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revision 235 by ph10, Tue Sep 11 11:39:14 2007 UTC revision 513 by ph10, Mon May 3 11:13:37 2010 UTC
# Line 9  are described in detail below. There is Line 9  are described in detail below. There is
9  .\" HREF  .\" HREF
10  \fBpcresyntax\fP  \fBpcresyntax\fP
11  .\"  .\"
12  page. Perl's regular expressions are described in its own documentation, and  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18  regular expressions in general are covered in a number of books, some of which  regular expressions in general are covered in a number of books, some of which
19  have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",  have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20  published by O'Reilly, covers regular expressions in great detail. This  published by O'Reilly, covers regular expressions in great detail. This
21  description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 character strings. To use this,
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  PCRE must be built to include UTF-8 support, and you must call
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  places below. There is also a summary of UTF-8 features in the  is also a special sequence that can be given at the start of a pattern:
28    .sp
29      (*UTF8)
30    .sp
31    Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32    option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33    pattern matching is mentioned in several places below. There is also a summary
34    of UTF-8 features in the
35  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
36  .\" </a>  .\" </a>
37  section on UTF-8 support  section on UTF-8 support
# Line 71  string with one of the following five se Line 83  string with one of the following five se
83    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
84    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
85  .sp  .sp
86  These override the default and the options given to \fBpcre_compile()\fP. For  These override the default and the options given to \fBpcre_compile()\fP or
87  example, on a Unix system where LF is the default newline sequence, the pattern  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
88    newline sequence, the pattern
89  .sp  .sp
90    (*CR)a.b    (*CR)a.b
91  .sp  .sp
# Line 89  this can be changed; see the description Line 102  this can be changed; see the description
102  .\" </a>  .\" </a>
103  "Newline sequences"  "Newline sequences"
104  .\"  .\"
105  below.  below. A change of \eR setting can be combined with a change of newline
106    convention.
107  .  .
108  .  .
109  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
# Line 193  The \eQ...\eE sequence is recognized bot Line 207  The \eQ...\eE sequence is recognized bot
207  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
208  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
209  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
210  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is often easier to use
211  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
212  .sp  .sp
213    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
214    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
# Line 204  represents: Line 217  represents:
217    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
218    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
219    \et        tab (hex 09)    \et        tab (hex 09)
220    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
221    \exhh      character with hex code hh    \exhh      character with hex code hh
222    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
223  .sp  .sp
# Line 282  zero, because no more than three octal d Line 295  zero, because no more than three octal d
295  .P  .P
296  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
297  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
298  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
299  sequences \eR and \eX are interpreted as the characters "R" and "X",  \eB, \eR, and \eX are not special inside a character class. Like any other
300  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
301  meanings  "R", and "X" by default, but cause an error if the PCRE_EXTRA option is set.
302    Outside a character class, these sequences have different meanings
303  .\" HTML <a href="#uniextseq">  .\" HTML <a href="#uniextseq">
304  .\" </a>  .\" </a>
305  (see below).  (see below).
# Line 309  parenthesized subpatterns. Line 323  parenthesized subpatterns.
323  .\"  .\"
324  .  .
325  .  .
326    .SS "Absolute and relative subroutine calls"
327    .rs
328    .sp
329    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
330    a number enclosed either in angle brackets or single quotes, is an alternative
331    syntax for referencing a subpattern as a "subroutine". Details are discussed
332    .\" HTML <a href="#onigurumasubroutines">
333    .\" </a>
334    later.
335    .\"
336    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
337    synonymous. The former is a back reference; the latter is a
338    .\" HTML <a href="#subpatternsassubroutines">
339    .\" </a>
340    subroutine
341    .\"
342    call.
343    .
344    .
345  .SS "Generic character types"  .SS "Generic character types"
346  .rs  .rs
347  .sp  .sp
# Line 344  In UTF-8 mode, characters with values gr Line 377  In UTF-8 mode, characters with values gr
377  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
378  character property support is available. These sequences retain their original  character property support is available. These sequences retain their original
379  meanings from before UTF-8 support was available, mainly for efficiency  meanings from before UTF-8 support was available, mainly for efficiency
380  reasons.  reasons. Note that this also affects \eb, because it is defined in terms of \ew
381    and \eW.
382  .P  .P
383  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
384  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  other sequences, these do match certain high-valued codepoints in UTF-8 mode.
# Line 426  recognized. Line 460  recognized.
460  .P  .P
461  It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the  It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
462  complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF  complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
463  either at compile time or when the pattern is matched. This can be made the  either at compile time or when the pattern is matched. (BSR is an abbrevation
464  default when PCRE is built; if this is the case, the other behaviour can be  for "backslash R".) This can be made the default when PCRE is built; if this is
465  requested via the PCRE_BSR_UNICODE option. It is also possible to specify these  the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
466  settings by starting a pattern string with one of the following sequences:  It is also possible to specify these settings by starting a pattern string with
467    one of the following sequences:
468  .sp  .sp
469    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
470    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
471  .sp  .sp
472  These override the default and the options given to \fBpcre_compile()\fP, but  These override the default and the options given to \fBpcre_compile()\fP or
473  they can be overridden by options given to \fBpcre_exec()\fP. Note that these  \fBpcre_compile2()\fP, but they can be overridden by options given to
474  special settings, which are not Perl-compatible, are recognized only at the  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
475  very start of a pattern, and that they must be in upper case. If more than one  which are not Perl-compatible, are recognized only at the very start of a
476  of them is present, the last one is used.  pattern, and that they must be in upper case. If more than one of them is
477  .P  present, the last one is used. They can be combined with a change of newline
478  Inside a character class, \eR matches the letter "R".  convention, for example, a pattern can start with:
479    .sp
480      (*ANY)(*BSR_ANYCRLF)
481    .sp
482    Inside a character class, \eR is treated as an unrecognized escape sequence,
483    and so matches the letter "R" by default, but causes an error if PCRE_EXTRA is
484    set.
