/[pcre]/code/trunk/doc/pcre.txt
ViewVC logotype

Diff of /code/trunk/doc/pcre.txt

Parent Directory Parent Directory | Revision Log Revision Log | View Patch Patch

revision 53 by nigel, Sat Feb 24 21:39:42 2007 UTC revision 75 by nigel, Sat Feb 24 21:40:37 2007 UTC
# Line 1  Line 1 
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    PCRE(3)                                                                PCRE(3)
10    
11    
12    
13  NAME  NAME
14       pcre - Perl-compatible regular expressions.         PCRE - Perl-compatible regular expressions
15    
16    INTRODUCTION
17    
18           The  PCRE  library is a set of functions that implement regular expres-
19           sion pattern matching using the same syntax and semantics as Perl, with
20           just  a  few  differences.  The current implementation of PCRE (release
21           5.x) corresponds approximately with Perl  5.8,  including  support  for
22           UTF-8 encoded strings and Unicode general category properties. However,
23           this support has to be explicitly enabled; it is not the default.
24    
25           PCRE is written in C and released as a C library. A  number  of  people
26           have  written  wrappers and interfaces of various kinds. A C++ class is
27           included in these contributions, which can  be  found  in  the  Contrib
28           directory at the primary FTP site, which is:
29    
30           ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
31    
32           Details  of  exactly which Perl regular expression features are and are
33           not supported by PCRE are given in separate documents. See the pcrepat-
34           tern and pcrecompat pages.
35    
36           Some  features  of  PCRE can be included, excluded, or changed when the
37           library is built. The pcre_config() function makes it  possible  for  a
38           client  to  discover  which  features are available. The features them-
39           selves are described in the pcrebuild page. Documentation about  build-
40           ing  PCRE for various operating systems can be found in the README file
41           in the source distribution.
42    
43    
44    USER DOCUMENTATION
45    
46           The user documentation for PCRE comprises a number  of  different  sec-
47           tions.  In the "man" format, each of these is a separate "man page". In
48           the HTML format, each is a separate page, linked from the  index  page.
49           In  the  plain text format, all the sections are concatenated, for ease
50           of searching. The sections are as follows:
51    
52             pcre              this document
53             pcreapi           details of PCRE's native API
54             pcrebuild         options for building PCRE
55             pcrecallout       details of the callout feature
56             pcrecompat        discussion of Perl compatibility
57             pcregrep          description of the pcregrep command
58             pcrepartial       details of the partial matching facility
59             pcrepattern       syntax and semantics of supported
60                                 regular expressions
61             pcreperform       discussion of performance issues
62             pcreposix         the POSIX-compatible API
63             pcreprecompile    details of saving and re-using precompiled patterns
64             pcresample        discussion of the sample program
65             pcretest          description of the pcretest testing command
66    
67  SYNOPSIS         In  addition,  in the "man" and HTML formats, there is a short page for
68       #include <pcre.h>         each library function, listing its arguments and results.
69    
      pcre *pcre_compile(const char *pattern, int options,  
           const char **errptr, int *erroffset,  
           const unsigned char *tableptr);  
70    
71       pcre_extra *pcre_study(const pcre *code, int options,  LIMITATIONS
           const char **errptr);  
72    
73       int pcre_exec(const pcre *code, const pcre_extra *extra,         There are some size limitations in PCRE but it is hoped that they  will
74            const char *subject, int length, int startoffset,         never in practice be relevant.
           int options, int *ovector, int ovecsize);  
75    
76       int pcre_copy_substring(const char *subject, int *ovector,         The  maximum  length of a compiled pattern is 65539 (sic) bytes if PCRE
77            int stringcount, int stringnumber, char *buffer,         is compiled with the default internal linkage size of 2. If you want to
78            int buffersize);         process  regular  expressions  that are truly enormous, you can compile
79           PCRE with an internal linkage size of 3 or 4 (see the  README  file  in
80           the  source  distribution and the pcrebuild documentation for details).
81           In these cases the limit is substantially larger.  However,  the  speed
82           of execution will be slower.
83    
84           All values in repeating quantifiers must be less than 65536.  The maxi-
85           mum number of capturing subpatterns is 65535.
86    
87           There is no limit to the number of non-capturing subpatterns,  but  the
88           maximum  depth  of  nesting  of  all kinds of parenthesized subpattern,
89           including capturing subpatterns, assertions, and other types of subpat-
90           tern, is 200.
91    
92           The  maximum  length of a subject string is the largest positive number
93           that an integer variable can hold. However, PCRE uses recursion to han-
94           dle  subpatterns  and indefinite repetition. This means that the avail-
95           able stack space may limit the size of a subject  string  that  can  be
96           processed by certain patterns.
97    
98    
99    UTF-8 AND UNICODE PROPERTY SUPPORT
100    
101           From  release  3.3,  PCRE  has  had  some support for character strings
102           encoded in the UTF-8 format. For release 4.0 this was greatly  extended
103           to  cover  most common requirements, and in release 5.0 additional sup-
104           port for Unicode general category properties was added.
105    
106           In order process UTF-8 strings, you must build PCRE  to  include  UTF-8
107           support  in  the  code,  and, in addition, you must call pcre_compile()
108           with the PCRE_UTF8 option flag. When you do this, both the pattern  and
109           any  subject  strings  that are matched against it are treated as UTF-8
110           strings instead of just strings of bytes.
111    
112           If you compile PCRE with UTF-8 support, but do not use it at run  time,
113           the  library will be a bit bigger, but the additional run time overhead
114           is limited to testing the PCRE_UTF8 flag in several places,  so  should
115           not be very large.
116    
117           If PCRE is built with Unicode character property support (which implies
118           UTF-8 support), the escape sequences \p{..}, \P{..}, and  \X  are  sup-
119           ported.  The available properties that can be tested are limited to the
120           general category properties such as Lu for an upper case letter  or  Nd
121           for  a decimal number. A full list is given in the pcrepattern documen-
122           tation. The PCRE library is increased in size by about 90K when Unicode
123           property support is included.
124    
125           The following comments apply when PCRE is running in UTF-8 mode:
126    
127           1.  When you set the PCRE_UTF8 flag, the strings passed as patterns and
128           subjects are checked for validity on entry to the  relevant  functions.
129           If an invalid UTF-8 string is passed, an error return is given. In some
130           situations, you may already know  that  your  strings  are  valid,  and
131           therefore want to skip these checks in order to improve performance. If
132           you set the PCRE_NO_UTF8_CHECK flag at compile time  or  at  run  time,
133           PCRE  assumes  that  the  pattern or subject it is given (respectively)
134           contains only valid UTF-8 codes. In this case, it does not diagnose  an
135           invalid  UTF-8 string. If you pass an invalid UTF-8 string to PCRE when
136           PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program  may
137           crash.
138    
139           2. In a pattern, the escape sequence \x{...}, where the contents of the
140           braces is a string of hexadecimal digits, is  interpreted  as  a  UTF-8
141           character  whose code number is the given hexadecimal number, for exam-
142           ple: \x{1234}. If a non-hexadecimal digit appears between  the  braces,
143           the item is not recognized.  This escape sequence can be used either as
144           a literal, or within a character class.
145    
146           3. The original hexadecimal escape sequence, \xhh, matches  a  two-byte
147           UTF-8 character if the value is greater than 127.
148    
149           4.  Repeat quantifiers apply to complete UTF-8 characters, not to indi-
150           vidual bytes, for example: \x{100}{3}.
151    
152           5. The dot metacharacter matches one UTF-8 character instead of a  sin-
153           gle byte.
154    
155           6.  The  escape sequence \C can be used to match a single byte in UTF-8
156           mode, but its use can lead to some strange effects.
157    
158           7. The character escapes \b, \B, \d, \D, \s, \S, \w, and  \W  correctly
159           test  characters of any code value, but the characters that PCRE recog-
160           nizes as digits, spaces, or word characters  remain  the  same  set  as
161           before, all with values less than 256. This remains true even when PCRE
162           includes Unicode property support, because to do otherwise  would  slow
163           down  PCRE in many common cases. If you really want to test for a wider
164           sense of, say, "digit", you must use Unicode  property  tests  such  as
165           \p{Nd}.
166    
167           8.  Similarly,  characters that match the POSIX named character classes
168           are all low-valued characters.
169    
170           9. Case-insensitive matching applies only to  characters  whose  values
171           are  less than 128, unless PCRE is built with Unicode property support.
172           Even when Unicode property support is available, PCRE  still  uses  its
173           own  character  tables when checking the case of low-valued characters,
174           so as not to degrade performance.  The Unicode property information  is
175           used only for characters with higher values.
176    
      int pcre_get_substring(const char *subject, int *ovector,  
           int stringcount, int stringnumber,  
           const char **stringptr);  
177    
178       int pcre_get_substring_list(const char *subject,  AUTHOR
           int *ovector, int stringcount, const char ***listptr);  
179    
180       void pcre_free_substring(const char *stringptr);         Philip Hazel <ph10@cam.ac.uk>
181           University Computing Service,
182           Cambridge CB2 3QG, England.
183           Phone: +44 1223 334714
184    
185    Last updated: 09 September 2004
186    Copyright (c) 1997-2004 University of Cambridge.
187    -----------------------------------------------------------------------------
188    
189       void pcre_free_substring_list(const char **stringptr);  PCRE(3)                                                                PCRE(3)
190    
      const unsigned char *pcre_maketables(void);  
191    
      int pcre_fullinfo(const pcre *code, const pcre_extra *extra,  
           int what, void *where);  
192    
193       int pcre_info(const pcre *code, int *optptr, *firstcharptr);  NAME
194           PCRE - Perl-compatible regular expressions
195    
196       char *pcre_version(void);  PCRE BUILD-TIME OPTIONS
197    
198       void *(*pcre_malloc)(size_t);         This  document  describes  the  optional  features  of PCRE that can be
199           selected when the library is compiled. They are all selected, or  dese-
200           lected, by providing options to the configure script that is run before
201           the make command. The complete list of  options  for  configure  (which
202           includes  the  standard  ones such as the selection of the installation
203           directory) can be obtained by running
204    
205             ./configure --help
206    
207           The following sections describe certain options whose names begin  with
208           --enable  or  --disable. These settings specify changes to the defaults
209           for the configure command. Because of the  way  that  configure  works,
210           --enable  and  --disable  always  come  in  pairs, so the complementary
211           option always exists as well, but as it specifies the  default,  it  is
212           not described.
213    
      void (*pcre_free)(void *);  
214    
215    UTF-8 SUPPORT
216    
217           To build PCRE with support for UTF-8 character strings, add
218    
219             --enable-utf8
220    
221           to  the  configure  command.  Of  itself, this does not make PCRE treat
222           strings as UTF-8. As well as compiling PCRE with this option, you  also
223           have  have to set the PCRE_UTF8 option when you call the pcre_compile()
224           function.
225    
226    
227    UNICODE CHARACTER PROPERTY SUPPORT
228    
229           UTF-8 support allows PCRE to process character values greater than  255
230           in  the  strings that it handles. On its own, however, it does not pro-
231           vide any facilities for accessing the properties of such characters. If
232           you  want  to  be able to use the pattern escapes \P, \p, and \X, which
233           refer to Unicode character properties, you must add
234    
235             --enable-unicode-properties
236    
237           to the configure command. This implies UTF-8 support, even if you  have
238           not explicitly requested it.
239    
240           Including  Unicode  property  support  adds around 90K of tables to the
241           PCRE library, approximately doubling its size. Only the  general  cate-
242           gory  properties  such as Lu and Nd are supported. Details are given in
243           the pcrepattern documentation.
244    
245    
246    CODE VALUE OF NEWLINE
247    
248           By default, PCRE treats character 10 (linefeed) as the newline  charac-
249           ter. This is the normal newline character on Unix-like systems. You can
250           compile PCRE to use character 13 (carriage return) instead by adding
251    
252             --enable-newline-is-cr
253    
254           to the configure command. For completeness there is  also  a  --enable-
255           newline-is-lf  option,  which explicitly specifies linefeed as the new-
256           line character.
257    
258    
259    BUILDING SHARED AND STATIC LIBRARIES
260    
261           The PCRE building process uses libtool to build both shared and  static
262           Unix  libraries by default. You can suppress one of these by adding one
263           of
264    
265             --disable-shared
266             --disable-static
267    
268           to the configure command, as required.
269    
270    
271    POSIX MALLOC USAGE
272    
273           When PCRE is called through the POSIX interface (see the pcreposix doc-
274           umentation),  additional  working  storage  is required for holding the
275           pointers to capturing substrings, because PCRE requires three  integers
276           per  substring,  whereas  the POSIX interface provides only two. If the
277           number of expected substrings is small, the wrapper function uses space
278           on the stack, because this is faster than using malloc() for each call.
279           The default threshold above which the stack is no longer used is 10; it
280           can be changed by adding a setting such as
281    
282             --with-posix-malloc-threshold=20
283    
284           to the configure command.
285    
286    
287    LIMITING PCRE RESOURCE USAGE
288    
289           Internally,  PCRE has a function called match(), which it calls repeat-
290           edly (possibly recursively) when matching a pattern. By controlling the
291           maximum  number  of  times  this function may be called during a single
292           matching operation, a limit can be placed on the resources  used  by  a
293           single  call  to  pcre_exec(). The limit can be changed at run time, as
294           described in the pcreapi documentation. The default is 10 million,  but
295           this can be changed by adding a setting such as
296    
297             --with-match-limit=500000
298    
299           to the configure command.
300    
301    
302    HANDLING VERY LARGE PATTERNS
303    
304           Within  a  compiled  pattern,  offset values are used to point from one
305           part to another (for example, from an opening parenthesis to an  alter-
306           nation  metacharacter).  By default, two-byte values are used for these
307           offsets, leading to a maximum size for a  compiled  pattern  of  around
308           64K.  This  is sufficient to handle all but the most gigantic patterns.
309           Nevertheless, some people do want to process enormous patterns,  so  it
310           is  possible  to compile PCRE to use three-byte or four-byte offsets by
311           adding a setting such as
312    
313             --with-link-size=3
314    
315           to the configure command. The value given must be 2,  3,  or  4.  Using
316           longer  offsets slows down the operation of PCRE because it has to load
317           additional bytes when handling them.
318    
319           If you build PCRE with an increased link size, test 2 (and  test  5  if
320           you  are using UTF-8) will fail. Part of the output of these tests is a
321           representation of the compiled pattern, and this changes with the  link
322           size.
323    
324    
325    AVOIDING EXCESSIVE STACK USAGE
326    
327           PCRE  implements  backtracking while matching by making recursive calls
328           to an internal function called match(). In environments where the  size
329           of the stack is limited, this can severely limit PCRE's operation. (The
330           Unix environment does not usually suffer from this problem.) An  alter-
331           native  approach  that  uses  memory  from  the  heap to remember data,
332           instead of using recursive function calls, has been implemented to work
333           round  this  problem. If you want to build a version of PCRE that works
334           this way, add
335    
336             --disable-stack-for-recursion
337    
338           to the configure command. With this configuration, PCRE  will  use  the
339           pcre_stack_malloc  and pcre_stack_free variables to call memory manage-
340           ment functions. Separate functions are provided because  the  usage  is
341           very  predictable:  the  block sizes requested are always the same, and
342           the blocks are always freed in reverse order. A calling  program  might
343           be  able  to implement optimized functions that perform better than the
344           standard malloc() and  free()  functions.  PCRE  runs  noticeably  more
345           slowly when built in this way.
346    
347    
348    USING EBCDIC CODE
349    
350           PCRE  assumes  by  default that it will run in an environment where the
351           character code is ASCII (or Unicode, which is  a  superset  of  ASCII).
352           PCRE  can,  however,  be  compiled  to  run in an EBCDIC environment by
353           adding
354    
355             --enable-ebcdic
356    
357           to the configure command.
358    
359    Last updated: 09 September 2004
360    Copyright (c) 1997-2004 University of Cambridge.
361    -----------------------------------------------------------------------------
362    
363    PCRE(3)                                                                PCRE(3)
364    
365    
366    
367    NAME
368           PCRE - Perl-compatible regular expressions
369    
370    PCRE NATIVE API
371    
372           #include <pcre.h>
373    
374           pcre *pcre_compile(const char *pattern, int options,
375                const char **errptr, int *erroffset,
376                const unsigned char *tableptr);
377    
378           pcre_extra *pcre_study(const pcre *code, int options,
379                const char **errptr);
380    
381           int pcre_exec(const pcre *code, const pcre_extra *extra,
382                const char *subject, int length, int startoffset,
383                int options, int *ovector, int ovecsize);
384    
385           int pcre_copy_named_substring(const pcre *code,
386                const char *subject, int *ovector,
387                int stringcount, const char *stringname,
388                char *buffer, int buffersize);
389    
390           int pcre_copy_substring(const char *subject, int *ovector,
391                int stringcount, int stringnumber, char *buffer,
392                int buffersize);
393    
394           int pcre_get_named_substring(const pcre *code,
395                const char *subject, int *ovector,
396                int stringcount, const char *stringname,
397                const char **stringptr);
398    
399           int pcre_get_stringnumber(const pcre *code,
400                const char *name);
401    
402           int pcre_get_substring(const char *subject, int *ovector,
403                int stringcount, int stringnumber,
404                const char **stringptr);
405    
406           int pcre_get_substring_list(const char *subject,
407                int *ovector, int stringcount, const char ***listptr);
408    
409           void pcre_free_substring(const char *stringptr);
410    
411           void pcre_free_substring_list(const char **stringptr);
412    
413           const unsigned char *pcre_maketables(void);
414    
415           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
416                int what, void *where);
417    
418           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
419    
420           int pcre_config(int what, void *where);
421    
422           char *pcre_version(void);
423    
424           void *(*pcre_malloc)(size_t);
425    
426           void (*pcre_free)(void *);
427    
428           void *(*pcre_stack_malloc)(size_t);
429    
430           void (*pcre_stack_free)(void *);
431    
432           int (*pcre_callout)(pcre_callout_block *);
433    
 DESCRIPTION  
      The PCRE library is a set of functions that implement  regu-  
      lar  expression  pattern  matching using the same syntax and  
      semantics as Perl  5,  with  just  a  few  differences  (see  
   
      below).  The  current  implementation  corresponds  to  Perl  
      5.005, with some additional features  from  later  versions.  
      This  includes  some  experimental,  incomplete  support for  
      UTF-8 encoded strings. Details of exactly what is  and  what  
      is not supported are given below.  
   
