/[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 171 by ph10, Mon Jun 4 14:28:58 2007 UTC revision 469 by ph10, Mon Oct 19 14:38:48 2009 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. Some of the features discussed below are not available when  Perl-compatible. Some of the features discussed below are not available when
50  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of 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  alternative function, and how it differs from the normal function, are
52  discussed in the  discussed in the
# Line 40  discussed in the Line 56  discussed in the
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
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 does not affect what the \eR escape sequence matches. By
99    default, this is any Unicode newline sequence, for Perl compatibility. However,
100    this can be changed; see the description of \eR in the section entitled
101    .\" HTML <a href="#newlineseq">
102    .\" </a>
103    "Newline sequences"
104    .\"
105    below. A change of \eR setting can be combined with a change of newline
106    convention.
107    .
108    .
109  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
110  .rs  .rs
111  .sp  .sp
# Line 141  The \eQ...\eE sequence is recognized bot Line 207  The \eQ...\eE sequence is recognized bot
207  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
208  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
209  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
210  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is often easier to use
211  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
212  .sp  .sp
213    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
214    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
215    \ee        escape (hex 1B)    \ee        escape (hex 1B)
216    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
217    \en        newline (hex 0A)    \en        linefeed (hex 0A)
218    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
219    \et        tab (hex 09)    \et        tab (hex 09)
220    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
# Line 164  Thus \ecz becomes hex 1A, but \ec{ becom Line 229  Thus \ecz becomes hex 1A, but \ec{ becom
229  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
230  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{
231  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
232  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
233  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
234  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
235  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
236  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
237    there is no terminating }, this form of escape is not recognized. Instead, the
238    initial \ex will be interpreted as a basic hexadecimal escape, with no
239    following digits, giving a character whose value is zero.
240  .P  .P
241  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
242  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 240  meanings Line 308  meanings
308  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
309  .rs  .rs
310  .sp  .sp
311  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
312  in braces, is an absolute or relative back reference. A named back reference  enclosed in braces, is an absolute or relative back reference. A named back
313  can be coded as \eg{name}. Back references are discussed  reference can be coded as \eg{name}. Back references are discussed
314  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
315  .\" </a>  .\" </a>
316  later,  later,
# Line 254  parenthesized subpatterns. Line 322  parenthesized subpatterns.
322  .\"  .\"
323  .  .
324  .  .
325    .SS "Absolute and relative subroutine calls"
326    .rs
327    .sp
328    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
329    a number enclosed either in angle brackets or single quotes, is an alternative
330    syntax for referencing a subpattern as a "subroutine". Details are discussed
331    .\" HTML <a href="#onigurumasubroutines">
332    .\" </a>
333    later.
334    .\"
335    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
336    synonymous. The former is a back reference; the latter is a
337    .\" HTML <a href="#subpatternsassubroutines">
338    .\" </a>
339    subroutine
340    .\"
341    call.
342    .
343    .
344  .SS "Generic character types"  .SS "Generic character types"
345  .rs  .rs
346  .sp  .sp
# Line 262  following are always recognized: Line 349  following are always recognized:
349  .sp  .sp
350    \ed     any decimal digit    \ed     any decimal digit
351    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
352      \eh     any horizontal whitespace character
353      \eH     any character that is not a horizontal whitespace character
354    \es     any whitespace character    \es     any whitespace character
355    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
356      \ev     any vertical whitespace character
357      \eV     any character that is not a vertical whitespace character
358    \ew     any "word" character    \ew     any "word" character
359    \eW     any "non-word" character    \eW     any "non-word" character
360  .sp  .sp
# Line 277  there is no character to match. Line 368  there is no character to match.
368  .P  .P
369  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).
370  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
371  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
372  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
373  does.)  does.
374    .P
375    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
376    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
377    character property support is available. These sequences retain their original
378    meanings from before UTF-8 support was available, mainly for efficiency
379    reasons. Note that this also affects \eb, because it is defined in terms of \ew
380    and \eW.
381    .P
382    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
383    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
384    The horizontal space characters are:
385    .sp
386      U+0009     Horizontal tab
387      U+0020     Space
388      U+00A0     Non-break space
389      U+1680     Ogham space mark
390      U+180E     Mongolian vowel separator
391      U+2000     En quad
392      U+2001     Em quad
393      U+2002     En space
394      U+2003     Em space
395      U+2004     Three-per-em space
396      U+2005     Four-per-em space
397      U+2006     Six-per-em space
398      U+2007     Figure space
399      U+2008     Punctuation space
400      U+2009     Thin space
401      U+200A     Hair space
402      U+202F     Narrow no-break space
403      U+205F     Medium mathematical space
404      U+3000     Ideographic space
405    .sp
406    The vertical space characters are:
407    .sp
408      U+000A     Linefeed
409      U+000B     Vertical tab
410      U+000C     Formfeed
411      U+000D     Carriage return
412      U+0085     Next line
413      U+2028     Line separator
414      U+2029     Paragraph separator
415  .P  .P
416  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
417  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 295  in the Line 427  in the
427  .\"  .\"
428  page). For example, in a French locale such as "fr_FR" in Unix-like systems,  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
429  or "french" in Windows, some character codes greater than 128 are used for  or "french" in Windows, some character codes greater than 128 are used for
430  accented letters, and these are matched by \ew.  accented letters, and these are matched by \ew. The use of locales with Unicode
431  .P  is discouraged.
