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revision 210 by ph10, Mon Aug 6 15:23:29 2007 UTC revision 211 by ph10, Thu Aug 9 09:52:43 2007 UTC
# Line 155  UTF-8 AND UNICODE PROPERTY SUPPORT Line 155  UTF-8 AND UNICODE PROPERTY SUPPORT
155         optionally  be  prefixed by "Is", for compatibility with Perl 5.6. PCRE         optionally  be  prefixed by "Is", for compatibility with Perl 5.6. PCRE
156         does not support this.         does not support this.
157    
158         The following comments apply when PCRE is running in UTF-8 mode:     Validity of UTF-8 strings
159    
160         1. When you set the PCRE_UTF8 flag, the strings passed as patterns  and         When you set the PCRE_UTF8 flag, the strings  passed  as  patterns  and
161         subjects  are  checked for validity on entry to the relevant functions.         subjects are (by default) checked for validity on entry to the relevant
162         If an invalid UTF-8 string is passed, an error return is given. In some         functions. From release 7.3 of PCRE, the check is according  the  rules
163         situations,  you  may  already  know  that  your strings are valid, and         of  RFC  3629, which are themselves derived from the Unicode specifica-
164         therefore want to skip these checks in order to improve performance. If         tion. Earlier releases of PCRE followed the rules of  RFC  2279,  which
165         you  set  the  PCRE_NO_UTF8_CHECK  flag at compile time or at run time,         allows  the  full range of 31-bit values (0 to 0x7FFFFFFF). The current
166         PCRE assumes that the pattern or subject  it  is  given  (respectively)         check allows only values in the range U+0 to U+10FFFF, excluding U+D800
167         contains  only valid UTF-8 codes. In this case, it does not diagnose an         to U+DFFF.
168         invalid UTF-8 string. If you pass an invalid UTF-8 string to PCRE  when  
169         PCRE_NO_UTF8_CHECK  is set, the results are undefined. Your program may         The  excluded  code  points are the "Low Surrogate Area" of Unicode, of
170         crash.         which the Unicode Standard says this: "The Low Surrogate Area does  not
171           contain  any  character  assignments,  consequently  no  character code
172           charts or namelists are provided for this area. Surrogates are reserved
173           for  use  with  UTF-16 and then must be used in pairs." The code points
174           that are encoded by UTF-16 pairs  are  available  as  independent  code
175           points  in  the  UTF-8  encoding.  (In other words, the whole surrogate
176           thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
177    
178           If an  invalid  UTF-8  string  is  passed  to  PCRE,  an  error  return
179           (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know
180           that your strings are valid, and therefore want to skip these checks in
181           order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at
182           compile time or at run time, PCRE assumes that the pattern  or  subject
183           it  is  given  (respectively)  contains only valid UTF-8 codes. In this
184           case, it does not diagnose an invalid UTF-8 string.
185    
186           If you pass an invalid UTF-8 string  when  PCRE_NO_UTF8_CHECK  is  set,
187           what  happens  depends on why the string is invalid. If the string con-
188           forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
189           string  of  characters  in  the  range 0 to 0x7FFFFFFF. In other words,
190           apart from the initial validity test, PCRE (when in UTF-8 mode) handles
191           strings  according  to  the more liberal rules of RFC 2279. However, if
192           the string does not even conform to RFC 2279, the result is  undefined.
193           Your program may crash.
194    
195           If  you  want  to  process  strings  of  values  in the full range 0 to
196           0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you  can
197           set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
198           this situation, you will have to apply your own validity check.
199    
200         2. An unbraced hexadecimal escape sequence (such  as  \xb3)  matches  a     General comments about UTF-8 mode
201    
202           1. An unbraced hexadecimal escape sequence (such  as  \xb3)  matches  a
203         two-byte UTF-8 character if the value is greater than 127.         two-byte UTF-8 character if the value is greater than 127.
204    
205         3.  Octal  numbers  up to \777 are recognized, and match two-byte UTF-8         2.  Octal  numbers  up to \777 are recognized, and match two-byte UTF-8
206         characters for values greater than \177.         characters for values greater than \177.
207    
208         4. Repeat quantifiers apply to complete UTF-8 characters, not to  indi-         3. Repeat quantifiers apply to complete UTF-8 characters, not to  indi-
209         vidual bytes, for example: \x{100}{3}.         vidual bytes, for example: \x{100}{3}.
210    
211         5.  The dot metacharacter matches one UTF-8 character instead of a sin-         4.  The dot metacharacter matches one UTF-8 character instead of a sin-
212         gle byte.         gle byte.
213    
214         6. The escape sequence \C can be used to match a single byte  in  UTF-8         5. The escape sequence \C can be used to match a single byte  in  UTF-8
215         mode,  but  its  use can lead to some strange effects. This facility is         mode,  but  its  use can lead to some strange effects. This facility is
216         not available in the alternative matching function, pcre_dfa_exec().         not available in the alternative matching function, pcre_dfa_exec().
217    
218         7. The character escapes \b, \B, \d, \D, \s, \S, \w, and  \W  correctly         6. The character escapes \b, \B, \d, \D, \s, \S, \w, and  \W  correctly
219         test  characters of any code value, but the characters that PCRE recog-         test  characters of any code value, but the characters that PCRE recog-
220         nizes as digits, spaces, or word characters  remain  the  same  set  as         nizes as digits, spaces, or word characters  remain  the  same  set  as
221         before, all with values less than 256. This remains true even when PCRE         before, all with values less than 256. This remains true even when PCRE
# Line 194  UTF-8 AND UNICODE PROPERTY SUPPORT Line 224  UTF-8 AND UNICODE PROPERTY SUPPORT
224         sense of, say, "digit", you must use Unicode  property  tests  such  as         sense of, say, "digit", you must use Unicode  property  tests  such  as
225         \p{Nd}.         \p{Nd}.
226    
227         8.  Similarly,  characters that match the POSIX named character classes         7.  Similarly,  characters that match the POSIX named character classes
228         are all low-valued characters.         are all low-valued characters.
229    
230         9. However, the Perl 5.10 horizontal and vertical  whitespace  matching         8. However, the Perl 5.10 horizontal and vertical  whitespace  matching
231         escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-         escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
232         acters.         acters.
233    
234         10. Case-insensitive matching applies only to characters  whose  values         9. Case-insensitive matching applies only to  characters  whose  values
235         are  less than 128, unless PCRE is built with Unicode property support.         are  less than 128, unless PCRE is built with Unicode property support.
236         Even when Unicode property support is available, PCRE  still  uses  its         Even when Unicode property support is available, PCRE  still  uses  its
237         own  character  tables when checking the case of low-valued characters,         own  character  tables when checking the case of low-valued characters,
# Line 226  AUTHOR Line 256  AUTHOR
256    
257  REVISION  REVISION
258    
259         Last updated: 06 August 2007         Last updated: 09 August 2007
260         Copyright (c) 1997-2007 University of Cambridge.         Copyright (c) 1997-2007 University of Cambridge.
261  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
262    
# Line 627  THE ALTERNATIVE MATCHING ALGORITHM Line 657  THE ALTERNATIVE MATCHING ALGORITHM
657         6.  Callouts  are  supported, but the value of the capture_top field is         6.  Callouts  are  supported, but the value of the capture_top field is
658         always 1, and the value of the capture_last field is always -1.         always 1, and the value of the capture_last field is always -1.
659    
660         7.  The \C escape sequence, which (in the standard algorithm) matches a         7. The \C escape sequence, which (in the standard algorithm) matches  a
661         single  byte, even in UTF-8 mode, is not supported because the alterna-         single  byte, even in UTF-8 mode, is not supported because the alterna-
662         tive algorithm moves through the subject  string  one  character  at  a         tive algorithm moves through the subject  string  one  character  at  a
663         time, for all active paths through the tree.         time, for all active paths through the tree.
664    
665           8.  None  of  the  backtracking control verbs such as (*PRUNE) are sup-
666           ported.
667    
668    
669  ADVANTAGES OF THE ALTERNATIVE ALGORITHM  ADVANTAGES OF THE ALTERNATIVE ALGORITHM
670    
671         Using  the alternative matching algorithm provides the following advan-         Using the alternative matching algorithm provides the following  advan-
672         tages:         tages:
673    
674         1. All possible matches (at a single point in the subject) are automat-         1. All possible matches (at a single point in the subject) are automat-
675         ically  found,  and  in particular, the longest match is found. To find         ically found, and in particular, the longest match is  found.  To  find
676         more than one match using the standard algorithm, you have to do kludgy         more than one match using the standard algorithm, you have to do kludgy
677         things with callouts.         things with callouts.
678    
679         2.  There is much better support for partial matching. The restrictions         2. There is much better support for partial matching. The  restrictions
680         on the content of the pattern that apply when using the standard  algo-         on  the content of the pattern that apply when using the standard algo-
681         rithm  for  partial matching do not apply to the alternative algorithm.         rithm for partial matching do not apply to the  alternative  algorithm.
682         For non-anchored patterns, the starting position of a partial match  is         For  non-anchored patterns, the starting position of a partial match is
683         available.         available.
684    
685         3.  Because  the  alternative  algorithm  scans the subject string just         3. Because the alternative algorithm  scans  the  subject  string  just
686         once, and never needs to backtrack, it is possible to  pass  very  long         once,  and  never  needs to backtrack, it is possible to pass very long
687         subject  strings  to  the matching function in several pieces, checking         subject strings to the matching function in  several  pieces,  checking
688         for partial matching each time.         for partial matching each time.
689    
690    
# Line 659  DISADVANTAGES OF THE ALTERNATIVE ALGORIT Line 692  DISADVANTAGES OF THE ALTERNATIVE ALGORIT
692    
693         The alternative algorithm suffers from a number of disadvantages:         The alternative algorithm suffers from a number of disadvantages:
694    
695         1. It is substantially slower than  the  standard  algorithm.  This  is         1.  It  is  substantially  slower  than the standard algorithm. This is
696         partly  because  it has to search for all possible matches, but is also         partly because it has to search for all possible matches, but  is  also
697         because it is less susceptible to optimization.         because it is less susceptible to optimization.
698    
699         2. Capturing parentheses and back references are not supported.         2. Capturing parentheses and back references are not supported.
# Line 678  AUTHOR Line 711  AUTHOR
711    
712  REVISION  REVISION
713    
714         Last updated: 29 May 2007         Last updated: 08 August 2007
715         Copyright (c) 1997-2007 University of Cambridge.         Copyright (c) 1997-2007 University of Cambridge.
716  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
717    
# Line 1222  COMPILING A PATTERN Line 1255  COMPILING A PATTERN
1255           PCRE_NO_UTF8_CHECK           PCRE_NO_UTF8_CHECK
1256    
1257         When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is         When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1258         automatically  checked. If an invalid UTF-8 sequence of bytes is found,         automatically  checked.  There  is  a  discussion about the validity of
1259         pcre_compile() returns an error. If you already know that your  pattern         UTF-8 strings in the main pcre page. If an invalid  UTF-8  sequence  of
1260         is  valid, and you want to skip this check for performance reasons, you         bytes  is  found,  pcre_compile() returns an error. If you already know
1261         can set the PCRE_NO_UTF8_CHECK option. When it is set,  the  effect  of         that your pattern is valid, and you want to skip this check for perfor-
1262         passing an invalid UTF-8 string as a pattern is undefined. It may cause         mance  reasons,  you  can set the PCRE_NO_UTF8_CHECK option. When it is
1263         your program to crash.  Note that this option can  also  be  passed  to         set, the effect of passing an invalid UTF-8  string  as  a  pattern  is
1264         pcre_exec()  and pcre_dfa_exec(), to suppress the UTF-8 validity check-         undefined.  It  may  cause your program to crash. Note that this option
1265         ing of subject strings.         can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress  the
1266           UTF-8 validity checking of subject strings.
1267    
1268    
1269  COMPILATION ERROR CODES  COMPILATION ERROR CODES
1270    
1271         The following table lists the error  codes  than  may  be  returned  by         The  following  table  lists  the  error  codes than may be returned by
1272         pcre_compile2(),  along with the error messages that may be returned by         pcre_compile2(), along with the error messages that may be returned  by
1273         both compiling functions. As PCRE has developed, some error codes  have         both  compiling functions. As PCRE has developed, some error codes have
1274         fallen out of use. To avoid confusion, they have not been re-used.         fallen out of use. To avoid confusion, they have not been re-used.
1275    
1276            0  no error            0  no error
# Line 1292  COMPILATION ERROR CODES Line 1326  COMPILATION ERROR CODES
1326           50  [this code is not in use]           50  [this code is not in use]
1327           51  octal value is greater than \377 (not in UTF-8 mode)           51  octal value is greater than \377 (not in UTF-8 mode)
1328           52  internal error: overran compiling workspace           52  internal error: overran compiling workspace
1329           53   internal  error:  previously-checked  referenced  subpattern not           53  internal  error:  previously-checked  referenced  subpattern  not
1330         found         found
1331           54  DEFINE group contains more than one branch           54  DEFINE group contains more than one branch
1332           55  repeating a DEFINE group is not allowed           55  repeating a DEFINE group is not allowed
# Line 1307  STUDYING A PATTERN Line 1341  STUDYING A PATTERN
1341         pcre_extra *pcre_study(const pcre *code, int options         pcre_extra *pcre_study(const pcre *code, int options
1342              const char **errptr);              const char **errptr);
1343    
1344         If a compiled pattern is going to be used several times,  it  is  worth         If  a  compiled  pattern is going to be used several times, it is worth
1345         spending more time analyzing it in order to speed up the time taken for         spending more time analyzing it in order to speed up the time taken for
1346         matching. The function pcre_study() takes a pointer to a compiled  pat-         matching.  The function pcre_study() takes a pointer to a compiled pat-
1347         tern as its first argument. If studying the pattern produces additional         tern as its first argument. If studying the pattern produces additional
1348         information that will help speed up matching,  pcre_study()  returns  a         information  that  will  help speed up matching, pcre_study() returns a
1349         pointer  to a pcre_extra block, in which the study_data field points to         pointer to a pcre_extra block, in which the study_data field points  to
1350         the results of the study.         the results of the study.
1351    
1352         The  returned  value  from  pcre_study()  can  be  passed  directly  to         The  returned  value  from  pcre_study()  can  be  passed  directly  to
1353         pcre_exec().  However,  a  pcre_extra  block also contains other fields         pcre_exec(). However, a pcre_extra block  also  contains  other  fields
1354         that can be set by the caller before the block  is  passed;  these  are         that  can  be  set  by the caller before the block is passed; these are
1355         described below in the section on matching a pattern.         described below in the section on matching a pattern.
1356    
1357         If  studying  the  pattern  does not produce any additional information         If studying the pattern does not  produce  any  additional  information
1358         pcre_study() returns NULL. In that circumstance, if the calling program         pcre_study() returns NULL. In that circumstance, if the calling program
1359         wants  to  pass  any of the other fields to pcre_exec(), it must set up         wants to pass any of the other fields to pcre_exec(), it  must  set  up
1360         its own pcre_extra block.         its own pcre_extra block.
1361    
1362         The second argument of pcre_study() contains option bits.  At  present,         The  second  argument of pcre_study() contains option bits. At present,
1363         no options are defined, and this argument should always be zero.         no options are defined, and this argument should always be zero.
1364    
1365         The  third argument for pcre_study() is a pointer for an error message.         The third argument for pcre_study() is a pointer for an error  message.
1366         If studying succeeds (even if no data is  returned),  the  variable  it         If  studying  succeeds  (even  if no data is returned), the variable it
1367         points  to  is  set  to NULL. Otherwise it is set to point to a textual         points to is set to NULL. Otherwise it is set to  point  to  a  textual
1368         error message. This is a static string that is part of the library. You         error message. This is a static string that is part of the library. You
1369         must  not  try  to  free it. You should test the error pointer for NULL         must not try to free it. You should test the  error  pointer  for  NULL
1370         after calling pcre_study(), to be sure that it has run successfully.         after calling pcre_study(), to be sure that it has run successfully.
1371    
1372         This is a typical call to pcre_study():         This is a typical call to pcre_study():
# Line 1344  STUDYING A PATTERN Line 1378  STUDYING A PATTERN
1378             &error);        /* set to NULL or points to a message */             &error);        /* set to NULL or points to a message */
1379    
1380         At present, studying a pattern is useful only for non-anchored patterns         At present, studying a pattern is useful only for non-anchored patterns
1381         that  do not have a single fixed starting character. A bitmap of possi-         that do not have a single fixed starting character. A bitmap of  possi-
1382         ble starting bytes is created.         ble starting bytes is created.
1383    
1384    
1385  LOCALE SUPPORT  LOCALE SUPPORT
1386    
1387         PCRE handles caseless matching, and determines whether  characters  are         PCRE  handles  caseless matching, and determines whether characters are
1388         letters,  digits, or whatever, by reference to a set of tables, indexed         letters, digits, or whatever, by reference to a set of tables,  indexed
1389         by character value. When running in UTF-8 mode, this  applies  only  to         by  character  value.  When running in UTF-8 mode, this applies only to
1390         characters  with  codes  less than 128. Higher-valued codes never match         characters with codes less than 128. Higher-valued  codes  never  match
1391         escapes such as \w or \d, but can be tested with \p if  PCRE  is  built         escapes  such  as  \w or \d, but can be tested with \p if PCRE is built
1392         with  Unicode  character property support. The use of locales with Uni-         with Unicode character property support. The use of locales  with  Uni-
1393         code is discouraged. If you are handling characters with codes  greater         code  is discouraged. If you are handling characters with codes greater
1394         than  128, you should either use UTF-8 and Unicode, or use locales, but         than 128, you should either use UTF-8 and Unicode, or use locales,  but
1395         not try to mix the two.         not try to mix the two.
1396    
1397         PCRE contains an internal set of tables that are used  when  the  final         PCRE  contains  an  internal set of tables that are used when the final
1398         argument  of  pcre_compile()  is  NULL.  These  are sufficient for many         argument of pcre_compile() is  NULL.  These  are  sufficient  for  many
1399         applications.  Normally, the internal tables recognize only ASCII char-         applications.  Normally, the internal tables recognize only ASCII char-
1400         acters. However, when PCRE is built, it is possible to cause the inter-         acters. However, when PCRE is built, it is possible to cause the inter-
1401         nal tables to be rebuilt in the default "C" locale of the local system,         nal tables to be rebuilt in the default "C" locale of the local system,
1402         which may cause them to be different.         which may cause them to be different.
1403    
1404         The  internal tables can always be overridden by tables supplied by the         The internal tables can always be overridden by tables supplied by  the
1405         application that calls PCRE. These may be created in a different locale         application that calls PCRE. These may be created in a different locale
1406         from  the  default.  As more and more applications change to using Uni-         from the default. As more and more applications change  to  using  Uni-
1407         code, the need for this locale support is expected to die away.         code, the need for this locale support is expected to die away.
1408    
1409         External tables are built by calling  the  pcre_maketables()  function,         External  tables  are  built by calling the pcre_maketables() function,
1410         which  has no arguments, in the relevant locale. The result can then be         which has no arguments, in the relevant locale. The result can then  be
1411         passed to pcre_compile() or pcre_exec()  as  often  as  necessary.  For         passed  to  pcre_compile()  or  pcre_exec()  as often as necessary. For
1412         example,  to  build  and use tables that are appropriate for the French         example, to build and use tables that are appropriate  for  the  French
1413         locale (where accented characters with  values  greater  than  128  are         locale  (where  accented  characters  with  values greater than 128 are
1414         treated as letters), the following code could be used:         treated as letters), the following code could be used:
1415    
1416           setlocale(LC_CTYPE, "fr_FR");           setlocale(LC_CTYPE, "fr_FR");
1417           tables = pcre_maketables();           tables = pcre_maketables();
1418           re = pcre_compile(..., tables);           re = pcre_compile(..., tables);
1419    
1420         The  locale  name "fr_FR" is used on Linux and other Unix-like systems;         The locale name "fr_FR" is used on Linux and other  Unix-like  systems;
1421         if you are using Windows, the name for the French locale is "french".         if you are using Windows, the name for the French locale is "french".
1422    
1423         When pcre_maketables() runs, the tables are built  in  memory  that  is         When  pcre_maketables()  runs,  the  tables are built in memory that is
1424         obtained  via  pcre_malloc. It is the caller's responsibility to ensure         obtained via pcre_malloc. It is the caller's responsibility  to  ensure
1425         that the memory containing the tables remains available for as long  as         that  the memory containing the tables remains available for as long as
1426         it is needed.         it is needed.
1427    
1428         The pointer that is passed to pcre_compile() is saved with the compiled         The pointer that is passed to pcre_compile() is saved with the compiled
1429         pattern, and the same tables are used via this pointer by  pcre_study()         pattern,  and the same tables are used via this pointer by pcre_study()
1430         and normally also by pcre_exec(). Thus, by default, for any single pat-         and normally also by pcre_exec(). Thus, by default, for any single pat-
1431         tern, compilation, studying and matching all happen in the same locale,         tern, compilation, studying and matching all happen in the same locale,
1432         but different patterns can be compiled in different locales.         but different patterns can be compiled in different locales.
1433    
1434         It  is  possible to pass a table pointer or NULL (indicating the use of         It is possible to pass a table pointer or NULL (indicating the  use  of
1435         the internal tables) to pcre_exec(). Although  not  intended  for  this         the  internal  tables)  to  pcre_exec(). Although not intended for this
1436         purpose,  this facility could be used to match a pattern in a different         purpose, this facility could be used to match a pattern in a  different
1437         locale from the one in which it was compiled. Passing table pointers at         locale from the one in which it was compiled. Passing table pointers at
1438         run time is discussed below in the section on matching a pattern.         run time is discussed below in the section on matching a pattern.
1439    
# Line 1409  INFORMATION ABOUT A PATTERN Line 1443  INFORMATION ABOUT A PATTERN
1443         int pcre_fullinfo(const pcre *code, const pcre_extra *extra,         int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1444              int what, void *where);              int what, void *where);
1445    
1446         The  pcre_fullinfo() function returns information about a compiled pat-         The pcre_fullinfo() function returns information about a compiled  pat-
1447         tern. It replaces the obsolete pcre_info() function, which is neverthe-         tern. It replaces the obsolete pcre_info() function, which is neverthe-
1448         less retained for backwards compability (and is documented below).         less retained for backwards compability (and is documented below).
1449    
1450         The  first  argument  for  pcre_fullinfo() is a pointer to the compiled         The first argument for pcre_fullinfo() is a  pointer  to  the  compiled
1451         pattern. The second argument is the result of pcre_study(), or NULL  if         pattern.  The second argument is the result of pcre_study(), or NULL if
1452         the  pattern  was not studied. The third argument specifies which piece         the pattern was not studied. The third argument specifies  which  piece
1453         of information is required, and the fourth argument is a pointer  to  a         of  information  is required, and the fourth argument is a pointer to a
1454         variable  to  receive  the  data. The yield of the function is zero for         variable to receive the data. The yield of the  function  is  zero  for
1455         success, or one of the following negative numbers:         success, or one of the following negative numbers:
1456    
1457           PCRE_ERROR_NULL       the argument code was NULL           PCRE_ERROR_NULL       the argument code was NULL
# Line 1425  INFORMATION ABOUT A PATTERN Line 1459  INFORMATION ABOUT A PATTERN
1459           PCRE_ERROR_BADMAGIC   the "magic number" was not found           PCRE_ERROR_BADMAGIC   the "magic number" was not found
1460           PCRE_ERROR_BADOPTION  the value of what was invalid           PCRE_ERROR_BADOPTION  the value of what was invalid
1461    
1462         The "magic number" is placed at the start of each compiled  pattern  as         The  "magic  number" is placed at the start of each compiled pattern as
1463         an  simple check against passing an arbitrary memory pointer. Here is a         an simple check against passing an arbitrary memory pointer. Here is  a
1464         typical call of pcre_fullinfo(), to obtain the length of  the  compiled         typical  call  of pcre_fullinfo(), to obtain the length of the compiled
1465         pattern:         pattern:
1466    
1467           int rc;           int rc;
# Line 1438  INFORMATION ABOUT A PATTERN Line 1472  INFORMATION ABOUT A PATTERN
1472             PCRE_INFO_SIZE,   /* what is required */             PCRE_INFO_SIZE,   /* what is required */
1473             &length);         /* where to put the data */             &length);         /* where to put the data */
1474    
1475         The  possible  values for the third argument are defined in pcre.h, and         The possible values for the third argument are defined in  pcre.h,  and
1476         are as follows:         are as follows:
1477    
1478           PCRE_INFO_BACKREFMAX           PCRE_INFO_BACKREFMAX
1479    
1480         Return the number of the highest back reference  in  the  pattern.  The         Return  the  number  of  the highest back reference in the pattern. The
1481         fourth  argument  should  point to an int variable. Zero is returned if         fourth argument should point to an int variable. Zero  is  returned  if
1482         there are no back references.         there are no back references.
