/[pcre]/code/trunk/doc/pcrepattern.3
ViewVC logotype

Diff of /code/trunk/doc/pcrepattern.3

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

revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 514 by ph10, Mon May 3 12:54:22 2010 UTC
# Line 4  PCRE - Perl-compatible regular expressio Line 4  PCRE - Perl-compatible regular expressio
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18    regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    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 30  The remainder of this document discusses Line 46  The remainder of this document discusses
46  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
47  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
48  \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
49  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
50  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
51    alternative function, and how it differs from the normal function, are
52    discussed in the
53  .\" HREF  .\" HREF
54  \fBpcrematching\fP  \fBpcrematching\fP
55  .\"  .\"
56  page.  page.
57    .
58    .
59    .SH "NEWLINE CONVENTIONS"
60    .rs
61    .sp
62    PCRE supports five different conventions for indicating line breaks in
63    strings: a single CR (carriage return) character, a single LF (linefeed)
64    character, the two-character sequence CRLF, any of the three preceding, or any
65    Unicode newline sequence. The
66    .\" HREF
67    \fBpcreapi\fP
68    .\"
69    page has
70    .\" HTML <a href="pcreapi.html#newlines">
71    .\" </a>
72    further discussion
73    .\"
74    about newlines, and shows how to set the newline convention in the
75    \fIoptions\fP arguments for the compiling and matching functions.
76  .P  .P
77    It is also possible to specify a newline convention by starting a pattern
78    string with one of the following five sequences:
79    .sp
80      (*CR)        carriage return
81      (*LF)        linefeed
82      (*CRLF)      carriage return, followed by linefeed
83      (*ANYCRLF)   any of the three above
84      (*ANY)       all Unicode newline sequences
85    .sp
86    These override the default and the options given to \fBpcre_compile()\fP or
87    \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
88    newline sequence, the pattern
89    .sp
90      (*CR)a.b
91    .sp
92    changes the convention to CR. That pattern matches "a\enb" because LF is no
93    longer a newline. Note that these special settings, which are not
94    Perl-compatible, are recognized only at the very start of a pattern, and that
95    they must be in upper case. If more than one of them is present, the last one
96    is used.
97    .P
98    The newline convention affects the interpretation of the dot metacharacter when
99    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
100    affect what the \eR escape sequence matches. By default, this is any Unicode
101    newline sequence, for Perl compatibility. However, this can be changed; see the
102    description of \eR in the section entitled
103    .\" HTML <a href="#newlineseq">
104    .\" </a>
105    "Newline sequences"
106    .\"
107    below. A change of \eR setting can be combined with a change of newline
108    convention.
109    .
110    .
111    .SH "CHARACTERS AND METACHARACTERS"
112    .rs
113    .sp
114  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
115  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
116  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 134  interpreted in some special way.
134  .P  .P
135  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
136  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
137  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
138  as follows:  are as follows:
139  .sp  .sp
140    \e      general escape character with several uses    \e      general escape character with several uses
141    ^      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 166  a character class the only metacharacter
166  .sp  .sp
167  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
168  .  .
169    .
170  .SH BACKSLASH  .SH BACKSLASH
171  .rs  .rs
172  .sp  .sp
# Line 134  The \eQ...\eE sequence is recognized bot Line 209  The \eQ...\eE sequence is recognized bot
209  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
210  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
211  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
212  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
213  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:  
214  .sp  .sp
215    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
216    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
217    \ee        escape (hex 1B)    \ee        escape (hex 1B)
218    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
219    \en        newline (hex 0A)    \en        linefeed (hex 0A)
220    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
221    \et        tab (hex 09)    \et        tab (hex 09)
222    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
223    \exhh      character with hex code hh    \exhh      character with hex code hh
224    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
225  .sp  .sp
# Line 157  Thus \ecz becomes hex 1A, but \ec{ becom Line 231  Thus \ecz becomes hex 1A, but \ec{ becom
231  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
232  upper or lower case). Any number of hexadecimal digits may appear between \ex{  upper or lower case). Any number of hexadecimal digits may appear between \ex{
233  and }, but the value of the character code must be less than 256 in non-UTF-8  and }, but the value of the character code must be less than 256 in non-UTF-8
234  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
235  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
236  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
237  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
238  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
239    there is no terminating }, this form of escape is not recognized. Instead, the
240    initial \ex will be interpreted as a basic hexadecimal escape, with no
241    following digits, giving a character whose value is zero.
242  .P  .P
243  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
244  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex. There is no difference in the way they are handled. For
# Line 190  parenthesized subpatterns. Line 267  parenthesized subpatterns.
267  .P  .P
268  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
269  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
270  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
271  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
272  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
273  to \e777 are permitted. For example:  to \e777 are permitted. For example:
# Line 220  zero, because no more than three octal d Line 297  zero, because no more than three octal d
297  .P  .P
298  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
299  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
300  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
301  sequence \eX is interpreted as the character "X". Outside a character class,  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
302  these sequences have different meanings  unrecognized escape sequences, they are treated as the literal characters "B",
303  .\" HTML <a href="#uniextseq">  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
304    set. Outside a character class, these sequences have different meanings.
305    .
306    .
307    .SS "Absolute and relative back references"
308    .rs
309    .sp
310    The sequence \eg followed by an unsigned or a negative number, optionally
311    enclosed in braces, is an absolute or relative back reference. A named back
312    reference can be coded as \eg{name}. Back references are discussed
313    .\" HTML <a href="#backreferences">
314  .\" </a>  .\" </a>
315  (see below).  later,
316    .\"
317    following the discussion of
318    .\" HTML <a href="#subpattern">
319    .\" </a>
320    parenthesized subpatterns.
321    .\"
322    .
323    .
324    .SS "Absolute and relative subroutine calls"
325    .rs
326    .sp
327    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
328    a number enclosed either in angle brackets or single quotes, is an alternative
329    syntax for referencing a subpattern as a "subroutine". Details are discussed
330    .\" HTML <a href="#onigurumasubroutines">
331    .\" </a>
332    later.
333  .\"  .\"
334    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
335    synonymous. The former is a back reference; the latter is a
336    .\" HTML <a href="#subpatternsassubroutines">
337    .\" </a>
338    subroutine
339    .\"
340    call.
341  .  .
342  .  .
343  .SS "Generic character types"  .SS "Generic character types"
344  .rs  .rs
345  .sp  .sp
346  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
347  .sp  .sp
348    \ed     any decimal digit    \ed     any decimal digit
349    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
350      \eh     any horizontal whitespace character
351      \eH     any character that is not a horizontal whitespace character
352    \es     any whitespace character    \es     any whitespace character
353    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
354      \ev     any vertical whitespace character
355      \eV     any character that is not a vertical whitespace character
356    \ew     any "word" character    \ew     any "word" character
357    \eW     any "non-word" character    \eW     any "non-word" character
358  .sp  .sp
359  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
360  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
361    .\" HTML <a href="#fullstopdot">
362    .\" </a>
363    the "." metacharacter
364    .\"
365    when PCRE_DOTALL is not set.
366    .P
367    Each pair of lower and upper case escape sequences partitions the complete set
368    of characters into two disjoint sets. Any given character matches one, and only
369    one, of each pair.
370  .P  .P
371  These character type sequences can appear both inside and outside character  These character type sequences can appear both inside and outside character
372  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
# Line 252  there is no character to match. Line 375  there is no character to match.
375  .P  .P
376  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).