485  .  .
486  .  .
487  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 475  Those that are not part of an identified Line 516  Those that are not part of an identified
516  .P  .P
517  Arabic,  Arabic,
518  Armenian,  Armenian,
519    Avestan,
520  Balinese,  Balinese,
521    Bamum,
522  Bengali,  Bengali,
523  Bopomofo,  Bopomofo,
524  Braille,  Braille,
525  Buginese,  Buginese,
526  Buhid,  Buhid,
527  Canadian_Aboriginal,  Canadian_Aboriginal,
528    Carian,
529    Cham,
530  Cherokee,  Cherokee,
531  Common,  Common,
532  Coptic,  Coptic,
# Line 490  Cypriot, Line 535  Cypriot,
535  Cyrillic,  Cyrillic,
536  Deseret,  Deseret,
537  Devanagari,  Devanagari,
538    Egyptian_Hieroglyphs,
539  Ethiopic,  Ethiopic,
540  Georgian,  Georgian,
541  Glagolitic,  Glagolitic,
# Line 502  Hangul, Line 548  Hangul,
548  Hanunoo,  Hanunoo,
549  Hebrew,  Hebrew,
550  Hiragana,  Hiragana,
551    Imperial_Aramaic,
552  Inherited,  Inherited,
553    Inscriptional_Pahlavi,
554    Inscriptional_Parthian,
555    Javanese,
556    Kaithi,
557  Kannada,  Kannada,
558  Katakana,  Katakana,
559    Kayah_Li,
560  Kharoshthi,  Kharoshthi,
561  Khmer,  Khmer,
562  Lao,  Lao,
563  Latin,  Latin,
564    Lepcha,
565  Limbu,  Limbu,
566  Linear_B,  Linear_B,
567    Lisu,
568    Lycian,
569    Lydian,
570  Malayalam,  Malayalam,
571    Meetei_Mayek,
572  Mongolian,  Mongolian,
573  Myanmar,  Myanmar,
574  New_Tai_Lue,  New_Tai_Lue,
# Line 519  Nko, Line 576  Nko,
576  Ogham,  Ogham,
577  Old_Italic,  Old_Italic,
578  Old_Persian,  Old_Persian,
579    Old_South_Arabian,
580    Old_Turkic,
581    Ol_Chiki,
582  Oriya,  Oriya,
583  Osmanya,  Osmanya,
584  Phags_Pa,  Phags_Pa,
585  Phoenician,  Phoenician,
586    Rejang,
587  Runic,  Runic,
588    Samaritan,
589    Saurashtra,
590  Shavian,  Shavian,
591  Sinhala,  Sinhala,
592    Sundanese,
593  Syloti_Nagri,  Syloti_Nagri,
594  Syriac,  Syriac,
595  Tagalog,  Tagalog,
596  Tagbanwa,  Tagbanwa,
597  Tai_Le,  Tai_Le,
598    Tai_Tham,
599    Tai_Viet,
600  Tamil,  Tamil,
601  Telugu,  Telugu,
602  Thaana,  Thaana,
# Line 538  Thai, Line 604  Thai,
604  Tibetan,  Tibetan,
605  Tifinagh,  Tifinagh,
606  Ugaritic,  Ugaritic,
607    Vai,
608  Yi.  Yi.
609  .P  .P
610  Each character has exactly one general category property, specified by a  Each character has exactly one general category property, specified by a
# Line 610  cannot be tested by PCRE, unless UTF-8 v Line 677  cannot be tested by PCRE, unless UTF-8 v
677  .\" HREF  .\" HREF
678  \fBpcreapi\fP  \fBpcreapi\fP
679  .\"  .\"
680  page).  page). Perl does not support the Cs property.
681  .P  .P
682  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The long synonyms for property names that Perl supports (such as \ep{Letter})
683  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
684  properties with "Is".  properties with "Is".
685  .P  .P
# Line 673  For example, when the pattern Line 740  For example, when the pattern
740    (foo)\eKbar    (foo)\eKbar
741  .sp  .sp
742  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
743    .P
744    Perl documents that the use of \eK within assertions is "not well defined". In
745    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
746    ignored in negative assertions.
747  .  .
748  .  .
749  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 697  The backslashed assertions are: Line 768  The backslashed assertions are:
768    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
769    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
770  .sp  .sp
771  These assertions may not appear in character classes (but note that \eb has a  Inside a character class, \eb has a different meaning; it matches the backspace
772  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
773    default it matches the corresponding literal character (for example, \eB
774    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
775    escape sequence" error is generated instead.
776  .P  .P
777  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
778  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
779  \ew and the other matches \eW), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
780  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. Neither PCRE nor Perl has a
781    separte "start of word" or "end of word" metasequence. However, whatever
782    follows \eb normally determines which it is. For example, the fragment
783    \eba matches "a" at the start of a word.
784  .P  .P
785  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
786  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 835  the lookbehind. Line 912  the lookbehind.
912  .rs  .rs
913  .sp  .sp
914  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
915  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special by default.
916  closing square bracket is required as a member of the class, it should be the  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
917  first data character in the class (after an initial circumflex, if present) or  bracket causes a compile-time error. If a closing square bracket is required as
918  escaped with a backslash.  a member of the class, it should be the first data character in the class
919    (after an initial circumflex, if present) or escaped with a backslash.