1483    
1484           PCRE_INFO_CAPTURECOUNT           PCRE_INFO_CAPTURECOUNT
1485    
1486         Return the number of capturing subpatterns in the pattern.  The  fourth         Return  the  number of capturing subpatterns in the pattern. The fourth
1487         argument should point to an int variable.         argument should point to an int variable.
1488    
1489           PCRE_INFO_DEFAULT_TABLES           PCRE_INFO_DEFAULT_TABLES
1490    
1491         Return  a pointer to the internal default character tables within PCRE.         Return a pointer to the internal default character tables within  PCRE.
1492         The fourth argument should point to an unsigned char *  variable.  This         The  fourth  argument should point to an unsigned char * variable. This
1493         information call is provided for internal use by the pcre_study() func-         information call is provided for internal use by the pcre_study() func-
1494         tion. External callers can cause PCRE to use  its  internal  tables  by         tion.  External  callers  can  cause PCRE to use its internal tables by
1495         passing a NULL table pointer.         passing a NULL table pointer.
1496    
1497           PCRE_INFO_FIRSTBYTE           PCRE_INFO_FIRSTBYTE
1498    
1499         Return  information  about  the first byte of any matched string, for a         Return information about the first byte of any matched  string,  for  a
1500         non-anchored pattern. The fourth argument should point to an int  vari-         non-anchored  pattern. The fourth argument should point to an int vari-
1501         able.  (This option used to be called PCRE_INFO_FIRSTCHAR; the old name         able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old  name
1502         is still recognized for backwards compatibility.)         is still recognized for backwards compatibility.)
1503    
1504         If there is a fixed first byte, for example, from  a  pattern  such  as         If  there  is  a  fixed first byte, for example, from a pattern such as
1505         (cat|cow|coyote), its value is returned. Otherwise, if either         (cat|cow|coyote), its value is returned. Otherwise, if either
1506    
1507         (a)  the pattern was compiled with the PCRE_MULTILINE option, and every         (a) the pattern was compiled with the PCRE_MULTILINE option, and  every
1508         branch starts with "^", or         branch starts with "^", or
1509    
1510         (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not         (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1511         set (if it were set, the pattern would be anchored),         set (if it were set, the pattern would be anchored),
1512    
1513         -1  is  returned, indicating that the pattern matches only at the start         -1 is returned, indicating that the pattern matches only at  the  start
1514         of a subject string or after any newline within the  string.  Otherwise         of  a  subject string or after any newline within the string. Otherwise
1515         -2 is returned. For anchored patterns, -2 is returned.         -2 is returned. For anchored patterns, -2 is returned.
1516    
1517           PCRE_INFO_FIRSTTABLE           PCRE_INFO_FIRSTTABLE
1518    
1519         If  the pattern was studied, and this resulted in the construction of a         If the pattern was studied, and this resulted in the construction of  a
1520         256-bit table indicating a fixed set of bytes for the first byte in any         256-bit table indicating a fixed set of bytes for the first byte in any
1521         matching  string, a pointer to the table is returned. Otherwise NULL is         matching string, a pointer to the table is returned. Otherwise NULL  is
1522         returned. The fourth argument should point to an unsigned char *  vari-         returned.  The fourth argument should point to an unsigned char * vari-
1523         able.         able.
1524    
1525           PCRE_INFO_JCHANGED           PCRE_INFO_JCHANGED
1526    
1527         Return  1  if the (?J) option setting is used in the pattern, otherwise         Return 1 if the (?J) option setting is used in the  pattern,  otherwise
1528         0. The fourth argument should point to an int variable. The (?J) inter-         0. The fourth argument should point to an int variable. The (?J) inter-
1529         nal option setting changes the local PCRE_DUPNAMES option.         nal option setting changes the local PCRE_DUPNAMES option.
1530    
1531           PCRE_INFO_LASTLITERAL           PCRE_INFO_LASTLITERAL
1532    
1533         Return  the  value of the rightmost literal byte that must exist in any         Return the value of the rightmost literal byte that must exist  in  any
1534         matched string, other than at its  start,  if  such  a  byte  has  been         matched  string,  other  than  at  its  start,  if such a byte has been
1535         recorded. The fourth argument should point to an int variable. If there         recorded. The fourth argument should point to an int variable. If there
1536         is no such byte, -1 is returned. For anchored patterns, a last  literal         is  no such byte, -1 is returned. For anchored patterns, a last literal
1537         byte  is  recorded only if it follows something of variable length. For         byte is recorded only if it follows something of variable  length.  For
1538         example, for the pattern /^a\d+z\d+/ the returned value is "z", but for         example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1539         /^a\dz\d/ the returned value is -1.         /^a\dz\d/ the returned value is -1.
1540    
# Line 1508  INFORMATION ABOUT A PATTERN Line 1542  INFORMATION ABOUT A PATTERN
1542           PCRE_INFO_NAMEENTRYSIZE           PCRE_INFO_NAMEENTRYSIZE
1543           PCRE_INFO_NAMETABLE           PCRE_INFO_NAMETABLE
1544    
1545         PCRE  supports the use of named as well as numbered capturing parenthe-         PCRE supports the use of named as well as numbered capturing  parenthe-
1546         ses. The names are just an additional way of identifying the  parenthe-         ses.  The names are just an additional way of identifying the parenthe-
1547         ses, which still acquire numbers. Several convenience functions such as         ses, which still acquire numbers. Several convenience functions such as
1548         pcre_get_named_substring() are provided for  extracting  captured  sub-         pcre_get_named_substring()  are  provided  for extracting captured sub-
1549         strings  by  name. It is also possible to extract the data directly, by         strings by name. It is also possible to extract the data  directly,  by
1550         first converting the name to a number in order to  access  the  correct         first  converting  the  name to a number in order to access the correct
1551         pointers in the output vector (described with pcre_exec() below). To do         pointers in the output vector (described with pcre_exec() below). To do
1552         the conversion, you need  to  use  the  name-to-number  map,  which  is         the  conversion,  you  need  to  use  the  name-to-number map, which is
1553         described by these three values.         described by these three values.
1554    
1555         The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT         The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1556         gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size         gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1557         of  each  entry;  both  of  these  return  an int value. The entry size         of each entry; both of these  return  an  int  value.  The  entry  size
1558         depends on the length of the longest name. PCRE_INFO_NAMETABLE  returns         depends  on the length of the longest name. PCRE_INFO_NAMETABLE returns
1559         a  pointer  to  the  first  entry of the table (a pointer to char). The         a pointer to the first entry of the table  (a  pointer  to  char).  The
1560         first two bytes of each entry are the number of the capturing parenthe-         first two bytes of each entry are the number of the capturing parenthe-
1561         sis,  most  significant byte first. The rest of the entry is the corre-         sis, most significant byte first. The rest of the entry is  the  corre-
1562         sponding name, zero terminated. The names are  in  alphabetical  order.         sponding  name,  zero  terminated. The names are in alphabetical order.
1563         When PCRE_DUPNAMES is set, duplicate names are in order of their paren-         When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
1564         theses numbers. For example, consider  the  following  pattern  (assume         theses  numbers.  For  example,  consider the following pattern (assume
1565         PCRE_EXTENDED  is  set,  so  white  space  -  including  newlines  - is         PCRE_EXTENDED is  set,  so  white  space  -  including  newlines  -  is
1566         ignored):         ignored):
1567    
1568           (?<date> (?<year>(\d\d)?\d\d) -           (?<date> (?<year>(\d\d)?\d\d) -
1569           (?<month>\d\d) - (?<day>\d\d) )           (?<month>\d\d) - (?<day>\d\d) )
1570    
1571         There are four named subpatterns, so the table has  four  entries,  and         There  are  four  named subpatterns, so the table has four entries, and
1572         each  entry  in the table is eight bytes long. The table is as follows,         each entry in the table is eight bytes long. The table is  as  follows,
1573         with non-printing bytes shows in hexadecimal, and undefined bytes shown         with non-printing bytes shows in hexadecimal, and undefined bytes shown
1574         as ??:         as ??:
1575    
# Line 1544  INFORMATION ABOUT A PATTERN Line 1578  INFORMATION ABOUT A PATTERN
1578           00 04 m  o  n  t  h  00           00 04 m  o  n  t  h  00
1579           00 02 y  e  a  r  00 ??           00 02 y  e  a  r  00 ??
1580    
1581         When  writing  code  to  extract  data from named subpatterns using the         When writing code to extract data  from  named  subpatterns  using  the
1582         name-to-number map, remember that the length of the entries  is  likely         name-to-number  map,  remember that the length of the entries is likely
1583         to be different for each compiled pattern.         to be different for each compiled pattern.
1584    
1585           PCRE_INFO_OKPARTIAL           PCRE_INFO_OKPARTIAL
1586    
1587         Return  1 if the pattern can be used for partial matching, otherwise 0.         Return 1 if the pattern can be used for partial matching, otherwise  0.
1588         The fourth argument should point to an int  variable.  The  pcrepartial         The  fourth  argument  should point to an int variable. The pcrepartial
1589         documentation  lists  the restrictions that apply to patterns when par-         documentation lists the restrictions that apply to patterns  when  par-
1590         tial matching is used.         tial matching is used.
1591    
1592           PCRE_INFO_OPTIONS           PCRE_INFO_OPTIONS
1593    
1594         Return a copy of the options with which the pattern was  compiled.  The         Return  a  copy of the options with which the pattern was compiled. The
1595         fourth  argument  should  point to an unsigned long int variable. These         fourth argument should point to an unsigned long  int  variable.  These
1596         option bits are those specified in the call to pcre_compile(), modified         option bits are those specified in the call to pcre_compile(), modified
1597         by any top-level option settings at the start of the pattern itself. In         by any top-level option settings at the start of the pattern itself. In
1598         other words, they are the options that will be in force  when  matching         other  words,  they are the options that will be in force when matching
1599         starts.  For  example, if the pattern /(?im)abc(?-i)d/ is compiled with         starts. For example, if the pattern /(?im)abc(?-i)d/ is  compiled  with
1600         the PCRE_EXTENDED option, the result is PCRE_CASELESS,  PCRE_MULTILINE,         the  PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
1601         and PCRE_EXTENDED.         and PCRE_EXTENDED.
1602    
1603         A  pattern  is  automatically  anchored by PCRE if all of its top-level         A pattern is automatically anchored by PCRE if  all  of  its  top-level
1604         alternatives begin with one of the following:         alternatives begin with one of the following:
1605    
1606           ^     unless PCRE_MULTILINE is set           ^     unless PCRE_MULTILINE is set
# Line 1580  INFORMATION ABOUT A PATTERN Line 1614  INFORMATION ABOUT A PATTERN
1614    
1615           PCRE_INFO_SIZE           PCRE_INFO_SIZE
1616    
1617         Return  the  size  of the compiled pattern, that is, the value that was         Return the size of the compiled pattern, that is, the  value  that  was
1618         passed as the argument to pcre_malloc() when PCRE was getting memory in         passed as the argument to pcre_malloc() when PCRE was getting memory in
1619         which to place the compiled data. The fourth argument should point to a         which to place the compiled data. The fourth argument should point to a
1620         size_t variable.         size_t variable.
# Line 1588  INFORMATION ABOUT A PATTERN Line 1622  INFORMATION ABOUT A PATTERN
1622           PCRE_INFO_STUDYSIZE           PCRE_INFO_STUDYSIZE
1623    
1624         Return the size of the data block pointed to by the study_data field in         Return the size of the data block pointed to by the study_data field in
1625         a  pcre_extra  block.  That  is,  it  is  the  value that was passed to         a pcre_extra block. That is,  it  is  the  value  that  was  passed  to
1626         pcre_malloc() when PCRE was getting memory into which to place the data         pcre_malloc() when PCRE was getting memory into which to place the data
1627         created  by  pcre_study(). The fourth argument should point to a size_t         created by pcre_study(). The fourth argument should point to  a  size_t
1628         variable.         variable.
1629    
1630    
# Line 1598  OBSOLETE INFO FUNCTION Line 1632  OBSOLETE INFO FUNCTION
1632    
1633         int pcre_info(const pcre *code, int *optptr, int *firstcharptr);         int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1634    
1635         The pcre_info() function is now obsolete because its interface  is  too         The  pcre_info()  function is now obsolete because its interface is too
1636         restrictive  to return all the available data about a compiled pattern.         restrictive to return all the available data about a compiled  pattern.
1637         New  programs  should  use  pcre_fullinfo()  instead.  The   yield   of         New   programs   should  use  pcre_fullinfo()  instead.  The  yield  of
1638         pcre_info()  is the number of capturing subpatterns, or one of the fol-         pcre_info() is the number of capturing subpatterns, or one of the  fol-
1639         lowing negative numbers:         lowing negative numbers:
1640    
1641           PCRE_ERROR_NULL       the argument code was NULL           PCRE_ERROR_NULL       the argument code was NULL
1642           PCRE_ERROR_BADMAGIC   the "magic number" was not found           PCRE_ERROR_BADMAGIC   the "magic number" was not found
1643    
1644         If the optptr argument is not NULL, a copy of the  options  with  which         If  the  optptr  argument is not NULL, a copy of the options with which
1645         the  pattern  was  compiled  is placed in the integer it points to (see         the pattern was compiled is placed in the integer  it  points  to  (see
1646         PCRE_INFO_OPTIONS above).         PCRE_INFO_OPTIONS above).
1647    
1648         If the pattern is not anchored and the  firstcharptr  argument  is  not         If  the  pattern  is  not anchored and the firstcharptr argument is not
1649         NULL,  it is used to pass back information about the first character of         NULL, it is used to pass back information about the first character  of
1650         any matched string (see PCRE_INFO_FIRSTBYTE above).         any matched string (see PCRE_INFO_FIRSTBYTE above).
1651    
1652    
# Line 1620  REFERENCE COUNTS Line 1654  REFERENCE COUNTS
1654    
1655         int pcre_refcount(pcre *code, int adjust);         int pcre_refcount(pcre *code, int adjust);
1656    
1657         The pcre_refcount() function is used to maintain a reference  count  in         The  pcre_refcount()  function is used to maintain a reference count in
1658         the data block that contains a compiled pattern. It is provided for the         the data block that contains a compiled pattern. It is provided for the
1659         benefit of applications that  operate  in  an  object-oriented  manner,         benefit  of  applications  that  operate  in an object-oriented manner,
1660         where different parts of the application may be using the same compiled         where different parts of the application may be using the same compiled
1661         pattern, but you want to free the block when they are all done.         pattern, but you want to free the block when they are all done.
1662    
1663         When a pattern is compiled, the reference count field is initialized to         When a pattern is compiled, the reference count field is initialized to
1664         zero.   It is changed only by calling this function, whose action is to         zero.  It is changed only by calling this function, whose action is  to
1665         add the adjust value (which may be positive or  negative)  to  it.  The         add  the  adjust  value  (which may be positive or negative) to it. The
1666         yield of the function is the new value. However, the value of the count         yield of the function is the new value. However, the value of the count
1667         is constrained to lie between 0 and 65535, inclusive. If the new  value         is  constrained to lie between 0 and 65535, inclusive. If the new value
1668         is outside these limits, it is forced to the appropriate limit value.         is outside these limits, it is forced to the appropriate limit value.
1669    
1670         Except  when it is zero, the reference count is not correctly preserved         Except when it is zero, the reference count is not correctly  preserved
1671         if a pattern is compiled on one host and then  transferred  to  a  host         if  a  pattern  is  compiled on one host and then transferred to a host
1672         whose byte-order is different. (This seems a highly unlikely scenario.)         whose byte-order is different. (This seems a highly unlikely scenario.)
1673    
1674    
# Line 1644  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 1678  MATCHING A PATTERN: THE TRADITIONAL FUNC
1678              const char *subject, int length, int startoffset,              const char *subject, int length, int startoffset,
1679              int options, int *ovector, int ovecsize);              int options, int *ovector, int ovecsize);
1680    
1681         The function pcre_exec() is called to match a subject string against  a         The  function pcre_exec() is called to match a subject string against a
1682         compiled  pattern, which is passed in the code argument. If the pattern         compiled pattern, which is passed in the code argument. If the  pattern
1683         has been studied, the result of the study should be passed in the extra         has been studied, the result of the study should be passed in the extra
1684         argument.  This  function is the main matching facility of the library,         argument. This function is the main matching facility of  the  library,
1685         and it operates in a Perl-like manner. For specialist use there is also         and it operates in a Perl-like manner. For specialist use there is also
1686         an  alternative matching function, which is described below in the sec-         an alternative matching function, which is described below in the  sec-
1687         tion about the pcre_dfa_exec() function.         tion about the pcre_dfa_exec() function.
1688    
1689         In most applications, the pattern will have been compiled (and  option-         In  most applications, the pattern will have been compiled (and option-
1690         ally  studied)  in the same process that calls pcre_exec(). However, it         ally studied) in the same process that calls pcre_exec().  However,  it
1691         is possible to save compiled patterns and study data, and then use them         is possible to save compiled patterns and study data, and then use them
1692         later  in  different processes, possibly even on different hosts. For a         later in different processes, possibly even on different hosts.  For  a
1693         discussion about this, see the pcreprecompile documentation.         discussion about this, see the pcreprecompile documentation.
1694    
1695         Here is an example of a simple call to pcre_exec():         Here is an example of a simple call to pcre_exec():
# Line 1674  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 1708  MATCHING A PATTERN: THE TRADITIONAL FUNC
1708    
1709     Extra data for pcre_exec()     Extra data for pcre_exec()
1710    
1711         If the extra argument is not NULL, it must point to a  pcre_extra  data         If  the  extra argument is not NULL, it must point to a pcre_extra data
1712         block.  The pcre_study() function returns such a block (when it doesn't         block. The pcre_study() function returns such a block (when it  doesn't
1713         return NULL), but you can also create one for yourself, and pass  addi-         return  NULL), but you can also create one for yourself, and pass addi-
1714         tional  information  in it. The pcre_extra block contains the following         tional information in it. The pcre_extra block contains  the  following
1715         fields (not necessarily in this order):         fields (not necessarily in this order):
1716    
1717           unsigned long int flags;           unsigned long int flags;
# Line 1687  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 1721  MATCHING A PATTERN: THE TRADITIONAL FUNC
1721           void *callout_data;           void *callout_data;
1722           const unsigned char *tables;           const unsigned char *tables;
1723    
1724         The flags field is a bitmap that specifies which of  the  other  fields         The  flags  field  is a bitmap that specifies which of the other fields
1725         are set. The flag bits are:         are set. The flag bits are:
1726    
1727           PCRE_EXTRA_STUDY_DATA           PCRE_EXTRA_STUDY_DATA
# Line 1696  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 1730  MATCHING A PATTERN: THE TRADITIONAL FUNC
1730           PCRE_EXTRA_CALLOUT_DATA           PCRE_EXTRA_CALLOUT_DATA
1731           PCRE_EXTRA_TABLES           PCRE_EXTRA_TABLES
1732    
1733         Other  flag  bits should be set to zero. The study_data field is set in         Other flag bits should be set to zero. The study_data field is  set  in
1734         the pcre_extra block that is returned by  pcre_study(),  together  with         the  pcre_extra  block  that is returned by pcre_study(), together with
1735         the appropriate flag bit. You should not set this yourself, but you may         the appropriate flag bit. You should not set this yourself, but you may
1736         add to the block by setting the other fields  and  their  corresponding         add  to  the  block by setting the other fields and their corresponding
1737         flag bits.         flag bits.
1738    
1739         The match_limit field provides a means of preventing PCRE from using up         The match_limit field provides a means of preventing PCRE from using up
1740         a vast amount of resources when running patterns that are not going  to         a  vast amount of resources when running patterns that are not going to
1741         match,  but  which  have  a very large number of possibilities in their         match, but which have a very large number  of  possibilities  in  their
1742         search trees. The classic  example  is  the  use  of  nested  unlimited         search  trees.  The  classic  example  is  the  use of nested unlimited
1743         repeats.         repeats.
1744    
1745         Internally,  PCRE uses a function called match() which it calls repeat-         Internally, PCRE uses a function called match() which it calls  repeat-
1746         edly (sometimes recursively). The limit set by match_limit  is  imposed         edly  (sometimes  recursively). The limit set by match_limit is imposed
1747         on  the  number  of times this function is called during a match, which         on the number of times this function is called during  a  match,  which
1748         has the effect of limiting the amount of  backtracking  that  can  take         has  the  effect  of  limiting the amount of backtracking that can take
1749         place. For patterns that are not anchored, the count restarts from zero         place. For patterns that are not anchored, the count restarts from zero
1750         for each position in the subject string.         for each position in the subject string.
1751    
1752         The default value for the limit can be set  when  PCRE  is  built;  the         The  default  value  for  the  limit can be set when PCRE is built; the
1753         default  default  is 10 million, which handles all but the most extreme         default default is 10 million, which handles all but the  most  extreme
1754         cases. You can override the default  by  suppling  pcre_exec()  with  a         cases.  You  can  override  the  default by suppling pcre_exec() with a
1755         pcre_extra     block    in    which    match_limit    is    set,    and         pcre_extra    block    in    which    match_limit    is    set,     and
1756         PCRE_EXTRA_MATCH_LIMIT is set in the  flags  field.  If  the  limit  is         PCRE_EXTRA_MATCH_LIMIT  is  set  in  the  flags  field. If the limit is
1757         exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.         exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1758    
1759         The  match_limit_recursion field is similar to match_limit, but instead         The match_limit_recursion field is similar to match_limit, but  instead
1760         of limiting the total number of times that match() is called, it limits         of limiting the total number of times that match() is called, it limits
1761         the  depth  of  recursion. The recursion depth is a smaller number than         the depth of recursion. The recursion depth is a  smaller  number  than
1762         the total number of calls, because not all calls to match() are  recur-         the  total number of calls, because not all calls to match() are recur-
1763         sive.  This limit is of use only if it is set smaller than match_limit.         sive.  This limit is of use only if it is set smaller than match_limit.
1764    
1765         Limiting the recursion depth limits the amount of  stack  that  can  be         Limiting  the  recursion  depth  limits the amount of stack that can be
1766         used, or, when PCRE has been compiled to use memory on the heap instead         used, or, when PCRE has been compiled to use memory on the heap instead
1767         of the stack, the amount of heap memory that can be used.         of the stack, the amount of heap memory that can be used.
1768    
1769         The default value for match_limit_recursion can be  set  when  PCRE  is         The  default  value  for  match_limit_recursion can be set when PCRE is
1770         built;  the  default  default  is  the  same  value  as the default for         built; the default default  is  the  same  value  as  the  default  for
1771         match_limit. You can override the default by suppling pcre_exec()  with         match_limit.  You can override the default by suppling pcre_exec() with
1772         a   pcre_extra   block  in  which  match_limit_recursion  is  set,  and         a  pcre_extra  block  in  which  match_limit_recursion  is   set,   and
1773         PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in  the  flags  field.  If  the         PCRE_EXTRA_MATCH_LIMIT_RECURSION  is  set  in  the  flags field. If the
1774         limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.         limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1775    
1776         The  pcre_callout  field is used in conjunction with the "callout" fea-         The pcre_callout field is used in conjunction with the  "callout"  fea-
1777         ture, which is described in the pcrecallout documentation.         ture, which is described in the pcrecallout documentation.
