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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  .\" HTML <a href="pcre.html#utf8support">  .sp
29  .\" </a>    (*UTF8)
30  section on UTF-8 support  .sp
31  .\"  Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32  in the main  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  .\" HREF  .\" HREF
36  \fBpcre\fP  \fBpcreunicode\fP
37  .\"  .\"
38  page.  page.
39  .P  .P
40    Another special sequence that may appear at the start of a pattern or in
41    combination with (*UTF8) is:
42    .sp
43      (*UCP)
44    .sp
45    This has the same effect as setting the PCRE_UCP option: it causes sequences
46    such as \ed and \ew to use Unicode properties to determine character types,
47    instead of recognizing only characters with codes less than 128 via a lookup
48    table.
49    .P
50    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
51    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
52    also some more of these special sequences that are concerned with the handling
53    of newlines; they are described below.
54    .P
55  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
56  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
57  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
58  \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
59  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
60  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
61    alternative function, and how it differs from the normal function, are
62    discussed in the
63  .\" HREF  .\" HREF
64  \fBpcrematching\fP  \fBpcrematching\fP
65  .\"  .\"
66  page.  page.
67  .  .
68  .  .
69    .\" HTML <a name="newlines"></a>
70    .SH "NEWLINE CONVENTIONS"
71    .rs
72    .sp
73    PCRE supports five different conventions for indicating line breaks in
74    strings: a single CR (carriage return) character, a single LF (linefeed)
75    character, the two-character sequence CRLF, any of the three preceding, or any
76    Unicode newline sequence. The
77    .\" HREF
78    \fBpcreapi\fP
79    .\"
80    page has
81    .\" HTML <a href="pcreapi.html#newlines">
82    .\" </a>
83    further discussion
84    .\"
85    about newlines, and shows how to set the newline convention in the
86    \fIoptions\fP arguments for the compiling and matching functions.
87    .P
88    It is also possible to specify a newline convention by starting a pattern
89    string with one of the following five sequences:
90    .sp
91      (*CR)        carriage return
92      (*LF)        linefeed
93      (*CRLF)      carriage return, followed by linefeed
94      (*ANYCRLF)   any of the three above
95      (*ANY)       all Unicode newline sequences
96    .sp
97    These override the default and the options given to \fBpcre_compile()\fP or
98    \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
99    newline sequence, the pattern
100    .sp
101      (*CR)a.b
102    .sp
103    changes the convention to CR. That pattern matches "a\enb" because LF is no
104    longer a newline. Note that these special settings, which are not
105    Perl-compatible, are recognized only at the very start of a pattern, and that
106    they must be in upper case. If more than one of them is present, the last one
107    is used.
108    .P
109    The newline convention affects the interpretation of the dot metacharacter when
110    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
111    affect what the \eR escape sequence matches. By default, this is any Unicode
112    newline sequence, for Perl compatibility. However, this can be changed; see the
113    description of \eR in the section entitled
114    .\" HTML <a href="#newlineseq">
115    .\" </a>
116    "Newline sequences"
117    .\"
118    below. A change of \eR setting can be combined with a change of newline
119    convention.
120    .
121    .
122  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
123  .rs  .rs
124  .sp  .sp
# Line 101  The following sections describe the use Line 182  The following sections describe the use
182  .rs  .rs
183  .sp  .sp
184  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
185  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
186  may have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
187  outside character classes.  both inside and outside character classes.
188  .P  .P
189  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
190  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 111  otherwise be interpreted as a metacharac Line 192  otherwise be interpreted as a metacharac
192  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
193  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
194  .P  .P
195    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
196    backslash. All other characters (in particular, those whose codepoints are
197    greater than 127) are treated as literals.
198    .P
199  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
200  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
201  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
# Line 130  Perl, $ and @ cause variable interpolati Line 215  Perl, $ and @ cause variable interpolati
215    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
216  .sp  .sp
217  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
218    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
219    by \eE later in the pattern, the literal interpretation continues to the end of
220    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
221    a character class, this causes an error, because the character class is not
222    terminated.
223  .  .
224  .  .
225  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 139  The \eQ...\eE sequence is recognized bot Line 229  The \eQ...\eE sequence is recognized bot
229  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
230  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
231  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
232  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
233  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:  
234  .sp  .sp
235    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
236    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
237    \ee        escape (hex 1B)    \ee        escape (hex 1B)
238    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
239    \en        newline (hex 0A)    \en        linefeed (hex 0A)
240    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
241    \et        tab (hex 09)    \et        tab (hex 09)
242    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
243    \exhh      character with hex code hh    \exhh      character with hex code hh
244    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
245  .sp  .sp
246  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
247  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
248  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
249  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
250    than 127, a compile-time error occurs. This locks out non-ASCII characters in
251    both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
252    values are valid. A lower case letter is converted to upper case, and then the
253    0xc0 bits are flipped.)
254  .P  .P
255  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
256  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{
257  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
258  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
259  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
260  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
261  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
262  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
263    there is no terminating }, this form of escape is not recognized. Instead, the
264    initial \ex will be interpreted as a basic hexadecimal escape, with no
265    following digits, giving a character whose value is zero.
266  .P  .P
267  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
268  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 225  zero, because no more than three octal d Line 321  zero, because no more than three octal d
321  .P  .P
322  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
323  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
324  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
325  sequences \eR and \eX are interpreted as the characters "R" and "X",  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
326  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
327  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
328  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
 .\" </a>  
 (see below).  
 .\"  
329  .  .
330  .  .
331  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
332  .rs  .rs
333  .sp  .sp
334  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
335  in braces, is an absolute or relative back reference. Back references are  enclosed in braces, is an absolute or relative back reference. A named back
336  discussed  reference can be coded as \eg{name}. Back references are discussed
337  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
338  .\" </a>  .\" </a>
339  later,  later,
# Line 252  parenthesized subpatterns. Line 345  parenthesized subpatterns.
345  .\"  .\"
346  .  .
347  .  .
348    .SS "Absolute and relative subroutine calls"
349    .rs
350    .sp
351    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
352    a number enclosed either in angle brackets or single quotes, is an alternative
353    syntax for referencing a subpattern as a "subroutine". Details are discussed
354    .\" HTML <a href="#onigurumasubroutines">
355    .\" </a>
356    later.
357    .\"
358    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
359    synonymous. The former is a back reference; the latter is a
360    .\" HTML <a href="#subpatternsassubroutines">
361    .\" </a>
362    subroutine
363    .\"
364    call.
365    .
366    .
367    .\" HTML <a name="genericchartypes"></a>
368  .SS "Generic character types"  .SS "Generic character types"
369  .rs  .rs
370  .sp  .sp
371  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
372  .sp  .sp
373    \ed     any decimal digit    \ed     any decimal digit
374    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
375      \eh     any horizontal whitespace character
376      \eH     any character that is not a horizontal whitespace character
377    \es     any whitespace character    \es     any whitespace character
378    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
379      \ev     any vertical whitespace character
380      \eV     any character that is not a vertical whitespace character
381    \ew     any "word" character    \ew     any "word" character
382    \eW     any "non-word" character    \eW     any "non-word" character
383  .sp  .sp
384  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
385  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
386    .\" HTML <a href="#fullstopdot">
387    .\" </a>
388    the "." metacharacter
389    .\"
390    when PCRE_DOTALL is not set.
391  .P  .P
392  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
393    of characters into two disjoint sets. Any given character matches one, and only
394    one, of each pair. The sequences can appear both inside and outside character
395  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
396  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
397  there is no character to match.  there is no character to match.
398  .P  .P
399  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).
400  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
401  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
402  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
403  does.)  does.
404  .P  .P
405  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character that is a letter or digit.
406  letter or digit. The definition of letters and digits is controlled by PCRE's  By default, the definition of letters and digits is controlled by PCRE's
407  low-valued character tables, and may vary if locale-specific matching is taking  low-valued character tables, and may vary if locale-specific matching is taking
408  place (see  place (see
409  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
# Line 293  in the Line 416  in the
416  .\"  .\"
417  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,
418  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
419  accented letters, and these are matched by \ew.  accented letters, and these are then matched by \ew. The use of locales with
420    Unicode is discouraged.
421  .P  .P
422  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  By default, in UTF-8 mode, characters with values greater than 128 never match
423  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
424  character property support is available. The use of locales with Unicode is  their original meanings from before UTF-8 support was available, mainly for
425  discouraged.  efficiency reasons. However, if PCRE is compiled with Unicode property support,
426    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
427    properties are used to determine character types, as follows:
428    .sp
429      \ed  any character that \ep{Nd} matches (decimal digit)
430      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
431      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
432    .sp
433    The upper case escapes match the inverse sets of characters. Note that \ed
434    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
435    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
436    \eB because they are defined in terms of \ew and \eW. Matching these sequences
437    is noticeably slower when PCRE_UCP is set.
