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

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

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

revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 556 by ph10, Tue Oct 26 11:06:44 2010 UTC
# Line 4  PCRE - Perl-compatible regular expressio Line 4  PCRE - Perl-compatible regular expressio
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18    regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 character strings. To use this,
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  PCRE must be built to include UTF-8 support, and you must call
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  places below. There is also a summary of UTF-8 features in the  is also a special sequence that can be given at the start of a pattern:
28    .sp
29      (*UTF8)
30    .sp
31    Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32    option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33    pattern matching is mentioned in several places below. There is also a summary
34    of UTF-8 features in the
35  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
36  .\" </a>  .\" </a>
37  section on UTF-8 support  section on UTF-8 support
# Line 26  in the main Line 42  in the main
42  .\"  .\"
43  page.  page.
44  .P  .P
45    Another special sequence that may appear at the start of a pattern or in
46    combination with (*UTF8) is:
47    .sp
48      (*UCP)
49    .sp
50    This has the same effect as setting the PCRE_UCP option: it causes sequences
51    such as \ed and \ew to use Unicode properties to determine character types,
52    instead of recognizing only characters with codes less than 128 via a lookup
53    table.
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  .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"
123    .rs
124    .sp
125  A regular expression is a pattern that is matched against a subject string from  A regular expression is a pattern that is matched against a subject string from
126  left to right. Most characters stand for themselves in a pattern, and match the  left to right. Most characters stand for themselves in a pattern, and match the
127  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
# Line 60  interpreted in some special way. Line 145  interpreted in some special way.
145  .P  .P
146  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
147  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
148  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
149  as follows:  are as follows:
150  .sp  .sp
151    \e      general escape character with several uses    \e      general escape character with several uses
152    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
# Line 92  a character class the only metacharacter Line 177  a character class the only metacharacter
177  .sp  .sp
178  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
179  .  .
180    .
181  .SH BACKSLASH  .SH BACKSLASH
182  .rs  .rs
183  .sp  .sp
# Line 125  Perl, $ and @ cause variable interpolati Line 211  Perl, $ and @ cause variable interpolati
211    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
212  .sp  .sp
213  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
214    An isolated \eE that is not preceded by \eQ is ignored.
215  .  .
216  .  .
217  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 134  The \eQ...\eE sequence is recognized bot Line 221  The \eQ...\eE sequence is recognized bot
221  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
222  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
223  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
224  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
225  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:  
226  .sp  .sp
227    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
228    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
229    \ee        escape (hex 1B)    \ee        escape (hex 1B)
230    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
231    \en        newline (hex 0A)    \en        linefeed (hex 0A)
232    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
233    \et        tab (hex 09)    \et        tab (hex 09)
234    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
235    \exhh      character with hex code hh    \exhh      character with hex code hh
236    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
237  .sp  .sp
# Line 157  Thus \ecz becomes hex 1A, but \ec{ becom Line 243  Thus \ecz becomes hex 1A, but \ec{ becom
243  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
244  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{
245  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
246  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
247  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
248  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
249  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
250  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
251    there is no terminating }, this form of escape is not recognized. Instead, the
252    initial \ex will be interpreted as a basic hexadecimal escape, with no
253    following digits, giving a character whose value is zero.
254  .P  .P
255  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
256  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex. There is no difference in the way they are handled. For
# Line 190  parenthesized subpatterns. Line 279  parenthesized subpatterns.
279  .P  .P
280  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number is greater than 9 and there
281  have not been that many capturing subpatterns, PCRE re-reads up to three octal  have not been that many capturing subpatterns, PCRE re-reads up to three octal
282  digits following the backslash, ane uses them to generate a data character. Any  digits following the backslash, and uses them to generate a data character. Any
283  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
284  character specified in octal must be less than \e400. In UTF-8 mode, values up  character specified in octal must be less than \e400. In UTF-8 mode, values up
285  to \e777 are permitted. For example:  to \e777 are permitted. For example:
# Line 220  zero, because no more than three octal d Line 309  zero, because no more than three octal d
309  .P  .P
310  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
311  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
312  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
313  sequence \eX is interpreted as the character "X". Outside a character class,  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
314  these sequences have different meanings  unrecognized escape sequences, they are treated as the literal characters "B",
315  .\" HTML <a href="#uniextseq">  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
316    set. Outside a character class, these sequences have different meanings.
317    .
318    .
319    .SS "Absolute and relative back references"
320    .rs
321    .sp
322    The sequence \eg followed by an unsigned or a negative number, optionally
323    enclosed in braces, is an absolute or relative back reference. A named back
324    reference can be coded as \eg{name}. Back references are discussed
325    .\" HTML <a href="#backreferences">
326  .\" </a>  .\" </a>
327  (see below).  later,
328    .\"
329    following the discussion of
330    .\" HTML <a href="#subpattern">
331    .\" </a>
332    parenthesized subpatterns.
333    .\"
334    .
335    .
336    .SS "Absolute and relative subroutine calls"
337    .rs
338    .sp
339    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
340    a number enclosed either in angle brackets or single quotes, is an alternative
341    syntax for referencing a subpattern as a "subroutine". Details are discussed
342    .\" HTML <a href="#onigurumasubroutines">
343    .\" </a>
344    later.
345    .\"
346    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
347    synonymous. The former is a back reference; the latter is a
348    .\" HTML <a href="#subpatternsassubroutines">
349    .\" </a>
350    subroutine
351  .\"  .\"
352    call.
353  .  .
354  .  .
355    .\" HTML <a name="genericchartypes"></a>
356  .SS "Generic character types"  .SS "Generic character types"
357  .rs  .rs
358  .sp  .sp
359  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
360  .sp  .sp
361    \ed     any decimal digit    \ed     any decimal digit
362    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
363      \eh     any horizontal whitespace character
364      \eH     any character that is not a horizontal whitespace character
365    \es     any whitespace character    \es     any whitespace character
366    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
367      \ev     any vertical whitespace character
368      \eV     any character that is not a vertical whitespace character
369    \ew     any "word" character    \ew     any "word" character
370    \eW     any "non-word" character    \eW     any "non-word" character
371  .sp  .sp
372  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.