1778    
1779         The tables field  is  used  to  pass  a  character  tables  pointer  to         The  tables  field  is  used  to  pass  a  character  tables pointer to
1780         pcre_exec();  this overrides the value that is stored with the compiled         pcre_exec(); this overrides the value that is stored with the  compiled
1781         pattern. A non-NULL value is stored with the compiled pattern  only  if         pattern.  A  non-NULL value is stored with the compiled pattern only if
1782         custom  tables  were  supplied to pcre_compile() via its tableptr argu-         custom tables were supplied to pcre_compile() via  its  tableptr  argu-
1783         ment.  If NULL is passed to pcre_exec() using this mechanism, it forces         ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
1784         PCRE's  internal  tables  to be used. This facility is helpful when re-         PCRE's internal tables to be used. This facility is  helpful  when  re-
1785         using patterns that have been saved after compiling  with  an  external         using  patterns  that  have been saved after compiling with an external
1786         set  of  tables,  because  the  external tables might be at a different         set of tables, because the external tables  might  be  at  a  different
1787         address when pcre_exec() is called. See the  pcreprecompile  documenta-         address  when  pcre_exec() is called. See the pcreprecompile documenta-
1788         tion for a discussion of saving compiled patterns for later use.         tion for a discussion of saving compiled patterns for later use.
1789    
1790     Option bits for pcre_exec()     Option bits for pcre_exec()
1791    
1792         The  unused  bits of the options argument for pcre_exec() must be zero.         The unused bits of the options argument for pcre_exec() must  be  zero.
1793         The only bits that may  be  set  are  PCRE_ANCHORED,  PCRE_NEWLINE_xxx,         The  only  bits  that  may  be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
1794         PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,   PCRE_NO_UTF8_CHECK   and         PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,   PCRE_NO_UTF8_CHECK   and
1795         PCRE_PARTIAL.         PCRE_PARTIAL.
1796    
1797           PCRE_ANCHORED           PCRE_ANCHORED
1798    
1799         The PCRE_ANCHORED option limits pcre_exec() to matching  at  the  first         The  PCRE_ANCHORED  option  limits pcre_exec() to matching at the first
1800         matching  position.  If  a  pattern was compiled with PCRE_ANCHORED, or         matching position. If a pattern was  compiled  with  PCRE_ANCHORED,  or
1801         turned out to be anchored by virtue of its contents, it cannot be  made         turned  out to be anchored by virtue of its contents, it cannot be made
1802         unachored at matching time.         unachored at matching time.
1803    
1804           PCRE_NEWLINE_CR           PCRE_NEWLINE_CR
# Line 1773  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 1807  MATCHING A PATTERN: THE TRADITIONAL FUNC
1807           PCRE_NEWLINE_ANYCRLF           PCRE_NEWLINE_ANYCRLF
1808           PCRE_NEWLINE_ANY           PCRE_NEWLINE_ANY
1809    
1810         These  options  override  the  newline  definition  that  was chosen or         These options override  the  newline  definition  that  was  chosen  or
1811         defaulted when the pattern was compiled. For details, see the  descrip-         defaulted  when the pattern was compiled. For details, see the descrip-
1812         tion  of  pcre_compile()  above.  During  matching,  the newline choice         tion of pcre_compile()  above.  During  matching,  the  newline  choice
1813         affects the behaviour of the dot, circumflex,  and  dollar  metacharac-         affects  the  behaviour  of the dot, circumflex, and dollar metacharac-
1814         ters.  It may also alter the way the match position is advanced after a         ters. It may also alter the way the match position is advanced after  a
1815         match  failure  for  an  unanchored  pattern.  When  PCRE_NEWLINE_CRLF,         match  failure  for  an  unanchored  pattern.  When  PCRE_NEWLINE_CRLF,
1816         PCRE_NEWLINE_ANYCRLF,  or  PCRE_NEWLINE_ANY is set, and a match attempt         PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is set, and a  match  attempt
1817         fails when the current position is at a CRLF sequence, the match  posi-         fails  when the current position is at a CRLF sequence, the match posi-
1818         tion  is  advanced by two characters instead of one, in other words, to         tion is advanced by two characters instead of one, in other  words,  to
1819         after the CRLF.         after the CRLF.
1820    
1821           PCRE_NOTBOL           PCRE_NOTBOL
1822    
1823         This option specifies that first character of the subject string is not         This option specifies that first character of the subject string is not
1824         the  beginning  of  a  line, so the circumflex metacharacter should not         the beginning of a line, so the  circumflex  metacharacter  should  not
1825         match before it. Setting this without PCRE_MULTILINE (at compile  time)         match  before it. Setting this without PCRE_MULTILINE (at compile time)
1826         causes  circumflex  never to match. This option affects only the behav-         causes circumflex never to match. This option affects only  the  behav-
1827         iour of the circumflex metacharacter. It does not affect \A.         iour of the circumflex metacharacter. It does not affect \A.
1828    
1829           PCRE_NOTEOL           PCRE_NOTEOL
1830    
1831         This option specifies that the end of the subject string is not the end         This option specifies that the end of the subject string is not the end
1832         of  a line, so the dollar metacharacter should not match it nor (except         of a line, so the dollar metacharacter should not match it nor  (except
1833         in multiline mode) a newline immediately before it. Setting this  with-         in  multiline mode) a newline immediately before it. Setting this with-
1834         out PCRE_MULTILINE (at compile time) causes dollar never to match. This         out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1835         option affects only the behaviour of the dollar metacharacter. It  does         option  affects only the behaviour of the dollar metacharacter. It does
1836         not affect \Z or \z.         not affect \Z or \z.
1837    
1838           PCRE_NOTEMPTY           PCRE_NOTEMPTY
1839    
1840         An empty string is not considered to be a valid match if this option is         An empty string is not considered to be a valid match if this option is
1841         set. If there are alternatives in the pattern, they are tried.  If  all         set.  If  there are alternatives in the pattern, they are tried. If all
1842         the  alternatives  match  the empty string, the entire match fails. For         the alternatives match the empty string, the entire  match  fails.  For
1843         example, if the pattern         example, if the pattern
1844    
1845           a?b?           a?b?
1846    
1847         is applied to a string not beginning with "a" or "b",  it  matches  the         is  applied  to  a string not beginning with "a" or "b", it matches the
1848         empty  string at the start of the subject. With PCRE_NOTEMPTY set, this         empty string at the start of the subject. With PCRE_NOTEMPTY set,  this
1849         match is not valid, so PCRE searches further into the string for occur-         match is not valid, so PCRE searches further into the string for occur-
1850         rences of "a" or "b".         rences of "a" or "b".
1851    
1852         Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-         Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1853         cial case of a pattern match of the empty  string  within  its  split()         cial  case  of  a  pattern match of the empty string within its split()
1854         function,  and  when  using  the /g modifier. It is possible to emulate         function, and when using the /g modifier. It  is  possible  to  emulate
1855         Perl's behaviour after matching a null string by first trying the match         Perl's behaviour after matching a null string by first trying the match
1856         again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then         again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1857         if that fails by advancing the starting offset (see below)  and  trying         if  that  fails by advancing the starting offset (see below) and trying
1858         an ordinary match again. There is some code that demonstrates how to do         an ordinary match again. There is some code that demonstrates how to do
1859         this in the pcredemo.c sample program.         this in the pcredemo.c sample program.
1860    
1861           PCRE_NO_UTF8_CHECK           PCRE_NO_UTF8_CHECK
1862    
1863         When PCRE_UTF8 is set at compile time, the validity of the subject as a         When PCRE_UTF8 is set at compile time, the validity of the subject as a
1864         UTF-8  string is automatically checked when pcre_exec() is subsequently         UTF-8 string is automatically checked when pcre_exec() is  subsequently
1865         called.  The value of startoffset is also checked  to  ensure  that  it         called.   The  value  of  startoffset is also checked to ensure that it
1866         points  to the start of a UTF-8 character. If an invalid UTF-8 sequence         points to the start of a UTF-8 character. There is a  discussion  about
1867         of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If         the  validity  of  UTF-8 strings in the section on UTF-8 support in the
1868         startoffset  contains  an  invalid  value, PCRE_ERROR_BADUTF8_OFFSET is         main pcre page. If  an  invalid  UTF-8  sequence  of  bytes  is  found,
1869         returned.         pcre_exec()  returns  the error PCRE_ERROR_BADUTF8. If startoffset con-
1870           tains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.
1871    
1872         If you already know that your subject is valid, and you  want  to  skip         If you already know that your subject is valid, and you  want  to  skip
1873         these    checks    for   performance   reasons,   you   can   set   the         these    checks    for   performance   reasons,   you   can   set   the
# Line 2416  AUTHOR Line 2451  AUTHOR
2451    
2452  REVISION  REVISION
2453    
2454         Last updated: 30 July 2007         Last updated: 09 August 2007
2455         Copyright (c) 1997-2007 University of Cambridge.         Copyright (c) 1997-2007 University of Cambridge.
2456  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
2457    
# Line 2669  DIFFERENCES BETWEEN PCRE AND PERL Line 2704  DIFFERENCES BETWEEN PCRE AND PERL
2704         matching  "aba"  against  the  pattern  /^(a(b)?)+$/  in Perl leaves $2         matching  "aba"  against  the  pattern  /^(a(b)?)+$/  in Perl leaves $2
2705         unset, but in PCRE it is set to "b".         unset, but in PCRE it is set to "b".
2706    
2707         11. PCRE provides some extensions to the Perl regular expression facil-         11.  PCRE  does  support  Perl  5.10's  backtracking  verbs  (*ACCEPT),
2708           (*FAIL),  (*F),  (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in
2709           the forms without an  argument.  PCRE  does  not  support  (*MARK).  If
2710           (*ACCEPT)  is within capturing parentheses, PCRE does not set that cap-
2711           ture group; this is different to Perl.
2712    
2713           12. PCRE provides some extensions to the Perl regular expression facil-
2714         ities.   Perl  5.10  will  include new features that are not in earlier         ities.   Perl  5.10  will  include new features that are not in earlier
2715         versions, some of which (such as named parentheses) have been  in  PCRE         versions, some of which (such as named parentheses) have been  in  PCRE
2716         for some time. This list is with respect to Perl 5.10:         for some time. This list is with respect to Perl 5.10:
# Line 2715  AUTHOR Line 2756  AUTHOR
2756    
2757  REVISION  REVISION
2758    
2759         Last updated: 13 June 2007         Last updated: 08 August 2007
2760         Copyright (c) 1997-2007 University of Cambridge.         Copyright (c) 1997-2007 University of Cambridge.
2761  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
2762    
# Line 2878  BACKSLASH Line 2919  BACKSLASH
2919         After  \x, from zero to two hexadecimal digits are read (letters can be         After  \x, from zero to two hexadecimal digits are read (letters can be
2920         in upper or lower case). Any number of hexadecimal  digits  may  appear         in upper or lower case). Any number of hexadecimal  digits  may  appear
2921         between  \x{  and  },  but the value of the character code must be less         between  \x{  and  },  but the value of the character code must be less
2922         than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode (that is,         than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
2923         the  maximum  hexadecimal  value is 7FFFFFFF). If characters other than         the  maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger
2924         hexadecimal digits appear between \x{ and }, or if there is  no  termi-         than the largest Unicode code point, which is 10FFFF.
2925         nating  }, this form of escape is not recognized.  Instead, the initial  
2926         \x will be interpreted as a basic hexadecimal escape, with no following         If characters other than hexadecimal digits appear between \x{  and  },
2927         digits, giving a character whose value is zero.         or if there is no terminating }, this form of escape is not recognized.
2928           Instead, the initial \x will be  interpreted  as  a  basic  hexadecimal
2929           escape,  with  no  following  digits, giving a character whose value is
2930           zero.
2931    
2932         Characters whose value is less than 256 can be defined by either of the         Characters whose value is less than 256 can be defined by either of the
2933         two syntaxes for \x. There is no difference in the way  they  are  han-         two  syntaxes  for  \x. There is no difference in the way they are han-
2934         dled. For example, \xdc is exactly the same as \x{dc}.         dled. For example, \xdc is exactly the same as \x{dc}.
2935    
2936         After  \0  up  to two further octal digits are read. If there are fewer         After \0 up to two further octal digits are read. If  there  are  fewer
2937         than two digits, just  those  that  are  present  are  used.  Thus  the         than  two  digits,  just  those  that  are  present  are used. Thus the
2938         sequence \0\x\07 specifies two binary zeros followed by a BEL character         sequence \0\x\07 specifies two binary zeros followed by a BEL character
2939         (code value 7). Make sure you supply two digits after the initial  zero         (code  value 7). Make sure you supply two digits after the initial zero
2940         if the pattern character that follows is itself an octal digit.         if the pattern character that follows is itself an octal digit.
2941    
2942         The handling of a backslash followed by a digit other than 0 is compli-         The handling of a backslash followed by a digit other than 0 is compli-
2943         cated.  Outside a character class, PCRE reads it and any following dig-         cated.  Outside a character class, PCRE reads it and any following dig-
2944         its  as  a  decimal  number. If the number is less than 10, or if there         its as a decimal number. If the number is less than  10,  or  if  there
2945         have been at least that many previous capturing left parentheses in the         have been at least that many previous capturing left parentheses in the
2946         expression,  the  entire  sequence  is  taken  as  a  back reference. A         expression, the entire  sequence  is  taken  as  a  back  reference.  A
2947         description of how this works is given later, following the  discussion         description  of how this works is given later, following the discussion
2948         of parenthesized subpatterns.         of parenthesized subpatterns.
2949    
2950         Inside  a  character  class, or if the decimal number is greater than 9         Inside a character class, or if the decimal number is  greater  than  9
2951         and there have not been that many capturing subpatterns, PCRE  re-reads         and  there have not been that many capturing subpatterns, PCRE re-reads
2952         up to three octal digits following the backslash, and uses them to gen-         up to three octal digits following the backslash, and uses them to gen-
2953         erate a data character. Any subsequent digits stand for themselves.  In         erate  a data character. Any subsequent digits stand for themselves. In
2954         non-UTF-8  mode,  the  value  of a character specified in octal must be         non-UTF-8 mode, the value of a character specified  in  octal  must  be
2955         less than \400. In UTF-8 mode, values up to  \777  are  permitted.  For         less  than  \400.  In  UTF-8 mode, values up to \777 are permitted. For
2956         example:         example:
2957    
2958           \040   is another way of writing a space           \040   is another way of writing a space
# Line 2926  BACKSLASH Line 2970  BACKSLASH
2970           \81    is either a back reference, or a binary zero           \81    is either a back reference, or a binary zero
2971                     followed by the two characters "8" and "1"                     followed by the two characters "8" and "1"
2972    
2973         Note  that  octal  values of 100 or greater must not be introduced by a         Note that octal values of 100 or greater must not be  introduced  by  a
2974         leading zero, because no more than three octal digits are ever read.         leading zero, because no more than three octal digits are ever read.
2975    
2976         All the sequences that define a single character value can be used both         All the sequences that define a single character value can be used both
2977         inside  and  outside character classes. In addition, inside a character         inside and outside character classes. In addition, inside  a  character
2978         class, the sequence \b is interpreted as the backspace  character  (hex         class,  the  sequence \b is interpreted as the backspace character (hex
2979         08),  and the sequences \R and \X are interpreted as the characters "R"         08), and the sequences \R and \X are interpreted as the characters  "R"
2980         and "X", respectively. Outside a character class, these sequences  have         and  "X", respectively. Outside a character class, these sequences have
2981         different meanings (see below).         different meanings (see below).
2982    
2983     Absolute and relative back references     Absolute and relative back references
2984    
2985         The  sequence  \g followed by an unsigned or a negative number, option-         The sequence \g followed by an unsigned or a negative  number,  option-
2986         ally enclosed in braces, is an absolute or relative back  reference.  A         ally  enclosed  in braces, is an absolute or relative back reference. A
2987         named back reference can be coded as \g{name}. Back references are dis-         named back reference can be coded as \g{name}. Back references are dis-
2988         cussed later, following the discussion of parenthesized subpatterns.         cussed later, following the discussion of parenthesized subpatterns.
2989    
# Line 2960  BACKSLASH Line 3004  BACKSLASH
3004           \W     any "non-word" character           \W     any "non-word" character
3005    
3006         Each pair of escape sequences partitions the complete set of characters         Each pair of escape sequences partitions the complete set of characters
3007         into two disjoint sets. Any given character matches one, and only  one,         into  two disjoint sets. Any given character matches one, and only one,
3008         of each pair.         of each pair.
3009    
3010         These character type sequences can appear both inside and outside char-         These character type sequences can appear both inside and outside char-
3011         acter classes. They each match one character of the  appropriate  type.         acter  classes.  They each match one character of the appropriate type.
3012         If  the current matching point is at the end of the subject string, all         If the current matching point is at the end of the subject string,  all
3013         of them fail, since there is no character to match.         of them fail, since there is no character to match.
3014    
3015         For compatibility with Perl, \s does not match the VT  character  (code         For  compatibility  with Perl, \s does not match the VT character (code
3016         11).   This makes it different from the the POSIX "space" class. The \s         11).  This makes it different from the the POSIX "space" class. The  \s
3017         characters are HT (9), LF (10), FF (12), CR (13), and  space  (32).  If         characters  are  HT  (9), LF (10), FF (12), CR (13), and space (32). If
3018         "use locale;" is included in a Perl script, \s may match the VT charac-         "use locale;" is included in a Perl script, \s may match the VT charac-
3019         ter. In PCRE, it never does.         ter. In PCRE, it never does.
3020    
3021         In UTF-8 mode, characters with values greater than 128 never match  \d,         In  UTF-8 mode, characters with values greater than 128 never match \d,
3022         \s, or \w, and always match \D, \S, and \W. This is true even when Uni-         \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
3023         code character property support is available.  These  sequences  retain         code  character  property  support is available. These sequences retain
3024         their original meanings from before UTF-8 support was available, mainly         their original meanings from before UTF-8 support was available, mainly
3025         for efficiency reasons.         for efficiency reasons.
3026    
3027         The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to         The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
3028         the  other  sequences, these do match certain high-valued codepoints in         the other sequences, these do match certain high-valued  codepoints  in
3029         UTF-8 mode.  The horizontal space characters are:         UTF-8 mode.  The horizontal space characters are:
3030    
3031           U+0009     Horizontal tab           U+0009     Horizontal tab
# Line 3015  BACKSLASH Line 3059  BACKSLASH
3059           U+2029     Paragraph separator           U+2029     Paragraph separator
3060    
3061         A "word" character is an underscore or any character less than 256 that         A "word" character is an underscore or any character less than 256 that
3062         is  a  letter  or  digit.  The definition of letters and digits is con-         is a letter or digit. The definition of  letters  and  digits  is  con-
3063         trolled by PCRE's low-valued character tables, and may vary if  locale-         trolled  by PCRE's low-valued character tables, and may vary if locale-
3064         specific  matching is taking place (see "Locale support" in the pcreapi         specific matching is taking place (see "Locale support" in the  pcreapi
3065         page). For example, in a French locale such  as  "fr_FR"  in  Unix-like         page).  For  example,  in  a French locale such as "fr_FR" in Unix-like
3066         systems,  or "french" in Windows, some character codes greater than 128         systems, or "french" in Windows, some character codes greater than  128
3067         are used for accented letters, and these are matched by \w. The use  of         are  used for accented letters, and these are matched by \w. The use of
3068         locales with Unicode is discouraged.         locales with Unicode is discouraged.
3069    
3070     Newline sequences     Newline sequences
3071    
3072         Outside  a  character class, the escape sequence \R matches any Unicode         Outside a character class, the escape sequence \R matches  any  Unicode
3073         newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R  is         newline  sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is
3074         equivalent to the following:         equivalent to the following:
3075    
3076           (?>\r\n|\n|\x0b|\f|\r|\x85)           (?>\r\n|\n|\x0b|\f|\r|\x85)
3077    
3078         This  is  an  example  of an "atomic group", details of which are given         This is an example of an "atomic group", details  of  which  are  given
3079         below.  This particular group matches either the two-character sequence         below.  This particular group matches either the two-character sequence
3080         CR  followed  by  LF,  or  one  of  the single characters LF (linefeed,         CR followed by LF, or  one  of  the  single  characters  LF  (linefeed,
3081         U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage         U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
3082         return, U+000D), or NEL (next line, U+0085). The two-character sequence         return, U+000D), or NEL (next line, U+0085). The two-character sequence
3083         is treated as a single unit that cannot be split.         is treated as a single unit that cannot be split.
3084    
3085         In UTF-8 mode, two additional characters whose codepoints  are  greater         In  UTF-8  mode, two additional characters whose codepoints are greater
3086         than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-         than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
3087         rator, U+2029).  Unicode character property support is not  needed  for         rator,  U+2029).   Unicode character property support is not needed for
3088         these characters to be recognized.         these characters to be recognized.
3089    
3090         Inside a character class, \R matches the letter "R".         Inside a character class, \R matches the letter "R".
# Line 3048  BACKSLASH Line 3092  BACKSLASH
3092     Unicode character properties     Unicode character properties
3093    
3094         When PCRE is built with Unicode character property support, three addi-         When PCRE is built with Unicode character property support, three addi-
3095         tional escape sequences that match characters with specific  properties         tional  escape sequences that match characters with specific properties
3096         are  available.   When not in UTF-8 mode, these sequences are of course         are available.  When not in UTF-8 mode, these sequences are  of  course
3097         limited to testing characters whose codepoints are less than  256,  but         limited  to  testing characters whose codepoints are less than 256, but
3098         they do work in this mode.  The extra escape sequences are:         they do work in this mode.  The extra escape sequences are:
3099    
3100           \p{xx}   a character with the xx property           \p{xx}   a character with the xx property
3101           \P{xx}   a character without the xx property           \P{xx}   a character without the xx property
3102           \X       an extended Unicode sequence           \X       an extended Unicode sequence
3103    
3104         The  property  names represented by xx above are limited to the Unicode         The property names represented by xx above are limited to  the  Unicode
3105         script names, the general category properties, and "Any", which matches         script names, the general category properties, and "Any", which matches
3106         any character (including newline). Other properties such as "InMusical-         any character (including newline). Other properties such as "InMusical-
3107         Symbols" are not currently supported by PCRE. Note  that  \P{Any}  does         Symbols"  are  not  currently supported by PCRE. Note that \P{Any} does
3108         not match any characters, so always causes a match failure.         not match any characters, so always causes a match failure.
3109    
3110         Sets of Unicode characters are defined as belonging to certain scripts.         Sets of Unicode characters are defined as belonging to certain scripts.
3111         A character from one of these sets can be matched using a script  name.         A  character from one of these sets can be matched using a script name.
