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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 182 by ph10, Wed Jun 13 15:09:54 2007 UTC
# Line 30  The remainder of this document discusses Line 30  The remainder of this document discusses
30  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
31  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
32  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
33  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
34  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
35    alternative function, and how it differs from the normal function, are
36    discussed in the
37  .\" HREF  .\" HREF
38  \fBpcrematching\fP  \fBpcrematching\fP
39  .\"  .\"
40  page.  page.
41  .P  .
42    .
43    .SH "CHARACTERS AND METACHARACTERS"
44    .rs
45    .sp
46  A regular expression is a pattern that is matched against a subject string from  A regular expression is a pattern that is matched against a subject string from
47  left to right. Most characters stand for themselves in a pattern, and match the  left to right. Most characters stand for themselves in a pattern, and match the
48  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
# Line 60  interpreted in some special way. Line 66  interpreted in some special way.
66  .P  .P
67  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
68  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
69  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
70  as follows:  are as follows:
71  .sp  .sp
72    \e      general escape character with several uses    \e      general escape character with several uses
73    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
# Line 92  a character class the only metacharacter Line 98  a character class the only metacharacter
98  .sp  .sp
99  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
100  .  .
101    .
102  .SH BACKSLASH  .SH BACKSLASH
103  .rs  .rs
104  .sp  .sp
# Line 190  parenthesized subpatterns. Line 197  parenthesized subpatterns.
197  .P  .P
198  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number is greater than 9 and there
199  have not been that many capturing subpatterns, PCRE re-reads up to three octal  have not been that many capturing subpatterns, PCRE re-reads up to three octal
200  digits following the backslash, ane uses them to generate a data character. Any  digits following the backslash, and uses them to generate a data character. Any
201  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
202  character specified in octal must be less than \e400. In UTF-8 mode, values up  character specified in octal must be less than \e400. In UTF-8 mode, values up
203  to \e777 are permitted. For example:  to \e777 are permitted. For example:
# Line 221  zero, because no more than three octal d Line 228  zero, because no more than three octal d
228  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
229  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
230  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08), and the
231  sequence \eX is interpreted as the character "X". Outside a character class,  sequences \eR and \eX are interpreted as the characters "R" and "X",
232  these sequences have different meanings  respectively. Outside a character class, these sequences have different
233    meanings
234  .\" HTML <a href="#uniextseq">  .\" HTML <a href="#uniextseq">
235  .\" </a>  .\" </a>
236  (see below).  (see below).
237  .\"  .\"
238  .  .
239  .  .
240    .SS "Absolute and relative back references"
241    .rs
242    .sp
243    The sequence \eg followed by a positive or negative number, optionally enclosed
244    in braces, is an absolute or relative back reference. A named back reference
245    can be coded as \eg{name}. Back references are discussed
246    .\" HTML <a href="#backreferences">
247    .\" </a>
248    later,
249    .\"
250    following the discussion of
251    .\" HTML <a href="#subpattern">
252    .\" </a>
253    parenthesized subpatterns.
254    .\"
255    .
256    .
257  .SS "Generic character types"  .SS "Generic character types"
258  .rs  .rs
259  .sp  .sp
260  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types. The
261  following are always recognized:  following are always recognized:
262  .sp  .sp
263    \ed     any decimal digit    \ed     any decimal digit
264    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
265      \eh     any horizontal whitespace character
266      \eH     any character that is not a horizontal whitespace character
267    \es     any whitespace character    \es     any whitespace character
268    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
269      \ev     any vertical whitespace character
270      \eV     any character that is not a vertical whitespace character
271    \ew     any "word" character    \ew     any "word" character
272    \eW     any "non-word" character    \eW     any "non-word" character
273  .sp  .sp
# Line 252  there is no character to match. Line 281  there is no character to match.
281  .P  .P
282  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
283  This makes it different from the the POSIX "space" class. The \es characters  This makes it different from the the POSIX "space" class. The \es characters
284  are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
285  included in a Perl script, \es may match the VT character. In PCRE, it never  included in a Perl script, \es may match the VT character. In PCRE, it never
286  does.)  does.
287    .P
288    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
289    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
290    character property support is available. These sequences retain their original
291    meanings from before UTF-8 support was available, mainly for efficiency
292    reasons.
