             PCRE_EXTRA_MATCH_LIMIT
1456             PCRE_EXTRA_MATCH_LIMIT_RECURSION
1457             PCRE_EXTRA_CALLOUT_DATA
1458             PCRE_EXTRA_TABLES
1459    
1460           Other flag bits should be set to zero. The study_data field is  set  in
1461           the  pcre_extra  block  that is returned by pcre_study(), together with
1462           the appropriate flag bit. You should not set this yourself, but you may
1463           add  to  the  block by setting the other fields and their corresponding
1464           flag bits.
1465    
1466           The match_limit field provides a means of preventing PCRE from using up
1467           a  vast amount of resources when running patterns that are not going to
1468           match, but which have a very large number  of  possibilities  in  their
1469           search  trees.  The  classic  example  is  the  use of nested unlimited
1470           repeats.
1471    
1472           Internally, PCRE uses a function called match() which it calls  repeat-
1473           edly  (sometimes  recursively). The limit set by match_limit is imposed
1474           on the number of times this function is called during  a  match,  which
1475           has  the  effect  of  limiting the amount of backtracking that can take
1476           place. For patterns that are not anchored, the count restarts from zero
1477           for each position in the subject string.
1478    
1479           The  default  value  for  the  limit can be set when PCRE is built; the
1480           default default is 10 million, which handles all but the  most  extreme
1481           cases.  You  can  override  the  default by suppling pcre_exec() with a
1482           pcre_extra    block    in    which    match_limit    is    set,     and
1483           PCRE_EXTRA_MATCH_LIMIT  is  set  in  the  flags  field. If the limit is
1484           exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1485    
1486           The match_limit_recursion field is similar to match_limit, but  instead
1487           of limiting the total number of times that match() is called, it limits
1488           the depth of recursion. The recursion depth is a  smaller  number  than
1489           the  total number of calls, because not all calls to match() are recur-
1490           sive.  This limit is of use only if it is set smaller than match_limit.
1491    
1492           Limiting  the  recursion  depth  limits the amount of stack that can be
1493           used, or, when PCRE has been compiled to use memory on the heap instead
1494           of the stack, the amount of heap memory that can be used.
1495    
1496           The  default  value  for  match_limit_recursion can be set when PCRE is
1497           built; the default default  is  the  same  value  as  the  default  for
1498           match_limit.  You can override the default by suppling pcre_exec() with
1499           a  pcre_extra  block  in  which  match_limit_recursion  is   set,   and
1500           PCRE_EXTRA_MATCH_LIMIT_RECURSION  is  set  in  the  flags field. If the
1501           limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1502    
1503           The pcre_callout field is used in conjunction with the  "callout"  fea-
1504           ture, which is described in the pcrecallout documentation.
1505    
1506           The  tables  field  is  used  to  pass  a  character  tables pointer to
1507           pcre_exec(); this overrides the value that is stored with the  compiled
1508           pattern.  A  non-NULL value is stored with the compiled pattern only if
1509           custom tables were supplied to pcre_compile() via  its  tableptr  argu-
1510           ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
1511           PCRE's internal tables to be used. This facility is  helpful  when  re-
1512           using  patterns  that  have been saved after compiling with an external
1513           set of tables, because the external tables  might  be  at  a  different
1514           address  when  pcre_exec() is called. See the pcreprecompile documenta-
1515           tion for a discussion of saving compiled patterns for later use.
1516    
1517       Option bits for pcre_exec()
1518    
1519           The unused bits of the options argument for pcre_exec() must  be  zero.
1520           The   only  bits  that  may  be  set  are  PCRE_ANCHORED,  PCRE_NOTBOL,
1521           PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and PCRE_PARTIAL.
1522    
1523             PCRE_ANCHORED
1524    
1525           The PCRE_ANCHORED option limits pcre_exec() to matching  at  the  first
1526           matching  position.  If  a  pattern was compiled with PCRE_ANCHORED, or
1527           turned out to be anchored by virtue of its contents, it cannot be  made
1528           unachored at matching time.
1529    
1530             PCRE_NOTBOL
1531    
1532           This option specifies that first character of the subject string is not
1533           the beginning of a line, so the  circumflex  metacharacter  should  not
1534           match  before it. Setting this without PCRE_MULTILINE (at compile time)
1535           causes circumflex never to match. This option affects only  the  behav-
1536           iour of the circumflex metacharacter. It does not affect \A.
1537    
1538             PCRE_NOTEOL
1539    
1540           This option specifies that the end of the subject string is not the end
1541           of a line, so the dollar metacharacter should not match it nor  (except
1542           in  multiline mode) a newline immediately before it. Setting this with-
1543           out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1544           option  affects only the behaviour of the dollar metacharacter. It does
1545           not affect \Z or \z.
1546    
1547             PCRE_NOTEMPTY
1548    
1549           An empty string is not considered to be a valid match if this option is
1550           set.  If  there are alternatives in the pattern, they are tried. If all
1551           the alternatives match the empty string, the entire  match  fails.  For
1552           example, if the pattern
1553    
1554             a?b?
1555    
1556           is  applied  to  a string not beginning with "a" or "b", it matches the
1557           empty string at the start of the subject. With PCRE_NOTEMPTY set,  this
1558           match is not valid, so PCRE searches further into the string for occur-
1559           rences of "a" or "b".
1560    
1561           Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1562           cial  case  of  a  pattern match of the empty string within its split()
1563           function, and when using the /g modifier. It  is  possible  to  emulate
1564           Perl's behaviour after matching a null string by first trying the match
1565           again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1566           if  that  fails by advancing the starting offset (see below) and trying
1567           an ordinary match again. There is some code that demonstrates how to do
1568           this in the pcredemo.c sample program.
1569    
1570             PCRE_NO_UTF8_CHECK
1571    
1572           When PCRE_UTF8 is set at compile time, the validity of the subject as a
1573           UTF-8 string is automatically checked when pcre_exec() is  subsequently
1574           called.   The  value  of  startoffset is also checked to ensure that it
1575           points to the start of a UTF-8 character. If an invalid UTF-8  sequence
1576           of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If
1577           startoffset contains an  invalid  value,  PCRE_ERROR_BADUTF8_OFFSET  is
1578           returned.
1579    
1580           If  you  already  know that your subject is valid, and you want to skip
1581           these   checks   for   performance   reasons,   you   can    set    the
1582           PCRE_NO_UTF8_CHECK  option  when calling pcre_exec(). You might want to
1583           do this for the second and subsequent calls to pcre_exec() if  you  are
1584           making  repeated  calls  to  find  all  the matches in a single subject
1585           string. However, you should be  sure  that  the  value  of  startoffset
1586           points  to  the  start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
1587           set, the effect of passing an invalid UTF-8 string as a subject,  or  a
1588           value  of startoffset that does not point to the start of a UTF-8 char-
1589           acter, is undefined. Your program may crash.
1590    
1591             PCRE_PARTIAL
1592    
1593           This option turns on the  partial  matching  feature.  If  the  subject
1594           string  fails to match the pattern, but at some point during the match-
1595           ing process the end of the subject was reached (that  is,  the  subject
1596           partially  matches  the  pattern and the failure to match occurred only
1597           because there were not enough subject characters), pcre_exec()  returns
1598           PCRE_ERROR_PARTIAL  instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
1599           used, there are restrictions on what may appear in the  pattern.  These
1600           are discussed in the pcrepartial documentation.
1601    
1602       The string to be matched by pcre_exec()
1603    
1604           The  subject string is passed to pcre_exec() as a pointer in subject, a
1605           length in length, and a starting byte offset in startoffset.  In  UTF-8
1606           mode,  the  byte  offset  must point to the start of a UTF-8 character.
1607           Unlike the pattern string, the subject may contain binary  zero  bytes.
1608           When  the starting offset is zero, the search for a match starts at the
1609           beginning of the subject, and this is by far the most common case.
1610    
1611           A non-zero starting offset is useful when searching for  another  match
1612           in  the same subject by calling pcre_exec() again after a previous suc-
1613           cess.  Setting startoffset differs from just passing over  a  shortened
1614           string  and  setting  PCRE_NOTBOL  in the case of a pattern that begins
1615           with any kind of lookbehind. For example, consider the pattern
1616    
1617             \Biss\B
1618    
1619           which finds occurrences of "iss" in the middle of  words.  (\B  matches
1620           only  if  the  current position in the subject is not a word boundary.)
1621           When applied to the string "Mississipi" the first call  to  pcre_exec()
1622           finds  the  first  occurrence. If pcre_exec() is called again with just
1623           the remainder of the subject,  namely  "issipi",  it  does  not  match,
1624           because \B is always false at the start of the subject, which is deemed
1625           to be a word boundary. However, if pcre_exec()  is  passed  the  entire
1626           string again, but with startoffset set to 4, it finds the second occur-
1627           rence of "iss" because it is able to look behind the starting point  to
1628           discover that it is preceded by a letter.
1629    
1630           If  a  non-zero starting offset is passed when the pattern is anchored,
1631           one attempt to match at the given offset is made. This can only succeed
1632           if  the  pattern  does  not require the match to be at the start of the
1633           subject.
1634    
1635       How pcre_exec() returns captured substrings
1636    
1637           In general, a pattern matches a certain portion of the subject, and  in
1638           addition,  further  substrings  from  the  subject may be picked out by
1639           parts of the pattern. Following the usage  in  Jeffrey  Friedl's  book,
1640           this  is  called "capturing" in what follows, and the phrase "capturing
1641           subpattern" is used for a fragment of a pattern that picks out  a  sub-
1642           string.  PCRE  supports several other kinds of parenthesized subpattern
1643           that do not cause substrings to be captured.
1644    
1645           Captured substrings are returned to the caller via a vector of  integer
1646           offsets  whose  address is passed in ovector. The number of elements in
1647           the vector is passed in ovecsize, which must be a non-negative  number.
1648           Note: this argument is NOT the size of ovector in bytes.
1649    
1650           The  first  two-thirds of the vector is used to pass back captured sub-
1651           strings, each substring using a pair of integers. The  remaining  third
1652           of  the  vector is used as workspace by pcre_exec() while matching cap-
1653           turing subpatterns, and is not available for passing back  information.
1654           The  length passed in ovecsize should always be a multiple of three. If
1655           it is not, it is rounded down.
1656    
1657           When a match is successful, information about  captured  substrings  is
1658           returned  in  pairs  of integers, starting at the beginning of ovector,
1659           and continuing up to two-thirds of its length at the  most.  The  first
1660           element of a pair is set to the offset of the first character in a sub-
1661           string, and the second is set to the  offset  of  the  first  character
1662           after  the  end  of  a  substring. The first pair, ovector[0] and ovec-
1663           tor[1], identify the portion of  the  subject  string  matched  by  the
1664           entire  pattern.  The next pair is used for the first capturing subpat-
1665           tern, and so on. The value returned by pcre_exec()  is  the  number  of
1666           pairs  that  have  been set. If there are no capturing subpatterns, the
1667           return value from a successful match is 1,  indicating  that  just  the
1668           first pair of offsets has been set.
1669    
1670           Some  convenience  functions  are  provided for extracting the captured
1671           substrings as separate strings. These are described  in  the  following
1672           section.
1673    
1674           It  is  possible  for  an capturing subpattern number n+1 to match some
1675           part of the subject when subpattern n has not been  used  at  all.  For
1676           example, if the string "abc" is matched against the pattern (a|(z))(bc)
1677           subpatterns 1 and 3 are matched, but 2 is not. When this happens,  both
1678           offset values corresponding to the unused subpattern are set to -1.
1679    
1680           If a capturing subpattern is matched repeatedly, it is the last portion
1681           of the string that it matched that is returned.
1682    
1683           If the vector is too small to hold all the captured substring  offsets,
1684           it is used as far as possible (up to two-thirds of its length), and the
1685           function returns a value of zero. In particular, if the substring  off-
1686           sets are not of interest, pcre_exec() may be called with ovector passed
1687           as NULL and ovecsize as zero. However, if  the  pattern  contains  back
1688           references  and  the  ovector is not big enough to remember the related
1689           substrings, PCRE has to get additional memory for use during  matching.
1690           Thus it is usually advisable to supply an ovector.
1691    
1692           Note  that  pcre_info() can be used to find out how many capturing sub-
1693           patterns there are in a compiled pattern. The smallest size for ovector
1694           that  will  allow for n captured substrings, in addition to the offsets
1695           of the substring matched by the whole pattern, is (n+1)*3.
1696    
1697       Return values from pcre_exec()
1698    
1699           If pcre_exec() fails, it returns a negative number. The  following  are
1700           defined in the header file:
1701    
1702             PCRE_ERROR_NOMATCH        (-1)
1703    
1704           The subject string did not match the pattern.
1705    
1706             PCRE_ERROR_NULL           (-2)
1707    
1708           Either  code  or  subject  was  passed as NULL, or ovector was NULL and
1709           ovecsize was not zero.
1710    
1711             PCRE_ERROR_BADOPTION      (-3)
1712    
1713           An unrecognized bit was set in the options argument.
1714    
1715             PCRE_ERROR_BADMAGIC       (-4)
1716    
1717           PCRE stores a 4-byte "magic number" at the start of the compiled  code,
1718           to catch the case when it is passed a junk pointer and to detect when a
1719           pattern that was compiled in an environment of one endianness is run in
1720           an  environment  with the other endianness. This is the error that PCRE
1721           gives when the magic number is not present.
1722    
1723             PCRE_ERROR_UNKNOWN_NODE   (-5)
1724    
1725           While running the pattern match, an unknown item was encountered in the
1726           compiled  pattern.  This  error  could be caused by a bug in PCRE or by
1727           overwriting of the compiled pattern.
1728    
1729             PCRE_ERROR_NOMEMORY       (-6)
1730    
1731           If a pattern contains back references, but the ovector that  is  passed
1732           to pcre_exec() is not big enough to remember the referenced substrings,
1733           PCRE gets a block of memory at the start of matching to  use  for  this
1734           purpose.  If the call via pcre_malloc() fails, this error is given. The
1735           memory is automatically freed at the end of matching.
1736    
1737             PCRE_ERROR_NOSUBSTRING    (-7)
1738    
1739           This error is used by the pcre_copy_substring(),  pcre_get_substring(),
1740           and  pcre_get_substring_list()  functions  (see  below).  It  is  never
1741           returned by pcre_exec().
1742    
1743             PCRE_ERROR_MATCHLIMIT     (-8)
1744    
1745           The backtracking limit, as specified by  the  match_limit  field  in  a
1746           pcre_extra  structure  (or  defaulted) was reached. See the description
1747           above.
1748    
1749             PCRE_ERROR_RECURSIONLIMIT (-21)
1750    
1751           The internal recursion limit, as specified by the match_limit_recursion
1752           field  in  a  pcre_extra  structure (or defaulted) was reached. See the
1753           description above.
1754    
1755             PCRE_ERROR_CALLOUT        (-9)
1756    
1757           This error is never generated by pcre_exec() itself. It is provided for
1758           use  by  callout functions that want to yield a distinctive error code.
1759           See the pcrecallout documentation for details.
1760    
1761             PCRE_ERROR_BADUTF8        (-10)