3112         For example:         For example:
3113    
3114           \p{Greek}           \p{Greek}
3115           \P{Han}           \P{Han}
3116    
3117         Those  that are not part of an identified script are lumped together as         Those that are not part of an identified script are lumped together  as
3118         "Common". The current list of scripts is:         "Common". The current list of scripts is:
3119    
3120         Arabic,  Armenian,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,         Arabic,  Armenian,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,
3121         Buhid,   Canadian_Aboriginal,   Cherokee,  Common,  Coptic,  Cuneiform,         Buhid,  Canadian_Aboriginal,  Cherokee,  Common,   Coptic,   Cuneiform,
3122         Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,         Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
3123         Gothic,  Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-         Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew,  Hira-
3124         gana, Inherited, Kannada,  Katakana,  Kharoshthi,  Khmer,  Lao,  Latin,         gana,  Inherited,  Kannada,  Katakana,  Kharoshthi,  Khmer, Lao, Latin,
3125         Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,         Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,
3126         Ogham, Old_Italic, Old_Persian, Oriya, Osmanya,  Phags_Pa,  Phoenician,         Ogham,  Old_Italic,  Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
3127         Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,         Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,
3128         Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.         Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
3129    
3130         Each character has exactly one general category property, specified  by         Each  character has exactly one general category property, specified by
3131         a two-letter abbreviation. For compatibility with Perl, negation can be         a two-letter abbreviation. For compatibility with Perl, negation can be
3132         specified by including a circumflex between the opening brace  and  the         specified  by  including a circumflex between the opening brace and the
3133         property name. For example, \p{^Lu} is the same as \P{Lu}.         property name. For example, \p{^Lu} is the same as \P{Lu}.
3134    
3135         If only one letter is specified with \p or \P, it includes all the gen-         If only one letter is specified with \p or \P, it includes all the gen-
3136         eral category properties that start with that letter. In this case,  in         eral  category properties that start with that letter. In this case, in
3137         the  absence of negation, the curly brackets in the escape sequence are         the absence of negation, the curly brackets in the escape sequence  are
3138         optional; these two examples have the same effect:         optional; these two examples have the same effect:
3139    
3140           \p{L}           \p{L}
# Line 3142  BACKSLASH Line 3186  BACKSLASH
3186           Zp    Paragraph separator           Zp    Paragraph separator
3187           Zs    Space separator           Zs    Space separator
3188    
3189         The special property L& is also supported: it matches a character  that         The  special property L& is also supported: it matches a character that
3190         has  the  Lu,  Ll, or Lt property, in other words, a letter that is not         has the Lu, Ll, or Lt property, in other words, a letter  that  is  not
3191         classified as a modifier or "other".         classified as a modifier or "other".
3192    
3193         The long synonyms for these properties  that  Perl  supports  (such  as         The  Cs  (Surrogate)  property  applies only to characters in the range
3194         \p{Letter})  are  not  supported by PCRE, nor is it permitted to prefix         U+D800 to U+DFFF. Such characters are not valid in UTF-8  strings  (see
3195           RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
3196           ing has been turned off (see the discussion  of  PCRE_NO_UTF8_CHECK  in
3197           the pcreapi page).
3198    
3199           The  long  synonyms  for  these  properties that Perl supports (such as
3200           \p{Letter}) are not supported by PCRE, nor is it  permitted  to  prefix
3201         any of these properties with "Is".         any of these properties with "Is".
3202    
3203         No character that is in the Unicode table has the Cn (unassigned) prop-         No character that is in the Unicode table has the Cn (unassigned) prop-
3204         erty.  Instead, this property is assumed for any code point that is not         erty.  Instead, this property is assumed for any code point that is not
3205         in the Unicode table.         in the Unicode table.
3206    
3207         Specifying caseless matching does not affect  these  escape  sequences.         Specifying  caseless  matching  does not affect these escape sequences.
3208         For example, \p{Lu} always matches only upper case letters.         For example, \p{Lu} always matches only upper case letters.
3209    
3210         The  \X  escape  matches  any number of Unicode characters that form an         The \X escape matches any number of Unicode  characters  that  form  an
3211         extended Unicode sequence. \X is equivalent to         extended Unicode sequence. \X is equivalent to
3212    
3213           (?>\PM\pM*)           (?>\PM\pM*)
3214    
3215         That is, it matches a character without the "mark"  property,  followed         That  is,  it matches a character without the "mark" property, followed
3216         by  zero  or  more  characters with the "mark" property, and treats the         by zero or more characters with the "mark"  property,  and  treats  the
3217         sequence as an atomic group (see below).  Characters  with  the  "mark"         sequence  as  an  atomic group (see below).  Characters with the "mark"
3218         property  are  typically  accents  that affect the preceding character.         property are typically accents that  affect  the  preceding  character.
3219         None of them have codepoints less than 256, so  in  non-UTF-8  mode  \X         None  of  them  have  codepoints less than 256, so in non-UTF-8 mode \X
3220         matches any one character.         matches any one character.
3221    
3222         Matching  characters  by Unicode property is not fast, because PCRE has         Matching characters by Unicode property is not fast, because  PCRE  has
3223         to search a structure that contains  data  for  over  fifteen  thousand         to  search  a  structure  that  contains data for over fifteen thousand
3224         characters. That is why the traditional escape sequences such as \d and         characters. That is why the traditional escape sequences such as \d and
3225         \w do not use Unicode properties in PCRE.         \w do not use Unicode properties in PCRE.
3226    
3227     Resetting the match start     Resetting the match start
3228    
3229         The escape sequence \K, which is a Perl 5.10 feature, causes any previ-         The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
3230         ously  matched  characters  not  to  be  included  in the final matched         ously matched characters not  to  be  included  in  the  final  matched
3231         sequence. For example, the pattern:         sequence. For example, the pattern:
3232    
3233           foo\Kbar           foo\Kbar
3234    
3235         matches "foobar", but reports that it has matched "bar".  This  feature         matches  "foobar",  but reports that it has matched "bar". This feature
3236         is  similar  to  a lookbehind assertion (described below).  However, in         is similar to a lookbehind assertion (described  below).   However,  in
3237         this case, the part of the subject before the real match does not  have         this  case, the part of the subject before the real match does not have
3238         to  be of fixed length, as lookbehind assertions do. The use of \K does         to be of fixed length, as lookbehind assertions do. The use of \K  does
3239         not interfere with the setting of captured  substrings.   For  example,         not  interfere  with  the setting of captured substrings.  For example,
3240         when the pattern         when the pattern
3241    
3242           (foo)\Kbar           (foo)\Kbar
# Line 3195  BACKSLASH Line 3245  BACKSLASH
3245    
3246     Simple assertions     Simple assertions
3247    
3248         The  final use of backslash is for certain simple assertions. An asser-         The final use of backslash is for certain simple assertions. An  asser-
3249         tion specifies a condition that has to be met at a particular point  in         tion  specifies a condition that has to be met at a particular point in
3250         a  match, without consuming any characters from the subject string. The         a match, without consuming any characters from the subject string.  The
3251         use of subpatterns for more complicated assertions is described  below.         use  of subpatterns for more complicated assertions is described below.
3252         The backslashed assertions are:         The backslashed assertions are:
3253    
3254           \b     matches at a word boundary           \b     matches at a word boundary
# Line 3209  BACKSLASH Line 3259  BACKSLASH
3259           \z     matches only at the end of the subject           \z     matches only at the end of the subject
3260           \G     matches at the first matching position in the subject           \G     matches at the first matching position in the subject
3261    
3262         These  assertions may not appear in character classes (but note that \b         These assertions may not appear in character classes (but note that  \b
3263         has a different meaning, namely the backspace character, inside a char-         has a different meaning, namely the backspace character, inside a char-
3264         acter class).         acter class).
3265    
3266         A  word  boundary is a position in the subject string where the current         A word boundary is a position in the subject string where  the  current
3267         character and the previous character do not both match \w or  \W  (i.e.         character  and  the previous character do not both match \w or \W (i.e.
3268         one  matches  \w  and the other matches \W), or the start or end of the         one matches \w and the other matches \W), or the start or  end  of  the
3269         string if the first or last character matches \w, respectively.         string if the first or last character matches \w, respectively.
3270    
3271         The \A, \Z, and \z assertions differ from  the  traditional  circumflex         The  \A,  \Z,  and \z assertions differ from the traditional circumflex
3272         and dollar (described in the next section) in that they only ever match         and dollar (described in the next section) in that they only ever match
3273         at the very start and end of the subject string, whatever  options  are         at  the  very start and end of the subject string, whatever options are
3274         set.  Thus,  they are independent of multiline mode. These three asser-         set. Thus, they are independent of multiline mode. These  three  asser-
3275         tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which         tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
3276         affect  only the behaviour of the circumflex and dollar metacharacters.         affect only the behaviour of the circumflex and dollar  metacharacters.
3277         However, if the startoffset argument of pcre_exec() is non-zero,  indi-         However,  if the startoffset argument of pcre_exec() is non-zero, indi-
3278         cating that matching is to start at a point other than the beginning of         cating that matching is to start at a point other than the beginning of
3279         the subject, \A can never match. The difference between \Z  and  \z  is         the  subject,  \A  can never match. The difference between \Z and \z is
3280         that \Z matches before a newline at the end of the string as well as at         that \Z matches before a newline at the end of the string as well as at
3281         the very end, whereas \z matches only at the end.         the very end, whereas \z matches only at the end.
3282    
3283         The \G assertion is true only when the current matching position is  at         The  \G assertion is true only when the current matching position is at
3284         the  start point of the match, as specified by the startoffset argument         the start point of the match, as specified by the startoffset  argument
3285         of pcre_exec(). It differs from \A when the  value  of  startoffset  is         of  pcre_exec().  It  differs  from \A when the value of startoffset is
3286         non-zero.  By calling pcre_exec() multiple times with appropriate argu-         non-zero. By calling pcre_exec() multiple times with appropriate  argu-
3287         ments, you can mimic Perl's /g option, and it is in this kind of imple-         ments, you can mimic Perl's /g option, and it is in this kind of imple-
3288         mentation where \G can be useful.         mentation where \G can be useful.
3289    
3290         Note,  however,  that  PCRE's interpretation of \G, as the start of the         Note, however, that PCRE's interpretation of \G, as the  start  of  the
3291         current match, is subtly different from Perl's, which defines it as the         current match, is subtly different from Perl's, which defines it as the
3292         end  of  the  previous  match. In Perl, these can be different when the         end of the previous match. In Perl, these can  be  different  when  the
3293         previously matched string was empty. Because PCRE does just  one  match         previously  matched  string was empty. Because PCRE does just one match
3294         at a time, it cannot reproduce this behaviour.         at a time, it cannot reproduce this behaviour.
3295    
3296         If  all  the alternatives of a pattern begin with \G, the expression is         If all the alternatives of a pattern begin with \G, the  expression  is
3297         anchored to the starting match position, and the "anchored" flag is set         anchored to the starting match position, and the "anchored" flag is set
3298         in the compiled regular expression.         in the compiled regular expression.
3299    
# Line 3251  BACKSLASH Line 3301  BACKSLASH
3301  CIRCUMFLEX AND DOLLAR  CIRCUMFLEX AND DOLLAR
3302    
3303         Outside a character class, in the default matching mode, the circumflex         Outside a character class, in the default matching mode, the circumflex
3304         character is an assertion that is true only  if  the  current  matching         character  is  an  assertion  that is true only if the current matching
3305         point  is  at the start of the subject string. If the startoffset argu-         point is at the start of the subject string. If the  startoffset  argu-
3306         ment of pcre_exec() is non-zero, circumflex  can  never  match  if  the         ment  of  pcre_exec()  is  non-zero,  circumflex can never match if the
3307         PCRE_MULTILINE  option  is  unset. Inside a character class, circumflex         PCRE_MULTILINE option is unset. Inside a  character  class,  circumflex
3308         has an entirely different meaning (see below).         has an entirely different meaning (see below).
3309    
3310         Circumflex need not be the first character of the pattern if  a  number         Circumflex  need  not be the first character of the pattern if a number
3311         of  alternatives are involved, but it should be the first thing in each         of alternatives are involved, but it should be the first thing in  each
3312         alternative in which it appears if the pattern is ever  to  match  that         alternative  in  which  it appears if the pattern is ever to match that
3313         branch.  If all possible alternatives start with a circumflex, that is,         branch. If all possible alternatives start with a circumflex, that  is,
3314         if the pattern is constrained to match only at the start  of  the  sub-         if  the  pattern  is constrained to match only at the start of the sub-
3315         ject,  it  is  said  to be an "anchored" pattern. (There are also other         ject, it is said to be an "anchored" pattern.  (There  are  also  other
3316         constructs that can cause a pattern to be anchored.)         constructs that can cause a pattern to be anchored.)
3317    
3318         A dollar character is an assertion that is true  only  if  the  current         A  dollar  character  is  an assertion that is true only if the current
3319         matching  point  is  at  the  end of the subject string, or immediately         matching point is at the end of  the  subject  string,  or  immediately
3320         before a newline at the end of the string (by default). Dollar need not         before a newline at the end of the string (by default). Dollar need not
3321         be  the  last  character of the pattern if a number of alternatives are         be the last character of the pattern if a number  of  alternatives  are
3322         involved, but it should be the last item in  any  branch  in  which  it         involved,  but  it  should  be  the last item in any branch in which it
3323         appears. Dollar has no special meaning in a character class.         appears. Dollar has no special meaning in a character class.
3324    
3325         The  meaning  of  dollar  can be changed so that it matches only at the         The meaning of dollar can be changed so that it  matches  only  at  the
3326         very end of the string, by setting the  PCRE_DOLLAR_ENDONLY  option  at         very  end  of  the string, by setting the PCRE_DOLLAR_ENDONLY option at
3327         compile time. This does not affect the \Z assertion.         compile time. This does not affect the \Z assertion.
3328    
3329         The meanings of the circumflex and dollar characters are changed if the         The meanings of the circumflex and dollar characters are changed if the
3330         PCRE_MULTILINE option is set. When  this  is  the  case,  a  circumflex         PCRE_MULTILINE  option  is  set.  When  this  is the case, a circumflex
3331         matches  immediately after internal newlines as well as at the start of         matches immediately after internal newlines as well as at the start  of
3332         the subject string. It does not match after a  newline  that  ends  the         the  subject  string.  It  does not match after a newline that ends the
3333         string.  A dollar matches before any newlines in the string, as well as         string. A dollar matches before any newlines in the string, as well  as
3334         at the very end, when PCRE_MULTILINE is set. When newline is  specified         at  the very end, when PCRE_MULTILINE is set. When newline is specified
3335         as  the  two-character  sequence CRLF, isolated CR and LF characters do         as the two-character sequence CRLF, isolated CR and  LF  characters  do
3336         not indicate newlines.         not indicate newlines.
3337    
3338         For example, the pattern /^abc$/ matches the subject string  "def\nabc"         For  example, the pattern /^abc$/ matches the subject string "def\nabc"
3339         (where  \n  represents a newline) in multiline mode, but not otherwise.         (where \n represents a newline) in multiline mode, but  not  otherwise.
3340         Consequently, patterns that are anchored in single  line  mode  because         Consequently,  patterns  that  are anchored in single line mode because
3341         all  branches  start  with  ^ are not anchored in multiline mode, and a         all branches start with ^ are not anchored in  multiline  mode,  and  a
3342         match for circumflex is  possible  when  the  startoffset  argument  of         match  for  circumflex  is  possible  when  the startoffset argument of
3343         pcre_exec()  is  non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if         pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is  ignored  if
3344         PCRE_MULTILINE is set.         PCRE_MULTILINE is set.
3345    
3346         Note that the sequences \A, \Z, and \z can be used to match  the  start         Note  that  the sequences \A, \Z, and \z can be used to match the start
3347         and  end of the subject in both modes, and if all branches of a pattern         and end of the subject in both modes, and if all branches of a  pattern
3348         start with \A it is always anchored, whether or not  PCRE_MULTILINE  is         start  with  \A it is always anchored, whether or not PCRE_MULTILINE is
3349         set.         set.
3350    
3351    
3352  FULL STOP (PERIOD, DOT)  FULL STOP (PERIOD, DOT)
3353    
3354         Outside a character class, a dot in the pattern matches any one charac-         Outside a character class, a dot in the pattern matches any one charac-
3355         ter in the subject string except (by default) a character  that  signi-         ter  in  the subject string except (by default) a character that signi-
3356         fies  the  end  of  a line. In UTF-8 mode, the matched character may be         fies the end of a line. In UTF-8 mode, the  matched  character  may  be
3357         more than one byte long.         more than one byte long.
3358    
3359         When a line ending is defined as a single character, dot never  matches         When  a line ending is defined as a single character, dot never matches
3360         that  character; when the two-character sequence CRLF is used, dot does         that character; when the two-character sequence CRLF is used, dot  does
3361         not match CR if it is immediately followed  by  LF,  but  otherwise  it         not  match  CR  if  it  is immediately followed by LF, but otherwise it
3362         matches  all characters (including isolated CRs and LFs). When any Uni-         matches all characters (including isolated CRs and LFs). When any  Uni-
3363         code line endings are being recognized, dot does not match CR or LF  or         code  line endings are being recognized, dot does not match CR or LF or
3364         any of the other line ending characters.         any of the other line ending characters.
3365    
3366         The  behaviour  of  dot  with regard to newlines can be changed. If the         The behaviour of dot with regard to newlines can  be  changed.  If  the
3367         PCRE_DOTALL option is set, a dot matches  any  one  character,  without         PCRE_DOTALL  option  is  set,  a dot matches any one character, without
3368         exception. If the two-character sequence CRLF is present in the subject         exception. If the two-character sequence CRLF is present in the subject
3369         string, it takes two dots to match it.         string, it takes two dots to match it.
3370    
3371         The handling of dot is entirely independent of the handling of  circum-         The  handling of dot is entirely independent of the handling of circum-
3372         flex  and  dollar,  the  only relationship being that they both involve         flex and dollar, the only relationship being  that  they  both  involve
3373         newlines. Dot has no special meaning in a character class.         newlines. Dot has no special meaning in a character class.
3374    
3375    
3376  MATCHING A SINGLE BYTE  MATCHING A SINGLE BYTE
3377    
3378         Outside a character class, the escape sequence \C matches any one byte,         Outside a character class, the escape sequence \C matches any one byte,
3379         both  in  and  out  of  UTF-8 mode. Unlike a dot, it always matches any         both in and out of UTF-8 mode. Unlike a  dot,  it  always  matches  any
3380         line-ending characters. The feature is provided in  Perl  in  order  to         line-ending  characters.  The  feature  is provided in Perl in order to
3381         match  individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-         match individual bytes in UTF-8 mode. Because it breaks up UTF-8  char-
3382         acters into individual bytes, what remains in the string may be a  mal-         acters  into individual bytes, what remains in the string may be a mal-
3383         formed  UTF-8  string.  For this reason, the \C escape sequence is best         formed UTF-8 string. For this reason, the \C escape  sequence  is  best
3384         avoided.         avoided.
3385    
3386         PCRE does not allow \C to appear in  lookbehind  assertions  (described         PCRE  does  not  allow \C to appear in lookbehind assertions (described
3387         below),  because  in UTF-8 mode this would make it impossible to calcu-         below), because in UTF-8 mode this would make it impossible  to  calcu-
3388         late the length of the lookbehind.         late the length of the lookbehind.
3389    
3390    
# Line 3343  SQUARE BRACKETS AND CHARACTER CLASSES Line 3393  SQUARE BRACKETS AND CHARACTER CLASSES
3393         An opening square bracket introduces a character class, terminated by a         An opening square bracket introduces a character class, terminated by a
3394         closing square bracket. A closing square bracket on its own is not spe-         closing square bracket. A closing square bracket on its own is not spe-
3395         cial. If a closing square bracket is required as a member of the class,         cial. If a closing square bracket is required as a member of the class,
3396         it  should  be  the first data character in the class (after an initial         it should be the first data character in the class  (after  an  initial
3397         circumflex, if present) or escaped with a backslash.         circumflex, if present) or escaped with a backslash.
3398    
3399         A character class matches a single character in the subject.  In  UTF-8         A  character  class matches a single character in the subject. In UTF-8
3400         mode,  the character may occupy more than one byte. A matched character         mode, the character may occupy more than one byte. A matched  character
3401         must be in the set of characters defined by the class, unless the first         must be in the set of characters defined by the class, unless the first
3402         character  in  the  class definition is a circumflex, in which case the         character in the class definition is a circumflex, in  which  case  the
3403         subject character must not be in the set defined by  the  class.  If  a         subject  character  must  not  be in the set defined by the class. If a
3404         circumflex  is actually required as a member of the class, ensure it is         circumflex is actually required as a member of the class, ensure it  is
3405         not the first character, or escape it with a backslash.         not the first character, or escape it with a backslash.
3406    
3407         For example, the character class [aeiou] matches any lower case  vowel,         For  example, the character class [aeiou] matches any lower case vowel,
3408         while  [^aeiou]  matches  any character that is not a lower case vowel.         while [^aeiou] matches any character that is not a  lower  case  vowel.
3409         Note that a circumflex is just a convenient notation for specifying the         Note that a circumflex is just a convenient notation for specifying the
3410         characters  that  are in the class by enumerating those that are not. A         characters that are in the class by enumerating those that are  not.  A
3411         class that starts with a circumflex is not an assertion: it still  con-         class  that starts with a circumflex is not an assertion: it still con-
3412         sumes  a  character  from the subject string, and therefore it fails if         sumes a character from the subject string, and therefore  it  fails  if
3413         the current pointer is at the end of the string.         the current pointer is at the end of the string.
3414    
3415         In UTF-8 mode, characters with values greater than 255 can be  included         In  UTF-8 mode, characters with values greater than 255 can be included
3416         in  a  class as a literal string of bytes, or by using the \x{ escaping         in a class as a literal string of bytes, or by using the  \x{  escaping
3417         mechanism.         mechanism.
3418    
3419         When caseless matching is set, any letters in a  class  represent  both         When  caseless  matching  is set, any letters in a class represent both
3420         their  upper  case  and lower case versions, so for example, a caseless         their upper case and lower case versions, so for  example,  a  caseless
3421         [aeiou] matches "A" as well as "a", and a caseless  [^aeiou]  does  not         [aeiou]  matches  "A"  as well as "a", and a caseless [^aeiou] does not
3422         match  "A", whereas a caseful version would. In UTF-8 mode, PCRE always         match "A", whereas a caseful version would. In UTF-8 mode, PCRE  always
3423         understands the concept of case for characters whose  values  are  less         understands  the  concept  of case for characters whose values are less
3424         than  128, so caseless matching is always possible. For characters with         than 128, so caseless matching is always possible. For characters  with
3425         higher values, the concept of case is supported  if  PCRE  is  compiled         higher  values,  the  concept  of case is supported if PCRE is compiled
3426         with  Unicode  property support, but not otherwise.  If you want to use         with Unicode property support, but not otherwise.  If you want  to  use
3427         caseless matching for characters 128 and above, you  must  ensure  that         caseless  matching  for  characters 128 and above, you must ensure that
3428         PCRE  is  compiled  with Unicode property support as well as with UTF-8         PCRE is compiled with Unicode property support as well  as  with  UTF-8
3429         support.         support.
3430    
3431         Characters that might indicate line breaks are  never  treated  in  any         Characters  that  might  indicate  line breaks are never treated in any
3432         special  way  when  matching  character  classes,  whatever line-ending         special way  when  matching  character  classes,  whatever  line-ending
3433         sequence is in  use,  and  whatever  setting  of  the  PCRE_DOTALL  and         sequence  is  in  use,  and  whatever  setting  of  the PCRE_DOTALL and
3434         PCRE_MULTILINE options is used. A class such as [^a] always matches one         PCRE_MULTILINE options is used. A class such as [^a] always matches one
3435         of these characters.         of these characters.
3436    
3437         The minus (hyphen) character can be used to specify a range of  charac-         The  minus (hyphen) character can be used to specify a range of charac-
3438         ters  in  a  character  class.  For  example,  [d-m] matches any letter         ters in a character  class.  For  example,  [d-m]  matches  any  letter
3439         between d and m, inclusive. If a  minus  character  is  required  in  a         between  d  and  m,  inclusive.  If  a minus character is required in a
3440         class,  it  must  be  escaped  with a backslash or appear in a position         class, it must be escaped with a backslash  or  appear  in  a  position
3441         where it cannot be interpreted as indicating a range, typically as  the         where  it cannot be interpreted as indicating a range, typically as the
3442         first or last character in the class.         first or last character in the class.