 In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  
 \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.  
432  .  .
433  .  .
434    .\" HTML <a name="newlineseq"></a>
435  .SS "Newline sequences"  .SS "Newline sequences"
436  .rs  .rs
437  .sp  .sp
438  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
439  sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
440  the following:  equivalent to the following:
441  .sp  .sp
442    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
443  .sp  .sp
# Line 328  are added: LS (line separator, U+2028) a Line 457  are added: LS (line separator, U+2028) a
457  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
458  recognized.  recognized.
459  .P  .P
460    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
461    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
462    either at compile time or when the pattern is matched. (BSR is an abbrevation
463    for "backslash R".) This can be made the default when PCRE is built; if this is
464    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
465    It is also possible to specify these settings by starting a pattern string with
466    one of the following sequences:
467    .sp
468      (*BSR_ANYCRLF)   CR, LF, or CRLF only
469      (*BSR_UNICODE)   any Unicode newline sequence
470    .sp
471    These override the default and the options given to \fBpcre_compile()\fP or
472    \fBpcre_compile2()\fP, but they can be overridden by options given to
473    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
474    which are not Perl-compatible, are recognized only at the very start of a
475    pattern, and that they must be in upper case. If more than one of them is
476    present, the last one is used. They can be combined with a change of newline
477    convention, for example, a pattern can start with:
478    .sp
479      (*ANY)(*BSR_ANYCRLF)
480    .sp
481  Inside a character class, \eR matches the letter "R".  Inside a character class, \eR matches the letter "R".
482  .  .
483  .  .
# Line 336  Inside a character class, \eR matches th Line 486  Inside a character class, \eR matches th
486  .rs  .rs
487  .sp  .sp
488  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
489  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
490  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
491    characters whose codepoints are less than 256, but they do work in this mode.
492    The extra escape sequences are:
493  .sp  .sp
494    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
495    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
# Line 489  The special property L& is also supporte Line 641  The special property L& is also supporte
641  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
642  a modifier or "other".  a modifier or "other".
643  .P  .P
644  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
645    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
646    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
647    (see the discussion of PCRE_NO_UTF8_CHECK in the
648    .\" HREF
649    \fBpcreapi\fP
650    .\"
651    page). Perl does not support the Cs property.
652    .P
653    The long synonyms for property names that Perl supports (such as \ep{Letter})
654  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
655  properties with "Is".  properties with "Is".
656  .P  .P
# Line 513  atomic group Line 674  atomic group
674  (see below).  (see below).
675  .\"  .\"
676  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
677  preceding character.  preceding character. None of them have codepoints less than 256, so in
678    non-UTF-8 mode \eX matches any one character.
679  .P  .P
680  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
681  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 525  properties in PCRE. Line 687  properties in PCRE.
687  .SS "Resetting the match start"  .SS "Resetting the match start"
688  .rs  .rs
689  .sp  .sp
690  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
691  matched characters not to be included in the final matched sequence. For  matched characters not to be included in the final matched sequence. For
692  example, the pattern:  example, the pattern:
693  .sp  .sp
694    foo\eKbar    foo\eKbar
695  .sp  .sp
696  matches "foobar", but reports that it has matched "bar". This feature is  matches "foobar", but reports that it has matched "bar". This feature is
697  similar to a lookbehind assertion  similar to a lookbehind assertion
698  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
699  .\" </a>  .\" </a>
700  (described below).  (described below).
701  .\"  .\"
702  However, in this case, the part of the subject before the real match does not  However, in this case, the part of the subject before the real match does not
703  have to be of fixed length, as lookbehind assertions do. The use of \eK does  have to be of fixed length, as lookbehind assertions do. The use of \eK does
704  not interfere with the setting of  not interfere with the setting of
705  .\" HTML <a href="#subpattern">  .\" HTML <a href="#subpattern">
706  .\" </a>  .\" </a>
707  captured substrings.  captured substrings.
708  .\"  .\"
709  For example, when the pattern  For example, when the pattern
710  .sp  .sp
711    (foo)\eKbar    (foo)\eKbar
712  .sp  .sp
713  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
714  .  .
715  .  .