377  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
378  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
379  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
380  does.)  does.
381    .P
382    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
383    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
384    character property support is available. These sequences retain their original
385    meanings from before UTF-8 support was available, mainly for efficiency
386    reasons. Note that this also affects \eb, because it is defined in terms of \ew
387    and \eW.
388    .P
389    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
390    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
391    The horizontal space characters are:
392    .sp
393      U+0009     Horizontal tab
394      U+0020     Space
395      U+00A0     Non-break space
396      U+1680     Ogham space mark
397      U+180E     Mongolian vowel separator
398      U+2000     En quad
399      U+2001     Em quad
400      U+2002     En space
401      U+2003     Em space
402      U+2004     Three-per-em space
403      U+2005     Four-per-em space
404      U+2006     Six-per-em space
405      U+2007     Figure space
406      U+2008     Punctuation space
407      U+2009     Thin space
408      U+200A     Hair space
409      U+202F     Narrow no-break space
410      U+205F     Medium mathematical space
411      U+3000     Ideographic space
412    .sp
413    The vertical space characters are:
414    .sp
415      U+000A     Linefeed
416      U+000B     Vertical tab
417      U+000C     Formfeed
418      U+000D     Carriage return
419      U+0085     Next line
420      U+2028     Line separator
421      U+2029     Paragraph separator
422  .P  .P
423  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
424  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 432  in the
432  .\" HREF  .\" HREF
433  \fBpcreapi\fP  \fBpcreapi\fP
434  .\"  .\"
435  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,
436  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
437  .P  accented letters, and these are matched by \ew. The use of locales with Unicode
438  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  is discouraged.
439  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .
440  character property support is available. The use of locales with Unicode is  .
441  discouraged.  .\" HTML <a name="newlineseq"></a>
442    .SS "Newline sequences"
443    .rs
444    .sp
445    Outside a character class, by default, the escape sequence \eR matches any
446    Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
447    equivalent to the following:
448    .sp
449      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
450    .sp
451    This is an example of an "atomic group", details of which are given
452    .\" HTML <a href="#atomicgroup">
453    .\" </a>
454    below.
455    .\"
456    This particular group matches either the two-character sequence CR followed by
457    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
458    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
459    line, U+0085). The two-character sequence is treated as a single unit that
460    cannot be split.
461    .P
462    In UTF-8 mode, two additional characters whose codepoints are greater than 255
463    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
464    Unicode character property support is not needed for these characters to be
465    recognized.
466    .P
467    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
468    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
469    either at compile time or when the pattern is matched. (BSR is an abbrevation
470    for "backslash R".) This can be made the default when PCRE is built; if this is
471    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
472    It is also possible to specify these settings by starting a pattern string with
473    one of the following sequences:
474    .sp
475      (*BSR_ANYCRLF)   CR, LF, or CRLF only
476      (*BSR_UNICODE)   any Unicode newline sequence
477    .sp
478    These override the default and the options given to \fBpcre_compile()\fP or
479    \fBpcre_compile2()\fP, but they can be overridden by options given to
480    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
481    which are not Perl-compatible, are recognized only at the very start of a
482    pattern, and that they must be in upper case. If more than one of them is
483    present, the last one is used. They can be combined with a change of newline
484    convention, for example, a pattern can start with:
485    .sp
486      (*ANY)(*BSR_ANYCRLF)
487    .sp
488    Inside a character class, \eR is treated as an unrecognized escape sequence,
489    and so matches the letter "R" by default, but causes an error if PCRE_EXTRA is
490    set.
491  .  .
492  .  .
493  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 282  discouraged. Line 495  discouraged.
495  .rs  .rs
496  .sp  .sp
497  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
498  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
499  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
500    characters whose codepoints are less than 256, but they do work in this mode.
501    The extra escape sequences are:
502  .sp  .sp
503    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
504    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
# Line 307  Those that are not part of an identified Line 522  Those that are not part of an identified
522  .P  .P
523  Arabic,  Arabic,
524  Armenian,  Armenian,
525    Avestan,
526    Balinese,
527    Bamum,
528  Bengali,  Bengali,
529  Bopomofo,  Bopomofo,
530  Braille,  Braille,
531  Buginese,  Buginese,
532  Buhid,  Buhid,
533  Canadian_Aboriginal,  Canadian_Aboriginal,
534    Carian,
535    Cham,
536  Cherokee,  Cherokee,
537  Common,  Common,
538  Coptic,  Coptic,
539    Cuneiform,
540  Cypriot,  Cypriot,
541  Cyrillic,  Cyrillic,
542  Deseret,  Deseret,
543  Devanagari,  Devanagari,
544    Egyptian_Hieroglyphs,
545  Ethiopic,  Ethiopic,
546  Georgian,  Georgian,
547  Glagolitic,  Glagolitic,
# Line 332  Hangul, Line 554  Hangul,
554  Hanunoo,  Hanunoo,
555  Hebrew,  Hebrew,
556  Hiragana,  Hiragana,
557    Imperial_Aramaic,
558  Inherited,  Inherited,
559    Inscriptional_Pahlavi,
560    Inscriptional_Parthian,
561    Javanese,
562    Kaithi,
563  Kannada,  Kannada,
564  Katakana,  Katakana,
565    Kayah_Li,
566  Kharoshthi,  Kharoshthi,
567  Khmer,  Khmer,
568  Lao,  Lao,
569  Latin,  Latin,
570    Lepcha,
571  Limbu,  Limbu,
572  Linear_B,  Linear_B,
573    Lisu,
574    Lycian,
575    Lydian,
576  Malayalam,  Malayalam,
577    Meetei_Mayek,
578  Mongolian,  Mongolian,
579  Myanmar,  Myanmar,
580  New_Tai_Lue,  New_Tai_Lue,
581    Nko,
582  Ogham,  Ogham,
583  Old_Italic,  Old_Italic,
584  Old_Persian,  Old_Persian,
585    Old_South_Arabian,
586    Old_Turkic,
587    Ol_Chiki,
588  Oriya,  Oriya,
589  Osmanya,  Osmanya,
590    Phags_Pa,
591    Phoenician,
592    Rejang,
593  Runic,  Runic,
594    Samaritan,
595    Saurashtra,
596  Shavian,  Shavian,
597  Sinhala,  Sinhala,
598    Sundanese,
599  Syloti_Nagri,  Syloti_Nagri,
600  Syriac,  Syriac,
601  Tagalog,  Tagalog,
602  Tagbanwa,  Tagbanwa,
603  Tai_Le,  Tai_Le,
604    Tai_Tham,
605    Tai_Viet,
606  Tamil,  Tamil,
607  Telugu,  Telugu,
608  Thaana,  Thaana,
# Line 365  Thai, Line 610  Thai,
610  Tibetan,  Tibetan,
611  Tifinagh,  Tifinagh,
612  Ugaritic,  Ugaritic,
613    Vai,
614  Yi.  Yi.
615  .P  .P
616  Each character has exactly one general category property, specified by a  Each character has exactly one general category property, specified by a
# Line 430  The special property L& is also supporte Line 676  The special property L& is also supporte
676  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
677  a modifier or "other".  a modifier or "other".
678  .P  .P
679  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The Cs (Surrogate) property applies only to characters in the range U+D800 to
680    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
681    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
682    (see the discussion of PCRE_NO_UTF8_CHECK in the
683    .\" HREF
684    \fBpcreapi\fP
685    .\"
686    page). Perl does not support the Cs property.