3443    
3444         It is not possible to have the literal character "]" as the end charac-         It is not possible to have the literal character "]" as the end charac-
3445         ter of a range. A pattern such as [W-]46] is interpreted as a class  of         ter  of a range. A pattern such as [W-]46] is interpreted as a class of
3446         two  characters ("W" and "-") followed by a literal string "46]", so it         two characters ("W" and "-") followed by a literal string "46]", so  it
3447         would match "W46]" or "-46]". However, if the "]"  is  escaped  with  a         would  match  "W46]"  or  "-46]". However, if the "]" is escaped with a
3448         backslash  it is interpreted as the end of range, so [W-\]46] is inter-         backslash it is interpreted as the end of range, so [W-\]46] is  inter-
3449         preted as a class containing a range followed by two other  characters.         preted  as a class containing a range followed by two other characters.
3450         The  octal or hexadecimal representation of "]" can also be used to end         The octal or hexadecimal representation of "]" can also be used to  end
3451         a range.         a range.
3452    
3453         Ranges operate in the collating sequence of character values. They  can         Ranges  operate in the collating sequence of character values. They can
3454         also   be  used  for  characters  specified  numerically,  for  example         also  be  used  for  characters  specified  numerically,  for   example
3455         [\000-\037]. In UTF-8 mode, ranges can include characters whose  values         [\000-\037].  In UTF-8 mode, ranges can include characters whose values
3456         are greater than 255, for example [\x{100}-\x{2ff}].         are greater than 255, for example [\x{100}-\x{2ff}].
3457    
3458         If a range that includes letters is used when caseless matching is set,         If a range that includes letters is used when caseless matching is set,
3459         it matches the letters in either case. For example, [W-c] is equivalent         it matches the letters in either case. For example, [W-c] is equivalent
3460         to  [][\\^_`wxyzabc],  matched  caselessly,  and  in non-UTF-8 mode, if         to [][\\^_`wxyzabc], matched caselessly,  and  in  non-UTF-8  mode,  if
3461         character tables for a French locale are in  use,  [\xc8-\xcb]  matches         character  tables  for  a French locale are in use, [\xc8-\xcb] matches
3462         accented  E  characters in both cases. In UTF-8 mode, PCRE supports the         accented E characters in both cases. In UTF-8 mode, PCRE  supports  the
3463         concept of case for characters with values greater than 128  only  when         concept  of  case for characters with values greater than 128 only when
3464         it is compiled with Unicode property support.         it is compiled with Unicode property support.
3465    
3466         The  character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear         The character types \d, \D, \p, \P, \s, \S, \w, and \W may also  appear
3467         in a character class, and add the characters that  they  match  to  the         in  a  character  class,  and add the characters that they match to the
3468         class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-         class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
3469         flex can conveniently be used with the upper case  character  types  to         flex  can  conveniently  be used with the upper case character types to
3470         specify  a  more  restricted  set of characters than the matching lower         specify a more restricted set of characters  than  the  matching  lower
3471         case type. For example, the class [^\W_] matches any letter  or  digit,         case  type.  For example, the class [^\W_] matches any letter or digit,
3472         but not underscore.         but not underscore.
3473    
3474         The  only  metacharacters  that are recognized in character classes are         The only metacharacters that are recognized in  character  classes  are
3475         backslash, hyphen (only where it can be  interpreted  as  specifying  a         backslash,  hyphen  (only  where  it can be interpreted as specifying a
3476         range),  circumflex  (only  at the start), opening square bracket (only         range), circumflex (only at the start), opening  square  bracket  (only
3477         when it can be interpreted as introducing a POSIX class name - see  the         when  it can be interpreted as introducing a POSIX class name - see the
3478         next  section),  and  the  terminating closing square bracket. However,         next section), and the terminating  closing  square  bracket.  However,
3479         escaping other non-alphanumeric characters does no harm.         escaping other non-alphanumeric characters does no harm.
3480    
3481    
3482  POSIX CHARACTER CLASSES  POSIX CHARACTER CLASSES
3483    
3484         Perl supports the POSIX notation for character classes. This uses names         Perl supports the POSIX notation for character classes. This uses names
3485         enclosed  by  [: and :] within the enclosing square brackets. PCRE also         enclosed by [: and :] within the enclosing square brackets.  PCRE  also
3486         supports this notation. For example,         supports this notation. For example,
3487    
3488           [01[:alpha:]%]           [01[:alpha:]%]
# Line 3455  POSIX CHARACTER CLASSES Line 3505  POSIX CHARACTER CLASSES
3505           word     "word" characters (same as \w)           word     "word" characters (same as \w)
3506           xdigit   hexadecimal digits           xdigit   hexadecimal digits
3507    
3508         The  "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),         The "space" characters are HT (9), LF (10), VT (11), FF (12), CR  (13),
3509         and space (32). Notice that this list includes the VT  character  (code         and  space  (32). Notice that this list includes the VT character (code
3510         11). This makes "space" different to \s, which does not include VT (for         11). This makes "space" different to \s, which does not include VT (for
3511         Perl compatibility).         Perl compatibility).
3512    
3513         The name "word" is a Perl extension, and "blank"  is  a  GNU  extension         The  name  "word"  is  a Perl extension, and "blank" is a GNU extension
3514         from  Perl  5.8. Another Perl extension is negation, which is indicated         from Perl 5.8. Another Perl extension is negation, which  is  indicated
3515         by a ^ character after the colon. For example,         by a ^ character after the colon. For example,
3516    
3517           [12[:^digit:]]           [12[:^digit:]]
3518    
3519         matches "1", "2", or any non-digit. PCRE (and Perl) also recognize  the         matches  "1", "2", or any non-digit. PCRE (and Perl) also recognize the
3520         POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but         POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
3521         these are not supported, and an error is given if they are encountered.         these are not supported, and an error is given if they are encountered.
3522    
# Line 3476  POSIX CHARACTER CLASSES Line 3526  POSIX CHARACTER CLASSES
3526    
3527  VERTICAL BAR  VERTICAL BAR
3528    
3529         Vertical bar characters are used to separate alternative patterns.  For         Vertical  bar characters are used to separate alternative patterns. For
3530         example, the pattern         example, the pattern
3531    
3532           gilbert|sullivan           gilbert|sullivan
3533    
3534         matches  either "gilbert" or "sullivan". Any number of alternatives may         matches either "gilbert" or "sullivan". Any number of alternatives  may
3535         appear, and an empty  alternative  is  permitted  (matching  the  empty         appear,  and  an  empty  alternative  is  permitted (matching the empty
3536         string). The matching process tries each alternative in turn, from left         string). The matching process tries each alternative in turn, from left
3537         to right, and the first one that succeeds is used. If the  alternatives         to  right, and the first one that succeeds is used. If the alternatives
3538         are  within a subpattern (defined below), "succeeds" means matching the         are within a subpattern (defined below), "succeeds" means matching  the
3539         rest of the main pattern as well as the alternative in the  subpattern.         rest  of the main pattern as well as the alternative in the subpattern.
3540    
3541    
3542  INTERNAL OPTION SETTING  INTERNAL OPTION SETTING
3543    
3544         The  settings  of  the  PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and         The settings of the  PCRE_CASELESS,  PCRE_MULTILINE,  PCRE_DOTALL,  and
3545         PCRE_EXTENDED options can be changed  from  within  the  pattern  by  a         PCRE_EXTENDED  options  can  be  changed  from  within the pattern by a
3546         sequence  of  Perl  option  letters  enclosed between "(?" and ")". The         sequence of Perl option letters enclosed  between  "(?"  and  ")".  The
3547         option letters are         option letters are
3548    
3549           i  for PCRE_CASELESS           i  for PCRE_CASELESS
# Line 3503  INTERNAL OPTION SETTING Line 3553  INTERNAL OPTION SETTING
3553    
3554         For example, (?im) sets caseless, multiline matching. It is also possi-         For example, (?im) sets caseless, multiline matching. It is also possi-
3555         ble to unset these options by preceding the letter with a hyphen, and a         ble to unset these options by preceding the letter with a hyphen, and a
3556         combined setting and unsetting such as (?im-sx), which sets  PCRE_CASE-         combined  setting and unsetting such as (?im-sx), which sets PCRE_CASE-
3557         LESS  and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,         LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and  PCRE_EXTENDED,
3558         is also permitted. If a  letter  appears  both  before  and  after  the         is  also  permitted.  If  a  letter  appears  both before and after the
3559         hyphen, the option is unset.         hyphen, the option is unset.
3560    
3561         When  an option change occurs at top level (that is, not inside subpat-         When an option change occurs at top level (that is, not inside  subpat-
3562         tern parentheses), the change applies to the remainder of  the  pattern         tern  parentheses),  the change applies to the remainder of the pattern
3563         that follows.  If the change is placed right at the start of a pattern,         that follows.  If the change is placed right at the start of a pattern,
3564         PCRE extracts it into the global options (and it will therefore show up         PCRE extracts it into the global options (and it will therefore show up
3565         in data extracted by the pcre_fullinfo() function).         in data extracted by the pcre_fullinfo() function).
3566    
3567         An  option  change  within a subpattern (see below for a description of         An option change within a subpattern (see below for  a  description  of
3568         subpatterns) affects only that part of the current pattern that follows         subpatterns) affects only that part of the current pattern that follows
3569         it, so         it, so
3570    
3571           (a(?i)b)c           (a(?i)b)c
3572    
3573         matches abc and aBc and no other strings (assuming PCRE_CASELESS is not         matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
3574         used).  By this means, options can be made to have  different  settings         used).   By  this means, options can be made to have different settings
3575         in  different parts of the pattern. Any changes made in one alternative         in different parts of the pattern. Any changes made in one  alternative
3576         do carry on into subsequent branches within the  same  subpattern.  For         do  carry  on  into subsequent branches within the same subpattern. For
3577         example,         example,
3578    
3579           (a(?i)b|c)           (a(?i)b|c)
3580    
3581         matches  "ab",  "aB",  "c",  and "C", even though when matching "C" the         matches "ab", "aB", "c", and "C", even though  when  matching  "C"  the
3582         first branch is abandoned before the option setting.  This  is  because         first  branch  is  abandoned before the option setting. This is because
3583         the  effects  of option settings happen at compile time. There would be         the effects of option settings happen at compile time. There  would  be
3584         some very weird behaviour otherwise.         some very weird behaviour otherwise.
3585    
3586         The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and  PCRE_EXTRA         The  PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
3587         can  be changed in the same way as the Perl-compatible options by using         can be changed in the same way as the Perl-compatible options by  using
3588         the characters J, U and X respectively.         the characters J, U and X respectively.
3589    
3590    
# Line 3547  SUBPATTERNS Line 3597  SUBPATTERNS
3597    
3598           cat(aract|erpillar|)           cat(aract|erpillar|)
3599    
3600         matches  one  of the words "cat", "cataract", or "caterpillar". Without         matches one of the words "cat", "cataract", or  "caterpillar".  Without
3601         the parentheses, it would match  "cataract",  "erpillar"  or  an  empty         the  parentheses,  it  would  match  "cataract", "erpillar" or an empty
3602         string.         string.
3603    
3604         2.  It  sets  up  the  subpattern as a capturing subpattern. This means         2. It sets up the subpattern as  a  capturing  subpattern.  This  means
3605         that, when the whole pattern  matches,  that  portion  of  the  subject         that,  when  the  whole  pattern  matches,  that portion of the subject
3606         string that matched the subpattern is passed back to the caller via the         string that matched the subpattern is passed back to the caller via the
3607         ovector argument of pcre_exec(). Opening parentheses are  counted  from         ovector  argument  of pcre_exec(). Opening parentheses are counted from
3608         left  to  right  (starting  from 1) to obtain numbers for the capturing         left to right (starting from 1) to obtain  numbers  for  the  capturing
3609         subpatterns.         subpatterns.
3610    
3611         For example, if the string "the red king" is matched against  the  pat-         For  example,  if the string "the red king" is matched against the pat-
3612         tern         tern
3613    
3614           the ((red|white) (king|queen))           the ((red|white) (king|queen))
# Line 3566  SUBPATTERNS Line 3616  SUBPATTERNS
3616         the captured substrings are "red king", "red", and "king", and are num-         the captured substrings are "red king", "red", and "king", and are num-
3617         bered 1, 2, and 3, respectively.         bered 1, 2, and 3, respectively.
3618    
3619         The fact that plain parentheses fulfil  two  functions  is  not  always         The  fact  that  plain  parentheses  fulfil two functions is not always
3620         helpful.   There are often times when a grouping subpattern is required         helpful.  There are often times when a grouping subpattern is  required
3621         without a capturing requirement. If an opening parenthesis is  followed         without  a capturing requirement. If an opening parenthesis is followed
3622         by  a question mark and a colon, the subpattern does not do any captur-         by a question mark and a colon, the subpattern does not do any  captur-
3623         ing, and is not counted when computing the  number  of  any  subsequent         ing,  and  is  not  counted when computing the number of any subsequent
3624         capturing  subpatterns. For example, if the string "the white queen" is         capturing subpatterns. For example, if the string "the white queen"  is
3625         matched against the pattern         matched against the pattern
3626    
3627           the ((?:red|white) (king|queen))           the ((?:red|white) (king|queen))
# Line 3579  SUBPATTERNS Line 3629  SUBPATTERNS
3629         the captured substrings are "white queen" and "queen", and are numbered         the captured substrings are "white queen" and "queen", and are numbered
3630         1 and 2. The maximum number of capturing subpatterns is 65535.         1 and 2. The maximum number of capturing subpatterns is 65535.
3631    
3632         As  a  convenient shorthand, if any option settings are required at the         As a convenient shorthand, if any option settings are required  at  the
3633         start of a non-capturing subpattern,  the  option  letters  may  appear         start  of  a  non-capturing  subpattern,  the option letters may appear
3634         between the "?" and the ":". Thus the two patterns         between the "?" and the ":". Thus the two patterns
3635    
3636           (?i:saturday|sunday)           (?i:saturday|sunday)
3637           (?:(?i)saturday|sunday)           (?:(?i)saturday|sunday)
3638    
3639         match exactly the same set of strings. Because alternative branches are         match exactly the same set of strings. Because alternative branches are
3640         tried from left to right, and options are not reset until  the  end  of         tried  from  left  to right, and options are not reset until the end of
3641         the  subpattern is reached, an option setting in one branch does affect         the subpattern is reached, an option setting in one branch does  affect
3642         subsequent branches, so the above patterns match "SUNDAY"  as  well  as         subsequent  branches,  so  the above patterns match "SUNDAY" as well as
3643         "Saturday".         "Saturday".
3644    
3645    
3646  DUPLICATE SUBPATTERN NUMBERS  DUPLICATE SUBPATTERN NUMBERS
3647    
3648         Perl 5.10 introduced a feature whereby each alternative in a subpattern         Perl 5.10 introduced a feature whereby each alternative in a subpattern
3649         uses the same numbers for its capturing parentheses. Such a  subpattern         uses  the same numbers for its capturing parentheses. Such a subpattern
3650         starts  with (?| and is itself a non-capturing subpattern. For example,         starts with (?| and is itself a non-capturing subpattern. For  example,
3651         consider this pattern:         consider this pattern:
3652    
3653           (?|(Sat)ur|(Sun))day           (?|(Sat)ur|(Sun))day
3654    
3655         Because the two alternatives are inside a (?| group, both sets of  cap-         Because  the two alternatives are inside a (?| group, both sets of cap-
3656         turing  parentheses  are  numbered one. Thus, when the pattern matches,         turing parentheses are numbered one. Thus, when  the  pattern  matches,
3657         you can look at captured substring number  one,  whichever  alternative         you  can  look  at captured substring number one, whichever alternative
3658         matched.  This  construct  is useful when you want to capture part, but         matched. This construct is useful when you want to  capture  part,  but
3659         not all, of one of a number of alternatives. Inside a (?| group, paren-         not all, of one of a number of alternatives. Inside a (?| group, paren-
3660         theses  are  numbered as usual, but the number is reset at the start of         theses are numbered as usual, but the number is reset at the  start  of
3661         each branch. The numbers of any capturing buffers that follow the  sub-         each  branch. The numbers of any capturing buffers that follow the sub-
3662         pattern  start after the highest number used in any branch. The follow-         pattern start after the highest number used in any branch. The  follow-
3663         ing example is taken from the Perl documentation.  The  numbers  under-         ing  example  is taken from the Perl documentation.  The numbers under-
3664         neath show in which buffer the captured content will be stored.         neath show in which buffer the captured content will be stored.
3665    
3666           # before  ---------------branch-reset----------- after           # before  ---------------branch-reset----------- after
3667           / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x           / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
3668           # 1            2         2  3        2     3     4           # 1            2         2  3        2     3     4
3669    
3670         A  backreference  or  a  recursive call to a numbered subpattern always         A backreference or a recursive call to  a  numbered  subpattern  always
3671         refers to the first one in the pattern with the given number.         refers to the first one in the pattern with the given number.
3672    
3673         An alternative approach to using this "branch reset" feature is to  use         An  alternative approach to using this "branch reset" feature is to use
3674         duplicate named subpatterns, as described in the next section.         duplicate named subpatterns, as described in the next section.
3675    
3676    
3677  NAMED SUBPATTERNS  NAMED SUBPATTERNS
3678    
3679         Identifying  capturing  parentheses  by number is simple, but it can be         Identifying capturing parentheses by number is simple, but  it  can  be
3680         very hard to keep track of the numbers in complicated  regular  expres-         very  hard  to keep track of the numbers in complicated regular expres-
3681         sions.  Furthermore,  if  an  expression  is  modified, the numbers may         sions. Furthermore, if an  expression  is  modified,  the  numbers  may
3682         change. To help with this difficulty, PCRE supports the naming of  sub-         change.  To help with this difficulty, PCRE supports the naming of sub-
3683         patterns. This feature was not added to Perl until release 5.10. Python         patterns. This feature was not added to Perl until release 5.10. Python
3684         had the feature earlier, and PCRE introduced it at release  4.0,  using         had  the  feature earlier, and PCRE introduced it at release 4.0, using
3685         the  Python syntax. PCRE now supports both the Perl and the Python syn-         the Python syntax. PCRE now supports both the Perl and the Python  syn-
3686         tax.         tax.
3687    
3688         In PCRE, a subpattern can be named in one of three  ways:  (?<name>...)         In  PCRE,  a subpattern can be named in one of three ways: (?<name>...)
3689         or  (?'name'...)  as in Perl, or (?P<name>...) as in Python. References         or (?'name'...) as in Perl, or (?P<name>...) as in  Python.  References
3690         to capturing parentheses from other parts of the pattern, such as back-         to capturing parentheses from other parts of the pattern, such as back-
3691         references,  recursion,  and conditions, can be made by name as well as         references, recursion, and conditions, can be made by name as  well  as
3692         by number.         by number.
3693    
3694         Names consist of up to  32  alphanumeric  characters  and  underscores.         Names  consist  of  up  to  32 alphanumeric characters and underscores.
3695         Named  capturing  parentheses  are  still  allocated numbers as well as         Named capturing parentheses are still  allocated  numbers  as  well  as
3696         names, exactly as if the names were not present. The PCRE API  provides         names,  exactly as if the names were not present. The PCRE API provides
3697         function calls for extracting the name-to-number translation table from         function calls for extracting the name-to-number translation table from
3698         a compiled pattern. There is also a convenience function for extracting         a compiled pattern. There is also a convenience function for extracting
3699         a captured substring by name.         a captured substring by name.
3700    
3701         By  default, a name must be unique within a pattern, but it is possible         By default, a name must be unique within a pattern, but it is  possible
3702         to relax this constraint by setting the PCRE_DUPNAMES option at compile         to relax this constraint by setting the PCRE_DUPNAMES option at compile
3703         time.  This  can  be useful for patterns where only one instance of the         time. This can be useful for patterns where only one  instance  of  the
3704         named parentheses can match. Suppose you want to match the  name  of  a         named  parentheses  can  match. Suppose you want to match the name of a
3705         weekday,  either as a 3-letter abbreviation or as the full name, and in         weekday, either as a 3-letter abbreviation or as the full name, and  in
3706         both cases you want to extract the abbreviation. This pattern (ignoring         both cases you want to extract the abbreviation. This pattern (ignoring
3707         the line breaks) does the job:         the line breaks) does the job:
3708    
# Line 3662  NAMED SUBPATTERNS Line 3712  NAMED SUBPATTERNS
3712           (?<DN>Thu)(?:rsday)?|           (?<DN>Thu)(?:rsday)?|
3713           (?<DN>Sat)(?:urday)?           (?<DN>Sat)(?:urday)?
3714    
3715         There  are  five capturing substrings, but only one is ever set after a         There are five capturing substrings, but only one is ever set  after  a
3716         match.  (An alternative way of solving this problem is to use a "branch         match.  (An alternative way of solving this problem is to use a "branch
3717         reset" subpattern, as described in the previous section.)         reset" subpattern, as described in the previous section.)
3718    
3719         The  convenience  function  for extracting the data by name returns the         The convenience function for extracting the data by  name  returns  the
3720         substring for the first (and in this example, the only)  subpattern  of         substring  for  the first (and in this example, the only) subpattern of
3721         that  name  that  matched.  This saves searching to find which numbered         that name that matched. This saves searching  to  find  which  numbered
3722         subpattern it was. If you make a reference to a non-unique  named  sub-         subpattern  it  was. If you make a reference to a non-unique named sub-
3723         pattern  from elsewhere in the pattern, the one that corresponds to the         pattern from elsewhere in the pattern, the one that corresponds to  the
3724         lowest number is used. For further details of the interfaces  for  han-         lowest  number  is used. For further details of the interfaces for han-
3725         dling named subpatterns, see the pcreapi documentation.         dling named subpatterns, see the pcreapi documentation.
3726    
3727    
3728  REPETITION  REPETITION
3729    
3730         Repetition  is  specified  by  quantifiers, which can follow any of the         Repetition is specified by quantifiers, which can  follow  any  of  the
3731         following items:         following items:
3732    
3733           a literal data character           a literal data character
# Line 3690  REPETITION Line 3740  REPETITION
3740           a back reference (see next section)           a back reference (see next section)
3741           a parenthesized subpattern (unless it is an assertion)           a parenthesized subpattern (unless it is an assertion)
3742    
3743         The general repetition quantifier specifies a minimum and maximum  num-         The  general repetition quantifier specifies a minimum and maximum num-
3744         ber  of  permitted matches, by giving the two numbers in curly brackets         ber of permitted matches, by giving the two numbers in  curly  brackets
3745         (braces), separated by a comma. The numbers must be  less  than  65536,         (braces),  separated  by  a comma. The numbers must be less than 65536,
3746         and the first must be less than or equal to the second. For example:         and the first must be less than or equal to the second. For example:
3747    
3748           z{2,4}           z{2,4}
3749    
3750         matches  "zz",  "zzz",  or  "zzzz". A closing brace on its own is not a         matches "zz", "zzz", or "zzzz". A closing brace on its  own  is  not  a
3751         special character. If the second number is omitted, but  the  comma  is         special  character.  If  the second number is omitted, but the comma is
3752         present,  there  is  no upper limit; if the second number and the comma         present, there is no upper limit; if the second number  and  the  comma
3753         are both omitted, the quantifier specifies an exact number of  required         are  both omitted, the quantifier specifies an exact number of required
3754         matches. Thus         matches. Thus
3755    
3756           [aeiou]{3,}           [aeiou]{3,}
# Line 3709  REPETITION Line 3759  REPETITION
3759    
3760           \d{8}           \d{8}
3761    
3762         matches  exactly  8  digits. An opening curly bracket that appears in a         matches exactly 8 digits. An opening curly bracket that  appears  in  a
3763         position where a quantifier is not allowed, or one that does not  match         position  where a quantifier is not allowed, or one that does not match
3764         the  syntax of a quantifier, is taken as a literal character. For exam-         the syntax of a quantifier, is taken as a literal character. For  exam-
3765         ple, {,6} is not a quantifier, but a literal string of four characters.         ple, {,6} is not a quantifier, but a literal string of four characters.