373  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
374    .\" HTML <a href="#fullstopdot">
375    .\" </a>
376    the "." metacharacter
377    .\"
378    when PCRE_DOTALL is not set.
379  .P  .P
380  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
381    of characters into two disjoint sets. Any given character matches one, and only
382    one, of each pair. The sequences can appear both inside and outside character
383  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
384  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
385  there is no character to match.  there is no character to match.
386  .P  .P
387  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).
388  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
389  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
390  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
391  does.)  does.
392  .P  .P
393  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.
394  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
395  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
396  place (see  place (see
397  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
# Line 268  in the Line 402  in the
402  .\" HREF  .\" HREF
403  \fBpcreapi\fP  \fBpcreapi\fP
404  .\"  .\"
405  page). For example, in the "fr_FR" (French) locale, some character codes  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
406  greater than 128 are used for accented letters, and these are matched by \ew.  or "french" in Windows, some character codes greater than 128 are used for
407  .P  accented letters, and these are then matched by \ew. The use of locales with
408  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  Unicode is discouraged.
409  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .P
410  character property support is available. The use of locales with Unicode is  By default, in UTF-8 mode, characters with values greater than 128 never match
411  discouraged.  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
412    their original meanings from before UTF-8 support was available, mainly for
413    efficiency reasons. However, if PCRE is compiled with Unicode property support,
414    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
415    properties are used to determine character types, as follows:
416    .sp
417      \ed  any character that \ep{Nd} matches (decimal digit)
418      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
419      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
420    .sp
421    The upper case escapes match the inverse sets of characters. Note that \ed
422    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
423    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
424    \eB because they are defined in terms of \ew and \eW. Matching these sequences
425    is noticeably slower when PCRE_UCP is set.
426    .P
427    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
428    other sequences, which match only ASCII characters by default, these always
429    match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is
430    set. The horizontal space characters are:
431    .sp
432      U+0009     Horizontal tab
433      U+0020     Space
434      U+00A0     Non-break space
435      U+1680     Ogham space mark
436      U+180E     Mongolian vowel separator
437      U+2000     En quad
438      U+2001     Em quad
439      U+2002     En space
440      U+2003     Em space
441      U+2004     Three-per-em space
442      U+2005     Four-per-em space
443      U+2006     Six-per-em space
444      U+2007     Figure space
445      U+2008     Punctuation space
446      U+2009     Thin space
447      U+200A     Hair space
448      U+202F     Narrow no-break space
449      U+205F     Medium mathematical space
450      U+3000     Ideographic space
451    .sp
452    The vertical space characters are:
453    .sp
454      U+000A     Linefeed
455      U+000B     Vertical tab
456      U+000C     Formfeed
457      U+000D     Carriage return
458      U+0085     Next line
459      U+2028     Line separator
460      U+2029     Paragraph separator
461    .
462    .
463    .\" HTML <a name="newlineseq"></a>
464    .SS "Newline sequences"
465    .rs
466    .sp
467    Outside a character class, by default, the escape sequence \eR matches any
468    Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
469    equivalent to the following:
470    .sp
471      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
472    .sp
473    This is an example of an "atomic group", details of which are given
474    .\" HTML <a href="#atomicgroup">
475    .\" </a>
476    below.
477    .\"
478    This particular group matches either the two-character sequence CR followed by
479    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
480    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
481    line, U+0085). The two-character sequence is treated as a single unit that
482    cannot be split.
483    .P
484    In UTF-8 mode, two additional characters whose codepoints are greater than 255
485    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
486    Unicode character property support is not needed for these characters to be
487    recognized.
488    .P
489    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
490    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
491    either at compile time or when the pattern is matched. (BSR is an abbrevation
492    for "backslash R".) This can be made the default when PCRE is built; if this is
493    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
494    It is also possible to specify these settings by starting a pattern string with
495    one of the following sequences:
496    .sp
497      (*BSR_ANYCRLF)   CR, LF, or CRLF only
498      (*BSR_UNICODE)   any Unicode newline sequence
499    .sp
500    These override the default and the options given to \fBpcre_compile()\fP or
501    \fBpcre_compile2()\fP, but they can be overridden by options given to
502    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
503    which are not Perl-compatible, are recognized only at the very start of a
504    pattern, and that they must be in upper case. If more than one of them is
505    present, the last one is used. They can be combined with a change of newline
506    convention; for example, a pattern can start with:
507    .sp
508      (*ANY)(*BSR_ANYCRLF)
509    .sp
510    They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
511    a character class, \eR is treated as an unrecognized escape sequence, and so
512    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
513  .  .
514  .  .
515  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 282  discouraged. Line 517  discouraged.
517  .rs  .rs
518  .sp  .sp
519  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
520  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
521  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
522    characters whose codepoints are less than 256, but they do work in this mode.
523    The extra escape sequences are:
524  .sp  .sp
525    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
526    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
527    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
528  .sp  .sp
529  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
530  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
531  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
532  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
533  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
534    .\" </a>
535    next section).
536    .\"
537    Other Perl properties such as "InMusicalSymbols" are not currently supported by
538    PCRE. Note that \eP{Any} does not match any characters, so always causes a
539    match failure.