      PCRE has its own native API,  which  is  described  in  this  
      document.  There  is  also  a  set of wrapper functions that  
      correspond to the POSIX regular expression API.   These  are  
      described in the pcreposix documentation.  
   
      The native API function prototypes are defined in the header  
      file  pcre.h,  and  on  Unix  systems  the library itself is  
      called libpcre.a, so can be accessed by adding -lpcre to the  
      command  for  linking  an  application  which  calls it. The  
      header file defines the macros PCRE_MAJOR and PCRE_MINOR  to  
      contain the major and minor release numbers for the library.  
      Applications can use these to include support for  different  
      releases.  
   
      The functions pcre_compile(), pcre_study(), and  pcre_exec()  
      are  used  for compiling and matching regular expressions. A  
      sample program that demonstrates the simplest way  of  using  
      them  is  given  in the file pcredemo.c. The last section of  
      this man page describes how to run it.  
   
      The functions  pcre_copy_substring(),  pcre_get_substring(),  
      and  pcre_get_substring_list() are convenience functions for  
      extracting  captured  substrings  from  a  matched   subject  
      string; pcre_free_substring() and pcre_free_substring_list()  
      are also provided, to free the  memory  used  for  extracted  
      strings.  
   
      The function pcre_maketables() is used (optionally) to build  
      a  set of character tables in the current locale for passing  
      to pcre_compile().  
   
      The function pcre_fullinfo() is used to find out information  
      about a compiled pattern; pcre_info() is an obsolete version  
      which returns only some of the available information, but is  
      retained   for   backwards   compatibility.    The  function  
      pcre_version() returns a pointer to a string containing  the  
      version of PCRE and its date of release.  
   
      The global variables  pcre_malloc  and  pcre_free  initially  
      contain the entry points of the standard malloc() and free()  
      functions respectively. PCRE  calls  the  memory  management  
      functions  via  these  variables,  so  a calling program can  
      replace them if it  wishes  to  intercept  the  calls.  This  
      should be done before calling any PCRE functions.  
   
   
   
 MULTI-THREADING  
      The PCRE functions can be used in  multi-threading  applica-  
      tions, with the proviso that the memory management functions  
      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.  
434    
435    PCRE API OVERVIEW
436    
437           PCRE has its own native API, which is described in this document. There
438           is also a set of wrapper functions that correspond to the POSIX regular
439           expression API.  These are described in the pcreposix documentation.
440    
441           The  native  API  function  prototypes  are  defined in the header file
442           pcre.h, and on Unix systems the library itself is  called  libpcre.  It
443           can normally be accessed by adding -lpcre to the command for linking an
444           application  that  uses  PCRE.  The  header  file  defines  the  macros
445           PCRE_MAJOR  and  PCRE_MINOR to contain the major and minor release num-
446           bers for the library.  Applications can use these  to  include  support
447           for different releases of PCRE.
448    
449           The  functions  pcre_compile(),  pcre_study(), and pcre_exec() are used
450           for compiling and matching regular expressions. A sample  program  that
451           demonstrates  the  simplest  way  of using them is provided in the file
452           called pcredemo.c in the source distribution. The pcresample documenta-
453           tion describes how to run it.
454    
455           In  addition  to  the  main compiling and matching functions, there are
456           convenience functions for extracting captured substrings from a matched
457           subject string.  They are:
458    
459             pcre_copy_substring()
460             pcre_copy_named_substring()
461             pcre_get_substring()
462             pcre_get_named_substring()
463             pcre_get_substring_list()
464             pcre_get_stringnumber()
465    
466           pcre_free_substring() and pcre_free_substring_list() are also provided,
467           to free the memory used for extracted strings.
468    
469           The function pcre_maketables() is used to  build  a  set  of  character
470           tables   in  the  current  locale  for  passing  to  pcre_compile()  or
471           pcre_exec().  This is an optional facility that is  provided  for  spe-
472           cialist use. Most commonly, no special tables are passed, in which case
473           internal tables that are generated when PCRE is built are used.
474    
475           The function pcre_fullinfo() is used to find out  information  about  a
476           compiled  pattern; pcre_info() is an obsolete version that returns only
477           some of the available information, but is retained for  backwards  com-
478           patibility.   The function pcre_version() returns a pointer to a string
479           containing the version of PCRE and its date of release.
480    
481           The global variables pcre_malloc and pcre_free  initially  contain  the
482           entry  points  of  the  standard malloc() and free() functions, respec-
483           tively. PCRE calls the memory management functions via these variables,
484           so  a  calling  program  can replace them if it wishes to intercept the
485           calls. This should be done before calling any PCRE functions.
486    
487           The global variables pcre_stack_malloc  and  pcre_stack_free  are  also
488           indirections  to  memory  management functions. These special functions
489           are used only when PCRE is compiled to use  the  heap  for  remembering
490           data,  instead  of recursive function calls. This is a non-standard way
491           of building PCRE, for use in environments  that  have  limited  stacks.
492           Because  of  the greater use of memory management, it runs more slowly.
493           Separate functions are provided so that special-purpose  external  code
494           can be used for this case. When used, these functions are always called
495           in a stack-like manner (last obtained, first  freed),  and  always  for
496           memory blocks of the same size.
497    
498           The global variable pcre_callout initially contains NULL. It can be set
499           by the caller to a "callout" function, which PCRE  will  then  call  at
500           specified  points during a matching operation. Details are given in the
501           pcrecallout documentation.
502    
503    
504    MULTITHREADING
505    
506           The PCRE functions can be used in  multi-threading  applications,  with
507           the  proviso  that  the  memory  management  functions  pointed  to  by
508           pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
509           callout function pointed to by pcre_callout, are shared by all threads.
510    
511           The compiled form of a regular expression is not altered during  match-
512           ing, so the same compiled pattern can safely be used by several threads
513           at once.
514    
515    
516    SAVING PRECOMPILED PATTERNS FOR LATER USE
517    
518           The compiled form of a regular expression can be saved and re-used at a
519           later  time,  possibly by a different program, and even on a host other
520           than the one on which  it  was  compiled.  Details  are  given  in  the
521           pcreprecompile documentation.
522    
523    
524    CHECKING BUILD-TIME OPTIONS
525    
526           int pcre_config(int what, void *where);
527    
528           The  function pcre_config() makes it possible for a PCRE client to dis-
529           cover which optional features have been compiled into the PCRE library.
530           The  pcrebuild documentation has more details about these optional fea-
531           tures.
532    
533           The first argument for pcre_config() is an  integer,  specifying  which
534           information is required; the second argument is a pointer to a variable
535           into which the information is  placed.  The  following  information  is
536           available:
537    
538             PCRE_CONFIG_UTF8
539    
540           The  output is an integer that is set to one if UTF-8 support is avail-
541           able; otherwise it is set to zero.
542    
543             PCRE_CONFIG_UNICODE_PROPERTIES
544    
545           The output is an integer that is set to  one  if  support  for  Unicode
546           character properties is available; otherwise it is set to zero.
547    
548             PCRE_CONFIG_NEWLINE
549    
550           The  output  is an integer that is set to the value of the code that is
551           used for the newline character. It is either linefeed (10) or  carriage
552           return  (13),  and  should  normally be the standard character for your
553           operating system.
554    
555             PCRE_CONFIG_LINK_SIZE
556    
557           The output is an integer that contains the number  of  bytes  used  for
558           internal linkage in compiled regular expressions. The value is 2, 3, or
559           4. Larger values allow larger regular expressions to  be  compiled,  at
560           the  expense  of  slower matching. The default value of 2 is sufficient
561           for all but the most massive patterns, since  it  allows  the  compiled
562           pattern to be up to 64K in size.
563    
564             PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
565    
566           The  output  is  an integer that contains the threshold above which the
567           POSIX interface uses malloc() for output vectors. Further  details  are
568           given in the pcreposix documentation.
569    
570             PCRE_CONFIG_MATCH_LIMIT
571    
572           The output is an integer that gives the default limit for the number of
573           internal matching function calls in a  pcre_exec()  execution.  Further
574           details are given with pcre_exec() below.
575    
576             PCRE_CONFIG_STACKRECURSE
577    
578           The  output  is  an integer that is set to one if internal recursion is
579           implemented by recursive function calls that use the stack to  remember
580           their state. This is the usual way that PCRE is compiled. The output is
581           zero if PCRE was compiled to use blocks of data on the heap instead  of
582           recursive   function   calls.   In  this  case,  pcre_stack_malloc  and
583           pcre_stack_free are called to manage memory blocks on  the  heap,  thus
584           avoiding the use of the stack.
585    
586    
587  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 the returned  
      block; this is a typedef for a structure whose contents  are  
      not  externally  defined. It is up to the caller to free the  
      memory when it is no longer required.  
   
      Although the compiled code of a PCRE regex  is  relocatable,  
      that is, it does not depend on memory location, the complete  
      pcre data block is not fully relocatable,  because  it  con-  
      tains  a  copy of the tableptr argument, which is an address  
      (see below).  
   
      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.  
   
      This code fragment shows a typical straightforward  call  to  
      pcre_compile():  
   
        pcre *re;  
        const char *error;  
        int erroffset;  
        re = pcre_compile(  
          "^A.*Z",          /* the pattern */  
          0,                /* default options */  
          &error,           /* for error message */  
          &erroffset,       /* for error offset */  
          NULL);            /* use default character tables */  
   
      The following 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.  
   
        PCRE_UTF8  
   
      This option causes PCRE to regard both the pattern  and  the  
      subject  as strings of UTF-8 characters instead of just byte  
      strings. However, it is available  only  if  PCRE  has  been  
      built  to  include  UTF-8  support.  If not, the use of this  
      option provokes an error. Support for UTF-8 is new,  experi-  
      mental,  and incomplete.  Details of exactly what it entails  
      are given below.  
588    
589           pcre *pcre_compile(const char *pattern, int options,
590                const char **errptr, int *erroffset,
591                const unsigned char *tableptr);
592    
593           The  function  pcre_compile()  is  called  to compile a pattern into an
594           internal form. The pattern is a C string terminated by a  binary  zero,
595           and  is  passed in the pattern argument. A pointer to a single block of
596           memory that is obtained via pcre_malloc is returned. This contains  the
597           compiled  code  and  related  data.  The  pcre  type is defined for the
598           returned block; this is a typedef for a structure  whose  contents  are
599           not  externally defined. It is up to the caller to free the memory when
600           it is no longer required.
601    
602           Although the compiled code of a PCRE regex is relocatable, that is,  it
603           does not depend on memory location, the complete pcre data block is not
604           fully relocatable, because it may contain a copy of the tableptr  argu-
605           ment, which is an address (see below).
606    
607           The options argument contains independent bits that affect the compila-
608           tion. It should be zero if  no  options  are  required.  The  available
609           options  are  described  below. Some of them, in particular, those that
610           are compatible with Perl, can also be set and  unset  from  within  the
611           pattern  (see  the  detailed  description in the pcrepattern documenta-
612           tion). For these options, the contents of the options  argument  speci-
613           fies  their initial settings at the start of compilation and execution.
614           The PCRE_ANCHORED option can be set at the time of matching as well  as
615           at compile time.
616    
617           If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
618           if compilation of a pattern fails,  pcre_compile()  returns  NULL,  and
619           sets the variable pointed to by errptr to point to a textual error mes-
620           sage. The offset from the start of the pattern to the  character  where
621           the  error  was  discovered  is  placed  in  the variable pointed to by
622           erroffset, which must not be NULL. If it  is,  an  immediate  error  is
623           given.
624    
625           If  the  final  argument, tableptr, is NULL, PCRE uses a default set of
626           character tables that are  built  when  PCRE  is  compiled,  using  the
627           default  C  locale.  Otherwise, tableptr must be an address that is the
628           result of a call to pcre_maketables(). This value is  stored  with  the
629           compiled  pattern,  and used again by pcre_exec(), unless another table
630           pointer is passed to it. For more discussion, see the section on locale
631           support below.
632    
633           This  code  fragment  shows a typical straightforward call to pcre_com-
634           pile():
635    
636             pcre *re;
637             const char *error;
638             int erroffset;
639             re = pcre_compile(
640               "^A.*Z",          /* the pattern */
641               0,                /* default options */
642               &error,           /* for error message */
643               &erroffset,       /* for error offset */
644               NULL);            /* use default character tables */
645    
646           The following names for option bits are defined in  the  pcre.h  header
647           file:
648    
649             PCRE_ANCHORED
650    
651           If this bit is set, the pattern is forced to be "anchored", that is, it
652           is constrained to match only at the first matching point in the  string
653           that  is being searched (the "subject string"). This effect can also be
654           achieved by appropriate constructs in the pattern itself, which is  the
655           only way to do it in Perl.
656    
657             PCRE_AUTO_CALLOUT
658    
659           If this bit is set, pcre_compile() automatically inserts callout items,
660           all with number 255, before each pattern item. For  discussion  of  the
661           callout facility, see the pcrecallout documentation.
662    
663             PCRE_CASELESS
664    
665           If  this  bit is set, letters in the pattern match both upper and lower
666           case letters. It is equivalent to Perl's  /i  option,  and  it  can  be
667           changed  within  a  pattern  by  a (?i) option setting. When running in
668           UTF-8 mode, case support for high-valued characters is  available  only
669           when PCRE is built with Unicode character property support.
670    
671             PCRE_DOLLAR_ENDONLY
672    
673           If  this bit is set, a dollar metacharacter in the pattern matches only
674           at the end of the subject string. Without this option,  a  dollar  also
675           matches  immediately before the final character if it is a newline (but
676           not before any  other  newlines).  The  PCRE_DOLLAR_ENDONLY  option  is
677           ignored if PCRE_MULTILINE is set. There is no equivalent to this option
678           in Perl, and no way to set it within a pattern.
679    
680             PCRE_DOTALL
681    
682           If this bit is set, a dot metacharater in the pattern matches all char-
683           acters,  including  newlines.  Without  it, newlines are excluded. This
684           option is equivalent to Perl's /s option, and it can be changed  within
685           a  pattern  by  a  (?s)  option  setting. A negative class such as [^a]
686           always matches a newline character, independent of the setting of  this
687           option.
688    
689             PCRE_EXTENDED
690    
691           If  this  bit  is  set,  whitespace  data characters in the pattern are
692           totally ignored except  when  escaped  or  inside  a  character  class.
693           Whitespace  does  not  include the VT character (code 11). In addition,
694           characters between an unescaped # outside a  character  class  and  the
695           next newline character, inclusive, are also ignored. This is equivalent
696           to Perl's /x option, and it can be changed within a pattern by  a  (?x)
697           option setting.
698    
699           This  option  makes  it possible to include comments inside complicated
700           patterns.  Note, however, that this applies only  to  data  characters.
701           Whitespace   characters  may  never  appear  within  special  character
702           sequences in a pattern, for  example  within  the  sequence  (?(  which
703           introduces a conditional subpattern.
704    
705             PCRE_EXTRA
706    
707           This  option  was invented in order to turn on additional functionality
708           of PCRE that is incompatible with Perl, but it  is  currently  of  very
709           little  use. When set, any backslash in a pattern that is followed by a
710           letter that has no special meaning  causes  an  error,  thus  reserving
711           these  combinations  for  future  expansion.  By default, as in Perl, a
712           backslash followed by a letter with no special meaning is treated as  a
713           literal.  There  are  at  present  no other features controlled by this
714           option. It can also be set by a (?X) option setting within a pattern.
715    
716             PCRE_MULTILINE
717    
718           By default, PCRE treats the subject string as consisting  of  a  single
719           line  of characters (even if it actually contains newlines). The "start
720           of line" metacharacter (^) matches only at the  start  of  the  string,
721           while  the  "end  of line" metacharacter ($) matches only at the end of
722           the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
723           is set). This is the same as Perl.
724    
725           When  PCRE_MULTILINE  it  is set, the "start of line" and "end of line"
726           constructs match immediately following or immediately before  any  new-
727           line  in the subject string, respectively, as well as at the very start
728           and end. This is equivalent to Perl's /m option, and it can be  changed
729           within a pattern by a (?m) option setting. If there are no "\n" charac-
730           ters in a subject string, or no occurrences of ^ or  $  in  a  pattern,
731           setting PCRE_MULTILINE has no effect.
732    
733             PCRE_NO_AUTO_CAPTURE
734    
735           If this option is set, it disables the use of numbered capturing paren-
736           theses in the pattern. Any opening parenthesis that is not followed  by
737           ?  behaves as if it were followed by ?: but named parentheses can still
738           be used for capturing (and they acquire  numbers  in  the  usual  way).
739           There is no equivalent of this option in Perl.
740    
741             PCRE_UNGREEDY
742    
743           This  option  inverts  the "greediness" of the quantifiers so that they
744           are not greedy by default, but become greedy if followed by "?". It  is
745           not  compatible  with Perl. It can also be set by a (?U) option setting
746           within the pattern.
747    
748             PCRE_UTF8
749    
750           This option causes PCRE to regard both the pattern and the  subject  as
751           strings  of  UTF-8 characters instead of single-byte character strings.
752           However, it is available only when PCRE is built to include UTF-8  sup-
753           port.  If not, the use of this option provokes an error. Details of how
754           this option changes the behaviour of PCRE are given in the  section  on
755           UTF-8 support in the main pcre page.
756    
757             PCRE_NO_UTF8_CHECK
758    
759           When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
760           automatically checked. If an invalid UTF-8 sequence of bytes is  found,
761           pcre_compile()  returns an error. If you already know that your pattern
762           is valid, and you want to skip this check for performance reasons,  you
763           can  set  the  PCRE_NO_UTF8_CHECK option. When it is set, the effect of
764           passing an invalid UTF-8 string as a pattern is undefined. It may cause
765           your  program  to  crash.   Note that this option can also be passed to
766           pcre_exec(),  to  suppress  the  UTF-8  validity  checking  of  subject
767           strings.
768    
769    
770  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 typedef for  
      a  structure  with  hidden  contents)  containing additional  
      information  about  the  pattern;  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.  
   
