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revision 333 by ph10, Thu Apr 10 19:55:57 2008 UTC revision 491 by ph10, Mon Mar 1 17:45:08 2010 UTC
# Line 21  published by O'Reilly, covers regular ex Line 21  published by O'Reilly, covers regular ex
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 76  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 199  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 210  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 318  parenthesized subpatterns. Line 325  parenthesized subpatterns.
325  .SS "Absolute and relative subroutine calls"  .SS "Absolute and relative subroutine calls"
326  .rs  .rs
327  .sp  .sp
328  For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or  For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
329  a number enclosed either in angle brackets or single quotes, is an alternative  a number enclosed either in angle brackets or single quotes, is an alternative
330  syntax for referencing a subpattern as a "subroutine". Details are discussed  syntax for referencing a subpattern as a "subroutine". Details are discussed
331  .\" HTML <a href="#onigurumasubroutines">  .\" HTML <a href="#onigurumasubroutines">
332  .\" </a>  .\" </a>
333  later.  later.
334  .\"  .\"
335  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
336  synonymous. The former is a back reference; the latter is a subroutine call.  synonymous. The former is a back reference; the latter is a
337    .\" HTML <a href="#subpatternsassubroutines">
338    .\" </a>
339    subroutine
340    .\"
341    call.
342  .  .
343  .  .
344  .SS "Generic character types"  .SS "Generic character types"
# Line 364  In UTF-8 mode, characters with values gr Line 376  In UTF-8 mode, characters with values gr
376  \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
377  character property support is available. These sequences retain their original  character property support is available. These sequences retain their original
378  meanings from before UTF-8 support was available, mainly for efficiency  meanings from before UTF-8 support was available, mainly for efficiency
379  reasons.  reasons. Note that this also affects \eb, because it is defined in terms of \ew
380    and \eW.
381  .P  .P
382  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
383  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 455  one of the following sequences: Line 468  one of the following sequences:
468    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
469    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
470  .sp  .sp
471  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
472  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
473  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,
474  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
475  of them is present, the last one is used. They can be combined with a change of  pattern, and that they must be in upper case. If more than one of them is
476  newline convention, for example, a pattern can start with:  present, the last one is used. They can be combined with a change of newline
477    convention, for example, a pattern can start with:
478  .sp  .sp
479    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
480  .sp  .sp
# Line 499  Those that are not part of an identified Line 513  Those that are not part of an identified
513  .P  .P
514  Arabic,  Arabic,
515  Armenian,  Armenian,
516    Avestan,
517  Balinese,  Balinese,
518    Bamum,
519  Bengali,  Bengali,
520  Bopomofo,  Bopomofo,
521  Braille,  Braille,
522  Buginese,  Buginese,
523  Buhid,  Buhid,
524  Canadian_Aboriginal,  Canadian_Aboriginal,
525    Carian,
526    Cham,
527  Cherokee,  Cherokee,
528  Common,  Common,
529  Coptic,  Coptic,
# Line 514  Cypriot, Line 532  Cypriot,
532  Cyrillic,  Cyrillic,
533  Deseret,  Deseret,
534  Devanagari,  Devanagari,
535    Egyptian_Hieroglyphs,
536  Ethiopic,  Ethiopic,
537  Georgian,  Georgian,
538  Glagolitic,  Glagolitic,
# Line 526  Hangul, Line 545  Hangul,
545  Hanunoo,  Hanunoo,
546  Hebrew,  Hebrew,
547  Hiragana,  Hiragana,
548    Imperial_Aramaic,
549  Inherited,  Inherited,
550    Inscriptional_Pahlavi,
551    Inscriptional_Parthian,
552    Javanese,
553    Kaithi,
554  Kannada,  Kannada,
555  Katakana,  Katakana,
556    Kayah_Li,
557  Kharoshthi,  Kharoshthi,
558  Khmer,  Khmer,
559  Lao,  Lao,
560  Latin,  Latin,
561    Lepcha,
562  Limbu,  Limbu,
563  Linear_B,  Linear_B,
564    Lisu,
565    Lycian,
566    Lydian,
567  Malayalam,  Malayalam,
568    Meetei_Mayek,
569  Mongolian,  Mongolian,
570  Myanmar,  Myanmar,
571  New_Tai_Lue,  New_Tai_Lue,
# Line 543  Nko, Line 573  Nko,
573  Ogham,  Ogham,
574  Old_Italic,  Old_Italic,
575  Old_Persian,  Old_Persian,
576    Old_South_Arabian,
577    Old_Turkic,
578    Ol_Chiki,
579  Oriya,  Oriya,
580  Osmanya,  Osmanya,
581  Phags_Pa,  Phags_Pa,
582  Phoenician,  Phoenician,
583    Rejang,
584  Runic,  Runic,
585    Samaritan,
586    Saurashtra,
587  Shavian,  Shavian,
588  Sinhala,  Sinhala,
589    Sundanese,
590  Syloti_Nagri,  Syloti_Nagri,
591  Syriac,  Syriac,
592  Tagalog,  Tagalog,
593  Tagbanwa,  Tagbanwa,