716  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 579  different meaning, namely the backspace Line 741  different meaning, namely the backspace
741  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
742  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
743  \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
744  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. Neither PCRE nor Perl has a
745    separte "start of word" or "end of word" metasequence. However, whatever
746    follows \eb normally determines which it is. For example, the fragment
747    \eba matches "a" at the start of a word.
748  .P  .P
749  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
750  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 711  the lookbehind. Line 876  the lookbehind.
876  .rs  .rs
877  .sp  .sp
878  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
879  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.
880  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
881  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
882  escaped with a backslash.  a member of the class, it should be the first data character in the class
883    (after an initial circumflex, if present) or escaped with a backslash.
884  .P  .P
885  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
886  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
887  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
888  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
889  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
890  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 728  For example, the character class [aeiou] Line 894  For example, the character class [aeiou]
894  [^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
895  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
896  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
897  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
898  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
899  string.  string.
900  .P  .P
# Line 742  caseful version would. In UTF-8 mode, PC Line 908  caseful version would. In UTF-8 mode, PC
908  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
909  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
910  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
911  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,
912  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
913  UTF-8 support.  with UTF-8 support.
914  .P  .P
915  Characters that might indicate line breaks are never treated in any special way  Characters that might indicate line breaks are never treated in any special way
916  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 863  alternative in the subpattern. Line 1029  alternative in the subpattern.
1029  .rs  .rs
1030  .sp  .sp
1031  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1032  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
1033  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1034    The option letters are
1035  .sp  .sp
1036    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1037    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 878  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1045  PCRE_MULTILINE while unsetting PCRE_DOTA
1045  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
1046  unset.  unset.
1047  .P  .P
1048  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
1049  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
1050  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1051  the global options (and it will therefore show up in data extracted by the  .P
1052  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1053    subpattern parentheses), the change applies to the remainder of the pattern
1054    that follows. If the change is placed right at the start of a pattern, PCRE
1055    extracts it into the global options (and it will therefore show up in data
1056    extracted by the \fBpcre_fullinfo()\fP function).
1057  .P  .P
1058  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1059  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
# Line 901  branch is abandoned before the option se Line 1072  branch is abandoned before the option se
1072  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1073  behaviour otherwise.  behaviour otherwise.
1074  .P  .P
1075  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
1076  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
1077  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1078    the application has set or what has been defaulted. Details are given in the
1079    section entitled
1080    .\" HTML <a href="#newlineseq">
1081    .\" </a>
1082    "Newline sequences"
1083    .\"
1084    above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1085    mode; this is equivalent to setting the PCRE_UTF8 option.
1086  .  .
1087  .  .
1088  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 958  is reached, an option setting in one bra Line 1137  is reached, an option setting in one bra
1137  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1138  .  .
1139  .  .
1140    .\" HTML <a name="dupsubpatternnumber"></a>
1141    .SH "DUPLICATE SUBPATTERN NUMBERS"
1142    .rs
1143    .sp
1144    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1145    the same numbers for its capturing parentheses. Such a subpattern starts with
1146    (?| and is itself a non-capturing subpattern. For example, consider this
1147    pattern:
1148    .sp
1149      (?|(Sat)ur|(Sun))day
1150    .sp
1151    Because the two alternatives are inside a (?| group, both sets of capturing
1152    parentheses are numbered one. Thus, when the pattern matches, you can look
1153    at captured substring number one, whichever alternative matched. This construct
1154    is useful when you want to capture part, but not all, of one of a number of
1155    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1156    number is reset at the start of each branch. The numbers of any capturing
1157    buffers that follow the subpattern start after the highest number used in any
1158    branch. The following example is taken from the Perl documentation.
1159    The numbers underneath show in which buffer the captured content will be
1160    stored.
1161    .sp
1162      # before  ---------------branch-reset----------- after
1163      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1164      # 1            2         2  3        2     3     4
1165    .sp
1166    A backreference to a numbered subpattern uses the most recent value that is set
1167    for that number by any subpattern. The following pattern matches "abcabc" or
1168    "defdef":
1169    .sp
1170      /(?|(abc)|(def))\e1/
1171    .sp
1172    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1173    refers to the first one in the pattern with the given number. The following
1174    pattern matches "abcabc" or "defabc":
1175    .sp
1176      /(?|(abc)|(def))(?1)/
1177    .sp
1178    If a
1179    .\" HTML <a href="#conditions">
1180    .\" </a>
1181    condition test
1182    .\"
1183    for a subpattern's having matched refers to a non-unique number, the test is
1184    true if any of the subpatterns of that number have matched.
1185    .P
1186    An alternative approach to using this "branch reset" feature is to use
1187    duplicate named subpatterns, as described in the next section.
1188    .
1189    .