      This is a typical call to pcre_study():  
   
        pcre_extra *pe;  
        pe = pcre_study(  
          re,             /* result of pcre_compile() */  
          0,              /* no options exist */  
          &error);        /* set to NULL or points to a message */  
   
      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.  
771    
772           pcre_extra *pcre_study(const pcre *code, int options,
773                const char **errptr);
774    
775           If  a  compiled  pattern is going to be used several times, it is worth
776           spending more time analyzing it in order to speed up the time taken for
777           matching.  The function pcre_study() takes a pointer to a compiled pat-
778           tern as its first argument. If studying the pattern produces additional
779           information  that  will  help speed up matching, pcre_study() returns a
780           pointer to a pcre_extra block, in which the study_data field points  to
781           the results of the study.
782    
783           The  returned  value  from  pcre_study()  can  be  passed  directly  to
784           pcre_exec(). However, a pcre_extra block  also  contains  other  fields
785           that  can  be  set  by the caller before the block is passed; these are
786           described below in the section on matching a pattern.
787    
788           If studying the pattern does not produce  any  additional  information,
789           pcre_study() returns NULL. In that circumstance, if the calling program
790           wants to pass any of the other fields to pcre_exec(), it  must  set  up
791           its own pcre_extra block.
792    
793           The  second  argument of pcre_study() contains option bits. At present,
794           no options are defined, and this argument should always be zero.
795    
796           The third argument for pcre_study() is a pointer for an error  message.
797           If  studying  succeeds  (even  if no data is returned), the variable it
798           points to is set to NULL. Otherwise it points to a textual  error  mes-
799           sage.  You should therefore test the error pointer for NULL after call-
800           ing pcre_study(), to be sure that it has run successfully.
801    
802           This is a typical call to pcre_study():
803    
804             pcre_extra *pe;
805             pe = pcre_study(
806               re,             /* result of pcre_compile() */
807               0,              /* no options exist */
808               &error);        /* set to NULL or points to a message */
809    
810           At present, studying a pattern is useful only for non-anchored patterns
811           that  do not have a single fixed starting character. A bitmap of possi-
812           ble starting bytes is created.
813    
814    
815  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.  
816    
817           PCRE handles caseless matching, and determines whether  characters  are
818           letters,  digits, or whatever, by reference to a set of tables, indexed
819           by character value. (When running in UTF-8 mode, this applies  only  to
820           characters  with  codes  less than 128. Higher-valued codes never match
821           escapes such as \w or \d, but can be tested with \p if  PCRE  is  built
822           with Unicode character property support.)
823    
824           An  internal set of tables is created in the default C locale when PCRE
825           is built. This is used when the final  argument  of  pcre_compile()  is
826           NULL,  and  is  sufficient for many applications. An alternative set of
827           tables can, however, be supplied. These may be created in  a  different
828           locale  from the default. As more and more applications change to using
829           Unicode, the need for this locale support is expected to die away.
830    
831           External tables are built by calling  the  pcre_maketables()  function,
832           which  has no arguments, in the relevant locale. The result can then be
833           passed to pcre_compile() or pcre_exec()  as  often  as  necessary.  For
834           example,  to  build  and use tables that are appropriate for the French
835           locale (where accented characters with  values  greater  than  128  are
836           treated as letters), the following code could be used:
837    
838             setlocale(LC_CTYPE, "fr_FR");
839             tables = pcre_maketables();
840             re = pcre_compile(..., tables);
841    
842           When  pcre_maketables()  runs,  the  tables are built in memory that is
843           obtained via pcre_malloc. It is the caller's responsibility  to  ensure
844           that  the memory containing the tables remains available for as long as
845           it is needed.
846    
847           The pointer that is passed to pcre_compile() is saved with the compiled
848           pattern,  and the same tables are used via this pointer by pcre_study()
849           and normally also by pcre_exec(). Thus, by default, for any single pat-
850           tern, compilation, studying and matching all happen in the same locale,
851           but different patterns can be compiled in different locales.
852    
853           It is possible to pass a table pointer or NULL (indicating the  use  of
854           the  internal  tables)  to  pcre_exec(). Although not intended for this
855           purpose, this facility could be used to match a pattern in a  different
856           locale from the one in which it was compiled. Passing table pointers at
857           run time is discussed below in the section on matching a pattern.
858    
859    
860  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  
   
      Here is a typical call of  pcre_fullinfo(),  to  obtain  the  
      length of the compiled pattern:  
   
        int rc;  
        unsigned long int length;  
        rc = pcre_fullinfo(  
          re,               /* result of pcre_compile() */  
          pe,               /* result of pcre_study(), or NULL */  
          PCRE_INFO_SIZE,   /* what is required */  
          &length);         /* where to put the data */  
   
      The possible values for the third argument  are  defined  in  
      pcre.h, and are as follows:  
   
