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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 227 by ph10, Tue Aug 21 15:00:15 2007 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. Perl's regular expressions are described in its own documentation, and
13    regular expressions in general are covered in a number of books, some of which
14    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
15    published by O'Reilly, covers regular expressions in great detail. This
16    description of PCRE's regular expressions is intended as reference material.
17  .P  .P
18  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,
19  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, you must
# Line 30  The remainder of this document discusses Line 34  The remainder of this document discusses
34  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
35  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
36  \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
37  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
38  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
39    alternative function, and how it differs from the normal function, are
40    discussed in the
41  .\" HREF  .\" HREF
42  \fBpcrematching\fP  \fBpcrematching\fP
43  .\"  .\"
44  page.  page.
45    .
46    .
47    .SH "NEWLINE CONVENTIONS"
48    .rs
49    .sp
50    PCRE supports five different conventions for indicating line breaks in
51    strings: a single CR (carriage return) character, a single LF (linefeed)
52    character, the two-character sequence CRLF, any of the three preceding, or any
53    Unicode newline sequence. The
54    .\" HREF
55    \fBpcreapi\fP
56    .\"
57    page has
58    .\" HTML <a href="pcreapi.html#newlines">
59    .\" </a>
60    further discussion
61    .\"
62    about newlines, and shows how to set the newline convention in the
63    \fIoptions\fP arguments for the compiling and matching functions.
64  .P  .P
65    It is also possible to specify a newline convention by starting a pattern
66    string with one of the following five sequences:
67    .sp
68      (*CR)        carriage return
69      (*LF)        linefeed
70      (*CRLF)      carriage return, followed by linefeed
71      (*ANYCRLF)   any of the three above
72      (*ANY)       all Unicode newline sequences
73    .sp
74    These override the default and the options given to \fBpcre_compile()\fP. For
75    example, on a Unix system where LF is the default newline sequence, the pattern
76    .sp
77      (*CR)a.b
78    .sp
79    changes the convention to CR. That pattern matches "a\enb" because LF is no
80    longer a newline. Note that these special settings, which are not
81    Perl-compatible, are recognized only at the very start of a pattern, and that
82    they must be in upper case.
83    .
84    .
85    .SH "CHARACTERS AND METACHARACTERS"
86    .rs
87    .sp
88  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
89  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
90  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 108  interpreted in some special way.
108  .P  .P
109  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
110  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
111  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
112  as follows:  are as follows:
113  .sp  .sp
114    \e      general escape character with several uses    \e      general escape character with several uses
115    ^      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 140  a character class the only metacharacter
140  .sp  .sp
141  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
142  .  .
143    .
144  .SH BACKSLASH  .SH BACKSLASH
145  .rs  .rs
146  .sp  .sp
# Line 142  represents: Line 191  represents:
191    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
192    \ee        escape (hex 1B)    \ee        escape (hex 1B)
193    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
194    \en        newline (hex 0A)    \en        linefeed (hex 0A)
195    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
196    \et        tab (hex 09)    \et        tab (hex 09)
197    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
# Line 157  Thus \ecz becomes hex 1A, but \ec{ becom Line 206  Thus \ecz becomes hex 1A, but \ec{ becom
206  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
207  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{
208  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
209  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
210  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
211  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
212  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
213  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
214    there is no terminating }, this form of escape is not recognized. Instead, the
215    initial \ex will be interpreted as a basic hexadecimal escape, with no
216    following digits, giving a character whose value is zero.
217  .P  .P
218  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
219  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 242  parenthesized subpatterns.
242  .P  .P
243  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
244  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
245  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
246  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
247  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
248  to \e777 are permitted. For example:  to \e777 are permitted. For example:
# Line 221  zero, because no more than three octal d Line 273  zero, because no more than three octal d
273  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
274  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
275  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08), and the
276  sequence \eX is interpreted as the character "X". Outside a character class,  sequences \eR and \eX are interpreted as the characters "R" and "X",
277  these sequences have different meanings  respectively. Outside a character class, these sequences have different
278    meanings
279  .\" HTML <a href="#uniextseq">  .\" HTML <a href="#uniextseq">
280  .\" </a>  .\" </a>
281  (see below).  (see below).
282  .\"  .\"
283  .  .
284  .  .
285    .SS "Absolute and relative back references"
286    .rs
287    .sp
288    The sequence \eg followed by an unsigned or a negative number, optionally
289    enclosed in braces, is an absolute or relative back reference. A named back
290    reference can be coded as \eg{name}. Back references are discussed
291    .\" HTML <a href="#backreferences">
292    .\" </a>
293    later,
294    .\"
295    following the discussion of
296    .\" HTML <a href="#subpattern">
297    .\" </a>
298    parenthesized subpatterns.