293    .P
294    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
295    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
296    The horizontal space characters are:
297    .sp
298      U+0009     Horizontal tab
299      U+0020     Space
300      U+00A0     Non-break space
301      U+1680     Ogham space mark
302      U+180E     Mongolian vowel separator
303      U+2000     En quad
304      U+2001     Em quad
305      U+2002     En space
306      U+2003     Em space
307      U+2004     Three-per-em space
308      U+2005     Four-per-em space
309      U+2006     Six-per-em space
310      U+2007     Figure space
311      U+2008     Punctuation space
312      U+2009     Thin space
313      U+200A     Hair space
314      U+202F     Narrow no-break space
315      U+205F     Medium mathematical space
316      U+3000     Ideographic space
317    .sp
318    The vertical space characters are:
319    .sp
320      U+000A     Linefeed
321      U+000B     Vertical tab
322      U+000C     Formfeed
323      U+000D     Carriage return
324      U+0085     Next line
325      U+2028     Line separator
326      U+2029     Paragraph separator
327  .P  .P
328  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character less than 256 that is a
329  letter or digit. The definition of letters and digits is controlled by PCRE's  letter or digit. The definition of letters and digits is controlled by PCRE's
# Line 268  in the Line 337  in the
337  .\" HREF  .\" HREF
338  \fBpcreapi\fP  \fBpcreapi\fP
339  .\"  .\"
340  page). For example, in the "fr_FR" (French) locale, some character codes  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
341  greater than 128 are used for accented letters, and these are matched by \ew.  or "french" in Windows, some character codes greater than 128 are used for
342    accented letters, and these are matched by \ew. The use of locales with Unicode
343    is discouraged.
344    .
345    .
346    .SS "Newline sequences"
347    .rs
348    .sp
349    Outside a character class, the escape sequence \eR matches any Unicode newline
350    sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to
351    the following:
352    .sp
353      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
354    .sp
355    This is an example of an "atomic group", details of which are given
356    .\" HTML <a href="#atomicgroup">
357    .\" </a>
358    below.
359    .\"
360    This particular group matches either the two-character sequence CR followed by
361    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
362    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
363    line, U+0085). The two-character sequence is treated as a single unit that
364    cannot be split.
365    .P
366    In UTF-8 mode, two additional characters whose codepoints are greater than 255
367    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
368    Unicode character property support is not needed for these characters to be
369    recognized.
370  .P  .P
371  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  Inside a character class, \eR matches the letter "R".
 \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  
 character property support is available. The use of locales with Unicode is  
 discouraged.  
372  .  .
373  .  .
374  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 307  Those that are not part of an identified Line 401  Those that are not part of an identified
401  .P  .P
402  Arabic,  Arabic,
403  Armenian,  Armenian,
404    Balinese,
405  Bengali,  Bengali,
406  Bopomofo,  Bopomofo,
407  Braille,  Braille,
# Line 316  Canadian_Aboriginal, Line 411  Canadian_Aboriginal,
411  Cherokee,  Cherokee,
412  Common,  Common,
413  Coptic,  Coptic,
414    Cuneiform,
415  Cypriot,  Cypriot,
416  Cyrillic,  Cyrillic,
417  Deseret,  Deseret,
# Line 345  Malayalam, Line 441  Malayalam,
441  Mongolian,  Mongolian,
442  Myanmar,  Myanmar,
443  New_Tai_Lue,  New_Tai_Lue,
444    Nko,
445  Ogham,  Ogham,
446  Old_Italic,  Old_Italic,
447  Old_Persian,  Old_Persian,
448  Oriya,  Oriya,
449  Osmanya,  Osmanya,
450    Phags_Pa,
451    Phoenician,
452  Runic,  Runic,
453  Shavian,  Shavian,
454  Sinhala,  Sinhala,
# Line 462  why the traditional escape sequences suc Line 561  why the traditional escape sequences suc
561  properties in PCRE.  properties in PCRE.
562  .  .
563  .  .
564    .\" HTML <a name="resetmatchstart"></a>
565    .SS "Resetting the match start"
566    .rs
567    .sp
568    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
569    matched characters not to be included in the final matched sequence. For
570    example, the pattern:
571    .sp
572      foo\eKbar
573    .sp
574    matches "foobar", but reports that it has matched "bar". This feature is
575    similar to a lookbehind assertion
576    .\" HTML <a href="#lookbehind">
577    .\" </a>
578    (described below).
579    .\"
580    However, in this case, the part of the subject before the real match does not
581    have to be of fixed length, as lookbehind assertions do. The use of \eK does
582    not interfere with the setting of
583    .\" HTML <a href="#subpattern">
584    .\" </a>
585    captured substrings.
586    .\"
587    For example, when the pattern
588    .sp
589      (foo)\eKbar
590    .sp
591    matches "foobar", the first substring is still set to "foo".
592    .
593    .