594  Tai_Le,  Tai_Le,
595    Tai_Tham,
596    Tai_Viet,
597  Tamil,  Tamil,
598  Telugu,  Telugu,
599  Thaana,  Thaana,
# Line 562  Thai, Line 601  Thai,
601  Tibetan,  Tibetan,
602  Tifinagh,  Tifinagh,
603  Ugaritic,  Ugaritic,
604    Vai,
605  Yi.  Yi.
606  .P  .P
607  Each character has exactly one general category property, specified by a  Each character has exactly one general category property, specified by a
# Line 634  cannot be tested by PCRE, unless UTF-8 v Line 674  cannot be tested by PCRE, unless UTF-8 v
674  .\" HREF  .\" HREF
675  \fBpcreapi\fP  \fBpcreapi\fP
676  .\"  .\"
677  page).  page). Perl does not support the Cs property.
678  .P  .P
679  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})
680  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
681  properties with "Is".  properties with "Is".
682  .P  .P
# Line 727  different meaning, namely the backspace Line 767  different meaning, namely the backspace
767  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
768  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
769  \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
770  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. Neither PCRE nor Perl has a
771    separte "start of word" or "end of word" metasequence. However, whatever
772    follows \eb normally determines which it is. For example, the fragment
773    \eba matches "a" at the start of a word.
774  .P  .P
775  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
776  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 859  the lookbehind. Line 902  the lookbehind.
902  .rs  .rs
903  .sp  .sp
904  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
905  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.
906  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
907  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
908  escaped with a backslash.  a member of the class, it should be the first data character in the class
909    (after an initial circumflex, if present) or escaped with a backslash.
910  .P  .P
911  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
912  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
913  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
914  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
915  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
916  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 876  For example, the character class [aeiou] Line 920  For example, the character class [aeiou]
920  [^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
921  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
922  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
923  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
924  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
925  string.  string.
926  .P  .P
# Line 890  caseful version would. In UTF-8 mode, PC Line 934  caseful version would. In UTF-8 mode, PC
934  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
935  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
936  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
937  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,
938  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
939  UTF-8 support.  with UTF-8 support.
940  .P  .P
941  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
942  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 1031  The PCRE-specific options PCRE_DUPNAMES, Line 1075  The PCRE-specific options PCRE_DUPNAMES,
1075  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
1076  J, U and X respectively.  J, U and X respectively.
1077  .P  .P
1078  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
1079  parentheses), the change applies to the remainder of the pattern that follows.  subpattern parentheses), the change applies to the remainder of the pattern
1080  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
1081  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
1082  \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1083  .P  .P
1084  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1085  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 1056  behaviour otherwise. Line 1100  behaviour otherwise.
1100  .P  .P
1101  \fBNote:\fP There are other PCRE-specific options that can be set by the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1102  application when the compile or match functions are called. In some cases the  application when the compile or match functions are called. In some cases the
1103  pattern can contain special leading sequences to override what the application  pattern can contain special leading sequences such as (*CRLF) to override what
1104  has set or what has been defaulted. Details are given in the section entitled  the application has set or what has been defaulted. Details are given in the
1105    section entitled
1106  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
1107  .\" </a>  .\" </a>
1108  "Newline sequences"  "Newline sequences"
1109  .\"  .\"
1110  above.  above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1111    mode; this is equivalent to setting the PCRE_UTF8 option.