540  .P  .P
541  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
542  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 307  Those that are not part of an identified Line 550  Those that are not part of an identified
550  .P  .P
551  Arabic,  Arabic,
552  Armenian,  Armenian,
553    Avestan,
554    Balinese,
555    Bamum,
556  Bengali,  Bengali,
557  Bopomofo,  Bopomofo,
558  Braille,  Braille,
559  Buginese,  Buginese,
560  Buhid,  Buhid,
561  Canadian_Aboriginal,  Canadian_Aboriginal,
562    Carian,
563    Cham,
564  Cherokee,  Cherokee,
565  Common,  Common,
566  Coptic,  Coptic,
567    Cuneiform,
568  Cypriot,  Cypriot,
569  Cyrillic,  Cyrillic,
570  Deseret,  Deseret,
571  Devanagari,  Devanagari,
572    Egyptian_Hieroglyphs,
573  Ethiopic,  Ethiopic,
574  Georgian,  Georgian,
575  Glagolitic,  Glagolitic,
# Line 332  Hangul, Line 582  Hangul,
582  Hanunoo,  Hanunoo,
583  Hebrew,  Hebrew,
584  Hiragana,  Hiragana,
585    Imperial_Aramaic,
586  Inherited,  Inherited,
587    Inscriptional_Pahlavi,
588    Inscriptional_Parthian,
589    Javanese,
590    Kaithi,
591  Kannada,  Kannada,
592  Katakana,  Katakana,
593    Kayah_Li,
594  Kharoshthi,  Kharoshthi,
595  Khmer,  Khmer,
596  Lao,  Lao,
597  Latin,  Latin,
598    Lepcha,
599  Limbu,  Limbu,
600  Linear_B,  Linear_B,
601    Lisu,
602    Lycian,
603    Lydian,
604  Malayalam,  Malayalam,
605    Meetei_Mayek,
606  Mongolian,  Mongolian,
607  Myanmar,  Myanmar,
608  New_Tai_Lue,  New_Tai_Lue,
609    Nko,
610  Ogham,  Ogham,
611  Old_Italic,  Old_Italic,
612  Old_Persian,  Old_Persian,
613    Old_South_Arabian,
614    Old_Turkic,
615    Ol_Chiki,
616  Oriya,  Oriya,
617  Osmanya,  Osmanya,
618    Phags_Pa,
619    Phoenician,
620    Rejang,
621  Runic,  Runic,
622    Samaritan,
623    Saurashtra,
624  Shavian,  Shavian,
625  Sinhala,  Sinhala,
626    Sundanese,
627  Syloti_Nagri,  Syloti_Nagri,
628  Syriac,  Syriac,
629  Tagalog,  Tagalog,
630  Tagbanwa,  Tagbanwa,
631  Tai_Le,  Tai_Le,
632    Tai_Tham,
633    Tai_Viet,
634  Tamil,  Tamil,
635  Telugu,  Telugu,
636  Thaana,  Thaana,
# Line 365  Thai, Line 638  Thai,
638  Tibetan,  Tibetan,
639  Tifinagh,  Tifinagh,
640  Ugaritic,  Ugaritic,
641    Vai,
642  Yi.  Yi.
643  .P  .P
644  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
645  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
646  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
647  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
648  .P  .P
649  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
650  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 430  The special property L& is also supporte Line 704  The special property L& is also supporte
704  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
705  a modifier or "other".  a modifier or "other".
706  .P  .P
707  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
708    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
709    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
710    (see the discussion of PCRE_NO_UTF8_CHECK in the
711    .\" HREF
712    \fBpcreapi\fP
713    .\"
714    page). Perl does not support the Cs property.
715    .P
716    The long synonyms for property names that Perl supports (such as \ep{Letter})
717  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
718  properties with "Is".  properties with "Is".
719  .P  .P
# Line 454  atomic group Line 737  atomic group
737  (see below).  (see below).
738  .\"  .\"
739  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
740  preceding character.  preceding character. None of them have codepoints less than 256, so in
741    non-UTF-8 mode \eX matches any one character.
742  .P  .P
743  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
744  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
745  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
746  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
747    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
748    (*UCP).
749    .
750    .
751    .\" HTML <a name="extraprops"></a>
752    .SS PCRE's additional properties
753    .rs
754    .sp
755    As well as the standard Unicode properties described in the previous
756    section, PCRE supports four more that make it possible to convert traditional
757    escape sequences such as \ew and \es and POSIX character classes to use Unicode
758    properties. PCRE uses these non-standard, non-Perl properties internally when
759    PCRE_UCP is set. They are:
760    .sp
761      Xan   Any alphanumeric character
762      Xps   Any POSIX space character
763      Xsp   Any Perl space character
764      Xwd   Any Perl "word" character
765    .sp
766    Xan matches characters that have either the L (letter) or the N (number)
767    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
768    carriage return, and any other character that has the Z (separator) property.
769    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
770    same characters as Xan, plus underscore.
771    .
772    .
773    .\" HTML <a name="resetmatchstart"></a>
774    .SS "Resetting the match start"
775    .rs
776    .sp
777    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
778    matched characters not to be included in the final matched sequence. For
779    example, the pattern:
780    .sp
781      foo\eKbar
782    .sp
783    matches "foobar", but reports that it has matched "bar". This feature is
784    similar to a lookbehind assertion
785    .\" HTML <a href="#lookbehind">
786    .\" </a>
787    (described below).
788    .\"
789    However, in this case, the part of the subject before the real match does not
790    have to be of fixed length, as lookbehind assertions do. The use of \eK does
791    not interfere with the setting of
792    .\" HTML <a href="#subpattern">
793    .\" </a>
794    captured substrings.
795    .\"
796    For example, when the pattern
797    .sp
798      (foo)\eKbar
799    .sp
800    matches "foobar", the first substring is still set to "foo".
801    .P
802    Perl documents that the use of \eK within assertions is "not well defined". In
803    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
804    ignored in negative assertions.
805  .  .
806  .  .