1762    
1763           A string that contains an invalid UTF-8 byte sequence was passed  as  a
1764           subject.
1765    
1766             PCRE_ERROR_BADUTF8_OFFSET (-11)
1767    
1768           The UTF-8 byte sequence that was passed as a subject was valid, but the
1769           value of startoffset did not point to the beginning of a UTF-8  charac-
1770           ter.
1771    
1772             PCRE_ERROR_PARTIAL        (-12)
1773    
1774           The  subject  string did not match, but it did match partially. See the
1775           pcrepartial documentation for details of partial matching.
1776    
1777             PCRE_ERROR_BADPARTIAL     (-13)
1778    
1779           The PCRE_PARTIAL option was used with  a  compiled  pattern  containing
1780           items  that are not supported for partial matching. See the pcrepartial
1781           documentation for details of partial matching.
1782    
1783             PCRE_ERROR_INTERNAL       (-14)
1784    
1785           An unexpected internal error has occurred. This error could  be  caused
1786           by a bug in PCRE or by overwriting of the compiled pattern.
1787    
1788             PCRE_ERROR_BADCOUNT       (-15)
1789    
1790           This  error is given if the value of the ovecsize argument is negative.
1791    
1792    
1793    EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
1794    
1795           int pcre_copy_substring(const char *subject, int *ovector,
1796                int stringcount, int stringnumber, char *buffer,
1797                int buffersize);
1798    
1799           int pcre_get_substring(const char *subject, int *ovector,
1800                int stringcount, int stringnumber,
1801                const char **stringptr);
1802    
1803           int pcre_get_substring_list(const char *subject,
1804                int *ovector, int stringcount, const char ***listptr);
1805    
1806           Captured substrings can be  accessed  directly  by  using  the  offsets
1807           returned  by  pcre_exec()  in  ovector.  For convenience, the functions
1808           pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
1809           string_list()  are  provided for extracting captured substrings as new,
1810           separate, zero-terminated strings. These functions identify  substrings
1811           by  number.  The  next section describes functions for extracting named
1812           substrings. A substring  that  contains  a  binary  zero  is  correctly
1813           extracted  and  has  a further zero added on the end, but the result is
1814           not, of course, a C string.
1815    
1816           The first three arguments are the same for all  three  of  these  func-
1817           tions:  subject  is  the subject string that has just been successfully
1818           matched, ovector is a pointer to the vector of integer offsets that was
1819           passed to pcre_exec(), and stringcount is the number of substrings that
1820           were captured by the match, including the substring  that  matched  the
1821           entire regular expression. This is the value returned by pcre_exec() if
1822           it is greater than zero. If pcre_exec() returned zero, indicating  that
1823           it  ran out of space in ovector, the value passed as stringcount should
1824           be the number of elements in the vector divided by three.
1825    
1826           The functions pcre_copy_substring() and pcre_get_substring() extract  a
1827           single  substring,  whose  number  is given as stringnumber. A value of
1828           zero extracts the substring that matched the  entire  pattern,  whereas
1829           higher  values  extract  the  captured  substrings.  For pcre_copy_sub-
1830           string(), the string is placed in buffer,  whose  length  is  given  by
1831           buffersize,  while  for  pcre_get_substring()  a new block of memory is
1832           obtained via pcre_malloc, and its address is  returned  via  stringptr.
1833           The  yield  of  the function is the length of the string, not including
1834           the terminating zero, or one of
1835    
1836             PCRE_ERROR_NOMEMORY       (-6)
1837    
1838           The buffer was too small for pcre_copy_substring(), or the  attempt  to
1839           get memory failed for pcre_get_substring().
1840    
1841             PCRE_ERROR_NOSUBSTRING    (-7)
1842    
1843           There is no substring whose number is stringnumber.
1844    
1845           The  pcre_get_substring_list()  function  extracts  all  available sub-
1846           strings and builds a list of pointers to them. All this is  done  in  a
1847           single block of memory that is obtained via pcre_malloc. The address of
1848           the memory block is returned via listptr, which is also  the  start  of
1849           the  list  of  string pointers. The end of the list is marked by a NULL
1850           pointer. The yield of the function is zero if all went well, or
1851    
1852             PCRE_ERROR_NOMEMORY       (-6)
1853    
1854           if the attempt to get the memory block failed.
1855    
1856           When any of these functions encounter a substring that is unset,  which
1857           can  happen  when  capturing subpattern number n+1 matches some part of
1858           the subject, but subpattern n has not been used at all, they return  an
1859           empty string. This can be distinguished from a genuine zero-length sub-
1860           string by inspecting the appropriate offset in ovector, which is  nega-
1861           tive for unset substrings.
1862    
1863           The  two convenience functions pcre_free_substring() and pcre_free_sub-
1864           string_list() can be used to free the memory  returned  by  a  previous
1865           call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
1866           tively. They do nothing more than  call  the  function  pointed  to  by
1867           pcre_free,  which  of course could be called directly from a C program.
1868           However, PCRE is used in some situations where it is linked via a  spe-
1869           cial  interface  to  another  programming  language  which  cannot  use
1870           pcre_free directly; it is for these cases that the functions  are  pro-
1871           vided.
1872    
1873    
1874    EXTRACTING CAPTURED SUBSTRINGS BY NAME
1875    
1876           int pcre_get_stringnumber(const pcre *code,
1877                const char *name);
1878    
1879           int pcre_copy_named_substring(const pcre *code,
1880                const char *subject, int *ovector,
1881                int stringcount, const char *stringname,
1882                char *buffer, int buffersize);
1883    
1884           int pcre_get_named_substring(const pcre *code,
1885                const char *subject, int *ovector,
1886                int stringcount, const char *stringname,
1887                const char **stringptr);
1888    
1889           To  extract a substring by name, you first have to find associated num-
1890           ber.  For example, for this pattern
1891    
1892             (a+)b(?P<xxx>\d+)...
1893    
1894           the number of the subpattern called "xxx" is 2. You can find the number
1895           from the name by calling pcre_get_stringnumber(). The first argument is
1896           the compiled pattern, and the second is the  name.  The  yield  of  the
1897           function  is  the  subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if
1898           there is no subpattern of that name.
1899    
1900           Given the number, you can extract the substring directly, or use one of
1901           the functions described in the previous section. For convenience, there
1902           are also two functions that do the whole job.
1903    
1904           Most   of   the   arguments    of    pcre_copy_named_substring()    and
1905           pcre_get_named_substring()  are  the  same  as  those for the similarly
1906           named functions that extract by number. As these are described  in  the
1907           previous  section,  they  are not re-described here. There are just two
1908           differences:
1909    
1910           First, instead of a substring number, a substring name is  given.  Sec-
1911           ond, there is an extra argument, given at the start, which is a pointer
1912           to the compiled pattern. This is needed in order to gain access to  the
1913           name-to-number translation table.
1914    
1915           These  functions call pcre_get_stringnumber(), and if it succeeds, they
1916           then call pcre_copy_substring() or pcre_get_substring(),  as  appropri-
1917           ate.
1918    
1919    
1920    FINDING ALL POSSIBLE MATCHES
1921    
1922           The  traditional  matching  function  uses a similar algorithm to Perl,
1923           which stops when it finds the first match, starting at a given point in
1924           the  subject.  If you want to find all possible matches, or the longest
1925           possible match, consider using the alternative matching  function  (see
1926           below)  instead.  If you cannot use the alternative function, but still
1927           need to find all possible matches, you can kludge it up by  making  use
1928           of the callout facility, which is described in the pcrecallout documen-
1929           tation.
1930    
1931           What you have to do is to insert a callout right at the end of the pat-
1932           tern.   When your callout function is called, extract and save the cur-
1933           rent matched substring. Then return  1,  which  forces  pcre_exec()  to
1934           backtrack  and  try other alternatives. Ultimately, when it runs out of
1935           matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
1936    
1937    
1938    MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
1939    
1940           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
1941                const char *subject, int length, int startoffset,
1942                int options, int *ovector, int ovecsize,
1943                int *workspace, int wscount);
1944    
1945           The function pcre_dfa_exec()  is  called  to  match  a  subject  string
1946           against  a compiled pattern, using a "DFA" matching algorithm. This has
1947           different characteristics to the normal algorithm, and is not  compati-
1948           ble with Perl. Some of the features of PCRE patterns are not supported.
1949           Nevertheless, there are times when this kind of matching can be useful.
1950           For  a  discussion of the two matching algorithms, see the pcrematching
1951           documentation.
1952    
1953           The arguments for the pcre_dfa_exec() function  are  the  same  as  for
1954           pcre_exec(), plus two extras. The ovector argument is used in a differ-
1955           ent way, and this is described below. The other  common  arguments  are
1956           used  in  the  same way as for pcre_exec(), so their description is not
1957           repeated here.
1958    
1959           The two additional arguments provide workspace for  the  function.  The
1960           workspace  vector  should  contain at least 20 elements. It is used for
1961           keeping  track  of  multiple  paths  through  the  pattern  tree.  More
1962           workspace  will  be  needed for patterns and subjects where there are a
1963           lot of possible matches.
1964    
1965           Here is an example of a simple call to pcre_dfa_exec():
1966    
1967             int rc;
1968             int ovector[10];
1969             int wspace[20];
1970             rc = pcre_dfa_exec(
1971               re,             /* result of pcre_compile() */
1972               NULL,           /* we didn't study the pattern */
1973               "some string",  /* the subject string */
1974               11,             /* the length of the subject string */
1975               0,              /* start at offset 0 in the subject */
1976               0,              /* default options */
1977               ovector,        /* vector of integers for substring information */
1978               10,             /* number of elements (NOT size in bytes) */
1979               wspace,         /* working space vector */
1980               20);            /* number of elements (NOT size in bytes) */
1981    
1982       Option bits for pcre_dfa_exec()
1983    
1984           The unused bits of the options argument  for  pcre_dfa_exec()  must  be
1985           zero.  The  only  bits  that may be set are PCRE_ANCHORED, PCRE_NOTBOL,
1986           PCRE_NOTEOL,    PCRE_NOTEMPTY,    PCRE_NO_UTF8_CHECK,     PCRE_PARTIAL,
1987           PCRE_DFA_SHORTEST,  and  PCRE_DFA_RESTART.  All  but  the last three of
1988           these are the same as for pcre_exec(),  so  their  description  is  not
1989           repeated here.
1990    
1991             PCRE_PARTIAL
1992    
1993           This  has  the  same general effect as it does for pcre_exec(), but the
1994           details  are  slightly  different.  When  PCRE_PARTIAL   is   set   for
1995           pcre_dfa_exec(),  the  return code PCRE_ERROR_NOMATCH is converted into
1996           PCRE_ERROR_PARTIAL if the end of the subject  is  reached,  there  have
1997           been no complete matches, but there is still at least one matching pos-
1998           sibility. The portion of the string that provided the partial match  is
1999           set as the first matching string.
2000    
2001             PCRE_DFA_SHORTEST
2002    
2003           Setting  the  PCRE_DFA_SHORTEST option causes the matching algorithm to
2004           stop as soon as it has found one match. Because  of  the  way  the  DFA
2005           algorithm works, this is necessarily the shortest possible match at the
2006           first possible matching point in the subject string.
2007    
2008             PCRE_DFA_RESTART
2009    
2010           When pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option,  and
2011           returns  a  partial  match, it is possible to call it again, with addi-
2012           tional subject characters, and have it continue with  the  same  match.
2013           The  PCRE_DFA_RESTART  option requests this action; when it is set, the
2014           workspace and wscount options must reference the same vector as  before
2015           because  data  about  the  match so far is left in them after a partial
2016           match. There is more discussion of this  facility  in  the  pcrepartial
2017           documentation.
2018    
2019       Successful returns from pcre_dfa_exec()
2020    
2021           When  pcre_dfa_exec()  succeeds, it may have matched more than one sub-
2022           string in the subject. Note, however, that all the matches from one run
2023           of  the  function  start  at the same point in the subject. The shorter
2024           matches are all initial substrings of the longer matches. For  example,
2025           if the pattern
2026    
2027             <.*>
2028    
2029           is matched against the string
2030    
2031             This is <something> <something else> <something further> no more
2032    
2033           the three matched strings are
2034    
2035             <something>
2036             <something> <something else>
2037             <something> <something else> <something further>
2038    
2039           On  success,  the  yield of the function is a number greater than zero,
2040           which is the number of matched substrings.  The  substrings  themselves
2041           are  returned  in  ovector. Each string uses two elements; the first is
2042           the offset to the start, and the second is the offset to the  end.  All
2043           the strings have the same start offset. (Space could have been saved by
2044           giving this only once, but it was decided to retain some  compatibility
2045           with  the  way pcre_exec() returns data, even though the meaning of the
2046           strings is different.)
2047    
2048           The strings are returned in reverse order of length; that is, the long-
2049           est  matching  string is given first. If there were too many matches to
2050           fit into ovector, the yield of the function is zero, and the vector  is
2051           filled with the longest matches.
2052    
2053       Error returns from pcre_dfa_exec()
2054    
2055           The  pcre_dfa_exec()  function returns a negative number when it fails.
2056           Many of the errors are the same  as  for  pcre_exec(),  and  these  are
2057           described  above.   There are in addition the following errors that are
2058           specific to pcre_dfa_exec():
2059    
2060             PCRE_ERROR_DFA_UITEM      (-16)
2061    
2062           This return is given if pcre_dfa_exec() encounters an item in the  pat-
2063           tern  that  it  does not support, for instance, the use of \C or a back
2064           reference.
2065    
2066             PCRE_ERROR_DFA_UCOND      (-17)
2067    
2068           This return is given if pcre_dfa_exec() encounters a condition item  in
2069           a  pattern  that  uses  a back reference for the condition. This is not
2070           supported.
2071    
2072             PCRE_ERROR_DFA_UMLIMIT    (-18)
2073    
2074           This return is given if pcre_dfa_exec() is called with an  extra  block
2075           that contains a setting of the match_limit field. This is not supported
2076           (it is meaningless).
2077    
2078             PCRE_ERROR_DFA_WSSIZE     (-19)
2079    
2080           This return is given if  pcre_dfa_exec()  runs  out  of  space  in  the
2081           workspace vector.
2082    
2083             PCRE_ERROR_DFA_RECURSE    (-20)
2084    
2085           When  a  recursive subpattern is processed, the matching function calls
2086           itself recursively, using private vectors for  ovector  and  workspace.
2087           This  error  is  given  if  the output vector is not large enough. This
2088           should be extremely rare, as a vector of size 1000 is used.
2089    
2090    Last updated: 18 January 2006
2091    Copyright (c) 1997-2006 University of Cambridge.
2092    ------------------------------------------------------------------------------
2093    
2094  MATCHING A PATTERN  
2095       The function pcre_exec() is called to match a subject string  PCRECALLOUT(3)                                                  PCRECALLOUT(3)
      against  a pre-compiled pattern, which is passed in the code  
      argument. If the pattern has been studied, the result of the  
      study should be passed in the extra argument. Otherwise this  
      must be NULL.  
   