687    .P
688    The long synonyms for property names that Perl supports (such as \ep{Letter})
689  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
690  properties with "Is".  properties with "Is".
691  .P  .P
# Line 454  atomic group Line 709  atomic group
709  (see below).  (see below).
710  .\"  .\"
711  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
712  preceding character.  preceding character. None of them have codepoints less than 256, so in
713    non-UTF-8 mode \eX matches any one character.
714  .P  .P
715  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
716  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
# Line 462  why the traditional escape sequences suc Line 718  why the traditional escape sequences suc
718  properties in PCRE.  properties in PCRE.
719  .  .
720  .  .
721    .\" HTML <a name="resetmatchstart"></a>
722    .SS "Resetting the match start"
723    .rs
724    .sp
725    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
726    matched characters not to be included in the final matched sequence. For
727    example, the pattern:
728    .sp
729      foo\eKbar
730    .sp
731    matches "foobar", but reports that it has matched "bar". This feature is
732    similar to a lookbehind assertion
733    .\" HTML <a href="#lookbehind">
734    .\" </a>
735    (described below).
736    .\"
737    However, in this case, the part of the subject before the real match does not
738    have to be of fixed length, as lookbehind assertions do. The use of \eK does
739    not interfere with the setting of
740    .\" HTML <a href="#subpattern">
741    .\" </a>
742    captured substrings.
743    .\"
744    For example, when the pattern
745    .sp
746      (foo)\eKbar
747    .sp
748    matches "foobar", the first substring is still set to "foo".
749    .P
750    Perl documents that the use of \eK within assertions is "not well defined". In
751    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
752    ignored in negative assertions.
753    .
754    .
755  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
756  .SS "Simple assertions"  .SS "Simple assertions"
757  .rs  .rs
758  .sp  .sp
759  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
760  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,
761  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
762  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 478  The backslashed assertions are: Line 768  The backslashed assertions are:
768  .sp  .sp
769    \eb     matches at a word boundary    \eb     matches at a word boundary
770    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
771    \eA     matches at start of subject    \eA     matches at the start of the subject
772    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
773    \ez     matches at end of subject            also matches before a newline at the end of the subject
774    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
775  .sp    \eG     matches at the first matching position in the subject
776  These assertions may not appear in character classes (but note that \eb has a  .sp
777  different meaning, namely the backspace character, inside a character class).  Inside a character class, \eb has a different meaning; it matches the backspace
778    character. If any other of these assertions appears in a character class, by
779    default it matches the corresponding literal character (for example, \eB
780    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
781    escape sequence" error is generated instead.
782  .P  .P
783  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
784  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
785  \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
786  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. Neither PCRE nor Perl has a
787    separte "start of word" or "end of word" metasequence. However, whatever
788    follows \eb normally determines which it is. For example, the fragment
789    \eba matches "a" at the start of a word.
790  .P  .P
791  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
792  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 573  end of the subject in both modes, and if Line 870  end of the subject in both modes, and if
870  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
871  .  .
872  .  .
873  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
874    .SH "FULL STOP (PERIOD, DOT) AND \eN"
875  .rs  .rs
876  .sp  .sp
877  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
878  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
879  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.
880  a line ending is defined as a single character (CR or LF), dot never matches  .P
881  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
882  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
883  characters (including isolated CRs and LFs).  if it is immediately followed by LF, but otherwise it matches all characters
884    (including isolated CRs and LFs). When any Unicode line endings are being
885    recognized, dot does not match CR or LF or any of the other line ending
886    characters.
887  .P  .P
888  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
889  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
890  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
891    to match it.
892  .P  .P
893  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
894  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
895  special meaning in a character class.  special meaning in a character class.
896    .P
897    The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not
898    set. In other words, it matches any one character except one that signifies the
899    end of a line.
900  .  .
901  .  .
902  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
903  .rs  .rs
904  .sp  .sp
905  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
906  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
907  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
908  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,
909  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,
910  sequence is best avoided.  the \eC escape sequence is best avoided.
911  .P  .P
912  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
913  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 617  the lookbehind. Line 923  the lookbehind.
923  .rs  .rs
924  .sp  .sp
925  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
926  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.
927  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
928  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
929  escaped with a backslash.  a member of the class, it should be the first data character in the class
930    (after an initial circumflex, if present) or escaped with a backslash.
931  .P  .P
932  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
933  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
934  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
935  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
936  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
937  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 634  For example, the character class [aeiou] Line 941  For example, the character class [aeiou]
941  [^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
942  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
943  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
944  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
945  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
946  string.  string.
947  .P  .P
# Line 648  caseful version would. In UTF-8 mode, PC Line 955  caseful version would. In UTF-8 mode, PC
955  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
956  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
957  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
958  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,
959  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
960  UTF-8 support.  with UTF-8 support.
961  .P  .P
962  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
963  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
964  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
965  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
966  .P  .P
967  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
968  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 986  example [\ex{100}-\ex{2ff}].
986  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
987  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
988  [][\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
989  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
990  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
991  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
992  property support.  property support.
# Line 769  alternative in the subpattern. Line 1076  alternative in the subpattern.
1076  .rs  .rs
1077  .sp  .sp
1078  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1079  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1080  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1081    The option letters are
1082  .sp  .sp
1083    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1084    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 784  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1092  PCRE_MULTILINE while unsetting PCRE_DOTA
1092  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
1093  unset.  unset.
1094  .P  .P
1095  When an option change occurs at top level (that is, not inside subpattern  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1096  parentheses), the change applies to the remainder of the pattern that follows.  changed in the same way as the Perl-compatible options by using the characters
1097  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
 the global options (and it will therefore show up in data extracted by the  
 \fBpcre_fullinfo()\fP function).  
1098  .P  .P
1099  An option change within a subpattern affects only that part of the current  When one of these option changes occurs at top level (that is, not inside
1100  pattern that follows it, so  subpattern parentheses), the change applies to the remainder of the pattern
1101    that follows. If the change is placed right at the start of a pattern, PCRE
1102    extracts it into the global options (and it will therefore show up in data
1103    extracted by the \fBpcre_fullinfo()\fP function).
1104    .P
1105    An option change within a subpattern (see below for a description of
1106    subpatterns) affects only that part of the current pattern that follows it, so
1107  .sp  .sp
1108    (a(?i)b)c    (a(?i)b)c
1109  .sp  .sp
# Line 807  branch is abandoned before the option se Line 1119  branch is abandoned before the option se
1119  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1120  behaviour otherwise.  behaviour otherwise.
1121  .P  .P
1122  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be  \fBNote:\fP There are other PCRE-specific options that can be set by the
1123  changed in the same way as the Perl-compatible options by using the characters  application when the compile or match functions are called. In some cases the
1124  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1125    the application has set or what has been defaulted. Details are given in the
1126    section entitled
1127    .\" HTML <a href="#newlineseq">
1128    .\" </a>
1129    "Newline sequences"
1130    .\"
1131    above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1132    mode; this is equivalent to setting the PCRE_UTF8 option.
1133  .  .
1134  .  .
1135  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 824  Turning part of a pattern into a subpatt Line 1144  Turning part of a pattern into a subpatt
1144    cat(aract|erpillar|)    cat(aract|erpillar|)
1145  .sp  .sp
1146  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1147  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1148  .sp  .sp
1149  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
1150  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 1169  the string "the white queen" is matched
1169    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1170  .sp  .sp
1171  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
1172  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.  