3766    
3767         In  UTF-8  mode,  quantifiers  apply to UTF-8 characters rather than to         In UTF-8 mode, quantifiers apply to UTF-8  characters  rather  than  to
3768         individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-         individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
3769         acters, each of which is represented by a two-byte sequence. Similarly,         acters, each of which is represented by a two-byte sequence. Similarly,
3770         when Unicode property support is available, \X{3} matches three Unicode         when Unicode property support is available, \X{3} matches three Unicode
3771         extended  sequences,  each of which may be several bytes long (and they         extended sequences, each of which may be several bytes long  (and  they
3772         may be of different lengths).         may be of different lengths).
3773    
3774         The quantifier {0} is permitted, causing the expression to behave as if         The quantifier {0} is permitted, causing the expression to behave as if
3775         the previous item and the quantifier were not present.         the previous item and the quantifier were not present.
3776    
3777         For  convenience, the three most common quantifiers have single-charac-         For convenience, the three most common quantifiers have  single-charac-
3778         ter abbreviations:         ter abbreviations:
3779    
3780           *    is equivalent to {0,}           *    is equivalent to {0,}
3781           +    is equivalent to {1,}           +    is equivalent to {1,}
3782           ?    is equivalent to {0,1}           ?    is equivalent to {0,1}
3783    
3784         It is possible to construct infinite loops by  following  a  subpattern         It  is  possible  to construct infinite loops by following a subpattern
3785         that can match no characters with a quantifier that has no upper limit,         that can match no characters with a quantifier that has no upper limit,
3786         for example:         for example:
3787    
3788           (a?)*           (a?)*
3789    
3790         Earlier versions of Perl and PCRE used to give an error at compile time         Earlier versions of Perl and PCRE used to give an error at compile time
3791         for  such  patterns. However, because there are cases where this can be         for such patterns. However, because there are cases where this  can  be
3792         useful, such patterns are now accepted, but if any  repetition  of  the         useful,  such  patterns  are now accepted, but if any repetition of the
3793         subpattern  does in fact match no characters, the loop is forcibly bro-         subpattern does in fact match no characters, the loop is forcibly  bro-
3794         ken.         ken.
3795    
3796         By default, the quantifiers are "greedy", that is, they match  as  much         By  default,  the quantifiers are "greedy", that is, they match as much
3797         as  possible  (up  to  the  maximum number of permitted times), without         as possible (up to the maximum  number  of  permitted  times),  without
3798         causing the rest of the pattern to fail. The classic example  of  where         causing  the  rest of the pattern to fail. The classic example of where
3799         this gives problems is in trying to match comments in C programs. These         this gives problems is in trying to match comments in C programs. These
3800         appear between /* and */ and within the comment,  individual  *  and  /         appear  between  /*  and  */ and within the comment, individual * and /
3801         characters  may  appear. An attempt to match C comments by applying the         characters may appear. An attempt to match C comments by  applying  the
3802         pattern         pattern
3803    
3804           /\*.*\*/           /\*.*\*/
# Line 3757  REPETITION Line 3807  REPETITION
3807    
3808           /* first comment */  not comment  /* second comment */           /* first comment */  not comment  /* second comment */
3809    
3810         fails, because it matches the entire string owing to the greediness  of         fails,  because it matches the entire string owing to the greediness of
3811         the .*  item.         the .*  item.
3812    
3813         However,  if  a quantifier is followed by a question mark, it ceases to         However, if a quantifier is followed by a question mark, it  ceases  to
3814         be greedy, and instead matches the minimum number of times possible, so         be greedy, and instead matches the minimum number of times possible, so
3815         the pattern         the pattern
3816    
3817           /\*.*?\*/           /\*.*?\*/
3818    
3819         does  the  right  thing with the C comments. The meaning of the various         does the right thing with the C comments. The meaning  of  the  various
3820         quantifiers is not otherwise changed,  just  the  preferred  number  of         quantifiers  is  not  otherwise  changed,  just the preferred number of
3821         matches.   Do  not  confuse this use of question mark with its use as a         matches.  Do not confuse this use of question mark with its  use  as  a
3822         quantifier in its own right. Because it has two uses, it can  sometimes         quantifier  in its own right. Because it has two uses, it can sometimes
3823         appear doubled, as in         appear doubled, as in
3824    
3825           \d??\d           \d??\d
# Line 3777  REPETITION Line 3827  REPETITION
3827         which matches one digit by preference, but can match two if that is the         which matches one digit by preference, but can match two if that is the
3828         only way the rest of the pattern matches.         only way the rest of the pattern matches.
3829    
3830         If the PCRE_UNGREEDY option is set (an option that is not available  in         If  the PCRE_UNGREEDY option is set (an option that is not available in
3831         Perl),  the  quantifiers are not greedy by default, but individual ones         Perl), the quantifiers are not greedy by default, but  individual  ones
3832         can be made greedy by following them with a  question  mark.  In  other         can  be  made  greedy  by following them with a question mark. In other
3833         words, it inverts the default behaviour.         words, it inverts the default behaviour.
3834    
3835         When  a  parenthesized  subpattern  is quantified with a minimum repeat         When a parenthesized subpattern is quantified  with  a  minimum  repeat
3836         count that is greater than 1 or with a limited maximum, more memory  is         count  that is greater than 1 or with a limited maximum, more memory is
3837         required  for  the  compiled  pattern, in proportion to the size of the         required for the compiled pattern, in proportion to  the  size  of  the
3838         minimum or maximum.         minimum or maximum.
3839    
3840         If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-         If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
3841         alent  to  Perl's  /s) is set, thus allowing the dot to match newlines,         alent to Perl's /s) is set, thus allowing the dot  to  match  newlines,
3842         the pattern is implicitly anchored, because whatever  follows  will  be         the  pattern  is  implicitly anchored, because whatever follows will be
3843         tried  against every character position in the subject string, so there         tried against every character position in the subject string, so  there
3844         is no point in retrying the overall match at  any  position  after  the         is  no  point  in  retrying the overall match at any position after the
3845         first.  PCRE  normally treats such a pattern as though it were preceded         first. PCRE normally treats such a pattern as though it  were  preceded
3846         by \A.         by \A.
3847    
3848         In cases where it is known that the subject  string  contains  no  new-         In  cases  where  it  is known that the subject string contains no new-
3849         lines,  it  is  worth setting PCRE_DOTALL in order to obtain this opti-         lines, it is worth setting PCRE_DOTALL in order to  obtain  this  opti-
3850         mization, or alternatively using ^ to indicate anchoring explicitly.         mization, or alternatively using ^ to indicate anchoring explicitly.
3851    
3852         However, there is one situation where the optimization cannot be  used.         However,  there is one situation where the optimization cannot be used.
3853         When  .*   is  inside  capturing  parentheses that are the subject of a         When .*  is inside capturing parentheses that  are  the  subject  of  a
3854         backreference elsewhere in the pattern, a match at the start  may  fail         backreference  elsewhere  in the pattern, a match at the start may fail
3855         where a later one succeeds. Consider, for example:         where a later one succeeds. Consider, for example:
3856    
3857           (.*)abc\1           (.*)abc\1
3858    
3859         If  the subject is "xyz123abc123" the match point is the fourth charac-         If the subject is "xyz123abc123" the match point is the fourth  charac-
3860         ter. For this reason, such a pattern is not implicitly anchored.         ter. For this reason, such a pattern is not implicitly anchored.
3861    
3862         When a capturing subpattern is repeated, the value captured is the sub-         When a capturing subpattern is repeated, the value captured is the sub-
# Line 3815  REPETITION Line 3865  REPETITION
3865           (tweedle[dume]{3}\s*)+           (tweedle[dume]{3}\s*)+
3866    
3867         has matched "tweedledum tweedledee" the value of the captured substring         has matched "tweedledum tweedledee" the value of the captured substring
3868         is "tweedledee". However, if there are  nested  capturing  subpatterns,         is  "tweedledee".  However,  if there are nested capturing subpatterns,
3869         the  corresponding captured values may have been set in previous itera-         the corresponding captured values may have been set in previous  itera-
3870         tions. For example, after         tions. For example, after
3871    
3872           /(a|(b))+/           /(a|(b))+/
# Line 3826  REPETITION Line 3876  REPETITION
3876    
3877  ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
3878    
3879         With both maximizing ("greedy") and minimizing ("ungreedy"  or  "lazy")         With  both  maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
3880         repetition,  failure  of what follows normally causes the repeated item         repetition, failure of what follows normally causes the  repeated  item
3881         to be re-evaluated to see if a different number of repeats  allows  the         to  be  re-evaluated to see if a different number of repeats allows the
3882         rest  of  the pattern to match. Sometimes it is useful to prevent this,         rest of the pattern to match. Sometimes it is useful to  prevent  this,
3883         either to change the nature of the match, or to cause it  fail  earlier         either  to  change the nature of the match, or to cause it fail earlier
3884         than  it otherwise might, when the author of the pattern knows there is         than it otherwise might, when the author of the pattern knows there  is
3885         no point in carrying on.         no point in carrying on.
3886    
3887         Consider, for example, the pattern \d+foo when applied to  the  subject         Consider,  for  example, the pattern \d+foo when applied to the subject
3888         line         line
3889    
3890           123456bar           123456bar
3891    
3892         After matching all 6 digits and then failing to match "foo", the normal         After matching all 6 digits and then failing to match "foo", the normal
3893         action of the matcher is to try again with only 5 digits  matching  the         action  of  the matcher is to try again with only 5 digits matching the
3894         \d+  item,  and  then  with  4,  and  so on, before ultimately failing.         \d+ item, and then with  4,  and  so  on,  before  ultimately  failing.
3895         "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides         "Atomic  grouping"  (a  term taken from Jeffrey Friedl's book) provides
3896         the  means for specifying that once a subpattern has matched, it is not         the means for specifying that once a subpattern has matched, it is  not
3897         to be re-evaluated in this way.         to be re-evaluated in this way.
3898    
3899         If we use atomic grouping for the previous example, the  matcher  gives         If  we  use atomic grouping for the previous example, the matcher gives
3900         up  immediately  on failing to match "foo" the first time. The notation         up immediately on failing to match "foo" the first time.  The  notation
3901         is a kind of special parenthesis, starting with (?> as in this example:         is a kind of special parenthesis, starting with (?> as in this example:
3902    
3903           (?>\d+)foo           (?>\d+)foo
3904    
3905         This  kind  of  parenthesis "locks up" the  part of the pattern it con-         This kind of parenthesis "locks up" the  part of the  pattern  it  con-
3906         tains once it has matched, and a failure further into  the  pattern  is         tains  once  it  has matched, and a failure further into the pattern is
3907         prevented  from  backtracking into it. Backtracking past it to previous         prevented from backtracking into it. Backtracking past it  to  previous
3908         items, however, works as normal.         items, however, works as normal.
3909    
3910         An alternative description is that a subpattern of  this  type  matches         An  alternative  description  is that a subpattern of this type matches
3911         the  string  of  characters  that an identical standalone pattern would         the string of characters that an  identical  standalone  pattern  would
3912         match, if anchored at the current point in the subject string.         match, if anchored at the current point in the subject string.
3913    
3914         Atomic grouping subpatterns are not capturing subpatterns. Simple cases         Atomic grouping subpatterns are not capturing subpatterns. Simple cases
3915         such as the above example can be thought of as a maximizing repeat that         such as the above example can be thought of as a maximizing repeat that
3916         must swallow everything it can. So, while both \d+ and  \d+?  are  pre-         must  swallow  everything  it can. So, while both \d+ and \d+? are pre-
3917         pared  to  adjust  the number of digits they match in order to make the         pared to adjust the number of digits they match in order  to  make  the
3918         rest of the pattern match, (?>\d+) can only match an entire sequence of         rest of the pattern match, (?>\d+) can only match an entire sequence of
3919         digits.         digits.
3920    
3921         Atomic  groups in general can of course contain arbitrarily complicated         Atomic groups in general can of course contain arbitrarily  complicated
3922         subpatterns, and can be nested. However, when  the  subpattern  for  an         subpatterns,  and  can  be  nested. However, when the subpattern for an
3923         atomic group is just a single repeated item, as in the example above, a         atomic group is just a single repeated item, as in the example above, a
3924         simpler notation, called a "possessive quantifier" can  be  used.  This         simpler  notation,  called  a "possessive quantifier" can be used. This
3925         consists  of  an  additional  + character following a quantifier. Using         consists of an additional + character  following  a  quantifier.  Using
3926         this notation, the previous example can be rewritten as         this notation, the previous example can be rewritten as
3927    
3928           \d++foo           \d++foo
# Line 3882  ATOMIC GROUPING AND POSSESSIVE QUANTIFIE Line 3932  ATOMIC GROUPING AND POSSESSIVE QUANTIFIE
3932    
3933           (abc|xyz){2,3}+           (abc|xyz){2,3}+
3934    
3935         Possessive   quantifiers   are   always  greedy;  the  setting  of  the         Possessive  quantifiers  are  always  greedy;  the   setting   of   the
3936         PCRE_UNGREEDY option is ignored. They are a convenient notation for the         PCRE_UNGREEDY option is ignored. They are a convenient notation for the
3937         simpler  forms  of atomic group. However, there is no difference in the         simpler forms of atomic group. However, there is no difference  in  the
3938         meaning of a possessive quantifier and  the  equivalent  atomic  group,         meaning  of  a  possessive  quantifier and the equivalent atomic group,
3939         though  there  may  be a performance difference; possessive quantifiers         though there may be a performance  difference;  possessive  quantifiers
3940         should be slightly faster.         should be slightly faster.
3941    
3942         The possessive quantifier syntax is an extension to the Perl  5.8  syn-         The  possessive  quantifier syntax is an extension to the Perl 5.8 syn-
3943         tax.   Jeffrey  Friedl  originated the idea (and the name) in the first         tax.  Jeffrey Friedl originated the idea (and the name)  in  the  first
3944         edition of his book. Mike McCloskey liked it, so implemented it when he         edition of his book. Mike McCloskey liked it, so implemented it when he
3945         built  Sun's Java package, and PCRE copied it from there. It ultimately         built Sun's Java package, and PCRE copied it from there. It  ultimately
3946         found its way into Perl at release 5.10.         found its way into Perl at release 5.10.
3947    
3948         PCRE has an optimization that automatically "possessifies" certain sim-         PCRE has an optimization that automatically "possessifies" certain sim-
3949         ple  pattern  constructs.  For  example, the sequence A+B is treated as         ple pattern constructs. For example, the sequence  A+B  is  treated  as
3950         A++B because there is no point in backtracking into a sequence  of  A's         A++B  because  there is no point in backtracking into a sequence of A's
3951         when B must follow.         when B must follow.
3952    
3953         When  a  pattern  contains an unlimited repeat inside a subpattern that         When a pattern contains an unlimited repeat inside  a  subpattern  that
3954         can itself be repeated an unlimited number of  times,  the  use  of  an         can  itself  be  repeated  an  unlimited number of times, the use of an
3955         atomic  group  is  the  only way to avoid some failing matches taking a         atomic group is the only way to avoid some  failing  matches  taking  a
3956         very long time indeed. The pattern         very long time indeed. The pattern
3957    
3958           (\D+|<\d+>)*[!?]           (\D+|<\d+>)*[!?]
3959    
3960         matches an unlimited number of substrings that either consist  of  non-         matches  an  unlimited number of substrings that either consist of non-
3961         digits,  or  digits  enclosed in <>, followed by either ! or ?. When it         digits, or digits enclosed in <>, followed by either ! or  ?.  When  it
3962         matches, it runs quickly. However, if it is applied to         matches, it runs quickly. However, if it is applied to
3963    
3964           aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa           aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
3965    
3966         it takes a long time before reporting  failure.  This  is  because  the         it  takes  a  long  time  before reporting failure. This is because the
3967         string  can be divided between the internal \D+ repeat and the external         string can be divided between the internal \D+ repeat and the  external
3968         * repeat in a large number of ways, and all  have  to  be  tried.  (The         *  repeat  in  a  large  number of ways, and all have to be tried. (The
3969         example  uses  [!?]  rather than a single character at the end, because         example uses [!?] rather than a single character at  the  end,  because
3970         both PCRE and Perl have an optimization that allows  for  fast  failure         both  PCRE  and  Perl have an optimization that allows for fast failure
3971         when  a single character is used. They remember the last single charac-         when a single character is used. They remember the last single  charac-
3972         ter that is required for a match, and fail early if it is  not  present         ter  that  is required for a match, and fail early if it is not present
3973         in  the  string.)  If  the pattern is changed so that it uses an atomic         in the string.) If the pattern is changed so that  it  uses  an  atomic
3974         group, like this:         group, like this:
3975    
3976           ((?>\D+)|<\d+>)*[!?]           ((?>\D+)|<\d+>)*[!?]
3977    
3978         sequences of non-digits cannot be broken, and failure happens  quickly.         sequences  of non-digits cannot be broken, and failure happens quickly.
3979    
3980    
3981  BACK REFERENCES  BACK REFERENCES
3982    
3983         Outside a character class, a backslash followed by a digit greater than         Outside a character class, a backslash followed by a digit greater than
3984         0 (and possibly further digits) is a back reference to a capturing sub-         0 (and possibly further digits) is a back reference to a capturing sub-
3985         pattern  earlier  (that is, to its left) in the pattern, provided there         pattern earlier (that is, to its left) in the pattern,  provided  there
3986         have been that many previous capturing left parentheses.         have been that many previous capturing left parentheses.
3987    
3988         However, if the decimal number following the backslash is less than 10,         However, if the decimal number following the backslash is less than 10,
3989         it  is  always  taken  as a back reference, and causes an error only if         it is always taken as a back reference, and causes  an  error  only  if
3990         there are not that many capturing left parentheses in the  entire  pat-         there  are  not that many capturing left parentheses in the entire pat-
3991         tern.  In  other words, the parentheses that are referenced need not be         tern. In other words, the parentheses that are referenced need  not  be
3992         to the left of the reference for numbers less than 10. A "forward  back         to  the left of the reference for numbers less than 10. A "forward back
3993         reference"  of  this  type can make sense when a repetition is involved         reference" of this type can make sense when a  repetition  is  involved
3994         and the subpattern to the right has participated in an  earlier  itera-         and  the  subpattern to the right has participated in an earlier itera-
3995         tion.         tion.
3996    
3997         It  is  not  possible to have a numerical "forward back reference" to a         It is not possible to have a numerical "forward back  reference"  to  a
3998         subpattern whose number is 10 or  more  using  this  syntax  because  a         subpattern  whose  number  is  10  or  more using this syntax because a
3999         sequence  such  as  \50 is interpreted as a character defined in octal.         sequence such as \50 is interpreted as a character  defined  in  octal.
4000         See the subsection entitled "Non-printing characters" above for further         See the subsection entitled "Non-printing characters" above for further
4001         details  of  the  handling of digits following a backslash. There is no         details of the handling of digits following a backslash.  There  is  no
4002         such problem when named parentheses are used. A back reference  to  any         such  problem  when named parentheses are used. A back reference to any
4003         subpattern is possible using named parentheses (see below).         subpattern is possible using named parentheses (see below).
4004    
4005         Another  way  of  avoiding  the ambiguity inherent in the use of digits         Another way of avoiding the ambiguity inherent in  the  use  of  digits
4006         following a backslash is to use the \g escape sequence, which is a fea-         following a backslash is to use the \g escape sequence, which is a fea-
4007         ture  introduced  in  Perl  5.10.  This  escape  must be followed by an         ture introduced in Perl 5.10.  This  escape  must  be  followed  by  an
4008         unsigned number or a negative number, optionally  enclosed  in  braces.         unsigned  number  or  a negative number, optionally enclosed in braces.
4009         These examples are all identical:         These examples are all identical:
4010    
4011           (ring), \1           (ring), \1
4012           (ring), \g1           (ring), \g1
4013           (ring), \g{1}           (ring), \g{1}
4014    
4015         An  unsigned number specifies an absolute reference without the ambigu-         An unsigned number specifies an absolute reference without the  ambigu-
4016         ity that is present in the older syntax. It is also useful when literal         ity that is present in the older syntax. It is also useful when literal
4017         digits follow the reference. A negative number is a relative reference.         digits follow the reference. A negative number is a relative reference.
4018         Consider this example:         Consider this example:
# Line 3970  BACK REFERENCES Line 4020  BACK REFERENCES
4020           (abc(def)ghi)\g{-1}           (abc(def)ghi)\g{-1}
4021    
4022         The sequence \g{-1} is a reference to the most recently started captur-         The sequence \g{-1} is a reference to the most recently started captur-
4023         ing  subpattern  before \g, that is, is it equivalent to \2. Similarly,         ing subpattern before \g, that is, is it equivalent to  \2.  Similarly,
4024         \g{-2} would be equivalent to \1. The use of relative references can be         \g{-2} would be equivalent to \1. The use of relative references can be
4025         helpful  in  long  patterns,  and  also in patterns that are created by         helpful in long patterns, and also in  patterns  that  are  created  by
4026         joining together fragments that contain references within themselves.         joining together fragments that contain references within themselves.
4027    
4028         A back reference matches whatever actually matched the  capturing  sub-         A  back  reference matches whatever actually matched the capturing sub-
4029         pattern  in  the  current subject string, rather than anything matching         pattern in the current subject string, rather  than  anything  matching
4030         the subpattern itself (see "Subpatterns as subroutines" below for a way         the subpattern itself (see "Subpatterns as subroutines" below for a way
4031         of doing that). So the pattern         of doing that). So the pattern
4032    
4033           (sens|respons)e and \1ibility           (sens|respons)e and \1ibility
4034    
4035         matches  "sense and sensibility" and "response and responsibility", but         matches "sense and sensibility" and "response and responsibility",  but
4036         not "sense and responsibility". If caseful matching is in force at  the         not  "sense and responsibility". If caseful matching is in force at the
4037         time  of the back reference, the case of letters is relevant. For exam-         time of the back reference, the case of letters is relevant. For  exam-
4038         ple,         ple,
4039    
4040           ((?i)rah)\s+\1           ((?i)rah)\s+\1
4041    
4042         matches "rah rah" and "RAH RAH", but not "RAH  rah",  even  though  the         matches  "rah  rah"  and  "RAH RAH", but not "RAH rah", even though the
4043         original capturing subpattern is matched caselessly.         original capturing subpattern is matched caselessly.
4044    
4045         There  are  several  different ways of writing back references to named         There are several different ways of writing back  references  to  named
4046         subpatterns. The .NET syntax \k{name} and the Perl syntax  \k<name>  or         subpatterns.  The  .NET syntax \k{name} and the Perl syntax \k<name> or
4047         \k'name'  are supported, as is the Python syntax (?P=name). Perl 5.10's         \k'name' are supported, as is the Python syntax (?P=name). Perl  5.10's
4048         unified back reference syntax, in which \g can be used for both numeric         unified back reference syntax, in which \g can be used for both numeric
4049         and  named  references,  is  also supported. We could rewrite the above         and named references, is also supported. We  could  rewrite  the  above
4050         example in any of the following ways:         example in any of the following ways:
4051    
4052           (?<p1>(?i)rah)\s+\k<p1>           (?<p1>(?i)rah)\s+\k<p1>
# Line 4004  BACK REFERENCES Line 4054  BACK REFERENCES
4054           (?P<p1>(?i)rah)\s+(?P=p1)           (?P<p1>(?i)rah)\s+(?P=p1)
4055           (?<p1>(?i)rah)\s+\g{p1}           (?<p1>(?i)rah)\s+\g{p1}
4056    
4057         A subpattern that is referenced by  name  may  appear  in  the  pattern         A  subpattern  that  is  referenced  by  name may appear in the pattern
4058         before or after the reference.         before or after the reference.