594  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
595  .SS "Simple assertions"  .SS "Simple assertions"
596  .rs  .rs
597  .sp  .sp
598  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
599  specifies a condition that has to be met at a particular point in a match,  specifies a condition that has to be met at a particular point in a match,
600  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
601  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 478  The backslashed assertions are: Line 607  The backslashed assertions are:
607  .sp  .sp
608    \eb     matches at a word boundary    \eb     matches at a word boundary
609    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
610    \eA     matches at start of subject    \eA     matches at the start of the subject
611    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
612    \ez     matches at end of subject            also matches before a newline at the end of the subject
613    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
614      \eG     matches at the first matching position in the subject
615  .sp  .sp
616  These assertions may not appear in character classes (but note that \eb has a  These assertions may not appear in character classes (but note that \eb has a
617  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
# Line 578  end of the subject in both modes, and if Line 708  end of the subject in both modes, and if
708  .sp  .sp
709  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
710  the subject string except (by default) a character that signifies the end of a  the subject string except (by default) a character that signifies the end of a
711  line. In UTF-8 mode, the matched character may be more than one byte long. When  line. In UTF-8 mode, the matched character may be more than one byte long.
712  a line ending is defined as a single character (CR or LF), dot never matches  .P
713  that character; when the two-character sequence CRLF is used, dot does not  When a line ending is defined as a single character, dot never matches that
714  match CR if it is immediately followed by LF, but otherwise it matches all  character; when the two-character sequence CRLF is used, dot does not match CR
715  characters (including isolated CRs and LFs).  if it is immediately followed by LF, but otherwise it matches all characters
716    (including isolated CRs and LFs). When any Unicode line endings are being
717    recognized, dot does not match CR or LF or any of the other line ending
718    characters.
719  .P  .P
720  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
721  option is set, a dot matches any one character, without exception. If newline  option is set, a dot matches any one character, without exception. If the
722  is defined as the two-character sequence CRLF, it takes two dots to match it.  two-character sequence CRLF is present in the subject string, it takes two dots
723    to match it.
724  .P  .P
725  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
726  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
# Line 597  special meaning in a character class. Line 731  special meaning in a character class.
731  .rs  .rs
732  .sp  .sp
733  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
734  in and out of UTF-8 mode. Unlike a dot, it always matches CR and LF. The  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
735  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  characters. The feature is provided in Perl in order to match individual bytes
736  Because it breaks up UTF-8 characters into individual bytes, what remains in  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
737  the string may be a malformed UTF-8 string. For this reason, the \eC escape  what remains in the string may be a malformed UTF-8 string. For this reason,
738  sequence is best avoided.  the \eC escape sequence is best avoided.
739  .P  .P
740  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
741  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 652  If you want to use caseless matching for Line 786  If you want to use caseless matching for
786  ensure that PCRE is compiled with Unicode property support as well as with  ensure that PCRE is compiled with Unicode property support as well as with
787  UTF-8 support.  UTF-8 support.
788  .P  .P
789  Characters that might indicate line breaks (CR and LF) are never treated in any  Characters that might indicate line breaks are never treated in any special way
790  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
791  in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
792  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
793  .P  .P
794  The minus (hyphen) character can be used to specify a range of characters in a  The minus (hyphen) character can be used to specify a range of characters in a
795  character class. For example, [d-m] matches any letter between d and m,  character class. For example, [d-m] matches any letter between d and m,
# Line 679  example [\ex{100}-\ex{2ff}]. Line 813  example [\ex{100}-\ex{2ff}].
813  If a range that includes letters is used when caseless matching is set, it  If a range that includes letters is used when caseless matching is set, it
814  matches the letters in either case. For example, [W-c] is equivalent to  matches the letters in either case. For example, [W-c] is equivalent to
815  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
816  tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
817  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
818  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
819  property support.  property support.
# Line 790  If the change is placed right at the sta Line 924  If the change is placed right at the sta
924  the global options (and it will therefore show up in data extracted by the  the global options (and it will therefore show up in data extracted by the
925  \fBpcre_fullinfo()\fP function).  \fBpcre_fullinfo()\fP function).
926  .P  .P
927  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
928  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
929  .sp  .sp
930    (a(?i)b)c    (a(?i)b)c
931  .sp  .sp
# Line 824  Turning part of a pattern into a subpatt Line 958  Turning part of a pattern into a subpatt
958    cat(aract|erpillar|)    cat(aract|erpillar|)
959  .sp  .sp
960  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
961  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
962  .sp  .sp
963  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
964  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
# Line 849  the string "the white queen" is matched Line 983  the string "the white queen" is matched
983    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
984  .sp  .sp
985  the captured substrings are "white queen" and "queen", and are numbered 1 and  the captured substrings are "white queen" and "queen", and are numbered 1 and
986  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
 of nesting of all subpatterns, both capturing and non-capturing, is 200.  
987  .P  .P
988  As a convenient shorthand, if any option settings are required at the start of  As a convenient shorthand, if any option settings are required at the start of
989  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
# Line 865  is reached, an option setting in one bra Line 998  is reached, an option setting in one bra
998  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
999  .  .