299    .\"
300    .
301    .
302  .SS "Generic character types"  .SS "Generic character types"
303  .rs  .rs
304  .sp  .sp
305  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types. The
306  following are always recognized:  following are always recognized:
307  .sp  .sp
308    \ed     any decimal digit    \ed     any decimal digit
309    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
310      \eh     any horizontal whitespace character
311      \eH     any character that is not a horizontal whitespace character
312    \es     any whitespace character    \es     any whitespace character
313    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
314      \ev     any vertical whitespace character
315      \eV     any character that is not a vertical whitespace character
316    \ew     any "word" character    \ew     any "word" character
317    \eW     any "non-word" character    \eW     any "non-word" character
318  .sp  .sp
# Line 252  there is no character to match. Line 326  there is no character to match.
326  .P  .P
327  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).
328  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
329  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
330  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
331  does.)  does.
332    .P
333    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
334    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
335    character property support is available. These sequences retain their original
336    meanings from before UTF-8 support was available, mainly for efficiency
337    reasons.
338    .P
339    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
340    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
341    The horizontal space characters are:
342    .sp
343      U+0009     Horizontal tab
344      U+0020     Space
345      U+00A0     Non-break space
346      U+1680     Ogham space mark
347      U+180E     Mongolian vowel separator
348      U+2000     En quad
349      U+2001     Em quad
350      U+2002     En space
351      U+2003     Em space
352      U+2004     Three-per-em space
353      U+2005     Four-per-em space
354      U+2006     Six-per-em space
355      U+2007     Figure space
356      U+2008     Punctuation space
357      U+2009     Thin space
358      U+200A     Hair space
359      U+202F     Narrow no-break space
360      U+205F     Medium mathematical space
361      U+3000     Ideographic space
362    .sp
363    The vertical space characters are:
364    .sp
365      U+000A     Linefeed
366      U+000B     Vertical tab
367      U+000C     Formfeed
368      U+000D     Carriage return
369      U+0085     Next line
370      U+2028     Line separator
371      U+2029     Paragraph separator
372  .P  .P
373  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
374  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 382  in the
382  .\" HREF  .\" HREF
383  \fBpcreapi\fP  \fBpcreapi\fP
384  .\"  .\"
385  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,
386  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
387    accented letters, and these are matched by \ew. The use of locales with Unicode
388    is discouraged.
389    .
390    .
391    .SS "Newline sequences"
392    .rs
393    .sp
394    Outside a character class, the escape sequence \eR matches any Unicode newline
395    sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to
396    the following:
397    .sp
398      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
399    .sp
400    This is an example of an "atomic group", details of which are given
401    .\" HTML <a href="#atomicgroup">
402    .\" </a>
403    below.
404    .\"
405    This particular group matches either the two-character sequence CR followed by
406    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
407    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
408    line, U+0085). The two-character sequence is treated as a single unit that
409    cannot be split.
410    .P
411    In UTF-8 mode, two additional characters whose codepoints are greater than 255
412    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
413    Unicode character property support is not needed for these characters to be
414    recognized.
415  .P  .P
416  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  Inside a character class, \eR matches the letter "R".
 \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  
 character property support is available. The use of locales with Unicode is  
 discouraged.  
417  .  .
418  .  .
419  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 282  discouraged. Line 421  discouraged.
421  .rs  .rs
422  .sp  .sp
423  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
424  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
425  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
426    characters whose codepoints are less than 256, but they do work in this mode.
427    The extra escape sequences are:
428  .sp  .sp
429    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
430    \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 448  Those that are not part of an identified
448  .P  .P
449  Arabic,  Arabic,
450  Armenian,  Armenian,
451    Balinese,
452  Bengali,  Bengali,
453  Bopomofo,  Bopomofo,
454  Braille,  Braille,
# Line 316  Canadian_Aboriginal, Line 458  Canadian_Aboriginal,
458  Cherokee,  Cherokee,
459  Common,  Common,
460  Coptic,  Coptic,
461    Cuneiform,
462  Cypriot,  Cypriot,
463  Cyrillic,  Cyrillic,
464  Deseret,  Deseret,
# Line 345  Malayalam, Line 488  Malayalam,
488  Mongolian,  Mongolian,
489  Myanmar,  Myanmar,
490  New_Tai_Lue,  New_Tai_Lue,
491    Nko,
492  Ogham,  Ogham,
493  Old_Italic,  Old_Italic,
494  Old_Persian,  Old_Persian,
495  Oriya,  Oriya,
496  Osmanya,  Osmanya,
497    Phags_Pa,
498    Phoenician,
499  Runic,  Runic,
500  Shavian,  Shavian,
501  Sinhala,  Sinhala,
# Line 430  The special property L& is also supporte Line 576  The special property L& is also supporte
576  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
577  a modifier or "other".  a modifier or "other".