4059    
4060         There  may be more than one back reference to the same subpattern. If a         There may be more than one back reference to the same subpattern. If  a
4061         subpattern has not actually been used in a particular match,  any  back         subpattern  has  not actually been used in a particular match, any back
4062         references to it always fail. For example, the pattern         references to it always fail. For example, the pattern
4063    
4064           (a|(bc))\2           (a|(bc))\2
4065    
4066         always  fails if it starts to match "a" rather than "bc". Because there         always fails if it starts to match "a" rather than "bc". Because  there
4067         may be many capturing parentheses in a pattern,  all  digits  following         may  be  many  capturing parentheses in a pattern, all digits following
4068         the  backslash  are taken as part of a potential back reference number.         the backslash are taken as part of a potential back  reference  number.
4069         If the pattern continues with a digit character, some delimiter must be         If the pattern continues with a digit character, some delimiter must be
4070         used  to  terminate  the back reference. If the PCRE_EXTENDED option is         used to terminate the back reference. If the  PCRE_EXTENDED  option  is
4071         set, this can be whitespace.  Otherwise an  empty  comment  (see  "Com-         set,  this  can  be  whitespace.  Otherwise an empty comment (see "Com-
4072         ments" below) can be used.         ments" below) can be used.
4073    
4074         A  back reference that occurs inside the parentheses to which it refers         A back reference that occurs inside the parentheses to which it  refers
4075         fails when the subpattern is first used, so, for example,  (a\1)  never         fails  when  the subpattern is first used, so, for example, (a\1) never
4076         matches.   However,  such references can be useful inside repeated sub-         matches.  However, such references can be useful inside  repeated  sub-
4077         patterns. For example, the pattern         patterns. For example, the pattern
4078    
4079           (a|b\1)+           (a|b\1)+
4080    
4081         matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-         matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
4082         ation  of  the  subpattern,  the  back  reference matches the character         ation of the subpattern,  the  back  reference  matches  the  character
4083         string corresponding to the previous iteration. In order  for  this  to         string  corresponding  to  the previous iteration. In order for this to
4084         work,  the  pattern must be such that the first iteration does not need         work, the pattern must be such that the first iteration does  not  need
4085         to match the back reference. This can be done using alternation, as  in         to  match the back reference. This can be done using alternation, as in
4086         the example above, or by a quantifier with a minimum of zero.         the example above, or by a quantifier with a minimum of zero.
4087    
4088    
4089  ASSERTIONS  ASSERTIONS
4090    
4091         An  assertion  is  a  test on the characters following or preceding the         An assertion is a test on the characters  following  or  preceding  the
4092         current matching point that does not actually consume  any  characters.         current  matching  point that does not actually consume any characters.
4093         The  simple  assertions  coded  as  \b, \B, \A, \G, \Z, \z, ^ and $ are         The simple assertions coded as \b, \B, \A, \G, \Z,  \z,  ^  and  $  are
4094         described above.         described above.
4095    
4096         More complicated assertions are coded as  subpatterns.  There  are  two         More  complicated  assertions  are  coded as subpatterns. There are two
4097         kinds:  those  that  look  ahead of the current position in the subject         kinds: those that look ahead of the current  position  in  the  subject
4098         string, and those that look  behind  it.  An  assertion  subpattern  is         string,  and  those  that  look  behind  it. An assertion subpattern is
4099         matched  in  the  normal way, except that it does not cause the current         matched in the normal way, except that it does not  cause  the  current
4100         matching position to be changed.         matching position to be changed.
4101    
4102         Assertion subpatterns are not capturing subpatterns,  and  may  not  be         Assertion  subpatterns  are  not  capturing subpatterns, and may not be
4103         repeated,  because  it  makes no sense to assert the same thing several         repeated, because it makes no sense to assert the  same  thing  several
4104         times. If any kind of assertion contains capturing  subpatterns  within         times.  If  any kind of assertion contains capturing subpatterns within
4105         it,  these are counted for the purposes of numbering the capturing sub-         it, these are counted for the purposes of numbering the capturing  sub-
4106         patterns in the whole pattern.  However, substring capturing is carried         patterns in the whole pattern.  However, substring capturing is carried
4107         out  only  for  positive assertions, because it does not make sense for         out only for positive assertions, because it does not  make  sense  for
4108         negative assertions.         negative assertions.
4109    
4110     Lookahead assertions     Lookahead assertions
# Line 4064  ASSERTIONS Line 4114  ASSERTIONS
4114    
4115           \w+(?=;)           \w+(?=;)
4116    
4117         matches  a word followed by a semicolon, but does not include the semi-         matches a word followed by a semicolon, but does not include the  semi-
4118         colon in the match, and         colon in the match, and
4119    
4120           foo(?!bar)           foo(?!bar)
4121    
4122         matches any occurrence of "foo" that is not  followed  by  "bar".  Note         matches  any  occurrence  of  "foo" that is not followed by "bar". Note
4123         that the apparently similar pattern         that the apparently similar pattern
4124    
4125           (?!foo)bar           (?!foo)bar
4126    
4127         does  not  find  an  occurrence  of "bar" that is preceded by something         does not find an occurrence of "bar"  that  is  preceded  by  something
4128         other than "foo"; it finds any occurrence of "bar" whatsoever,  because         other  than "foo"; it finds any occurrence of "bar" whatsoever, because
4129         the assertion (?!foo) is always true when the next three characters are         the assertion (?!foo) is always true when the next three characters are
4130         "bar". A lookbehind assertion is needed to achieve the other effect.         "bar". A lookbehind assertion is needed to achieve the other effect.
4131    
4132         If you want to force a matching failure at some point in a pattern, the         If you want to force a matching failure at some point in a pattern, the
4133         most  convenient  way  to  do  it  is with (?!) because an empty string         most convenient way to do it is  with  (?!)  because  an  empty  string
4134         always matches, so an assertion that requires there not to be an  empty         always  matches, so an assertion that requires there not to be an empty
4135         string must always fail.         string must always fail.
4136    
4137     Lookbehind assertions     Lookbehind assertions
4138    
4139         Lookbehind  assertions start with (?<= for positive assertions and (?<!         Lookbehind assertions start with (?<= for positive assertions and  (?<!
4140         for negative assertions. For example,         for negative assertions. For example,
4141    
4142           (?<!foo)bar           (?<!foo)bar
4143    
4144         does find an occurrence of "bar" that is not  preceded  by  "foo".  The         does  find  an  occurrence  of "bar" that is not preceded by "foo". The
4145         contents  of  a  lookbehind  assertion are restricted such that all the         contents of a lookbehind assertion are restricted  such  that  all  the
4146         strings it matches must have a fixed length. However, if there are sev-         strings it matches must have a fixed length. However, if there are sev-
4147         eral  top-level  alternatives,  they  do  not all have to have the same         eral top-level alternatives, they do not all  have  to  have  the  same
4148         fixed length. Thus         fixed length. Thus
4149    
4150           (?<=bullock|donkey)           (?<=bullock|donkey)
# Line 4103  ASSERTIONS Line 4153  ASSERTIONS
4153    
4154           (?<!dogs?|cats?)           (?<!dogs?|cats?)
4155    
4156         causes an error at compile time. Branches that match  different  length         causes  an  error at compile time. Branches that match different length
4157         strings  are permitted only at the top level of a lookbehind assertion.         strings are permitted only at the top level of a lookbehind  assertion.
4158         This is an extension compared with  Perl  (at  least  for  5.8),  which         This  is  an  extension  compared  with  Perl (at least for 5.8), which
4159         requires  all branches to match the same length of string. An assertion         requires all branches to match the same length of string. An  assertion
4160         such as         such as
4161    
4162           (?<=ab(c|de))           (?<=ab(c|de))
4163    
4164         is not permitted, because its single top-level  branch  can  match  two         is  not  permitted,  because  its single top-level branch can match two
4165         different  lengths,  but  it is acceptable if rewritten to use two top-         different lengths, but it is acceptable if rewritten to  use  two  top-
4166         level branches:         level branches:
4167    
4168           (?<=abc|abde)           (?<=abc|abde)
4169    
4170         In some cases, the Perl 5.10 escape sequence \K (see above) can be used         In some cases, the Perl 5.10 escape sequence \K (see above) can be used
4171         instead  of  a lookbehind assertion; this is not restricted to a fixed-         instead of a lookbehind assertion; this is not restricted to  a  fixed-
4172         length.         length.
4173    
4174         The implementation of lookbehind assertions is, for  each  alternative,         The  implementation  of lookbehind assertions is, for each alternative,
4175         to  temporarily  move the current position back by the fixed length and         to temporarily move the current position back by the fixed  length  and
4176         then try to match. If there are insufficient characters before the cur-         then try to match. If there are insufficient characters before the cur-
4177         rent position, the assertion fails.         rent position, the assertion fails.
4178    
4179         PCRE does not allow the \C escape (which matches a single byte in UTF-8         PCRE does not allow the \C escape (which matches a single byte in UTF-8
4180         mode) to appear in lookbehind assertions, because it makes it  impossi-         mode)  to appear in lookbehind assertions, because it makes it impossi-
4181         ble  to  calculate the length of the lookbehind. The \X and \R escapes,         ble to calculate the length of the lookbehind. The \X and  \R  escapes,
4182         which can match different numbers of bytes, are also not permitted.         which can match different numbers of bytes, are also not permitted.
4183    
4184         Possessive quantifiers can  be  used  in  conjunction  with  lookbehind         Possessive  quantifiers  can  be  used  in  conjunction with lookbehind
4185         assertions  to  specify  efficient  matching  at the end of the subject         assertions to specify efficient matching at  the  end  of  the  subject
4186         string. Consider a simple pattern such as         string. Consider a simple pattern such as
4187    
4188           abcd$           abcd$
4189    
4190         when applied to a long string that does  not  match.  Because  matching         when  applied  to  a  long string that does not match. Because matching
4191         proceeds from left to right, PCRE will look for each "a" in the subject         proceeds from left to right, PCRE will look for each "a" in the subject
4192         and then see if what follows matches the rest of the  pattern.  If  the         and  then  see  if what follows matches the rest of the pattern. If the
4193         pattern is specified as         pattern is specified as
4194    
4195           ^.*abcd$           ^.*abcd$
4196    
4197         the  initial .* matches the entire string at first, but when this fails         the initial .* matches the entire string at first, but when this  fails
4198         (because there is no following "a"), it backtracks to match all but the         (because there is no following "a"), it backtracks to match all but the
4199         last  character,  then all but the last two characters, and so on. Once         last character, then all but the last two characters, and so  on.  Once
4200         again the search for "a" covers the entire string, from right to  left,         again  the search for "a" covers the entire string, from right to left,
4201         so we are no better off. However, if the pattern is written as         so we are no better off. However, if the pattern is written as
4202    
4203           ^.*+(?<=abcd)           ^.*+(?<=abcd)
4204    
4205         there  can  be  no backtracking for the .*+ item; it can match only the         there can be no backtracking for the .*+ item; it can  match  only  the
4206         entire string. The subsequent lookbehind assertion does a  single  test         entire  string.  The subsequent lookbehind assertion does a single test
4207         on  the last four characters. If it fails, the match fails immediately.         on the last four characters. If it fails, the match fails  immediately.
4208         For long strings, this approach makes a significant difference  to  the         For  long  strings, this approach makes a significant difference to the
4209         processing time.         processing time.
4210    
4211     Using multiple assertions     Using multiple assertions
# Line 4164  ASSERTIONS Line 4214  ASSERTIONS
4214    
4215           (?<=\d{3})(?<!999)foo           (?<=\d{3})(?<!999)foo
4216    
4217         matches  "foo" preceded by three digits that are not "999". Notice that         matches "foo" preceded by three digits that are not "999". Notice  that
4218         each of the assertions is applied independently at the  same  point  in         each  of  the  assertions is applied independently at the same point in
4219         the  subject  string.  First  there  is a check that the previous three         the subject string. First there is a  check  that  the  previous  three
4220         characters are all digits, and then there is  a  check  that  the  same         characters  are  all  digits,  and  then there is a check that the same
4221         three characters are not "999".  This pattern does not match "foo" pre-         three characters are not "999".  This pattern does not match "foo" pre-
4222         ceded by six characters, the first of which are  digits  and  the  last         ceded  by  six  characters,  the first of which are digits and the last
4223         three  of  which  are not "999". For example, it doesn't match "123abc-         three of which are not "999". For example, it  doesn't  match  "123abc-
4224         foo". A pattern to do that is         foo". A pattern to do that is
4225    
4226           (?<=\d{3}...)(?<!999)foo           (?<=\d{3}...)(?<!999)foo
4227    
4228         This time the first assertion looks at the  preceding  six  characters,         This  time  the  first assertion looks at the preceding six characters,
4229         checking that the first three are digits, and then the second assertion         checking that the first three are digits, and then the second assertion
4230         checks that the preceding three characters are not "999".         checks that the preceding three characters are not "999".
4231    
# Line 4183  ASSERTIONS Line 4233  ASSERTIONS
4233    
4234           (?<=(?<!foo)bar)baz           (?<=(?<!foo)bar)baz
4235    
4236         matches an occurrence of "baz" that is preceded by "bar" which in  turn         matches  an occurrence of "baz" that is preceded by "bar" which in turn
4237         is not preceded by "foo", while         is not preceded by "foo", while
4238    
4239           (?<=\d{3}(?!999)...)foo           (?<=\d{3}(?!999)...)foo
4240    
4241         is  another pattern that matches "foo" preceded by three digits and any         is another pattern that matches "foo" preceded by three digits and  any
4242         three characters that are not "999".         three characters that are not "999".
4243    
4244    
4245  CONDITIONAL SUBPATTERNS  CONDITIONAL SUBPATTERNS
4246    
4247         It is possible to cause the matching process to obey a subpattern  con-         It  is possible to cause the matching process to obey a subpattern con-
4248         ditionally  or to choose between two alternative subpatterns, depending         ditionally or to choose between two alternative subpatterns,  depending
4249         on the result of an assertion, or whether a previous capturing  subpat-         on  the result of an assertion, or whether a previous capturing subpat-
4250         tern  matched  or not. The two possible forms of conditional subpattern         tern matched or not. The two possible forms of  conditional  subpattern
4251         are         are
4252    
4253           (?(condition)yes-pattern)           (?(condition)yes-pattern)
4254           (?(condition)yes-pattern|no-pattern)           (?(condition)yes-pattern|no-pattern)
4255    
4256         If the condition is satisfied, the yes-pattern is used;  otherwise  the         If  the  condition is satisfied, the yes-pattern is used; otherwise the
4257         no-pattern  (if  present)  is used. If there are more than two alterna-         no-pattern (if present) is used. If there are more  than  two  alterna-
4258         tives in the subpattern, a compile-time error occurs.         tives in the subpattern, a compile-time error occurs.
4259    
4260         There are four kinds of condition: references  to  subpatterns,  refer-         There  are  four  kinds of condition: references to subpatterns, refer-
4261         ences to recursion, a pseudo-condition called DEFINE, and assertions.         ences to recursion, a pseudo-condition called DEFINE, and assertions.
4262    
4263     Checking for a used subpattern by number     Checking for a used subpattern by number
4264    
4265         If  the  text between the parentheses consists of a sequence of digits,         If the text between the parentheses consists of a sequence  of  digits,
4266         the condition is true if the capturing subpattern of  that  number  has         the  condition  is  true if the capturing subpattern of that number has
4267         previously  matched.  An  alternative notation is to precede the digits         previously matched. An alternative notation is to  precede  the  digits
4268         with a plus or minus sign. In this case, the subpattern number is rela-         with a plus or minus sign. In this case, the subpattern number is rela-
4269         tive rather than absolute.  The most recently opened parentheses can be         tive rather than absolute.  The most recently opened parentheses can be
4270         referenced by (?(-1), the next most recent by (?(-2),  and  so  on.  In         referenced  by  (?(-1),  the  next most recent by (?(-2), and so on. In
4271         looping constructs it can also make sense to refer to subsequent groups         looping constructs it can also make sense to refer to subsequent groups
4272         with constructs such as (?(+2).         with constructs such as (?(+2).
4273    
4274         Consider the following pattern, which  contains  non-significant  white         Consider  the  following  pattern, which contains non-significant white
4275         space to make it more readable (assume the PCRE_EXTENDED option) and to         space to make it more readable (assume the PCRE_EXTENDED option) and to
4276         divide it into three parts for ease of discussion:         divide it into three parts for ease of discussion:
4277    
4278           ( \( )?    [^()]+    (?(1) \) )           ( \( )?    [^()]+    (?(1) \) )
4279    
4280         The first part matches an optional opening  parenthesis,  and  if  that         The  first  part  matches  an optional opening parenthesis, and if that
4281         character is present, sets it as the first captured substring. The sec-         character is present, sets it as the first captured substring. The sec-
4282         ond part matches one or more characters that are not  parentheses.  The         ond  part  matches one or more characters that are not parentheses. The
4283         third part is a conditional subpattern that tests whether the first set         third part is a conditional subpattern that tests whether the first set
4284         of parentheses matched or not. If they did, that is, if subject started         of parentheses matched or not. If they did, that is, if subject started
4285         with an opening parenthesis, the condition is true, and so the yes-pat-         with an opening parenthesis, the condition is true, and so the yes-pat-
4286         tern is executed and a  closing  parenthesis  is  required.  Otherwise,         tern  is  executed  and  a  closing parenthesis is required. Otherwise,
4287         since  no-pattern  is  not  present, the subpattern matches nothing. In         since no-pattern is not present, the  subpattern  matches  nothing.  In
4288         other words,  this  pattern  matches  a  sequence  of  non-parentheses,         other  words,  this  pattern  matches  a  sequence  of non-parentheses,
4289         optionally enclosed in parentheses.         optionally enclosed in parentheses.
4290    
4291         If  you  were  embedding  this pattern in a larger one, you could use a         If you were embedding this pattern in a larger one,  you  could  use  a
4292         relative reference:         relative reference:
4293    
4294           ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...           ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...
4295    
4296         This makes the fragment independent of the parentheses  in  the  larger         This  makes  the  fragment independent of the parentheses in the larger
4297         pattern.         pattern.
4298    
4299     Checking for a used subpattern by name     Checking for a used subpattern by name
4300    
4301         Perl  uses  the  syntax  (?(<name>)...) or (?('name')...) to test for a         Perl uses the syntax (?(<name>)...) or (?('name')...)  to  test  for  a
4302         used subpattern by name. For compatibility  with  earlier  versions  of         used  subpattern  by  name.  For compatibility with earlier versions of
4303         PCRE,  which  had this facility before Perl, the syntax (?(name)...) is         PCRE, which had this facility before Perl, the syntax  (?(name)...)  is
4304         also recognized. However, there is a possible ambiguity with this  syn-         also  recognized. However, there is a possible ambiguity with this syn-
4305         tax,  because  subpattern  names  may  consist entirely of digits. PCRE         tax, because subpattern names may  consist  entirely  of  digits.  PCRE
4306         looks first for a named subpattern; if it cannot find one and the  name         looks  first for a named subpattern; if it cannot find one and the name
4307         consists  entirely  of digits, PCRE looks for a subpattern of that num-         consists entirely of digits, PCRE looks for a subpattern of  that  num-
4308         ber, which must be greater than zero. Using subpattern names that  con-         ber,  which must be greater than zero. Using subpattern names that con-
4309         sist entirely of digits is not recommended.         sist entirely of digits is not recommended.
4310    
4311         Rewriting the above example to use a named subpattern gives this:         Rewriting the above example to use a named subpattern gives this:
# Line 4266  CONDITIONAL SUBPATTERNS Line 4316  CONDITIONAL SUBPATTERNS
4316     Checking for pattern recursion     Checking for pattern recursion
4317    
4318         If the condition is the string (R), and there is no subpattern with the         If the condition is the string (R), and there is no subpattern with the
4319         name R, the condition is true if a recursive call to the whole  pattern         name  R, the condition is true if a recursive call to the whole pattern
4320         or any subpattern has been made. If digits or a name preceded by amper-         or any subpattern has been made. If digits or a name preceded by amper-
4321         sand follow the letter R, for example:         sand follow the letter R, for example:
4322    
4323           (?(R3)...) or (?(R&name)...)           (?(R3)...) or (?(R&name)...)
4324    
4325         the condition is true if the most recent recursion is into the  subpat-         the  condition is true if the most recent recursion is into the subpat-
4326         tern  whose  number or name is given. This condition does not check the         tern whose number or name is given. This condition does not  check  the
4327         entire recursion stack.         entire recursion stack.
4328    
4329         At "top level", all these recursion test conditions are  false.  Recur-         At  "top  level", all these recursion test conditions are false. Recur-
4330         sive patterns are described below.         sive patterns are described below.
4331    
4332     Defining subpatterns for use by reference only     Defining subpatterns for use by reference only
4333    
4334         If  the  condition  is  the string (DEFINE), and there is no subpattern         If the condition is the string (DEFINE), and  there  is  no  subpattern
4335         with the name DEFINE, the condition is  always  false.  In  this  case,         with  the  name  DEFINE,  the  condition is always false. In this case,
4336         there  may  be  only  one  alternative  in the subpattern. It is always         there may be only one alternative  in  the  subpattern.  It  is  always
4337         skipped if control reaches this point  in  the  pattern;  the  idea  of         skipped  if  control  reaches  this  point  in the pattern; the idea of
4338         DEFINE  is that it can be used to define "subroutines" that can be ref-         DEFINE is that it can be used to define "subroutines" that can be  ref-
4339         erenced from elsewhere. (The use of "subroutines" is described  below.)         erenced  from elsewhere. (The use of "subroutines" is described below.)
4340         For  example,  a pattern to match an IPv4 address could be written like         For example, a pattern to match an IPv4 address could be  written  like
4341         this (ignore whitespace and line breaks):         this (ignore whitespace and line breaks):
4342    
4343           (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )           (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
4344           \b (?&byte) (\.(?&byte)){3} \b           \b (?&byte) (\.(?&byte)){3} \b
4345    
4346         The first part of the pattern is a DEFINE group inside which a  another         The  first part of the pattern is a DEFINE group inside which a another
4347         group  named "byte" is defined. This matches an individual component of         group named "byte" is defined. This matches an individual component  of
4348         an IPv4 address (a number less than 256). When  matching  takes  place,         an  IPv4  address  (a number less than 256). When matching takes place,
4349         this  part  of  the pattern is skipped because DEFINE acts like a false         this part of the pattern is skipped because DEFINE acts  like  a  false
4350         condition.         condition.
4351    
4352         The rest of the pattern uses references to the named group to match the         The rest of the pattern uses references to the named group to match the
4353         four  dot-separated  components of an IPv4 address, insisting on a word         four dot-separated components of an IPv4 address, insisting on  a  word
4354         boundary at each end.         boundary at each end.
4355    
4356     Assertion conditions     Assertion conditions
4357    
4358         If the condition is not in any of the above  formats,  it  must  be  an         If  the  condition  is  not  in any of the above formats, it must be an
4359         assertion.   This may be a positive or negative lookahead or lookbehind         assertion.  This may be a positive or negative lookahead or  lookbehind
4360         assertion. Consider  this  pattern,  again  containing  non-significant         assertion.  Consider  this  pattern,  again  containing non-significant
4361         white space, and with the two alternatives on the second line:         white space, and with the two alternatives on the second line:
4362    
4363           (?(?=[^a-z]*[a-z])           (?(?=[^a-z]*[a-z])
4364           \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )           \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )
4365    
4366         The  condition  is  a  positive  lookahead  assertion  that  matches an         The condition  is  a  positive  lookahead  assertion  that  matches  an
4367         optional sequence of non-letters followed by a letter. In other  words,         optional  sequence of non-letters followed by a letter. In other words,
4368         it  tests  for the presence of at least one letter in the subject. If a         it tests for the presence of at least one letter in the subject.  If  a
4369         letter is found, the subject is matched against the first  alternative;         letter  is found, the subject is matched against the first alternative;
4370         otherwise  it  is  matched  against  the  second.  This pattern matches         otherwise it is  matched  against  the  second.  This  pattern  matches
4371         strings in one of the two forms dd-aaa-dd or dd-dd-dd,  where  aaa  are         strings  in  one  of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
4372         letters and dd are digits.         letters and dd are digits.