1000  .  .
1001    .SH "DUPLICATE SUBPATTERN NUMBERS"
1002    .rs
1003    .sp
1004    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1005    the same numbers for its capturing parentheses. Such a subpattern starts with
1006    (?| and is itself a non-capturing subpattern. For example, consider this
1007    pattern:
1008    .sp
1009      (?|(Sat)ur|(Sun))day
1010    .sp
1011    Because the two alternatives are inside a (?| group, both sets of capturing
1012    parentheses are numbered one. Thus, when the pattern matches, you can look
1013    at captured substring number one, whichever alternative matched. This construct
1014    is useful when you want to capture part, but not all, of one of a number of
1015    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1016    number is reset at the start of each branch. The numbers of any capturing
1017    buffers that follow the subpattern start after the highest number used in any
1018    branch. The following example is taken from the Perl documentation.
1019    The numbers underneath show in which buffer the captured content will be
1020    stored.
1021    .sp
1022      # before  ---------------branch-reset----------- after
1023      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1024      # 1            2         2  3        2     3     4
1025    .sp
1026    A backreference or a recursive call to a numbered subpattern always refers to
1027    the first one in the pattern with the given number.
1028    .P
1029    An alternative approach to using this "branch reset" feature is to use
1030    duplicate named subpatterns, as described in the next section.
1031    .
1032    .
1033  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1034  .rs  .rs
1035  .sp  .sp
1036  Identifying capturing parentheses by number is simple, but it can be very hard  Identifying capturing parentheses by number is simple, but it can be very hard
1037  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1038  if an expression is modified, the numbers may change. To help with this  if an expression is modified, the numbers may change. To help with this
1039  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1040  not provide. The Python syntax (?P<name>...) is used. References to capturing  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1041    introduced it at release 4.0, using the Python syntax. PCRE now supports both
1042    the Perl and the Python syntax.
1043    .P
1044    In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1045    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1046  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1047  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1048  .\" </a>  .\" </a>
# Line 890  conditions, Line 1060  conditions,
1060  can be made by name as well as by number.  can be made by name as well as by number.
1061  .P  .P
1062  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
1063  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
1064  API provides function calls for extracting the name-to-number translation table  if the names were not present. The PCRE API provides function calls for
1065  from a compiled pattern. There is also a convenience function for extracting a  extracting the name-to-number translation table from a compiled pattern. There
1066  captured substring by name.  is also a convenience function for extracting a captured substring by name.
1067  .P  .P
1068  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
1069  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. This can
# Line 902  match. Suppose you want to match the nam Line 1072  match. Suppose you want to match the nam
1072  abbreviation or as the full name, and in both cases you want to extract the  abbreviation or as the full name, and in both cases you want to extract the
1073  abbreviation. This pattern (ignoring the line breaks) does the job:  abbreviation. This pattern (ignoring the line breaks) does the job:
1074  .sp  .sp
1075    (?P<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1076    (?P<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
1077    (?P<DN>Wed)(?:nesday)?|    (?<DN>Wed)(?:nesday)?|
1078    (?P<DN>Thu)(?:rsday)?|    (?<DN>Thu)(?:rsday)?|
1079    (?P<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1080  .sp  .sp
1081  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
1082    (An alternative way of solving this problem is to use a "branch reset"
1083    subpattern, as described in the previous section.)
1084    .P
1085  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1086  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
1087  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was. If you
1088  make a reference to a non-unique named subpattern from elsewhere in the  make a reference to a non-unique named subpattern from elsewhere in the
1089  pattern, the one that corresponds to the lowest number is used. For further  pattern, the one that corresponds to the lowest number is used. For further
# Line 928  Repetition is specified by quantifiers, Line 1101  Repetition is specified by quantifiers,
1101  items:  items:
1102  .sp  .sp
1103    a literal data character    a literal data character
1104    the . metacharacter    the dot metacharacter
1105    the \eC escape sequence    the \eC escape sequence
1106    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1107      the \eR escape sequence
1108    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1109    a character class    a character class
1110    a back reference (see next section)    a back reference (see next section)
# Line 968  which may be several bytes long (and the Line 1142  which may be several bytes long (and the
1142  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
1143  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1144  .P  .P
1145  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1146  quantifiers have single-character abbreviations:  abbreviations:
1147  .sp  .sp
1148    *    is equivalent to {0,}    *    is equivalent to {0,}
1149    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1017  own right. Because it has two uses, it c Line 1191  own right. Because it has two uses, it c
1191  which matches one digit by preference, but can match two if that is the only  which matches one digit by preference, but can match two if that is the only
1192  way the rest of the pattern matches.  way the rest of the pattern matches.