        PCRE_INFO_OPTIONS  
   
      Return a copy of the options with which the pattern was com-  
      piled.  The fourth argument should point to an unsigned long  
      int variable. These option bits are those specified  in  the  
      call  to  pcre_compile(),  modified  by any top-level option  
      settings  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).  
861    
862           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
863                int what, void *where);
864    
865           The pcre_fullinfo() function returns information about a compiled  pat-
866           tern. It replaces the obsolete pcre_info() function, which is neverthe-
867           less retained for backwards compability (and is documented below).
868    
869           The first argument for pcre_fullinfo() is a  pointer  to  the  compiled
870           pattern.  The second argument is the result of pcre_study(), or NULL if
871           the pattern was not studied. The third argument specifies  which  piece
872           of  information  is required, and the fourth argument is a pointer to a
873           variable to receive the data. The yield of the  function  is  zero  for
874           success, or one of the following negative numbers:
875    
876             PCRE_ERROR_NULL       the argument code was NULL
877                                   the argument where was NULL
878             PCRE_ERROR_BADMAGIC   the "magic number" was not found
879             PCRE_ERROR_BADOPTION  the value of what was invalid
880    
881           The  "magic  number" is placed at the start of each compiled pattern as
882           an simple check against passing an arbitrary memory pointer. Here is  a
883           typical  call  of pcre_fullinfo(), to obtain the length of the compiled
884           pattern:
885    
886             int rc;
887             unsigned long int length;
888             rc = pcre_fullinfo(
889               re,               /* result of pcre_compile() */
890               pe,               /* result of pcre_study(), or NULL */
891               PCRE_INFO_SIZE,   /* what is required */
892               &length);         /* where to put the data */
893    
894           The possible values for the third argument are defined in  pcre.h,  and
895           are as follows:
896    
897             PCRE_INFO_BACKREFMAX
898    
899           Return  the  number  of  the highest back reference in the pattern. The
900           fourth argument should point to an int variable. Zero  is  returned  if
901           there are no back references.
902    
903             PCRE_INFO_CAPTURECOUNT
904    
905           Return  the  number of capturing subpatterns in the pattern. The fourth
906           argument should point to an int variable.
907    
908             PCRE_INFO_DEFAULTTABLES
909    
910           Return a pointer to the internal default character tables within  PCRE.
911           The  fourth  argument should point to an unsigned char * variable. This
912           information call is provided for internal use by the pcre_study() func-
913           tion.  External  callers  can  cause PCRE to use its internal tables by
914           passing a NULL table pointer.
915    
916             PCRE_INFO_FIRSTBYTE
917    
918           Return information about the first byte of any matched  string,  for  a
919           non-anchored    pattern.    (This    option    used    to   be   called
920           PCRE_INFO_FIRSTCHAR; the old name is  still  recognized  for  backwards
921           compatibility.)
922    
923           If  there  is  a  fixed first byte, for example, from a pattern such as
924           (cat|cow|coyote), it is returned in the integer pointed  to  by  where.
925           Otherwise, if either
926    
927           (a)  the pattern was compiled with the PCRE_MULTILINE option, and every
928           branch starts with "^", or
929    
930           (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
931           set (if it were set, the pattern would be anchored),
932    
933           -1  is  returned, indicating that the pattern matches only at the start
934           of a subject string or after any newline within the  string.  Otherwise
935           -2 is returned. For anchored patterns, -2 is returned.
936    
937             PCRE_INFO_FIRSTTABLE
938    
939           If  the pattern was studied, and this resulted in the construction of a
940           256-bit table indicating a fixed set of bytes for the first byte in any
941           matching  string, a pointer to the table is returned. Otherwise NULL is
942           returned. The fourth argument should point to an unsigned char *  vari-
943           able.
944    
945             PCRE_INFO_LASTLITERAL
946    
947           Return  the  value of the rightmost literal byte that must exist in any
948           matched string, other than at its  start,  if  such  a  byte  has  been
949           recorded. The fourth argument should point to an int variable. If there
950           is no such byte, -1 is returned. For anchored patterns, a last  literal
951           byte  is  recorded only if it follows something of variable length. For
952           example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
953           /^a\dz\d/ the returned value is -1.
954    
955             PCRE_INFO_NAMECOUNT
956             PCRE_INFO_NAMEENTRYSIZE
957             PCRE_INFO_NAMETABLE
958    
959           PCRE  supports the use of named as well as numbered capturing parenthe-
960           ses. The names are just an additional way of identifying the  parenthe-
961           ses,  which  still  acquire  numbers.  A  convenience  function  called
962           pcre_get_named_substring() is provided  for  extracting  an  individual
963           captured  substring  by  name.  It is also possible to extract the data
964           directly, by first converting the name to a number in order  to  access
965           the  correct  pointers in the output vector (described with pcre_exec()
966           below). To do the conversion, you need to use the  name-to-number  map,
967           which is described by these three values.
968    
969           The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
970           gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
971           of  each  entry;  both  of  these  return  an int value. The entry size
972           depends on the length of the longest name. PCRE_INFO_NAMETABLE  returns
973           a  pointer  to  the  first  entry of the table (a pointer to char). The
974           first two bytes of each entry are the number of the capturing parenthe-
975           sis,  most  significant byte first. The rest of the entry is the corre-
976           sponding name, zero terminated. The names are  in  alphabetical  order.
977           For  example,  consider  the following pattern (assume PCRE_EXTENDED is
978           set, so white space - including newlines - is ignored):
979    
980             (?P<date> (?P<year>(\d\d)?\d\d) -
981             (?P<month>\d\d) - (?P<day>\d\d) )
982    
983           There are four named subpatterns, so the table has  four  entries,  and
984           each  entry  in the table is eight bytes long. The table is as follows,
985           with non-printing bytes shows in hexadecimal, and undefined bytes shown
986           as ??:
987    
988             00 01 d  a  t  e  00 ??
989             00 05 d  a  y  00 ?? ??
990             00 04 m  o  n  t  h  00
991             00 02 y  e  a  r  00 ??
992    
993           When  writing  code  to  extract  data from named subpatterns using the
994           name-to-number map, remember that the length of each entry is likely to
995           be different for each compiled pattern.
996    
997             PCRE_INFO_OPTIONS
998    
999           Return  a  copy of the options with which the pattern was compiled. The
1000           fourth argument should point to an unsigned long  int  variable.  These
1001           option bits are those specified in the call to pcre_compile(), modified
1002           by any top-level option settings within the pattern itself.
1003    
1004           A pattern is automatically anchored by PCRE if  all  of  its  top-level
1005           alternatives begin with one of the following:
1006    
1007             ^     unless PCRE_MULTILINE is set
1008             \A    always
1009             \G    always
1010             .*    if PCRE_DOTALL is set and there are no back
1011                     references to the subpattern in which .* appears
1012    
1013           For such patterns, the PCRE_ANCHORED bit is set in the options returned
1014           by pcre_fullinfo().
1015    
1016             PCRE_INFO_SIZE
1017    
1018           Return the size of the compiled pattern, that is, the  value  that  was
1019           passed as the argument to pcre_malloc() when PCRE was getting memory in
1020           which to place the compiled data. The fourth argument should point to a
1021           size_t variable.
1022    
1023             PCRE_INFO_STUDYSIZE
1024    
1025           Return the size of the data block pointed to by the study_data field in
1026           a pcre_extra block. That is,  it  is  the  value  that  was  passed  to
1027           pcre_malloc() when PCRE was getting memory into which to place the data
1028           created by pcre_study(). The fourth argument should point to  a  size_t
1029           variable.
1030    
1031    
1032    OBSOLETE INFO FUNCTION
1033    
1034           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1035    
1036           The  pcre_info()  function is now obsolete because its interface is too
1037           restrictive to return all the available data about a compiled  pattern.
1038           New   programs   should  use  pcre_fullinfo()  instead.  The  yield  of
1039           pcre_info() is the number of capturing subpatterns, or one of the  fol-
1040           lowing negative numbers:
1041    
1042             PCRE_ERROR_NULL       the argument code was NULL
1043             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1044    
1045           If  the  optptr  argument is not NULL, a copy of the options with which
1046           the pattern was compiled is placed in the integer  it  points  to  (see
1047           PCRE_INFO_OPTIONS above).
1048    
1049           If  the  pattern  is  not anchored and the firstcharptr argument is not
1050           NULL, it is used to pass back information about the first character  of
1051           any matched string (see PCRE_INFO_FIRSTBYTE above).
1052    
1053    
1054  MATCHING A PATTERN  MATCHING A PATTERN
      The function pcre_exec() is called to match a subject string  
1055    
1056           int pcre_exec(const pcre *code, const pcre_extra *extra,
1057                const char *subject, int length, int startoffset,
1058                int options, int *ovector, int ovecsize);
1059    
1060           The  function pcre_exec() is called to match a subject string against a
1061           compiled pattern, which is passed in the code argument. If the  pattern
1062           has been studied, the result of the study should be passed in the extra
1063           argument.
1064    
1065           In most applications, the pattern will have been compiled (and  option-
1066           ally  studied)  in the same process that calls pcre_exec(). However, it
1067           is possible to save compiled patterns and study data, and then use them
1068           later  in  different processes, possibly even on different hosts. For a
1069           discussion about this, see the pcreprecompile documentation.
1070    
1071           Here is an example of a simple call to pcre_exec():
1072    
1073             int rc;
1074             int ovector[30];
1075             rc = pcre_exec(
1076               re,             /* result of pcre_compile() */
1077               NULL,           /* we didn't study the pattern */
1078               "some string",  /* the subject string */
1079               11,             /* the length of the subject string */
1080               0,              /* start at offset 0 in the subject */
1081               0,              /* default options */
1082               ovector,        /* vector of integers for substring information */
1083               30);            /* number of elements in the vector  (NOT  size  in
1084           bytes) */
1085    
1086       Extra data for pcre_exec()
1087    
1088           If  the  extra argument is not NULL, it must point to a pcre_extra data
1089           block. The pcre_study() function returns such a block (when it  doesn't
1090           return  NULL), but you can also create one for yourself, and pass addi-
1091           tional information in it. The fields in a pcre_extra block are as  fol-
1092           lows:
1093    
1094             unsigned long int flags;
1095             void *study_data;
1096             unsigned long int match_limit;
1097             void *callout_data;
1098             const unsigned char *tables;
1099    
1100           The  flags  field  is a bitmap that specifies which of the other fields
1101           are set. The flag bits are:
1102    
1103             PCRE_EXTRA_STUDY_DATA
1104             PCRE_EXTRA_MATCH_LIMIT
1105             PCRE_EXTRA_CALLOUT_DATA
1106             PCRE_EXTRA_TABLES
1107    
1108           Other flag bits should be set to zero. The study_data field is  set  in
1109           the  pcre_extra  block  that is returned by pcre_study(), together with
1110           the appropriate flag bit. You should not set this yourself, but you may
1111           add  to  the  block by setting the other fields and their corresponding
1112           flag bits.
1113    
1114           The match_limit field provides a means of preventing PCRE from using up
1115           a  vast amount of resources when running patterns that are not going to
1116           match, but which have a very large number  of  possibilities  in  their
1117           search  trees.  The  classic  example  is  the  use of nested unlimited
1118           repeats.
1119    
1120           Internally, PCRE uses a function called match() which it calls  repeat-
1121           edly  (sometimes  recursively).  The  limit is imposed on the number of
1122           times this function is called during a match, which has the  effect  of
1123           limiting  the amount of recursion and backtracking that can take place.
1124           For patterns that are not anchored, the count starts from zero for each
1125           position in the subject string.
1126    
1127           The  default  limit  for the library can be set when PCRE is built; the
1128           default default is 10 million, which handles all but the  most  extreme
1129           cases.  You  can  reduce  the  default  by  suppling pcre_exec() with a
1130           pcre_extra block in which match_limit is set to a  smaller  value,  and
1131           PCRE_EXTRA_MATCH_LIMIT  is  set  in  the  flags  field. If the limit is
1132           exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1133    
1134           The pcre_callout field is used in conjunction with the  "callout"  fea-
1135           ture, which is described in the pcrecallout documentation.
1136    
1137           The  tables  field  is  used  to  pass  a  character  tables pointer to
1138           pcre_exec(); this overrides the value that is stored with the  compiled
1139           pattern.  A  non-NULL value is stored with the compiled pattern only if
1140           custom tables were supplied to pcre_compile() via  its  tableptr  argu-
1141           ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
1142           PCRE's internal tables to be used. This facility is  helpful  when  re-
1143           using  patterns  that  have been saved after compiling with an external
1144           set of tables, because the external tables  might  be  at  a  different
1145           address  when  pcre_exec() is called. See the pcreprecompile documenta-
1146           tion for a discussion of saving compiled patterns for later use.
1147    
1148       Option bits for pcre_exec()
1149    
1150           The unused bits of the options argument for pcre_exec() must  be  zero.
1151           The   only  bits  that  may  be  set  are  PCRE_ANCHORED,  PCRE_NOTBOL,
1152           PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and PCRE_PARTIAL.
1153    
1154             PCRE_ANCHORED
1155    
1156           The PCRE_ANCHORED option limits pcre_exec() to matching  at  the  first
1157           matching  position.  If  a  pattern was compiled with PCRE_ANCHORED, or
1158           turned out to be anchored by virtue of its contents, it cannot be  made
1159           unachored at matching time.
1160    
1161             PCRE_NOTBOL
1162    
1163           This option specifies that first character of the subject string is not
1164           the beginning of a line, so the  circumflex  metacharacter  should  not
1165           match  before it. Setting this without PCRE_MULTILINE (at compile time)
1166           causes  circumflex  never  to  match.  This  option  affects  only  the
1167           behaviour of the circumflex metacharacter. It does not affect \A.
1168    
1169             PCRE_NOTEOL
1170    
1171           This option specifies that the end of the subject string is not the end
1172           of a line, so the dollar metacharacter should not match it nor  (except
1173           in  multiline mode) a newline immediately before it. Setting this with-
1174           out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1175           option  affects only the behaviour of the dollar metacharacter. It does
1176           not affect \Z or \z.
1177    
1178             PCRE_NOTEMPTY
1179    
1180           An empty string is not considered to be a valid match if this option is
1181           set.  If  there are alternatives in the pattern, they are tried. If all
1182           the alternatives match the empty string, the entire  match  fails.  For
1183           example, if the pattern
1184    
1185             a?b?
1186    
1187           is  applied  to  a string not beginning with "a" or "b", it matches the
1188           empty string at the start of the subject. With PCRE_NOTEMPTY set,  this
1189           match is not valid, so PCRE searches further into the string for occur-
1190           rences of "a" or "b".
1191    
1192           Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1193           cial  case  of  a  pattern match of the empty string within its split()
1194           function, and when using the /g modifier. It  is  possible  to  emulate
1195           Perl's behaviour after matching a null string by first trying the match
1196           again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1197           if  that  fails by advancing the starting offset (see below) and trying
1198           an ordinary match again. There is some code that demonstrates how to do
1199           this in the pcredemo.c sample program.
1200    
1201             PCRE_NO_UTF8_CHECK
1202    
1203           When PCRE_UTF8 is set at compile time, the validity of the subject as a
1204           UTF-8 string is automatically checked when pcre_exec() is  subsequently
1205           called.   The  value  of  startoffset is also checked to ensure that it
1206           points to the start of a UTF-8 character. If an invalid UTF-8  sequence
1207           of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If
1208           startoffset contains an  invalid  value,  PCRE_ERROR_BADUTF8_OFFSET  is
1209           returned.
1210    
1211           If  you  already  know that your subject is valid, and you want to skip
1212           these   checks   for   performance   reasons,   you   can    set    the
1213           PCRE_NO_UTF8_CHECK  option  when calling pcre_exec(). You might want to
1214           do this for the second and subsequent calls to pcre_exec() if  you  are
1215           making  repeated  calls  to  find  all  the matches in a single subject
1216           string. However, you should be  sure  that  the  value  of  startoffset
1217           points  to  the  start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
1218           set, the effect of passing an invalid UTF-8 string as a subject,  or  a
1219           value  of startoffset that does not point to the start of a UTF-8 char-
1220           acter, is undefined. Your program may crash.
1221    
1222             PCRE_PARTIAL
1223    
1224           This option turns on the  partial  matching  feature.  If  the  subject
1225           string  fails to match the pattern, but at some point during the match-
1226           ing process the end of the subject was reached (that  is,  the  subject
1227           partially  matches  the  pattern and the failure to match occurred only
1228           because there were not enough subject characters), pcre_exec()  returns
1229           PCRE_ERROR_PARTIAL  instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
1230           used, there are restrictions on what may appear in the  pattern.  These
1231           are discussed in the pcrepartial documentation.
1232    
1233       The string to be matched by pcre_exec()
1234    
1235           The  subject string is passed to pcre_exec() as a pointer in subject, a
1236           length in length, and a starting byte offset in startoffset.  In  UTF-8
1237           mode,  the  byte  offset  must point to the start of a UTF-8 character.
1238           Unlike the pattern string, the subject may contain binary  zero  bytes.
1239           When  the starting offset is zero, the search for a match starts at the
1240           beginning of the subject, and this is by far the most common case.
1241    
1242           A non-zero starting offset is useful when searching for  another  match
1243           in  the same subject by calling pcre_exec() again after a previous suc-
1244           cess.  Setting startoffset differs from just passing over  a  shortened
1245           string  and  setting  PCRE_NOTBOL  in the case of a pattern that begins
1246           with any kind of lookbehind. For example, consider the pattern
1247    
1248             \Biss\B
1249    
1250           which finds occurrences of "iss" in the middle of  words.  (\B  matches
1251           only  if  the  current position in the subject is not a word boundary.)
1252           When applied to the string "Mississipi" the first call  to  pcre_exec()
1253           finds  the  first  occurrence. If pcre_exec() is called again with just
1254           the remainder of the subject,  namely  "issipi",  it  does  not  match,
1255           because \B is always false at the start of the subject, which is deemed
1256           to be a word boundary. However, if pcre_exec()  is  passed  the  entire
1257           string again, but with startoffset set to 4, it finds the second occur-
1258           rence of "iss" because it is able to look behind the starting point  to
1259           discover that it is preceded by a letter.
1260    
1261           If  a  non-zero starting offset is passed when the pattern is anchored,
1262           one attempt to match at the given offset is made. This can only succeed
1263           if  the  pattern  does  not require the match to be at the start of the
1264           subject.
1265    
1266       How pcre_exec() returns captured substrings
1267    
1268           In general, a pattern matches a certain portion of the subject, and  in
1269           addition,  further  substrings  from  the  subject may be picked out by
1270           parts of the pattern. Following the usage  in  Jeffrey  Friedl's  book,
1271           this  is  called "capturing" in what follows, and the phrase "capturing
1272           subpattern" is used for a fragment of a pattern that picks out  a  sub-
1273           string.  PCRE  supports several other kinds of parenthesized subpattern
1274           that do not cause substrings to be captured.
1275    
1276           Captured substrings are returned to the caller via a vector of  integer
1277           offsets  whose  address is passed in ovector. The number of elements in
1278           the vector is passed in ovecsize, which must be a non-negative  number.
1279           Note: this argument is NOT the size of ovector in bytes.
1280    
1281           The  first  two-thirds of the vector is used to pass back captured sub-
1282           strings, each substring using a pair of integers. The  remaining  third
1283           of  the  vector is used as workspace by pcre_exec() while matching cap-
1284           turing subpatterns, and is not available for passing back  information.
1285           The  length passed in ovecsize should always be a multiple of three. If
1286           it is not, it is rounded down.
1287    
1288           When a match is successful, information about  captured  substrings  is
1289           returned  in  pairs  of integers, starting at the beginning of ovector,
1290           and continuing up to two-thirds of its length at the  most.  The  first
1291           element of a pair is set to the offset of the first character in a sub-
1292           string, and the second is set to the  offset  of  the  first  character
1293           after  the  end  of  a  substring. The first pair, ovector[0] and ovec-
1294           tor[1], identify the portion of  the  subject  string  matched  by  the
1295           entire  pattern.  The next pair is used for the first capturing subpat-
1296           tern, and so on. The value returned by pcre_exec()  is  the  number  of
1297           pairs  that  have  been set. If there are no capturing subpatterns, the
1298           return value from a successful match is 1,  indicating  that  just  the
1299           first pair of offsets has been set.
1300    
1301           Some  convenience  functions  are  provided for extracting the captured
1302           substrings as separate strings. These are described  in  the  following
1303           section.
1304    
1305           It  is  possible  for  an capturing subpattern number n+1 to match some
1306           part of the subject when subpattern n has not been  used  at  all.  For
1307           example, if the string "abc" is matched against the pattern (a|(z))(bc)
1308           subpatterns 1 and 3 are matched, but 2 is not. When this happens,  both
1309           offset values corresponding to the unused subpattern are set to -1.
1310    
1311           If a capturing subpattern is matched repeatedly, it is the last portion
1312           of the string that it matched that is returned.
1313    
1314           If the vector is too small to hold all the captured substring  offsets,
1315           it is used as far as possible (up to two-thirds of its length), and the
1316           function returns a value of zero. In particular, if the substring  off-
1317           sets are not of interest, pcre_exec() may be called with ovector passed
1318           as NULL and ovecsize as zero. However, if  the  pattern  contains  back
1319           references  and  the  ovector is not big enough to remember the related
1320           substrings, PCRE has to get additional memory for use during  matching.
1321           Thus it is usually advisable to supply an ovector.
1322    
1323           Note  that  pcre_info() can be used to find out how many capturing sub-
1324           patterns there are in a compiled pattern. The smallest size for ovector
1325           that  will  allow for n captured substrings, in addition to the offsets
1326           of the substring matched by the whole pattern, is (n+1)*3.
1327    
1328       Return values from pcre_exec()
1329    
1330           If pcre_exec() fails, it returns a negative number. The  following  are
1331           defined in the header file:
1332    
1333             PCRE_ERROR_NOMATCH        (-1)
1334    
1335           The subject string did not match the pattern.
1336    
1337             PCRE_ERROR_NULL           (-2)
1338    
1339           Either  code  or  subject  was  passed as NULL, or ovector was NULL and
1340           ovecsize was not zero.
1341    
1342             PCRE_ERROR_BADOPTION      (-3)
1343    
1344           An unrecognized bit was set in the options argument.
1345    
1346             PCRE_ERROR_BADMAGIC       (-4)
1347    
1348           PCRE stores a 4-byte "magic number" at the start of the compiled  code,
1349           to catch the case when it is passed a junk pointer and to detect when a
1350           pattern that was compiled in an environment of one endianness is run in
1351           an  environment  with the other endianness. This is the error that PCRE
1352           gives when the magic number is not present.
1353    
1354             PCRE_ERROR_UNKNOWN_NODE   (-5)
1355    
1356           While running the pattern match, an unknown item was encountered in the
1357           compiled  pattern.  This  error  could be caused by a bug in PCRE or by
1358           overwriting of the compiled pattern.
1359    
1360             PCRE_ERROR_NOMEMORY       (-6)
1361    
1362           If a pattern contains back references, but the ovector that  is  passed
1363           to pcre_exec() is not big enough to remember the referenced substrings,
1364           PCRE gets a block of memory at the start of matching to  use  for  this
1365           purpose.  If the call via pcre_malloc() fails, this error is given. The
1366           memory is automatically freed at the end of matching.
1367    
1368             PCRE_ERROR_NOSUBSTRING    (-7)
1369    
1370           This error is used by the pcre_copy_substring(),  pcre_get_substring(),
1371           and  pcre_get_substring_list()  functions  (see  below).  It  is  never
1372           returned by pcre_exec().
1373    
1374             PCRE_ERROR_MATCHLIMIT     (-8)
1375    
1376           The recursion and backtracking limit, as specified by  the  match_limit
1377           field  in  a  pcre_extra  structure (or defaulted) was reached. See the
1378           description above.
1379    
1380             PCRE_ERROR_CALLOUT        (-9)
1381    
1382           This error is never generated by pcre_exec() itself. It is provided for
1383           use  by  callout functions that want to yield a distinctive error code.
1384           See the pcrecallout documentation for details.
1385    
1386             PCRE_ERROR_BADUTF8        (-10)
1387    
1388           A string that contains an invalid UTF-8 byte sequence was passed  as  a
1389           subject.
1390    
1391             PCRE_ERROR_BADUTF8_OFFSET (-11)
1392    
1393           The UTF-8 byte sequence that was passed as a subject was valid, but the
1394           value of startoffset did not point to the beginning of a UTF-8  charac-
1395           ter.
1396    
1397             PCRE_ERROR_PARTIAL (-12)
1398    
1399           The  subject  string did not match, but it did match partially. See the
1400           pcrepartial documentation for details of partial matching.
1401    
1402             PCRE_ERROR_BAD_PARTIAL (-13)
1403    
1404           The PCRE_PARTIAL option was used with  a  compiled  pattern  containing
1405           items  that are not supported for partial matching. See the pcrepartial
1406           documentation for details of partial matching.
1407    
1408             PCRE_ERROR_INTERNAL (-14)
1409    
1410           An unexpected internal error has occurred. This error could  be  caused
1411           by a bug in PCRE or by overwriting of the compiled pattern.
1412    
1413             PCRE_ERROR_BADCOUNT (-15)
1414    
1415           This  error is given if the value of the ovecsize argument is negative.
1416    
1417    
1418    EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
1419    
1420           int pcre_copy_substring(const char *subject, int *ovector,
1421                int stringcount, int stringnumber, char *buffer,
1422                int buffersize);
1423    
1424           int pcre_get_substring(const char *subject, int *ovector,
1425                int stringcount, int stringnumber,
1426                const char **stringptr);
1427    
1428           int pcre_get_substring_list(const char *subject,
1429                int *ovector, int stringcount, const char ***listptr);
1430    
1431           Captured substrings can be  accessed  directly  by  using  the  offsets
1432           returned  by  pcre_exec()  in  ovector.  For convenience, the functions
1433           pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
1434           string_list()  are  provided for extracting captured substrings as new,
1435           separate, zero-terminated strings. These functions identify  substrings
1436           by  number.  The  next section describes functions for extracting named
1437           substrings. A substring  that  contains  a  binary  zero  is  correctly
1438           extracted  and  has  a further zero added on the end, but the result is
1439           not, of course, a C string.
1440    
1441           The first three arguments are the same for all  three  of  these  func-
1442           tions:  subject  is  the subject string that has just been successfully
1443           matched, ovector is a pointer to the vector of integer offsets that was
1444           passed to pcre_exec(), and stringcount is the number of substrings that
1445           were captured by the match, including the substring  that  matched  the
1446           entire regular expression. This is the value returned by pcre_exec() if
1447           it is greater than zero. If pcre_exec() returned zero, indicating  that
1448           it  ran out of space in ovector, the value passed as stringcount should
1449           be the number of elements in the vector divided by three.
1450    
1451           The functions pcre_copy_substring() and pcre_get_substring() extract  a
1452           single  substring,  whose  number  is given as stringnumber. A value of
1453           zero extracts the substring that matched the  entire  pattern,  whereas
1454           higher  values  extract  the  captured  substrings.  For pcre_copy_sub-
1455           string(), the string is placed in buffer,  whose  length  is  given  by
1456           buffersize,  while  for  pcre_get_substring()  a new block of memory is
1457           obtained via pcre_malloc, and its address is  returned  via  stringptr.
1458           The  yield  of  the function is the length of the string, not including
1459           the terminating zero, or one of
1460    
1461             PCRE_ERROR_NOMEMORY       (-6)
1462    
1463           The buffer was too small for pcre_copy_substring(), or the  attempt  to
1464           get memory failed for pcre_get_substring().
1465    
1466             PCRE_ERROR_NOSUBSTRING    (-7)
1467    
1468           There is no substring whose number is stringnumber.
1469    
1470           The  pcre_get_substring_list()  function  extracts  all  available sub-
1471           strings and builds a list of pointers to them. All this is  done  in  a
1472           single block of memory that is obtained via pcre_malloc. The address of
1473           the memory block is returned via listptr, which is also  the  start  of
1474           the  list  of  string pointers. The end of the list is marked by a NULL
1475           pointer. The yield of the function is zero if all went well, or
1476    
1477             PCRE_ERROR_NOMEMORY       (-6)
1478    
1479           if the attempt to get the memory block failed.
1480    
1481           When any of these functions encounter a substring that is unset,  which
1482           can  happen  when  capturing subpattern number n+1 matches some part of
1483           the subject, but subpattern n has not been used at all, they return  an
1484           empty string. This can be distinguished from a genuine zero-length sub-
1485           string by inspecting the appropriate offset in ovector, which is  nega-
1486           tive for unset substrings.
1487    
1488           The  two convenience functions pcre_free_substring() and pcre_free_sub-
1489           string_list() can be used to free the memory  returned  by  a  previous
1490           call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
1491           tively. They do nothing more than  call  the  function  pointed  to  by
1492           pcre_free,  which  of course could be called directly from a C program.
1493           However, PCRE is used in some situations where it is linked via a  spe-
1494           cial  interface  to  another  programming  language  which  cannot  use
1495           pcre_free directly; it is  for  these  cases  that  the  functions  are
1496           provided.
1497    
1498    
1499    EXTRACTING CAPTURED SUBSTRINGS BY NAME
1500    
1501           int pcre_get_stringnumber(const pcre *code,
1502                const char *name);
1503    
1504           int pcre_copy_named_substring(const pcre *code,
1505                const char *subject, int *ovector,
1506                int stringcount, const char *stringname,
1507                char *buffer, int buffersize);
1508    
1509           int pcre_get_named_substring(const pcre *code,
1510                const char *subject, int *ovector,
1511                int stringcount, const char *stringname,
1512                const char **stringptr);
1513    
1514           To  extract a substring by name, you first have to find associated num-
1515           ber.  For example, for this pattern
1516    
1517             (a+)b(?<xxx>\d+)...
1518    
1519           the number of the subpattern called "xxx" is 2. You can find the number
1520           from the name by calling pcre_get_stringnumber(). The first argument is
1521           the compiled pattern, and the second is the  name.  The  yield  of  the
1522           function  is  the  subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if
1523           there is no subpattern of that name.
1524    
1525           Given the number, you can extract the substring directly, or use one of
1526           the functions described in the previous section. For convenience, there
1527           are also two functions that do the whole job.
1528    
1529           Most   of   the   arguments    of    pcre_copy_named_substring()    and
1530           pcre_get_named_substring()  are  the  same  as  those for the similarly
1531           named functions that extract by number. As these are described  in  the
1532           previous  section,  they  are not re-described here. There are just two
1533           differences:
1534    
1535           First, instead of a substring number, a substring name is  given.  Sec-
1536           ond, there is an extra argument, given at the start, which is a pointer
1537           to the compiled pattern. This is needed in order to gain access to  the
1538           name-to-number translation table.
1539    
1540           These  functions call pcre_get_stringnumber(), and if it succeeds, they
1541           then call pcre_copy_substring() or pcre_get_substring(),  as  appropri-
1542           ate.
1543    
1544    Last updated: 09 September 2004
1545    Copyright (c) 1997-2004 University of Cambridge.
1546    -----------------------------------------------------------------------------
1547    
1548    PCRE(3)                                                                PCRE(3)
1549    
1550    
1551    
1552  SunOS 5.8                 Last change:                          9  NAME
1553           PCRE - Perl-compatible regular expressions
1554    
1555    PCRE CALLOUTS
1556    
1557           int (*pcre_callout)(pcre_callout_block *);
1558    
1559       against  a pre-compiled pattern, which is passed in the code         PCRE provides a feature called "callout", which is a means of temporar-
1560       argument. If the pattern has been studied, the result of the         ily passing control to the caller of PCRE  in  the  middle  of  pattern
1561       study should be passed in the extra argument. Otherwise this         matching.  The  caller of PCRE provides an external function by putting
1562       must be NULL.         its entry point in the global variable pcre_callout. By  default,  this
1563           variable contains NULL, which disables all calling out.
1564       Here is an example of a simple call to pcre_exec():  
1565           Within  a  regular  expression,  (?C) indicates the points at which the
1566         int rc;         external function is to be called.  Different  callout  points  can  be
1567         int ovector[30];         identified  by  putting  a number less than 256 after the letter C. The
1568         rc = pcre_exec(         default value is zero.  For  example,  this  pattern  has  two  callout
1569           re,             /* result of pcre_compile() */         points:
1570           NULL,           /* we didn't study the pattern */  
1571           "some string",  /* the subject string */           (?C1)eabc(?C2)def
1572           11,             /* the length of the subject string */  
1573           0,              /* start at offset 0 in the subject */         If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() is
1574           0,              /* default options */         called, PCRE automatically  inserts  callouts,  all  with  number  255,
1575           ovector,        /* vector for substring information */         before  each  item in the pattern. For example, if PCRE_AUTO_CALLOUT is
1576           30);            /* number of elements in the vector */         used with the pattern
1577    
1578       The PCRE_ANCHORED option can be passed in the options  argu-           A(\d{2}|--)
1579       ment,  whose unused bits must be zero. However, if a pattern  
1580       was  compiled  with  PCRE_ANCHORED,  or  turned  out  to  be         it is processed as if it were
1581       anchored  by  virtue  of  its  contents,  it  cannot be made  
1582       unachored at matching time.         (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
1583    
1584       There are also three further options that can be set only at         Notice that there is a callout before and after  each  parenthesis  and
1585       matching time:         alternation  bar.  Automatic  callouts  can  be  used  for tracking the
1586           progress of pattern matching. The pcretest command has an  option  that
1587         PCRE_NOTBOL         sets  automatic callouts; when it is used, the output indicates how the
1588           pattern is matched. This is useful information when you are  trying  to
1589       The first character of the string is not the beginning of  a         optimize the performance of a particular pattern.
1590       line,  so  the  circumflex  metacharacter  should  not match  
1591       before it. Setting this without PCRE_MULTILINE  (at  compile  
1592       time) causes circumflex never to match.  MISSING CALLOUTS
1593    
1594         PCRE_NOTEOL         You  should  be  aware  that,  because of optimizations in the way PCRE
1595           matches patterns, callouts sometimes do not happen. For example, if the
1596       The end of the string is not the end of a line, so the  dol-         pattern is
1597       lar  metacharacter should not match it nor (except in multi-  
1598       line mode) a newline immediately  before  it.  Setting  this           ab(?C4)cd
1599       without PCRE_MULTILINE (at compile time) causes dollar never  
1600       to match.         PCRE knows that any matching string must contain the letter "d". If the
1601           subject string is "abyz", the lack of "d" means that  matching  doesn't
1602         PCRE_NOTEMPTY         ever  start,  and  the  callout is never reached. However, with "abyd",
1603           though the result is still no match, the callout is obeyed.
1604       An empty string is not considered to be  a  valid  match  if  
1605       this  option  is  set. If there are alternatives in the pat-  
1606       tern, they are tried. If  all  the  alternatives  match  the  THE CALLOUT INTERFACE
1607       empty  string,  the  entire match fails. For example, if the  
1608       pattern         During matching, when PCRE reaches a callout point, the external  func-
1609           tion  defined  by pcre_callout is called (if it is set). The only argu-
1610         a?b?         ment is a pointer to a pcre_callout block. This structure contains  the
1611           following fields:
1612       is applied to a string not beginning with  "a"  or  "b",  it  
1613       matches  the  empty string at the start of the subject. With           int          version;
1614       PCRE_NOTEMPTY set, this match is not valid, so PCRE searches           int          callout_number;
1615       further into the string for occurrences of "a" or "b".           int         *offset_vector;
1616             const char  *subject;
1617       Perl has no direct equivalent of PCRE_NOTEMPTY, but it  does           int          subject_length;
1618       make  a  special case of a pattern match of the empty string           int          start_match;
1619       within its split() function, and when using the /g modifier.           int          current_position;
1620       It  is possible to emulate Perl's behaviour after matching a           int          capture_top;
1621       null string by first trying the  match  again  at  the  same           int          capture_last;
1622       offset  with  PCRE_NOTEMPTY  set,  and then if that fails by           void        *callout_data;
1623       advancing the starting offset  (see  below)  and  trying  an           int          pattern_position;
1624       ordinary match again.           int          next_item_length;
1625    
1626       The subject string is passed as  a  pointer  in  subject,  a         The  version  field  is an integer containing the version number of the
1627       length  in  length,  and  a  starting offset in startoffset.         block format. The initial version was 0; the current version is 1.  The
1628       Unlike the pattern string, the subject  may  contain  binary         version  number  will  change  again in future if additional fields are
1629       zero  characters.  When  the  starting  offset  is zero, the         added, but the intention is never to remove any of the existing fields.
1630       search for a match starts at the beginning of  the  subject,  
1631       and this is by far the most common case.         The  callout_number  field  contains the number of the callout, as com-
1632           piled into the pattern (that is, the number after ?C for  manual  call-
1633       A non-zero starting offset  is  useful  when  searching  for         outs, and 255 for automatically generated callouts).
1634       another  match  in  the  same subject by calling pcre_exec()  
1635       again after a previous success.  Setting startoffset differs         The  offset_vector field is a pointer to the vector of offsets that was
1636       from  just  passing  over  a  shortened  string  and setting         passed by the caller to pcre_exec(). The contents can be  inspected  in
1637       PCRE_NOTBOL in the case of a pattern that  begins  with  any         order  to extract substrings that have been matched so far, in the same
1638       kind of lookbehind. For example, consider the pattern         way as for extracting substrings after a match has completed.
1639    
1640         \Biss\B         The subject and subject_length fields contain copies of the values that
1641           were passed to pcre_exec().
1642       which finds occurrences of "iss" in the middle of words. (\B  
1643       matches only if the current position in the subject is not a         The  start_match  field contains the offset within the subject at which
1644       word boundary.) When applied to the string "Mississipi"  the         the current match attempt started. If the pattern is not anchored,  the
1645       first  call  to  pcre_exec()  finds the first occurrence. If         callout function may be called several times from the same point in the
1646       pcre_exec() is called again with just the remainder  of  the         pattern for different starting points in the subject.
1647       subject,  namely  "issipi", it does not match, because \B is  
1648       always false at the start of the subject, which is deemed to         The current_position field contains the offset within  the  subject  of
1649       be  a  word  boundary. However, if pcre_exec() is passed the         the current match pointer.
1650       entire string again, but with startoffset set to 4, it finds  
1651       the  second  occurrence  of "iss" because it is able to look         The  capture_top field contains one more than the number of the highest
1652       behind the starting point to discover that it is preceded by         numbered captured substring so far. If no  substrings  have  been  cap-
1653       a letter.         tured, the value of capture_top is one.
1654    
1655       If a non-zero starting offset is passed when the pattern  is         The  capture_last  field  contains the number of the most recently cap-
1656       anchored, one attempt to match at the given offset is tried.         tured substring. If no substrings have been captured, its value is  -1.
1657       This can only succeed if the pattern does  not  require  the  
1658       match to be at the start of the subject.         The  callout_data  field contains a value that is passed to pcre_exec()
1659           by the caller specifically so that it can be passed back  in  callouts.
1660       In general, a pattern matches a certain portion of the  sub-         It  is  passed  in the pcre_callout field of the pcre_extra data struc-
1661       ject,  and  in addition, further substrings from the subject         ture. If no such data was  passed,  the  value  of  callout_data  in  a
1662       may be picked out by parts of  the  pattern.  Following  the         pcre_callout  block  is  NULL. There is a description of the pcre_extra
1663       usage  in  Jeffrey Friedl's book, this is called "capturing"         structure in the pcreapi documentation.
1664       in what follows, and the phrase  "capturing  subpattern"  is  
1665       used for a fragment of a pattern that picks out a substring.         The pattern_position field is present from version 1 of the  pcre_call-
1666       PCRE supports several other kinds of  parenthesized  subpat-         out structure. It contains the offset to the next item to be matched in
1667       tern that do not cause substrings to be captured.         the pattern string.
1668    
1669       Captured substrings are returned to the caller via a  vector         The next_item_length field is present from version 1 of the  pcre_call-
1670       of  integer  offsets whose address is passed in ovector. The         out structure. It contains the length of the next item to be matched in
1671       number of elements in the vector is passed in ovecsize.  The         the pattern string. When the callout immediately precedes  an  alterna-
1672       first two-thirds of the vector is used to pass back captured         tion  bar, a closing parenthesis, or the end of the pattern, the length
1673       substrings, each substring using a  pair  of  integers.  The         is zero. When the callout precedes an opening parenthesis,  the  length
1674       remaining  third  of  the  vector  is  used  as workspace by         is that of the entire subpattern.
1675       pcre_exec() while matching capturing subpatterns, and is not  
1676       available for passing back information. The length passed in         The  pattern_position  and next_item_length fields are intended to help
1677       ovecsize should always be a multiple of three. If it is not,         in distinguishing between different automatic callouts, which all  have
1678       it is rounded down.         the same callout number. However, they are set for all callouts.
1679    
1680       When a match has been successful, information about captured  
1681       substrings is returned in pairs of integers, starting at the  RETURN VALUES
1682       beginning of ovector, and continuing up to two-thirds of its  
1683       length  at  the  most. The first element of a pair is set to         The  external callout function returns an integer to PCRE. If the value
1684       the offset of the first character in a  substring,  and  the         is zero, matching proceeds as normal. If  the  value  is  greater  than
1685       second is set to the offset of the first character after the         zero,  matching  fails  at  the current point, but backtracking to test
1686       end of a substring. The first  pair,  ovector[0]  and  ovec-         other matching possibilities goes ahead, just as if a lookahead  asser-
1687       tor[1],  identify  the portion of the subject string matched         tion  had  failed.  If  the value is less than zero, the match is aban-
1688       by the entire pattern. The next pair is used for  the  first         doned, and pcre_exec() returns the negative value.
1689       capturing  subpattern,  and  so  on.  The  value returned by  
1690       pcre_exec() is the number of pairs that have  been  set.  If         Negative  values  should  normally  be   chosen   from   the   set   of
1691       there  are no capturing subpatterns, the return value from a         PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
1692       successful match is 1, indicating that just the  first  pair         dard "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT  is
1693       of offsets has been set.         reserved  for  use  by callout functions; it will never be used by PCRE
1694           itself.
1695       Some convenience functions are provided for  extracting  the  
1696       captured substrings as separate strings. These are described  Last updated: 09 September 2004
1697       in the following section.  Copyright (c) 1997-2004 University of Cambridge.
1698    -----------------------------------------------------------------------------
      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 memory is  obtained  via  
      pcre_malloc,  and its address is returned via stringptr. The  
      yield of the function is  the  length  of  the  string,  not  
      including the terminating zero, or one of  
   