438    .P
439    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
440    release 5.10. In contrast to the other sequences, which match only ASCII
441    characters by default, these always match certain high-valued codepoints in
442    UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
443    are:
444    .sp
445      U+0009     Horizontal tab
446      U+0020     Space
447      U+00A0     Non-break space
448      U+1680     Ogham space mark
449      U+180E     Mongolian vowel separator
450      U+2000     En quad
451      U+2001     Em quad
452      U+2002     En space
453      U+2003     Em space
454      U+2004     Three-per-em space
455      U+2005     Four-per-em space
456      U+2006     Six-per-em space
457      U+2007     Figure space
458      U+2008     Punctuation space
459      U+2009     Thin space
460      U+200A     Hair space
461      U+202F     Narrow no-break space
462      U+205F     Medium mathematical space
463      U+3000     Ideographic space
464    .sp
465    The vertical space characters are:
466    .sp
467      U+000A     Linefeed
468      U+000B     Vertical tab
469      U+000C     Formfeed
470      U+000D     Carriage return
471      U+0085     Next line
472      U+2028     Line separator
473      U+2029     Paragraph separator
474  .  .
475  .  .
476    .\" HTML <a name="newlineseq"></a>
477  .SS "Newline sequences"  .SS "Newline sequences"
478  .rs  .rs
479  .sp  .sp
480  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
481  sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to  Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
 the following:  
482  .sp  .sp
483    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
484  .sp  .sp
# Line 326  are added: LS (line separator, U+2028) a Line 498  are added: LS (line separator, U+2028) a
498  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
499  recognized.  recognized.
500  .P  .P
501  Inside a character class, \eR matches the letter "R".  It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
502    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
503    either at compile time or when the pattern is matched. (BSR is an abbrevation
504    for "backslash R".) This can be made the default when PCRE is built; if this is
505    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
506    It is also possible to specify these settings by starting a pattern string with
507    one of the following sequences:
508    .sp
509      (*BSR_ANYCRLF)   CR, LF, or CRLF only
510      (*BSR_UNICODE)   any Unicode newline sequence
511    .sp
512    These override the default and the options given to \fBpcre_compile()\fP or
513    \fBpcre_compile2()\fP, but they can be overridden by options given to
514    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
515    which are not Perl-compatible, are recognized only at the very start of a
516    pattern, and that they must be in upper case. If more than one of them is
517    present, the last one is used. They can be combined with a change of newline
518    convention; for example, a pattern can start with:
519    .sp
520      (*ANY)(*BSR_ANYCRLF)
521    .sp
522    They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
523    a character class, \eR is treated as an unrecognized escape sequence, and so
524    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
525  .  .
526  .  .
527  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 334  Inside a character class, \eR matches th Line 529  Inside a character class, \eR matches th
529  .rs  .rs
530  .sp  .sp
531  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
532  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
533  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
534    characters whose codepoints are less than 256, but they do work in this mode.
535    The extra escape sequences are:
536  .sp  .sp
537    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
538    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
539    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
540  .sp  .sp
541  The property names represented by \fIxx\fP above are limited to the Unicode  The property names represented by \fIxx\fP above are limited to the Unicode
542  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
543  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
544  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
545  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
546    .\" </a>
547    next section).
548    .\"
549    Other Perl properties such as "InMusicalSymbols" are not currently supported by
550    PCRE. Note that \eP{Any} does not match any characters, so always causes a
551    match failure.
552  .P  .P
553  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
554  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 359  Those that are not part of an identified Line 562  Those that are not part of an identified
562  .P  .P
563  Arabic,  Arabic,
564  Armenian,  Armenian,
565    Avestan,
566  Balinese,  Balinese,
567    Bamum,
568  Bengali,  Bengali,
569  Bopomofo,  Bopomofo,
570  Braille,  Braille,
571  Buginese,  Buginese,
572  Buhid,  Buhid,
573  Canadian_Aboriginal,  Canadian_Aboriginal,
574    Carian,
575    Cham,
576  Cherokee,  Cherokee,
577  Common,  Common,
578  Coptic,  Coptic,
# Line 374  Cypriot, Line 581  Cypriot,
581  Cyrillic,  Cyrillic,
582  Deseret,  Deseret,
583  Devanagari,  Devanagari,
584    Egyptian_Hieroglyphs,
585  Ethiopic,  Ethiopic,
586  Georgian,  Georgian,
587  Glagolitic,  Glagolitic,
# Line 386  Hangul, Line 594  Hangul,
594  Hanunoo,  Hanunoo,
595  Hebrew,  Hebrew,
596  Hiragana,  Hiragana,
597    Imperial_Aramaic,
598  Inherited,  Inherited,
599    Inscriptional_Pahlavi,
600    Inscriptional_Parthian,
601    Javanese,
602    Kaithi,
603  Kannada,  Kannada,
604  Katakana,  Katakana,
605    Kayah_Li,
606  Kharoshthi,  Kharoshthi,
607  Khmer,  Khmer,
608  Lao,  Lao,
609  Latin,  Latin,
610    Lepcha,
611  Limbu,  Limbu,
612  Linear_B,  Linear_B,
613    Lisu,
614    Lycian,
615    Lydian,
616  Malayalam,  Malayalam,
617    Meetei_Mayek,
618  Mongolian,  Mongolian,
619  Myanmar,  Myanmar,
620  New_Tai_Lue,  New_Tai_Lue,
# Line 403  Nko, Line 622  Nko,
622  Ogham,  Ogham,
623  Old_Italic,  Old_Italic,
624  Old_Persian,  Old_Persian,
625    Old_South_Arabian,
626    Old_Turkic,
627    Ol_Chiki,
628  Oriya,  Oriya,
629  Osmanya,  Osmanya,
630  Phags_Pa,  Phags_Pa,
631  Phoenician,  Phoenician,
632    Rejang,
633  Runic,  Runic,
634    Samaritan,
635    Saurashtra,
636  Shavian,  Shavian,
637  Sinhala,  Sinhala,
638    Sundanese,
639  Syloti_Nagri,  Syloti_Nagri,
640  Syriac,  Syriac,
641  Tagalog,  Tagalog,
642  Tagbanwa,  Tagbanwa,
643  Tai_Le,  Tai_Le,
644    Tai_Tham,
645    Tai_Viet,
646  Tamil,  Tamil,
647  Telugu,  Telugu,
648  Thaana,  Thaana,
# Line 422  Thai, Line 650  Thai,
650  Tibetan,  Tibetan,
651  Tifinagh,  Tifinagh,
652  Ugaritic,  Ugaritic,
653    Vai,
654  Yi.  Yi.
655  .P  .P
656  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
657  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
658  by including a circumflex between the opening brace and the property name. For  specified by including a circumflex between the opening brace and the property
659  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
660  .P  .P
661  If only one letter is specified with \ep or \eP, it includes all the general  If only one letter is specified with \ep or \eP, it includes all the general
662  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 487  The special property L& is also supporte Line 716  The special property L& is also supporte
716  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
717  a modifier or "other".  a modifier or "other".
718  .P  .P
719  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
720    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
721    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
722    (see the discussion of PCRE_NO_UTF8_CHECK in the
723    .\" HREF
724    \fBpcreapi\fP
725    .\"
726    page). Perl does not support the Cs property.
727    .P
728    The long synonyms for property names that Perl supports (such as \ep{Letter})
729  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
730  properties with "Is".  properties with "Is".
731  .P  .P
# Line 511  atomic group Line 749  atomic group
749  (see below).  (see below).
750  .\"  .\"
751  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
752  preceding character.  preceding character. None of them have codepoints less than 256, so in
753    non-UTF-8 mode \eX matches any one character.
754    .P
755    Note that recent versions of Perl have changed \eX to match what Unicode calls
756    an "extended grapheme cluster", which has a more complicated definition.
757  .P  .P
758  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
759  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
760  why the traditional escape sequences such as \ed and \ew do not use Unicode  why the traditional escape sequences such as \ed and \ew do not use Unicode
761  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
762    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
763    (*UCP).