1193  .P  .P
1194  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1195  the quantifiers are not greedy by default, but individual ones can be made  the quantifiers are not greedy by default, but individual ones can be made
1196  greedy by following them with a question mark. In other words, it inverts the  greedy by following them with a question mark. In other words, it inverts the
1197  default behaviour.  default behaviour.
# Line 1027  is greater than 1 or with a limited maxi Line 1201  is greater than 1 or with a limited maxi
1201  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1202  .P  .P
1203  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1204  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1205  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1206  character position in the subject string, so there is no point in retrying the  character position in the subject string, so there is no point in retrying the
1207  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
# Line 1039  alternatively using ^ to indicate anchor Line 1213  alternatively using ^ to indicate anchor
1213  .P  .P
1214  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1215  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1216  elsewhere in the pattern, a match at the start may fail, and a later one  elsewhere in the pattern, a match at the start may fail where a later one
1217  succeed. Consider, for example:  succeeds. Consider, for example:
1218  .sp  .sp
1219    (.*)abc\e1    (.*)abc\e1
1220  .sp  .sp
# Line 1066  matches "aba" the value of the second ca Line 1240  matches "aba" the value of the second ca
1240  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1241  .rs  .rs
1242  .sp  .sp
1243  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1244  normally causes the repeated item to be re-evaluated to see if a different  repetition, failure of what follows normally causes the repeated item to be
1245  number of repeats allows the rest of the pattern to match. Sometimes it is  re-evaluated to see if a different number of repeats allows the rest of the
1246  useful to prevent this, either to change the nature of the match, or to cause  pattern to match. Sometimes it is useful to prevent this, either to change the
1247  it fail earlier than it otherwise might, when the author of the pattern knows  nature of the match, or to cause it fail earlier than it otherwise might, when
1248  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1249  .P  .P
1250  Consider, for example, the pattern \ed+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1251  .sp  .sp
# Line 1083  item, and then with 4, and so on, before Line 1257  item, and then with 4, and so on, before
1257  (a term taken from Jeffrey Friedl's book) provides the means for specifying  (a term taken from Jeffrey Friedl's book) provides the means for specifying
1258  that once a subpattern has matched, it is not to be re-evaluated in this way.  that once a subpattern has matched, it is not to be re-evaluated in this way.
1259  .P  .P
1260  If we use atomic grouping for the previous example, the matcher would give up  If we use atomic grouping for the previous example, the matcher gives up
1261  immediately on failing to match "foo" the first time. The notation is a kind of  immediately on failing to match "foo" the first time. The notation is a kind of
1262  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1263  .sp  .sp
# Line 1115  previous example can be rewritten as Line 1289  previous example can be rewritten as
1289  .sp  .sp
1290  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1291  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1292  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning of a possessive
1293  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1294  .P  difference; possessive quantifiers should be slightly faster.
1295  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  .P
1296  Friedl originated the idea (and the name) in the first edition of his book.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1297  Mike McCloskey liked it, so implemented it when he built Sun's Java package,  Jeffrey Friedl originated the idea (and the name) in the first edition of his
1298  and PCRE copied it from there.  book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1299    package, and PCRE copied it from there. It ultimately found its way into Perl
1300    at release 5.10.
1301    .P
1302    PCRE has an optimization that automatically "possessifies" certain simple
1303    pattern constructs. For example, the sequence A+B is treated as A++B because
1304    there is no point in backtracking into a sequence of A's when B must follow.
1305  .P  .P
1306  When a pattern contains an unlimited repeat inside a subpattern that can itself  When a pattern contains an unlimited repeat inside a subpattern that can itself
1307  be repeated an unlimited number of times, the use of an atomic group is the  be repeated an unlimited number of times, the use of an atomic group is the
# Line 1167  numbers less than 10. A "forward back re Line 1347  numbers less than 10. A "forward back re
1347  when a repetition is involved and the subpattern to the right has participated  when a repetition is involved and the subpattern to the right has participated
1348  in an earlier iteration.  in an earlier iteration.
1349  .P  .P
1350  It is not possible to have a numerical "forward back reference" to subpattern  It is not possible to have a numerical "forward back reference" to a subpattern
1351  whose number is 10 or more. However, a back reference to any subpattern is  whose number is 10 or more using this syntax because a sequence such as \e50 is
1352  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1353  "Non-printing characters"  "Non-printing characters"
1354  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1355  .\" </a>  .\" </a>
1356  above  above
1357  .\"  .\"
1358  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1359    no such problem when named parentheses are used. A back reference to any
1360    subpattern is possible using named parentheses (see below).