920  .P  .P
921  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In UTF-8 mode, the
922  character may occupy more than one byte. A matched character must be in the set  character may be more than one byte long. A matched character must be in the
923  of characters defined by the class, unless the first character in the class  set of characters defined by the class, unless the first character in the class
924  definition is a circumflex, in which case the subject character must not be in  definition is a circumflex, in which case the subject character must not be in
925  the set defined by the class. If a circumflex is actually required as a member  the set defined by the class. If a circumflex is actually required as a member
926  of the class, ensure it is not the first character, or escape it with a  of the class, ensure it is not the first character, or escape it with a
# Line 852  For example, the character class [aeiou] Line 930  For example, the character class [aeiou]
930  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
931  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
932  are in the class by enumerating those that are not. A class that starts with a  are in the class by enumerating those that are not. A class that starts with a
933  circumflex is not an assertion: it still consumes a character from the subject  circumflex is not an assertion; it still consumes a character from the subject
934  string, and therefore it fails if the current pointer is at the end of the  string, and therefore it fails if the current pointer is at the end of the
935  string.  string.
936  .P  .P
# Line 866  caseful version would. In UTF-8 mode, PC Line 944  caseful version would. In UTF-8 mode, PC
944  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
945  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
946  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
947  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in UTF8-mode for characters 128 and above,
948  ensure that PCRE is compiled with Unicode property support as well as with  you must ensure that PCRE is compiled with Unicode property support as well as
949  UTF-8 support.  with UTF-8 support.
950  .P  .P
951  Characters that might indicate line breaks are never treated in any special way  Characters that might indicate line breaks are never treated in any special way
952  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 1007  The PCRE-specific options PCRE_DUPNAMES, Line 1085  The PCRE-specific options PCRE_DUPNAMES,
1085  changed in the same way as the Perl-compatible options by using the characters  changed in the same way as the Perl-compatible options by using the characters
1086  J, U and X respectively.  J, U and X respectively.
1087  .P  .P
1088  When an option change occurs at top level (that is, not inside subpattern  When one of these option changes occurs at top level (that is, not inside
1089  parentheses), the change applies to the remainder of the pattern that follows.  subpattern parentheses), the change applies to the remainder of the pattern
1090  If the change is placed right at the start of a pattern, PCRE extracts it into  that follows. If the change is placed right at the start of a pattern, PCRE
1091  the global options (and it will therefore show up in data extracted by the  extracts it into the global options (and it will therefore show up in data
1092  \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1093  .P  .P
1094  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1095  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
# Line 1029  matches "ab", "aB", "c", and "C", even t Line 1107  matches "ab", "aB", "c", and "C", even t
1107  branch is abandoned before the option setting. This is because the effects of  branch is abandoned before the option setting. This is because the effects of
1108  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1109  behaviour otherwise.  behaviour otherwise.
1110    .P
1111    \fBNote:\fP There are other PCRE-specific options that can be set by the
1112    application when the compile or match functions are called. In some cases the
1113    pattern can contain special leading sequences such as (*CRLF) to override what
1114    the application has set or what has been defaulted. Details are given in the
1115    section entitled
1116    .\" HTML <a href="#newlineseq">
1117    .\" </a>
1118    "Newline sequences"
1119    .\"
1120    above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1121    mode; this is equivalent to setting the PCRE_UTF8 option.
1122  .  .
1123  .  .
1124  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1083  is reached, an option setting in one bra Line 1173  is reached, an option setting in one bra
1173  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1174  .  .
1175  .  .
1176    .\" HTML <a name="dupsubpatternnumber"></a>
1177  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1178  .rs  .rs
1179  .sp  .sp
# Line 1108  stored. Line 1199  stored.
1199    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1200    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1201  .sp  .sp
1202  A backreference or a recursive call to a numbered subpattern always refers to  A back reference to a numbered subpattern uses the most recent value that is
1203  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1204    or "defdef":
1205    .sp
1206      /(?|(abc)|(def))\e1/
1207    .sp
1208    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1209    refers to the first one in the pattern with the given number. The following
1210    pattern matches "abcabc" or "defabc":
1211    .sp
1212      /(?|(abc)|(def))(?1)/
1213    .sp
1214    If a
1215    .\" HTML <a href="#conditions">
1216    .\" </a>
1217    condition test
1218    .\"
1219    for a subpattern's having matched refers to a non-unique number, the test is
1220    true if any of the subpatterns of that number have matched.
1221  .P  .P
1222  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1223  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
# Line 1124  if an expression is modified, the number Line 1232  if an expression is modified, the number
1232  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1233  added to Perl until release 5.10. Python had the feature earlier, and PCRE  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1234  introduced it at release 4.0, using the Python syntax. PCRE now supports both  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1235  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1236    have different names, but PCRE does not.
1237  .P  .P
1238  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1239  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1240  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1241  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1242  .\" </a>  .\" </a>
1243  backreferences,  back references,
1244  .\"  .\"
1245  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1246  .\" </a>  .\" </a>
# Line 1151  extracting the name-to-number translatio Line 1260  extracting the name-to-number translatio
1260  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1261  .P  .P
1262  By default, a name must be unique within a pattern, but it is possible to relax  By default, a name must be unique within a pattern, but it is possible to relax
1263  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1264  be useful for patterns where only one instance of the named parentheses can  names are also always permitted for subpatterns with the same number, set up as
1265  match. Suppose you want to match the name of a weekday, either as a 3-letter  described in the previous section.) Duplicate names can be useful for patterns
1266  abbreviation or as the full name, and in both cases you want to extract the  where only one instance of the named parentheses can match. Suppose you want to
1267  abbreviation. This pattern (ignoring the line breaks) does the job:  match the name of a weekday, either as a 3-letter abbreviation or as the full
1268    name, and in both cases you want to extract the abbreviation. This pattern
1269    (ignoring the line breaks) does the job:
1270  .sp  .sp
1271    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1272    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1169  subpattern, as described in the previous Line 1280  subpattern, as described in the previous
1280  .P  .P
1281  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1282  for the first (and in this example, the only) subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1283  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1284  make a reference to a non-unique named subpattern from elsewhere in the  .P
1285  pattern, the one that corresponds to the lowest number is used. For further  If you make a back reference to a non-unique named subpattern from elsewhere in
1286  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1287    used. In the absence of duplicate numbers (see the previous section) this is
1288    the one with the lowest number. If you use a named reference in a condition
1289    test (see the
1290    .\"
1291    .\" HTML <a href="#conditions">
1292    .\" </a>
1293    section about conditions
1294    .\"
1295    below), either to check whether a subpattern has matched, or to check for
1296    recursion, all subpatterns with the same name are tested. If the condition is
1297    true for any one of them, the overall condition is true. This is the same
1298    behaviour as testing by number. For further details of the interfaces for
1299    handling named subpatterns, see the
1300  .\" HREF  .\" HREF
1301  \fBpcreapi\fP  \fBpcreapi\fP
1302  .\"  .\"
1303  documentation.  documentation.