        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.  
   
      The  two  convenience  functions  pcre_free_substring()  and  
      pcre_free_substring_list()  can  be  used to free the memory  
      returned by  a  previous  call  of  pcre_get_substring()  or  
      pcre_get_substring_list(),  respectively.  They  do  nothing  
      more than call the function pointed to by  pcre_free,  which  
      of  course  could  be called directly from a C program. How-  
      ever, PCRE is used in some situations where it is linked via  
      a  special  interface  to another programming language which  
      cannot use pcre_free directly; it is for  these  cases  that  
      the functions are provided.  
1699    
1700    PCRE(3)                                                                PCRE(3)
1701    
1702    
 LIMITATIONS  
      There are some size limitations in PCRE but it is hoped that  
      they will never in practice be relevant.  The maximum length  
      of a compiled pattern is 65539 (sic) bytes.  All  values  in  
      repeating  quantifiers  must be less than 65536.  There max-  
      imum number of capturing subpatterns is 65535.  There is  no  
      limit  to  the  number of non-capturing subpatterns, but the  
      maximum depth of nesting of all kinds of parenthesized  sub-  
      pattern,  including  capturing  subpatterns, assertions, and  
      other types of subpattern, is 200.  
   
      The maximum length of a subject string is the largest  posi-  
      tive number that an integer variable can hold. However, PCRE  
      uses recursion to handle subpatterns and indefinite  repeti-  
      tion.  This  means  that the available stack space may limit  
      the size of a subject string that can be processed  by  cer-  
      tain patterns.  
   
   
   
 DIFFERENCES FROM PERL  
      The differences described here  are  with  respect  to  Perl  
      5.005.  
   
      1. By default, a whitespace character is any character  that  
      the  C  library  function isspace() recognizes, though it is  
      possible to compile PCRE  with  alternative  character  type  
      tables. Normally isspace() matches space, formfeed, newline,  
      carriage return, horizontal tab, and vertical tab. Perl 5 no  
      longer  includes vertical tab in its set of whitespace char-  
      acters. The \v escape that was in the Perl documentation for  
      a long time was never in fact recognized. However, the char-  
      acter itself was treated as whitespace at least up to 5.002.  
      In 5.004 and 5.005 it does not match \s.  
   
      2. PCRE does  not  allow  repeat  quantifiers  on  lookahead  
      assertions. Perl permits them, but they do not mean what you  
      might think. For example, (?!a){3} does not assert that  the  
      next  three characters are not "a". It just asserts that the  
      next character is not "a" three times.  
   
      3. Capturing subpatterns that occur inside  negative  looka-  
      head  assertions  are  counted,  but  their  entries  in the  
      offsets vector are never set. Perl sets its numerical  vari-  
      ables  from  any  such  patterns that are matched before the  
      assertion fails to match something (thereby succeeding), but  
      only  if  the negative lookahead assertion contains just one  
      branch.  
   
      4. Though binary zero characters are supported in  the  sub-  
      ject  string,  they  are  not  allowed  in  a pattern string  
      because it is passed as a normal  C  string,  terminated  by  
      zero. The escape sequence "\0" can be used in the pattern to  
      represent a binary zero.  
   
      5. The following Perl escape sequences  are  not  supported:  
      \l,  \u,  \L,  \U,  \E, \Q. In fact these are implemented by  
      Perl's general string-handling and are not part of its  pat-  
      tern matching engine.  
   
      6. The Perl \G assertion is  not  supported  as  it  is  not  
      relevant to single pattern matches.  
   
      7. Fairly obviously, PCRE does not support the (?{code}) and  
      (?p{code})  constructions. However, there is some experimen-  
      tal support for recursive patterns using the  non-Perl  item  
      (?R).  
   
      8. There are at the time of writing some  oddities  in  Perl  
      5.005_02  concerned  with  the  settings of captured strings  
      when part of a pattern is repeated.  For  example,  matching  
      "aba"  against the pattern /^(a(b)?)+$/ sets $2 to the value  
      "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves  $2  
      unset.    However,    if   the   pattern   is   changed   to  
      /^(aa(b(b))?)+$/ then $2 (and $3) are set.  
   
      In Perl 5.004 $2 is set in both cases, and that is also true  
      of PCRE. If in the future Perl changes to a consistent state  
      that is different, PCRE may change to follow.  
   
      9. Another as yet unresolved discrepancy  is  that  in  Perl  
      5.005_02  the  pattern /^(a)?(?(1)a|b)+$/ matches the string  
      "a", whereas in PCRE it does not.  However, in both Perl and  
      PCRE /^(a)?a/ matched against "a" leaves $1 unset.  
   
      10. PCRE  provides  some  extensions  to  the  Perl  regular  
      expression facilities:  
   
      (a) Although lookbehind assertions must match  fixed  length  
      strings,  each  alternative branch of a lookbehind assertion  
      can match a different length of string. Perl 5.005  requires  
      them all to have the same length.  
   
      (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is  not  
      set,  the  $ meta- character matches only at the very end of  
      the string.  
   
      (c) If PCRE_EXTRA is set, a backslash followed by  a  letter  
      with no special meaning is faulted.  
   
      (d) If PCRE_UNGREEDY is set, the greediness of  the  repeti-  
      tion  quantifiers  is inverted, that is, by default they are  
      not greedy, but if followed by a question mark they are.  
   
      (e) PCRE_ANCHORED can be used to force a pattern to be tried  
      only at the start of the subject.  
   
      (f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY  options  
      for pcre_exec() have no Perl equivalents.  
   
      (g) The (?R) construct allows for recursive pattern matching  
      (Perl  5.6 can do this using the (?p{code}) construct, which  
      PCRE cannot of course support.)  
   
   
   
 REGULAR EXPRESSION DETAILS  
      The syntax and semantics of  the  regular  expressions  sup-  
      ported  by PCRE are described below. Regular expressions are  
      also described in the Perl documentation and in a number  of  
      other  books,  some  of which have copious examples. Jeffrey  
      Friedl's  "Mastering  Regular  Expressions",  published   by  
      O'Reilly (ISBN 1-56592-257), covers them in great detail.  
   
      The description here is intended as reference documentation.  
      The basic operation of PCRE is on strings of bytes. However,  
      there is the beginnings of some support for UTF-8  character  
      strings.  To  use  this  support  you must configure PCRE to  
      include it, and then call pcre_compile() with the  PCRE_UTF8  
      option.  How  this affects the pattern matching is described  
      in the final section of this document.  
   
      A regular expression is a pattern that is matched against  a  
      subject string from left to right. Most characters stand for  
      themselves in a pattern, and match the corresponding charac-  
      ters in the subject. As a trivial example, the pattern  
   
        The quick brown fox  
   
      matches a portion of a subject string that is  identical  to  
      itself.  The  power  of  regular  expressions comes from the  
      ability to include alternatives and repetitions in the  pat-  
      tern.  These  are encoded in the pattern by the use of meta-  
      characters, which do not stand for  themselves  but  instead  
      are interpreted in some special way.  
   