1361    .P
1362    Another way of avoiding the ambiguity inherent in the use of digits following a
1363    backslash is to use the \eg escape sequence, which is a feature introduced in
1364    Perl 5.10. This escape must be followed by a positive or a negative number,
1365    optionally enclosed in braces. These examples are all identical:
1366    .sp
1367      (ring), \e1
1368      (ring), \eg1
1369      (ring), \eg{1}
1370    .sp
1371    A positive number specifies an absolute reference without the ambiguity that is
1372    present in the older syntax. It is also useful when literal digits follow the
1373    reference. A negative number is a relative reference. Consider this example:
1374    .sp
1375      (abc(def)ghi)\eg{-1}
1376    .sp
1377    The sequence \eg{-1} is a reference to the most recently started capturing
1378    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1379    would be equivalent to \e1. The use of relative references can be helpful in
1380    long patterns, and also in patterns that are created by joining together
1381    fragments that contain references within themselves.
1382  .P  .P
1383  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1384  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1197  back reference, the case of letters is r Line 1400  back reference, the case of letters is r
1400  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1401  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1402  .P  .P
1403  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1404  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1405    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1406    back reference syntax, in which \eg can be used for both numeric and named
1407    references, is also supported. We could rewrite the above example in any of
1408    the following ways:
1409  .sp  .sp
1410      (?<p1>(?i)rah)\es+\ek<p1>
1411      (?'p1'(?i)rah)\es+\ek{p1}
1412    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1413      (?<p1>(?i)rah)\es+\eg{p1}
1414  .sp  .sp
1415  A subpattern that is referenced by name may appear in the pattern before or  A subpattern that is referenced by name may appear in the pattern before or
1416  after the reference.  after the reference.
# Line 1323  lengths, but it is acceptable if rewritt Line 1533  lengths, but it is acceptable if rewritt
1533  .sp  .sp
1534    (?<=abc|abde)    (?<=abc|abde)
1535  .sp  .sp
1536    In some cases, the Perl 5.10 escape sequence \eK
1537    .\" HTML <a href="#resetmatchstart">
1538    .\" </a>
1539    (see above)
1540    .\"
1541    can be used instead of a lookbehind assertion; this is not restricted to a
1542    fixed-length.
1543    .P
1544  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1545  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed length and then try to
1546  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1547  match is deemed to fail.  assertion fails.
1548  .P  .P
1549  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1550  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1551  the length of the lookbehind. The \eX escape, which can match different numbers  the length of the lookbehind. The \eX and \eR escapes, which can match
1552  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1553  .P  .P
1554  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1555  efficient matching at the end of the subject string. Consider a simple pattern  specify efficient matching at the end of the subject string. Consider a simple
1556  such as  pattern such as
1557  .sp  .sp
1558    abcd$    abcd$
1559  .sp  .sp
# Line 1351  then all but the last two characters, an Line 1569  then all but the last two characters, an
1569  covers the entire string, from right to left, so we are no better off. However,  covers the entire string, from right to left, so we are no better off. However,
1570  if the pattern is written as  if the pattern is written as
1571  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1572    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1573  .sp  .sp
1574  there can be no backtracking for the .* item; it can match only the entire  there can be no backtracking for the .*+ item; it can match only the entire
1575  string. The subsequent lookbehind assertion does a single test on the last four  string. The subsequent lookbehind assertion does a single test on the last four
1576  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1577  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1413  If the condition is satisfied, the yes-p Line 1627  If the condition is satisfied, the yes-p
1627  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
1628  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1629  .P  .P
1630  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1631  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
1632  underscores, the condition is satisfied if the capturing subpattern of that  .
1633  number or name has previously matched. There is a possible ambiguity here,  .SS "Checking for a used subpattern by number"
1634  because subpattern names may consist entirely of digits. PCRE looks first for a  .rs
1635  named subpattern; if it cannot find one and the text consists entirely of  .sp
1636  digits, it looks for a subpattern of that number, which must be greater than  If the text between the parentheses consists of a sequence of digits, the
1637  zero. Using subpattern names that consist entirely of digits is not  condition is true if the capturing subpattern of that number has previously
1638  recommended.  matched. An alternative notation is to precede the digits with a plus or minus
1639    sign. In this case, the subpattern number is relative rather than absolute.
1640    The most recently opened parentheses can be referenced by (?(-1), the next most
1641    recent by (?(-2), and so on. In looping constructs it can also make sense to
1642    refer to subsequent groups with constructs such as (?(+2).
1643  .P  .P
1644  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1645  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 1437  or not. If they did, that is, if subject Line 1655  or not. If they did, that is, if subject
1655  the condition is true, and so the yes-pattern is executed and a closing  the condition is true, and so the yes-pattern is executed and a closing
1656  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1657  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1658  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
1659  named subpattern gives this:  .P
1660    If you were embedding this pattern in a larger one, you could use a relative
1661    reference:
1662  .sp  .sp
1663    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1664    .sp
1665    This makes the fragment independent of the parentheses in the larger pattern.
1666    .