1173  .P  .P
1174  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
1175  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 1184  is reached, an option setting in one bra
1184  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1185  .  .
1186  .  .
1187    .\" HTML <a name="dupsubpatternnumber"></a>
1188    .SH "DUPLICATE SUBPATTERN NUMBERS"
1189    .rs
1190    .sp
1191    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1192    the same numbers for its capturing parentheses. Such a subpattern starts with
1193    (?| and is itself a non-capturing subpattern. For example, consider this
1194    pattern:
1195    .sp
1196      (?|(Sat)ur|(Sun))day
1197    .sp
1198    Because the two alternatives are inside a (?| group, both sets of capturing
1199    parentheses are numbered one. Thus, when the pattern matches, you can look
1200    at captured substring number one, whichever alternative matched. This construct
1201    is useful when you want to capture part, but not all, of one of a number of
1202    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1203    number is reset at the start of each branch. The numbers of any capturing
1204    buffers that follow the subpattern start after the highest number used in any
1205    branch. The following example is taken from the Perl documentation.
1206    The numbers underneath show in which buffer the captured content will be
1207    stored.
1208    .sp
1209      # before  ---------------branch-reset----------- after
1210      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1211      # 1            2         2  3        2     3     4
1212    .sp
1213    A back reference to a numbered subpattern uses the most recent value that is
1214    set for that number by any subpattern. The following pattern matches "abcabc"
1215    or "defdef":
1216    .sp
1217      /(?|(abc)|(def))\e1/
1218    .sp
1219    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1220    refers to the first one in the pattern with the given number. The following
1221    pattern matches "abcabc" or "defabc":
1222    .sp
1223      /(?|(abc)|(def))(?1)/
1224    .sp
1225    If a
1226    .\" HTML <a href="#conditions">
1227    .\" </a>
1228    condition test
1229    .\"
1230    for a subpattern's having matched refers to a non-unique number, the test is
1231    true if any of the subpatterns of that number have matched.
1232    .P
1233    An alternative approach to using this "branch reset" feature is to use
1234    duplicate named subpatterns, as described in the next section.
1235    .
1236    .
1237  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1238  .rs  .rs
1239  .sp  .sp
1240  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
1241  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1242  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
1243  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1244  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
1245    introduced it at release 4.0, using the Python syntax. PCRE now supports both
1246    the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1247    have different names, but PCRE does not.
1248    .P
1249    In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1250    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1251  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1252  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1253  .\" </a>  .\" </a>
1254  backreferences,  back references,
1255  .\"  .\"
1256  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1257  .\" </a>  .\" </a>
# Line 890  conditions, Line 1265  conditions,
1265  can be made by name as well as by number.  can be made by name as well as by number.
1266  .P  .P
1267  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
1268  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
1269  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
1270  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
1271  captured substring by name.  is also a convenience function for extracting a captured substring by name.
1272  .P  .P
1273  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
1274  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
1275  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
1276  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
1277  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
1278  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
1279  .sp  name, and in both cases you want to extract the abbreviation. This pattern
1280    (?P<DN>Mon|Fri|Sun)(?:day)?|  (ignoring the line breaks) does the job:
1281    (?P<DN>Tue)(?:sday)?|  .sp
1282    (?P<DN>Wed)(?:nesday)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1283    (?P<DN>Thu)(?:rsday)?|    (?<DN>Tue)(?:sday)?|
1284    (?P<DN>Sat)(?:urday)?    (?<DN>Wed)(?:nesday)?|
1285      (?<DN>Thu)(?:rsday)?|
1286      (?<DN>Sat)(?:urday)?
1287  .sp  .sp
1288  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.
1289    (An alternative way of solving this problem is to use a "branch reset"
1290    subpattern, as described in the previous section.)
1291    .P
1292  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1293  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
1294  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1295  make a reference to a non-unique named subpattern from elsewhere in the  .P
1296  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
1297  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1298    used. In the absence of duplicate numbers (see the previous section) this is
1299    the one with the lowest number. If you use a named reference in a condition
1300    test (see the
1301    .\"
1302    .\" HTML <a href="#conditions">
1303    .\" </a>
1304    section about conditions
1305    .\"
1306    below), either to check whether a subpattern has matched, or to check for
1307    recursion, all subpatterns with the same name are tested. If the condition is
1308    true for any one of them, the overall condition is true. This is the same
1309    behaviour as testing by number. For further details of the interfaces for
1310    handling named subpatterns, see the
1311  .\" HREF  .\" HREF
1312  \fBpcreapi\fP  \fBpcreapi\fP
1313  .\"  .\"
1314  documentation.  documentation.
1315    .P
1316    \fBWarning:\fP You cannot use different names to distinguish between two
1317    subpatterns with the same number because PCRE uses only the numbers when
1318    matching. For this reason, an error is given at compile time if different names
1319    are given to subpatterns with the same number. However, you can give the same
1320    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1321  .  .
1322  .  .
1323  .SH REPETITION  .SH REPETITION
# Line 928  Repetition is specified by quantifiers, Line 1327  Repetition is specified by quantifiers,
1327  items:  items:
1328  .sp  .sp
1329    a literal data character    a literal data character
1330    the . metacharacter    the dot metacharacter
1331    the \eC escape sequence    the \eC escape sequence
1332    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1333      the \eR escape sequence
1334    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1335    a character class    a character class
1336    a back reference (see next section)    a back reference (see next section)
1337    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1338      a recursive or "subroutine" call to a subpattern
1339  .sp  .sp
1340  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1341  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 966  support is available, \eX{3} matches thr Line 1367  support is available, \eX{3} matches thr
1367  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).
1368  .P  .P
1369  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
1370  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1371    subpatterns that are referenced as
1372    .\" HTML <a href="#subpatternsassubroutines">
1373    .\" </a>
1374    subroutines
1375    .\"
1376    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1377    quantifier are omitted from the compiled pattern.
1378  .P  .P
1379  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1380  quantifiers have single-character abbreviations:  abbreviations:
1381  .sp  .sp
1382    *    is equivalent to {0,}    *    is equivalent to {0,}
1383    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1017  own right. Because it has two uses, it c Line 1425  own right. Because it has two uses, it c
1425  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
1426  way the rest of the pattern matches.  way the rest of the pattern matches.
1427  .P  .P
1428  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),
1429  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
1430  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
1431  default behaviour.  default behaviour.
# Line 1027  is greater than 1 or with a limited maxi Line 1435  is greater than 1 or with a limited maxi
1435  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1436  .P  .P
1437  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
1438  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
1439  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1440  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
1441  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 1038  worth setting PCRE_DOTALL in order to ob Line 1446  worth setting PCRE_DOTALL in order to ob
1446  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1447  .P  .P
1448  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1449  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1450  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
1451  succeed. Consider, for example:  succeeds. Consider, for example:
1452  .sp  .sp
1453    (.*)abc\e1    (.*)abc\e1
1454  .sp  .sp
# Line 1066  matches "aba" the value of the second ca Line 1474  matches "aba" the value of the second ca
1474  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1475  .rs  .rs
1476  .sp  .sp
1477  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1478  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
1479  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
1480  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
1481  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
1482  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1483  .P  .P
1484  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
1485  .sp  .sp
# Line 1083  item, and then with 4, and so on, before Line 1491  item, and then with 4, and so on, before
1491  (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
1492  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.