1112  .  .
1113  .  .
1114  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1117  is reached, an option setting in one bra Line 1163  is reached, an option setting in one bra
1163  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1164  .  .
1165  .  .
1166    .\" HTML <a name="dupsubpatternnumber"></a>
1167  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1168  .rs  .rs
1169  .sp  .sp
# Line 1142  stored. Line 1189  stored.
1189    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1190    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1191  .sp  .sp
1192  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
1193  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1194    or "defdef":
1195    .sp
1196      /(?|(abc)|(def))\e1/
1197    .sp
1198    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1199    refers to the first one in the pattern with the given number. The following
1200    pattern matches "abcabc" or "defabc":
1201    .sp
1202      /(?|(abc)|(def))(?1)/
1203    .sp
1204    If a
1205    .\" HTML <a href="#conditions">
1206    .\" </a>
1207    condition test
1208    .\"
1209    for a subpattern's having matched refers to a non-unique number, the test is
1210    true if any of the subpatterns of that number have matched.
1211  .P  .P
1212  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1213  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
# Line 1158  if an expression is modified, the number Line 1222  if an expression is modified, the number
1222  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1223  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
1224  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
1225  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1226    have different names, but PCRE does not.
1227  .P  .P
1228  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
1229  (?'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
1230  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1231  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1232  .\" </a>  .\" </a>
1233  backreferences,  back references,
1234  .\"  .\"
1235  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1236  .\" </a>  .\" </a>
# Line 1185  extracting the name-to-number translatio Line 1250  extracting the name-to-number translatio
1250  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1251  .P  .P
1252  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
1253  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
1254  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
1255  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
1256  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
1257  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
1258    name, and in both cases you want to extract the abbreviation. This pattern
1259    (ignoring the line breaks) does the job:
1260  .sp  .sp
1261    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1262    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1203  subpattern, as described in the previous Line 1270  subpattern, as described in the previous
1270  .P  .P
1271  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1272  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
1273  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1274  make a reference to a non-unique named subpattern from elsewhere in the  .P
1275  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
1276  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1277    used. In the absence of duplicate numbers (see the previous section) this is
1278    the one with the lowest number. If you use a named reference in a condition
1279    test (see the
1280    .\"
1281    .\" HTML <a href="#conditions">
1282    .\" </a>
1283    section about conditions
1284    .\"
1285    below), either to check whether a subpattern has matched, or to check for
1286    recursion, all subpatterns with the same name are tested. If the condition is
1287    true for any one of them, the overall condition is true. This is the same
1288    behaviour as testing by number. For further details of the interfaces for
1289    handling named subpatterns, see the
1290  .\" HREF  .\" HREF
1291  \fBpcreapi\fP  \fBpcreapi\fP
1292  .\"  .\"
1293  documentation.  documentation.
1294    .P
1295    \fBWarning:\fP You cannot use different names to distinguish between two
1296    subpatterns with the same number because PCRE uses only the numbers when
1297    matching. For this reason, an error is given at compile time if different names
1298    are given to subpatterns with the same number. However, you can give the same
1299    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1300  .  .
1301  .  .
1302  .SH REPETITION  .SH REPETITION
# Line 1228  items: Line 1314  items:
1314    a character class    a character class
1315    a back reference (see next section)    a back reference (see next section)
1316    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1317      a recursive or "subroutine" call to a subpattern
1318  .sp  .sp
1319  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1320  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1259  support is available, \eX{3} matches thr Line 1346  support is available, \eX{3} matches thr
1346  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).
1347  .P  .P
1348  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
1349  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1350    subpatterns that are referenced as
1351    .\" HTML <a href="#subpatternsassubroutines">
1352    .\" </a>
1353    subroutines
1354    .\"
1355    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1356    quantifier are omitted from the compiled pattern.