764    .
765    .
766    .\" HTML <a name="extraprops"></a>
767    .SS PCRE's additional properties
768    .rs
769    .sp
770    As well as the standard Unicode properties described in the previous
771    section, PCRE supports four more that make it possible to convert traditional
772    escape sequences such as \ew and \es and POSIX character classes to use Unicode
773    properties. PCRE uses these non-standard, non-Perl properties internally when
774    PCRE_UCP is set. They are:
775    .sp
776      Xan   Any alphanumeric character
777      Xps   Any POSIX space character
778      Xsp   Any Perl space character
779      Xwd   Any Perl "word" character
780    .sp
781    Xan matches characters that have either the L (letter) or the N (number)
782    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
783    carriage return, and any other character that has the Z (separator) property.
784    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
785    same characters as Xan, plus underscore.
786    .
787    .
788    .\" HTML <a name="resetmatchstart"></a>
789    .SS "Resetting the match start"
790    .rs
791    .sp
792    The escape sequence \eK causes any previously matched characters not to be
793    included in the final matched sequence. For example, the pattern:
794    .sp
795      foo\eKbar
796    .sp
797    matches "foobar", but reports that it has matched "bar". This feature is
798    similar to a lookbehind assertion
799    .\" HTML <a href="#lookbehind">
800    .\" </a>
801    (described below).
802    .\"
803    However, in this case, the part of the subject before the real match does not
804    have to be of fixed length, as lookbehind assertions do. The use of \eK does
805    not interfere with the setting of
806    .\" HTML <a href="#subpattern">
807    .\" </a>
808    captured substrings.
809    .\"
810    For example, when the pattern
811    .sp
812      (foo)\eKbar
813    .sp
814    matches "foobar", the first substring is still set to "foo".
815    .P
816    Perl documents that the use of \eK within assertions is "not well defined". In
817    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
818    ignored in negative assertions.
819  .  .
820  .  .
821  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 541  The backslashed assertions are: Line 840  The backslashed assertions are:
840    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
841    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
842  .sp  .sp
843  These assertions may not appear in character classes (but note that \eb has a  Inside a character class, \eb has a different meaning; it matches the backspace
844  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
845    default it matches the corresponding literal character (for example, \eB
846    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
847    escape sequence" error is generated instead.
848  .P  .P
849  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
850  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
851  \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
852  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
853    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
854    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
855    of word" or "end of word" metasequence. However, whatever follows \eb normally
856    determines which it is. For example, the fragment \eba matches "a" at the start
857    of a word.
858  .P  .P
859  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
860  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 631  end of the subject in both modes, and if Line 938  end of the subject in both modes, and if
938  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
939  .  .
940  .  .
941  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
942    .SH "FULL STOP (PERIOD, DOT) AND \eN"
943  .rs  .rs
944  .sp  .sp
945  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
# Line 653  to match it. Line 961  to match it.
961  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
962  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
963  special meaning in a character class.  special meaning in a character class.
964    .P
965    The escape sequence \eN behaves like a dot, except that it is not affected by
966    the PCRE_DOTALL option. In other words, it matches any character except one
967    that signifies the end of a line.
968  .  .
969  .  .
970  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
971  .rs  .rs
972  .sp  .sp
973  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
974  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  in and out of UTF-8 mode. Unlike a dot, it always matches line-ending
975  characters. The feature is provided in Perl in order to match individual bytes  characters. The feature is provided in Perl in order to match individual bytes
976  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  in UTF-8 mode, but it is unclear how it can usefully be used. Because \eC
977  what remains in the string may be a malformed UTF-8 string. For this reason,  breaks up characters into individual bytes, matching one byte with \eC in UTF-8
978  the \eC escape sequence is best avoided.  mode means that the rest of the string may start with a malformed UTF-8
979    character. This has undefined results, because PCRE assumes that it is dealing
980    with valid UTF-8 strings (and by default it checks this at the start of
981    processing unless the PCRE_NO_UTF8_CHECK option is used).
982  .P  .P
983  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
984  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 672  PCRE does not allow \eC to appear in loo Line 987  PCRE does not allow \eC to appear in loo
987  .\"  .\"
988  because in UTF-8 mode this would make it impossible to calculate the length of  because in UTF-8 mode this would make it impossible to calculate the length of
989  the lookbehind.  the lookbehind.
990    .P
991    In general, the \eC escape sequence is best avoided in UTF-8 mode. However, one
992    way of using it that avoids the problem of malformed UTF-8 characters is to
993    use a lookahead to check the length of the next character, as in this pattern
994    (ignore white space and line breaks):
995    .sp
996      (?| (?=[\ex00-\ex7f])(\eC) |
997          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
998          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
999          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1000    .sp
1001    A group that starts with (?| resets the capturing parentheses numbers in each
1002    alternative (see
1003    .\" HTML <a href="#dupsubpatternnumber">
1004    .\" </a>
1005    "Duplicate Subpattern Numbers"
1006    .\"
1007    below). The assertions at the start of each branch check the next UTF-8
1008    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1009    character's individual bytes are then captured by the appropriate number of
1010    groups.
1011  .  .
1012  .  .
1013  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 679  the lookbehind. Line 1015  the lookbehind.
1015  .rs  .rs
1016  .sp  .sp
1017  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1018  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.
1019  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
1020  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
1021  escaped with a backslash.  a member of the class, it should be the first data character in the class
1022    (after an initial circumflex, if present) or escaped with a backslash.
1023  .P  .P
1024  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
1025  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
1026  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
1027  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
1028  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
1029  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 696  For example, the character class [aeiou] Line 1033  For example, the character class [aeiou]
1033  [^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
1034  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1035  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
1036  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
1037  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
1038  string.  string.
1039  .P  .P
# Line 710  caseful version would. In UTF-8 mode, PC Line 1047  caseful version would. In UTF-8 mode, PC
1047  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
1048  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1049  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1050  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,
1051  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
1052  UTF-8 support.  with UTF-8 support.
1053  .P  .P
1054  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
1055  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 746  characters in both cases. In UTF-8 mode, Line 1083  characters in both cases. In UTF-8 mode,
1083  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1084  property support.  property support.
1085  .P  .P
1086  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1087  in a character class, and add the characters that they match to the class. For  \eV, \ew, and \eW may appear in a character class, and add the characters that
1088  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1089  conveniently be used with the upper case character types to specify a more  digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1090  restricted set of characters than the matching lower case type. For example,  and their upper case partners, just as it does when they appear outside a
1091  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1092    .\" HTML <a href="#genericchartypes">
1093    .\" </a>
1094    "Generic character types"
1095    .\"
1096    above. The escape sequence \eb has a different meaning inside a character
1097    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1098    are not special inside a character class. Like any other unrecognized escape
1099    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1100    default, but cause an error if the PCRE_EXTRA option is set.
1101    .P
1102    A circumflex can conveniently be used with the upper case character types to
1103    specify a more restricted set of characters than the matching lower case type.
1104    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1105    whereas [\ew] includes underscore. A positive character class should be read as
1106    "something OR something OR ..." and a negative class as "NOT something AND NOT
1107    something AND NOT ...".
1108  .P  .P
1109  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1110  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 771  this notation. For example, Line 1124  this notation. For example,
1124    [01[:alpha:]%]    [01[:alpha:]%]
1125  .sp  .sp
1126  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1127  are  are:
1128  .sp  .sp
1129    alnum    letters and digits    alnum    letters and digits
1130    alpha    letters    alpha    letters
# Line 782  are Line 1135  are
1135    graph    printing characters, excluding space    graph    printing characters, excluding space
1136    lower    lower case letters    lower    lower case letters
1137    print    printing characters, including space    print    printing characters, including space
1138    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1139    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1140    upper    upper case letters    upper    upper case letters
1141    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 803  matches "1", "2", or any non-digit. PCRE Line 1156  matches "1", "2", or any non-digit. PCRE
1156  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1157  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1158  .P  .P
1159  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF-8 mode, characters with values greater than 128 do not match
1160  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1161    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1162    character properties are used. This is achieved by replacing the POSIX classes
1163    by other sequences, as follows:
1164    .sp
1165      [:alnum:]  becomes  \ep{Xan}
1166      [:alpha:]  becomes  \ep{L}
1167      [:blank:]  becomes  \eh
1168      [:digit:]  becomes  \ep{Nd}
1169      [:lower:]  becomes  \ep{Ll}
1170      [:space:]  becomes  \ep{Xps}
1171      [:upper:]  becomes  \ep{Lu}
1172      [:word:]   becomes  \ep{Xwd}
1173    .sp
1174    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1175    classes are unchanged, and match only characters with code points less than
1176    128.