4373    
4374    
4375  COMMENTS  COMMENTS
4376    
4377         The  sequence (?# marks the start of a comment that continues up to the         The sequence (?# marks the start of a comment that continues up to  the
4378         next closing parenthesis. Nested parentheses  are  not  permitted.  The         next  closing  parenthesis.  Nested  parentheses are not permitted. The
4379         characters  that make up a comment play no part in the pattern matching         characters that make up a comment play no part in the pattern  matching
4380         at all.         at all.
4381    
4382         If the PCRE_EXTENDED option is set, an unescaped # character outside  a         If  the PCRE_EXTENDED option is set, an unescaped # character outside a
4383         character  class  introduces  a  comment  that continues to immediately         character class introduces a  comment  that  continues  to  immediately
4384         after the next newline in the pattern.         after the next newline in the pattern.
4385    
4386    
4387  RECURSIVE PATTERNS  RECURSIVE PATTERNS
4388    
4389         Consider the problem of matching a string in parentheses, allowing  for         Consider  the problem of matching a string in parentheses, allowing for
4390         unlimited  nested  parentheses.  Without the use of recursion, the best         unlimited nested parentheses. Without the use of  recursion,  the  best
4391         that can be done is to use a pattern that  matches  up  to  some  fixed         that  can  be  done  is  to use a pattern that matches up to some fixed
4392         depth  of  nesting.  It  is not possible to handle an arbitrary nesting         depth of nesting. It is not possible to  handle  an  arbitrary  nesting
4393         depth.         depth.
4394    
4395         For some time, Perl has provided a facility that allows regular expres-         For some time, Perl has provided a facility that allows regular expres-
4396         sions  to recurse (amongst other things). It does this by interpolating         sions to recurse (amongst other things). It does this by  interpolating
4397         Perl code in the expression at run time, and the code can refer to  the         Perl  code in the expression at run time, and the code can refer to the
4398         expression itself. A Perl pattern using code interpolation to solve the         expression itself. A Perl pattern using code interpolation to solve the
4399         parentheses problem can be created like this:         parentheses problem can be created like this:
4400    
# Line 4354  RECURSIVE PATTERNS Line 4404  RECURSIVE PATTERNS
4404         refers recursively to the pattern in which it appears.         refers recursively to the pattern in which it appears.
4405    
4406         Obviously, PCRE cannot support the interpolation of Perl code. Instead,         Obviously, PCRE cannot support the interpolation of Perl code. Instead,
4407         it supports special syntax for recursion of  the  entire  pattern,  and         it  supports  special  syntax  for recursion of the entire pattern, and
4408         also  for  individual  subpattern  recursion. After its introduction in         also for individual subpattern recursion.  After  its  introduction  in
4409         PCRE and Python, this kind of recursion was  introduced  into  Perl  at         PCRE  and  Python,  this  kind of recursion was introduced into Perl at
4410         release 5.10.         release 5.10.
4411    
4412         A  special  item  that consists of (? followed by a number greater than         A special item that consists of (? followed by a  number  greater  than
4413         zero and a closing parenthesis is a recursive call of the subpattern of         zero and a closing parenthesis is a recursive call of the subpattern of
4414         the  given  number, provided that it occurs inside that subpattern. (If         the given number, provided that it occurs inside that  subpattern.  (If
4415         not, it is a "subroutine" call, which is described  in  the  next  sec-         not,  it  is  a  "subroutine" call, which is described in the next sec-
4416         tion.)  The special item (?R) or (?0) is a recursive call of the entire         tion.) The special item (?R) or (?0) is a recursive call of the  entire
4417         regular expression.         regular expression.
4418    
4419         In PCRE (like Python, but unlike Perl), a recursive subpattern call  is         In  PCRE (like Python, but unlike Perl), a recursive subpattern call is
4420         always treated as an atomic group. That is, once it has matched some of         always treated as an atomic group. That is, once it has matched some of
4421         the subject string, it is never re-entered, even if it contains untried         the subject string, it is never re-entered, even if it contains untried
4422         alternatives and there is a subsequent matching failure.         alternatives and there is a subsequent matching failure.
4423    
4424         This  PCRE  pattern  solves  the nested parentheses problem (assume the         This PCRE pattern solves the nested  parentheses  problem  (assume  the
4425         PCRE_EXTENDED option is set so that white space is ignored):         PCRE_EXTENDED option is set so that white space is ignored):
4426    
4427           \( ( (?>[^()]+) | (?R) )* \)           \( ( (?>[^()]+) | (?R) )* \)
4428    
4429         First it matches an opening parenthesis. Then it matches any number  of         First  it matches an opening parenthesis. Then it matches any number of
4430         substrings  which  can  either  be  a sequence of non-parentheses, or a         substrings which can either be a  sequence  of  non-parentheses,  or  a
4431         recursive match of the pattern itself (that is, a  correctly  parenthe-         recursive  match  of the pattern itself (that is, a correctly parenthe-
4432         sized substring).  Finally there is a closing parenthesis.         sized substring).  Finally there is a closing parenthesis.
4433    
4434         If  this  were  part of a larger pattern, you would not want to recurse         If this were part of a larger pattern, you would not  want  to  recurse
4435         the entire pattern, so instead you could use this:         the entire pattern, so instead you could use this:
4436    
4437           ( \( ( (?>[^()]+) | (?1) )* \) )           ( \( ( (?>[^()]+) | (?1) )* \) )
4438    
4439         We have put the pattern into parentheses, and caused the  recursion  to         We  have  put the pattern into parentheses, and caused the recursion to
4440         refer to them instead of the whole pattern.         refer to them instead of the whole pattern.
4441    
4442         In  a  larger  pattern,  keeping  track  of  parenthesis numbers can be         In a larger pattern,  keeping  track  of  parenthesis  numbers  can  be
4443         tricky. This is made easier by the use of relative references. (A  Perl         tricky.  This is made easier by the use of relative references. (A Perl
4444         5.10  feature.)   Instead  of  (?1)  in the pattern above you can write         5.10 feature.)  Instead of (?1) in the  pattern  above  you  can  write
4445         (?-2) to refer to the second most recently opened parentheses preceding         (?-2) to refer to the second most recently opened parentheses preceding
4446         the  recursion.  In  other  words,  a  negative number counts capturing         the recursion. In other  words,  a  negative  number  counts  capturing
4447         parentheses leftwards from the point at which it is encountered.         parentheses leftwards from the point at which it is encountered.
4448    
4449         It is also possible to refer to  subsequently  opened  parentheses,  by         It  is  also  possible  to refer to subsequently opened parentheses, by
4450         writing  references  such  as (?+2). However, these cannot be recursive         writing references such as (?+2). However, these  cannot  be  recursive
4451         because the reference is not inside the  parentheses  that  are  refer-         because  the  reference  is  not inside the parentheses that are refer-
4452         enced.  They  are  always  "subroutine" calls, as described in the next         enced. They are always "subroutine" calls, as  described  in  the  next
4453         section.         section.
4454    
4455         An alternative approach is to use named parentheses instead.  The  Perl         An  alternative  approach is to use named parentheses instead. The Perl
4456         syntax  for  this  is (?&name); PCRE's earlier syntax (?P>name) is also         syntax for this is (?&name); PCRE's earlier syntax  (?P>name)  is  also
4457         supported. We could rewrite the above example as follows:         supported. We could rewrite the above example as follows:
4458    
4459           (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )           (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
4460    
4461         If there is more than one subpattern with the same name,  the  earliest         If  there  is more than one subpattern with the same name, the earliest
4462         one is used.         one is used.
4463    
4464         This  particular  example pattern that we have been looking at contains         This particular example pattern that we have been looking  at  contains
4465         nested unlimited repeats, and so the use of atomic grouping for  match-         nested  unlimited repeats, and so the use of atomic grouping for match-
4466         ing  strings  of non-parentheses is important when applying the pattern         ing strings of non-parentheses is important when applying  the  pattern
4467         to strings that do not match. For example, when this pattern is applied         to strings that do not match. For example, when this pattern is applied
4468         to         to
4469    
4470           (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()           (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
4471    
4472         it  yields "no match" quickly. However, if atomic grouping is not used,         it yields "no match" quickly. However, if atomic grouping is not  used,
4473         the match runs for a very long time indeed because there  are  so  many         the  match  runs  for a very long time indeed because there are so many
4474         different  ways  the  + and * repeats can carve up the subject, and all         different ways the + and * repeats can carve up the  subject,  and  all
4475         have to be tested before failure can be reported.         have to be tested before failure can be reported.
4476    
4477         At the end of a match, the values set for any capturing subpatterns are         At the end of a match, the values set for any capturing subpatterns are
4478         those from the outermost level of the recursion at which the subpattern         those from the outermost level of the recursion at which the subpattern
4479         value is set.  If you want to obtain  intermediate  values,  a  callout         value  is  set.   If  you want to obtain intermediate values, a callout
4480         function  can be used (see below and the pcrecallout documentation). If         function can be used (see below and the pcrecallout documentation).  If
4481         the pattern above is matched against         the pattern above is matched against
4482    
4483           (ab(cd)ef)           (ab(cd)ef)
4484    
4485         the value for the capturing parentheses is  "ef",  which  is  the  last         the  value  for  the  capturing  parentheses is "ef", which is the last
4486         value  taken  on at the top level. If additional parentheses are added,         value taken on at the top level. If additional parentheses  are  added,
4487         giving         giving
4488    
4489           \( ( ( (?>[^()]+) | (?R) )* ) \)           \( ( ( (?>[^()]+) | (?R) )* ) \)
4490              ^                        ^              ^                        ^
4491              ^                        ^              ^                        ^
4492    
4493         the string they capture is "ab(cd)ef", the contents of  the  top  level         the  string  they  capture is "ab(cd)ef", the contents of the top level
4494         parentheses.  If there are more than 15 capturing parentheses in a pat-         parentheses. If there are more than 15 capturing parentheses in a  pat-
4495         tern, PCRE has to obtain extra memory to store data during a recursion,         tern, PCRE has to obtain extra memory to store data during a recursion,
4496         which  it  does  by  using pcre_malloc, freeing it via pcre_free after-         which it does by using pcre_malloc, freeing  it  via  pcre_free  after-
4497         wards. If  no  memory  can  be  obtained,  the  match  fails  with  the         wards.  If  no  memory  can  be  obtained,  the  match  fails  with the
4498         PCRE_ERROR_NOMEMORY error.         PCRE_ERROR_NOMEMORY error.
4499    
4500         Do  not  confuse  the (?R) item with the condition (R), which tests for         Do not confuse the (?R) item with the condition (R),  which  tests  for
4501         recursion.  Consider this pattern, which matches text in  angle  brack-         recursion.   Consider  this pattern, which matches text in angle brack-
4502         ets,  allowing for arbitrary nesting. Only digits are allowed in nested         ets, allowing for arbitrary nesting. Only digits are allowed in  nested
4503         brackets (that is, when recursing), whereas any characters are  permit-         brackets  (that is, when recursing), whereas any characters are permit-
4504         ted at the outer level.         ted at the outer level.
4505    
4506           < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >           < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >
4507    
4508         In  this  pattern, (?(R) is the start of a conditional subpattern, with         In this pattern, (?(R) is the start of a conditional  subpattern,  with
4509         two different alternatives for the recursive and  non-recursive  cases.         two  different  alternatives for the recursive and non-recursive cases.
4510         The (?R) item is the actual recursive call.         The (?R) item is the actual recursive call.
4511    
4512    
4513  SUBPATTERNS AS SUBROUTINES  SUBPATTERNS AS SUBROUTINES
4514    
4515         If the syntax for a recursive subpattern reference (either by number or         If the syntax for a recursive subpattern reference (either by number or
4516         by name) is used outside the parentheses to which it refers,  it  oper-         by  name)  is used outside the parentheses to which it refers, it oper-
4517         ates  like a subroutine in a programming language. The "called" subpat-         ates like a subroutine in a programming language. The "called"  subpat-
4518         tern may be defined before or after the reference. A numbered reference         tern may be defined before or after the reference. A numbered reference
4519         can be absolute or relative, as in these examples:         can be absolute or relative, as in these examples:
4520    
# Line 4476  SUBPATTERNS AS SUBROUTINES Line 4526  SUBPATTERNS AS SUBROUTINES
4526    
4527           (sens|respons)e and \1ibility           (sens|respons)e and \1ibility
4528    
4529         matches  "sense and sensibility" and "response and responsibility", but         matches "sense and sensibility" and "response and responsibility",  but
4530         not "sense and responsibility". If instead the pattern         not "sense and responsibility". If instead the pattern
4531    
4532           (sens|respons)e and (?1)ibility           (sens|respons)e and (?1)ibility
4533    
4534         is used, it does match "sense and responsibility" as well as the  other         is  used, it does match "sense and responsibility" as well as the other
4535         two  strings.  Another  example  is  given  in the discussion of DEFINE         two strings. Another example is  given  in  the  discussion  of  DEFINE
4536         above.         above.
4537    
4538         Like recursive subpatterns, a "subroutine" call is always treated as an         Like recursive subpatterns, a "subroutine" call is always treated as an
4539         atomic  group. That is, once it has matched some of the subject string,         atomic group. That is, once it has matched some of the subject  string,
4540         it is never re-entered, even if it contains  untried  alternatives  and         it  is  never  re-entered, even if it contains untried alternatives and
4541         there is a subsequent matching failure.         there is a subsequent matching failure.
4542    
4543         When  a  subpattern is used as a subroutine, processing options such as         When a subpattern is used as a subroutine, processing options  such  as
4544         case-independence are fixed when the subpattern is defined. They cannot         case-independence are fixed when the subpattern is defined. They cannot
4545         be changed for different calls. For example, consider this pattern:         be changed for different calls. For example, consider this pattern:
4546    
4547           (abc)(?i:(?-1))           (abc)(?i:(?-1))
4548    
4549         It  matches  "abcabc". It does not match "abcABC" because the change of         It matches "abcabc". It does not match "abcABC" because the  change  of
4550         processing option does not affect the called subpattern.         processing option does not affect the called subpattern.
4551    
4552    
4553  CALLOUTS  CALLOUTS
4554    
4555         Perl has a feature whereby using the sequence (?{...}) causes arbitrary         Perl has a feature whereby using the sequence (?{...}) causes arbitrary
4556         Perl  code to be obeyed in the middle of matching a regular expression.         Perl code to be obeyed in the middle of matching a regular  expression.
4557         This makes it possible, amongst other things, to extract different sub-         This makes it possible, amongst other things, to extract different sub-
4558         strings that match the same pair of parentheses when there is a repeti-         strings that match the same pair of parentheses when there is a repeti-
4559         tion.         tion.
4560    
4561         PCRE provides a similar feature, but of course it cannot obey arbitrary         PCRE provides a similar feature, but of course it cannot obey arbitrary
4562         Perl code. The feature is called "callout". The caller of PCRE provides         Perl code. The feature is called "callout". The caller of PCRE provides
4563         an external function by putting its entry point in the global  variable         an  external function by putting its entry point in the global variable
4564         pcre_callout.   By default, this variable contains NULL, which disables         pcre_callout.  By default, this variable contains NULL, which  disables
4565         all calling out.         all calling out.
4566    
4567         Within a regular expression, (?C) indicates the  points  at  which  the         Within  a  regular  expression,  (?C) indicates the points at which the
4568         external  function  is  to be called. If you want to identify different         external function is to be called. If you want  to  identify  different
4569         callout points, you can put a number less than 256 after the letter  C.         callout  points, you can put a number less than 256 after the letter C.
4570         The  default  value is zero.  For example, this pattern has two callout         The default value is zero.  For example, this pattern has  two  callout
4571         points:         points:
4572    
4573           (?C1)abc(?C2)def           (?C1)abc(?C2)def
4574    
4575         If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are         If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
4576         automatically  installed  before each item in the pattern. They are all         automatically installed before each item in the pattern. They  are  all
4577         numbered 255.         numbered 255.
4578    
4579         During matching, when PCRE reaches a callout point (and pcre_callout is         During matching, when PCRE reaches a callout point (and pcre_callout is
4580         set),  the  external function is called. It is provided with the number         set), the external function is called. It is provided with  the  number
4581         of the callout, the position in the pattern, and, optionally, one  item         of  the callout, the position in the pattern, and, optionally, one item
4582         of  data  originally supplied by the caller of pcre_exec(). The callout         of data originally supplied by the caller of pcre_exec().  The  callout
4583         function may cause matching to proceed, to backtrack, or to fail  alto-         function  may cause matching to proceed, to backtrack, or to fail alto-
4584         gether. A complete description of the interface to the callout function         gether. A complete description of the interface to the callout function
4585         is given in the pcrecallout documentation.         is given in the pcrecallout documentation.
4586    
4587    
4588    BACTRACKING CONTROL
4589    
4590           Perl  5.10 introduced a number of "Special Backtracking Control Verbs",
4591           which are described in the Perl documentation as "experimental and sub-
4592           ject  to  change or removal in a future version of Perl". It goes on to
4593           say: "Their usage in production code should be noted to avoid  problems
4594           during upgrades." The same remarks apply to the PCRE features described
4595           in this section.
4596    
4597           Since these verbs are specifically related to backtracking, they can be
4598           used  only  when  the pattern is to be matched using pcre_exec(), which
4599           uses a backtracking algorithm. They cause an error  if  encountered  by
4600           pcre_dfa_exec().
4601    
4602           The  new verbs make use of what was previously invalid syntax: an open-
4603           ing parenthesis followed by an asterisk. In Perl, they are generally of
4604           the form (*VERB:ARG) but PCRE does not support the use of arguments, so
4605           its general form is just (*VERB). Any number of these verbs  may  occur
4606           in a pattern. There are two kinds:
4607    
4608       Verbs that act immediately
4609    
4610           The following verbs act as soon as they are encountered:
4611    
4612              (*ACCEPT)
4613    
4614           This  verb causes the match to end successfully, skipping the remainder
4615           of the pattern. When inside a recursion, only the innermost pattern  is
4616           ended  immediately.  PCRE  differs  from  Perl  in  what happens if the
4617           (*ACCEPT) is inside capturing parentheses. In Perl, the data so far  is
4618           captured: in PCRE no data is captured. For example:
4619    
4620             A(A|B(*ACCEPT)|C)D
4621    
4622           This  matches  "AB", "AAD", or "ACD", but when it matches "AB", no data
4623           is captured.
4624    
4625             (*FAIL) or (*F)
4626    
4627           This verb causes the match to fail, forcing backtracking to  occur.  It
4628           is  equivalent to (?!) but easier to read. The Perl documentation notes
4629           that it is probably useful only when combined  with  (?{})  or  (??{}).
4630           Those  are,  of course, Perl features that are not present in PCRE. The
4631           nearest equivalent is the callout feature, as for example in this  pat-
4632           tern:
4633    
4634             a+(?C)(*FAIL)
4635    
4636           A  match  with the string "aaaa" always fails, but the callout is taken
4637           before each backtrack happens (in this example, 10 times).
4638    
4639       Verbs that act after backtracking
4640    
4641           The following verbs do nothing when they are encountered. Matching con-
4642           tinues  with what follows, but if there is no subsequent match, a fail-
4643           ure is forced.  The verbs  differ  in  exactly  what  kind  of  failure
4644           occurs.
4645    
4646             (*COMMIT)
4647    
4648           This  verb  causes  the whole match to fail outright if the rest of the
4649           pattern does not match. Even if the pattern is unanchored,  no  further
4650           attempts  to find a match by advancing the start point take place. Once
4651           (*COMMIT) has been passed, pcre_exec() is committed to finding a  match
4652           at the current starting point, or not at all. For example:
4653    
4654             a+(*COMMIT)b
4655    
4656           This  matches  "xxaab" but not "aacaab". It can be thought of as a kind
4657           of dynamic anchor, or "I've started, so I must finish."
4658    
4659             (*PRUNE)
4660    
4661           This verb causes the match to fail at the current position if the  rest
4662           of the pattern does not match. If the pattern is unanchored, the normal
4663           "bumpalong" advance to the next starting character then happens.  Back-
4664           tracking  can  occur as usual to the left of (*PRUNE), or when matching
4665           to the right of (*PRUNE), but if there is no match to the right,  back-
4666           tracking  cannot  cross (*PRUNE).  In simple cases, the use of (*PRUNE)
4667           is just an alternative to an atomic group or possessive quantifier, but
4668           there  are  some uses of (*PRUNE) that cannot be expressed in any other
4669           way.
4670    
4671             (*SKIP)
4672    
4673           This verb is like (*PRUNE), except that if the pattern  is  unanchored,
4674           the  "bumpalong" advance is not to the next character, but to the posi-
4675           tion in the subject where (*SKIP) was  encountered.  (*SKIP)  signifies
4676           that  whatever  text  was  matched leading up to it cannot be part of a
4677           successful match. Consider:
4678    
4679             a+(*SKIP)b
4680    
4681           If the subject is "aaaac...",  after  the  first  match  attempt  fails
4682           (starting  at  the  first  character in the string), the starting point
4683           skips on to start the next attempt at "c". Note that a possessive quan-
4684           tifer  does not have the same effect in this example; although it would
4685           suppress backtracking  during  the  first  match  attempt,  the  second
4686           attempt  would  start at the second character instead of skipping on to
4687           "c".
4688    
4689             (*THEN)
4690    
4691           This verb causes a skip to the next alternation if the rest of the pat-
4692           tern does not match. That is, it cancels pending backtracking, but only
4693           within the current alternation. Its name  comes  from  the  observation
4694           that it can be used for a pattern-based if-then-else block:
4695    
4696             ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
4697    
4698           If  the COND1 pattern matches, FOO is tried (and possibly further items
4699           after the end of the group if FOO succeeds);  on  failure  the  matcher
4700           skips  to  the second alternative and tries COND2, without backtracking
4701           into COND1. If (*THEN) is used outside  of  any  alternation,  it  acts
4702           exactly like (*PRUNE).
4703    
4704    
4705  SEE ALSO  SEE ALSO
4706    
4707         pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).         pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
# Line 4549  AUTHOR Line 4716  AUTHOR
4716    
4717  REVISION  REVISION
4718    
4719         Last updated: 06 August 2007         Last updated: 09 August 2007
4720         Copyright (c) 1997-2007 University of Cambridge.         Copyright (c) 1997-2007 University of Cambridge.
4721  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
4722    
# Line 4828  CONDITIONAL PATTERNS Line 4995  CONDITIONAL PATTERNS
4995           (?(assert)...  assertion condition           (?(assert)...  assertion condition
4996    
4997    
4998    BACKTRACKING CONTROL
4999    
5000           The following act immediately they are reached:
5001    
5002             (*ACCEPT)      force successful match
5003             (*FAIL)        force backtrack; synonym (*F)
5004    
5005           The following act only when a subsequent match failure causes  a  back-
5006           track to reach them. They all force a match failure, but they differ in
5007           what happens afterwards. Those that advance the start-of-match point do
5008           so only if the pattern is not anchored.
5009    
5010             (*COMMIT)      overall failure, no advance of starting point
5011             (*PRUNE)       advance to next starting character
5012             (*SKIP)        advance start to current matching position
5013             (*THEN)        local failure, backtrack to next alternation
5014    
5015    
5016  CALLOUTS  CALLOUTS
5017    
5018           (?C)      callout           (?C)      callout
# Line 4848  AUTHOR Line 5033  AUTHOR
5033    
5034  REVISION  REVISION
5035    
5036         Last updated: 06 August 2007         Last updated: 08 August 2007
5037         Copyright (c) 1997-2007 University of Cambridge.         Copyright (c) 1997-2007 University of Cambridge.
5038  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
5039    

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