1304    .P
1305    \fBWarning:\fP You cannot use different names to distinguish between two
1306    subpatterns with the same number because PCRE uses only the numbers when
1307    matching. For this reason, an error is given at compile time if different names
1308    are given to subpatterns with the same number. However, you can give the same
1309    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1310  .  .
1311  .  .
1312  .SH REPETITION  .SH REPETITION
# Line 1194  items: Line 1324  items:
1324    a character class    a character class
1325    a back reference (see next section)    a back reference (see next section)
1326    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1327      a recursive or "subroutine" call to a subpattern
1328  .sp  .sp
1329  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1330  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1225  support is available, \eX{3} matches thr Line 1356  support is available, \eX{3} matches thr
1356  which may be several bytes long (and they may be of different lengths).  which may be several bytes long (and they may be of different lengths).
1357  .P  .P
1358  The quantifier {0} is permitted, causing the expression to behave as if the  The quantifier {0} is permitted, causing the expression to behave as if the
1359  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1360    subpatterns that are referenced as
1361    .\" HTML <a href="#subpatternsassubroutines">
1362    .\" </a>
1363    subroutines
1364    .\"
1365    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1366    quantifier are omitted from the compiled pattern.
1367  .P  .P
1368  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1369  abbreviations:  abbreviations:
# Line 1297  worth setting PCRE_DOTALL in order to ob Line 1435  worth setting PCRE_DOTALL in order to ob
1435  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1436  .P  .P
1437  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1438  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1439  elsewhere in the pattern, a match at the start may fail where a later one  elsewhere in the pattern, a match at the start may fail where a later one
1440  succeeds. Consider, for example:  succeeds. Consider, for example:
1441  .sp  .sp
# Line 1508  after the reference. Line 1646  after the reference.
1646  .P  .P
1647  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
1648  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1649  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1650  .sp  .sp
1651    (a|(bc))\e2    (a|(bc))\e2
1652  .sp  .sp
1653  always fails if it starts to match "a" rather than "bc". Because there may be  always fails if it starts to match "a" rather than "bc". However, if the
1654  many capturing parentheses in a pattern, all digits following the backslash are  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1655  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1656  with a digit character, some delimiter must be used to terminate the back  .P
1657  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1658  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1659    If the pattern continues with a digit character, some delimiter must be used to
1660    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1661    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1662  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1663  .\" </a>  .\" </a>
1664  "Comments"  "Comments"
1665  .\"  .\"
1666  below) can be used.  below) can be used.
1667  .P  .
1668    .SS "Recursive back references"
1669    .rs
1670    .sp
1671  A back reference that occurs inside the parentheses to which it refers fails  A back reference that occurs inside the parentheses to which it refers fails
1672  when the subpattern is first used, so, for example, (a\e1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1673  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1537  to the previous iteration. In order for Line 1681  to the previous iteration. In order for
1681  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
1682  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1683  minimum of zero.  minimum of zero.
1684    .P
1685    Back references of this type cause the group that they reference to be treated
1686    as an
1687    .\" HTML <a href="#atomicgroup">
1688    .\" </a>
1689    atomic group.
1690    .\"
1691    Once the whole group has been matched, a subsequent matching failure cannot
1692    cause backtracking into the middle of the group.
1693  .  .
1694  .  .
1695  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1590  lookbehind assertion is needed to achiev Line 1743  lookbehind assertion is needed to achiev
1743  If you want to force a matching failure at some point in a pattern, the most  If you want to force a matching failure at some point in a pattern, the most
1744  convenient way to do it is with (?!) because an empty string always matches, so  convenient way to do it is with (?!) because an empty string always matches, so
1745  an assertion that requires there not to be an empty string must always fail.  an assertion that requires there not to be an empty string must always fail.
1746    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1747    synonym for (?!).
1748  .  .
1749  .  .
1750  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1614  is permitted, but Line 1769  is permitted, but
1769  .sp  .sp
1770  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1771  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1772  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (5.8 and 5.10), which requires all branches to
1773  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1774  .sp  .sp
1775    (?<=ab(c|de))    (?<=ab(c|de))
1776  .sp  .sp
1777  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
1778  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
1779    branches:
1780  .sp  .sp
1781    (?<=abc|abde)    (?<=abc|abde)
1782  .sp  .sp
# Line 1629  In some cases, the Perl 5.10 escape sequ Line 1785  In some cases, the Perl 5.10 escape sequ
1785  .\" </a>  .\" </a>
1786  (see above)  (see above)
1787  .\"  .\"
1788  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
1789  fixed-length.  restriction.
1790  .P  .P
1791  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1792  temporarily move the current position back by the fixed length and then try to  temporarily move the current position back by the fixed length and then try to
# Line 1642  to appear in lookbehind assertions, beca Line 1798  to appear in lookbehind assertions, beca
1798  the length of the lookbehind. The \eX and \eR escapes, which can match  the length of the lookbehind. The \eX and \eR escapes, which can match
1799  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1800  .P  .P
1801    .\" HTML <a href="#subpatternsassubroutines">
1802    .\" </a>
1803    "Subroutine"
1804    .\"
1805    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1806    as the subpattern matches a fixed-length string.