807  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
808  .SS "Simple assertions"  .SS "Simple assertions"
809  .rs  .rs
810  .sp  .sp
811  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
812  specifies a condition that has to be met at a particular point in a match,  specifies a condition that has to be met at a particular point in a match,
813  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
814  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 478  The backslashed assertions are: Line 820  The backslashed assertions are:
820  .sp  .sp
821    \eb     matches at a word boundary    \eb     matches at a word boundary
822    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
823    \eA     matches at start of subject    \eA     matches at the start of the subject
824    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
825    \ez     matches at end of subject            also matches before a newline at the end of the subject
826    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
827  .sp    \eG     matches at the first matching position in the subject
828  These assertions may not appear in character classes (but note that \eb has a  .sp
829  different meaning, namely the backspace character, inside a character class).  Inside a character class, \eb has a different meaning; it matches the backspace
830    character. If any other of these assertions appears in a character class, by
831    default it matches the corresponding literal character (for example, \eB
832    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
833    escape sequence" error is generated instead.
834  .P  .P
835  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
836  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
837  \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
838  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
839    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
840    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
841    of word" or "end of word" metasequence. However, whatever follows \eb normally
842    determines which it is. For example, the fragment \eba matches "a" at the start
843    of a word.
844  .P  .P
845  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
846  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 573  end of the subject in both modes, and if Line 924  end of the subject in both modes, and if
924  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
925  .  .
926  .  .
927  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
928    .SH "FULL STOP (PERIOD, DOT) AND \eN"
929  .rs  .rs
930  .sp  .sp
931  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
932  the subject string except (by default) a character that signifies the end of a  the subject string except (by default) a character that signifies the end of a
933  line. In UTF-8 mode, the matched character may be more than one byte long. When  line. In UTF-8 mode, the matched character may be more than one byte long.
934  a line ending is defined as a single character (CR or LF), dot never matches  .P
935  that character; when the two-character sequence CRLF is used, dot does not  When a line ending is defined as a single character, dot never matches that
936  match CR if it is immediately followed by LF, but otherwise it matches all  character; when the two-character sequence CRLF is used, dot does not match CR
937  characters (including isolated CRs and LFs).  if it is immediately followed by LF, but otherwise it matches all characters
938    (including isolated CRs and LFs). When any Unicode line endings are being
939    recognized, dot does not match CR or LF or any of the other line ending
940    characters.
941  .P  .P
942  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
943  option is set, a dot matches any one character, without exception. If newline  option is set, a dot matches any one character, without exception. If the
944  is defined as the two-character sequence CRLF, it takes two dots to match it.  two-character sequence CRLF is present in the subject string, it takes two dots
945    to match it.
946  .P  .P
947  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
948  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
949  special meaning in a character class.  special meaning in a character class.
950    .P
951    The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not
952    set. In other words, it matches any one character except one that signifies the
953    end of a line.
954  .  .
955  .  .
956  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
957  .rs  .rs
958  .sp  .sp
959  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
960  in and out of UTF-8 mode. Unlike a dot, it always matches CR and LF. The  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
961  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  characters. The feature is provided in Perl in order to match individual bytes
962  Because it breaks up UTF-8 characters into individual bytes, what remains in  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
963  the string may be a malformed UTF-8 string. For this reason, the \eC escape  what remains in the string may be a malformed UTF-8 string. For this reason,
964  sequence is best avoided.  the \eC escape sequence is best avoided.
965  .P  .P
966  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
967  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 617  the lookbehind. Line 977  the lookbehind.
977  .rs  .rs
978  .sp  .sp
979  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
980  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.
981  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
982  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
983  escaped with a backslash.  a member of the class, it should be the first data character in the class
984    (after an initial circumflex, if present) or escaped with a backslash.
985  .P  .P
986  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
987  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
988  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
989  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
990  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
991  of the class, ensure it is not the first character, or escape it with a  of the class, ensure it is not the first character, or escape it with a
# Line 634  For example, the character class [aeiou] Line 995  For example, the character class [aeiou]
995  [^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
996  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
997  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
998  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
999  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
1000  string.  string.
1001  .P  .P
# Line 648  caseful version would. In UTF-8 mode, PC Line 1009  caseful version would. In UTF-8 mode, PC
1009  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
1010  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1011  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1012  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,
1013  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
1014  UTF-8 support.  with UTF-8 support.
1015  .P  .P
1016  Characters that might indicate line breaks (CR and LF) are never treated in any  Characters that might indicate line breaks are never treated in any special way
1017  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
1018  in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1019  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
1020  .P  .P
1021  The minus (hyphen) character can be used to specify a range of characters in a  The minus (hyphen) character can be used to specify a range of characters in a
1022  character class. For example, [d-m] matches any letter between d and m,  character class. For example, [d-m] matches any letter between d and m,
# Line 679  example [\ex{100}-\ex{2ff}]. Line 1040  example [\ex{100}-\ex{2ff}].
1040  If a range that includes letters is used when caseless matching is set, it  If a range that includes letters is used when caseless matching is set, it
1041  matches the letters in either case. For example, [W-c] is equivalent to  matches the letters in either case. For example, [W-c] is equivalent to
1042  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1043  tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
1044  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1045  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
1046  property support.  property support.
1047  .P  .P
1048  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and
1049  in a character class, and add the characters that they match to the class. For  \eW may also appear in a character class, and add the characters that they
1050  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A
1051  conveniently be used with the upper case character types to specify a more  circumflex can conveniently be used with the upper case character types to
1052  restricted set of characters than the matching lower case type. For example,  specify a more restricted set of characters than the matching lower case type.
1053  the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore.