1177  .  .
1178  .  .
1179  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 831  alternative in the subpattern. Line 1200  alternative in the subpattern.
1200  .rs  .rs
1201  .sp  .sp
1202  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1203  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
1204  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1205    The option letters are
1206  .sp  .sp
1207    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1208    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 846  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1216  PCRE_MULTILINE while unsetting PCRE_DOTA
1216  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
1217  unset.  unset.
1218  .P  .P
1219  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
1220  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
1221  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1222  the global options (and it will therefore show up in data extracted by the  .P
1223  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1224    subpattern parentheses), the change applies to the remainder of the pattern
1225    that follows. If the change is placed right at the start of a pattern, PCRE
1226    extracts it into the global options (and it will therefore show up in data
1227    extracted by the \fBpcre_fullinfo()\fP function).
1228  .P  .P
1229  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1230  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1231  .sp  .sp
1232    (a(?i)b)c    (a(?i)b)c
1233  .sp  .sp
# Line 869  branch is abandoned before the option se Line 1243  branch is abandoned before the option se
1243  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1244  behaviour otherwise.  behaviour otherwise.
1245  .P  .P
1246  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
1247  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
1248  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1249    the application has set or what has been defaulted. Details are given in the
1250    section entitled
1251    .\" HTML <a href="#newlineseq">
1252    .\" </a>
1253    "Newline sequences"
1254    .\"
1255    above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1256    to set UTF-8 and Unicode property modes; they are equivalent to setting the
1257    PCRE_UTF8 and the PCRE_UCP options, respectively.
1258  .  .
1259  .  .
1260  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 885  Turning part of a pattern into a subpatt Line 1268  Turning part of a pattern into a subpatt
1268  .sp  .sp
1269    cat(aract|erpillar|)    cat(aract|erpillar|)
1270  .sp  .sp
1271  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1272  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1273  .sp  .sp
1274  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
1275  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
1276  subpattern is passed back to the caller via the \fIovector\fP argument of  subpattern is passed back to the caller via the \fIovector\fP argument of
1277  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1278  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1279  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1280  .sp  .sp
1281    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1282  .sp  .sp
# Line 926  is reached, an option setting in one bra Line 1308  is reached, an option setting in one bra
1308  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1309  .  .
1310  .  .
1311    .\" HTML <a name="dupsubpatternnumber"></a>
1312    .SH "DUPLICATE SUBPATTERN NUMBERS"
1313    .rs
1314    .sp
1315    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1316    the same numbers for its capturing parentheses. Such a subpattern starts with
1317    (?| and is itself a non-capturing subpattern. For example, consider this
1318    pattern:
1319    .sp
1320      (?|(Sat)ur|(Sun))day
1321    .sp
1322    Because the two alternatives are inside a (?| group, both sets of capturing
1323    parentheses are numbered one. Thus, when the pattern matches, you can look
1324    at captured substring number one, whichever alternative matched. This construct
1325    is useful when you want to capture part, but not all, of one of a number of
1326    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1327    number is reset at the start of each branch. The numbers of any capturing
1328    parentheses that follow the subpattern start after the highest number used in
1329    any branch. The following example is taken from the Perl documentation. The
1330    numbers underneath show in which buffer the captured content will be stored.
1331    .sp
1332      # before  ---------------branch-reset----------- after
1333      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1334      # 1            2         2  3        2     3     4
1335    .sp
1336    A back reference to a numbered subpattern uses the most recent value that is
1337    set for that number by any subpattern. The following pattern matches "abcabc"
1338    or "defdef":
1339    .sp
1340      /(?|(abc)|(def))\e1/
1341    .sp
1342    In contrast, a subroutine call to a numbered subpattern always refers to the
1343    first one in the pattern with the given number. The following pattern matches
1344    "abcabc" or "defabc":
1345    .sp
1346      /(?|(abc)|(def))(?1)/
1347    .sp
1348    If a
1349    .\" HTML <a href="#conditions">
1350    .\" </a>
1351    condition test
1352    .\"
1353    for a subpattern's having matched refers to a non-unique number, the test is
1354    true if any of the subpatterns of that number have matched.
1355    .P
1356    An alternative approach to using this "branch reset" feature is to use
1357    duplicate named subpatterns, as described in the next section.
1358    .
1359    .
1360  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1361  .rs  .rs
1362  .sp  .sp
# Line 935  if an expression is modified, the number Line 1366  if an expression is modified, the number
1366  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1367  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
1368  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
1369  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1370    have different names, but PCRE does not.
1371  .P  .P
1372  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
1373  (?'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
1374  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1375  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1376  .\" </a>  .\" </a>
1377  backreferences,  back references,
1378  .\"  .\"
1379  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1380  .\" </a>  .\" </a>
# Line 962  extracting the name-to-number translatio Line 1394  extracting the name-to-number translatio
1394  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1395  .P  .P
1396  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
1397  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
1398  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
1399  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
1400  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
1401  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
1402    name, and in both cases you want to extract the abbreviation. This pattern
1403    (ignoring the line breaks) does the job:
1404  .sp  .sp
1405    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1406    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 975  abbreviation. This pattern (ignoring the Line 1409  abbreviation. This pattern (ignoring the
1409    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1410  .sp  .sp
1411  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.
1412    (An alternative way of solving this problem is to use a "branch reset"
1413    subpattern, as described in the previous section.)
1414    .P
1415  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1416  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
1417  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1418  make a reference to a non-unique named subpattern from elsewhere in the  .P
1419  pattern, the one that corresponds to the lowest number is used. For further  If you make a back reference to a non-unique named subpattern from elsewhere in
1420  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1421    used. In the absence of duplicate numbers (see the previous section) this is
1422    the one with the lowest number. If you use a named reference in a condition
1423    test (see the
1424    .\"
1425    .\" HTML <a href="#conditions">
1426    .\" </a>
1427    section about conditions
1428    .\"
1429    below), either to check whether a subpattern has matched, or to check for
1430    recursion, all subpatterns with the same name are tested. If the condition is
1431    true for any one of them, the overall condition is true. This is the same
1432    behaviour as testing by number. For further details of the interfaces for
1433    handling named subpatterns, see the
1434  .\" HREF  .\" HREF
1435  \fBpcreapi\fP  \fBpcreapi\fP
1436  .\"  .\"
1437  documentation.  documentation.
1438    .P
1439    \fBWarning:\fP You cannot use different names to distinguish between two
1440    subpatterns with the same number because PCRE uses only the numbers when
1441    matching. For this reason, an error is given at compile time if different names
1442    are given to subpatterns with the same number. However, you can give the same
1443    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1444  .  .
1445  .  .
1446  .SH REPETITION  .SH REPETITION
# Line 998  items: Line 1454  items:
1454    the \eC escape sequence    the \eC escape sequence
1455    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1456    the \eR escape sequence    the \eR escape sequence
1457    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1458    a character class    a character class
1459    a back reference (see next section)    a back reference (see next section)
1460    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1461      a subroutine call to a subpattern (recursive or otherwise)
1462  .sp  .sp
1463  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1464  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1033  support is available, \eX{3} matches thr Line 1490  support is available, \eX{3} matches thr
1490  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).
1491  .P  .P
1492  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
1493  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1494    subpatterns that are referenced as
1495    .\" HTML <a href="#subpatternsassubroutines">
1496    .\" </a>
1497    subroutines
1498    .\"
1499    from elsewhere in the pattern (but see also the section entitled
1500    .\" HTML <a href="#subdefine">
1501    .\" </a>
1502    "Defining subpatterns for use by reference only"
1503    .\"
1504    below). Items other than subpatterns that have a {0} quantifier are omitted
1505    from the compiled pattern.