1807    .\" HTML <a href="#recursion">
1808    .\" </a>
1809    Recursion,
1810    .\"
1811    however, is not supported.
1812    .P
1813  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1814  specify efficient matching at the end of the subject string. Consider a simple  specify efficient matching of fixed-length strings at the end of subject
1815  pattern such as  strings. Consider a simple pattern such as
1816  .sp  .sp
1817    abcd$    abcd$
1818  .sp  .sp
# Line 1708  characters that are not "999". Line 1876  characters that are not "999".
1876  .sp  .sp
1877  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1878  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1879  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1880  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1881  .sp  .sp
1882    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1883    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
# Line 1725  recursion, a pseudo-condition called DEF Line 1893  recursion, a pseudo-condition called DEF
1893  .rs  .rs
1894  .sp  .sp
1895  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
1896  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
1897  matched. An alternative notation is to precede the digits with a plus or minus  matched. If there is more than one capturing subpattern with the same number
1898  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
1899  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
1900  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
1901  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
1902    section about duplicate subpattern numbers),
1903    .\"
1904    the condition is true if any of them have been set. An alternative notation is
1905    to precede the digits with a plus or minus sign. In this case, the subpattern
1906    number is relative rather than absolute. The most recently opened parentheses
1907    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1908    looping constructs it can also make sense to refer to subsequent groups with
1909    constructs such as (?(+2).
1910  .P  .P
1911  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1912  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1771  Rewriting the above example to use a nam Line 1947  Rewriting the above example to use a nam
1947  .sp  .sp
1948    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1949  .sp  .sp
1950    If the name used in a condition of this kind is a duplicate, the test is
1951    applied to all subpatterns of the same name, and is true if any one of them has
1952    matched.
1953  .  .
1954  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
1955  .rs  .rs
# Line 1782  letter R, for example: Line 1961  letter R, for example:
1961  .sp  .sp
1962    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
1963  .sp  .sp
1964  the condition is true if the most recent recursion is into the subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
1965  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
1966  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
1967    applied to all subpatterns of the same name, and is true if any one of them is
1968    the most recent recursion.
1969  .P  .P
1970  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
1971  patterns are described below.  .\" HTML <a href="#recursion">
1972    .\" </a>
1973    The syntax for recursive patterns
1974    .\"
1975    is described below.
1976  .  .
1977  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
1978  .rs  .rs
# Line 1796  If the condition is the string (DEFINE), Line 1981  If the condition is the string (DEFINE),
1981  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
1982  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
1983  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
1984  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  "subroutines" that can be referenced from elsewhere. (The use of
1985    .\" HTML <a href="#subpatternsassubroutines">
1986    .\" </a>
1987    "subroutines"
1988    .\"
1989  is described below.) For example, a pattern to match an IPv4 address could be  is described below.) For example, a pattern to match an IPv4 address could be
1990  written like this (ignore whitespace and line breaks):  written like this (ignore whitespace and line breaks):
1991  .sp  .sp
# Line 1806  written like this (ignore whitespace and Line 1995  written like this (ignore whitespace and
1995  The first part of the pattern is a DEFINE group inside which a another group  The first part of the pattern is a DEFINE group inside which a another group
1996  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
1997  address (a number less than 256). When matching takes place, this part of the  address (a number less than 256). When matching takes place, this part of the
1998  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
1999  .P  pattern uses references to the named group to match the four dot-separated
2000  The rest of the pattern uses references to the named group to match the four  components of an IPv4 address, insisting on a word boundary at each end.
 dot-separated components of an IPv4 address, insisting on a word boundary at  
 each end.  
2001  .  .
2002  .SS "Assertion conditions"  .SS "Assertion conditions"
2003  .rs  .rs
# Line 1867  recursively to the pattern in which it a Line 2054  recursively to the pattern in which it a
2054  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2055  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2056  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2057  this kind of recursion was introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2058  .P  .P
2059  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
2060  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive call of the subpattern of the given number,
2061  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  provided that it occurs inside that subpattern. (If not, it is a
2062    .\" HTML <a href="#subpatternsassubroutines">
2063    .\" </a>
2064    "subroutine"
2065    .\"
2066  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
2067  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2068  .P  .P
 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  
 treated as an atomic group. That is, once it has matched some of the subject  
 string, it is never re-entered, even if it contains untried alternatives and  
 there is a subsequent matching failure.  
 .P  
2069  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2070  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2071  .sp  .sp
2072    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2073  .sp  .sp
2074  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2075  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
2076  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2077  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2078    to avoid backtracking into sequences of non-parentheses.
2079  .P  .P
2080  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
2081  pattern, so instead you could use this:  pattern, so instead you could use this:
2082  .sp  .sp
2083    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2084  .sp  .sp
2085  We have put the pattern into parentheses, and caused the recursion to refer to  We have put the pattern into parentheses, and caused the recursion to refer to
2086  them instead of the whole pattern.  them instead of the whole pattern.
2087  .P  .P
2088  In a larger pattern, keeping track of parenthesis numbers can be tricky. This  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2089  is made easier by the use of relative references. (A Perl 5.10 feature.)  is made easier by the use of relative references (a Perl 5.10 feature).
2090  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
2091  most recently opened parentheses preceding the recursion. In other words, a  most recently opened parentheses preceding the recursion. In other words, a
2092  negative number counts capturing parentheses leftwards from the point at which  negative number counts capturing parentheses leftwards from the point at which
# Line 1908  it is encountered. Line 2095  it is encountered.
2095  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2096  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2097  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2098  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2099    .\" </a>
2100    "subroutine"
2101    .\"
2102    calls, as described in the next section.