1190  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1191  .rs  .rs
1192  .sp  .sp
# Line 967  if an expression is modified, the number Line 1196  if an expression is modified, the number
1196  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1197  added to Perl until release 5.10. Python had the feature earlier, and PCRE  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1198  introduced it at release 4.0, using the Python syntax. PCRE now supports both  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1199  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1200    have different names, but PCRE does not.
1201  .P  .P
1202  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1203  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
# Line 994  extracting the name-to-number translatio Line 1224  extracting the name-to-number translatio
1224  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1225  .P  .P
1226  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
1227  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
1228  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
1229  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
1230  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
1231  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
1232    name, and in both cases you want to extract the abbreviation. This pattern
1233    (ignoring the line breaks) does the job:
1234  .sp  .sp
1235    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1236    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1007  abbreviation. This pattern (ignoring the Line 1239  abbreviation. This pattern (ignoring the
1239    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1240  .sp  .sp
1241  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.
1242    (An alternative way of solving this problem is to use a "branch reset"
1243    subpattern, as described in the previous section.)
1244    .P
1245  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1246  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
1247  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1248  make a reference to a non-unique named subpattern from elsewhere in the  .P
1249  pattern, the one that corresponds to the lowest number is used. For further  If you make a backreference to a non-unique named subpattern from elsewhere in
1250  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1251    used. In the absence of duplicate numbers (see the previous section) this is
1252    the one with the lowest number. If you use a named reference in a condition
1253    test (see the
1254    .\"
1255    .\" HTML <a href="#conditions">
1256    .\" </a>
1257    section about conditions
1258    .\"
1259    below), either to check whether a subpattern has matched, or to check for
1260    recursion, all subpatterns with the same name are tested. If the condition is
1261    true for any one of them, the overall condition is true. This is the same
1262    behaviour as testing by number. For further details of the interfaces for
1263    handling named subpatterns, see the
1264  .\" HREF  .\" HREF
1265  \fBpcreapi\fP  \fBpcreapi\fP
1266  .\"  .\"
1267  documentation.  documentation.
1268    .P
1269    \fBWarning:\fP You cannot use different names to distinguish between two
1270    subpatterns with the same number because PCRE uses only the numbers when
1271    matching. For this reason, an error is given at compile time if different names
1272    are given to subpatterns with the same number. However, you can give the same
1273    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1274  .  .
1275  .  .
1276  .SH REPETITION  .SH REPETITION
# Line 1034  items: Line 1288  items:
1288    a character class    a character class
1289    a back reference (see next section)    a back reference (see next section)
1290    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1291      a recursive or "subroutine" call to a subpattern
1292  .sp  .sp
1293  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1294  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1065  support is available, \eX{3} matches thr Line 1320  support is available, \eX{3} matches thr
1320  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).
1321  .P  .P
1322  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
1323  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1324    subpatterns that are referenced as
1325    .\" HTML <a href="#subpatternsassubroutines">
1326    .\" </a>
1327    subroutines
1328    .\"
1329    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1330    quantifier are omitted from the compiled pattern.
1331  .P  .P
1332  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1333  abbreviations:  abbreviations:
# Line 1212  previous example can be rewritten as Line 1474  previous example can be rewritten as
1474  .sp  .sp
1475    \ed++foo    \ed++foo
1476  .sp  .sp
1477    Note that a possessive quantifier can be used with an entire group, for
1478    example:
1479    .sp
1480      (abc|xyz){2,3}+
1481    .sp
1482  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1483  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
1484  atomic group. However, there is no difference in the meaning of a possessive  atomic group. However, there is no difference in the meaning of a possessive
# Line 1286  subpattern is possible using named paren Line 1553  subpattern is possible using named paren
1553  .P  .P
1554  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1555  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence, which is a feature introduced in
1556  Perl 5.10. This escape must be followed by a positive or a negative number,  Perl 5.10. This escape must be followed by an unsigned number or a negative
1557  optionally enclosed in braces. These examples are all identical:  number, optionally enclosed in braces. These examples are all identical:
1558  .sp  .sp
1559    (ring), \e1    (ring), \e1
1560    (ring), \eg1    (ring), \eg1
1561    (ring), \eg{1}    (ring), \eg{1}
1562  .sp  .sp
1563  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1564  present in the older syntax. It is also useful when literal digits follow the  is present in the older syntax. It is also useful when literal digits follow
1565  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1566    example:
1567  .sp  .sp
1568    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1569  .sp  .sp
# Line 1342  after the reference. Line 1610  after the reference.
1610  .P  .P
1611  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
1612  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1613  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1614  .sp  .sp
1615    (a|(bc))\e2    (a|(bc))\e2
1616  .sp  .sp
1617  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
1618  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
1619  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1620  with a digit character, some delimiter must be used to terminate the back  .P
1621  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1622  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1623    If the pattern continues with a digit character, some delimiter must be used to
1624    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1625    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1626  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1627  .\" </a>  .\" </a>
1628  "Comments"  "Comments"
# Line 1424  lookbehind assertion is needed to achiev Line 1695  lookbehind assertion is needed to achiev
1695  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
1696  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
1697  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.