1506  .P  .P
1507  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1508  abbreviations:  abbreviations:
# Line 1105  worth setting PCRE_DOTALL in order to ob Line 1574  worth setting PCRE_DOTALL in order to ob
1574  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1575  .P  .P
1576  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1577  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1578  elsewhere in the pattern, a match at the start may fail where a later one  elsewhere in the pattern, a match at the start may fail where a later one
1579  succeeds. Consider, for example:  succeeds. Consider, for example:
1580  .sp  .sp
# Line 1180  previous example can be rewritten as Line 1649  previous example can be rewritten as
1649  .sp  .sp
1650    \ed++foo    \ed++foo
1651  .sp  .sp
1652    Note that a possessive quantifier can be used with an entire group, for
1653    example:
1654    .sp
1655      (abc|xyz){2,3}+
1656    .sp
1657  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1658  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
1659  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 1253  no such problem when named parentheses a Line 1727  no such problem when named parentheses a
1727  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1728  .P  .P
1729  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
1730  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence. This escape must be followed by an
1731  Perl 5.10. This escape must be followed by a positive or a negative number,  unsigned number or a negative number, optionally enclosed in braces. These
1732  optionally enclosed in braces. These examples are all identical:  examples are all identical:
1733  .sp  .sp
1734    (ring), \e1    (ring), \e1
1735    (ring), \eg1    (ring), \eg1
1736    (ring), \eg{1}    (ring), \eg{1}
1737  .sp  .sp
1738  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1739  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
1740  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1741    example:
1742  .sp  .sp
1743    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1744  .sp  .sp
1745  The sequence \eg{-1} is a reference to the most recently started capturing  The sequence \eg{-1} is a reference to the most recently started capturing
1746  subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}  subpattern before \eg, that is, is it equivalent to \e2 in this example.
1747  would be equivalent to \e1. The use of relative references can be helpful in  Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1748  long patterns, and also in patterns that are created by joining together  can be helpful in long patterns, and also in patterns that are created by
1749  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1750  .P  .P
1751  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1752  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1293  back reference, the case of letters is r Line 1768  back reference, the case of letters is r
1768  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1769  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1770  .P  .P
1771  Back references to named subpatterns use the Perl syntax \ek<name> or \ek'name'  There are several different ways of writing back references to named
1772  or the Python syntax (?P=name). We could rewrite the above example in either of  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1773    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1774    back reference syntax, in which \eg can be used for both numeric and named
1775    references, is also supported. We could rewrite the above example in any of
1776  the following ways:  the following ways:
1777  .sp  .sp
1778    (?<p1>(?i)rah)\es+\ek<p1>    (?<p1>(?i)rah)\es+\ek<p1>
1779      (?'p1'(?i)rah)\es+\ek{p1}
1780    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1781      (?<p1>(?i)rah)\es+\eg{p1}
1782  .sp  .sp
1783  A subpattern that is referenced by name may appear in the pattern before or  A subpattern that is referenced by name may appear in the pattern before or
1784  after the reference.  after the reference.
1785  .P  .P
1786  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
1787  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1788  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1789  .sp  .sp
1790    (a|(bc))\e2    (a|(bc))\e2
1791  .sp  .sp
1792  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
1793  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
1794  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1795  with a digit character, some delimiter must be used to terminate the back  .P
1796  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1797  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1798    If the pattern continues with a digit character, some delimiter must be used to
1799    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1800    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1801  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1802  .\" </a>  .\" </a>
1803  "Comments"  "Comments"
1804  .\"  .\"
1805  below) can be used.  below) can be used.
1806  .P  .
1807    .SS "Recursive back references"
1808    .rs
1809    .sp
1810  A back reference that occurs inside the parentheses to which it refers fails  A back reference that occurs inside the parentheses to which it refers fails
1811  when the subpattern is first used, so, for example, (a\e1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1812  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1334  to the previous iteration. In order for Line 1820  to the previous iteration. In order for
1820  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
1821  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1822  minimum of zero.  minimum of zero.
1823    .P
1824    Back references of this type cause the group that they reference to be treated
1825    as an
1826    .\" HTML <a href="#atomicgroup">
1827    .\" </a>
1828    atomic group.
1829    .\"
1830    Once the whole group has been matched, a subsequent matching failure cannot
1831    cause backtracking into the middle of the group.
1832  .  .
1833  .  .
1834  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1353  those that look ahead of the current pos Line 1848  those that look ahead of the current pos
1848  that look behind it. An assertion subpattern is matched in the normal way,  that look behind it. An assertion subpattern is matched in the normal way,
1849  except that it does not cause the current matching position to be changed.  except that it does not cause the current matching position to be changed.
1850  .P  .P
1851  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1852  because it makes no sense to assert the same thing several times. If any kind  contains capturing subpatterns within it, these are counted for the purposes of
1853  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1854  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1855  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1856  because it does not make sense for negative assertions.  .P
1857    For compatibility with Perl, assertion subpatterns may be repeated; though
1858    it makes no sense to assert the same thing several times, the side effect of
1859    capturing parentheses may occasionally be useful. In practice, there only three
1860    cases:
1861    .sp
1862    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1863    However, it may contain internal capturing parenthesized groups that are called
1864    from elsewhere via the
1865    .\" HTML <a href="#subpatternsassubroutines">
1866    .\" </a>
1867    subroutine mechanism.
1868    .\"
1869    .sp
1870    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1871    were {0,1}. At run time, the rest of the pattern match is tried with and
1872    without the assertion, the order depending on the greediness of the quantifier.
1873    .sp
1874    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1875    The assertion is obeyed just once when encountered during matching.
1876  .  .
1877  .  .
1878  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1387  lookbehind assertion is needed to achiev Line 1901  lookbehind assertion is needed to achiev
1901  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
1902  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
1903  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.
1904    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1905  .  .
1906  .  .
1907  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1411  is permitted, but Line 1926  is permitted, but
1926  .sp  .sp
1927  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1928  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
1929  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1930  match the same length of string. An assertion such as  length of string. An assertion such as
1931  .sp  .sp
1932    (?<=ab(c|de))    (?<=ab(c|de))
1933  .sp  .sp
1934  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
1935  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
1936    branches:
1937  .sp  .sp
1938    (?<=abc|abde)    (?<=abc|abde)
1939  .sp  .sp
1940    In some cases, the escape sequence \eK
1941    .\" HTML <a href="#resetmatchstart">
1942    .\" </a>
1943    (see above)
1944    .\"
1945    can be used instead of a lookbehind assertion to get round the fixed-length
1946    restriction.
1947    .P
1948  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1949  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
1950  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
# Line 1431  to appear in lookbehind assertions, beca Line 1955  to appear in lookbehind assertions, beca
1955  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
1956  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1957  .P  .P
1958    .\" HTML <a href="#subpatternsassubroutines">
1959    .\" </a>
1960    "Subroutine"
1961    .\"
1962    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1963    as the subpattern matches a fixed-length string.
1964    .\" HTML <a href="#recursion">
1965    .\" </a>
1966    Recursion,
1967    .\"
1968    however, is not supported.
1969    .P
1970  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1971  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
1972  pattern such as  strings. Consider a simple pattern such as
1973  .sp  .sp
1974    abcd$    abcd$
1975  .sp  .sp
# Line 1497  characters that are not "999". Line 2033  characters that are not "999".
2033  .sp  .sp
2034  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2035  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2036  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2037  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2038  .sp  .sp
2039    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2040    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2041  .sp  .sp
2042  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2043  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
2044  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2045    itself contain nested subpatterns of any form, including conditional
2046    subpatterns; the restriction to two alternatives applies only at the level of
2047    the condition. This pattern fragment is an example where the alternatives are
2048    complex:
2049    .sp
2050      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2051    .sp
2052  .P  .P
2053  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2054  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1514  recursion, a pseudo-condition called DEF Line 2057  recursion, a pseudo-condition called DEF
2057  .rs  .rs
2058  .sp  .sp
2059  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
2060  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2061  matched.  matched. If there is more than one capturing subpattern with the same number
2062    (see the earlier
2063    .\"
2064    .\" HTML <a href="#recursion">
2065    .\" </a>
2066    section about duplicate subpattern numbers),
2067    .\"
2068    the condition is true if any of them have matched. An alternative notation is
2069    to precede the digits with a plus or minus sign. In this case, the subpattern
2070    number is relative rather than absolute. The most recently opened parentheses
2071    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2072    loops it can also make sense to refer to subsequent groups. The next
2073    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2074    zero in any of these forms is not used; it provokes a compile-time error.)