1054  .P  .P
1055  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1056  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 709  this notation. For example, Line 1070  this notation. For example,
1070    [01[:alpha:]%]    [01[:alpha:]%]
1071  .sp  .sp
1072  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1073  are  are:
1074  .sp  .sp
1075    alnum    letters and digits    alnum    letters and digits
1076    alpha    letters    alpha    letters
# Line 720  are Line 1081  are
1081    graph    printing characters, excluding space    graph    printing characters, excluding space
1082    lower    lower case letters    lower    lower case letters
1083    print    printing characters, including space    print    printing characters, including space
1084    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1085    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1086    upper    upper case letters    upper    upper case letters
1087    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 741  matches "1", "2", or any non-digit. PCRE Line 1102  matches "1", "2", or any non-digit. PCRE
1102  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
1103  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1104  .P  .P
1105  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
1106  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1107    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1108    character properties are used. This is achieved by replacing the POSIX classes
1109    by other sequences, as follows:
1110    .sp
1111      [:alnum:]  becomes  \ep{Xan}
1112      [:alpha:]  becomes  \ep{L}
1113      [:blank:]  becomes  \eh
1114      [:digit:]  becomes  \ep{Nd}
1115      [:lower:]  becomes  \ep{Ll}
1116      [:space:]  becomes  \ep{Xps}
1117      [:upper:]  becomes  \ep{Lu}
1118      [:word:]   becomes  \ep{Xwd}
1119    .sp
1120    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1121    classes are unchanged, and match only characters with code points less than
1122    128.
1123  .  .
1124  .  .
1125  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 769  alternative in the subpattern. Line 1146  alternative in the subpattern.
1146  .rs  .rs
1147  .sp  .sp
1148  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1149  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
1150  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1151    The option letters are
1152  .sp  .sp
1153    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1154    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 784  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1162  PCRE_MULTILINE while unsetting PCRE_DOTA
1162  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
1163  unset.  unset.
1164  .P  .P
1165  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
1166  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
1167  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1168  the global options (and it will therefore show up in data extracted by the  .P
1169  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1170    subpattern parentheses), the change applies to the remainder of the pattern
1171    that follows. If the change is placed right at the start of a pattern, PCRE
1172    extracts it into the global options (and it will therefore show up in data
1173    extracted by the \fBpcre_fullinfo()\fP function).
1174  .P  .P
1175  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
1176  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
1177  .sp  .sp
1178    (a(?i)b)c    (a(?i)b)c
1179  .sp  .sp
# Line 807  branch is abandoned before the option se Line 1189  branch is abandoned before the option se
1189  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1190  behaviour otherwise.  behaviour otherwise.
1191  .P  .P
1192  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
1193  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
1194  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1195    the application has set or what has been defaulted. Details are given in the
1196    section entitled
1197    .\" HTML <a href="#newlineseq">
1198    .\" </a>
1199    "Newline sequences"
1200    .\"
1201    above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1202    to set UTF-8 and Unicode property modes; they are equivalent to setting the
1203    PCRE_UTF8 and the PCRE_UCP options, respectively.
1204  .  .
1205  .  .
1206  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 824  Turning part of a pattern into a subpatt Line 1215  Turning part of a pattern into a subpatt
1215    cat(aract|erpillar|)    cat(aract|erpillar|)
1216  .sp  .sp
1217  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1218  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1219  .sp  .sp
1220  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
1221  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
# Line 849  the string "the white queen" is matched Line 1240  the string "the white queen" is matched
1240    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1241  .sp  .sp
1242  the captured substrings are "white queen" and "queen", and are numbered 1 and  the captured substrings are "white queen" and "queen", and are numbered 1 and
1243  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
 of nesting of all subpatterns, both capturing and non-capturing, is 200.  
1244  .P  .P
1245  As a convenient shorthand, if any option settings are required at the start of  As a convenient shorthand, if any option settings are required at the start of
1246  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
# Line 865  is reached, an option setting in one bra Line 1255  is reached, an option setting in one bra
1255  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1256  .  .
1257  .  .
1258    .\" HTML <a name="dupsubpatternnumber"></a>
1259    .SH "DUPLICATE SUBPATTERN NUMBERS"
1260    .rs
1261    .sp
1262    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1263    the same numbers for its capturing parentheses. Such a subpattern starts with
1264    (?| and is itself a non-capturing subpattern. For example, consider this
1265    pattern:
1266    .sp
1267      (?|(Sat)ur|(Sun))day
1268    .sp
1269    Because the two alternatives are inside a (?| group, both sets of capturing
1270    parentheses are numbered one. Thus, when the pattern matches, you can look
1271    at captured substring number one, whichever alternative matched. This construct
1272    is useful when you want to capture part, but not all, of one of a number of
1273    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1274    number is reset at the start of each branch. The numbers of any capturing
1275    buffers that follow the subpattern start after the highest number used in any
1276    branch. The following example is taken from the Perl documentation.
1277    The numbers underneath show in which buffer the captured content will be
1278    stored.
1279    .sp
1280      # before  ---------------branch-reset----------- after
1281      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1282      # 1            2         2  3        2     3     4
1283    .sp
1284    A back reference to a numbered subpattern uses the most recent value that is
1285    set for that number by any subpattern. The following pattern matches "abcabc"
1286    or "defdef":
1287    .sp
1288      /(?|(abc)|(def))\e1/
1289    .sp
1290    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1291    refers to the first one in the pattern with the given number. The following
1292    pattern matches "abcabc" or "defabc":
1293    .sp
1294      /(?|(abc)|(def))(?1)/
1295    .sp
1296    If a
1297    .\" HTML <a href="#conditions">
1298    .\" </a>
1299    condition test
1300    .\"
1301    for a subpattern's having matched refers to a non-unique number, the test is
1302    true if any of the subpatterns of that number have matched.
1303    .P
1304    An alternative approach to using this "branch reset" feature is to use
1305    duplicate named subpatterns, as described in the next section.
1306    .
1307    .
1308  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1309  .rs  .rs
1310  .sp  .sp
1311  Identifying capturing parentheses by number is simple, but it can be very hard  Identifying capturing parentheses by number is simple, but it can be very hard
1312  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1313  if an expression is modified, the numbers may change. To help with this  if an expression is modified, the numbers may change. To help with this
1314  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1315  not provide. The Python syntax (?P<name>...) is used. References to capturing  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1316    introduced it at release 4.0, using the Python syntax. PCRE now supports both
1317    the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1318    have different names, but PCRE does not.
