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# 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 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 289  In UTF-8 mode, characters with values gr Line 376  In UTF-8 mode, characters with values gr
376  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
377  character property support is available. These sequences retain their original  character property support is available. These sequences retain their original
378  meanings from before UTF-8 support was available, mainly for efficiency  meanings from before UTF-8 support was available, mainly for efficiency
379  reasons.  reasons. Note that this also affects \eb, because it is defined in terms of \ew
380    and \eW.
381  .P  .P
382  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
383  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  other sequences, these do match certain high-valued codepoints in UTF-8 mode.
# Line 343  accented letters, and these are matched Line 431  accented letters, and these are matched
431  is discouraged.  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 a Perl 5.10 feature. 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 368  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 376  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 529  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 553  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 619  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 751  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 768  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 782  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 903  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 918  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 941  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 998  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"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1142  .rs  .rs
1143  .sp  .sp
# Line 1023  stored. Line 1163  stored.
1163    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1164    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1165  .sp  .sp
1166  A backreference or a recursive call to a numbered subpattern always refers to  A backreference to a numbered subpattern uses the most recent value that is set
1167  the first one in the pattern with the given number.  for that number by any subpattern. The following pattern matches "abcabc" or
1168  .P  "defdef":
1169  An alternative approach to using this "branch reset" feature is to use  .sp
1170      /(?|(abc)|(def))\1/
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    An alternative approach to using the "branch reset" feature is to use
1179  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
1180  .  .
1181  .  .
# Line 1066  extracting the name-to-number translatio Line 1215  extracting the name-to-number translatio
1215  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1216  .P  .P
1217  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
1218  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
1219  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
1220  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
1221  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
1222  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
1223    name, and in both cases you want to extract the abbreviation. This pattern
1224    (ignoring the line breaks) does the job:
1225  .sp  .sp
1226    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1227    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1092  details of the interfaces for handling n Line 1243  details of the interfaces for handling n
1243  \fBpcreapi\fP  \fBpcreapi\fP
1244  .\"  .\"
1245  documentation.  documentation.
1246    .P
1247    \fBWarning:\fP You cannot use different names to distinguish between two
1248    subpatterns with the same number because PCRE uses only the numbers when
1249    matching. For this reason, an error is given at compile time if different names
1250    are given to subpatterns with the same number. However, you can give the same
1251    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1252  .  .
1253  .  .
1254  .SH REPETITION  .SH REPETITION
# Line 1109  items: Line 1266  items:
1266    a character class    a character class
1267    a back reference (see next section)    a back reference (see next section)
1268    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1269      a recursive or "subroutine" call to a subpattern
1270  .sp  .sp
1271  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1272  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1140  support is available, \eX{3} matches thr Line 1298  support is available, \eX{3} matches thr
1298  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).
1299  .P  .P
1300  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
1301  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1302    subpatterns that are referenced as
1303    .\" HTML <a href="#subpatternsassubroutines">
1304    .\" </a>
1305    subroutines
1306    .\"
1307    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1308    quantifier are omitted from the compiled pattern.
1309  .P  .P
1310  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1311  abbreviations:  abbreviations:
# Line 1287  previous example can be rewritten as Line 1452  previous example can be rewritten as
1452  .sp  .sp
1453    \ed++foo    \ed++foo
1454  .sp  .sp
1455    Note that a possessive quantifier can be used with an entire group, for
1456    example:
1457    .sp
1458      (abc|xyz){2,3}+
1459    .sp
1460  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1461  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
1462  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 1361  subpattern is possible using named paren Line 1531  subpattern is possible using named paren
1531  .P  .P
1532  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
1533  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
1534  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
1535  optionally enclosed in braces. These examples are all identical:  number, optionally enclosed in braces. These examples are all identical:
1536  .sp  .sp
1537    (ring), \e1    (ring), \e1
1538    (ring), \eg1    (ring), \eg1
1539    (ring), \eg{1}    (ring), \eg{1}
1540  .sp  .sp
1541  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1542  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
1543  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1544    example:
1545  .sp  .sp
1546    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1547  .sp  .sp
# Line 1417  after the reference. Line 1588  after the reference.
1588  .P  .P
1589  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
1590  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1591  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1592  .sp  .sp
1593    (a|(bc))\e2    (a|(bc))\e2
1594  .sp  .sp
1595  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
1596  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
1597  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1598  with a digit character, some delimiter must be used to terminate the back  .P
1599  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1600  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1601    If the pattern continues with a digit character, some delimiter must be used to
1602    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1603    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1604  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1605  .\" </a>  .\" </a>
1606  "Comments"  "Comments"
# Line 1499  lookbehind assertion is needed to achiev Line 1673  lookbehind assertion is needed to achiev
1673  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
1674  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
1675  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.
1676    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1677    synonym for (?!).
1678  .  .
1679  .  .
1680  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1523  is permitted, but Line 1699  is permitted, but
1699  .sp  .sp
1700  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1701  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
1702  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
1703  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1704  .sp  .sp
1705    (?<=ab(c|de))    (?<=ab(c|de))
1706  .sp  .sp
1707  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
1708  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
1709    branches:
1710  .sp  .sp
1711    (?<=abc|abde)    (?<=abc|abde)
1712  .sp  .sp
# Line 1538  In some cases, the Perl 5.10 escape sequ Line 1715  In some cases, the Perl 5.10 escape sequ
1715  .\" </a>  .\" </a>
1716  (see above)  (see above)
1717  .\"  .\"
1718  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
1719  fixed-length.  restriction.
1720  .P  .P
1721  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1722  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 1551  to appear in lookbehind assertions, beca Line 1728  to appear in lookbehind assertions, beca
1728  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
1729  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1730  .P  .P
1731    .\" HTML <a href="#subpatternsassubroutines">
1732    .\" </a>
1733    "Subroutine"
1734    .\"
1735    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1736    as the subpattern matches a fixed-length string.
1737    .\" HTML <a href="#recursion">
1738    .\" </a>
1739    Recursion,
1740    .\"
1741    however, is not supported.
1742    .P
1743  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1744  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
1745  pattern such as  strings. Consider a simple pattern such as
1746  .sp  .sp
1747    abcd$    abcd$
1748  .sp  .sp
# Line 1617  characters that are not "999". Line 1806  characters that are not "999".
1806  .sp  .sp
1807  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1808  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1809  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1810  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1811  .sp  .sp
1812    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1813    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
# Line 1634  recursion, a pseudo-condition called DEF Line 1823  recursion, a pseudo-condition called DEF
1823  .rs  .rs
1824  .sp  .sp
1825  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
1826  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
1827  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
1828  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
1829  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
1830  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
1831  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
1832    section about duplicate subpattern numbers),
1833    .\"
1834    the condition is true if any of them have been set. An alternative notation is
1835    to precede the digits with a plus or minus sign. In this case, the subpattern
1836    number is relative rather than absolute. The most recently opened parentheses
1837    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1838    looping constructs it can also make sense to refer to subsequent groups with
1839    constructs such as (?(+2).
1840  .P  .P
1841  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1842  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 1691  letter R, for example: Line 1888  letter R, for example:
1888  .sp  .sp
1889    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
1890  .sp  .sp
1891  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
1892  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
1893  stack.  stack.
1894  .P  .P
1895  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
1896  patterns are described below.  .\" HTML <a href="#recursion">
1897    .\" </a>
1898    Recursive patterns
1899    .\"
1900    are described below.
1901  .  .
1902  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
1903  .rs  .rs
# Line 1705  If the condition is the string (DEFINE), Line 1906  If the condition is the string (DEFINE),
1906  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
1907  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
1908  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
1909  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  "subroutines" that can be referenced from elsewhere. (The use of
1910    .\" HTML <a href="#subpatternsassubroutines">
1911    .\" </a>
1912    "subroutines"
1913    .\"
1914  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
1915  written like this (ignore whitespace and line breaks):  written like this (ignore whitespace and line breaks):
1916  .sp  .sp
# Line 1715  written like this (ignore whitespace and Line 1920  written like this (ignore whitespace and
1920  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
1921  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
1922  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
1923  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
1924  .P  pattern uses references to the named group to match the four dot-separated
1925  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.  