1698    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1699    synonym for (?!).
1700  .  .
1701  .  .
1702  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1448  is permitted, but Line 1721  is permitted, but
1721  .sp  .sp
1722  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1723  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
1724  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
1725  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1726  .sp  .sp
1727    (?<=ab(c|de))    (?<=ab(c|de))
1728  .sp  .sp
1729  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
1730  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
1731    branches:
1732  .sp  .sp
1733    (?<=abc|abde)    (?<=abc|abde)
1734  .sp  .sp
1735  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the Perl 5.10 escape sequence \eK
1736  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1737  .\" </a>  .\" </a>
1738  (see above)  (see above)
1739  .\"  .\"
1740  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
1741  fixed-length.  restriction.
1742  .P  .P
1743  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1744  temporarily move the current position back by the fixed length and then try to  temporarily move the current position back by the fixed length and then try to
# Line 1476  to appear in lookbehind assertions, beca Line 1750  to appear in lookbehind assertions, beca
1750  the length of the lookbehind. The \eX and \eR escapes, which can match  the length of the lookbehind. The \eX and \eR escapes, which can match
1751  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1752  .P  .P
1753    .\" HTML <a href="#subpatternsassubroutines">
1754    .\" </a>
1755    "Subroutine"
1756    .\"
1757    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1758    as the subpattern matches a fixed-length string.
1759    .\" HTML <a href="#recursion">
1760    .\" </a>
1761    Recursion,
1762    .\"
1763    however, is not supported.
1764    .P
1765  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1766  specify efficient matching at the end of the subject string. Consider a simple  specify efficient matching of fixed-length strings at the end of subject
1767  pattern such as  strings. Consider a simple pattern such as
1768  .sp  .sp
1769    abcd$    abcd$
1770  .sp  .sp
# Line 1542  characters that are not "999". Line 1828  characters that are not "999".
1828  .sp  .sp
1829  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1830  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1831  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1832  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1833  .sp  .sp
1834    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1835    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
# Line 1559  recursion, a pseudo-condition called DEF Line 1845  recursion, a pseudo-condition called DEF
1845  .rs  .rs
1846  .sp  .sp
1847  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
1848  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
1849  matched. An alternative notation is to precede the digits with a plus or minus  matched. If there is more than one capturing subpattern with the same number
1850  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
1851  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
1852  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
1853  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
1854    section about duplicate subpattern numbers),
1855    .\"
1856    the condition is true if any of them have been set. An alternative notation is
1857    to precede the digits with a plus or minus sign. In this case, the subpattern
1858    number is relative rather than absolute. The most recently opened parentheses
1859    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1860    looping constructs it can also make sense to refer to subsequent groups with
1861    constructs such as (?(+2).
1862  .P  .P
1863  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1864  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 1582  parenthesis is required. Otherwise, sinc Line 1876  parenthesis is required. Otherwise, sinc
1876  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1877  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1878  .P  .P
1879  If you were embedding this pattern in a larger one, you could use a relative  If you were embedding this pattern in a larger one, you could use a relative
1880  reference:  reference:
1881  .sp  .sp
1882    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
# Line 1605  Rewriting the above example to use a nam Line 1899  Rewriting the above example to use a nam
1899  .sp  .sp
1900    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1901  .sp  .sp
1902    If the name used in a condition of this kind is a duplicate, the test is
1903    applied to all subpatterns of the same name, and is true if any one of them has
1904    matched.
1905  .  .
1906  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
1907  .rs  .rs
# Line 1616  letter R, for example: Line 1913  letter R, for example:
1913  .sp  .sp
1914    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
1915  .sp  .sp
1916  the condition is true if the most recent recursion is into the subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
1917  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
1918  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
1919    applied to all subpatterns of the same name, and is true if any one of them is
1920    the most recent recursion.
1921  .P  .P
1922  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
1923  patterns are described below.  .\" HTML <a href="#recursion">
1924    .\" </a>
1925    The syntax for recursive patterns
1926    .\"
1927    is described below.
1928  .  .