1319    .P
1320    In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1321    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1322  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1323  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1324  .\" </a>  .\" </a>
1325  backreferences,  back references,
1326  .\"  .\"
1327  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1328  .\" </a>  .\" </a>
# Line 890  conditions, Line 1336  conditions,
1336  can be made by name as well as by number.  can be made by name as well as by number.
1337  .P  .P
1338  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
1339  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
1340  API provides function calls for extracting the name-to-number translation table  if the names were not present. The PCRE API provides function calls for
1341  from a compiled pattern. There is also a convenience function for extracting a  extracting the name-to-number translation table from a compiled pattern. There
1342  captured substring by name.  is also a convenience function for extracting a captured substring by name.
1343  .P  .P
1344  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
1345  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
1346  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
1347  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
1348  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
1349  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
1350  .sp  name, and in both cases you want to extract the abbreviation. This pattern
1351    (?P<DN>Mon|Fri|Sun)(?:day)?|  (ignoring the line breaks) does the job:
1352    (?P<DN>Tue)(?:sday)?|  .sp
1353    (?P<DN>Wed)(?:nesday)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1354    (?P<DN>Thu)(?:rsday)?|    (?<DN>Tue)(?:sday)?|
1355    (?P<DN>Sat)(?:urday)?    (?<DN>Wed)(?:nesday)?|
1356      (?<DN>Thu)(?:rsday)?|
1357      (?<DN>Sat)(?:urday)?
1358  .sp  .sp
1359  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.
1360    (An alternative way of solving this problem is to use a "branch reset"
1361    subpattern, as described in the previous section.)
1362    .P
1363  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1364  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
1365  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1366  make a reference to a non-unique named subpattern from elsewhere in the  .P
1367  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
1368  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1369    used. In the absence of duplicate numbers (see the previous section) this is
1370    the one with the lowest number. If you use a named reference in a condition
1371    test (see the
1372    .\"
1373    .\" HTML <a href="#conditions">
1374    .\" </a>
1375    section about conditions
1376    .\"
1377    below), either to check whether a subpattern has matched, or to check for
1378    recursion, all subpatterns with the same name are tested. If the condition is
1379    true for any one of them, the overall condition is true. This is the same
1380    behaviour as testing by number. For further details of the interfaces for
1381    handling named subpatterns, see the
1382  .\" HREF  .\" HREF
1383  \fBpcreapi\fP  \fBpcreapi\fP
1384  .\"  .\"
1385  documentation.  documentation.
1386    .P
1387    \fBWarning:\fP You cannot use different names to distinguish between two
1388    subpatterns with the same number because PCRE uses only the numbers when
1389    matching. For this reason, an error is given at compile time if different names
1390    are given to subpatterns with the same number. However, you can give the same
1391    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1392  .  .
1393  .  .
1394  .SH REPETITION  .SH REPETITION
# Line 928  Repetition is specified by quantifiers, Line 1398  Repetition is specified by quantifiers,
1398  items:  items:
1399  .sp  .sp
1400    a literal data character    a literal data character
1401    the . metacharacter    the dot metacharacter
1402    the \eC escape sequence    the \eC escape sequence
1403    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1404      the \eR escape sequence
1405    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1406    a character class    a character class
1407    a back reference (see next section)    a back reference (see next section)
1408    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1409      a recursive or "subroutine" call to a subpattern
1410  .sp  .sp
1411  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1412  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 966  support is available, \eX{3} matches thr Line 1438  support is available, \eX{3} matches thr
1438  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).
1439  .P  .P
1440  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
1441  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1442    subpatterns that are referenced as
1443    .\" HTML <a href="#subpatternsassubroutines">
1444    .\" </a>
1445    subroutines
1446    .\"
1447    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1448    quantifier are omitted from the compiled pattern.
1449  .P  .P
1450  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1451  quantifiers have single-character abbreviations:  abbreviations:
1452  .sp  .sp
1453    *    is equivalent to {0,}    *    is equivalent to {0,}
1454    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1017  own right. Because it has two uses, it c Line 1496  own right. Because it has two uses, it c
1496  which matches one digit by preference, but can match two if that is the only  which matches one digit by preference, but can match two if that is the only
1497  way the rest of the pattern matches.  way the rest of the pattern matches.
1498  .P  .P
1499  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1500  the quantifiers are not greedy by default, but individual ones can be made  the quantifiers are not greedy by default, but individual ones can be made
1501  greedy by following them with a question mark. In other words, it inverts the  greedy by following them with a question mark. In other words, it inverts the
1502  default behaviour.  default behaviour.
# Line 1027  is greater than 1 or with a limited maxi Line 1506  is greater than 1 or with a limited maxi
1506  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1507  .P  .P
1508  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1509  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1510  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1511  character position in the subject string, so there is no point in retrying the  character position in the subject string, so there is no point in retrying the
1512  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
# Line 1038  worth setting PCRE_DOTALL in order to ob Line 1517  worth setting PCRE_DOTALL in order to ob
1517  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1518  .P  .P
1519  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1520  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1521  elsewhere in the pattern, a match at the start may fail, and a later one  elsewhere in the pattern, a match at the start may fail where a later one
1522  succeed. Consider, for example:  succeeds. Consider, for example:
1523  .sp  .sp
1524    (.*)abc\e1    (.*)abc\e1
1525  .sp  .sp
# Line 1066  matches "aba" the value of the second ca Line 1545  matches "aba" the value of the second ca
1545  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1546  .rs  .rs
1547  .sp  .sp
1548  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1549  normally causes the repeated item to be re-evaluated to see if a different  repetition, failure of what follows normally causes the repeated item to be
1550  number of repeats allows the rest of the pattern to match. Sometimes it is  re-evaluated to see if a different number of repeats allows the rest of the
1551  useful to prevent this, either to change the nature of the match, or to cause  pattern to match. Sometimes it is useful to prevent this, either to change the
1552  it fail earlier than it otherwise might, when the author of the pattern knows  nature of the match, or to cause it fail earlier than it otherwise might, when
1553  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1554  .P  .P
1555  Consider, for example, the pattern \ed+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1556  .sp  .sp
# Line 1083  item, and then with 4, and so on, before Line 1562  item, and then with 4, and so on, before
1562  (a term taken from Jeffrey Friedl's book) provides the means for specifying  (a term taken from Jeffrey Friedl's book) provides the means for specifying
1563  that once a subpattern has matched, it is not to be re-evaluated in this way.  that once a subpattern has matched, it is not to be re-evaluated in this way.