      The PCRE_ANCHORED option can be passed in the options  argu-  
      ment,  whose unused bits must be zero. However, if a pattern  
      was  compiled  with  PCRE_ANCHORED,  or  turned  out  to  be  
      anchored  by  virtue  of  its  contents,  it  cannot be made  
      unachored at matching time.  
   
      There are also three further options that can be set only at  
      matching time:  
   
        PCRE_NOTBOL  
   
      The first character of the string is not the beginning of  a  
      line,  so  the  circumflex  metacharacter  should  not match  
      before it. Setting this without PCRE_MULTILINE  (at  compile  
      time) causes circumflex never to match.  
   
        PCRE_NOTEOL  
   
      The end of the string is not the end of a line, so the  dol-  
      lar  metacharacter should not match it nor (except in multi-  
      line mode) a newline immediately  before  it.  Setting  this  
      without PCRE_MULTILINE (at compile time) causes dollar never  
      to match.  
   
        PCRE_NOTEMPTY  
   
      An empty string is not considered to be  a  valid  match  if  
      this  option  is  set. If there are alternatives in the pat-  
      tern, they are tried. If  all  the  alternatives  match  the  
      empty  string,  the  entire match fails. For example, if the  
      pattern  
   
        a?b?  
   
      is applied to a string not beginning with  "a"  or  "b",  it  
      matches  the  empty string at the start of the subject. With  
      PCRE_NOTEMPTY set, this match is not valid, so PCRE searches  
      further into the string for occurrences of "a" or "b".  
   
      Perl has no direct equivalent of PCRE_NOTEMPTY, but it  does  
      make  a  special case of a pattern match of the empty string  
      within its split() function, and when using the /g modifier.  
      It  is possible to emulate Perl's behaviour after matching a  
      null string by first trying the  match  again  at  the  same  
      offset  with  PCRE_NOTEMPTY  set,  and then if that fails by  
      advancing the starting offset  (see  below)  and  trying  an  
      ordinary match again.  
   
      The subject string is passed as  a  pointer  in  subject,  a  
      length  in  length,  and  a  starting offset in startoffset.  
      Unlike the pattern string, it may contain binary zero  char-  
      acters.  When  the starting offset is zero, the search for a  
      match starts at the beginning of the subject, and this is by  
      far the most common case.  
   
      A non-zero starting offset  is  useful  when  searching  for  
      another  match  in  the  same subject by calling pcre_exec()  
      again after a previous success.  Setting startoffset differs  
      from  just  passing  over  a  shortened  string  and setting  
      PCRE_NOTBOL in the case of a pattern that  begins  with  any  
      kind of lookbehind. For example, consider the pattern  
   
        \Biss\B  
   
      which finds occurrences of "iss" in the middle of words. (\B  
      matches only if the current position in the subject is not a  
      word boundary.) When applied to the string "Mississipi"  the  
      first  call  to  pcre_exec()  finds the first occurrence. If  
      pcre_exec() is called again with just the remainder  of  the  
      subject,  namely  "issipi", it does not match, because \B is  
      always false at the start of the subject, which is deemed to  
      be  a  word  boundary. However, if pcre_exec() is passed the  
      entire string again, but with startoffset set to 4, it finds  
      the  second  occurrence  of "iss" because it is able to look  
      behind the starting point to discover that it is preceded by  
      a letter.  
   
      If a non-zero starting offset is passed when the pattern  is  
      anchored, one attempt to match at the given offset is tried.  
      This can only succeed if the pattern does  not  require  the  
      match to be at the start of the subject.  
   
      In general, a pattern matches a certain portion of the  sub-  
      ject,  and  in addition, further substrings from the subject  
      may be picked out by parts of  the  pattern.  Following  the  
      usage  in  Jeffrey Friedl's book, this is called "capturing"  
      in what follows, and the phrase  "capturing  subpattern"  is  
      used for a fragment of a pattern that picks out a substring.  
      PCRE supports several other kinds of  parenthesized  subpat-  
      tern that do not cause substrings to be captured.  
   
      Captured substrings are returned to the caller via a  vector  
      of  integer  offsets whose address is passed in ovector. The  
      number of elements in the vector is passed in ovecsize.  The  
      first two-thirds of the vector is used to pass back captured  
      substrings, each substring using a  pair  of  integers.  The  
      remaining  third  of  the  vector  is  used  as workspace by  
      pcre_exec() while matching capturing subpatterns, and is not  
      available for passing back information. The length passed in  
      ovecsize should always be a multiple of three. If it is not,  
      it is rounded down.  
   
      When a match has been successful, information about captured  
      substrings is returned in pairs of integers, starting at the  
      beginning of ovector, and continuing up to two-thirds of its  
      length  at  the  most. The first element of a pair is set to  
      the offset of the first character in a  substring,  and  the  
      second is set to the offset of the first character after the  
      end of a substring. The first  pair,  ovector[0]  and  ovec-  
      tor[1],  identify  the portion of the subject string matched  
      by the entire pattern. The next pair is used for  the  first  
      capturing  subpattern,  and  so  on.  The  value returned by  
      pcre_exec() is the number of pairs that have  been  set.  If  
      there  are no capturing subpatterns, the return value from a  
      successful match is 1, indicating that just the  first  pair  
      of offsets has been set.  
   