578  .P  .P
579    The Cs (Surrogate) property applies only to characters in the range U+D800 to
580    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
581    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
582    (see the discussion of PCRE_NO_UTF8_CHECK in the
583    .\" HREF
584    \fBpcreapi\fP
585    .\"
586    page).
587    .P
588  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The long synonyms for these properties that Perl supports (such as \ep{Letter})
589  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
590  properties with "Is".  properties with "Is".
# Line 454  atomic group Line 609  atomic group
609  (see below).  (see below).
610  .\"  .\"
611  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
612  preceding character.  preceding character. None of them have codepoints less than 256, so in
613    non-UTF-8 mode \eX matches any one character.
614  .P  .P
615  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
616  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 618  why the traditional escape sequences suc
618  properties in PCRE.  properties in PCRE.
619  .  .
620  .  .
621    .\" HTML <a name="resetmatchstart"></a>
622    .SS "Resetting the match start"
623    .rs
624    .sp
625    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
626    matched characters not to be included in the final matched sequence. For
627    example, the pattern:
628    .sp
629      foo\eKbar
630    .sp
631    matches "foobar", but reports that it has matched "bar". This feature is
632    similar to a lookbehind assertion
633    .\" HTML <a href="#lookbehind">
634    .\" </a>
635    (described below).
636    .\"
637    However, in this case, the part of the subject before the real match does not
638    have to be of fixed length, as lookbehind assertions do. The use of \eK does
639    not interfere with the setting of
640    .\" HTML <a href="#subpattern">
641    .\" </a>
642    captured substrings.
643    .\"
644    For example, when the pattern
645    .sp
646      (foo)\eKbar
647    .sp
648    matches "foobar", the first substring is still set to "foo".
649    .
650    .
651  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
652  .SS "Simple assertions"  .SS "Simple assertions"
653  .rs  .rs
654  .sp  .sp
655  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
656  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,
657  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
658  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 478  The backslashed assertions are: Line 664  The backslashed assertions are:
664  .sp  .sp
665    \eb     matches at a word boundary    \eb     matches at a word boundary
666    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
667    \eA     matches at start of subject    \eA     matches at the start of the subject
668    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
669    \ez     matches at end of subject            also matches before a newline at the end of the subject
670    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
671      \eG     matches at the first matching position in the subject
672  .sp  .sp
673  These assertions may not appear in character classes (but note that \eb has a  These assertions may not appear in character classes (but note that \eb has a
674  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
# Line 578  end of the subject in both modes, and if Line 765  end of the subject in both modes, and if
765  .sp  .sp
766  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
767  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
768  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.
769  a line ending is defined as a single character (CR or LF), dot never matches  .P
770  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
771  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
772  characters (including isolated CRs and LFs).  if it is immediately followed by LF, but otherwise it matches all characters
773    (including isolated CRs and LFs). When any Unicode line endings are being
774    recognized, dot does not match CR or LF or any of the other line ending
775    characters.
776  .P  .P
777  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
778  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
779  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
780    to match it.
781  .P  .P
782  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
783  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
# Line 597  special meaning in a character class. Line 788  special meaning in a character class.
788  .rs  .rs
789  .sp  .sp
790  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
791  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
792  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
793  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,
794  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,
795  sequence is best avoided.  the \eC escape sequence is best avoided.
796  .P  .P
797  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
798  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 652  If you want to use caseless matching for Line 843  If you want to use caseless matching for
843  ensure that PCRE is compiled with Unicode property support as well as with  ensure that PCRE is compiled with Unicode property support as well as with
844  UTF-8 support.  UTF-8 support.
845  .P  .P
846  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
847  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
848  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
849  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
850  .P  .P
851  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
852  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 870  example [\ex{100}-\ex{2ff}].
870  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
871  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
872  [][\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
873  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
874  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
875  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
876  property support.  property support.
# Line 790  If the change is placed right at the sta Line 981  If the change is placed right at the sta
981  the global options (and it will therefore show up in data extracted by the  the global options (and it will therefore show up in data extracted by the
982  \fBpcre_fullinfo()\fP function).  \fBpcre_fullinfo()\fP function).