2103  .P  .P
2104  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2105  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2106  could rewrite the above example as follows:  could rewrite the above example as follows:
2107  .sp  .sp
2108    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2109  .sp  .sp
2110  If there is more than one subpattern with the same name, the earliest one is  If there is more than one subpattern with the same name, the earliest one is
2111  used.  used.
2112  .P  .P
2113  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2114  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of a possessive quantifier for matching
2115  non-parentheses is important when applying the pattern to strings that do not  strings of non-parentheses is important when applying the pattern to strings
2116  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2117  .sp  .sp
2118    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2119  .sp  .sp
2120  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if a possessive quantifier is not used,
2121  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
2122  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
2123  before failure can be reported.  before failure can be reported.
2124  .P  .P
2125  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values of capturing parentheses are those from
2126  from the outermost level of the recursion at which the subpattern value is set.  the outermost level. If you want to obtain intermediate values, a callout
2127  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2128  .\" HREF  .\" HREF
2129  \fBpcrecallout\fP  \fBpcrecallout\fP
2130  .\"  .\"
# Line 1942  documentation). If the pattern above is Line 2132  documentation). If the pattern above is
2132  .sp  .sp
2133    (ab(cd)ef)    (ab(cd)ef)
2134  .sp  .sp
2135  the value for the capturing parentheses is "ef", which is the last value taken  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2136  on at the top level. If additional parentheses are added, giving  the last value taken on at the top level. If a capturing subpattern is not
2137  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2138    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2139       ^                        ^  .P
2140       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2141  .sp  obtain extra memory to store data during a recursion, which it does by using
2142  the string they capture is "ab(cd)ef", the contents of the top level  \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2143  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
 has to obtain extra memory to store data during a recursion, which it does by  
 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
2144  .P  .P
2145  Do not confuse the (?R) item with the condition (R), which tests for recursion.  Do not confuse the (?R) item with the condition (R), which tests for recursion.
2146  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1967  different alternatives for the recursive Line 2154  different alternatives for the recursive
2154  is the actual recursive call.  is the actual recursive call.
2155  .  .
2156  .  .
2157    .\" HTML <a name="recursiondifference"></a>
2158    .SS "Recursion difference from Perl"
2159    .rs
2160    .sp
2161    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2162    treated as an atomic group. That is, once it has matched some of the subject
2163    string, it is never re-entered, even if it contains untried alternatives and
2164    there is a subsequent matching failure. This can be illustrated by the
2165    following pattern, which purports to match a palindromic string that contains
2166    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2167    .sp
2168      ^(.|(.)(?1)\e2)$
2169    .sp
2170    The idea is that it either matches a single character, or two identical
2171    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2172    it does not if the pattern is longer than three characters. Consider the
2173    subject string "abcba":
2174    .P
2175    At the top level, the first character is matched, but as it is not at the end
2176    of the string, the first alternative fails; the second alternative is taken
2177    and the recursion kicks in. The recursive call to subpattern 1 successfully
2178    matches the next character ("b"). (Note that the beginning and end of line
2179    tests are not part of the recursion).
2180    .P
2181    Back at the top level, the next character ("c") is compared with what
2182    subpattern 2 matched, which was "a". This fails. Because the recursion is
2183    treated as an atomic group, there are now no backtracking points, and so the
2184    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2185    try the second alternative.) However, if the pattern is written with the
2186    alternatives in the other order, things are different:
2187    .sp
2188      ^((.)(?1)\e2|.)$
2189    .sp
2190    This time, the recursing alternative is tried first, and continues to recurse
2191    until it runs out of characters, at which point the recursion fails. But this
2192    time we do have another alternative to try at the higher level. That is the big
2193    difference: in the previous case the remaining alternative is at a deeper
2194    recursion level, which PCRE cannot use.
2195    .P
2196    To change the pattern so that matches all palindromic strings, not just those
2197    with an odd number of characters, it is tempting to change the pattern to this:
2198    .sp
2199      ^((.)(?1)\e2|.?)$
2200    .sp
2201    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2202    deeper recursion has matched a single character, it cannot be entered again in
2203    order to match an empty string. The solution is to separate the two cases, and
2204    write out the odd and even cases as alternatives at the higher level:
2205    .sp
2206      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2207    .sp
2208    If you want to match typical palindromic phrases, the pattern has to ignore all
2209    non-word characters, which can be done like this:
2210    .sp
2211      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2212    .sp
2213    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2214    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2215    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2216    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2217    more) to match typical phrases, and Perl takes so long that you think it has
2218    gone into a loop.
2219    .P
2220    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2221    string does not start with a palindrome that is shorter than the entire string.
2222    For example, although "abcba" is correctly matched, if the subject is "ababa",
2223    PCRE finds the palindrome "aba" at the start, then fails at top level because
2224    the end of the string does not follow. Once again, it cannot jump back into the
2225    recursion to try other alternatives, so the entire match fails.
2226    .
2227    .
2228  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2229  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2230  .rs  .rs
# Line 1993  matches "sense and sensibility" and "res Line 2251  matches "sense and sensibility" and "res
2251  is used, it does match "sense and responsibility" as well as the other two  is used, it does match "sense and responsibility" as well as the other two
2252  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2253  .P  .P
2254  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2255  group. That is, once it has matched some of the subject string, it is never  group. That is, once it has matched some of the subject string, it is never
2256  re-entered, even if it contains untried alternatives and there is a subsequent  re-entered, even if it contains untried alternatives and there is a subsequent
2257  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2258    call revert to their previous values afterwards.
2259  .P  .P
2260  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2261  case-independence are fixed when the subpattern is defined. They cannot be  case-independence are fixed when the subpattern is defined. They cannot be
# Line 2008  It matches "abcabc". It does not match " Line 2267  It matches "abcabc". It does not match "
2267  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2268  .  .
2269  .  .