      Some convenience functions are provided for  extracting  the  
      captured substrings as separate strings. These are described  
      in the following section.  
   
      It is possible for an capturing  subpattern  number  n+1  to  
      match  some  part  of  the subject when subpattern n has not  
      been used at all.  For  example,  if  the  string  "abc"  is  
      matched  against the pattern (a|(z))(bc) subpatterns 1 and 3  
      are matched, but 2 is not. When this  happens,  both  offset  
      values corresponding to the unused subpattern are set to -1.  
   
      If a capturing subpattern is matched repeatedly, it  is  the  
      last  portion  of  the  string  that  it  matched  that gets  
      returned.  
   
      If the vector is too small to hold  all  the  captured  sub-  
      strings,  it is used as far as possible (up to two-thirds of  
      its length), and the function returns a value  of  zero.  In  
      particular,  if  the  substring offsets are not of interest,  
      pcre_exec() may be called with ovector passed  as  NULL  and  
      ovecsize  as  zero.  However,  if  the pattern contains back  
      references and the ovector isn't big enough to remember  the  
      related  substrings,  PCRE  has to get additional memory for  
      use during matching. Thus it is usually advisable to  supply  
      an ovector.  
   
      Note that pcre_info() can be used to find out how many  cap-  
      turing  subpatterns  there  are  in  a compiled pattern. The  
      smallest size for ovector that will  allow  for  n  captured  
      substrings  in  addition  to  the  offsets  of the substring  
      matched by the whole pattern is (n+1)*3.  
   
      If pcre_exec() fails, it returns a negative number. The fol-  
      lowing are defined in the header file:  
   
        PCRE_ERROR_NOMATCH        (-1)  
   
      The subject string did not match the pattern.  
   
        PCRE_ERROR_NULL           (-2)  
   
      Either code or subject was passed as NULL,  or  ovector  was  
      NULL and ovecsize was not zero.  
   
        PCRE_ERROR_BADOPTION      (-3)  
   
      An unrecognized bit was set in the options argument.  
   
        PCRE_ERROR_BADMAGIC       (-4)  
   
      PCRE stores a 4-byte "magic number" at the start of the com-  
      piled  code,  to  catch  the  case  when it is passed a junk  
      pointer. This is the error it gives when  the  magic  number  
      isn't present.  
   
        PCRE_ERROR_UNKNOWN_NODE   (-5)  
   
      While running the pattern match, an unknown item was encoun-  
      tered in the compiled pattern. This error could be caused by  
      a bug in PCRE or by overwriting of the compiled pattern.  
   
        PCRE_ERROR_NOMEMORY       (-6)  
   
      If a pattern contains back references, but the ovector  that  
      is  passed  to pcre_exec() is not big enough to remember the  
      referenced substrings, PCRE gets a block of  memory  at  the  
      start  of  matching to use for this purpose. If the call via  
      pcre_malloc() fails, this error  is  given.  The  memory  is  
      freed at the end of matching.  
   
   
   
 EXTRACTING CAPTURED SUBSTRINGS  
      Captured substrings can be accessed directly  by  using  the  
      offsets returned by pcre_exec() in ovector. For convenience,  
      the functions  pcre_copy_substring(),  pcre_get_substring(),  
      and  pcre_get_substring_list()  are  provided for extracting  
      captured  substrings  as  new,   separate,   zero-terminated  
      strings.   A  substring  that  contains  a  binary  zero  is  
      correctly extracted and has a further zero added on the end,  
      but the result does not, of course, function as a C string.  
   
      The first three arguments are the same for all  three  func-  
      tions:  subject  is  the  subject string which has just been  
      successfully matched, ovector is a pointer to the vector  of  
      integer   offsets   that  was  passed  to  pcre_exec(),  and  
      stringcount is the number of substrings that  were  captured  
      by  the  match,  including  the  substring  that matched the  
      entire regular expression. This is  the  value  returned  by  
      pcre_exec  if  it  is  greater  than  zero.  If  pcre_exec()  
      returned zero, indicating that it ran out of space in  ovec-  
      tor,  the  value passed as stringcount should be the size of  
      the vector divided by three.  
   
      The functions pcre_copy_substring() and pcre_get_substring()  
      extract a single substring, whose number is given as string-  
      number. A value of zero extracts the substring that  matched  
      the entire pattern, while higher values extract the captured  
      substrings. For pcre_copy_substring(), the string is  placed  
      in  buffer,  whose  length is given by buffersize, while for  
      pcre_get_substring() a new block of store  is  obtained  via  
      pcre_malloc,  and its address is returned via stringptr. The  
      yield of the function is  the  length  of  the  string,  not  
      including the terminating zero, or one of  
   
        PCRE_ERROR_NOMEMORY       (-6)  
   
      The buffer was too small for pcre_copy_substring(),  or  the  
      attempt to get memory failed for pcre_get_substring().  
   
        PCRE_ERROR_NOSUBSTRING    (-7)  
   
      There is no substring whose number is stringnumber.  
   
      The pcre_get_substring_list() function extracts  all  avail-  
      able  substrings  and builds a list of pointers to them. All  
      this is done in a single block of memory which  is  obtained  
      via pcre_malloc. The address of the memory block is returned  
      via listptr, which is also the start of the list  of  string  
      pointers.  The  end of the list is marked by a NULL pointer.  
      The yield of the function is zero if all went well, or  
   
        PCRE_ERROR_NOMEMORY       (-6)  
   
      if the attempt to get the memory block failed.  
   
      When any of these functions encounter a  substring  that  is  
      unset, which can happen when capturing subpattern number n+1  
      matches some part of the subject, but subpattern n  has  not  
      been  used  at all, they return an empty string. This can be  
      distinguished  from  a  genuine  zero-length  substring   by  
      inspecting the appropriate offset in ovector, which is nega-  
      tive for unset substrings.  
2096    
2097    
2098    NAME
2099           PCRE - Perl-compatible regular expressions
2100    
2101    
2102  LIMITATIONS  PCRE CALLOUTS
2103       There are some size limitations in PCRE but it is hoped that  
2104       they will never in practice be relevant.  The maximum length         int (*pcre_callout)(pcre_callout_block *);
2105       of a compiled pattern is 65539 (sic) bytes.  All  values  in  
2106       repeating  quantifiers must be less than 65536.  The maximum         PCRE provides a feature called "callout", which is a means of temporar-
2107       number of capturing subpatterns is 99.  The  maximum  number         ily passing control to the caller of PCRE  in  the  middle  of  pattern
2108       of  all  parenthesized subpatterns, including capturing sub-         matching.  The  caller of PCRE provides an external function by putting
2109       patterns, assertions, and other types of subpattern, is 200.         its entry point in the global variable pcre_callout. By  default,  this
2110           variable contains NULL, which disables all calling out.
2111       The maximum length of a subject string is the largest  posi-  
2112       tive number that an integer variable can hold. However, PCRE         Within  a  regular  expression,  (?C) indicates the points at which the
2113       uses recursion to handle subpatterns and indefinite  repeti-         external function is to be called.  Different  callout  points  can  be
2114       tion.  This  means  that the available stack space may limit         identified  by  putting  a number less than 256 after the letter C. The
2115       the size of a subject string that can be processed  by  cer-         default value is zero.  For  example,  this  pattern  has  two  callout
2116       tain patterns.         points:
2117    
2118             (?C1)eabc(?C2)def
2119    
2120  DIFFERENCES FROM PERL         If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() is
2121       The differences described here  are  with  respect  to  Perl         called, PCRE automatically  inserts  callouts,  all  with  number  255,
2122       5.005.         before  each  item in the pattern. For example, if PCRE_AUTO_CALLOUT is
2123           used with the pattern
2124       1. By default, a whitespace character is any character  that  
2125       the  C  library  function isspace() recognizes, though it is           A(\d{2}|--)
2126       possible to compile PCRE  with  alternative  character  type  
2127       tables. Normally isspace() matches space, formfeed, newline,         it is processed as if it were
2128       carriage return, horizontal tab, and vertical tab. Perl 5 no  
2129       longer  includes vertical tab in its set of whitespace char-         (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2130       acters. The \v escape that was in the Perl documentation for  
2131       a long time was never in fact recognized. However, the char-         Notice that there is a callout before and after  each  parenthesis  and
2132       acter itself was treated as whitespace at least up to 5.002.         alternation  bar.  Automatic  callouts  can  be  used  for tracking the
2133       In 5.004 and 5.005 it does not match \s.         progress of pattern matching. The pcretest command has an  option  that
2134           sets  automatic callouts; when it is used, the output indicates how the
2135       2. PCRE does  not  allow  repeat  quantifiers  on  lookahead         pattern is matched. This is useful information when you are  trying  to
2136       assertions. Perl permits them, but they do not mean what you         optimize the performance of a particular pattern.
2137       might think. For example, (?!a){3} does not assert that  the  
2138       next  three characters are not "a". It just asserts that the  
2139       next character is not "a" three times.  MISSING CALLOUTS
2140    
2141       3. Capturing subpatterns that occur inside  negative  looka-         You  should  be  aware  that,  because of optimizations in the way PCRE
2142       head  assertions  are  counted,  but  their  entries  in the         matches patterns, callouts sometimes do not happen. For example, if the
2143       offsets vector are never set. Perl sets its numerical  vari-         pattern is
2144       ables  from  any  such  patterns that are matched before the  
2145       assertion fails to match something (thereby succeeding), but           ab(?C4)cd
2146       only  if  the negative lookahead assertion contains just one  
2147       branch.         PCRE knows that any matching string must contain the letter "d". If the
2148           subject string is "abyz", the lack of "d" means that  matching  doesn't
2149       4. Though binary zero characters are supported in  the  sub-         ever  start,  and  the  callout is never reached. However, with "abyd",
2150       ject  string,  they  are  not  allowed  in  a pattern string         though the result is still no match, the callout is obeyed.
2151       because it is passed as a normal  C  string,  terminated  by  
2152       zero. The escape sequence "\0" can be used in the pattern to  
2153       represent a binary zero.  THE CALLOUT INTERFACE
2154    
2155       5. The following Perl escape sequences  are  not  supported:         During matching, when PCRE reaches a callout point, the external  func-
2156       \l,  \u,  \L,  \U,  \E, \Q. In fact these are implemented by         tion  defined by pcre_callout is called (if it is set). This applies to
2157       Perl's general string-handling and are not part of its  pat-         both the pcre_exec() and the pcre_dfa_exec()  matching  functions.  The
2158       tern matching engine.         only  argument  to  the callout function is a pointer to a pcre_callout
2159           block. This structure contains the following fields:
2160       6. The Perl \G assertion is  not  supported  as  it  is  not  
2161       relevant to single pattern matches.           int          version;
2162             int          callout_number;
2163       7. Fairly obviously, PCRE does not support the (?{code}) and           int         *offset_vector;
2164       (?p{code})  constructions. However, there is some experimen-           const char  *subject;
2165       tal support for recursive patterns using the  non-Perl  item           int          subject_length;
2166       (?R).           int          start_match;
2167       8. There are at the time of writing some  oddities  in  Perl           int          current_position;
2168       5.005_02  concerned  with  the  settings of captured strings           int          capture_top;
2169       when part of a pattern is repeated.  For  example,  matching           int          capture_last;
2170       "aba"  against the pattern /^(a(b)?)+$/ sets $2 to the value           void        *callout_data;
2171       "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves  $2           int          pattern_position;
2172       unset.    However,    if   the   pattern   is   changed   to           int          next_item_length;
2173       /^(aa(b(b))?)+$/ then $2 (and $3) are set.  
2174           The version field is an integer containing the version  number  of  the
2175       In Perl 5.004 $2 is set in both cases, and that is also true         block  format. The initial version was 0; the current version is 1. The
2176       of PCRE. If in the future Perl changes to a consistent state         version number will change again in future  if  additional  fields  are
2177       that is different, PCRE may change to follow.         added, but the intention is never to remove any of the existing fields.
2178    
2179       9. Another as yet unresolved discrepancy  is  that  in  Perl         The callout_number field contains the number of the  callout,  as  com-
2180       5.005_02  the  pattern /^(a)?(?(1)a|b)+$/ matches the string         piled  into  the pattern (that is, the number after ?C for manual call-
2181       "a", whereas in PCRE it does not.  However, in both Perl and         outs, and 255 for automatically generated callouts).
2182       PCRE /^(a)?a/ matched against "a" leaves $1 unset.  
2183           The offset_vector field is a pointer to the vector of offsets that  was
2184       10. PCRE  provides  some  extensions  to  the  Perl  regular         passed   by   the   caller  to  pcre_exec()  or  pcre_dfa_exec().  When
2185       expression facilities:         pcre_exec() is used, the contents can be inspected in order to  extract
2186           substrings  that  have  been  matched  so  far,  in the same way as for
2187       (a) Although lookbehind assertions must match  fixed  length         extracting substrings after a match has completed. For  pcre_dfa_exec()
2188       strings,  each  alternative branch of a lookbehind assertion         this field is not useful.
2189       can match a different length of string. Perl 5.005  requires  
2190       them all to have the same length.         The subject and subject_length fields contain copies of the values that
2191           were passed to pcre_exec().
2192       (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is  not  
2193       set,  the  $ meta- character matches only at the very end of         The start_match field contains the offset within the subject  at  which
2194       the string.         the  current match attempt started. If the pattern is not anchored, the
2195           callout function may be called several times from the same point in the
2196       (c) If PCRE_EXTRA is set, a backslash followed by  a  letter         pattern for different starting points in the subject.
2197       with no special meaning is faulted.  
2198           The  current_position  field  contains the offset within the subject of
2199       (d) If PCRE_UNGREEDY is set, the greediness of  the  repeti-         the current match pointer.
2200       tion  quantifiers  is inverted, that is, by default they are  
2201       not greedy, but if followed by a question mark they are.         When the pcre_exec() function is used, the capture_top  field  contains
2202           one  more than the number of the highest numbered captured substring so
2203       (e) PCRE_ANCHORED can be used to force a pattern to be tried         far. If no substrings have been captured, the value of  capture_top  is
2204       only at the start of the subject.         one.  This  is always the case when pcre_dfa_exec() is used, because it
2205           does not support captured substrings.
2206       (f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY  options  
2207       for pcre_exec() have no Perl equivalents.         The capture_last field contains the number of the  most  recently  cap-
2208           tured  substring. If no substrings have been captured, its value is -1.
2209       (g) The (?R) construct allows for recursive pattern matching         This is always the case when pcre_dfa_exec() is used.
2210       (Perl  5.6 can do this using the (?p{code}) construct, which  
2211       PCRE cannot of course support.)         The callout_data field contains a value that is passed  to  pcre_exec()
2212           or  pcre_dfa_exec() specifically so that it can be passed back in call-
2213           outs. It is passed in the pcre_callout field  of  the  pcre_extra  data
2214           structure.  If  no such data was passed, the value of callout_data in a
2215  REGULAR EXPRESSION DETAILS         pcre_callout block is NULL. There is a description  of  the  pcre_extra
2216       The syntax and semantics of  the  regular  expressions  sup-         structure in the pcreapi documentation.
2217       ported  by PCRE are described below. Regular expressions are  
2218       also described in the Perl documentation and in a number  of         The  pattern_position field is present from version 1 of the pcre_call-
2219           out structure. It contains the offset to the next item to be matched in
2220       other  books,  some  of which have copious examples. Jeffrey         the pattern string.
2221       Friedl's  "Mastering  Regular  Expressions",  published   by  
2222       O'Reilly  (ISBN  1-56592-257),  covers them in great detail.         The  next_item_length field is present from version 1 of the pcre_call-
2223       The description here is intended as reference documentation.         out structure. It contains the length of the next item to be matched in
2224           the  pattern  string. When the callout immediately precedes an alterna-
2225       A regular expression is a pattern that is matched against  a         tion bar, a closing parenthesis, or the end of the pattern, the  length
2226       subject string from left to right. Most characters stand for         is  zero.  When the callout precedes an opening parenthesis, the length
2227       themselves in a pattern, and match the corresponding charac-         is that of the entire subpattern.
2228       ters in the subject. As a trivial example, the pattern  
2229           The pattern_position and next_item_length fields are intended  to  help
2230         The quick brown fox         in  distinguishing between different automatic callouts, which all have
2231           the same callout number. However, they are set for all callouts.
2232       matches a portion of a subject string that is  identical  to  
2233       itself.  The  power  of  regular  expressions comes from the  
2234       ability to include alternatives and repetitions in the  pat-  RETURN VALUES
2235       tern.  These  are encoded in the pattern by the use of meta-  
2236       characters, which do not stand for  themselves  but  instead         The external callout function returns an integer to PCRE. If the  value
2237       are interpreted in some special way.         is  zero,  matching  proceeds  as  normal. If the value is greater than
2238           zero, matching fails at the current point, but  the  testing  of  other
2239       There are two different sets of meta-characters: those  that         matching possibilities goes ahead, just as if a lookahead assertion had
2240       are  recognized anywhere in the pattern except within square         failed. If the value is less than zero, the  match  is  abandoned,  and
2241       brackets, and those that are recognized in square  brackets.         pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2242       Outside square brackets, the meta-characters are as follows:  
2243           Negative   values   should   normally   be   chosen  from  the  set  of
2244         \      general escape character with several uses         PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2245         ^      assert start of  subject  (or  line,  in  multiline         dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is
2246       mode)         reserved for use by callout functions; it will never be  used  by  PCRE
2247         $      assert end of subject (or line, in multiline mode)         itself.
2248         .      match any character except newline (by default)  
2249         [      start character class definition  Last updated: 28 February 2005
2250         |      start of alternative branch  Copyright (c) 1997-2005 University of Cambridge.
2251         (      start subpattern  ------------------------------------------------------------------------------
2252         )      end subpattern  
2253         ?      extends the meaning of (  
2254                also 0 or 1 quantifier  PCRECOMPAT(3)                                                    PCRECOMPAT(3)
               also quantifier minimizer  
        *      0 or more quantifier  
        +      1 or more quantifier  
        {      start min/max quantifier  
   