1357  .P  .P
1358  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1359  abbreviations:  abbreviations:
# Line 1331  worth setting PCRE_DOTALL in order to ob Line 1425  worth setting PCRE_DOTALL in order to ob
1425  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1426  .P  .P
1427  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1428  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1429  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
1430  succeeds. Consider, for example:  succeeds. Consider, for example:
1431  .sp  .sp
# Line 1542  after the reference. Line 1636  after the reference.
1636  .P  .P
1637  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
1638  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1639  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1640  .sp  .sp
1641    (a|(bc))\e2    (a|(bc))\e2
1642  .sp  .sp
1643  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
1644  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
1645  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1646  with a digit character, some delimiter must be used to terminate the back  .P
1647  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1648  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1649    If the pattern continues with a digit character, some delimiter must be used to
1650    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1651    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1652  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1653  .\" </a>  .\" </a>
1654  "Comments"  "Comments"
1655  .\"  .\"
1656  below) can be used.  below) can be used.
1657  .P  .
1658    .SS "Recursive back references"
1659    .rs
1660    .sp
1661  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
1662  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.
1663  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1571  to the previous iteration. In order for Line 1671  to the previous iteration. In order for
1671  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
1672  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
1673  minimum of zero.  minimum of zero.
1674    .P
1675    Back references of this type cause the group that they reference to be treated
1676    as an
1677    .\" HTML <a href="#atomicgroup">
1678    .\" </a>
1679    atomic group.
1680    .\"
1681    Once the whole group has been matched, a subsequent matching failure cannot
1682    cause backtracking into the middle of the group.
1683  .  .
1684  .  .
1685  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1624  lookbehind assertion is needed to achiev Line 1733  lookbehind assertion is needed to achiev
1733  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
1734  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
1735  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.
1736    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1737    synonym for (?!).
1738  .  .
1739  .  .
1740  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1648  is permitted, but Line 1759  is permitted, but
1759  .sp  .sp
1760  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1761  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
1762  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
1763  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1764  .sp  .sp
1765    (?<=ab(c|de))    (?<=ab(c|de))
1766  .sp  .sp
1767  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
1768  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
1769    branches:
1770  .sp  .sp
1771    (?<=abc|abde)    (?<=abc|abde)
1772  .sp  .sp
# Line 1663  In some cases, the Perl 5.10 escape sequ Line 1775  In some cases, the Perl 5.10 escape sequ
1775  .\" </a>  .\" </a>
1776  (see above)  (see above)
1777  .\"  .\"
1778  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
1779  fixed-length.  restriction.
1780  .P  .P
1781  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1782  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 1676  to appear in lookbehind assertions, beca Line 1788  to appear in lookbehind assertions, beca
1788  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
1789  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1790  .P  .P
1791    .\" HTML <a href="#subpatternsassubroutines">
1792    .\" </a>
1793    "Subroutine"
1794    .\"
1795    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1796    as the subpattern matches a fixed-length string.
1797    .\" HTML <a href="#recursion">
1798    .\" </a>
1799    Recursion,
1800    .\"
1801    however, is not supported.
1802    .P
1803  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1804  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
1805  pattern such as  strings. Consider a simple pattern such as
1806  .sp  .sp
1807    abcd$    abcd$
1808  .sp  .sp
# Line 1742  characters that are not "999". Line 1866  characters that are not "999".
1866  .sp  .sp
1867  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1868  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1869  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1870  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1871  .sp  .sp
1872    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1873    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
# Line 1759  recursion, a pseudo-condition called DEF Line 1883  recursion, a pseudo-condition called DEF
1883  .rs  .rs
1884  .sp  .sp
1885  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
1886  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
1887  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
1888  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
1889  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
1890  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
1891  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
1892    section about duplicate subpattern numbers),
1893    .\"
1894    the condition is true if any of them have been set. An alternative notation is
1895    to precede the digits with a plus or minus sign. In this case, the subpattern
1896    number is relative rather than absolute. The most recently opened parentheses
1897    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1898    looping constructs it can also make sense to refer to subsequent groups with
1899    constructs such as (?(+2).