983  .P  .P
984  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
985  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
986  .sp  .sp
987    (a(?i)b)c    (a(?i)b)c
988  .sp  .sp
# Line 824  Turning part of a pattern into a subpatt Line 1015  Turning part of a pattern into a subpatt
1015    cat(aract|erpillar|)    cat(aract|erpillar|)
1016  .sp  .sp
1017  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1018  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1019  .sp  .sp
1020  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
1021  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 1040  the string "the white queen" is matched
1040    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1041  .sp  .sp
1042  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
1043  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.  
1044  .P  .P
1045  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
1046  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 1055  is reached, an option setting in one bra
1055  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1056  .  .
1057  .  .
1058    .SH "DUPLICATE SUBPATTERN NUMBERS"
1059    .rs
1060    .sp
1061    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1062    the same numbers for its capturing parentheses. Such a subpattern starts with
1063    (?| and is itself a non-capturing subpattern. For example, consider this
1064    pattern:
1065    .sp
1066      (?|(Sat)ur|(Sun))day
1067    .sp
1068    Because the two alternatives are inside a (?| group, both sets of capturing
1069    parentheses are numbered one. Thus, when the pattern matches, you can look
1070    at captured substring number one, whichever alternative matched. This construct
1071    is useful when you want to capture part, but not all, of one of a number of
1072    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1073    number is reset at the start of each branch. The numbers of any capturing
1074    buffers that follow the subpattern start after the highest number used in any
1075    branch. The following example is taken from the Perl documentation.
1076    The numbers underneath show in which buffer the captured content will be
1077    stored.
1078    .sp
1079      # before  ---------------branch-reset----------- after
1080      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1081      # 1            2         2  3        2     3     4
1082    .sp
1083    A backreference or a recursive call to a numbered subpattern always refers to
1084    the first one in the pattern with the given number.
1085    .P
1086    An alternative approach to using this "branch reset" feature is to use
1087    duplicate named subpatterns, as described in the next section.
1088    .
1089    .
1090  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1091  .rs  .rs
1092  .sp  .sp
1093  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
1094  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1095  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
1096  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1097  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
1098    introduced it at release 4.0, using the Python syntax. PCRE now supports both
1099    the Perl and the Python syntax.
1100    .P
1101    In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1102    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1103  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1104  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1105  .\" </a>  .\" </a>
# Line 890  conditions, Line 1117  conditions,
1117  can be made by name as well as by number.  can be made by name as well as by number.
1118  .P  .P
1119  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
1120  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
1121  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
1122  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
1123  captured substring by name.  is also a convenience function for extracting a captured substring by name.
1124  .P  .P
1125  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
1126  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. This can
# Line 902  match. Suppose you want to match the nam Line 1129  match. Suppose you want to match the nam
1129  abbreviation or as the full name, and in both cases you want to extract the  abbreviation or as the full name, and in both cases you want to extract the
1130  abbreviation. This pattern (ignoring the line breaks) does the job:  abbreviation. This pattern (ignoring the line breaks) does the job:
1131  .sp  .sp
1132    (?P<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1133    (?P<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
1134    (?P<DN>Wed)(?:nesday)?|    (?<DN>Wed)(?:nesday)?|
1135    (?P<DN>Thu)(?:rsday)?|    (?<DN>Thu)(?:rsday)?|
1136    (?P<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1137  .sp  .sp
1138  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.
1139    (An alternative way of solving this problem is to use a "branch reset"
1140    subpattern, as described in the previous section.)
1141    .P
1142  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1143  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
1144  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was. If you
1145  make a reference to a non-unique named subpattern from elsewhere in the  make a reference to a non-unique named subpattern from elsewhere in the
1146  pattern, the one that corresponds to the lowest number is used. For further  pattern, the one that corresponds to the lowest number is used. For further
# Line 928  Repetition is specified by quantifiers, Line 1158  Repetition is specified by quantifiers,
1158  items:  items:
1159  .sp  .sp
1160    a literal data character    a literal data character
1161    the . metacharacter    the dot metacharacter
1162    the \eC escape sequence    the \eC escape sequence
1163    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1164      the \eR escape sequence
1165    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1166    a character class    a character class
1167    a back reference (see next section)    a back reference (see next section)
# Line 968  which may be several bytes long (and the Line 1199  which may be several bytes long (and the
1199  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
1200  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1201  .P  .P
1202  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1203  quantifiers have single-character abbreviations:  abbreviations:
1204  .sp  .sp
1205    *    is equivalent to {0,}    *    is equivalent to {0,}
1206    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1017  own right. Because it has two uses, it c Line 1248  own right. Because it has two uses, it c
1248  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
1249  way the rest of the pattern matches.  way the rest of the pattern matches.