2270    .\" HTML <a name="onigurumasubroutines"></a>
2271    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2272    .rs
2273    .sp
2274    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2275    a number enclosed either in angle brackets or single quotes, is an alternative
2276    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2277    are two of the examples used above, rewritten using this syntax:
2278    .sp
2279      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2280      (sens|respons)e and \eg'1'ibility
2281    .sp
2282    PCRE supports an extension to Oniguruma: if a number is preceded by a
2283    plus or a minus sign it is taken as a relative reference. For example:
2284    .sp
2285      (abc)(?i:\eg<-1>)
2286    .sp
2287    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2288    synonymous. The former is a back reference; the latter is a subroutine call.
2289    .
2290    .
2291  .SH CALLOUTS  .SH CALLOUTS
2292  .rs  .rs
2293  .sp  .sp
# Line 2044  description of the interface to the call Line 2324  description of the interface to the call
2324  documentation.  documentation.
2325  .  .
2326  .  .
2327    .\" HTML <a name="backtrackcontrol"></a>
2328  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2329  .rs  .rs
2330  .sp  .sp
# Line 2053  or removal in a future version of Perl". Line 2334  or removal in a future version of Perl".
2334  production code should be noted to avoid problems during upgrades." The same  production code should be noted to avoid problems during upgrades." The same
2335  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2336  .P  .P
2337  Since these verbs are specifically related to backtracking, they can be used  Since these verbs are specifically related to backtracking, most of them can be
2338  only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a  used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2339  backtracking algorithm. They cause an error if encountered by  a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2340    failing negative assertion, they cause an error if encountered by
2341  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2342  .P  .P
2343    If any of these verbs are used in an assertion or subroutine subpattern
2344    (including recursive subpatterns), their effect is confined to that subpattern;
2345    it does not extend to the surrounding pattern. Note that such subpatterns are
2346    processed as anchored at the point where they are tested.
2347    .P
2348  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2349  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2350  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2351  form is just (*VERB). Any number of these verbs may occur in a pattern. There  depending on whether or not an argument is present. An name is a sequence of
2352  are two kinds:  letters, digits, and underscores. If the name is empty, that is, if the closing
2353    parenthesis immediately follows the colon, the effect is as if the colon were
2354    not there. Any number of these verbs may occur in a pattern.
2355    .P
2356    PCRE contains some optimizations that are used to speed up matching by running
2357    some checks at the start of each match attempt. For example, it may know the
2358    minimum length of matching subject, or that a particular character must be
2359    present. When one of these optimizations suppresses the running of a match, any
2360    included backtracking verbs will not, of course, be processed. You can suppress
2361    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2362    when calling \fBpcre_exec()\fP.
2363    .
2364  .  .
2365  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2366  .rs  .rs
2367  .sp  .sp
2368  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2369    followed by a name.
2370  .sp  .sp
2371     (*ACCEPT)     (*ACCEPT)
2372  .sp  .sp
2373  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2374  pattern. When inside a recursion, only the innermost pattern is ended  pattern. When inside a recursion, only the innermost pattern is ended
2375  immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2376  capturing parentheses. In Perl, the data so far is captured: in PCRE no data is  captured. (This feature was added to PCRE at release 8.00.) For example:
 captured. For example:  
2377  .sp  .sp
2378    A(A|B(*ACCEPT)|C)D    A((?:A|B(*ACCEPT)|C)D)
2379  .sp  .sp
2380  This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2381  captured.  the outer parentheses.
2382  .sp  .sp
2383    (*FAIL) or (*F)    (*FAIL) or (*F)
2384  .sp  .sp
# Line 2095  callout feature, as for example in this Line 2393  callout feature, as for example in this
2393  A match with the string "aaaa" always fails, but the callout is taken before  A match with the string "aaaa" always fails, but the callout is taken before
2394  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2395  .  .
2396    .
2397    .SS "Recording which path was taken"
2398    .rs
2399    .sp
2400    There is one verb whose main purpose is to track how a match was arrived at,
2401    though it also has a secondary use in conjunction with advancing the match
2402    starting point (see (*SKIP) below).
2403    .sp
2404      (*MARK:NAME) or (*:NAME)
2405    .sp
2406    A name is always required with this verb. There may be as many instances of
2407    (*MARK) as you like in a pattern, and their names do not have to be unique.
2408    .P
2409    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2410    to the caller via the \fIpcre_extra\fP data structure, as described in the
2411    .\" HTML <a href="pcreapi.html#extradata">
2412    .\" </a>
2413    section on \fIpcre_extra\fP
2414    .\"
2415    in the
2416    .\" HREF
2417    \fBpcreapi\fP
2418    .\"
2419    documentation. No data is returned for a partial match. Here is an example of
2420    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2421    outputting of (*MARK) data:
2422    .sp
2423      /X(*MARK:A)Y|X(*MARK:B)Z/K
2424      XY
2425       0: XY
2426      MK: A
2427      XZ
2428       0: XZ
2429      MK: B
2430    .sp
2431    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2432    indicates which of the two alternatives matched. This is a more efficient way
2433    of obtaining this information than putting each alternative in its own
2434    capturing parentheses.
2435    .P
2436    A name may also be returned after a failed match if the final path through the
2437    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2438    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2439    starting point for matching is advanced, the final check is often with an empty
2440    string, causing a failure before (*MARK) is reached. For example:
2441    .sp
2442      /X(*MARK:A)Y|X(*MARK:B)Z/K
2443      XP
2444      No match
2445    .sp
2446    There are three potential starting points for this match (starting with X,
2447    starting with P, and with an empty string). If the pattern is anchored, the
2448    result is different:
2449    .sp
2450      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2451      XP
2452      No match, mark = B
2453    .sp
2454    PCRE's start-of-match optimizations can also interfere with this. For example,
2455    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2456    subject length for a match, a shorter subject will not be scanned at all.