1929  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
1930  .rs  .rs
# Line 1630  If the condition is the string (DEFINE), Line 1933  If the condition is the string (DEFINE),
1933  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
1934  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
1935  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
1936  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  "subroutines" that can be referenced from elsewhere. (The use of
1937    .\" HTML <a href="#subpatternsassubroutines">
1938    .\" </a>
1939    "subroutines"
1940    .\"
1941  is described below.) For example, a pattern to match an IPv4 address could be  is described below.) For example, a pattern to match an IPv4 address could be
1942  written like this (ignore whitespace and line breaks):  written like this (ignore whitespace and line breaks):
1943  .sp  .sp
# Line 1640  written like this (ignore whitespace and Line 1947  written like this (ignore whitespace and
1947  The first part of the pattern is a DEFINE group inside which a another group  The first part of the pattern is a DEFINE group inside which a another group
1948  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
1949  address (a number less than 256). When matching takes place, this part of the  address (a number less than 256). When matching takes place, this part of the
1950  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
1951  .P  pattern uses references to the named group to match the four dot-separated
1952  The rest of the pattern uses references to the named group to match the four  components of an IPv4 address, insisting on a word boundary at each end.
 dot-separated components of an IPv4 address, insisting on a word boundary at  
 each end.  
1953  .  .
1954  .SS "Assertion conditions"  .SS "Assertion conditions"
1955  .rs  .rs
# Line 1701  recursively to the pattern in which it a Line 2006  recursively to the pattern in which it a
2006  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2007  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2008  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2009  this kind of recursion was introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2010  .P  .P
2011  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
2012  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive call of the subpattern of the given number,
2013  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  provided that it occurs inside that subpattern. (If not, it is a
2014    .\" HTML <a href="#subpatternsassubroutines">
2015    .\" </a>
2016    "subroutine"
2017    .\"
2018  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
2019  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2020  .P  .P
 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  
 treated as an atomic group. That is, once it has matched some of the subject  
 string, it is never re-entered, even if it contains untried alternatives and  
 there is a subsequent matching failure.  
 .P  
2021  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2022  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2023  .sp  .sp
2024    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2025  .sp  .sp
2026  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2027  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
2028  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2029  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2030    to avoid backtracking into sequences of non-parentheses.
2031  .P  .P
2032  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
2033  pattern, so instead you could use this:  pattern, so instead you could use this:
2034  .sp  .sp
2035    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2036  .sp  .sp
2037  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
2038  them instead of the whole pattern.  them instead of the whole pattern.
2039  .P  .P
2040  In a larger pattern, keeping track of parenthesis numbers can be tricky. This  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2041  is made easier by the use of relative references. (A Perl 5.10 feature.)  is made easier by the use of relative references (a Perl 5.10 feature).
2042  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
2043  most recently opened parentheses preceding the recursion. In other words, a  most recently opened parentheses preceding the recursion. In other words, a
2044  negative number counts capturing parentheses leftwards from the point at which  negative number counts capturing parentheses leftwards from the point at which
# Line 1742  it is encountered. Line 2047  it is encountered.
2047  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2048  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2049  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2050  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2051    .\" </a>
2052    "subroutine"
2053    .\"
2054    calls, as described in the next section.
2055  .P  .P
2056  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2057  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2058  could rewrite the above example as follows:  could rewrite the above example as follows:
2059  .sp  .sp
2060    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2061  .sp  .sp
2062  If there is more than one subpattern with the same name, the earliest one is  If there is more than one subpattern with the same name, the earliest one is
2063  used.  used.
2064  .P  .P
2065  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2066  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of a possessive quantifier for matching
2067  non-parentheses is important when applying the pattern to strings that do not  strings of non-parentheses is important when applying the pattern to strings
2068  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2069  .sp  .sp
2070    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2071  .sp  .sp
2072  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,
2073  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
2074  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
2075  before failure can be reported.  before failure can be reported.
2076  .P  .P
2077  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
2078  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
2079  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2080  .\" HREF  .\" HREF
2081  \fBpcrecallout\fP  \fBpcrecallout\fP
2082  .\"  .\"
# Line 1776  documentation). If the pattern above is Line 2084  documentation). If the pattern above is
2084  .sp  .sp
2085    (ab(cd)ef)    (ab(cd)ef)
2086  .sp  .sp
2087  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
2088  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
2089  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2090    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2091       ^                        ^  .P
2092       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2093  .sp  obtain extra memory to store data during a recursion, which it does by using
2094  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
2095  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.  
2096  .P  .P
2097  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.
2098  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1801  different alternatives for the recursive Line 2106  different alternatives for the recursive
2106  is the actual recursive call.  is the actual recursive call.
2107  .  .
2108  .  .
2109    .\" HTML <a name="recursiondifference"></a>
2110    .SS "Recursion difference from Perl"
2111    .rs
2112    .sp
2113    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2114    treated as an atomic group. That is, once it has matched some of the subject
2115    string, it is never re-entered, even if it contains untried alternatives and
2116    there is a subsequent matching failure. This can be illustrated by the
2117    following pattern, which purports to match a palindromic string that contains
2118    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2119    .sp
2120      ^(.|(.)(?1)\e2)$
2121    .sp
2122    The idea is that it either matches a single character, or two identical
2123    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2124    it does not if the pattern is longer than three characters. Consider the
2125    subject string "abcba":
2126    .P
2127    At the top level, the first character is matched, but as it is not at the end
2128    of the string, the first alternative fails; the second alternative is taken
2129    and the recursion kicks in. The recursive call to subpattern 1 successfully
2130    matches the next character ("b"). (Note that the beginning and end of line
2131    tests are not part of the recursion).