1250  .P  .P
1251  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),
1252  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
1253  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
1254  default behaviour.  default behaviour.
# Line 1027  is greater than 1 or with a limited maxi Line 1258  is greater than 1 or with a limited maxi
1258  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1259  .P  .P
1260  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
1261  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
1262  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1263  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
1264  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
# Line 1039  alternatively using ^ to indicate anchor Line 1270  alternatively using ^ to indicate anchor
1270  .P  .P
1271  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1272  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1273  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
1274  succeed. Consider, for example:  succeeds. Consider, for example:
1275  .sp  .sp
1276    (.*)abc\e1    (.*)abc\e1
1277  .sp  .sp
# Line 1066  matches "aba" the value of the second ca Line 1297  matches "aba" the value of the second ca
1297  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1298  .rs  .rs
1299  .sp  .sp
1300  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1301  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
1302  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
1303  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
1304  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
1305  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1306  .P  .P
1307  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
1308  .sp  .sp
# Line 1083  item, and then with 4, and so on, before Line 1314  item, and then with 4, and so on, before
1314  (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
1315  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.
1316  .P  .P
1317  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
1318  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
1319  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1320  .sp  .sp
# Line 1113  previous example can be rewritten as Line 1344  previous example can be rewritten as
1344  .sp  .sp
1345    \ed++foo    \ed++foo
1346  .sp  .sp
1347    Note that a possessive quantifier can be used with an entire group, for
1348    example:
1349    .sp
1350      (abc|xyz){2,3}+
1351    .sp
1352  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1353  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
1354  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
1355  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1356  .P  difference; possessive quantifiers should be slightly faster.
1357  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  .P
1358  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.
1359  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
1360  and PCRE copied it from there.  book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1361    package, and PCRE copied it from there. It ultimately found its way into Perl
1362    at release 5.10.
1363    .P
1364    PCRE has an optimization that automatically "possessifies" certain simple
1365    pattern constructs. For example, the sequence A+B is treated as A++B because
1366    there is no point in backtracking into a sequence of A's when B must follow.
1367  .P  .P
1368  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
1369  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 1409  numbers less than 10. A "forward back re
1409  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
1410  in an earlier iteration.  in an earlier iteration.
1411  .P  .P
1412  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
1413  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
1414  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1415  "Non-printing characters"  "Non-printing characters"
1416  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1417  .\" </a>  .\" </a>
1418  above  above
1419  .\"  .\"
1420  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1421    no such problem when named parentheses are used. A back reference to any
1422    subpattern is possible using named parentheses (see below).
1423    .P
1424    Another way of avoiding the ambiguity inherent in the use of digits following a
1425    backslash is to use the \eg escape sequence, which is a feature introduced in
1426    Perl 5.10. This escape must be followed by an unsigned number or a negative
1427    number, optionally enclosed in braces. These examples are all identical:
1428    .sp
1429      (ring), \e1
1430      (ring), \eg1
1431      (ring), \eg{1}
1432    .sp
1433    An unsigned number specifies an absolute reference without the ambiguity that
1434    is present in the older syntax. It is also useful when literal digits follow
1435    the reference. A negative number is a relative reference. Consider this
1436    example:
1437    .sp
1438      (abc(def)ghi)\eg{-1}
1439    .sp
1440    The sequence \eg{-1} is a reference to the most recently started capturing
1441    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1442    would be equivalent to \e1. The use of relative references can be helpful in
1443    long patterns, and also in patterns that are created by joining together
1444    fragments that contain references within themselves.
1445  .P  .P
1446  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1447  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 1463  back reference, the case of letters is r
1463  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
1464  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1465  .P  .P
1466  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1467  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1468    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1469    back reference syntax, in which \eg can be used for both numeric and named
1470    references, is also supported. We could rewrite the above example in any of
1471    the following ways:
1472  .sp  .sp
1473      (?<p1>(?i)rah)\es+\ek<p1>
1474      (?'p1'(?i)rah)\es+\ek{p1}
1475    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1476      (?<p1>(?i)rah)\es+\eg{p1}
1477  .sp  .sp
1478  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
1479  after the reference.  after the reference.
# Line 1323  lengths, but it is acceptable if rewritt Line 1596  lengths, but it is acceptable if rewritt
1596  .sp  .sp
1597    (?<=abc|abde)    (?<=abc|abde)
1598  .sp  .sp
1599    In some cases, the Perl 5.10 escape sequence \eK
1600    .\" HTML <a href="#resetmatchstart">
1601    .\" </a>
1602    (see above)
1603    .\"
1604    can be used instead of a lookbehind assertion; this is not restricted to a
1605    fixed-length.