2457    .P
2458    Note that similar anomalies (though different in detail) exist in Perl, no
2459    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2460    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2461    .
2462    .
2463  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2464  .rs  .rs
2465  .sp  .sp
2466  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2467  with what follows, but if there is no subsequent match, a failure is forced.  with what follows, but if there is no subsequent match, causing a backtrack to
2468  The verbs differ in exactly what kind of failure occurs.  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2469    the verb. However, when one of these verbs appears inside an atomic group, its
2470    effect is confined to that group, because once the group has been matched,
2471    there is never any backtracking into it. In this situation, backtracking can
2472    "jump back" to the left of the entire atomic group. (Remember also, as stated
2473    above, that this localization also applies in subroutine calls and assertions.)
2474    .P
2475    These verbs differ in exactly what kind of failure occurs when backtracking
2476    reaches them.
2477  .sp  .sp
2478    (*COMMIT)    (*COMMIT)
2479  .sp  .sp
2480  This verb causes the whole match to fail outright if the rest of the pattern  This verb, which may not be followed by a name, causes the whole match to fail
2481  does not match. Even if the pattern is unanchored, no further attempts to find  outright if the rest of the pattern does not match. Even if the pattern is
2482  a match by advancing the start point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2483  passed, \fBpcre_exec()\fP is committed to finding a match at the current  take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2484  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2485  .sp  .sp
2486    a+(*COMMIT)b    a+(*COMMIT)b
2487  .sp  .sp
2488  This matches "xxaab" but not "aacaab". It can be thought of as a kind of  This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2489  dynamic anchor, or "I've started, so I must finish."  dynamic anchor, or "I've started, so I must finish." The name of the most
2490  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2491    (*PRUNE)  match failure.
2492  .sp  .P
2493  This verb causes the match to fail at the current position if the rest of the  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2494  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  unless PCRE's start-of-match optimizations are turned off, as shown in this
2495  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2496  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2497  if there is no match to the right, backtracking cannot cross (*PRUNE).    /(*COMMIT)abc/
2498  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    xyzabc
2499  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2500  be expressed in any other way.    xyzabc\eY
2501      No match
2502    .sp
2503    PCRE knows that any match must start with "a", so the optimization skips along
2504    the subject to "a" before running the first match attempt, which succeeds. When
2505    the optimization is disabled by the \eY escape in the second subject, the match
2506    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2507    starting points.
2508    .sp
2509      (*PRUNE) or (*PRUNE:NAME)
2510    .sp
2511    This verb causes the match to fail at the current starting position in the
2512    subject if the rest of the pattern does not match. If the pattern is
2513    unanchored, the normal "bumpalong" advance to the next starting character then
2514    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2515    reached, or when matching to the right of (*PRUNE), but if there is no match to
2516    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2517    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2518    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2519    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2520    match fails completely; the name is passed back if this is the final attempt.
2521    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2522    pattern (*PRUNE) has the same effect as (*COMMIT).
2523  .sp  .sp
2524    (*SKIP)    (*SKIP)
2525  .sp  .sp
2526  This verb is like (*PRUNE), except that if the pattern is unanchored, the  This verb, when given without a name, is like (*PRUNE), except that if the
2527  "bumpalong" advance is not to the next character, but to the position in the  pattern is unanchored, the "bumpalong" advance is not to the next character,
2528  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2529  was matched leading up to it cannot be part of a successful match. Consider:  signifies that whatever text was matched leading up to it cannot be part of a
2530    successful match. Consider:
2531  .sp  .sp
2532    a+(*SKIP)b    a+(*SKIP)b
2533  .sp  .sp
2534  If the subject is "aaaac...", after the first match attempt fails (starting at  If the subject is "aaaac...", after the first match attempt fails (starting at
2535  the first character in the string), the starting point skips on to start the  the first character in the string), the starting point skips on to start the
2536  next attempt at "c". Note that a possessive quantifer does not have the same  next attempt at "c". Note that a possessive quantifer does not have the same
2537  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
2538  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2539  instead of skipping on to "c".  instead of skipping on to "c".
2540  .sp  .sp
2541    (*THEN)    (*SKIP:NAME)
2542    .sp
2543    When (*SKIP) has an associated name, its behaviour is modified. If the
2544    following pattern fails to match, the previous path through the pattern is
2545    searched for the most recent (*MARK) that has the same name. If one is found,
2546    the "bumpalong" advance is to the subject position that corresponds to that
2547    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2548    matching name is found, normal "bumpalong" of one character happens (the
2549    (*SKIP) is ignored).
2550    .sp
2551      (*THEN) or (*THEN:NAME)
2552  .sp  .sp
2553  This verb causes a skip to the next alternation if the rest of the pattern does  This verb causes a skip to the next alternation if the rest of the pattern does
2554  not match. That is, it cancels pending backtracking, but only within the  not match. That is, it cancels pending backtracking, but only within the
# Line 2153  for a pattern-based if-then-else block: Line 2559  for a pattern-based if-then-else block:
2559  .sp  .sp
2560  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
2561  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure the matcher skips to the
2562  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2563  is used outside of any alternation, it acts exactly like (*PRUNE).  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2564    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2565    like (*PRUNE).
2566  .  .
2567  .  .
2568  .SH "SEE ALSO"  .SH "SEE ALSO"
2569  .rs  .rs
2570  .sp  .sp
2571  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2572    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2573  .  .
2574  .  .
2575  .SH AUTHOR  .SH AUTHOR
# Line 2177  Cambridge CB2 3QH, England. Line 2586  Cambridge CB2 3QH, England.
2586  .rs  .rs
2587  .sp  .sp
2588  .nf  .nf
2589  Last updated: 11 September 2007  Last updated: 03 May 2010
2590  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2591  .fi  .fi

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