      Part of a pattern that is in square  brackets  is  called  a  
      "character  class".  In  a  character  class  the only meta-  
      characters are:  
   
        \      general escape character  
        ^      negate the class, but only if the first character  
        -      indicates character range  
        ]      terminates the character class  
2255    
      The following sections describe  the  use  of  each  of  the  
      meta-characters.  
2256    
2257    NAME
2258           PCRE - Perl-compatible regular expressions
2259    
2260    
2261    DIFFERENCES BETWEEN PCRE AND PERL
2262    
2263           This  document describes the differences in the ways that PCRE and Perl
2264           handle regular expressions. The differences  described  here  are  with
2265           respect to Perl 5.8.
2266    
2267           1.  PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
2268           of what it does have are given in the section on UTF-8 support  in  the
2269           main pcre page.
2270    
2271           2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2272           permits them, but they do not mean what you might think.  For  example,
2273           (?!a){3} does not assert that the next three characters are not "a". It
2274           just asserts that the next character is not "a" three times.
2275    
2276           3. Capturing subpatterns that occur inside  negative  lookahead  asser-
2277           tions  are  counted,  but their entries in the offsets vector are never
2278           set. Perl sets its numerical variables from any such patterns that  are
2279           matched before the assertion fails to match something (thereby succeed-
2280           ing), but only if the negative lookahead assertion  contains  just  one
2281           branch.
2282    
2283           4.  Though  binary zero characters are supported in the subject string,
2284           they are not allowed in a pattern string because it is passed as a nor-
2285           mal C string, terminated by zero. The escape sequence \0 can be used in
2286           the pattern to represent a binary zero.
2287    
2288           5. The following Perl escape sequences are not supported: \l,  \u,  \L,
2289           \U, and \N. In fact these are implemented by Perl's general string-han-
2290           dling and are not part of its pattern matching engine. If any of  these
2291           are encountered by PCRE, an error is generated.
2292    
2293           6.  The Perl escape sequences \p, \P, and \X are supported only if PCRE
2294           is built with Unicode character property support. The  properties  that
2295           can  be tested with \p and \P are limited to the general category prop-
2296           erties such as Lu and Nd, script names such as Greek or  Han,  and  the
2297           derived properties Any and L&.
2298    
2299           7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2300           ters in between are treated as literals.  This  is  slightly  different
2301           from  Perl  in  that  $  and  @ are also handled as literals inside the
2302           quotes. In Perl, they cause variable interpolation (but of course  PCRE
2303           does not have variables). Note the following examples:
2304    
2305               Pattern            PCRE matches      Perl matches
2306    
2307               \Qabc$xyz\E        abc$xyz           abc followed by the
2308                                                      contents of $xyz
2309               \Qabc\$xyz\E       abc\$xyz          abc\$xyz
2310               \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
2311    
2312           The  \Q...\E  sequence  is recognized both inside and outside character
2313           classes.
2314    
2315           8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
2316           constructions.  However,  there is support for recursive patterns using
2317           the non-Perl items (?R),  (?number),  and  (?P>name).  Also,  the  PCRE
2318           "callout"  feature allows an external function to be called during pat-
2319           tern matching. See the pcrecallout documentation for details.
2320    
2321           9. There are some differences that are concerned with the  settings  of
2322           captured  strings  when  part  of  a  pattern is repeated. For example,
2323           matching "aba" against the  pattern  /^(a(b)?)+$/  in  Perl  leaves  $2
2324           unset, but in PCRE it is set to "b".
2325    
2326           10. PCRE provides some extensions to the Perl regular expression facil-
2327           ities:
2328    
2329           (a) Although lookbehind assertions must  match  fixed  length  strings,
2330           each alternative branch of a lookbehind assertion can match a different
2331           length of string. Perl requires them all to have the same length.
2332    
2333           (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the  $
2334           meta-character matches only at the very end of the string.
2335    
2336           (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
2337           cial meaning is faulted.
2338    
2339           (d) If PCRE_UNGREEDY is set, the greediness of the  repetition  quanti-
2340           fiers is inverted, that is, by default they are not greedy, but if fol-
2341           lowed by a question mark they are.
2342    
2343           (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
2344           tried only at the first matching position in the subject string.
2345    
2346           (f)  The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
2347           TURE options for pcre_exec() have no Perl equivalents.
2348    
2349           (g) The (?R), (?number), and (?P>name) constructs allows for  recursive
2350           pattern  matching  (Perl  can  do  this using the (?p{code}) construct,
2351           which PCRE cannot support.)
2352    
2353           (h) PCRE supports named capturing substrings, using the Python  syntax.
2354    
2355           (i)  PCRE  supports  the  possessive quantifier "++" syntax, taken from
2356           Sun's Java package.
2357    
2358           (j) The (R) condition, for testing recursion, is a PCRE extension.
2359    
2360           (k) The callout facility is PCRE-specific.
2361    
2362           (l) The partial matching facility is PCRE-specific.
2363    
2364           (m) Patterns compiled by PCRE can be saved and re-used at a later time,
2365           even on different hosts that have the other endianness.
2366    
2367           (n)  The  alternative  matching function (pcre_dfa_exec()) matches in a
2368           different way and is not Perl-compatible.
2369    
2370    Last updated: 24 January 2006
2371    Copyright (c) 1997-2006 University of Cambridge.
2372    ------------------------------------------------------------------------------
2373    
2374    
2375    PCREPATTERN(3)                                                  PCREPATTERN(3)
2376    
2377    
2378    NAME
2379           PCRE - Perl-compatible regular expressions
2380    
2381    
2382    PCRE REGULAR EXPRESSION DETAILS
2383    
2384           The  syntax  and semantics of the regular expressions supported by PCRE
2385           are described below. Regular expressions are also described in the Perl
2386           documentation  and  in  a  number  of books, some of which have copious
2387           examples.  Jeffrey Friedl's "Mastering Regular Expressions",  published
2388           by  O'Reilly, covers regular expressions in great detail. This descrip-
2389           tion of PCRE's regular expressions is intended as reference material.
2390    
2391           The original operation of PCRE was on strings of  one-byte  characters.
2392           However,  there is now also support for UTF-8 character strings. To use
2393           this, you must build PCRE to  include  UTF-8  support,  and  then  call
2394           pcre_compile()  with  the  PCRE_UTF8  option.  How this affects pattern
2395           matching is mentioned in several places below. There is also a  summary
2396           of  UTF-8  features  in  the  section on UTF-8 support in the main pcre
2397           page.
2398    
2399           The remainder of this document discusses the  patterns  that  are  sup-
2400           ported  by  PCRE when its main matching function, pcre_exec(), is used.
2401           From  release  6.0,   PCRE   offers   a   second   matching   function,
2402           pcre_dfa_exec(),  which matches using a different algorithm that is not
2403           Perl-compatible. The advantages and disadvantages  of  the  alternative
2404           function, and how it differs from the normal function, are discussed in
2405           the pcrematching page.
2406    
2407           A regular expression is a pattern that is  matched  against  a  subject
2408           string  from  left  to right. Most characters stand for themselves in a
2409           pattern, and match the corresponding characters in the  subject.  As  a
2410           trivial example, the pattern
2411    
2412             The quick brown fox
2413    
2414           matches a portion of a subject string that is identical to itself. When
2415           caseless matching is specified (the PCRE_CASELESS option), letters  are
2416           matched  independently  of case. In UTF-8 mode, PCRE always understands
2417           the concept of case for characters whose values are less than  128,  so
2418           caseless  matching  is always possible. For characters with higher val-
2419           ues, the concept of case is supported if PCRE is compiled with  Unicode
2420           property  support,  but  not  otherwise.   If  you want to use caseless
2421           matching for characters 128 and above, you must  ensure  that  PCRE  is
2422           compiled with Unicode property support as well as with UTF-8 support.
2423    
2424           The  power  of  regular  expressions  comes from the ability to include
2425           alternatives and repetitions in the pattern. These are encoded  in  the
2426           pattern by the use of metacharacters, which do not stand for themselves
2427           but instead are interpreted in some special way.
2428    
2429           There are two different sets of metacharacters: those that  are  recog-
2430           nized  anywhere in the pattern except within square brackets, and those
2431           that are recognized in square brackets. Outside  square  brackets,  the
2432           metacharacters are as follows:
2433    
2434             \      general escape character with several uses
2435             ^      assert start of string (or line, in multiline mode)
2436             $      assert end of string (or line, in multiline mode)
2437             .      match any character except newline (by default)
2438             [      start character class definition
2439             |      start of alternative branch
2440             (      start subpattern
2441             )      end subpattern
2442             ?      extends the meaning of (
2443                    also 0 or 1 quantifier
2444                    also quantifier minimizer
2445             *      0 or more quantifier
2446             +      1 or more quantifier
2447                    also "possessive quantifier"
2448             {      start min/max quantifier
2449    
2450           Part  of  a  pattern  that is in square brackets is called a "character
2451           class". In a character class the only metacharacters are:
2452    
2453             \      general escape character
2454             ^      negate the class, but only if the first character
2455             -      indicates character range
2456             [      POSIX character class (only if followed by POSIX
2457                      syntax)
2458             ]      terminates the character class
2459    
2460           The following sections describe the use of each of the  metacharacters.
2461    
2462    
2463  BACKSLASH  BACKSLASH
      The backslash character has several uses. Firstly, if it  is  
      followed  by  a  non-alphameric character, it takes away any  
      special  meaning  that  character  may  have.  This  use  of  
      backslash  as  an  escape  character applies both inside and  
      outside character classes.  
   