      There are two different sets of meta-characters: those  that  
      are  recognized anywhere in the pattern except within square  
      brackets, and those that are recognized in square  brackets.  
      Outside square brackets, the meta-characters are as follows:  
   
        \      general escape character with several uses  
        ^      assert start of  subject  (or  line,  in  multiline  
      mode)  
        $      assert end of subject (or line, in multiline mode)  
        .      match any character except newline (by default)  
        [      start character class definition  
        |      start of alternative branch  
        (      start subpattern  
        )      end subpattern  
        ?      extends the meaning of (  
               also 0 or 1 quantifier  
               also quantifier minimizer  
        *      0 or more quantifier  
        +      1 or more quantifier  
        {      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  
1703    
1704       The following sections describe  the  use  of  each  of  the  NAME
1705       meta-characters.         PCRE - Perl-compatible regular expressions
1706    
1707    DIFFERENCES BETWEEN PCRE AND PERL
1708    
1709           This  document describes the differences in the ways that PCRE and Perl
1710           handle regular expressions. The differences  described  here  are  with
1711           respect to Perl 5.8.
1712    
1713           1.  PCRE does not have full UTF-8 support. Details of what it does have
1714           are given in the section on UTF-8 support in the main pcre page.
1715    
1716           2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
1717           permits  them,  but they do not mean what you might think. For example,
1718           (?!a){3} does not assert that the next three characters are not "a". It
1719           just asserts that the next character is not "a" three times.
1720    
1721           3.  Capturing  subpatterns  that occur inside negative lookahead asser-
1722           tions are counted, but their entries in the offsets  vector  are  never
1723           set.  Perl sets its numerical variables from any such patterns that are
1724           matched before the assertion fails to match something (thereby succeed-
1725           ing),  but  only  if the negative lookahead assertion contains just one
1726           branch.
1727    
1728           4. Though binary zero characters are supported in the  subject  string,
1729           they are not allowed in a pattern string because it is passed as a nor-
1730           mal C string, terminated by zero. The escape sequence \0 can be used in
1731           the pattern to represent a binary zero.
1732    
1733           5.  The  following Perl escape sequences are not supported: \l, \u, \L,
1734           \U, and \N. In fact these are implemented by Perl's general string-han-
1735           dling  and are not part of its pattern matching engine. If any of these
1736           are encountered by PCRE, an error is generated.
1737    
1738           6. The Perl escape sequences \p, \P, and \X are supported only if  PCRE
1739           is  built  with Unicode character property support. The properties that
1740           can be tested with \p and \P are limited to the general category  prop-
1741           erties such as Lu and Nd.
1742    
1743           7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
1744           ters in between are treated as literals.  This  is  slightly  different
1745           from  Perl  in  that  $  and  @ are also handled as literals inside the
1746           quotes. In Perl, they cause variable interpolation (but of course  PCRE
1747           does not have variables). Note the following examples:
1748    
1749               Pattern            PCRE matches      Perl matches
1750    
1751               \Qabc$xyz\E        abc$xyz           abc followed by the
1752                                                      contents of $xyz
1753               \Qabc\$xyz\E       abc\$xyz          abc\$xyz
1754               \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
1755    
1756           The  \Q...\E  sequence  is recognized both inside and outside character
1757           classes.
1758    
1759           8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
1760           constructions.  However,  there is support for recursive patterns using
1761           the non-Perl items (?R),  (?number),  and  (?P>name).  Also,  the  PCRE
1762           "callout"  feature allows an external function to be called during pat-
1763           tern matching. See the pcrecallout documentation for details.
1764    
1765           9. There are some differences that are concerned with the  settings  of
1766           captured  strings  when  part  of  a  pattern is repeated. For example,
1767           matching "aba" against the  pattern  /^(a(b)?)+$/  in  Perl  leaves  $2
1768           unset, but in PCRE it is set to "b".
1769    
1770           10. PCRE provides some extensions to the Perl regular expression facil-
1771           ities:
1772    
1773           (a) Although lookbehind assertions must  match  fixed  length  strings,
1774           each alternative branch of a lookbehind assertion can match a different
1775           length of string. Perl requires them all to have the same length.
1776    
1777           (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the  $
1778           meta-character matches only at the very end of the string.
1779    
1780           (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
1781           cial meaning is faulted.
1782    
1783           (d) If PCRE_UNGREEDY is set, the greediness of the  repetition  quanti-
1784           fiers is inverted, that is, by default they are not greedy, but if fol-
1785           lowed by a question mark they are.
1786    
1787           (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
1788           tried only at the first matching position in the subject string.
1789    
1790           (f)  The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
1791           TURE options for pcre_exec() have no Perl equivalents.
1792    
1793           (g) The (?R), (?number), and (?P>name) constructs allows for  recursive
1794           pattern  matching  (Perl  can  do  this using the (?p{code}) construct,
1795           which PCRE cannot support.)
1796    
1797           (h) PCRE supports named capturing substrings, using the Python  syntax.
1798    
1799           (i)  PCRE  supports  the  possessive quantifier "++" syntax, taken from
1800           Sun's Java package.
1801    
1802           (j) The (R) condition, for testing recursion, is a PCRE extension.
1803    
1804           (k) The callout facility is PCRE-specific.
1805    
1806           (l) The partial matching facility is PCRE-specific.
1807    
1808           (m) Patterns compiled by PCRE can be saved and re-used at a later time,
1809           even on different hosts that have the other endianness.
1810    
1811    Last updated: 09 September 2004
1812    Copyright (c) 1997-2004 University of Cambridge.
1813    -----------------------------------------------------------------------------
1814    
1815    PCRE(3)                                                                PCRE(3)
1816    
1817    
1818    
1819    NAME
1820           PCRE - Perl-compatible regular expressions
1821    
1822    PCRE REGULAR EXPRESSION DETAILS
1823    
1824           The  syntax  and semantics of the regular expressions supported by PCRE
1825           are described below. Regular expressions are also described in the Perl
1826           documentation  and  in  a  number  of books, some of which have copious
1827           examples.  Jeffrey Friedl's "Mastering Regular Expressions",  published
1828           by  O'Reilly, covers regular expressions in great detail. This descrip-
1829           tion of PCRE's regular expressions is intended as reference material.
1830    
1831           The original operation of PCRE was on strings of  one-byte  characters.
1832           However,  there is now also support for UTF-8 character strings. To use
1833           this, you must build PCRE to  include  UTF-8  support,  and  then  call
1834           pcre_compile()  with  the  PCRE_UTF8  option.  How this affects pattern
1835           matching is mentioned in several places below. There is also a  summary
1836           of  UTF-8  features  in  the  section on UTF-8 support in the main pcre
1837           page.
1838    
1839           A regular expression is a pattern that is  matched  against  a  subject
1840           string  from  left  to right. Most characters stand for themselves in a
1841           pattern, and match the corresponding characters in the  subject.  As  a
1842           trivial example, the pattern
1843    
1844             The quick brown fox
1845    
1846           matches  a portion of a subject string that is identical to itself. The
1847           power of regular expressions comes from the ability to include alterna-
1848           tives  and repetitions in the pattern. These are encoded in the pattern
1849           by the use of metacharacters, which do not  stand  for  themselves  but
1850           instead are interpreted in some special way.
1851    
1852           There  are  two different sets of metacharacters: those that are recog-
1853           nized anywhere in the pattern except within square brackets, and  those
1854           that  are  recognized  in square brackets. Outside square brackets, the
1855           metacharacters are as follows:
1856    
1857             \      general escape character with several uses
1858             ^      assert start of string (or line, in multiline mode)
1859             $      assert end of string (or line, in multiline mode)
1860             .      match any character except newline (by default)
1861             [      start character class definition
1862             |      start of alternative branch
1863             (      start subpattern
1864             )      end subpattern
1865             ?      extends the meaning of (
1866                    also 0 or 1 quantifier
1867                    also quantifier minimizer
1868             *      0 or more quantifier
1869             +      1 or more quantifier
1870                    also "possessive quantifier"
1871             {      start min/max quantifier
1872    
1873           Part of a pattern that is in square brackets  is  called  a  "character
1874           class". In a character class the only metacharacters are:
1875    
1876             \      general escape character
1877             ^      negate the class, but only if the first character
1878             -      indicates character range
1879             [      POSIX character class (only if followed by POSIX
1880                      syntax)
1881             ]      terminates the character class
1882    
1883           The  following sections describe the use of each of the metacharacters.
1884    
1885    
1886  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.  
1887    
1888           The backslash character has several uses. Firstly, if it is followed by
1889           a  non-alphanumeric  character,  it takes away any special meaning that
1890           character may have. This  use  of  backslash  as  an  escape  character
1891           applies both inside and outside character classes.
1892    
1893           For  example,  if  you want to match a * character, you write \* in the
1894           pattern.  This escaping action applies whether  or  not  the  following
1895           character  would  otherwise be interpreted as a metacharacter, so it is
1896           always safe to precede a non-alphanumeric  with  backslash  to  specify
1897           that  it stands for itself. In particular, if you want to match a back-
1898           slash, you write \\.
1899    
1900           If a pattern is compiled with the PCRE_EXTENDED option,  whitespace  in
1901           the  pattern (other than in a character class) and characters between a
1902           # outside a character class and the next newline character are ignored.
1903           An  escaping backslash can be used to include a whitespace or # charac-
1904           ter as part of the pattern.
1905    
1906           If you want to remove the special meaning from a  sequence  of  charac-
1907           ters,  you can do so by putting them between \Q and \E. This is differ-
1908           ent from Perl in that $ and  @  are  handled  as  literals  in  \Q...\E
1909           sequences  in  PCRE, whereas in Perl, $ and @ cause variable interpola-
1910           tion. Note the following examples:
1911    
1912             Pattern            PCRE matches   Perl matches
1913    
1914             \Qabc$xyz\E        abc$xyz        abc followed by the
1915                                                 contents of $xyz
1916             \Qabc\$xyz\E       abc\$xyz       abc\$xyz
1917             \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
1918    
1919           The \Q...\E sequence is recognized both inside  and  outside  character
1920           classes.
1921    
1922       Non-printing characters
1923    
1924           A second use of backslash provides a way of encoding non-printing char-
1925           acters in patterns in a visible manner. There is no restriction on  the
1926           appearance  of non-printing characters, apart from the binary zero that
1927           terminates a pattern, but when a pattern  is  being  prepared  by  text
1928           editing,  it  is  usually  easier  to  use  one of the following escape
1929           sequences than the binary character it represents:
1930    
1931             \a        alarm, that is, the BEL character (hex 07)
1932             \cx       "control-x", where x is any character
1933             \e        escape (hex 1B)
1934             \f        formfeed (hex 0C)
1935             \n        newline (hex 0A)
1936             \r        carriage return (hex 0D)
1937             \t        tab (hex 09)
1938             \ddd      character with octal code ddd, or backreference
1939             \xhh      character with hex code hh
1940             \x{hhh..} character with hex code hhh... (UTF-8 mode only)
1941    
1942           The precise effect of \cx is as follows: if x is a lower  case  letter,
1943           it  is converted to upper case. Then bit 6 of the character (hex 40) is
1944           inverted.  Thus \cz becomes hex 1A, but \c{ becomes hex 3B,  while  \c;
1945           becomes hex 7B.
1946    
1947           After  \x, from zero to two hexadecimal digits are read (letters can be
1948           in upper or lower case). In UTF-8 mode, any number of hexadecimal  dig-
1949           its  may  appear between \x{ and }, but the value of the character code
1950           must be less than 2**31 (that is,  the  maximum  hexadecimal  value  is
1951           7FFFFFFF).  If  characters other than hexadecimal digits appear between
1952           \x{ and }, or if there is no terminating }, this form of escape is  not
1953           recognized. Instead, the initial \x will be interpreted as a basic hex-
1954           adecimal escape, with no following digits,  giving  a  character  whose
1955           value is zero.
1956    
1957           Characters whose value is less than 256 can be defined by either of the
1958           two syntaxes for \x when PCRE is in UTF-8 mode. There is no  difference
1959           in  the  way they are handled. For example, \xdc is exactly the same as
1960           \x{dc}.
1961    
1962           After \0 up to two further octal digits are read.  In  both  cases,  if
1963           there  are fewer than two digits, just those that are present are used.
1964           Thus the sequence \0\x\07 specifies two binary zeros followed by a  BEL
1965           character  (code  value  7).  Make sure you supply two digits after the
1966           initial zero if the pattern character that follows is itself  an  octal
1967           digit.
1968    
1969           The handling of a backslash followed by a digit other than 0 is compli-
1970           cated.  Outside a character class, PCRE reads it and any following dig-
1971           its  as  a  decimal  number. If the number is less than 10, or if there
1972           have been at least that many previous capturing left parentheses in the
1973           expression,  the  entire  sequence  is  taken  as  a  back reference. A
1974           description of how this works is given later, following the  discussion
1975           of parenthesized subpatterns.
1976    
1977           Inside  a  character  class, or if the decimal number is greater than 9
1978           and there have not been that many capturing subpatterns, PCRE  re-reads
1979           up  to three octal digits following the backslash, and generates a sin-
1980           gle byte from the least significant 8 bits of the value. Any subsequent
1981           digits stand for themselves.  For example:
1982    
1983             \040   is another way of writing a space
1984             \40    is the same, provided there are fewer than 40
1985                       previous capturing subpatterns
1986             \7     is always a back reference
1987             \11    might be a back reference, or another way of
1988                       writing a tab
1989             \011   is always a tab
1990             \0113  is a tab followed by the character "3"
1991             \113   might be a back reference, otherwise the
1992                       character with octal code 113
1993             \377   might be a back reference, otherwise
1994                       the byte consisting entirely of 1 bits
1995             \81    is either a back reference, or a binary zero
1996                       followed by the two characters "8" and "1"
1997    
1998           Note  that  octal  values of 100 or greater must not be introduced by a
1999           leading zero, because no more than three octal digits are ever read.
2000    
2001           All the sequences that define a single byte value  or  a  single  UTF-8
2002           character (in UTF-8 mode) can be used both inside and outside character
2003           classes. In addition, inside a character  class,  the  sequence  \b  is
2004           interpreted as the backspace character (hex 08), and the sequence \X is
2005           interpreted as the character "X".  Outside  a  character  class,  these
2006           sequences have different meanings (see below).
2007    
2008       Generic character types
2009    
2010           The  third  use of backslash is for specifying generic character types.
2011           The following are always recognized:
2012    
2013             \d     any decimal digit
2014             \D     any character that is not a decimal digit
2015             \s     any whitespace character
2016             \S     any character that is not a whitespace character
2017             \w     any "word" character
2018             \W     any "non-word" character
2019    
2020           Each pair of escape sequences partitions the complete set of characters
2021           into  two disjoint sets. Any given character matches one, and only one,
2022           of each pair.
2023    
2024           These character type sequences can appear both inside and outside char-
2025           acter  classes.  They each match one character of the appropriate type.
2026           If the current matching point is at the end of the subject string,  all
2027           of them fail, since there is no character to match.
2028    
2029           For  compatibility  with Perl, \s does not match the VT character (code
2030           11).  This makes it different from the the POSIX "space" class. The  \s
2031           characters are HT (9), LF (10), FF (12), CR (13), and space (32).
2032    
2033           A "word" character is an underscore or any character less than 256 that
2034           is a letter or digit. The definition of  letters  and  digits  is  con-
2035           trolled  by PCRE's low-valued character tables, and may vary if locale-
2036           specific matching is taking place (see "Locale support" in the  pcreapi
2037           page).  For  example,  in  the  "fr_FR" (French) locale, some character
2038           codes greater than 128 are used for accented  letters,  and  these  are
2039           matched by \w.
2040    
2041           In  UTF-8 mode, characters with values greater than 128 never match \d,
2042           \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
2043           code character property support is available.
2044    
2045       Unicode character properties
2046    
2047           When PCRE is built with Unicode character property support, three addi-
2048           tional escape sequences to match generic character types are  available
2049           when UTF-8 mode is selected. They are:
2050    
2051            \p{xx}   a character with the xx property
2052            \P{xx}   a character without the xx property
2053            \X       an extended Unicode sequence
2054    
2055           The  property  names represented by xx above are limited to the Unicode
2056           general category properties. Each character has exactly one such  prop-
2057           erty,  specified  by  a two-letter abbreviation. For compatibility with
2058           Perl, negation can be specified by including a circumflex  between  the
2059           opening  brace  and the property name. For example, \p{^Lu} is the same
2060           as \P{Lu}.
2061    
2062           If only one letter is specified with \p or  \P,  it  includes  all  the
2063           properties that start with that letter. In this case, in the absence of
2064           negation, the curly brackets in the escape sequence are optional; these
2065           two examples have the same effect:
2066    
2067             \p{L}
2068             \pL
2069    
2070           The following property codes are supported:
2071    
2072             C     Other
2073             Cc    Control
2074             Cf    Format
2075             Cn    Unassigned
2076             Co    Private use
2077             Cs    Surrogate
2078    
2079             L     Letter
2080             Ll    Lower case letter
2081             Lm    Modifier letter
2082             Lo    Other letter
2083             Lt    Title case letter
2084             Lu    Upper case letter
2085    
2086             M     Mark
2087             Mc    Spacing mark
2088             Me    Enclosing mark
2089             Mn    Non-spacing mark
2090    
2091             N     Number
2092             Nd    Decimal number
2093             Nl    Letter number
2094             No    Other number
2095    
2096             P     Punctuation
2097             Pc    Connector punctuation
2098             Pd    Dash punctuation
2099             Pe    Close punctuation
2100             Pf    Final punctuation
2101             Pi    Initial punctuation
2102             Po    Other punctuation
2103             Ps    Open punctuation
2104    
2105             S     Symbol
2106             Sc    Currency symbol
2107             Sk    Modifier symbol
2108             Sm    Mathematical symbol
2109             So    Other symbol
2110    
2111             Z     Separator
2112             Zl    Line separator
2113             Zp    Paragraph separator
2114             Zs    Space separator
2115    
2116           Extended  properties such as "Greek" or "InMusicalSymbols" are not sup-
2117           ported by PCRE.
2118    
2119           Specifying caseless matching does not affect  these  escape  sequences.
2120           For example, \p{Lu} always matches only upper case letters.
2121    
2122           The  \X  escape  matches  any number of Unicode characters that form an
2123           extended Unicode sequence. \X is equivalent to
2124    
2125             (?>\PM\pM*)
2126    
2127           That is, it matches a character without the "mark"  property,  followed
2128           by  zero  or  more  characters with the "mark" property, and treats the
2129           sequence as an atomic group (see below).  Characters  with  the  "mark"
2130           property are typically accents that affect the preceding character.
2131    
2132           Matching  characters  by Unicode property is not fast, because PCRE has
2133           to search a structure that contains  data  for  over  fifteen  thousand
2134           characters. That is why the traditional escape sequences such as \d and
2135           \w do not use Unicode properties in PCRE.
2136    
2137       Simple assertions
2138    
2139           The fourth use of backslash is for certain simple assertions. An asser-
2140           tion  specifies a condition that has to be met at a particular point in
2141           a match, without consuming any characters from the subject string.  The
2142           use  of subpatterns for more complicated assertions is described below.
2143           The backslashed assertions are:
2144    
2145             \b     matches at a word boundary
2146             \B     matches when not at a word boundary
2147             \A     matches at start of subject
2148             \Z     matches at end of subject or before newline at end
2149             \z     matches at end of subject
2150             \G     matches at first matching position in subject
2151    
2152           These assertions may not appear in character classes (but note that  \b
2153           has a different meaning, namely the backspace character, inside a char-
2154           acter class).
2155    
2156           A word boundary is a position in the subject string where  the  current
2157           character  and  the previous character do not both match \w or \W (i.e.
2158           one matches \w and the other matches \W), or the start or  end  of  the
2159           string if the first or last character matches \w, respectively.
2160    
2161           The  \A,  \Z,  and \z assertions differ from the traditional circumflex
2162           and dollar (described in the next section) in that they only ever match
2163           at  the  very start and end of the subject string, whatever options are
2164           set. Thus, they are independent of multiline mode. These  three  asser-
2165           tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
2166           affect only the behaviour of the circumflex and dollar  metacharacters.
2167           However,  if the startoffset argument of pcre_exec() is non-zero, indi-
2168           cating that matching is to start at a point other than the beginning of
2169           the  subject,  \A  can never match. The difference between \Z and \z is
2170           that \Z matches before a newline that is  the  last  character  of  the
2171           string  as well as at the end of the string, whereas \z matches only at
2172           the end.
2173    
2174           The \G assertion is true only when the current matching position is  at
2175           the  start point of the match, as specified by the startoffset argument
2176           of pcre_exec(). It differs from \A when the  value  of  startoffset  is
2177           non-zero.  By calling pcre_exec() multiple times with appropriate argu-
2178           ments, you can mimic Perl's /g option, and it is in this kind of imple-
2179           mentation where \G can be useful.
2180    
2181           Note,  however,  that  PCRE's interpretation of \G, as the start of the
2182           current match, is subtly different from Perl's, which defines it as the
2183           end  of  the  previous  match. In Perl, these can be different when the
2184           previously matched string was empty. Because PCRE does just  one  match
2185           at a time, it cannot reproduce this behaviour.
2186    
2187           If  all  the alternatives of a pattern begin with \G, the expression is
2188           anchored to the starting match position, and the "anchored" flag is set
2189           in the compiled regular expression.
2190    
2191    
2192  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 it is  always  anchored,  
      whether PCRE_MULTILINE is set or not.  
2193    
2194           Outside a character class, in the default matching mode, the circumflex
2195           character is an assertion that is true only  if  the  current  matching
2196           point  is  at the start of the subject string. If the startoffset argu-
2197           ment of pcre_exec() is non-zero, circumflex  can  never  match  if  the
2198           PCRE_MULTILINE  option  is  unset. Inside a character class, circumflex
2199           has an entirely different meaning (see below).
2200    
2201           Circumflex need not be the first character of the pattern if  a  number
2202           of  alternatives are involved, but it should be the first thing in each
2203           alternative in which it appears if the pattern is ever  to  match  that
2204           branch.  If all possible alternatives start with a circumflex, that is,
2205           if the pattern is constrained to match only at the start  of  the  sub-
2206           ject,  it  is  said  to be an "anchored" pattern. (There are also other
2207           constructs that can cause a pattern to be anchored.)
2208    
2209           A dollar character is an assertion that is true  only  if  the  current
2210           matching  point  is  at  the  end of the subject string, or immediately
2211           before a newline character that is the last character in the string (by
2212           default).  Dollar  need  not  be the last character of the pattern if a
2213           number of alternatives are involved, but it should be the last item  in
2214           any  branch  in  which  it appears.  Dollar has no special meaning in a
2215           character class.
2216    
2217           The meaning of dollar can be changed so that it  matches  only  at  the
2218           very  end  of  the string, by setting the PCRE_DOLLAR_ENDONLY option at
2219           compile time. This does not affect the \Z assertion.
2220    
2221           The meanings of the circumflex and dollar characters are changed if the
2222           PCRE_MULTILINE option is set. When this is the case, they match immedi-
2223           ately after and  immediately  before  an  internal  newline  character,
2224           respectively,  in addition to matching at the start and end of the sub-
2225           ject string. For example,  the  pattern  /^abc$/  matches  the  subject
2226           string  "def\nabc"  (where \n represents a newline character) in multi-
2227           line mode, but not otherwise.  Consequently, patterns that are anchored
2228           in  single line mode because all branches start with ^ are not anchored
2229           in multiline mode, and a match for  circumflex  is  possible  when  the
2230           startoffset   argument   of  pcre_exec()  is  non-zero.  The  PCRE_DOL-
2231           LAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
2232    
2233           Note that the sequences \A, \Z, and \z can be used to match  the  start
2234           and  end of the subject in both modes, and if all branches of a pattern
2235           start with \A it is always anchored, whether PCRE_MULTILINE is  set  or
2236           not.
2237    
2238    
2239  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.  
2240    
2241           Outside a character class, a dot in the pattern matches any one charac-
2242           ter in the subject, including a non-printing  character,  but  not  (by
2243           default)  newline.   In  UTF-8 mode, a dot matches any UTF-8 character,
2244           which might be more than one byte long, except (by default) newline. If
2245           the  PCRE_DOTALL  option  is set, dots match newlines as well. The han-
2246           dling of dot is entirely independent of the handling of circumflex  and
2247           dollar,  the  only  relationship  being  that they both involve newline
2248           characters. Dot has no special meaning in a character class.
2249    
2250    
2251    MATCHING A SINGLE BYTE
2252    
2253           Outside a character class, the escape sequence \C matches any one byte,
2254           both  in  and  out of UTF-8 mode. Unlike a dot, it can match a newline.
2255           The feature is provided in Perl in order to match individual  bytes  in
2256           UTF-8  mode.  Because  it  breaks  up  UTF-8 characters into individual
2257           bytes, what remains in the string may be a malformed UTF-8 string.  For
2258           this reason, the \C escape sequence is best avoided.
2259    
2260           PCRE  does  not  allow \C to appear in lookbehind assertions (described
2261           below), because in UTF-8 mode this would make it impossible  to  calcu-
2262           late the length of the lookbehind.
2263    
2264    
2265    SQUARE BRACKETS AND CHARACTER CLASSES
2266    
2267           An opening square bracket introduces a character class, terminated by a
2268           closing square bracket. A closing square bracket on its own is not spe-
2269           cial. If a closing square bracket is required as a member of the class,
2270           it should be the first data character in the class  (after  an  initial
2271           circumflex, if present) or escaped with a backslash.
2272    
2273           A  character  class matches a single character in the subject. In UTF-8
2274           mode, the character may occupy more than one byte. A matched  character
2275           must be in the set of characters defined by the class, unless the first
2276           character in the class definition is a circumflex, in  which  case  the
2277           subject  character  must  not  be in the set defined by the class. If a
2278           circumflex is actually required as a member of the class, ensure it  is
2279           not the first character, or escape it with a backslash.
2280    
2281           For  example, the character class [aeiou] matches any lower case vowel,
2282           while [^aeiou] matches any character that is not a  lower  case  vowel.
2283           Note that a circumflex is just a convenient notation for specifying the
2284           characters that are in the class by enumerating those that are  not.  A
2285           class  that starts with a circumflex is not an assertion: it still con-
2286           sumes a character from the subject string, and therefore  it  fails  if
2287           the current pointer is at the end of the string.
2288    
2289           In  UTF-8 mode, characters with values greater than 255 can be included
2290           in a class as a literal string of bytes, or by using the  \x{  escaping
2291           mechanism.
2292    
2293           When  caseless  matching  is set, any letters in a class represent both
2294           their upper case and lower case versions, so for  example,  a  caseless
2295           [aeiou]  matches  "A"  as well as "a", and a caseless [^aeiou] does not
2296           match "A", whereas a caseful version would. When running in UTF-8 mode,
2297           PCRE  supports  the  concept of case for characters with values greater
2298           than 128 only when it is compiled with Unicode property support.
2299    
2300           The newline character is never treated in any special way in  character
2301           classes,  whatever  the  setting  of  the PCRE_DOTALL or PCRE_MULTILINE
2302           options is. A class such as [^a] will always match a newline.
2303    
2304           The minus (hyphen) character can be used to specify a range of  charac-
2305           ters  in  a  character  class.  For  example,  [d-m] matches any letter
2306           between d and m, inclusive. If a  minus  character  is  required  in  a
2307           class,  it  must  be  escaped  with a backslash or appear in a position
2308           where it cannot be interpreted as indicating a range, typically as  the
2309           first or last character in the class.
2310    
2311           It is not possible to have the literal character "]" as the end charac-
2312           ter of a range. A pattern such as [W-]46] is interpreted as a class  of
2313           two  characters ("W" and "-") followed by a literal string "46]", so it
2314           would match "W46]" or "-46]". However, if the "]"  is  escaped  with  a
2315           backslash  it is interpreted as the end of range, so [W-\]46] is inter-
2316           preted as a class containing a range followed by two other  characters.
2317           The  octal or hexadecimal representation of "]" can also be used to end
2318           a range.
2319    
2320           Ranges operate in the collating sequence of character values. They  can
2321           also   be  used  for  characters  specified  numerically,  for  example
2322           [\000-\037]. In UTF-8 mode, ranges can include characters whose  values
2323           are greater than 255, for example [\x{100}-\x{2ff}].
2324    
2325           If a range that includes letters is used when caseless matching is set,
2326           it matches the letters in either case. For example, [W-c] is equivalent
2327           to  [][\\^_`wxyzabc],  matched  caselessly,  and  in non-UTF-8 mode, if
2328           character tables for the "fr_FR" locale are in use, [\xc8-\xcb] matches
2329           accented  E  characters in both cases. In UTF-8 mode, PCRE supports the
2330           concept of case for characters with values greater than 128  only  when
2331           it is compiled with Unicode property support.
2332    
2333           The  character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
2334           in a character class, and add the characters that  they  match  to  the
2335           class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
2336           flex can conveniently be used with the upper case  character  types  to
2337           specify  a  more  restricted  set of characters than the matching lower
2338           case type. For example, the class [^\W_] matches any letter  or  digit,
2339           but not underscore.
2340    
2341           The  only  metacharacters  that are recognized in character classes are
2342           backslash, hyphen (only where it can be  interpreted  as  specifying  a
2343           range),  circumflex  (only  at the start), opening square bracket (only
2344           when it can be interpreted as introducing a POSIX class name - see  the
2345           next  section),  and  the  terminating closing square bracket. However,
2346           escaping other non-alphanumeric characters does no harm.
2347    
2348    
2349  POSIX CHARACTER CLASSES  POSIX CHARACTER CLASSES
      Perl 5.6 (not yet released at the time of writing) is  going  
      to  support  the POSIX notation for character classes, which  
      uses names enclosed by  [:  and  :]   within  the  enclosing  
      square brackets. PCRE supports this notation. For example,  
   