1926  .  .
1927  .SS "Assertion conditions"  .SS "Assertion conditions"
1928  .rs  .rs
# Line 1776  recursively to the pattern in which it a Line 1979  recursively to the pattern in which it a
1979  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1980  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
1981  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
1982  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.
1983  .P  .P
1984  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
1985  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,
1986  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
1987    .\" HTML <a href="#subpatternsassubroutines">
1988    .\" </a>
1989    "subroutine"
1990    .\"
1991  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
1992  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
1993  .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  
1994  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
1995  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
1996  .sp  .sp
1997    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
1998  .sp  .sp
1999  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2000  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
2001  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2002  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2003    to avoid backtracking into sequences of non-parentheses.
2004  .P  .P
2005  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
2006  pattern, so instead you could use this:  pattern, so instead you could use this:
2007  .sp  .sp
2008    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2009  .sp  .sp
2010  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
2011  them instead of the whole pattern.  them instead of the whole pattern.
2012  .P  .P
2013  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
2014  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).
2015  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
2016  most recently opened parentheses preceding the recursion. In other words, a  most recently opened parentheses preceding the recursion. In other words, a
2017  negative number counts capturing parentheses leftwards from the point at which  negative number counts capturing parentheses leftwards from the point at which
# Line 1817  it is encountered. Line 2020  it is encountered.
2020  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2021  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2022  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2023  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2024    .\" </a>
2025    "subroutine"
2026    .\"
2027    calls, as described in the next section.
2028  .P  .P
2029  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2030  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
2031  could rewrite the above example as follows:  could rewrite the above example as follows:
2032  .sp  .sp
2033    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2034  .sp  .sp
2035  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
2036  used.  used.
2037  .P  .P
2038  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2039  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
2040  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
2041  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2042  .sp  .sp
2043    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2044  .sp  .sp
2045  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,
2046  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
2047  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
2048  before failure can be reported.  before failure can be reported.
# Line 1854  documentation). If the pattern above is Line 2061  documentation). If the pattern above is
2061  the value for the capturing parentheses is "ef", which is the last value taken  the value for the capturing parentheses is "ef", which is the last value taken
2062  on at the top level. If additional parentheses are added, giving  on at the top level. If additional parentheses are added, giving
2063  .sp  .sp
2064    \e( ( ( (?>[^()]+) | (?R) )* ) \e)    \e( ( ( [^()]++ | (?R) )* ) \e)
2065       ^                        ^       ^                        ^
2066       ^                        ^       ^                        ^
2067  .sp  .sp
# Line 1876  different alternatives for the recursive Line 2083  different alternatives for the recursive
2083  is the actual recursive call.  is the actual recursive call.
2084  .  .
2085  .  .
2086    .\" HTML <a name="recursiondifference"></a>
2087    .SS "Recursion difference from Perl"
2088    .rs
2089    .sp
2090    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2091    treated as an atomic group. That is, once it has matched some of the subject
2092    string, it is never re-entered, even if it contains untried alternatives and
2093    there is a subsequent matching failure. This can be illustrated by the
2094    following pattern, which purports to match a palindromic string that contains
2095    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2096    .sp
2097      ^(.|(.)(?1)\e2)$
2098    .sp
2099    The idea is that it either matches a single character, or two identical
2100    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2101    it does not if the pattern is longer than three characters. Consider the
2102    subject string "abcba":
2103    .P
2104    At the top level, the first character is matched, but as it is not at the end
2105    of the string, the first alternative fails; the second alternative is taken
2106    and the recursion kicks in. The recursive call to subpattern 1 successfully
2107    matches the next character ("b"). (Note that the beginning and end of line
2108    tests are not part of the recursion).