1606    .P
1607  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1608  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
1609  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1610  match is deemed to fail.  assertion fails.
1611  .P  .P
1612  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)
1613  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1614  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
1615  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1616  .P  .P
1617  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1618  efficient matching at the end of the subject string. Consider a simple pattern  specify efficient matching at the end of the subject string. Consider a simple
1619  such as  pattern such as
1620  .sp  .sp
1621    abcd$    abcd$
1622  .sp  .sp
# Line 1351  then all but the last two characters, an Line 1632  then all but the last two characters, an
1632  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,
1633  if the pattern is written as  if the pattern is written as
1634  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1635    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1636  .sp  .sp
1637  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
1638  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
1639  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1640  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1413  If the condition is satisfied, the yes-p Line 1690  If the condition is satisfied, the yes-p
1690  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
1691  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1692  .P  .P
1693  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1694  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
1695  underscores, the condition is satisfied if the capturing subpattern of that  .
1696  number or name has previously matched. There is a possible ambiguity here,  .SS "Checking for a used subpattern by number"
1697  because subpattern names may consist entirely of digits. PCRE looks first for a  .rs
1698  named subpattern; if it cannot find one and the text consists entirely of  .sp
1699  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
1700  zero. Using subpattern names that consist entirely of digits is not  condition is true if the capturing subpattern of that number has previously
1701  recommended.  matched. An alternative notation is to precede the digits with a plus or minus
1702    sign. In this case, the subpattern number is relative rather than absolute.
1703    The most recently opened parentheses can be referenced by (?(-1), the next most
1704    recent by (?(-2), and so on. In looping constructs it can also make sense to
1705    refer to subsequent groups with constructs such as (?(+2).
1706  .P  .P
1707  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1708  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 1718  or not. If they did, that is, if subject
1718  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
1719  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1720  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1721  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
1722  named subpattern gives this:  .P
1723    If you were embedding this pattern in a larger one, you could use a relative
1724    reference:
1725  .sp  .sp
1726    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1727    .sp
1728    This makes the fragment independent of the parentheses in the larger pattern.
1729    .
1730    .SS "Checking for a used subpattern by name"
1731    .rs
1732    .sp
1733    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1734    subpattern by name. For compatibility with earlier versions of PCRE, which had
1735    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1736    there is a possible ambiguity with this syntax, because subpattern names may
1737    consist entirely of digits. PCRE looks first for a named subpattern; if it
1738    cannot find one and the name consists entirely of digits, PCRE looks for a
1739    subpattern of that number, which must be greater than zero. Using subpattern
1740    names that consist entirely of digits is not recommended.
1741    .P
1742    Rewriting the above example to use a named subpattern gives this:
1743    .sp
1744      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1745    .sp
1746    .
1747    .SS "Checking for pattern recursion"
1748    .rs
1749  .sp  .sp
1750  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,
1751  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
1752  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
1753  Recursive patterns are described in the next section.  letter R, for example:
1754    .sp
1755      (?(R3)...) or (?(R&name)...)
1756    .sp
1757    the condition is true if the most recent recursion is into the subpattern whose
1758    number or name is given. This condition does not check the entire recursion
1759    stack.
1760  .P  .P
1761  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. Recursive
1762    patterns are described below.
1763    .
1764    .SS "Defining subpatterns for use by reference only"
1765    .rs
1766    .sp
1767    If the condition is the string (DEFINE), and there is no subpattern with the
1768    name DEFINE, the condition is always false. In this case, there may be only one
1769    alternative in the subpattern. It is always skipped if control reaches this
1770    point in the pattern; the idea of DEFINE is that it can be used to define
1771    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1772    is described below.) For example, a pattern to match an IPv4 address could be
1773    written like this (ignore whitespace and line breaks):
1774    .sp
1775      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1776      \eb (?&byte) (\e.(?&byte)){3} \eb
1777    .sp
1778    The first part of the pattern is a DEFINE group inside which a another group
1779    named "byte" is defined. This matches an individual component of an IPv4
1780    address (a number less than 256). When matching takes place, this part of the
1781    pattern is skipped because DEFINE acts like a false condition.
1782    .P
1783    The rest of the pattern uses references to the named group to match the four
1784    dot-separated components of an IPv4 address, insisting on a word boundary at
1785    each end.
1786    .