        [01[:alpha:]%]  
   
      matches "0", "1", any alphabetic character, or "%". The sup-  
      ported class names are  
   
        alnum    letters and digits  
        alpha    letters  
        ascii    character codes 0 - 127  
        cntrl    control characters  
        digit    decimal digits (same as \d)  
        graph    printing characters, excluding space  
        lower    lower case letters  
        print    printing characters, including space  
        punct    printing characters, excluding letters and digits  
        space    white space (same as \s)  
        upper    upper case letters  
        word     "word" characters (same as \w)  
        xdigit   hexadecimal digits  
   
      The names "ascii" and "word" are  Perl  extensions.  Another  
      Perl  extension is negation, which is indicated by a ^ char-  
      acter after the colon. For example,  
   
        [12[:^digit:]]  
   
      matches "1", "2", or any non-digit.  PCRE  (and  Perl)  also  
      recognize the POSIX syntax [.ch.] and [=ch=] where "ch" is a  
      "collating element", but these are  not  supported,  and  an  
      error is given if they are encountered.  
2350    
2351           Perl supports the POSIX notation for character classes. This uses names
2352           enclosed  by  [: and :] within the enclosing square brackets. PCRE also
2353           supports this notation. For example,
2354    
2355             [01[:alpha:]%]
2356    
2357           matches "0", "1", any alphabetic character, or "%". The supported class
2358           names are
2359    
2360             alnum    letters and digits
2361             alpha    letters
2362             ascii    character codes 0 - 127
2363             blank    space or tab only
2364             cntrl    control characters
2365             digit    decimal digits (same as \d)
2366             graph    printing characters, excluding space
2367             lower    lower case letters
2368             print    printing characters, including space
2369             punct    printing characters, excluding letters and digits
2370             space    white space (not quite the same as \s)
2371             upper    upper case letters
2372             word     "word" characters (same as \w)
2373             xdigit   hexadecimal digits
2374    
2375           The  "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
2376           and space (32). Notice that this list includes the VT  character  (code
2377           11). This makes "space" different to \s, which does not include VT (for
2378           Perl compatibility).
2379    
2380           The name "word" is a Perl extension, and "blank"  is  a  GNU  extension
2381           from  Perl  5.8. Another Perl extension is negation, which is indicated
2382           by a ^ character after the colon. For example,
2383    
2384             [12[:^digit:]]
2385    
2386           matches "1", "2", or any non-digit. PCRE (and Perl) also recognize  the
2387           POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
2388           these are not supported, and an error is given if they are encountered.
2389    
2390           In UTF-8 mode, characters with values greater than 128 do not match any
2391           of the POSIX character classes.
2392    
2393    
2394  VERTICAL BAR  VERTICAL BAR
      Vertical bar characters are  used  to  separate  alternative  
      patterns. For example, the pattern  
2395    
2396         gilbert|sullivan         Vertical bar characters are used to separate alternative patterns.  For
2397           example, the pattern
2398    
2399       matches either "gilbert" or "sullivan". Any number of alter-           gilbert|sullivan
      natives  may  appear,  and an empty alternative is permitted  
      (matching the empty string).   The  matching  process  tries  
      each  alternative in turn, from left to right, and the first  
      one that succeeds is used. If the alternatives are within  a  
      subpattern  (defined  below),  "succeeds" means matching the  
      rest of the main pattern as well as the alternative  in  the  
      subpattern.  
2400    
2401           matches  either "gilbert" or "sullivan". Any number of alternatives may
2402           appear, and an empty  alternative  is  permitted  (matching  the  empty
2403           string).   The  matching  process  tries each alternative in turn, from
2404           left to right, and the first one that succeeds is used. If the alterna-
2405           tives  are within a subpattern (defined below), "succeeds" means match-
2406           ing the rest of the main pattern as well as the alternative in the sub-
2407           pattern.
2408    
2409    
2410  INTERNAL OPTION SETTING  INTERNAL OPTION SETTING
      The settings of PCRE_CASELESS, PCRE_MULTILINE,  PCRE_DOTALL,  
      and  PCRE_EXTENDED can be changed from within the pattern by  
      a sequence of Perl option letters enclosed between "(?"  and  
      ")". The option letters are  
   
        i  for PCRE_CASELESS  
        m  for PCRE_MULTILINE  
        s  for PCRE_DOTALL  
        x  for PCRE_EXTENDED  
   
      For example, (?im) sets caseless, multiline matching. It  is  
      also possible to unset these options by preceding the letter  
      with a hyphen, and a combined setting and unsetting such  as  
      (?im-sx),  which sets PCRE_CASELESS and PCRE_MULTILINE while  
      unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  permitted.  
      If  a  letter  appears both before and after the hyphen, the  
      option is unset.  
   
      The scope of these option changes depends on  where  in  the  
      pattern  the  setting  occurs. For settings that are outside  
      any subpattern (defined below), the effect is the same as if  
      the  options were set or unset at the start of matching. The  
      following patterns all behave in exactly the same way:  
   
        (?i)abc  
        a(?i)bc  
        ab(?i)c  
        abc(?i)  
   
      which in turn is the same as compiling the pattern abc  with  
      PCRE_CASELESS  set.   In  other words, such "top level" set-  
      tings apply to the whole pattern  (unless  there  are  other  
      changes  inside subpatterns). If there is more than one set-  
      ting of the same option at top level, the rightmost  setting  
      is used.  
   
      If an option change occurs inside a subpattern,  the  effect  
      is  different.  This is a change of behaviour in Perl 5.005.  
      An option change inside a subpattern affects only that  part  
      of the subpattern that follows it, so  
   
        (a(?i)b)c  
   
      matches  abc  and  aBc  and  no  other   strings   (assuming  
      PCRE_CASELESS  is  not used).  By this means, options can be  
      made to have different settings in different  parts  of  the  
      pattern.  Any  changes  made  in one alternative do carry on  
      into subsequent branches within  the  same  subpattern.  For  
      example,  
   
        (a(?i)b|c)  
   
      matches "ab", "aB", "c", and "C", even though when  matching  
      "C" the first branch is abandoned before the option setting.  
      This is because the effects of  option  settings  happen  at  
      compile  time. There would be some very weird behaviour oth-  
      erwise.  
   