2075  .P  .P
2076  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2077  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 1526  three parts for ease of discussion: Line 2082  three parts for ease of discussion:
2082  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2083  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
2084  matches one or more characters that are not parentheses. The third part is a  matches one or more characters that are not parentheses. The third part is a
2085  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2086  or not. If they did, that is, if subject started with an opening parenthesis,  matched. If they did, that is, if subject started with an opening parenthesis,
2087  the condition is true, and so the yes-pattern is executed and a closing  the condition is true, and so the yes-pattern is executed and a closing
2088  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2089  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2090  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
2091    .P
2092    If you were embedding this pattern in a larger one, you could use a relative
2093    reference:
2094    .sp
2095      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
2096    .sp
2097    This makes the fragment independent of the parentheses in the larger pattern.
2098  .  .
2099  .SS "Checking for a used subpattern by name"  .SS "Checking for a used subpattern by name"
2100  .rs  .rs
# Line 1549  Rewriting the above example to use a nam Line 2112  Rewriting the above example to use a nam
2112  .sp  .sp
2113    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2114  .sp  .sp
2115    If the name used in a condition of this kind is a duplicate, the test is
2116    applied to all subpatterns of the same name, and is true if any one of them has
2117    matched.
2118  .  .
2119  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2120  .rs  .rs
# Line 1560  letter R, for example: Line 2126  letter R, for example:
2126  .sp  .sp
2127    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2128  .sp  .sp
2129  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
2130  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
2131  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2132    applied to all subpatterns of the same name, and is true if any one of them is
2133    the most recent recursion.
2134  .P  .P
2135  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2136  patterns are described below.  .\" HTML <a href="#recursion">
2137    .\" </a>
2138    The syntax for recursive patterns
2139    .\"
2140    is described below.
2141  .  .
2142    .\" HTML <a name="subdefine"></a>
2143  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2144  .rs  .rs
2145  .sp  .sp
# Line 1574  If the condition is the string (DEFINE), Line 2147  If the condition is the string (DEFINE),
2147  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
2148  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2149  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
2150  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2151  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2152  written like this (ignore whitespace and line breaks):  .\" </a>
2153    subroutines
2154    .\"
2155    is described below.) For example, a pattern to match an IPv4 address such as
2156    "192.168.23.245" could be written like this (ignore whitespace and line
2157    breaks):
2158  .sp  .sp
2159    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2160    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1584  written like this (ignore whitespace and Line 2162  written like this (ignore whitespace and
2162  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
2163  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2164  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
2165  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2166  .P  pattern uses references to the named group to match the four dot-separated
2167  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.  
2168  .  .
2169  .SS "Assertion conditions"  .SS "Assertion conditions"
2170  .rs  .rs
# Line 1613  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2189  dd-aaa-dd or dd-dd-dd, where aaa are let
2189  .SH COMMENTS  .SH COMMENTS
2190  .rs  .rs
2191  .sp  .sp
2192  The sequence (?# marks the start of a comment that continues up to the next  There are two ways of including comments in patterns that are processed by
2193  closing parenthesis. Nested parentheses are not permitted. The characters  PCRE. In both cases, the start of the comment must not be in a character class,
2194  that make up a comment play no part in the pattern matching at all.  nor in the middle of any other sequence of related characters such as (?: or a
2195    subpattern name or number. The characters that make up a comment play no part
2196    in the pattern matching.
2197  .P  .P
2198  If the PCRE_EXTENDED option is set, an unescaped # character outside a  The sequence (?# marks the start of a comment that continues up to the next
2199  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2200  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2201    this case continues to immediately after the next newline character or
2202    character sequence in the pattern. Which characters are interpreted as newlines
2203    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2204    sequence at the start of the pattern, as described in the section entitled
2205    .\" HTML <a href="#newlines">
2206    .\" </a>
2207    "Newline conventions"
2208    .\"
2209    above. Note that the end of this type of comment is a literal newline sequence
2210    in the pattern; escape sequences that happen to represent a newline do not
2211    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2212    default newline convention is in force:
2213    .sp
2214      abc #comment \en still comment
2215    .sp
2216    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2217    a newline in the pattern. The sequence \en is still literal at this stage, so
2218    it does not terminate the comment. Only an actual character with the code value
2219    0x0a (the default newline) does so.
2220  .  .
2221  .  .
2222  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1645  recursively to the pattern in which it a Line 2242  recursively to the pattern in which it a
2242  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2243  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2244  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2245  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.
2246  .P  .P
2247  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
2248  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive subroutine call of the subpattern of the
2249  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  given number, provided that it occurs inside that subpattern. (If not, it is a
2250    .\" HTML <a href="#subpatternsassubroutines">
2251    .\" </a>
2252    non-recursive subroutine
2253    .\"
2254  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
2255  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2256  .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  
2257  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2258  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2259  .sp  .sp
2260    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2261  .sp  .sp
2262  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2263  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
2264  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2265  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2266    to avoid backtracking into sequences of non-parentheses.
2267  .P  .P
2268  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
2269  pattern, so instead you could use this:  pattern, so instead you could use this:
2270  .sp  .sp
2271    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2272  .sp  .sp
2273  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
2274  them instead of the whole pattern.  them instead of the whole pattern.
2275  .P  .P
2276  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
2277  is made easier by the use of relative references. (A Perl 5.10 feature.)  is made easier by the use of relative references. Instead of (?1) in the
2278  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  pattern above you can write (?-2) to refer to the second most recently opened
2279  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2280  negative number counts capturing parentheses leftwards from the point at which  capturing parentheses leftwards from the point at which it is encountered.
 it is encountered.  
2281  .P  .P
2282  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2283  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2284  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2285  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2286    .\" </a>
2287    non-recursive subroutine
2288    .\"
2289    calls, as described in the next section.
2290  .P  .P
2291  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2292  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
2293  could rewrite the above example as follows:  could rewrite the above example as follows:
2294  .sp  .sp
2295    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2296  .sp  .sp
2297  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
2298  used.  used.
2299  .P  .P
2300  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2301  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
2302  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
2303  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2304  .sp  .sp
2305    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2306  .sp  .sp
2307  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,
2308  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
2309  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
2310  before failure can be reported.  before failure can be reported.
2311  .P  .P
2312  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
2313  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
2314  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  
2315  .\" HREF  .\" HREF
2316  \fBpcrecallout\fP  \fBpcrecallout\fP
2317  .\"  .\"
# Line 1720  documentation). If the pattern above is Line 2319  documentation). If the pattern above is
2319  .sp  .sp
2320    (ab(cd)ef)    (ab(cd)ef)
2321  .sp  .sp
2322  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
2323  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
2324  .sp  matched at the top level, its final captured value is unset, even if it was
2325    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2326       ^                        ^  .P
2327       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2328  .sp  obtain extra memory to store data during a recursion, which it does by using
2329  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
2330  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.  
2331  .P  .P
2332  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.
2333  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1745  different alternatives for the recursive Line 2341  different alternatives for the recursive
2341  is the actual recursive call.  is the actual recursive call.
2342  .  .
2343  .  .
2344    .\" HTML <a name="recursiondifference"></a>
2345    .SS "Differences in recursion processing between PCRE and Perl"
2346    .rs
2347    .sp
2348    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2349    (like Python, but unlike Perl), a recursive subpattern call is always treated
2350    as an atomic group. That is, once it has matched some of the subject string, it
2351    is never re-entered, even if it contains untried alternatives and there is a
2352    subsequent matching failure. This can be illustrated by the following pattern,
2353    which purports to match a palindromic string that contains an odd number of
2354    characters (for example, "a", "aba", "abcba", "abcdcba"):
2355    .sp
2356      ^(.|(.)(?1)\e2)$
2357    .sp
2358    The idea is that it either matches a single character, or two identical
2359    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2360    it does not if the pattern is longer than three characters. Consider the
2361    subject string "abcba":
2362    .P
2363    At the top level, the first character is matched, but as it is not at the end
2364    of the string, the first alternative fails; the second alternative is taken
2365    and the recursion kicks in. The recursive call to subpattern 1 successfully
2366    matches the next character ("b"). (Note that the beginning and end of line
2367    tests are not part of the recursion).
2368    .P
2369    Back at the top level, the next character ("c") is compared with what
2370    subpattern 2 matched, which was "a". This fails. Because the recursion is
2371    treated as an atomic group, there are now no backtracking points, and so the
2372    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2373    try the second alternative.) However, if the pattern is written with the
2374    alternatives in the other order, things are different:
2375    .sp
2376      ^((.)(?1)\e2|.)$
2377    .sp
2378    This time, the recursing alternative is tried first, and continues to recurse
2379    until it runs out of characters, at which point the recursion fails. But this
2380    time we do have another alternative to try at the higher level. That is the big
2381    difference: in the previous case the remaining alternative is at a deeper
2382    recursion level, which PCRE cannot use.