1787    .SS "Assertion conditions"
1788    .rs
1789    .sp
1790    If the condition is not in any of the above formats, it must be an assertion.
1791  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1792  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1793  alternatives on the second line:  alternatives on the second line:
# Line 1483  next newline in the pattern. Line 1823  next newline in the pattern.
1823  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1824  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1825  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
1826  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
1827  that allows regular expressions to recurse (amongst other things). It does this  .P
1828  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
1829  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
1830  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1831    pattern using code interpolation to solve the parentheses problem can be
1832    created like this:
1833  .sp  .sp
1834    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1835  .sp  .sp
1836  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
1837  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1838  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1839  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1840  .P  supports special syntax for recursion of the entire pattern, and also for
1841  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1842  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was introduced into Perl at release 5.10.
1843  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1844  "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
1845  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1846  .P  provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1847  A recursive subpattern call is always treated as an atomic group. That is, once  call, which is described in the next section.) The special item (?R) or (?0) is
1848  it has matched some of the subject string, it is never re-entered, even if  a recursive call of the entire regular expression.
1849  it contains untried alternatives and there is a subsequent matching failure.  .P
1850    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1851    treated as an atomic group. That is, once it has matched some of the subject
1852    string, it is never re-entered, even if it contains untried alternatives and
1853    there is a subsequent matching failure.
1854  .P  .P
1855  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
1856  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
# Line 1522  pattern, so instead you could use this: Line 1868  pattern, so instead you could use this:
1868    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1869  .sp  .sp
1870  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
1871  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1872  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1873  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
1874  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.)
1875  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
1876  .sp  most recently opened parentheses preceding the recursion. In other words, a
1877    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
1878  .sp  it is encountered.
1879  This particular example pattern contains nested unlimited repeats, and so the  .P
1880  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1881  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
1882  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1883    "subroutine" calls, as described in the next section.
1884    .P
1885    An alternative approach is to use named parentheses instead. The Perl syntax
1886    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1887    could rewrite the above example as follows:
1888    .sp
1889      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1890    .sp
1891    If there is more than one subpattern with the same name, the earliest one is
1892    used.
1893    .P
1894    This particular example pattern that we have been looking at contains nested
1895    unlimited repeats, and so the use of atomic grouping for matching strings of
1896    non-parentheses is important when applying the pattern to strings that do not
1897    match. For example, when this pattern is applied to
1898  .sp  .sp
1899    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1900  .sp  .sp
# Line 1545  before failure can be reported. Line 1906  before failure can be reported.
1906  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values set for any capturing subpatterns are those
1907  from the outermost level of the recursion at which the subpattern value is set.  from the outermost level of the recursion at which the subpattern value is set.
1908  If you want to obtain intermediate values, a callout function can be used (see  If you want to obtain intermediate values, a callout function can be used (see
1909  the next section and the  below and the
1910  .\" HREF  .\" HREF
1911  \fBpcrecallout\fP  \fBpcrecallout\fP
1912  .\"  .\"
# Line 1584  is the actual recursive call. Line 1945  is the actual recursive call.
1945  .sp  .sp
1946  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
1947  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
1948  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1949  pattern  before or after the reference. A numbered reference can be absolute or
1950    relative, as in these examples:
1951    .sp
1952      (...(absolute)...)...(?2)...
1953      (...(relative)...)...(?-1)...
1954      (...(?+1)...(relative)...
1955    .sp
1956    An earlier example pointed out that the pattern
1957  .sp  .sp
1958    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
1959  .sp  .sp
# Line 1595  matches "sense and sensibility" and "res Line 1963  matches "sense and sensibility" and "res
1963    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1964  .sp  .sp
1965  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
1966  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.  
1967  .P  .P
1968  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1969  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
1970  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
1971  matching failure.  matching failure.
1972    .P
1973    When a subpattern is used as a subroutine, processing options such as
1974    case-independence are fixed when the subpattern is defined. They cannot be
1975    changed for different calls. For example, consider this pattern:
1976    .sp
1977      (abc)(?i:(?-1))
1978    .sp
1979    It matches "abcabc". It does not match "abcABC" because the change of
1980    processing option does not affect the called subpattern.
1981  .  .
1982  .  .
1983  .SH CALLOUTS  .SH CALLOUTS
# Line 1622  function is to be called. If you want to Line 1998  function is to be called. If you want to
1998  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.
1999  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2000  .sp  .sp
2001    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2002  .sp  .sp
2003  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
2004  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 2014  description of the interface to the call
2014  \fBpcrecallout\fP  \fBpcrecallout\fP
2015  .\"  .\"
2016  documentation.  documentation.
2017    .
2018    .