      The PCRE-specific options PCRE_UNGREEDY and  PCRE_EXTRA  can  
      be changed in the same way as the Perl-compatible options by  
      using the characters U and X  respectively.  The  (?X)  flag  
      setting  is  special in that it must always occur earlier in  
      the pattern than any of the additional features it turns on,  
      even when it is at top level. It is best put at the start.  
2411    
2412           The  settings  of  the  PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
2413           PCRE_EXTENDED options can be changed  from  within  the  pattern  by  a
2414           sequence  of  Perl  option  letters  enclosed between "(?" and ")". The
2415           option letters are
2416    
2417             i  for PCRE_CASELESS
2418             m  for PCRE_MULTILINE
2419             s  for PCRE_DOTALL
2420             x  for PCRE_EXTENDED
2421    
2422           For example, (?im) sets caseless, multiline matching. It is also possi-
2423           ble to unset these options by preceding the letter with a hyphen, and a
2424           combined setting and unsetting such as (?im-sx), which sets  PCRE_CASE-
2425           LESS  and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
2426           is also permitted. If a  letter  appears  both  before  and  after  the
2427           hyphen, the option is unset.
2428    
2429           When  an option change occurs at top level (that is, not inside subpat-
2430           tern parentheses), the change applies to the remainder of  the  pattern
2431           that follows.  If the change is placed right at the start of a pattern,
2432           PCRE extracts it into the global options (and it will therefore show up
2433           in data extracted by the pcre_fullinfo() function).
2434    
2435           An option change within a subpattern affects only that part of the cur-
2436           rent pattern that follows it, so
2437    
2438             (a(?i)b)c
2439    
2440           matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
2441           used).   By  this means, options can be made to have different settings
2442           in different parts of the pattern. Any changes made in one  alternative
2443           do  carry  on  into subsequent branches within the same subpattern. For
2444           example,
2445    
2446             (a(?i)b|c)
2447    
2448           matches "ab", "aB", "c", and "C", even though  when  matching  "C"  the
2449           first  branch  is  abandoned before the option setting. This is because
2450           the effects of option settings happen at compile time. There  would  be
2451           some very weird behaviour otherwise.
2452    
2453           The  PCRE-specific  options PCRE_UNGREEDY and PCRE_EXTRA can be changed
2454           in the same way as the Perl-compatible options by using the  characters
2455           U  and X respectively. The (?X) flag setting is special in that it must
2456           always occur earlier in the pattern than any of the additional features
2457           it  turns on, even when it is at top level. It is best to put it at the
2458           start.
2459    
2460    
2461  SUBPATTERNS  SUBPATTERNS
      Subpatterns are delimited by parentheses  (round  brackets),  
      which can be nested.  Marking part of a pattern as a subpat-  
      tern does two things:  
   
      1. It localizes a set of alternatives. For example, the pat-  
      tern  
   
        cat(aract|erpillar|)  
   
      matches one of the words "cat",  "cataract",  or  "caterpil-  
      lar".  Without  the  parentheses, it would match "cataract",  
      "erpillar" or the empty string.  
   
      2. It sets up the subpattern as a capturing  subpattern  (as  
      defined  above).   When the whole pattern matches, that por-  
      tion of the subject string that matched  the  subpattern  is  
      passed  back  to  the  caller  via  the  ovector argument of  
      pcre_exec(). Opening parentheses are counted  from  left  to  
      right (starting from 1) to obtain the numbers of the captur-  
      ing subpatterns.  
   
      For example, if the string "the red king" is matched against  
      the pattern  
   
        the ((red|white) (king|queen))  
   
      the captured substrings are "red king", "red",  and  "king",  
      and are numbered 1, 2, and 3, respectively.  
   
      The fact that plain parentheses fulfil two functions is  not  
      always  helpful.  There are often times when a grouping sub-  
      pattern is required without a capturing requirement.  If  an  
      opening parenthesis is followed by "?:", the subpattern does  
      not do any capturing, and is not counted when computing  the  
      number of any subsequent capturing subpatterns. For example,  
      if the string "the white queen" is matched against the  pat-  
      tern  
   
        the ((?:red|white) (king|queen))  
   
      the captured substrings are "white queen" and  "queen",  and  
      are  numbered  1  and 2. The maximum number of captured sub-  
      strings is 99, and the maximum number  of  all  subpatterns,  
      both capturing and non-capturing, is 200.  
   
      As a  convenient  shorthand,  if  any  option  settings  are  
      required  at  the  start  of a non-capturing subpattern, the  
      option letters may appear between the "?" and the ":".  Thus  
      the two patterns  
   
        (?i:saturday|sunday)  
        (?:(?i)saturday|sunday)  
   
      match exactly the same set of strings.  Because  alternative  
      branches  are  tried from left to right, and options are not  
      reset until the end of the subpattern is reached, an  option  
      setting  in  one  branch does affect subsequent branches, so  
      the above patterns match "SUNDAY" as well as "Saturday".  
2462    
2463           Subpatterns are delimited by parentheses (round brackets), which can be
2464           nested.  Turning part of a pattern into a subpattern does two things:
2465    
2466           1. It localizes a set of alternatives. For example, the pattern
2467    
2468             cat(aract|erpillar|)
2469    
2470           matches  one  of the words "cat", "cataract", or "caterpillar". Without
2471           the parentheses, it would match "cataract",  "erpillar"  or  the  empty
2472           string.
2473    
2474           2.  It  sets  up  the  subpattern as a capturing subpattern. This means
2475           that, when the whole pattern  matches,  that  portion  of  the  subject
2476           string that matched the subpattern is passed back to the caller via the
2477           ovector argument of pcre_exec(). Opening parentheses are  counted  from
2478           left  to  right  (starting  from 1) to obtain numbers for the capturing
2479           subpatterns.
2480    
2481           For example, if the string "the red king" is matched against  the  pat-
2482           tern
2483    
2484             the ((red|white) (king|queen))
2485    
2486           the captured substrings are "red king", "red", and "king", and are num-
2487           bered 1, 2, and 3, respectively.
2488    
2489           The fact that plain parentheses fulfil  two  functions  is  not  always
2490           helpful.   There are often times when a grouping subpattern is required
2491           without a capturing requirement. If an opening parenthesis is  followed
2492           by  a question mark and a colon, the subpattern does not do any captur-
2493           ing, and is not counted when computing the  number  of  any  subsequent
2494           capturing  subpatterns. For example, if the string "the white queen" is
2495           matched against the pattern
2496    
2497             the ((?:red|white) (king|queen))
2498    
2499           the captured substrings are "white queen" and "queen", and are numbered
2500           1  and 2. The maximum number of capturing subpatterns is 65535, and the
2501           maximum depth of nesting of all subpatterns, both  capturing  and  non-
2502           capturing, is 200.
2503    
2504           As  a  convenient shorthand, if any option settings are required at the
2505           start of a non-capturing subpattern,  the  option  letters  may  appear
2506           between the "?" and the ":". Thus the two patterns
2507    
2508             (?i:saturday|sunday)
2509             (?:(?i)saturday|sunday)
2510    
2511           match exactly the same set of strings. Because alternative branches are
2512           tried from left to right, and options are not reset until  the  end  of
2513           the  subpattern is reached, an option setting in one branch does affect
2514           subsequent branches, so the above patterns match "SUNDAY"  as  well  as
2515           "Saturday".
2516    
2517    
2518    NAMED SUBPATTERNS
2519    
2520           Identifying  capturing  parentheses  by number is simple, but it can be
2521           very hard to keep track of the numbers in complicated  regular  expres-
2522           sions.  Furthermore,  if  an  expression  is  modified, the numbers may
2523           change. To help with this difficulty, PCRE supports the naming of  sub-
2524           patterns,  something  that  Perl  does  not  provide. The Python syntax
2525           (?P<name>...) is used. Names consist  of  alphanumeric  characters  and
2526           underscores, and must be unique within a pattern.
2527    
2528           Named  capturing  parentheses  are  still  allocated numbers as well as
2529           names. The PCRE API provides function calls for extracting the name-to-
2530           number  translation table from a compiled pattern. There is also a con-
2531           venience function for extracting a captured substring by name. For fur-
2532           ther details see the pcreapi documentation.
2533    
2534    
2535  REPETITION  REPETITION
      Repetition is specified by quantifiers, which can follow any  
      of the following items:  
2536    
2537         a single character, possibly escaped         Repetition  is  specified  by  quantifiers, which can follow any of the
2538         the . metacharacter         following items:
        a character class  
        a back reference (see next section)  
        a parenthesized subpattern (unless it is  an  assertion  -  
      see below)  
   
      The general repetition quantifier specifies  a  minimum  and  
      maximum  number  of  permitted  matches,  by  giving the two  
      numbers in curly brackets (braces), separated  by  a  comma.  
      The  numbers  must be less than 65536, and the first must be  
      less than or equal to the second. For example:  
   
        z{2,4}  
   
      matches "zz", "zzz", or "zzzz". A closing brace on  its  own  
      is not a special character. If the second number is omitted,  
      but the comma is present, there is no upper  limit;  if  the  
      second number and the comma are both omitted, the quantifier  
      specifies an exact number of required matches. Thus  
   
        [aeiou]{3,}  
   
      matches at least 3 successive vowels,  but  may  match  many  
      more, while  
   
        \d{8}  
   
      matches exactly 8 digits.  An  opening  curly  bracket  that  
      appears  in a position where a quantifier is not allowed, or  
      one that does not match the syntax of a quantifier, is taken  
      as  a literal character. For example, {,6} is not a quantif-  
      ier, but a literal string of four characters.  
      The quantifier {0} is permitted, causing the  expression  to  
      behave  as  if the previous item and the quantifier were not  
      present.  
   
      For convenience (and  historical  compatibility)  the  three  
      most common quantifiers have single-character abbreviations:  
   
        *    is equivalent to {0,}  
        +    is equivalent to {1,}  
        ?    is equivalent to {0,1}  
   
      It is possible to construct infinite loops  by  following  a  
      subpattern  that  can  match no characters with a quantifier  
      that has no upper limit, for example:  
   
        (a?)*  
   
      Earlier versions of Perl and PCRE used to give an  error  at  
      compile  time  for such patterns. However, because there are  
      cases where this  can  be  useful,  such  patterns  are  now  
      accepted,  but  if  any repetition of the subpattern does in  
      fact match no characters, the loop is forcibly broken.  
   
      By default, the quantifiers  are  "greedy",  that  is,  they  
      match  as much as possible (up to the maximum number of per-  
      mitted times), without causing the rest of  the  pattern  to  
      fail. The classic example of where this gives problems is in  
      trying to match comments in C programs. These appear between  
      the  sequences /* and */ and within the sequence, individual  
      * and / characters may appear. An attempt to  match  C  com-  
      ments by applying the pattern  
   
        /\*.*\*/  
   
      to the string  
   
        /* first command */  not comment  /* second comment */  
   
      fails, because it matches the entire  string  owing  to  the  
      greediness of the .*  item.  
   
      However, if a quantifier is followed by a question mark,  it  
      ceases  to be greedy, and instead matches the minimum number  
      of times possible, so the pattern  
   
        /\*.*?\*/  
   
      does the right thing with the C comments. The meaning of the  
      various  quantifiers is not otherwise changed, just the pre-  
      ferred number of matches.  Do not confuse this use of  ques-  
      tion  mark  with  its  use as a quantifier in its own right.  
      Because it has two uses, it can sometimes appear doubled, as  
      in  
   
        \d??\d  
   
      which matches one digit by preference, but can match two  if  
      that is the only way the rest of the pattern matches.  
   
      If the PCRE_UNGREEDY option is set (an option which  is  not  
      available  in  Perl),  the  quantifiers  are  not  greedy by  
      default, but individual ones can be made greedy by following  
      them  with  a  question mark. In other words, it inverts the  
      default behaviour.  
   
      When a parenthesized subpattern is quantified with a minimum  
      repeat  count  that is greater than 1 or with a limited max-  
      imum, more store is required for the  compiled  pattern,  in  
      proportion to the size of the minimum or maximum.  
   
      If a pattern starts with .* or  .{0,}  and  the  PCRE_DOTALL  
      option (equivalent to Perl's /s) is set, thus allowing the .  
      to match  newlines,  the  pattern  is  implicitly  anchored,  
      because whatever follows will be tried against every charac-  
      ter position in the subject string, so there is no point  in  
      retrying  the overall match at any position after the first.  
      PCRE treats such a pattern as though it were preceded by \A.  
      In  cases where it is known that the subject string contains  
      no newlines, it is worth setting PCRE_DOTALL when  the  pat-  
      tern begins with .* in order to obtain this optimization, or  
      alternatively using ^ to indicate anchoring explicitly.  
   
      When a capturing subpattern is repeated, the value  captured  
      is the substring that matched the final iteration. For exam-  
      ple, after  
   
        (tweedle[dume]{3}\s*)+  
   
      has matched "tweedledum tweedledee" the value  of  the  cap-  
      tured  substring  is  "tweedledee".  However,  if  there are  
      nested capturing  subpatterns,  the  corresponding  captured  
      values  may  have been set in previous iterations. For exam-  
      ple, after  
2539    
2540         /(a|(b))+/           a literal data character
2541             the . metacharacter
2542             the \C escape sequence
2543             the \X escape sequence (in UTF-8 mode with Unicode properties)
2544             an escape such as \d that matches a single character
2545             a character class
2546             a back reference (see next section)
2547             a parenthesized subpattern (unless it is an assertion)
2548    
2549       matches "aba" the value of the second captured substring  is         The general repetition quantifier specifies a minimum and maximum  num-
2550       "b".         ber  of  permitted matches, by giving the two numbers in curly brackets
2551           (braces), separated by a comma. The numbers must be  less  than  65536,
2552           and the first must be less than or equal to the second. For example:
2553    
2554             z{2,4}
2555    
2556           matches  "zz",  "zzz",  or  "zzzz". A closing brace on its own is not a
2557           special character. If the second number is omitted, but  the  comma  is
2558           present,  there  is  no upper limit; if the second number and the comma
2559           are both omitted, the quantifier specifies an exact number of  required
2560           matches. Thus
2561    
2562  BACK REFERENCES           [aeiou]{3,}
2563       Outside a character class, a backslash followed by  a  digit  
2564       greater  than  0  (and  possibly  further  digits) is a back         matches at least 3 successive vowels, but may match many more, while
2565    
2566             \d{8}
2567    
2568           matches  exactly  8  digits. An opening curly bracket that appears in a
2569           position where a quantifier is not allowed, or one that does not  match
2570           the  syntax of a quantifier, is taken as a literal character. For exam-
2571           ple, {,6} is not a quantifier, but a literal string of four characters.
2572    
2573           In  UTF-8  mode,  quantifiers  apply to UTF-8 characters rather than to
2574           individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
2575           acters, each of which is represented by a two-byte sequence. Similarly,
2576           when Unicode property support is available, \X{3} matches three Unicode
2577           extended  sequences,  each of which may be several bytes long (and they
2578           may be of different lengths).
2579    
2580           The quantifier {0} is permitted, causing the expression to behave as if
2581           the previous item and the quantifier were not present.
2582    
2583           For  convenience  (and  historical compatibility) the three most common
2584           quantifiers have single-character abbreviations:
2585    
2586             *    is equivalent to {0,}
2587             +    is equivalent to {1,}
2588             ?    is equivalent to {0,1}
2589    
2590           It is possible to construct infinite loops by  following  a  subpattern
2591           that can match no characters with a quantifier that has no upper limit,
2592           for example:
2593    
2594             (a?)*
2595    
2596           Earlier versions of Perl and PCRE used to give an error at compile time
2597           for  such  patterns. However, because there are cases where this can be
2598           useful, such patterns are now accepted, but if any  repetition  of  the
2599           subpattern  does in fact match no characters, the loop is forcibly bro-
2600           ken.
2601    
2602           By default, the quantifiers are "greedy", that is, they match  as  much
2603           as  possible  (up  to  the  maximum number of permitted times), without
2604           causing the rest of the pattern to fail. The classic example  of  where
2605           this gives problems is in trying to match comments in C programs. These
2606           appear between /* and */ and within the comment,  individual  *  and  /
2607           characters  may  appear. An attempt to match C comments by applying the
2608           pattern
2609    
2610             /\*.*\*/
2611    
2612           to the string
2613    
2614             /* first comment */  not comment  /* second comment */
2615    
2616           fails, because it matches the entire string owing to the greediness  of
2617           the .*  item.
2618    
2619           However,  if  a quantifier is followed by a question mark, it ceases to
2620           be greedy, and instead matches the minimum number of times possible, so
2621           the pattern
2622    
2623             /\*.*?\*/
2624    
2625           does  the  right  thing with the C comments. The meaning of the various
2626           quantifiers is not otherwise changed,  just  the  preferred  number  of
2627           matches.   Do  not  confuse this use of question mark with its use as a
2628           quantifier in its own right. Because it has two uses, it can  sometimes
2629           appear doubled, as in
2630    
2631             \d??\d
2632    
2633           which matches one digit by preference, but can match two if that is the
2634           only way the rest of the pattern matches.
2635    
2636           If the PCRE_UNGREEDY option is set (an option which is not available in
2637           Perl),  the  quantifiers are not greedy by default, but individual ones
2638           can be made greedy by following them with a  question  mark.  In  other
2639           words, it inverts the default behaviour.
2640    
2641           When  a  parenthesized  subpattern  is quantified with a minimum repeat
2642           count that is greater than 1 or with a limited maximum, more memory  is
2643           required  for  the  compiled  pattern, in proportion to the size of the
2644           minimum or maximum.
2645    
2646  SunOS 5.8                 Last change:                         30         If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
2647           alent  to Perl's /s) is set, thus allowing the . to match newlines, the
2648           pattern is implicitly anchored, because whatever follows will be  tried
2649           against  every character position in the subject string, so there is no
2650           point in retrying the overall match at any position  after  the  first.
2651           PCRE normally treats such a pattern as though it were preceded by \A.
2652    
2653           In  cases  where  it  is known that the subject string contains no new-
2654           lines, it is worth setting PCRE_DOTALL in order to  obtain  this  opti-
2655           mization, or alternatively using ^ to indicate anchoring explicitly.
2656