1900  .P  .P
1901  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1902  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 1805  Rewriting the above example to use a nam Line 1937  Rewriting the above example to use a nam
1937  .sp  .sp
1938    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1939  .sp  .sp
1940    If the name used in a condition of this kind is a duplicate, the test is
1941    applied to all subpatterns of the same name, and is true if any one of them has
1942    matched.
1943  .  .
1944  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
1945  .rs  .rs
# Line 1816  letter R, for example: Line 1951  letter R, for example:
1951  .sp  .sp
1952    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
1953  .sp  .sp
1954  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
1955  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
1956  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
1957    applied to all subpatterns of the same name, and is true if any one of them is
1958    the most recent recursion.
1959  .P  .P
1960  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
1961  patterns are described below.  .\" HTML <a href="#recursion">
1962    .\" </a>
1963    The syntax for recursive patterns
1964    .\"
1965    is described below.
1966  .  .
1967  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
1968  .rs  .rs
# Line 1830  If the condition is the string (DEFINE), Line 1971  If the condition is the string (DEFINE),
1971  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
1972  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
1973  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
1974  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  "subroutines" that can be referenced from elsewhere. (The use of
1975    .\" HTML <a href="#subpatternsassubroutines">
1976    .\" </a>
1977    "subroutines"
1978    .\"
1979  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
1980  written like this (ignore whitespace and line breaks):  written like this (ignore whitespace and line breaks):
1981  .sp  .sp
# Line 1840  written like this (ignore whitespace and Line 1985  written like this (ignore whitespace and
1985  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
1986  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
1987  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
1988  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
1989  .P  pattern uses references to the named group to match the four dot-separated
1990  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.  
1991  .  .
1992  .SS "Assertion conditions"  .SS "Assertion conditions"
1993  .rs  .rs
# Line 1901  recursively to the pattern in which it a Line 2044  recursively to the pattern in which it a
2044  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2045  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2046  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2047  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.
2048  .P  .P
2049  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
2050  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,
2051  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
2052    .\" HTML <a href="#subpatternsassubroutines">
2053    .\" </a>
2054    "subroutine"
2055    .\"
2056  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
2057  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2058  .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  
2059  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2060  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2061  .sp  .sp
2062    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2063  .sp  .sp
2064  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2065  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
2066  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2067  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2068    to avoid backtracking into sequences of non-parentheses.
2069  .P  .P
2070  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
2071  pattern, so instead you could use this:  pattern, so instead you could use this:
2072  .sp  .sp
2073    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2074  .sp  .sp
2075  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
2076  them instead of the whole pattern.  them instead of the whole pattern.
2077  .P  .P
2078  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
2079  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).
2080  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
2081  most recently opened parentheses preceding the recursion. In other words, a  most recently opened parentheses preceding the recursion. In other words, a
2082  negative number counts capturing parentheses leftwards from the point at which  negative number counts capturing parentheses leftwards from the point at which
# Line 1942  it is encountered. Line 2085  it is encountered.
2085  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2086  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2087  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2088  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2089    .\" </a>
2090    "subroutine"
2091    .\"
2092    calls, as described in the next section.
2093  .P  .P
2094  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2095  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
2096  could rewrite the above example as follows:  could rewrite the above example as follows:
2097  .sp  .sp
2098    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2099  .sp  .sp
2100  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
2101  used.  used.
2102  .P  .P
2103  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2104  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
2105  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
2106  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2107  .sp  .sp
2108    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2109  .sp  .sp
2110  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,
2111  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
2112  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
2113  before failure can be reported.  before failure can be reported.
2114  .P  .P
2115  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
2116  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
2117  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  
2118  .\" HREF  .\" HREF
2119  \fBpcrecallout\fP  \fBpcrecallout\fP
2120  .\"  .\"
# Line 1976  documentation). If the pattern above is Line 2122  documentation). If the pattern above is
2122  .sp  .sp
2123    (ab(cd)ef)    (ab(cd)ef)
2124  .sp  .sp
2125  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
2126  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
2127  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2128    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2129       ^                        ^  .P
2130       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2131  .sp  obtain extra memory to store data during a recursion, which it does by using
2132  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
2133  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.  