1564  .P  .P
1565  If we use atomic grouping for the previous example, the matcher would give up  If we use atomic grouping for the previous example, the matcher gives up
1566  immediately on failing to match "foo" the first time. The notation is a kind of  immediately on failing to match "foo" the first time. The notation is a kind of
1567  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1568  .sp  .sp
# Line 1113  previous example can be rewritten as Line 1592  previous example can be rewritten as
1592  .sp  .sp
1593    \ed++foo    \ed++foo
1594  .sp  .sp
1595    Note that a possessive quantifier can be used with an entire group, for
1596    example:
1597    .sp
1598      (abc|xyz){2,3}+
1599    .sp
1600  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1601  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
1602  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning of a possessive
1603  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1604  .P  difference; possessive quantifiers should be slightly faster.
1605  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  .P
1606  Friedl originated the idea (and the name) in the first edition of his book.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1607  Mike McCloskey liked it, so implemented it when he built Sun's Java package,  Jeffrey Friedl originated the idea (and the name) in the first edition of his
1608  and PCRE copied it from there.  book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1609    package, and PCRE copied it from there. It ultimately found its way into Perl
1610    at release 5.10.
1611    .P
1612    PCRE has an optimization that automatically "possessifies" certain simple
1613    pattern constructs. For example, the sequence A+B is treated as A++B because
1614    there is no point in backtracking into a sequence of A's when B must follow.
1615  .P  .P
1616  When a pattern contains an unlimited repeat inside a subpattern that can itself  When a pattern contains an unlimited repeat inside a subpattern that can itself
1617  be repeated an unlimited number of times, the use of an atomic group is the  be repeated an unlimited number of times, the use of an atomic group is the
# Line 1167  numbers less than 10. A "forward back re Line 1657  numbers less than 10. A "forward back re
1657  when a repetition is involved and the subpattern to the right has participated  when a repetition is involved and the subpattern to the right has participated
1658  in an earlier iteration.  in an earlier iteration.
1659  .P  .P
1660  It is not possible to have a numerical "forward back reference" to subpattern  It is not possible to have a numerical "forward back reference" to a subpattern
1661  whose number is 10 or more. However, a back reference to any subpattern is  whose number is 10 or more using this syntax because a sequence such as \e50 is
1662  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1663  "Non-printing characters"  "Non-printing characters"
1664  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1665  .\" </a>  .\" </a>
1666  above  above
1667  .\"  .\"
1668  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1669    no such problem when named parentheses are used. A back reference to any
1670    subpattern is possible using named parentheses (see below).
1671    .P
1672    Another way of avoiding the ambiguity inherent in the use of digits following a
1673    backslash is to use the \eg escape sequence, which is a feature introduced in
1674    Perl 5.10. This escape must be followed by an unsigned number or a negative
1675    number, optionally enclosed in braces. These examples are all identical:
1676    .sp
1677      (ring), \e1
1678      (ring), \eg1
1679      (ring), \eg{1}
1680    .sp
1681    An unsigned number specifies an absolute reference without the ambiguity that
1682    is present in the older syntax. It is also useful when literal digits follow
1683    the reference. A negative number is a relative reference. Consider this
1684    example:
1685    .sp
1686      (abc(def)ghi)\eg{-1}
1687    .sp
1688    The sequence \eg{-1} is a reference to the most recently started capturing
1689    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1690    would be equivalent to \e1. The use of relative references can be helpful in
1691    long patterns, and also in patterns that are created by joining together
1692    fragments that contain references within themselves.
1693  .P  .P
1694  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1695  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1197  back reference, the case of letters is r Line 1711  back reference, the case of letters is r
1711  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
1712  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1713  .P  .P
1714  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1715  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1716    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1717    back reference syntax, in which \eg can be used for both numeric and named
1718    references, is also supported. We could rewrite the above example in any of
1719    the following ways:
1720  .sp  .sp
1721      (?<p1>(?i)rah)\es+\ek<p1>
1722      (?'p1'(?i)rah)\es+\ek{p1}
1723    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1724      (?<p1>(?i)rah)\es+\eg{p1}
1725  .sp  .sp
1726  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
1727  after the reference.  after the reference.
1728  .P  .P
1729  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
1730  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1731  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1732  .sp  .sp
1733    (a|(bc))\e2    (a|(bc))\e2
1734  .sp  .sp
1735  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
1736  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
1737  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1738  with a digit character, some delimiter must be used to terminate the back  .P
1739  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1740  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1741    If the pattern continues with a digit character, some delimiter must be used to
1742    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1743    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1744  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1745  .\" </a>  .\" </a>
1746  "Comments"  "Comments"
1747  .\"  .\"
1748  below) can be used.  below) can be used.
1749  .P  .
1750    .SS "Recursive back references"
1751    .rs
1752    .sp
1753  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
1754  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.
1755  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1236  to the previous iteration. In order for Line 1763  to the previous iteration. In order for
1763  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
1764  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
1765  minimum of zero.  minimum of zero.