2132    .P
2133    Back at the top level, the next character ("c") is compared with what
2134    subpattern 2 matched, which was "a". This fails. Because the recursion is
2135    treated as an atomic group, there are now no backtracking points, and so the
2136    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2137    try the second alternative.) However, if the pattern is written with the
2138    alternatives in the other order, things are different:
2139    .sp
2140      ^((.)(?1)\e2|.)$
2141    .sp
2142    This time, the recursing alternative is tried first, and continues to recurse
2143    until it runs out of characters, at which point the recursion fails. But this
2144    time we do have another alternative to try at the higher level. That is the big
2145    difference: in the previous case the remaining alternative is at a deeper
2146    recursion level, which PCRE cannot use.
2147    .P
2148    To change the pattern so that matches all palindromic strings, not just those
2149    with an odd number of characters, it is tempting to change the pattern to this:
2150    .sp
2151      ^((.)(?1)\e2|.?)$
2152    .sp
2153    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2154    deeper recursion has matched a single character, it cannot be entered again in
2155    order to match an empty string. The solution is to separate the two cases, and
2156    write out the odd and even cases as alternatives at the higher level:
2157    .sp
2158      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2159    .sp
2160    If you want to match typical palindromic phrases, the pattern has to ignore all
2161    non-word characters, which can be done like this:
2162    .sp
2163      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2164    .sp
2165    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2166    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2167    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2168    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2169    more) to match typical phrases, and Perl takes so long that you think it has
2170    gone into a loop.
2171    .P
2172    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2173    string does not start with a palindrome that is shorter than the entire string.
2174    For example, although "abcba" is correctly matched, if the subject is "ababa",
2175    PCRE finds the palindrome "aba" at the start, then fails at top level because
2176    the end of the string does not follow. Once again, it cannot jump back into the
2177    recursion to try other alternatives, so the entire match fails.
2178    .
2179    .
2180  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2181  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2182  .rs  .rs
# Line 1813  relative, as in these examples: Line 2189  relative, as in these examples:
2189  .sp  .sp
2190    (...(absolute)...)...(?2)...    (...(absolute)...)...(?2)...
2191    (...(relative)...)...(?-1)...    (...(relative)...)...(?-1)...
2192    (...(?+1)...(relative)...    (...(?+1)...(relative)...
2193  .sp  .sp
2194  An earlier example pointed out that the pattern  An earlier example pointed out that the pattern
2195  .sp  .sp
# Line 1827  matches "sense and sensibility" and "res Line 2203  matches "sense and sensibility" and "res
2203  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
2204  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2205  .P  .P
2206  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2207  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
2208  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
2209  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2210    call revert to their previous values afterwards.
2211  .P  .P
2212  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2213  case-independence are fixed when the subpattern is defined. They cannot be  case-independence are fixed when the subpattern is defined. They cannot be
# Line 1842  It matches "abcabc". It does not match " Line 2219  It matches "abcabc". It does not match "
2219  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2220  .  .
2221  .  .
2222    .\" HTML <a name="onigurumasubroutines"></a>
2223    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2224    .rs
2225    .sp
2226    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2227    a number enclosed either in angle brackets or single quotes, is an alternative
2228    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2229    are two of the examples used above, rewritten using this syntax:
2230    .sp
2231      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2232      (sens|respons)e and \eg'1'ibility
2233    .sp
2234    PCRE supports an extension to Oniguruma: if a number is preceded by a
2235    plus or a minus sign it is taken as a relative reference. For example:
2236    .sp
2237      (abc)(?i:\eg<-1>)
2238    .sp
2239    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2240    synonymous. The former is a back reference; the latter is a subroutine call.
2241    .
2242    .
2243  .SH CALLOUTS  .SH CALLOUTS
2244  .rs  .rs
2245  .sp  .sp
# Line 1878  description of the interface to the call Line 2276  description of the interface to the call
2276  documentation.  documentation.
2277  .  .
2278  .  .
2279    .SH "BACKTRACKING CONTROL"
2280    .rs
2281    .sp
2282    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2283    are described in the Perl documentation as "experimental and subject to change
2284    or removal in a future version of Perl". It goes on to say: "Their usage in
2285    production code should be noted to avoid problems during upgrades." The same
2286    remarks apply to the PCRE features described in this section.