      For example, if you want to match a "*" character, you write  
      "\*" in the pattern. This applies whether or not the follow-  
      ing character would otherwise  be  interpreted  as  a  meta-  
      character,  so it is always safe to precede a non-alphameric  
      with "\" to specify that it stands for itself.  In  particu-  
      lar, if you want to match a backslash, you write "\\".  
   
      If a pattern is compiled with the PCRE_EXTENDED option, whi-  
      tespace in the pattern (other than in a character class) and  
      characters between a "#" outside a character class  and  the  
      next  newline  character  are ignored. An escaping backslash  
      can be used to include a whitespace or "#" character as part  
      of the pattern.  
   
      A second use of backslash provides a way  of  encoding  non-  
      printing  characters  in patterns in a visible manner. There  
      is no restriction on the appearance of non-printing  charac-  
      ters,  apart from the binary zero that terminates a pattern,  
      but when a pattern is being prepared by text editing, it  is  
      usually  easier to use one of the following escape sequences  
      than the binary character it represents:  
   
        \a     alarm, that is, the BEL character (hex 07)  
        \cx    "control-x", where x is any character  
        \e     escape (hex 1B)  
        \f     formfeed (hex 0C)  
        \n     newline (hex 0A)  
        \r     carriage return (hex 0D)  
        \t     tab (hex 09)  
        \xhh   character with hex code hh  
        \ddd   character with octal code ddd, or backreference  
   
      The precise effect of "\cx" is as follows: if "x" is a lower  
      case  letter,  it  is converted to upper case. Then bit 6 of  
      the character (hex 40) is inverted.  Thus "\cz" becomes  hex  
      1A, but "\c{" becomes hex 3B, while "\c;" becomes hex 7B.  
   
      After "\x", up to two hexadecimal digits are  read  (letters  
      can be in upper or lower case).  
   
      After "\0" up to two further octal digits are read. In  both  
      cases,  if  there are fewer than two digits, just those that  
      are present are used. Thus the sequence "\0\x\07"  specifies  
      two binary zeros followed by a BEL character.  Make sure you  
      supply two digits after the initial zero  if  the  character  
      that follows is itself an octal digit.  
   
      The handling of a backslash followed by a digit other than 0  
      is  complicated.   Outside  a character class, PCRE reads it  
      and any following digits as a decimal number. If the  number  
      is  less  than  10, or if there have been at least that many  
      previous capturing left parentheses in the  expression,  the  
      entire  sequence is taken as a back reference. A description  
      of how this works is given later, following  the  discussion  
      of parenthesized subpatterns.  
   
      Inside a character  class,  or  if  the  decimal  number  is  
      greater  than  9 and there have not been that many capturing  
      subpatterns, PCRE re-reads up to three octal digits  follow-  
      ing  the  backslash,  and  generates  a single byte from the  
      least significant 8 bits of the value. Any subsequent digits  
      stand for themselves.  For example:  
   
        \040   is another way of writing a space  
        \40    is the same, provided there are fewer than 40  
                  previous capturing subpatterns  
        \7     is always a back reference  
        \11    might be a back reference, or another way of  
                  writing a tab  
        \011   is always a tab  
        \0113  is a tab followed by the character "3"  
        \113   is the character with octal code 113 (since there  
                  can be no more than 99 back references)  
        \377   is a byte consisting entirely of 1 bits  
        \81    is either a back reference, or a binary zero  
                  followed by the two characters "8" and "1"  
   
      Note that octal values of 100 or greater must not be  intro-  
      duced  by  a  leading zero, because no more than three octal  
      digits are ever read.  
   
      All the sequences that define a single  byte  value  can  be  
      used both inside and outside character classes. In addition,  
      inside a character class, the sequence "\b"  is  interpreted  
      as  the  backspace  character  (hex 08). Outside a character  
      class it has a different meaning (see below).  
   
      The third use of backslash is for specifying generic charac-  
      ter types:  
   
        \d     any decimal digit  
        \D     any character that is not a decimal digit  
        \s     any whitespace character  
        \S     any character that is not a whitespace character  
        \w     any "word" character  
        \W     any "non-word" character  
   
      Each pair of escape sequences partitions the complete set of  
      characters  into  two  disjoint  sets.  Any  given character  
      matches one, and only one, of each pair.  
   
      A "word" character is any letter or digit or the  underscore  
      character,  that  is,  any  character which can be part of a  
      Perl "word". The definition of letters and  digits  is  con-  
      trolled  by PCRE's character tables, and may vary if locale-  
      specific matching is  taking  place  (see  "Locale  support"  
      above). For example, in the "fr" (French) locale, some char-  
      acter codes greater than 128 are used for accented  letters,  
      and these are matched by \w.  
   
      These character type sequences can appear  both  inside  and  
      outside  character classes. They each match one character of  
      the appropriate type. If the current matching  point  is  at  
      the end of the subject string, all of them fail, since there  
      is no character to match.  
   
      The fourth use of backslash is  for  certain  simple  asser-  
      tions. An assertion specifies a condition that has to be met  
      at a particular point in  a  match,  without  consuming  any  
      characters  from  the subject string. The use of subpatterns  
      for more complicated  assertions  is  described  below.  The  
      backslashed assertions are  
   
        \b     word boundary  
        \B     not a word boundary  
        \A     start of subject (independent of multiline mode)  
        \Z     end of subject or newline at  end  (independent  of  
      multiline mode)  
        \z     end of subject (independent of multiline mode)  
   
      These assertions may not appear in  character  classes  (but  
      note that "\b" has a different meaning, namely the backspace  
      character, inside a character class).  
   
      A word boundary is a position in the  subject  string  where  
      the current character and the previous character do not both  
      match \w or \W (i.e. one matches \w and  the  other  matches  
      \W),  or the start or end of the string if the first or last  
      character matches \w, respectively.  
   