2109    .P
2110    Back at the top level, the next character ("c") is compared with what
2111    subpattern 2 matched, which was "a". This fails. Because the recursion is
2112    treated as an atomic group, there are now no backtracking points, and so the
2113    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2114    try the second alternative.) However, if the pattern is written with the
2115    alternatives in the other order, things are different:
2116    .sp
2117      ^((.)(?1)\e2|.)$
2118    .sp
2119    This time, the recursing alternative is tried first, and continues to recurse
2120    until it runs out of characters, at which point the recursion fails. But this
2121    time we do have another alternative to try at the higher level. That is the big
2122    difference: in the previous case the remaining alternative is at a deeper
2123    recursion level, which PCRE cannot use.
2124    .P
2125    To change the pattern so that matches all palindromic strings, not just those
2126    with an odd number of characters, it is tempting to change the pattern to this:
2127    .sp
2128      ^((.)(?1)\e2|.?)$
2129    .sp
2130    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2131    deeper recursion has matched a single character, it cannot be entered again in
2132    order to match an empty string. The solution is to separate the two cases, and
2133    write out the odd and even cases as alternatives at the higher level:
2134    .sp
2135      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2136    .sp
2137    If you want to match typical palindromic phrases, the pattern has to ignore all
2138    non-word characters, which can be done like this:
2139    .sp
2140      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$
2141    .sp
2142    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2143    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2144    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2145    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2146    more) to match typical phrases, and Perl takes so long that you think it has
2147    gone into a loop.
2148    .P
2149    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2150    string does not start with a palindrome that is shorter than the entire string.
2151    For example, although "abcba" is correctly matched, if the subject is "ababa",
2152    PCRE finds the palindrome "aba" at the start, then fails at top level because
2153    the end of the string does not follow. Once again, it cannot jump back into the
2154    recursion to try other alternatives, so the entire match fails.
2155    .
2156    .
2157  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2158  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2159  .rs  .rs
# Line 1917  It matches "abcabc". It does not match " Line 2195  It matches "abcabc". It does not match "
2195  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2196  .  .
2197  .  .
2198    .\" HTML <a name="onigurumasubroutines"></a>
2199    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2200    .rs
2201    .sp
2202    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2203    a number enclosed either in angle brackets or single quotes, is an alternative
2204    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2205    are two of the examples used above, rewritten using this syntax:
2206    .sp
2207      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2208      (sens|respons)e and \eg'1'ibility
2209    .sp
2210    PCRE supports an extension to Oniguruma: if a number is preceded by a
2211    plus or a minus sign it is taken as a relative reference. For example:
2212    .sp
2213      (abc)(?i:\eg<-1>)
2214    .sp
2215    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2216    synonymous. The former is a back reference; the latter is a subroutine call.
2217    .
2218    .
2219  .SH CALLOUTS  .SH CALLOUTS
2220  .rs  .rs
2221  .sp  .sp
# Line 1953  description of the interface to the call Line 2252  description of the interface to the call
2252  documentation.  documentation.
2253  .  .
2254  .  .
2255    .SH "BACKTRACKING CONTROL"
2256    .rs
2257    .sp
2258    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2259    are described in the Perl documentation as "experimental and subject to change
2260    or removal in a future version of Perl". It goes on to say: "Their usage in
2261    production code should be noted to avoid problems during upgrades." The same
2262    remarks apply to the PCRE features described in this section.
2263    .P
2264    Since these verbs are specifically related to backtracking, most of them can be
2265    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2266    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2267    failing negative assertion, they cause an error if encountered by
2268    \fBpcre_dfa_exec()\fP.
2269    .P
2270    If any of these verbs are used in an assertion subpattern, their effect is
2271    confined to that subpattern; it does not extend to the surrounding pattern.
2272    Note that assertion subpatterns are processed as anchored at the point where
2273    they are tested.
2274    .P
2275    The new verbs make use of what was previously invalid syntax: an opening
2276    parenthesis followed by an asterisk. In Perl, they are generally of the form
2277    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2278    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2279    are two kinds:
2280    .