2287    .P
2288    Since these verbs are specifically related to backtracking, most of them can be
2289    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2290    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2291    failing negative assertion, they cause an error if encountered by
2292    \fBpcre_dfa_exec()\fP.
2293    .P
2294    If any of these verbs are used in an assertion or subroutine subpattern
2295    (including recursive subpatterns), their effect is confined to that subpattern;
2296    it does not extend to the surrounding pattern. Note that such subpatterns are
2297    processed as anchored at the point where they are tested.
2298    .P
2299    The new verbs make use of what was previously invalid syntax: an opening
2300    parenthesis followed by an asterisk. In Perl, they are generally of the form
2301    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2302    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2303    are two kinds:
2304    .
2305    .SS "Verbs that act immediately"
2306    .rs
2307    .sp
2308    The following verbs act as soon as they are encountered:
2309    .sp
2310       (*ACCEPT)
2311    .sp
2312    This verb causes the match to end successfully, skipping the remainder of the
2313    pattern. When inside a recursion, only the innermost pattern is ended
2314    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2315    captured. (This feature was added to PCRE at release 8.00.) For example:
2316    .sp
2317      A((?:A|B(*ACCEPT)|C)D)
2318    .sp
2319    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2320    the outer parentheses.
2321    .sp
2322      (*FAIL) or (*F)
2323    .sp
2324    This verb causes the match to fail, forcing backtracking to occur. It is
2325    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2326    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2327    Perl features that are not present in PCRE. The nearest equivalent is the
2328    callout feature, as for example in this pattern:
2329    .sp
2330      a+(?C)(*FAIL)
2331    .sp
2332    A match with the string "aaaa" always fails, but the callout is taken before
2333    each backtrack happens (in this example, 10 times).
2334    .
2335    .SS "Verbs that act after backtracking"
2336    .rs
2337    .sp
2338    The following verbs do nothing when they are encountered. Matching continues
2339    with what follows, but if there is no subsequent match, a failure is forced.
2340    The verbs differ in exactly what kind of failure occurs.
2341    .sp
2342      (*COMMIT)
2343    .sp
2344    This verb causes the whole match to fail outright if the rest of the pattern
2345    does not match. Even if the pattern is unanchored, no further attempts to find
2346    a match by advancing the starting point take place. Once (*COMMIT) has been
2347    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2348    starting point, or not at all. For example:
2349    .sp
2350      a+(*COMMIT)b
2351    .sp
2352    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2353    dynamic anchor, or "I've started, so I must finish."
2354    .sp
2355      (*PRUNE)
2356    .sp
2357    This verb causes the match to fail at the current position if the rest of the
2358    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2359    advance to the next starting character then happens. Backtracking can occur as
2360    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2361    if there is no match to the right, backtracking cannot cross (*PRUNE).
2362    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2363    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2364    be expressed in any other way.
2365    .sp
2366      (*SKIP)
2367    .sp
2368    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2369    "bumpalong" advance is not to the next character, but to the position in the
2370    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2371    was matched leading up to it cannot be part of a successful match. Consider:
2372    .sp
2373      a+(*SKIP)b
2374    .sp
2375    If the subject is "aaaac...", after the first match attempt fails (starting at
2376    the first character in the string), the starting point skips on to start the
2377    next attempt at "c". Note that a possessive quantifer does not have the same
2378    effect as this example; although it would suppress backtracking during the
2379    first match attempt, the second attempt would start at the second character
2380    instead of skipping on to "c".
2381    .sp
2382      (*THEN)
2383    .sp
2384    This verb causes a skip to the next alternation if the rest of the pattern does
2385    not match. That is, it cancels pending backtracking, but only within the
2386    current alternation. Its name comes from the observation that it can be used
2387    for a pattern-based if-then-else block:
2388    .sp
2389      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2390    .sp
2391    If the COND1 pattern matches, FOO is tried (and possibly further items after
2392    the end of the group if FOO succeeds); on failure the matcher skips to the
2393    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2394    is used outside of any alternation, it acts exactly like (*PRUNE).
2395    .
2396    .
2397  .SH "SEE ALSO"  .SH "SEE ALSO"
2398  .rs  .rs
2399  .sp  .sp
2400  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2401    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2402  .  .
2403  .  .
2404  .SH AUTHOR  .SH AUTHOR
# Line 1898  Cambridge CB2 3QH, England. Line 2415  Cambridge CB2 3QH, England.
2415  .rs  .rs
2416  .sp  .sp
2417  .nf  .nf
2418  Last updated: 29 May 2007  Last updated: 18 October 2009
2419  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2009 University of Cambridge.
2420  .fi  .fi

Legend:
Removed from v.171  
changed lines
  Added in v.469

webmaster@exim.org
ViewVC Help
Powered by ViewVC 1.1.12