1493  .P  .P
1494  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
1495  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
1496  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1497  .sp  .sp
# Line 1113  previous example can be rewritten as Line 1521  previous example can be rewritten as
1521  .sp  .sp
1522    \ed++foo    \ed++foo
1523  .sp  .sp
1524    Note that a possessive quantifier can be used with an entire group, for
1525    example:
1526    .sp
1527      (abc|xyz){2,3}+
1528    .sp
1529  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1530  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
1531  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
1532  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1533  .P  difference; possessive quantifiers should be slightly faster.
1534  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  .P
1535  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.
1536  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
1537  and PCRE copied it from there.  book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1538    package, and PCRE copied it from there. It ultimately found its way into Perl
1539    at release 5.10.
1540    .P
1541    PCRE has an optimization that automatically "possessifies" certain simple
1542    pattern constructs. For example, the sequence A+B is treated as A++B because
1543    there is no point in backtracking into a sequence of A's when B must follow.
1544  .P  .P
1545  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
1546  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 1586  numbers less than 10. A "forward back re
1586  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
1587  in an earlier iteration.  in an earlier iteration.
1588  .P  .P
1589  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
1590  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
1591  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1592  "Non-printing characters"  "Non-printing characters"
1593  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1594  .\" </a>  .\" </a>
1595  above  above
1596  .\"  .\"
1597  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1598    no such problem when named parentheses are used. A back reference to any
1599    subpattern is possible using named parentheses (see below).
1600    .P
1601    Another way of avoiding the ambiguity inherent in the use of digits following a
1602    backslash is to use the \eg escape sequence, which is a feature introduced in
1603    Perl 5.10. This escape must be followed by an unsigned number or a negative
1604    number, optionally enclosed in braces. These examples are all identical:
1605    .sp
1606      (ring), \e1
1607      (ring), \eg1
1608      (ring), \eg{1}
1609    .sp
1610    An unsigned number specifies an absolute reference without the ambiguity that
1611    is present in the older syntax. It is also useful when literal digits follow
1612    the reference. A negative number is a relative reference. Consider this
1613    example:
1614    .sp
1615      (abc(def)ghi)\eg{-1}
1616    .sp
1617    The sequence \eg{-1} is a reference to the most recently started capturing
1618    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1619    would be equivalent to \e1. The use of relative references can be helpful in
1620    long patterns, and also in patterns that are created by joining together
1621    fragments that contain references within themselves.
1622  .P  .P
1623  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1624  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 1640  back reference, the case of letters is r
1640  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
1641  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1642  .P  .P
1643  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1644  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1645    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1646    back reference syntax, in which \eg can be used for both numeric and named
1647    references, is also supported. We could rewrite the above example in any of
1648    the following ways:
1649  .sp  .sp
1650      (?<p1>(?i)rah)\es+\ek<p1>
1651      (?'p1'(?i)rah)\es+\ek{p1}
1652    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1653      (?<p1>(?i)rah)\es+\eg{p1}
1654  .sp  .sp
1655  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
1656  after the reference.  after the reference.
1657  .P  .P
1658  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
1659  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1660  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1661  .sp  .sp
1662    (a|(bc))\e2    (a|(bc))\e2
1663  .sp  .sp
1664  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
1665  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
1666  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1667  with a digit character, some delimiter must be used to terminate the back  .P
1668  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1669  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1670    If the pattern continues with a digit character, some delimiter must be used to
1671    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1672    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1673  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1674  .\" </a>  .\" </a>
1675  "Comments"  "Comments"
1676  .\"  .\"
1677  below) can be used.  below) can be used.
1678  .P  .
1679    .SS "Recursive back references"
1680    .rs
1681    .sp
1682  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
1683  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.
1684  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1236  to the previous iteration. In order for Line 1692  to the previous iteration. In order for
1692  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
1693  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
1694  minimum of zero.  minimum of zero.
1695    .P
1696    Back references of this type cause the group that they reference to be treated
1697    as an
1698    .\" HTML <a href="#atomicgroup">
1699    .\" </a>
1700    atomic group.
1701    .\"
1702    Once the whole group has been matched, a subsequent matching failure cannot
1703    cause backtracking into the middle of the group.
1704  .  .
1705  .  .
1706  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1289  lookbehind assertion is needed to achiev Line 1754  lookbehind assertion is needed to achiev
1754  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
1755  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
1756  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.
1757    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1758    synonym for (?!).
1759  .  .
1760  .  .
1761  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1313  is permitted, but Line 1780  is permitted, but
1780  .sp  .sp
1781  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1782  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
1783  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
1784  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1785  .sp  .sp
1786    (?<=ab(c|de))    (?<=ab(c|de))
1787  .sp  .sp
1788  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
1789  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
1790    branches:
1791  .sp  .sp
1792    (?<=abc|abde)    (?<=abc|abde)
1793  .sp  .sp
1794    In some cases, the Perl 5.10 escape sequence \eK
1795    .\" HTML <a href="#resetmatchstart">
1796    .\" </a>
1797    (see above)
1798    .\"
1799    can be used instead of a lookbehind assertion to get round the fixed-length
1800    restriction.
1801    .P
1802  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1803  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
1804  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1805  match is deemed to fail.  assertion fails.
1806  .P  .P
1807  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)
1808  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1809  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
1810  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1811    .P
1812    .\" HTML <a href="#subpatternsassubroutines">
1813    .\" </a>
1814    "Subroutine"
1815    .\"
1816    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1817    as the subpattern matches a fixed-length string.
1818    .\" HTML <a href="#recursion">
1819    .\" </a>
1820    Recursion,
1821    .\"
1822    however, is not supported.
1823  .P  .P
1824  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1825  efficient matching at the end of the subject string. Consider a simple pattern  specify efficient matching of fixed-length strings at the end of subject
1826  such as  strings. Consider a simple pattern such as
1827  .sp  .sp
1828    abcd$    abcd$
1829  .sp  .sp
# Line 1351  then all but the last two characters, an Line 1839  then all but the last two characters, an
1839  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,
1840  if the pattern is written as  if the pattern is written as
1841  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1842    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1843  .sp  .sp
1844  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
1845  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
1846  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1847  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1403  characters that are not "999". Line 1887  characters that are not "999".
1887  .sp  .sp
1888  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1889  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1890  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1891  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1892  .sp  .sp
1893    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1894    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
# Line 1413  If the condition is satisfied, the yes-p Line 1897  If the condition is satisfied, the yes-p
1897  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
1898  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1899  .P  .P
1900  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1901  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
1902  underscores, the condition is satisfied if the capturing subpattern of that  .
1903  number or name has previously matched. There is a possible ambiguity here,  .SS "Checking for a used subpattern by number"
1904  because subpattern names may consist entirely of digits. PCRE looks first for a  .rs
1905  named subpattern; if it cannot find one and the text consists entirely of  .sp
1906  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
1907  zero. Using subpattern names that consist entirely of digits is not  condition is true if a capturing subpattern of that number has previously
1908  recommended.  matched. If there is more than one capturing subpattern with the same number
1909    (see the earlier
1910    .\"
1911    .\" HTML <a href="#recursion">
1912    .\" </a>
1913    section about duplicate subpattern numbers),
1914    .\"
1915    the condition is true if any of them have been set. An alternative notation is
1916    to precede the digits with a plus or minus sign. In this case, the subpattern
1917    number is relative rather than absolute. The most recently opened parentheses
1918    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1919    looping constructs it can also make sense to refer to subsequent groups with
1920    constructs such as (?(+2).