2019    .SH "BACTRACKING CONTROL"
2020    .rs
2021    .sp
2022    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2023    are described in the Perl documentation as "experimental and subject to change
2024    or removal in a future version of Perl". It goes on to say: "Their usage in
2025    production code should be noted to avoid problems during upgrades." The same
2026    remarks apply to the PCRE features described in this section.
2027  .P  .P
2028  .in 0  Since these verbs are specifically related to backtracking, they can be used
2029  Last updated: 06 June 2006  only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
2030  .br  backtracking algorithm. They cause an error if encountered by
2031  Copyright (c) 1997-2006 University of Cambridge.  \fBpcre_dfa_exec()\fP.
2032    .P
2033    The new verbs make use of what was previously invalid syntax: an opening
2034    parenthesis followed by an asterisk. In Perl, they are generally of the form
2035    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2036    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2037    are two kinds:
2038    .
2039    .SS "Verbs that act immediately"
2040    .rs
2041    .sp
2042    The following verbs act as soon as they are encountered:
2043    .sp
2044       (*ACCEPT)
2045    .sp
2046    This verb causes the match to end successfully, skipping the remainder of the
2047    pattern. When inside a recursion, only the innermost pattern is ended
2048    immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2049    capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2050    captured. For example:
2051    .sp
2052      A(A|B(*ACCEPT)|C)D
2053    .sp
2054    This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2055    captured.
2056    .sp
2057      (*FAIL) or (*F)
2058    .sp
2059    This verb causes the match to fail, forcing backtracking to occur. It is
2060    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2061    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2062    Perl features that are not present in PCRE. The nearest equivalent is the
2063    callout feature, as for example in this pattern:
2064    .sp
2065      a+(?C)(*FAIL)
2066    .sp
2067    A match with the string "aaaa" always fails, but the callout is taken before
2068    each backtrack happens (in this example, 10 times).
2069    .
2070    .SS "Verbs that act after backtracking"
2071    .rs
2072    .sp
2073    The following verbs do nothing when they are encountered. Matching continues
2074    with what follows, but if there is no subsequent match, a failure is forced.
2075    The verbs differ in exactly what kind of failure occurs.
2076    .sp
2077      (*COMMIT)
2078    .sp
2079    This verb causes the whole match to fail outright if the rest of the pattern
2080    does not match. Even if the pattern is unanchored, no further attempts to find
2081    a match by advancing the start point take place. Once (*COMMIT) has been
2082    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2083    starting point, or not at all. For example:
2084    .sp
2085      a+(*COMMIT)b
2086    .sp
2087    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2088    dynamic anchor, or "I've started, so I must finish."
2089    .sp
2090      (*PRUNE)
2091    .sp
2092    This verb causes the match to fail at the current position if the rest of the
2093    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2094    advance to the next starting character then happens. Backtracking can occur as
2095    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2096    if there is no match to the right, backtracking cannot cross (*PRUNE).
2097    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2098    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2099    be expressed in any other way.
2100    .sp
2101      (*SKIP)
2102    .sp
2103    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2104    "bumpalong" advance is not to the next character, but to the position in the
2105    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2106    was matched leading up to it cannot be part of a successful match. Consider:
2107    .sp
2108      a+(*SKIP)b
2109    .sp
2110    If the subject is "aaaac...", after the first match attempt fails (starting at
2111    the first character in the string), the starting point skips on to start the
2112    next attempt at "c". Note that a possessive quantifer does not have the same
2113    effect in this example; although it would suppress backtracking during the
2114    first match attempt, the second attempt would start at the second character
2115    instead of skipping on to "c".
2116    .sp
2117      (*THEN)
2118    .sp
2119    This verb causes a skip to the next alternation if the rest of the pattern does
2120    not match. That is, it cancels pending backtracking, but only within the
2121    current alternation. Its name comes from the observation that it can be used
2122    for a pattern-based if-then-else block:
2123    .sp
2124      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2125    .sp
2126    If the COND1 pattern matches, FOO is tried (and possibly further items after
2127    the end of the group if FOO succeeds); on failure the matcher skips to the
2128    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2129    is used outside of any alternation, it acts exactly like (*PRUNE).
2130    .
2131    .
2132    .SH "SEE ALSO"
2133    .rs
2134    .sp
2135    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
2136    .
2137    .
2138    .SH AUTHOR
2139    .rs
2140    .sp
2141    .nf
2142    Philip Hazel
2143    University Computing Service
2144    Cambridge CB2 3QH, England.
2145    .fi
2146    .
2147    .
2148    .SH REVISION
2149    .rs
2150    .sp
2151    .nf
2152    Last updated: 21 August 2007
2153    Copyright (c) 1997-2007 University of Cambridge.
2154    .fi

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