1667    .SS "Checking for a used subpattern by name"
1668    .rs
1669    .sp
1670    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1671    subpattern by name. For compatibility with earlier versions of PCRE, which had
1672    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1673    there is a possible ambiguity with this syntax, because subpattern names may
1674    consist entirely of digits. PCRE looks first for a named subpattern; if it
1675    cannot find one and the name consists entirely of digits, PCRE looks for a
1676    subpattern of that number, which must be greater than zero. Using subpattern
1677    names that consist entirely of digits is not recommended.
1678    .P
1679    Rewriting the above example to use a named subpattern gives this:
1680    .sp
1681      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1682    .sp
1683    .
1684    .SS "Checking for pattern recursion"
1685    .rs
1686  .sp  .sp
1687  If the condition is the string (R), and there is no subpattern with the name R,  If the condition is the string (R), and there is no subpattern with the name R,
1688  the condition is satisfied if a recursive call to the pattern or subpattern has  the condition is true if a recursive call to the whole pattern or any
1689  been made. At "top level", the condition is false. This is a PCRE extension.  subpattern has been made. If digits or a name preceded by ampersand follow the
1690  Recursive patterns are described in the next section.  letter R, for example:
1691    .sp
1692      (?(R3)...) or (?(R&name)...)
1693    .sp
1694    the condition is true if the most recent recursion is into the subpattern whose
1695    number or name is given. This condition does not check the entire recursion
1696    stack.
1697    .P
1698    At "top level", all these recursion test conditions are false. Recursive
1699    patterns are described below.
1700    .
1701    .SS "Defining subpatterns for use by reference only"
1702    .rs
1703    .sp
1704    If the condition is the string (DEFINE), and there is no subpattern with the
1705    name DEFINE, the condition is always false. In this case, there may be only one
1706    alternative in the subpattern. It is always skipped if control reaches this
1707    point in the pattern; the idea of DEFINE is that it can be used to define
1708    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1709    is described below.) For example, a pattern to match an IPv4 address could be
1710    written like this (ignore whitespace and line breaks):
1711    .sp
1712      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1713      \eb (?&byte) (\e.(?&byte)){3} \eb
1714    .sp
1715    The first part of the pattern is a DEFINE group inside which a another group
1716    named "byte" is defined. This matches an individual component of an IPv4
1717    address (a number less than 256). When matching takes place, this part of the
1718    pattern is skipped because DEFINE acts like a false condition.
1719  .P  .P
1720  If the condition is not a sequence of digits or (R), it must be an assertion.  The rest of the pattern uses references to the named group to match the four
1721    dot-separated components of an IPv4 address, insisting on a word boundary at
1722    each end.
1723    .
1724    .SS "Assertion conditions"
1725    .rs
1726    .sp
1727    If the condition is not in any of the above formats, it must be an assertion.
1728  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1729  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1730  alternatives on the second line:  alternatives on the second line:
# Line 1483  next newline in the pattern. Line 1760  next newline in the pattern.
1760  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1761  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1762  be done is to use a pattern that matches up to some fixed depth of nesting. It  be done is to use a pattern that matches up to some fixed depth of nesting. It
1763  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
1764  that allows regular expressions to recurse (amongst other things). It does this  .P
1765  by interpolating Perl code in the expression at run time, and the code can  For some time, Perl has provided a facility that allows regular expressions to
1766  refer to the expression itself. A Perl pattern to solve the parentheses problem  recurse (amongst other things). It does this by interpolating Perl code in the
1767  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1768    pattern using code interpolation to solve the parentheses problem can be
1769    created like this:
1770  .sp  .sp
1771    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1772  .sp  .sp
1773  The (?p{...}) item interpolates Perl code at run time, and in this case refers  The (?p{...}) item interpolates Perl code at run time, and in this case refers
1774  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1775  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1776  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1777  .P  supports special syntax for recursion of the entire pattern, and also for
1778  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1779  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was introduced into Perl at release 5.10.
1780  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1781  "subroutine" call, which is described in the next section.) The special item  A special item that consists of (? followed by a number greater than zero and a
1782  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1783  .P  provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1784  A recursive subpattern call is always treated as an atomic group. That is, once  call, which is described in the next section.) The special item (?R) or (?0) is
1785  it has matched some of the subject string, it is never re-entered, even if  a recursive call of the entire regular expression.
1786  it contains untried alternatives and there is a subsequent matching failure.  .P
1787    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1788    treated as an atomic group. That is, once it has matched some of the subject
1789    string, it is never re-entered, even if it contains untried alternatives and
1790    there is a subsequent matching failure.