1921  .P  .P
1922  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1923  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 1933  or not. If they did, that is, if subject
1933  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
1934  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1935  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1936  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
1937  named subpattern gives this:  .P
1938    If you were embedding this pattern in a larger one, you could use a relative
1939    reference:
1940    .sp
1941      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1942    .sp
1943    This makes the fragment independent of the parentheses in the larger pattern.
1944    .
1945    .SS "Checking for a used subpattern by name"
1946    .rs
1947    .sp
1948    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1949    subpattern by name. For compatibility with earlier versions of PCRE, which had
1950    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1951    there is a possible ambiguity with this syntax, because subpattern names may
1952    consist entirely of digits. PCRE looks first for a named subpattern; if it
1953    cannot find one and the name consists entirely of digits, PCRE looks for a
1954    subpattern of that number, which must be greater than zero. Using subpattern
1955    names that consist entirely of digits is not recommended.
1956    .P
1957    Rewriting the above example to use a named subpattern gives this:
1958  .sp  .sp
1959    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1960    .sp
1961    If the name used in a condition of this kind is a duplicate, the test is
1962    applied to all subpatterns of the same name, and is true if any one of them has
1963    matched.
1964    .
1965    .SS "Checking for pattern recursion"
1966    .rs
1967  .sp  .sp
1968  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,
1969  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
1970  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
1971  Recursive patterns are described in the next section.  letter R, for example:
1972    .sp
1973      (?(R3)...) or (?(R&name)...)
1974    .sp
1975    the condition is true if the most recent recursion is into a subpattern whose
1976    number or name is given. This condition does not check the entire recursion
1977    stack. If the name used in a condition of this kind is a duplicate, the test is
1978    applied to all subpatterns of the same name, and is true if any one of them is
1979    the most recent recursion.
1980  .P  .P
1981  If the condition is not a sequence of digits or (R), it must be an assertion.  At "top level", all these recursion test conditions are false.
1982    .\" HTML <a href="#recursion">
1983    .\" </a>
1984    The syntax for recursive patterns
1985    .\"
1986    is described below.
1987    .
1988    .SS "Defining subpatterns for use by reference only"
1989    .rs
1990    .sp
1991    If the condition is the string (DEFINE), and there is no subpattern with the
1992    name DEFINE, the condition is always false. In this case, there may be only one
1993    alternative in the subpattern. It is always skipped if control reaches this
1994    point in the pattern; the idea of DEFINE is that it can be used to define
1995    "subroutines" that can be referenced from elsewhere. (The use of
1996    .\" HTML <a href="#subpatternsassubroutines">
1997    .\" </a>
1998    "subroutines"
1999    .\"
2000    is described below.) For example, a pattern to match an IPv4 address could be
2001    written like this (ignore whitespace and line breaks):
2002    .sp
2003      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2004      \eb (?&byte) (\e.(?&byte)){3} \eb
2005    .sp
2006    The first part of the pattern is a DEFINE group inside which a another group
2007    named "byte" is defined. This matches an individual component of an IPv4
2008    address (a number less than 256). When matching takes place, this part of the
2009    pattern is skipped because DEFINE acts like a false condition. The rest of the
2010    pattern uses references to the named group to match the four dot-separated
2011    components of an IPv4 address, insisting on a word boundary at each end.
2012    .
2013    .SS "Assertion conditions"
2014    .rs
2015    .sp
2016    If the condition is not in any of the above formats, it must be an assertion.
2017  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
2018  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
2019  alternatives on the second line:  alternatives on the second line:
# Line 1483  next newline in the pattern. Line 2049  next newline in the pattern.
2049  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2050  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2051  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
2052  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
2053  that allows regular expressions to recurse (amongst other things). It does this  .P
2054  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
2055  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
2056  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2057    pattern using code interpolation to solve the parentheses problem can be
2058    created like this:
2059  .sp  .sp
2060    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2061  .sp  .sp
2062  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
2063  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2064  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2065  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2066  .P  supports special syntax for recursion of the entire pattern, and also for
2067  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2068  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was subsequently introduced into Perl at release 5.10.
2069  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2070  "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
2071  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
2072  .P  provided that it occurs inside that subpattern. (If not, it is a
2073  A recursive subpattern call is always treated as an atomic group. That is, once  .\" HTML <a href="#subpatternsassubroutines">
2074  it has matched some of the subject string, it is never re-entered, even if  .\" </a>
2075  it contains untried alternatives and there is a subsequent matching failure.  "subroutine"
2076    .\"
2077    call, which is described in the next section.) The special item (?R) or (?0) is
2078    a recursive call of the entire regular expression.
2079  .P  .P
2080  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2081  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2082  .sp  .sp
2083    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2084  .sp  .sp
2085  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2086  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
2087  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2088  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2089    to avoid backtracking into sequences of non-parentheses.
2090  .P  .P
2091  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
2092  pattern, so instead you could use this:  pattern, so instead you could use this:
2093  .sp  .sp
2094    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2095  .sp  .sp
2096  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
2097  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2098  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2099  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
2100  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).
2101  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
2102  .sp  most recently opened parentheses preceding the recursion. In other words, a
2103    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
2104  .sp  it is encountered.
2105  This particular example pattern contains nested unlimited repeats, and so the  .P
2106  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
2107  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
2108  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
2109    .\" HTML <a href="#subpatternsassubroutines">
2110    .\" </a>
2111    "subroutine"
2112    .\"
2113    calls, as described in the next section.
2114    .P
2115    An alternative approach is to use named parentheses instead. The Perl syntax
2116    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2117    could rewrite the above example as follows:
2118    .sp
2119      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2120    .sp
2121    If there is more than one subpattern with the same name, the earliest one is
2122    used.
2123    .P
2124    This particular example pattern that we have been looking at contains nested
2125    unlimited repeats, and so the use of a possessive quantifier for matching
2126    strings of non-parentheses is important when applying the pattern to strings
2127    that do not match. For example, when this pattern is applied to
2128  .sp  .sp
2129    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2130  .sp  .sp
2131  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,
2132  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
2133  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
2134  before failure can be reported.  before failure can be reported.
2135  .P  .P
2136  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
2137  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
2138  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 the next section and the  
2139  .\" HREF  .\" HREF
2140  \fBpcrecallout\fP  \fBpcrecallout\fP
2141  .\"  .\"
# Line 1553  documentation). If the pattern above is Line 2143  documentation). If the pattern above is
2143  .sp  .sp
2144    (ab(cd)ef)    (ab(cd)ef)
2145  .sp  .sp
2146  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
2147  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
2148  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2149    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2150       ^                        ^  .P
2151       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2152  .sp  obtain extra memory to store data during a recursion, which it does by using
2153  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
2154  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.  
2155  .P  .P
2156  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.
2157  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1578  different alternatives for the recursive Line 2165  different alternatives for the recursive
2165  is the actual recursive call.  is the actual recursive call.
2166  .  .
2167  .  .