2383    .P
2384    To change the pattern so that it matches all palindromic strings, not just
2385    those with an odd number of characters, it is tempting to change the pattern to
2386    this:
2387    .sp
2388      ^((.)(?1)\e2|.?)$
2389    .sp
2390    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2391    deeper recursion has matched a single character, it cannot be entered again in
2392    order to match an empty string. The solution is to separate the two cases, and
2393    write out the odd and even cases as alternatives at the higher level:
2394    .sp
2395      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2396    .sp
2397    If you want to match typical palindromic phrases, the pattern has to ignore all
2398    non-word characters, which can be done like this:
2399    .sp
2400      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2401    .sp
2402    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2403    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2404    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2405    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2406    more) to match typical phrases, and Perl takes so long that you think it has
2407    gone into a loop.
2408    .P
2409    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2410    string does not start with a palindrome that is shorter than the entire string.
2411    For example, although "abcba" is correctly matched, if the subject is "ababa",
2412    PCRE finds the palindrome "aba" at the start, then fails at top level because
2413    the end of the string does not follow. Once again, it cannot jump back into the
2414    recursion to try other alternatives, so the entire match fails.
2415    .P
2416    The second way in which PCRE and Perl differ in their recursion processing is
2417    in the handling of captured values. In Perl, when a subpattern is called
2418    recursively or as a subpattern (see the next section), it has no access to any
2419    values that were captured outside the recursion, whereas in PCRE these values
2420    can be referenced. Consider this pattern:
2421    .sp
2422      ^(.)(\e1|a(?2))
2423    .sp
2424    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2425    then in the second group, when the back reference \e1 fails to match "b", the
2426    second alternative matches "a" and then recurses. In the recursion, \e1 does
2427    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2428    match because inside the recursive call \e1 cannot access the externally set
2429    value.
2430    .
2431    .
2432  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2433  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2434  .rs  .rs
2435  .sp  .sp
2436  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern call (either by number or by
2437  name) is used outside the parentheses to which it refers, it operates like a  name) is used outside the parentheses to which it refers, it operates like a
2438  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2439  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2440  relative, as in these examples:  relative, as in these examples:
2441  .sp  .sp
2442    (...(absolute)...)...(?2)...    (...(absolute)...)...(?2)...
2443    (...(relative)...)...(?-1)...    (...(relative)...)...(?-1)...
2444    (...(?+1)...(relative)...    (...(?+1)...(relative)...
2445  .sp  .sp
2446  An earlier example pointed out that the pattern  An earlier example pointed out that the pattern
2447  .sp  .sp
# Line 1771  matches "sense and sensibility" and "res Line 2455  matches "sense and sensibility" and "res
2455  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
2456  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2457  .P  .P
2458  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2459  group. That is, once it has matched some of the subject string, it is never  groups. That is, once a subroutine has matched some of the subject string, it
2460  re-entered, even if it contains untried alternatives and there is a subsequent  is never re-entered, even if it contains untried alternatives and there is a
2461  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2462  .P  subroutine call revert to their previous values afterwards.
2463  When a subpattern is used as a subroutine, processing options such as  .P
2464  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2465  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2466    different calls. For example, consider this pattern:
2467  .sp  .sp
2468    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2469  .sp  .sp
# Line 1786  It matches "abcabc". It does not match " Line 2471  It matches "abcabc". It does not match "
2471  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2472  .  .
2473  .  .
2474    .\" HTML <a name="onigurumasubroutines"></a>
2475    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2476    .rs
2477    .sp
2478    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2479    a number enclosed either in angle brackets or single quotes, is an alternative
2480    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2481    are two of the examples used above, rewritten using this syntax:
2482    .sp
2483      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2484      (sens|respons)e and \eg'1'ibility
2485    .sp
2486    PCRE supports an extension to Oniguruma: if a number is preceded by a
2487    plus or a minus sign it is taken as a relative reference. For example:
2488    .sp
2489      (abc)(?i:\eg<-1>)
2490    .sp
2491    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2492    synonymous. The former is a back reference; the latter is a subroutine call.
2493    .
2494    .
2495  .SH CALLOUTS  .SH CALLOUTS
2496  .rs  .rs
2497  .sp  .sp
# Line 1822  description of the interface to the call Line 2528  description of the interface to the call
2528  documentation.  documentation.
2529  .  .
2530  .  .
2531    .\" HTML <a name="backtrackcontrol"></a>
2532    .SH "BACKTRACKING CONTROL"
2533    .rs
2534    .sp
2535    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2536    are described in the Perl documentation as "experimental and subject to change
2537    or removal in a future version of Perl". It goes on to say: "Their usage in
2538    production code should be noted to avoid problems during upgrades." The same
2539    remarks apply to the PCRE features described in this section.
2540    .P
2541    Since these verbs are specifically related to backtracking, most of them can be
2542    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2543    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2544    failing negative assertion, they cause an error if encountered by
2545    \fBpcre_dfa_exec()\fP.
2546    .P
2547    If any of these verbs are used in an assertion or in a subpattern that is
2548    called as a subroutine (whether or not recursively), their effect is confined
2549    to that subpattern; it does not extend to the surrounding pattern, with one
2550    exception: a *MARK that is encountered in a positive assertion \fIis\fP passed
2551    back (compare capturing parentheses in assertions). Note that such subpatterns
2552    are processed as anchored at the point where they are tested. Note also that
2553    Perl's treatment of subroutines is different in some cases.
2554    .P
2555    The new verbs make use of what was previously invalid syntax: an opening
2556    parenthesis followed by an asterisk. They are generally of the form
2557    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2558    depending on whether or not an argument is present. A name is any sequence of
2559    characters that does not include a closing parenthesis. If the name is empty,
2560    that is, if the closing parenthesis immediately follows the colon, the effect
2561    is as if the colon were not there. Any number of these verbs may occur in a
2562    pattern.
2563    .P
2564    PCRE contains some optimizations that are used to speed up matching by running
2565    some checks at the start of each match attempt. For example, it may know the
2566    minimum length of matching subject, or that a particular character must be
2567    present. When one of these optimizations suppresses the running of a match, any
2568    included backtracking verbs will not, of course, be processed. You can suppress
2569    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2570    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2571    pattern with (*NO_START_OPT).
2572    .
2573    .
2574    .SS "Verbs that act immediately"
2575    .rs
2576    .sp
2577    The following verbs act as soon as they are encountered. They may not be
2578    followed by a name.
2579    .sp
2580       (*ACCEPT)
2581    .sp
2582    This verb causes the match to end successfully, skipping the remainder of the
2583    pattern. However, when it is inside a subpattern that is called as a
2584    subroutine, only that subpattern is ended successfully. Matching then continues
2585    at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2586    far is captured. For example:
2587    .sp
2588      A((?:A|B(*ACCEPT)|C)D)
2589    .sp
2590    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2591    the outer parentheses.
2592    .sp
2593      (*FAIL) or (*F)
2594    .sp
2595    This verb causes a matching failure, forcing backtracking to occur. It is
2596    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2597    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2598    Perl features that are not present in PCRE. The nearest equivalent is the
2599    callout feature, as for example in this pattern:
2600    .sp
2601      a+(?C)(*FAIL)
2602    .sp
2603    A match with the string "aaaa" always fails, but the callout is taken before
2604    each backtrack happens (in this example, 10 times).
2605    .
2606    .
2607    .SS "Recording which path was taken"
2608    .rs
2609    .sp
2610    There is one verb whose main purpose is to track how a match was arrived at,
2611    though it also has a secondary use in conjunction with advancing the match
2612    starting point (see (*SKIP) below).
2613    .sp
2614      (*MARK:NAME) or (*:NAME)
2615    .sp
2616    A name is always required with this verb. There may be as many instances of
2617    (*MARK) as you like in a pattern, and their names do not have to be unique.