      The \A, \Z, and \z assertions differ  from  the  traditional  
      circumflex  and  dollar  (described below) in that they only  
      ever match at the very start and end of the subject  string,  
      whatever  options  are  set.  They  are  not affected by the  
      PCRE_NOTBOL or PCRE_NOTEOL options. If the startoffset argu-  
      ment  of  pcre_exec()  is  non-zero, \A can never match. The  
      difference between \Z and \z is that  \Z  matches  before  a  
      newline  that is the last character of the string as well as  
      at the end of the string, whereas \z  matches  only  at  the  
      end.  
2464    
2465           The backslash character has several uses. Firstly, if it is followed by
2466           a non-alphanumeric character, it takes away any  special  meaning  that
2467           character  may  have.  This  use  of  backslash  as an escape character
2468           applies both inside and outside character classes.
2469    
2470           For example, if you want to match a * character, you write  \*  in  the
2471           pattern.   This  escaping  action  applies whether or not the following
2472           character would otherwise be interpreted as a metacharacter, so  it  is
2473           always  safe  to  precede  a non-alphanumeric with backslash to specify
2474           that it stands for itself. In particular, if you want to match a  back-
2475           slash, you write \\.
2476    
2477           If  a  pattern is compiled with the PCRE_EXTENDED option, whitespace in
2478           the pattern (other than in a character class) and characters between  a
2479           # outside a character class and the next newline character are ignored.
2480           An escaping backslash can be used to include a whitespace or #  charac-
2481           ter as part of the pattern.
2482    
2483           If  you  want  to remove the special meaning from a sequence of charac-
2484           ters, you can do so by putting them between \Q and \E. This is  differ-
2485           ent  from  Perl  in  that  $  and  @ are handled as literals in \Q...\E
2486           sequences in PCRE, whereas in Perl, $ and @ cause  variable  interpola-
2487           tion. Note the following examples:
2488    
2489             Pattern            PCRE matches   Perl matches
2490    
2491             \Qabc$xyz\E        abc$xyz        abc followed by the
2492                                                 contents of $xyz
2493             \Qabc\$xyz\E       abc\$xyz       abc\$xyz
2494             \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
2495    
2496           The  \Q...\E  sequence  is recognized both inside and outside character
2497           classes.
2498    
2499       Non-printing characters
2500    
2501           A second use of backslash provides a way of encoding non-printing char-
2502           acters  in patterns in a visible manner. There is no restriction on the
2503           appearance of non-printing characters, apart from the binary zero  that
2504           terminates  a  pattern,  but  when  a pattern is being prepared by text
2505           editing, it is usually easier  to  use  one  of  the  following  escape
2506           sequences than the binary character it represents:
2507    
2508             \a        alarm, that is, the BEL character (hex 07)
2509             \cx       "control-x", where x is any character
2510             \e        escape (hex 1B)
2511             \f        formfeed (hex 0C)
2512             \n        newline (hex 0A)
2513             \r        carriage return (hex 0D)
2514             \t        tab (hex 09)
2515             \ddd      character with octal code ddd, or backreference
2516             \xhh      character with hex code hh
2517             \x{hhh..} character with hex code hhh..
2518    
2519           The  precise  effect of \cx is as follows: if x is a lower case letter,
2520           it is converted to upper case. Then bit 6 of the character (hex 40)  is
2521           inverted.   Thus  \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
2522           becomes hex 7B.
2523    
2524           After \x, from zero to two hexadecimal digits are read (letters can  be
2525           in  upper  or  lower case). Any number of hexadecimal digits may appear
2526           between \x{ and }, but the value of the character  code  must  be  less
2527           than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode (that is,
2528           the maximum hexadecimal value is 7FFFFFFF). If  characters  other  than
2529           hexadecimal  digits  appear between \x{ and }, or if there is no termi-
2530           nating }, this form of escape is not recognized.  Instead, the  initial
2531           \x will be interpreted as a basic hexadecimal escape, with no following
2532           digits, giving a character whose value is zero.
2533    
2534           Characters whose value is less than 256 can be defined by either of the
2535           two  syntaxes  for  \x. There is no difference in the way they are han-
2536           dled. For example, \xdc is exactly the same as \x{dc}.
2537    
2538           After \0 up to two further octal digits are read.  In  both  cases,  if
2539           there  are fewer than two digits, just those that are present are used.
2540           Thus the sequence \0\x\07 specifies two binary zeros followed by a  BEL
2541           character  (code  value  7).  Make sure you supply two digits after the
2542           initial zero if the pattern character that follows is itself  an  octal
2543           digit.
2544    
2545           The handling of a backslash followed by a digit other than 0 is compli-
2546           cated.  Outside a character class, PCRE reads it and any following dig-
2547           its  as  a  decimal  number. If the number is less than 10, or if there
2548           have been at least that many previous capturing left parentheses in the
2549           expression,  the  entire  sequence  is  taken  as  a  back reference. A
2550           description of how this works is given later, following the  discussion
2551           of parenthesized subpatterns.
2552    
2553           Inside  a  character  class, or if the decimal number is greater than 9
2554           and there have not been that many capturing subpatterns, PCRE  re-reads
2555           up  to three octal digits following the backslash, and generates a sin-
2556           gle byte from the least significant 8 bits of the value. Any subsequent
2557           digits stand for themselves.  For example:
2558    
2559             \040   is another way of writing a space
2560             \40    is the same, provided there are fewer than 40
2561                       previous capturing subpatterns
2562             \7     is always a back reference
2563             \11    might be a back reference, or another way of
2564                       writing a tab
2565             \011   is always a tab
2566             \0113  is a tab followed by the character "3"
2567             \113   might be a back reference, otherwise the
2568                       character with octal code 113
2569             \377   might be a back reference, otherwise
2570                       the byte consisting entirely of 1 bits
2571             \81    is either a back reference, or a binary zero
2572                       followed by the two characters "8" and "1"
2573    
2574           Note  that  octal  values of 100 or greater must not be introduced by a
2575           leading zero, because no more than three octal digits are ever read.
2576    
2577           All the sequences that define a single byte value  or  a  single  UTF-8
2578           character (in UTF-8 mode) can be used both inside and outside character
2579           classes. In addition, inside a character  class,  the  sequence  \b  is
2580           interpreted as the backspace character (hex 08), and the sequence \X is
2581           interpreted as the character "X".  Outside  a  character  class,  these
2582           sequences have different meanings (see below).
2583    
2584       Generic character types
2585    
2586           The  third  use of backslash is for specifying generic character types.
2587           The following are always recognized:
2588    
2589             \d     any decimal digit
2590             \D     any character that is not a decimal digit
2591             \s     any whitespace character
2592             \S     any character that is not a whitespace character
2593             \w     any "word" character
2594             \W     any "non-word" character
2595    
2596           Each pair of escape sequences partitions the complete set of characters
2597           into  two disjoint sets. Any given character matches one, and only one,
2598           of each pair.
2599    
2600           These character type sequences can appear both inside and outside char-
2601           acter  classes.  They each match one character of the appropriate type.
2602           If the current matching point is at the end of the subject string,  all
2603           of them fail, since there is no character to match.
2604    
2605           For  compatibility  with Perl, \s does not match the VT character (code
2606           11).  This makes it different from the the POSIX "space" class. The  \s
2607           characters are HT (9), LF (10), FF (12), CR (13), and space (32).
2608    
2609           A "word" character is an underscore or any character less than 256 that
2610           is a letter or digit. The definition of  letters  and  digits  is  con-
2611           trolled  by PCRE's low-valued character tables, and may vary if locale-
2612           specific matching is taking place (see "Locale support" in the  pcreapi
2613           page).  For  example,  in  the  "fr_FR" (French) locale, some character
2614           codes greater than 128 are used for accented  letters,  and  these  are
2615           matched by \w.
2616    
2617           In  UTF-8 mode, characters with values greater than 128 never match \d,
2618           \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
2619           code  character  property support is available. The use of locales with
2620           Unicode is discouraged.
2621    
2622       Unicode character properties
2623    
2624           When PCRE is built with Unicode character property support, three addi-
2625           tional  escape  sequences  to  match character properties are available
2626           when UTF-8 mode is selected. They are:
2627    
2628             \p{xx}   a character with the xx property
2629             \P{xx}   a character without the xx property
2630             \X       an extended Unicode sequence
2631    
2632           The property names represented by xx above are limited to  the  Unicode
2633           script names, the general category properties, and "Any", which matches
2634           any character (including newline). Other properties such as "InMusical-
2635           Symbols"  are  not  currently supported by PCRE. Note that \P{Any} does
2636           not match any characters, so always causes a match failure.
2637    
2638           Sets of Unicode characters are defined as belonging to certain scripts.
2639           A  character from one of these sets can be matched using a script name.
2640           For example:
2641    
2642             \p{Greek}
2643             \P{Han}
2644    
2645           Those that are not part of an identified script are lumped together  as
2646           "Common". The current list of scripts is:
2647    
2648           Arabic,  Armenian,  Bengali,  Bopomofo, Braille, Buginese, Buhid, Cana-
2649           dian_Aboriginal, Cherokee, Common, Coptic, Cypriot, Cyrillic,  Deseret,
2650           Devanagari,  Ethiopic,  Georgian,  Glagolitic, Gothic, Greek, Gujarati,
2651           Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana,  Inherited,  Kannada,
2652           Katakana,  Kharoshthi,  Khmer,  Lao, Latin, Limbu, Linear_B, Malayalam,
2653           Mongolian, Myanmar, New_Tai_Lue, Ogham, Old_Italic, Old_Persian, Oriya,
2654           Osmanya,  Runic,  Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tag-
2655           banwa,  Tai_Le,  Tamil,  Telugu,  Thaana,  Thai,   Tibetan,   Tifinagh,
2656           Ugaritic, Yi.
2657    
2658           Each  character has exactly one general category property, specified by
2659           a two-letter abbreviation. For compatibility with Perl, negation can be
2660           specified  by  including a circumflex between the opening brace and the
2661           property name. For example, \p{^Lu} is the same as \P{Lu}.
2662    
2663           If only one letter is specified with \p or \P, it includes all the gen-
2664           eral  category properties that start with that letter. In this case, in
2665           the absence of negation, the curly brackets in the escape sequence  are
2666           optional; these two examples have the same effect:
2667    
2668             \p{L}
2669             \pL
2670    
2671           The following general category property codes are supported:
2672    
2673             C     Other
2674             Cc    Control
2675             Cf    Format
2676             Cn    Unassigned
2677             Co    Private use
2678             Cs    Surrogate
2679    
2680             L     Letter
2681             Ll    Lower case letter
2682             Lm    Modifier letter
2683             Lo    Other letter
2684             Lt    Title case letter
2685             Lu    Upper case letter
2686    
2687             M     Mark
2688             Mc    Spacing mark
2689             Me    Enclosing mark
2690             Mn    Non-spacing mark
2691    
2692             N     Number
2693             Nd    Decimal number
2694             Nl    Letter number
2695             No    Other number
2696    
2697             P     Punctuation
2698             Pc    Connector punctuation
2699             Pd    Dash punctuation
2700             Pe    Close punctuation
2701             Pf    Final punctuation
2702             Pi    Initial punctuation
2703             Po    Other punctuation
2704             Ps    Open punctuation
2705    
2706             S     Symbol
2707             Sc    Currency symbol
2708             Sk    Modifier symbol
2709             Sm    Mathematical symbol
2710             So    Other symbol
2711    
2712             Z     Separator
2713             Zl    Line separator
2714             Zp    Paragraph separator
2715             Zs    Space separator
2716    
2717           The  special property L& is also supported: it matches a character that
2718           has the Lu, Ll, or Lt property, in other words, a letter  that  is  not
2719           classified as a modifier or "other".
2720    
2721           The  long  synonyms  for  these  properties that Perl supports (such as
2722           \p{Letter}) are not supported by PCRE. Nor is is  permitted  to  prefix
2723           any of these properties with "Is".
2724    
2725           No character that is in the Unicode table has the Cn (unassigned) prop-
2726           erty.  Instead, this property is assumed for any code point that is not
2727           in the Unicode table.
2728    
2729           Specifying  caseless  matching  does not affect these escape sequences.
2730           For example, \p{Lu} always matches only upper case letters.
2731    
2732           The \X escape matches any number of Unicode  characters  that  form  an
2733           extended Unicode sequence. \X is equivalent to
2734    
2735             (?>\PM\pM*)
2736    
2737           That  is,  it matches a character without the "mark" property, followed
2738           by zero or more characters with the "mark"  property,  and  treats  the
2739           sequence  as  an  atomic group (see below).  Characters with the "mark"
2740           property are typically accents that affect the preceding character.
2741    
2742           Matching characters by Unicode property is not fast, because  PCRE  has
2743           to  search  a  structure  that  contains data for over fifteen thousand
2744           characters. That is why the traditional escape sequences such as \d and
2745           \w do not use Unicode properties in PCRE.
2746    
2747       Simple assertions
2748    
2749           The fourth use of backslash is for certain simple assertions. An asser-
2750           tion specifies a condition that has to be met at a particular point  in
2751           a  match, without consuming any characters from the subject string. The
2752           use of subpatterns for more complicated assertions is described  below.
2753           The backslashed assertions are:
2754    
2755             \b     matches at a word boundary
2756             \B     matches when not at a word boundary
2757             \A     matches at start of subject
2758             \Z     matches at end of subject or before newline at end
2759             \z     matches at end of subject
2760             \G     matches at first matching position in subject
2761    
2762           These  assertions may not appear in character classes (but note that \b
2763           has a different meaning, namely the backspace character, inside a char-
2764           acter class).
2765    
2766           A  word  boundary is a position in the subject string where the current
2767           character and the previous character do not both match \w or  \W  (i.e.
2768           one  matches  \w  and the other matches \W), or the start or end of the
2769           string if the first or last character matches \w, respectively.
2770    
2771           The \A, \Z, and \z assertions differ from  the  traditional  circumflex
2772           and dollar (described in the next section) in that they only ever match
2773           at the very start and end of the subject string, whatever  options  are
2774           set.  Thus,  they are independent of multiline mode. These three asser-
2775           tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
2776           affect  only the behaviour of the circumflex and dollar metacharacters.
2777           However, if the startoffset argument of pcre_exec() is non-zero,  indi-
2778           cating that matching is to start at a point other than the beginning of
2779           the subject, \A can never match. The difference between \Z  and  \z  is
2780           that  \Z  matches  before  a  newline that is the last character of the
2781           string as well as at the end of the string, whereas \z matches only  at
2782           the end.
2783    
2784           The  \G assertion is true only when the current matching position is at
2785           the start point of the match, as specified by the startoffset  argument
2786           of  pcre_exec().  It  differs  from \A when the value of startoffset is
2787           non-zero. By calling pcre_exec() multiple times with appropriate  argu-
2788           ments, you can mimic Perl's /g option, and it is in this kind of imple-
2789           mentation where \G can be useful.
2790    
2791           Note, however, that PCRE's interpretation of \G, as the  start  of  the
2792           current match, is subtly different from Perl's, which defines it as the
2793           end of the previous match. In Perl, these can  be  different  when  the
2794           previously  matched  string was empty. Because PCRE does just one match
2795           at a time, it cannot reproduce this behaviour.
2796    
2797           If all the alternatives of a pattern begin with \G, the  expression  is
2798           anchored to the starting match position, and the "anchored" flag is set
2799           in the compiled regular expression.
2800    
2801    
2802  CIRCUMFLEX AND DOLLAR  CIRCUMFLEX AND DOLLAR
      Outside a character class, in the default matching mode, the  
      circumflex  character  is an assertion which is true only if  
      the current matching point is at the start  of  the  subject  
      string.  If  the startoffset argument of pcre_exec() is non-  
      zero, circumflex can never match. Inside a character  class,  
      circumflex has an entirely different meaning (see below).  
   