2168    .\" HTML <a name="recursiondifference"></a>
2169    .SS "Recursion difference from Perl"
2170    .rs
2171    .sp
2172    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2173    treated as an atomic group. That is, once it has matched some of the subject
2174    string, it is never re-entered, even if it contains untried alternatives and
2175    there is a subsequent matching failure. This can be illustrated by the
2176    following pattern, which purports to match a palindromic string that contains
2177    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2178    .sp
2179      ^(.|(.)(?1)\e2)$
2180    .sp
2181    The idea is that it either matches a single character, or two identical
2182    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2183    it does not if the pattern is longer than three characters. Consider the
2184    subject string "abcba":
2185    .P
2186    At the top level, the first character is matched, but as it is not at the end
2187    of the string, the first alternative fails; the second alternative is taken
2188    and the recursion kicks in. The recursive call to subpattern 1 successfully
2189    matches the next character ("b"). (Note that the beginning and end of line
2190    tests are not part of the recursion).
2191    .P
2192    Back at the top level, the next character ("c") is compared with what
2193    subpattern 2 matched, which was "a". This fails. Because the recursion is
2194    treated as an atomic group, there are now no backtracking points, and so the
2195    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2196    try the second alternative.) However, if the pattern is written with the
2197    alternatives in the other order, things are different:
2198    .sp
2199      ^((.)(?1)\e2|.)$
2200    .sp
2201    This time, the recursing alternative is tried first, and continues to recurse
2202    until it runs out of characters, at which point the recursion fails. But this
2203    time we do have another alternative to try at the higher level. That is the big
2204    difference: in the previous case the remaining alternative is at a deeper
2205    recursion level, which PCRE cannot use.
2206    .P
2207    To change the pattern so that matches all palindromic strings, not just those
2208    with an odd number of characters, it is tempting to change the pattern to this:
2209    .sp
2210      ^((.)(?1)\e2|.?)$
2211    .sp
2212    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2213    deeper recursion has matched a single character, it cannot be entered again in
2214    order to match an empty string. The solution is to separate the two cases, and
2215    write out the odd and even cases as alternatives at the higher level:
2216    .sp
2217      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2218    .sp
2219    If you want to match typical palindromic phrases, the pattern has to ignore all
2220    non-word characters, which can be done like this:
2221    .sp
2222      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2223    .sp
2224    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2225    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2226    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2227    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2228    more) to match typical phrases, and Perl takes so long that you think it has
2229    gone into a loop.
2230    .P
2231    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2232    string does not start with a palindrome that is shorter than the entire string.
2233    For example, although "abcba" is correctly matched, if the subject is "ababa",
2234    PCRE finds the palindrome "aba" at the start, then fails at top level because
2235    the end of the string does not follow. Once again, it cannot jump back into the
2236    recursion to try other alternatives, so the entire match fails.
2237    .
2238    .
2239  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2240  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2241  .rs  .rs
2242  .sp  .sp
2243  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
2244  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
2245  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
2246  pattern  before or after the reference. A numbered reference can be absolute or
2247    relative, as in these examples:
2248    .sp
2249      (...(absolute)...)...(?2)...
2250      (...(relative)...)...(?-1)...
2251      (...(?+1)...(relative)...
2252    .sp
2253    An earlier example pointed out that the pattern
2254  .sp  .sp
2255    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2256  .sp  .sp
# Line 1595  matches "sense and sensibility" and "res Line 2260  matches "sense and sensibility" and "res
2260    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2261  .sp  .sp
2262  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
2263  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.  
2264  .P  .P
2265  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2266  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
2267  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
2268  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2269    call revert to their previous values afterwards.
2270    .P
2271    When a subpattern is used as a subroutine, processing options such as
2272    case-independence are fixed when the subpattern is defined. They cannot be
2273    changed for different calls. For example, consider this pattern:
2274    .sp
2275      (abc)(?i:(?-1))
2276    .sp
2277    It matches "abcabc". It does not match "abcABC" because the change of
2278    processing option does not affect the called subpattern.
2279    .
2280    .
2281    .\" HTML <a name="onigurumasubroutines"></a>
2282    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2283    .rs
2284    .sp
2285    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2286    a number enclosed either in angle brackets or single quotes, is an alternative
2287    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2288    are two of the examples used above, rewritten using this syntax:
2289    .sp
2290      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2291      (sens|respons)e and \eg'1'ibility
2292    .sp
2293    PCRE supports an extension to Oniguruma: if a number is preceded by a
2294    plus or a minus sign it is taken as a relative reference. For example:
2295    .sp
2296      (abc)(?i:\eg<-1>)
2297    .sp
2298    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2299    synonymous. The former is a back reference; the latter is a subroutine call.
2300  .  .
2301  .  .
2302  .SH CALLOUTS  .SH CALLOUTS
# Line 1622  function is to be called. If you want to Line 2317  function is to be called. If you want to
2317  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.
2318  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2319  .sp  .sp
2320    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2321  .sp  .sp
2322  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
2323  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 2333  description of the interface to the call
2333  \fBpcrecallout\fP  \fBpcrecallout\fP
2334  .\"  .\"
2335  documentation.  documentation.
2336    .
2337    .
2338    .\" HTML <a name="backtrackcontrol"></a>
2339    .SH "BACKTRACKING CONTROL"
2340    .rs
2341    .sp
2342    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2343    are described in the Perl documentation as "experimental and subject to change
2344    or removal in a future version of Perl". It goes on to say: "Their usage in
2345    production code should be noted to avoid problems during upgrades." The same
2346    remarks apply to the PCRE features described in this section.
2347    .P
2348    Since these verbs are specifically related to backtracking, most of them can be
2349    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2350    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2351    failing negative assertion, they cause an error if encountered by
2352    \fBpcre_dfa_exec()\fP.
2353  .P  .P
2354  .in 0  If any of these verbs are used in an assertion or subroutine subpattern
2355  Last updated: 06 June 2006  (including recursive subpatterns), their effect is confined to that subpattern;
2356  .br  it does not extend to the surrounding pattern. Note that such subpatterns are
2357  Copyright (c) 1997-2006 University of Cambridge.  processed as anchored at the point where they are tested.
2358    .P
2359    The new verbs make use of what was previously invalid syntax: an opening
2360    parenthesis followed by an asterisk. They are generally of the form
2361    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2362    depending on whether or not an argument is present. An name is a sequence of
2363    letters, digits, and underscores. If the name is empty, that is, if the closing
2364    parenthesis immediately follows the colon, the effect is as if the colon were
2365    not there. Any number of these verbs may occur in a pattern.
2366    .P
2367    PCRE contains some optimizations that are used to speed up matching by running
2368    some checks at the start of each match attempt. For example, it may know the
2369    minimum length of matching subject, or that a particular character must be
2370    present. When one of these optimizations suppresses the running of a match, any
2371    included backtracking verbs will not, of course, be processed. You can suppress
2372    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2373    when calling \fBpcre_exec()\fP.
2374    .
2375    .
2376    .SS "Verbs that act immediately"
2377    .rs
2378    .sp
2379    The following verbs act as soon as they are encountered. They may not be
2380    followed by a name.
2381    .sp
2382       (*ACCEPT)
2383    .sp
2384    This verb causes the match to end successfully, skipping the remainder of the
2385    pattern. When inside a recursion, only the innermost pattern is ended
2386    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2387    captured. (This feature was added to PCRE at release 8.00.) For example:
2388    .sp
2389      A((?:A|B(*ACCEPT)|C)D)
2390    .sp
2391    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2392    the outer parentheses.