2281    .SS "Verbs that act immediately"
2282    .rs
2283    .sp
2284    The following verbs act as soon as they are encountered:
2285    .sp
2286       (*ACCEPT)
2287    .sp
2288    This verb causes the match to end successfully, skipping the remainder of the
2289    pattern. When inside a recursion, only the innermost pattern is ended
2290    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2291    captured. (This feature was added to PCRE at release 8.00.) For example:
2292    .sp
2293      A((?:A|B(*ACCEPT)|C)D)
2294    .sp
2295    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2296    the outer parentheses.
2297    .sp
2298      (*FAIL) or (*F)
2299    .sp
2300    This verb causes the match to fail, forcing backtracking to occur. It is
2301    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2302    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2303    Perl features that are not present in PCRE. The nearest equivalent is the
2304    callout feature, as for example in this pattern:
2305    .sp
2306      a+(?C)(*FAIL)
2307    .sp
2308    A match with the string "aaaa" always fails, but the callout is taken before
2309    each backtrack happens (in this example, 10 times).
2310    .
2311    .SS "Verbs that act after backtracking"
2312    .rs
2313    .sp
2314    The following verbs do nothing when they are encountered. Matching continues
2315    with what follows, but if there is no subsequent match, a failure is forced.
2316    The verbs differ in exactly what kind of failure occurs.
2317    .sp
2318      (*COMMIT)
2319    .sp
2320    This verb causes the whole match to fail outright if the rest of the pattern
2321    does not match. Even if the pattern is unanchored, no further attempts to find
2322    a match by advancing the starting point take place. Once (*COMMIT) has been
2323    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2324    starting point, or not at all. For example:
2325    .sp
2326      a+(*COMMIT)b
2327    .sp
2328    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2329    dynamic anchor, or "I've started, so I must finish."
2330    .sp
2331      (*PRUNE)
2332    .sp
2333    This verb causes the match to fail at the current position if the rest of the
2334    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2335    advance to the next starting character then happens. Backtracking can occur as
2336    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2337    if there is no match to the right, backtracking cannot cross (*PRUNE).
2338    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2339    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2340    be expressed in any other way.
2341    .sp
2342      (*SKIP)
2343    .sp
2344    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2345    "bumpalong" advance is not to the next character, but to the position in the
2346    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2347    was matched leading up to it cannot be part of a successful match. Consider:
2348    .sp
2349      a+(*SKIP)b
2350    .sp
2351    If the subject is "aaaac...", after the first match attempt fails (starting at
2352    the first character in the string), the starting point skips on to start the
2353    next attempt at "c". Note that a possessive quantifer does not have the same
2354    effect as this example; although it would suppress backtracking during the
2355    first match attempt, the second attempt would start at the second character
2356    instead of skipping on to "c".
2357    .sp
2358      (*THEN)
2359    .sp
2360    This verb causes a skip to the next alternation if the rest of the pattern does
2361    not match. That is, it cancels pending backtracking, but only within the
2362    current alternation. Its name comes from the observation that it can be used
2363    for a pattern-based if-then-else block:
2364    .sp
2365      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2366    .sp
2367    If the COND1 pattern matches, FOO is tried (and possibly further items after
2368    the end of the group if FOO succeeds); on failure the matcher skips to the
2369    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2370    is used outside of any alternation, it acts exactly like (*PRUNE).
2371    .
2372    .
2373  .SH "SEE ALSO"  .SH "SEE ALSO"
2374  .rs  .rs
2375  .sp  .sp
2376  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2377    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2378  .  .
2379  .  .
2380  .SH AUTHOR  .SH AUTHOR
# Line 1973  Cambridge CB2 3QH, England. Line 2391  Cambridge CB2 3QH, England.
2391  .rs  .rs
2392  .sp  .sp
2393  .nf  .nf
2394  Last updated: 13 June 2007  Last updated: 03 October 2009
2395  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2009 University of Cambridge.
2396  .fi  .fi

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