      Circumflex need not be the first character of the pattern if  
      a  number of alternatives are involved, but it should be the  
      first thing in each alternative in which it appears  if  the  
      pattern is ever to match that branch. If all possible alter-  
      natives start with a circumflex, that is, if the pattern  is  
      constrained to match only at the start of the subject, it is  
      said to be an "anchored" pattern. (There are also other con-  
      structs that can cause a pattern to be anchored.)  
   
      A dollar character is an assertion which is true only if the  
      current  matching point is at the end of the subject string,  
      or immediately before a newline character that is  the  last  
      character in the string (by default). Dollar need not be the  
      last character of the pattern if a  number  of  alternatives  
      are  involved,  but it should be the last item in any branch  
      in which it appears.  Dollar has no  special  meaning  in  a  
      character class.  
   
      The meaning of dollar can be changed so that it matches only  
      at   the   very   end   of   the   string,  by  setting  the  
      PCRE_DOLLAR_ENDONLY option at compile or matching time. This  
      does not affect the \Z assertion.  
   
      The meanings of the circumflex  and  dollar  characters  are  
      changed  if  the  PCRE_MULTILINE option is set. When this is  
      the case,  they  match  immediately  after  and  immediately  
      before an internal "\n" character, respectively, in addition  
      to matching at the start and end of the subject string.  For  
      example,  the  pattern  /^abc$/  matches  the subject string  
      "def\nabc" in multiline  mode,  but  not  otherwise.  Conse-  
      quently,  patterns  that  are  anchored  in single line mode  
      because all branches start with "^" are not anchored in mul-  
      tiline mode, and a match for circumflex is possible when the  
      startoffset  argument  of  pcre_exec()  is   non-zero.   The  
      PCRE_DOLLAR_ENDONLY  option  is ignored if PCRE_MULTILINE is  
      set.  
   
      Note that the sequences \A, \Z, and \z can be used to  match  
      the  start  and end of the subject in both modes, and if all  
      branches of a pattern start with \A is it  always  anchored,  
      whether PCRE_MULTILINE is set or not.  
2803    
2804           Outside a character class, in the default matching mode, the circumflex
2805           character  is  an  assertion  that is true only if the current matching
2806           point is at the start of the subject string. If the  startoffset  argu-
2807           ment  of  pcre_exec()  is  non-zero,  circumflex can never match if the
2808           PCRE_MULTILINE option is unset. Inside a  character  class,  circumflex
2809           has an entirely different meaning (see below).
2810    
2811           Circumflex  need  not be the first character of the pattern if a number
2812           of alternatives are involved, but it should be the first thing in  each
2813           alternative  in  which  it appears if the pattern is ever to match that
2814           branch. If all possible alternatives start with a circumflex, that  is,
2815           if  the  pattern  is constrained to match only at the start of the sub-
2816           ject, it is said to be an "anchored" pattern.  (There  are  also  other
2817           constructs that can cause a pattern to be anchored.)
2818    
2819           A  dollar  character  is  an assertion that is true only if the current
2820           matching point is at the end of  the  subject  string,  or  immediately
2821           before a newline character that is the last character in the string (by
2822           default). Dollar need not be the last character of  the  pattern  if  a
2823           number  of alternatives are involved, but it should be the last item in
2824           any branch in which it appears.  Dollar has no  special  meaning  in  a
2825           character class.
2826    
2827           The  meaning  of  dollar  can be changed so that it matches only at the
2828           very end of the string, by setting the  PCRE_DOLLAR_ENDONLY  option  at
2829           compile time. This does not affect the \Z assertion.
2830    
2831           The meanings of the circumflex and dollar characters are changed if the
2832           PCRE_MULTILINE option is set. When this is the case, they match immedi-
2833           ately  after  and  immediately  before  an  internal newline character,
2834           respectively, in addition to matching at the start and end of the  sub-
2835           ject  string.  For  example,  the  pattern  /^abc$/ matches the subject
2836           string "def\nabc" (where \n represents a newline character)  in  multi-
2837           line mode, but not otherwise.  Consequently, patterns that are anchored
2838           in single line mode because all branches start with ^ are not  anchored
2839           in  multiline  mode,  and  a  match for circumflex is possible when the
2840           startoffset  argument  of  pcre_exec()  is  non-zero.   The   PCRE_DOL-
2841           LAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
2842    
2843           Note  that  the sequences \A, \Z, and \z can be used to match the start
2844           and end of the subject in both modes, and if all branches of a  pattern
2845           start  with  \A it is always anchored, whether PCRE_MULTILINE is set or
2846           not.
2847    
2848    
2849  FULL STOP (PERIOD, DOT)  FULL STOP (PERIOD, DOT)
      Outside a character class, a dot in the pattern matches  any  
      one character in the subject, including a non-printing char-  
      acter, but not (by default)  newline.   If  the  PCRE_DOTALL  
      option  is set, dots match newlines as well. The handling of  
      dot is entirely independent of the  handling  of  circumflex  
      and  dollar,  the  only  relationship  being  that they both  
      involve newline characters. Dot has no special meaning in  a  
      character class.  
   
   
   
 SQUARE BRACKETS  
      An opening square bracket introduces a character class, ter-  
      minated  by  a  closing  square  bracket.  A  closing square  
      bracket on its own is  not  special.  If  a  closing  square  
      bracket  is  required as a member of the class, it should be  
      the first data character in the class (after an initial cir-  
      cumflex, if present) or escaped with a backslash.  
   
      A character class matches a single character in the subject;  
      the  character  must  be in the set of characters defined by  
      the class, unless the first character in the class is a cir-  
      cumflex,  in which case the subject character must not be in  
      the set defined by the class. If a  circumflex  is  actually  
      required  as  a  member  of  the class, ensure it is not the  
      first character, or escape it with a backslash.  
   
      For example, the character class [aeiou] matches  any  lower  
      case vowel, while [^aeiou] matches any character that is not  
      a lower case vowel. Note that a circumflex is  just  a  con-  
      venient  notation for specifying the characters which are in  
      the class by enumerating those that are not. It  is  not  an  
      assertion:  it  still  consumes a character from the subject  
      string, and fails if the current pointer is at  the  end  of  
      the string.  
   
      When caseless matching  is  set,  any  letters  in  a  class  
      represent  both their upper case and lower case versions, so  
      for example, a caseless [aeiou] matches "A" as well as  "a",  
      and  a caseless [^aeiou] does not match "A", whereas a case-  
      ful version would.  
   
      The newline character is never treated in any special way in  
      character  classes,  whatever the setting of the PCRE_DOTALL  
      or PCRE_MULTILINE options is. A  class  such  as  [^a]  will  
      always match a newline.  
   
      The minus (hyphen) character can be used to specify a  range  
      of  characters  in  a  character  class.  For example, [d-m]  
      matches any letter between d and m, inclusive.  If  a  minus  
      character  is required in a class, it must be escaped with a  
      backslash or appear in a position where it cannot be  inter-  
      preted as indicating a range, typically as the first or last  
      character in the class.  
   
      It is not possible to have the literal character "]" as  the  
      end  character  of  a  range.  A  pattern such as [W-]46] is  
      interpreted as a class of two characters ("W" and "-")  fol-  
      lowed by a literal string "46]", so it would match "W46]" or  
      "-46]". However, if the "]" is escaped with a  backslash  it  
      is  interpreted  as  the end of range, so [W-\]46] is inter-  
      preted as a single class containing a range followed by  two  
      separate characters. The octal or hexadecimal representation  
      of "]" can also be used to end a range.  
   
      Ranges operate in ASCII collating sequence. They can also be  
      used  for  characters  specified  numerically,  for  example  
      [\000-\037]. If a range that includes letters is  used  when  
      caseless  matching  is set, it matches the letters in either  
      case. For example, [W-c] is equivalent  to  [][\^_`wxyzabc],  
      matched  caselessly,  and  if  character tables for the "fr"  
      locale are in use, [\xc8-\xcb] matches accented E characters  
      in both cases.  
   
      The character types \d, \D, \s, \S,  \w,  and  \W  may  also  
      appear  in  a  character  class, and add the characters that  
      they match to the class. For example, [\dABCDEF] matches any  
      hexadecimal  digit.  A  circumflex  can conveniently be used  
      with the upper case character types to specify a  more  res-  
      tricted set of characters than the matching lower case type.  
      For example, the class [^\W_] matches any letter  or  digit,  
      but not underscore.  
   
      All non-alphameric characters other than \,  -,  ^  (at  the  
      start)  and  the  terminating ] are non-special in character  
      classes, but it does no harm if they are escaped.  
2850    
2851           Outside a character class, a dot in the pattern matches any one charac-
2852           ter  in  the  subject,  including a non-printing character, but not (by
2853           default) newline.  In UTF-8 mode, a dot matches  any  UTF-8  character,
2854           which might be more than one byte long, except (by default) newline. If
2855           the PCRE_DOTALL option is set, dots match newlines as  well.  The  han-
2856           dling  of dot is entirely independent of the handling of circumflex and
2857           dollar, the only relationship being  that  they  both  involve  newline
2858           characters. Dot has no special meaning in a character class.
2859    
2860    
2861    MATCHING A SINGLE BYTE
2862    
2863           Outside a character class, the escape sequence \C matches any one byte,
2864           both in and out of UTF-8 mode. Unlike a dot, it can  match  a  newline.
2865           The  feature  is provided in Perl in order to match individual bytes in
2866           UTF-8 mode. Because it  breaks  up  UTF-8  characters  into  individual
2867           bytes,  what remains in the string may be a malformed UTF-8 string. For
2868           this reason, the \C escape sequence is best avoided.
2869    
2870           PCRE does not allow \C to appear in  lookbehind  assertions  (described
2871           below),  because  in UTF-8 mode this would make it impossible to calcu-
2872           late the length of the lookbehind.
2873