2393    .sp
2394      (*FAIL) or (*F)
2395    .sp
2396    This verb causes the match to fail, forcing backtracking to occur. It is
2397    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2398    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2399    Perl features that are not present in PCRE. The nearest equivalent is the
2400    callout feature, as for example in this pattern:
2401    .sp
2402      a+(?C)(*FAIL)
2403    .sp
2404    A match with the string "aaaa" always fails, but the callout is taken before
2405    each backtrack happens (in this example, 10 times).
2406    .
2407    .
2408    .SS "Recording which path was taken"
2409    .rs
2410    .sp
2411    There is one verb whose main purpose is to track how a match was arrived at,
2412    though it also has a secondary use in conjunction with advancing the match
2413    starting point (see (*SKIP) below).
2414    .sp
2415      (*MARK:NAME) or (*:NAME)
2416    .sp
2417    A name is always required with this verb. There may be as many instances of
2418    (*MARK) as you like in a pattern, and their names do not have to be unique.
2419    .P
2420    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2421    to the caller via the \fIpcre_extra\fP data structure, as described in the
2422    .\" HTML <a href="pcreapi.html#extradata">
2423    .\" </a>
2424    section on \fIpcre_extra\fP
2425    .\"
2426    in the
2427    .\" HREF
2428    \fBpcreapi\fP
2429    .\"
2430    documentation. No data is returned for a partial match. Here is an example of
2431    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2432    outputting of (*MARK) data:
2433    .sp
2434      /X(*MARK:A)Y|X(*MARK:B)Z/K
2435      XY
2436       0: XY
2437      MK: A
2438      XZ
2439       0: XZ
2440      MK: B
2441    .sp
2442    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2443    indicates which of the two alternatives matched. This is a more efficient way
2444    of obtaining this information than putting each alternative in its own
2445    capturing parentheses.
2446    .P
2447    A name may also be returned after a failed match if the final path through the
2448    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2449    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2450    starting point for matching is advanced, the final check is often with an empty
2451    string, causing a failure before (*MARK) is reached. For example:
2452    .sp
2453      /X(*MARK:A)Y|X(*MARK:B)Z/K
2454      XP
2455      No match
2456    .sp
2457    There are three potential starting points for this match (starting with X,
2458    starting with P, and with an empty string). If the pattern is anchored, the
2459    result is different:
2460    .sp
2461      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2462      XP
2463      No match, mark = B
2464    .sp
2465    PCRE's start-of-match optimizations can also interfere with this. For example,
2466    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2467    subject length for a match, a shorter subject will not be scanned at all.
2468    .P
2469    Note that similar anomalies (though different in detail) exist in Perl, no
2470    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2471    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2472    .
2473    .
2474    .SS "Verbs that act after backtracking"
2475    .rs
2476    .sp
2477    The following verbs do nothing when they are encountered. Matching continues
2478    with what follows, but if there is no subsequent match, causing a backtrack to
2479    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2480    the verb. However, when one of these verbs appears inside an atomic group, its
2481    effect is confined to that group, because once the group has been matched,
2482    there is never any backtracking into it. In this situation, backtracking can
2483    "jump back" to the left of the entire atomic group. (Remember also, as stated
2484    above, that this localization also applies in subroutine calls and assertions.)
2485    .P
2486    These verbs differ in exactly what kind of failure occurs when backtracking
2487    reaches them.
2488    .sp
2489      (*COMMIT)
2490    .sp
2491    This verb, which may not be followed by a name, causes the whole match to fail
2492    outright if the rest of the pattern does not match. Even if the pattern is
2493    unanchored, no further attempts to find a match by advancing the starting point
2494    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2495    finding a match at the current starting point, or not at all. For example:
2496    .sp
2497      a+(*COMMIT)b
2498    .sp
2499    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2500    dynamic anchor, or "I've started, so I must finish." The name of the most
2501    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2502    match failure.
2503    .P
2504    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2505    unless PCRE's start-of-match optimizations are turned off, as shown in this
2506    \fBpcretest\fP example:
2507    .sp
2508      /(*COMMIT)abc/
2509      xyzabc
2510       0: abc
2511      xyzabc\eY
2512      No match
2513    .sp
2514    PCRE knows that any match must start with "a", so the optimization skips along
2515    the subject to "a" before running the first match attempt, which succeeds. When
2516    the optimization is disabled by the \eY escape in the second subject, the match
2517    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2518    starting points.
2519    .sp
2520      (*PRUNE) or (*PRUNE:NAME)
2521    .sp
2522    This verb causes the match to fail at the current starting position in the
2523    subject if the rest of the pattern does not match. If the pattern is
2524    unanchored, the normal "bumpalong" advance to the next starting character then
2525    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2526    reached, or when matching to the right of (*PRUNE), but if there is no match to
2527    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2528    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2529    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2530    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2531    match fails completely; the name is passed back if this is the final attempt.
2532    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2533    pattern (*PRUNE) has the same effect as (*COMMIT).
2534    .sp
2535      (*SKIP)
2536    .sp
2537    This verb, when given without a name, is like (*PRUNE), except that if the
2538    pattern is unanchored, the "bumpalong" advance is not to the next character,
2539    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2540    signifies that whatever text was matched leading up to it cannot be part of a
2541    successful match. Consider:
2542    .sp
2543      a+(*SKIP)b
2544    .sp
2545    If the subject is "aaaac...", after the first match attempt fails (starting at
2546    the first character in the string), the starting point skips on to start the
2547    next attempt at "c". Note that a possessive quantifer does not have the same
2548    effect as this example; although it would suppress backtracking during the
2549    first match attempt, the second attempt would start at the second character
2550    instead of skipping on to "c".
2551    .sp
2552      (*SKIP:NAME)
2553    .sp
2554    When (*SKIP) has an associated name, its behaviour is modified. If the
2555    following pattern fails to match, the previous path through the pattern is
2556    searched for the most recent (*MARK) that has the same name. If one is found,
2557    the "bumpalong" advance is to the subject position that corresponds to that
2558    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2559    matching name is found, normal "bumpalong" of one character happens (the
2560    (*SKIP) is ignored).
2561    .sp
2562      (*THEN) or (*THEN:NAME)
2563    .sp
2564    This verb causes a skip to the next alternation if the rest of the pattern does
2565    not match. That is, it cancels pending backtracking, but only within the
2566    current alternation. Its name comes from the observation that it can be used
2567    for a pattern-based if-then-else block:
2568    .sp
2569      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2570    .sp
2571    If the COND1 pattern matches, FOO is tried (and possibly further items after
2572    the end of the group if FOO succeeds); on failure the matcher skips to the
2573    second alternative and tries COND2, without backtracking into COND1. The
2574    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2575    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2576    like (*PRUNE).
2577    .
2578    .
2579    .SH "SEE ALSO"
2580    .rs
2581    .sp
2582    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2583    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2584    .
2585    .
2586    .SH AUTHOR
2587    .rs
2588    .sp
2589    .nf
2590    Philip Hazel
2591    University Computing Service
2592    Cambridge CB2 3QH, England.
2593    .fi
2594    .
2595    .
2596    .SH REVISION
2597    .rs
2598    .sp
2599    .nf
2600    Last updated: 03 May 2010
2601    Copyright (c) 1997-2010 University of Cambridge.
2602    .fi

Legend:
Removed from v.91  
changed lines
  Added in v.514

  ViewVC Help
Powered by ViewVC 1.1.5