2618    .P
2619    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2620    to the caller via the \fIpcre_extra\fP data structure, as described in the
2621    .\" HTML <a href="pcreapi.html#extradata">
2622    .\" </a>
2623    section on \fIpcre_extra\fP
2624    .\"
2625    in the
2626    .\" HREF
2627    \fBpcreapi\fP
2628    .\"
2629    documentation. No data is returned for a partial match. Here is an example of
2630    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2631    outputting of (*MARK) data:
2632    .sp
2633      /X(*MARK:A)Y|X(*MARK:B)Z/K
2634      XY
2635       0: XY
2636      MK: A
2637      XZ
2638       0: XZ
2639      MK: B
2640    .sp
2641    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2642    indicates which of the two alternatives matched. This is a more efficient way
2643    of obtaining this information than putting each alternative in its own
2644    capturing parentheses.
2645    .P
2646    If (*MARK) is encountered in a positive assertion, its name is recorded and
2647    passed back if it is the last-encountered. This does not happen for negative
2648    assertions.
2649    .P
2650    A name may also be returned after a failed match if the final path through the
2651    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2652    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2653    starting point for matching is advanced, the final check is often with an empty
2654    string, causing a failure before (*MARK) is reached. For example:
2655    .sp
2656      /X(*MARK:A)Y|X(*MARK:B)Z/K
2657      XP
2658      No match
2659    .sp
2660    There are three potential starting points for this match (starting with X,
2661    starting with P, and with an empty string). If the pattern is anchored, the
2662    result is different:
2663    .sp
2664      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2665      XP
2666      No match, mark = B
2667    .sp
2668    PCRE's start-of-match optimizations can also interfere with this. For example,
2669    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2670    subject length for a match, a shorter subject will not be scanned at all.
2671    .P
2672    Note that similar anomalies (though different in detail) exist in Perl, no
2673    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2674    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2675    .
2676    .
2677    .SS "Verbs that act after backtracking"
2678    .rs
2679    .sp
2680    The following verbs do nothing when they are encountered. Matching continues
2681    with what follows, but if there is no subsequent match, causing a backtrack to
2682    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2683    the verb. However, when one of these verbs appears inside an atomic group, its
2684    effect is confined to that group, because once the group has been matched,
2685    there is never any backtracking into it. In this situation, backtracking can
2686    "jump back" to the left of the entire atomic group. (Remember also, as stated
2687    above, that this localization also applies in subroutine calls and assertions.)
2688    .P
2689    These verbs differ in exactly what kind of failure occurs when backtracking
2690    reaches them.
2691    .sp
2692      (*COMMIT)
2693    .sp
2694    This verb, which may not be followed by a name, causes the whole match to fail
2695    outright if the rest of the pattern does not match. Even if the pattern is
2696    unanchored, no further attempts to find a match by advancing the starting point
2697    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2698    finding a match at the current starting point, or not at all. For example:
2699    .sp
2700      a+(*COMMIT)b
2701    .sp
2702    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2703    dynamic anchor, or "I've started, so I must finish." The name of the most
2704    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2705    match failure.
2706    .P
2707    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2708    unless PCRE's start-of-match optimizations are turned off, as shown in this
2709    \fBpcretest\fP example:
2710    .sp
2711      /(*COMMIT)abc/
2712      xyzabc
2713       0: abc
2714      xyzabc\eY
2715      No match
2716    .sp
2717    PCRE knows that any match must start with "a", so the optimization skips along
2718    the subject to "a" before running the first match attempt, which succeeds. When
2719    the optimization is disabled by the \eY escape in the second subject, the match
2720    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2721    starting points.
2722    .sp
2723      (*PRUNE) or (*PRUNE:NAME)
2724    .sp
2725    This verb causes the match to fail at the current starting position in the
2726    subject if the rest of the pattern does not match. If the pattern is
2727    unanchored, the normal "bumpalong" advance to the next starting character then
2728    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2729    reached, or when matching to the right of (*PRUNE), but if there is no match to
2730    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2731    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2732    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2733    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2734    match fails completely; the name is passed back if this is the final attempt.
2735    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2736    pattern (*PRUNE) has the same effect as (*COMMIT).
2737    .sp
2738      (*SKIP)
2739    .sp
2740    This verb, when given without a name, is like (*PRUNE), except that if the
2741    pattern is unanchored, the "bumpalong" advance is not to the next character,
2742    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2743    signifies that whatever text was matched leading up to it cannot be part of a
2744    successful match. Consider:
2745    .sp
2746      a+(*SKIP)b
2747    .sp
2748    If the subject is "aaaac...", after the first match attempt fails (starting at
2749    the first character in the string), the starting point skips on to start the
2750    next attempt at "c". Note that a possessive quantifer does not have the same
2751    effect as this example; although it would suppress backtracking during the
2752    first match attempt, the second attempt would start at the second character
2753    instead of skipping on to "c".
2754    .sp
2755      (*SKIP:NAME)
2756    .sp
2757    When (*SKIP) has an associated name, its behaviour is modified. If the
2758    following pattern fails to match, the previous path through the pattern is
2759    searched for the most recent (*MARK) that has the same name. If one is found,
2760    the "bumpalong" advance is to the subject position that corresponds to that
2761    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2762    matching name is found, normal "bumpalong" of one character happens (that is,
2763    the (*SKIP) is ignored).
2764    .sp
2765      (*THEN) or (*THEN:NAME)
2766    .sp
2767    This verb causes a skip to the next innermost alternative if the rest of the
2768    pattern does not match. That is, it cancels pending backtracking, but only
2769    within the current alternative. Its name comes from the observation that it can
2770    be used for a pattern-based if-then-else block:
2771    .sp
2772      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2773    .sp
2774    If the COND1 pattern matches, FOO is tried (and possibly further items after
2775    the end of the group if FOO succeeds); on failure, the matcher skips to the
2776    second alternative and tries COND2, without backtracking into COND1. The
2777    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2778    overall match fails. If (*THEN) is not inside an alternation, it acts like
2779    (*PRUNE).
2780    .P
2781    Note that a subpattern that does not contain a | character is just a part of
2782    the enclosing alternative; it is not a nested alternation with only one
2783    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2784    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2785    pattern fragments that do not contain any | characters at this level:
2786    .sp
2787      A (B(*THEN)C) | D
2788    .sp
2789    If A and B are matched, but there is a failure in C, matching does not
2790    backtrack into A; instead it moves to the next alternative, that is, D.
2791    However, if the subpattern containing (*THEN) is given an alternative, it
2792    behaves differently:
2793    .sp
2794      A (B(*THEN)C | (*FAIL)) | D
2795    .sp
2796    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2797    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2798    because there are no more alternatives to try. In this case, matching does now
2799    backtrack into A.
2800    .P
2801    Note also that a conditional subpattern is not considered as having two
2802    alternatives, because only one is ever used. In other words, the | character in
2803    a conditional subpattern has a different meaning. Ignoring white space,
2804    consider:
2805    .sp
2806      ^.*? (?(?=a) a | b(*THEN)c )
2807    .sp
2808    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2809    it initially matches zero characters. The condition (?=a) then fails, the
2810    character "b" is matched, but "c" is not. At this point, matching does not
2811    backtrack to .*? as might perhaps be expected from the presence of the |
2812    character. The conditional subpattern is part of the single alternative that
2813    comprises the whole pattern, and so the match fails. (If there was a backtrack
2814    into .*?, allowing it to match "b", the match would succeed.)
2815    .P
2816    The verbs just described provide four different "strengths" of control when
2817    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2818    next alternative. (*PRUNE) comes next, failing the match at the current
2819    starting position, but allowing an advance to the next character (for an
2820    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2821    than one character. (*COMMIT) is the strongest, causing the entire match to
2822    fail.
2823    .P
2824    If more than one such verb is present in a pattern, the "strongest" one wins.
2825    For example, consider this pattern, where A, B, etc. are complex pattern
2826    fragments:
2827    .sp
2828      (A(*COMMIT)B(*THEN)C|D)
2829    .sp
2830    Once A has matched, PCRE is committed to this match, at the current starting
2831    position. If subsequently B matches, but C does not, the normal (*THEN) action
2832    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2833    overrides.
2834    .
2835    .
2836  .SH "SEE ALSO"  .SH "SEE ALSO"
2837  .rs  .rs
2838  .sp  .sp
2839  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2840    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2841  .  .
2842  .  .
2843  .SH AUTHOR  .SH AUTHOR
# Line 1842  Cambridge CB2 3QH, England. Line 2854  Cambridge CB2 3QH, England.
2854  .rs  .rs
2855  .sp  .sp
2856  .nf  .nf
2857  Last updated: 06 March 2007  Last updated: 19 October 2011
2858  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
2859  .fi  .fi

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