2134  .P  .P
2135  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.
2136  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 2001  different alternatives for the recursive Line 2144  different alternatives for the recursive
2144  is the actual recursive call.  is the actual recursive call.
2145  .  .
2146  .  .
2147    .\" HTML <a name="recursiondifference"></a>
2148    .SS "Recursion difference from Perl"
2149    .rs
2150    .sp
2151    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2152    treated as an atomic group. That is, once it has matched some of the subject
2153    string, it is never re-entered, even if it contains untried alternatives and
2154    there is a subsequent matching failure. This can be illustrated by the
2155    following pattern, which purports to match a palindromic string that contains
2156    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2157    .sp
2158      ^(.|(.)(?1)\e2)$
2159    .sp
2160    The idea is that it either matches a single character, or two identical
2161    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2162    it does not if the pattern is longer than three characters. Consider the
2163    subject string "abcba":
2164    .P
2165    At the top level, the first character is matched, but as it is not at the end
2166    of the string, the first alternative fails; the second alternative is taken
2167    and the recursion kicks in. The recursive call to subpattern 1 successfully
2168    matches the next character ("b"). (Note that the beginning and end of line
2169    tests are not part of the recursion).
2170    .P
2171    Back at the top level, the next character ("c") is compared with what
2172    subpattern 2 matched, which was "a". This fails. Because the recursion is
2173    treated as an atomic group, there are now no backtracking points, and so the
2174    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2175    try the second alternative.) However, if the pattern is written with the
2176    alternatives in the other order, things are different:
2177    .sp
2178      ^((.)(?1)\e2|.)$
2179    .sp
2180    This time, the recursing alternative is tried first, and continues to recurse
2181    until it runs out of characters, at which point the recursion fails. But this
2182    time we do have another alternative to try at the higher level. That is the big
2183    difference: in the previous case the remaining alternative is at a deeper
2184    recursion level, which PCRE cannot use.
2185    .P
2186    To change the pattern so that matches all palindromic strings, not just those
2187    with an odd number of characters, it is tempting to change the pattern to this:
2188    .sp
2189      ^((.)(?1)\e2|.?)$
2190    .sp
2191    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2192    deeper recursion has matched a single character, it cannot be entered again in
2193    order to match an empty string. The solution is to separate the two cases, and
2194    write out the odd and even cases as alternatives at the higher level:
2195    .sp
2196      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2197    .sp
2198    If you want to match typical palindromic phrases, the pattern has to ignore all
2199    non-word characters, which can be done like this:
2200    .sp
2201      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2202    .sp
2203    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2204    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2205    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2206    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2207    more) to match typical phrases, and Perl takes so long that you think it has
2208    gone into a loop.
2209    .P
2210    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2211    string does not start with a palindrome that is shorter than the entire string.
2212    For example, although "abcba" is correctly matched, if the subject is "ababa",
2213    PCRE finds the palindrome "aba" at the start, then fails at top level because
2214    the end of the string does not follow. Once again, it cannot jump back into the
2215    recursion to try other alternatives, so the entire match fails.
2216    .
2217    .
2218  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2219  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2220  .rs  .rs
# Line 2027  matches "sense and sensibility" and "res Line 2241  matches "sense and sensibility" and "res
2241  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
2242  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2243  .P  .P
2244  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2245  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
2246  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
2247  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2248    call revert to their previous values afterwards.