1791  .P  .P
1792  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
1793  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
# Line 1522  pattern, so instead you could use this: Line 1805  pattern, so instead you could use this:
1805    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1806  .sp  .sp
1807  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
1808  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1809  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1810  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1811  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references. (A Perl 5.10 feature.)
1812  named parentheses). We could rewrite the above example as follows:  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1813  .sp  most recently opened parentheses preceding the recursion. In other words, a
1814    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
1815  .sp  it is encountered.
1816  This particular example pattern contains nested unlimited repeats, and so the  .P
1817  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1818  when applying the pattern to strings that do not match. For example, when this  references such as (?+2). However, these cannot be recursive because the
1819  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1820    "subroutine" calls, as described in the next section.
1821    .P
1822    An alternative approach is to use named parentheses instead. The Perl syntax
1823    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1824    could rewrite the above example as follows:
1825    .sp
1826      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1827    .sp
1828    If there is more than one subpattern with the same name, the earliest one is
1829    used.
1830    .P
1831    This particular example pattern that we have been looking at contains nested
1832    unlimited repeats, and so the use of atomic grouping for matching strings of
1833    non-parentheses is important when applying the pattern to strings that do not
1834    match. For example, when this pattern is applied to
1835  .sp  .sp
1836    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1837  .sp  .sp
# Line 1545  before failure can be reported. Line 1843  before failure can be reported.
1843  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values set for any capturing subpatterns are those
1844  from the outermost level of the recursion at which the subpattern value is set.  from the outermost level of the recursion at which the subpattern value is set.
1845  If you want to obtain intermediate values, a callout function can be used (see  If you want to obtain intermediate values, a callout function can be used (see
1846  the next section and the  below and the
1847  .\" HREF  .\" HREF
1848  \fBpcrecallout\fP  \fBpcrecallout\fP
1849  .\"  .\"
# Line 1584  is the actual recursive call. Line 1882  is the actual recursive call.
1882  .sp  .sp
1883  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern reference (either by number or by
1884  name) is used outside the parentheses to which it refers, it operates like a  name) is used outside the parentheses to which it refers, it operates like a
1885  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1886  pattern  before or after the reference. A numbered reference can be absolute or
1887    relative, as in these examples:
1888    .sp
1889      (...(absolute)...)...(?2)...
1890      (...(relative)...)...(?-1)...
1891      (...(?+1)...(relative)...
1892    .sp
1893    An earlier example pointed out that the pattern
1894  .sp  .sp
1895    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
1896  .sp  .sp
# Line 1595  matches "sense and sensibility" and "res Line 1900  matches "sense and sensibility" and "res
1900    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1901  .sp  .sp
1902  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
1903  strings. Such references, if given numerically, must follow the subpattern to  strings. Another example is given in the discussion of DEFINE above.
 which they refer. However, named references can refer to later subpatterns.  
1904  .P  .P
1905  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1906  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
1907  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
1908  matching failure.  matching failure.
1909    .P
1910    When a subpattern is used as a subroutine, processing options such as
1911    case-independence are fixed when the subpattern is defined. They cannot be
1912    changed for different calls. For example, consider this pattern:
1913    .sp
1914      (abc)(?i:(?-1))
1915    .sp
1916    It matches "abcabc". It does not match "abcABC" because the change of
1917    processing option does not affect the called subpattern.
1918  .  .
1919  .  .
1920  .SH CALLOUTS  .SH CALLOUTS
# Line 1622  function is to be called. If you want to Line 1935  function is to be called. If you want to
1935  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
1936  For example, this pattern has two callout points:  For example, this pattern has two callout points:
1937  .sp  .sp
1938    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
1939  .sp  .sp
1940  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
1941  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
# Line 1638  description of the interface to the call Line 1951  description of the interface to the call
1951  \fBpcrecallout\fP  \fBpcrecallout\fP
1952  .\"  .\"
1953  documentation.  documentation.
1954  .P  .
1955  .in 0  .
1956  Last updated: 06 June 2006  .SH "SEE ALSO"
1957  .br  .rs
1958  Copyright (c) 1997-2006 University of Cambridge.  .sp
1959    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
1960    .
1961    .
1962    .SH AUTHOR
1963    .rs
1964    .sp
1965    .nf
1966    Philip Hazel
1967    University Computing Service
1968    Cambridge CB2 3QH, England.
1969    .fi
1970    .
1971    .
1972    .SH REVISION
1973    .rs
1974    .sp
1975    .nf
1976    Last updated: 13 June 2007
1977    Copyright (c) 1997-2007 University of Cambridge.
1978    .fi

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