2249  .P  .P
2250  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2251  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 2046  processing option does not affect the ca Line 2261  processing option does not affect the ca
2261  .SH "ONIGURUMA SUBROUTINE SYNTAX"  .SH "ONIGURUMA SUBROUTINE SYNTAX"
2262  .rs  .rs
2263  .sp  .sp
2264  For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or  For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2265  a number enclosed either in angle brackets or single quotes, is an alternative  a number enclosed either in angle brackets or single quotes, is an alternative
2266  syntax for referencing a subpattern as a subroutine, possibly recursively. Here  syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2267  are two of the examples used above, rewritten using this syntax:  are two of the examples used above, rewritten using this syntax:
2268  .sp  .sp
2269    (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )    (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2270    (sens|respons)e and \eg'1'ibility    (sens|respons)e and \eg'1'ibility
2271  .sp  .sp
2272  PCRE supports an extension to Oniguruma: if a number is preceded by a  PCRE supports an extension to Oniguruma: if a number is preceded by a
2273  plus or a minus sign it is taken as a relative reference. For example:  plus or a minus sign it is taken as a relative reference. For example:
2274  .sp  .sp
2275    (abc)(?i:\eg<-1>)    (abc)(?i:\eg<-1>)
2276  .sp  .sp
2277  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2278  synonymous. The former is a back reference; the latter is a subroutine call.  synonymous. The former is a back reference; the latter is a subroutine call.
2279  .  .
2280  .  .
# Line 2108  or removal in a future version of Perl". Line 2323  or removal in a future version of Perl".
2323  production code should be noted to avoid problems during upgrades." The same  production code should be noted to avoid problems during upgrades." The same
2324  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2325  .P  .P
2326  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
2327  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
2328  backtracking algorithm. They cause an error if encountered by  a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2329    failing negative assertion, they cause an error if encountered by
2330  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2331  .P  .P
2332    If any of these verbs are used in an assertion or subroutine subpattern
2333    (including recursive subpatterns), their effect is confined to that subpattern;
2334    it does not extend to the surrounding pattern. Note that such subpatterns are
2335    processed as anchored at the point where they are tested.
2336    .P
2337  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
2338  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. In Perl, they are generally of the form
2339  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB:ARG) but PCRE does not support the use of arguments, so its general
# Line 2128  The following verbs act as soon as they Line 2349  The following verbs act as soon as they
2349  .sp  .sp
2350  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
2351  pattern. When inside a recursion, only the innermost pattern is ended  pattern. When inside a recursion, only the innermost pattern is ended
2352  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
2353  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:  
2354  .sp  .sp
2355    A(A|B(*ACCEPT)|C)D    A((?:A|B(*ACCEPT)|C)D)
2356  .sp  .sp
2357  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
2358  captured.  the outer parentheses.
2359  .sp  .sp
2360    (*FAIL) or (*F)    (*FAIL) or (*F)
2361  .sp  .sp
# Line 2161  The verbs differ in exactly what kind of Line 2381  The verbs differ in exactly what kind of
2381  .sp  .sp
2382  This verb causes the whole match to fail outright if the rest of the pattern  This verb causes the whole match to fail outright if the rest of the pattern
2383  does not match. Even if the pattern is unanchored, no further attempts to find  does not match. Even if the pattern is unanchored, no further attempts to find
2384  a match by advancing the start point take place. Once (*COMMIT) has been  a match by advancing the starting point take place. Once (*COMMIT) has been
2385  passed, \fBpcre_exec()\fP is committed to finding a match at the current  passed, \fBpcre_exec()\fP is committed to finding a match at the current
2386  starting point, or not at all. For example:  starting point, or not at all. For example:
2387  .sp  .sp
# Line 2193  was matched leading up to it cannot be p Line 2413  was matched leading up to it cannot be p
2413  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
2414  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
2415  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
2416  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
2417  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2418  instead of skipping on to "c".  instead of skipping on to "c".
2419  .sp  .sp
# Line 2215  is used outside of any alternation, it a Line 2435  is used outside of any alternation, it a
2435  .SH "SEE ALSO"  .SH "SEE ALSO"
2436  .rs  .rs
2437  .sp  .sp
2438  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2439    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2440  .  .
2441  .  .
2442  .SH AUTHOR  .SH AUTHOR
# Line 2232  Cambridge CB2 3QH, England. Line 2453  Cambridge CB2 3QH, England.
2453  .rs  .rs
2454  .sp  .sp
2455  .nf  .nf
2456  Last updated: 10 April 2008  Last updated: 01 March 2010
2457  Copyright (c) 1997-2008 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2458  .fi  .fi

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