1766    .P
1767    Back references of this type cause the group that they reference to be treated
1768    as an
1769    .\" HTML <a href="#atomicgroup">
1770    .\" </a>
1771    atomic group.
1772    .\"
1773    Once the whole group has been matched, a subsequent matching failure cannot
1774    cause backtracking into the middle of the group.
1775  .  .
1776  .  .
1777  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1289  lookbehind assertion is needed to achiev Line 1825  lookbehind assertion is needed to achiev
1825  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
1826  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
1827  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.
1828    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1829    synonym for (?!).
1830  .  .
1831  .  .
1832  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1313  is permitted, but Line 1851  is permitted, but
1851  .sp  .sp
1852  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1853  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
1854  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (5.8 and 5.10), which requires all branches to
1855  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1856  .sp  .sp
1857    (?<=ab(c|de))    (?<=ab(c|de))
1858  .sp  .sp
1859  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
1860  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
1861    branches:
1862  .sp  .sp
1863    (?<=abc|abde)    (?<=abc|abde)
1864  .sp  .sp
1865    In some cases, the Perl 5.10 escape sequence \eK
1866    .\" HTML <a href="#resetmatchstart">
1867    .\" </a>
1868    (see above)
1869    .\"
1870    can be used instead of a lookbehind assertion to get round the fixed-length
1871    restriction.
1872    .P
1873  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1874  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed length and then try to
1875  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1876  match is deemed to fail.  assertion fails.
1877  .P  .P
1878  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1879  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1880  the length of the lookbehind. The \eX escape, which can match different numbers  the length of the lookbehind. The \eX and \eR escapes, which can match
1881  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1882    .P
1883    .\" HTML <a href="#subpatternsassubroutines">
1884    .\" </a>
1885    "Subroutine"
1886    .\"
1887    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1888    as the subpattern matches a fixed-length string.
1889    .\" HTML <a href="#recursion">
1890    .\" </a>
1891    Recursion,
1892    .\"
1893    however, is not supported.
1894  .P  .P
1895  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1896  efficient matching at the end of the subject string. Consider a simple pattern  specify efficient matching of fixed-length strings at the end of subject
1897  such as  strings. Consider a simple pattern such as
1898  .sp  .sp
1899    abcd$    abcd$
1900  .sp  .sp
# Line 1351  then all but the last two characters, an Line 1910  then all but the last two characters, an
1910  covers the entire string, from right to left, so we are no better off. However,  covers the entire string, from right to left, so we are no better off. However,
1911  if the pattern is written as  if the pattern is written as
1912  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1913    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1914  .sp  .sp
1915  there can be no backtracking for the .* item; it can match only the entire  there can be no backtracking for the .*+ item; it can match only the entire
1916  string. The subsequent lookbehind assertion does a single test on the last four  string. The subsequent lookbehind assertion does a single test on the last four
1917  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1918  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1403  characters that are not "999". Line 1958  characters that are not "999".
1958  .sp  .sp
1959  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1960  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1961  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1962  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1963  .sp  .sp
1964    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1965    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
# Line 1413  If the condition is satisfied, the yes-p Line 1968  If the condition is satisfied, the yes-p
1968  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
1969  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1970  .P  .P
1971  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1972  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
1973  underscores, the condition is satisfied if the capturing subpattern of that  .
1974  number or name has previously matched. There is a possible ambiguity here,  .SS "Checking for a used subpattern by number"
1975  because subpattern names may consist entirely of digits. PCRE looks first for a  .rs
1976  named subpattern; if it cannot find one and the text consists entirely of  .sp
1977  digits, it looks for a subpattern of that number, which must be greater than  If the text between the parentheses consists of a sequence of digits, the
1978  zero. Using subpattern names that consist entirely of digits is not  condition is true if a capturing subpattern of that number has previously
1979  recommended.  matched. If there is more than one capturing subpattern with the same number
1980    (see the earlier
1981    .\"
1982    .\" HTML <a href="#recursion">
1983    .\" </a>
1984    section about duplicate subpattern numbers),
1985    .\"
1986    the condition is true if any of them have been set. An alternative notation is
1987    to precede the digits with a plus or minus sign. In this case, the subpattern
1988    number is relative rather than absolute. The most recently opened parentheses
1989    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1990    looping constructs it can also make sense to refer to subsequent groups with
1991    constructs such as (?(+2).
1992  .P  .P
1993  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1994  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1437  or not. If they did, that is, if subject Line 2004  or not. If they did, that is, if subject
2004  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
2005  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2006  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2007  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
2008  named subpattern gives this:  .P
2009    If you were embedding this pattern in a larger one, you could use a relative
2010    reference:
2011  .sp  .sp
2012    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
2013    .sp
2014    This makes the fragment independent of the parentheses in the larger pattern.
2015    .
2016    .SS "Checking for a used subpattern by name"
2017    .rs
2018    .sp
2019    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2020    subpattern by name. For compatibility with earlier versions of PCRE, which had
2021    this facility before Perl, the syntax (?(name)...) is also recognized. However,
2022    there is a possible ambiguity with this syntax, because subpattern names may
2023    consist entirely of digits. PCRE looks first for a named subpattern; if it
2024    cannot find one and the name consists entirely of digits, PCRE looks for a
2025    subpattern of that number, which must be greater than zero. Using subpattern
2026    names that consist entirely of digits is not recommended.
2027    .P
2028    Rewriting the above example to use a named subpattern gives this:
2029    .sp
2030      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2031    .sp
2032    If the name used in a condition of this kind is a duplicate, the test is
2033    applied to all subpatterns of the same name, and is true if any one of them has
2034    matched.
2035    .
2036    .SS "Checking for pattern recursion"
2037    .rs
2038  .sp  .sp
2039  If the condition is the string (R), and there is no subpattern with the name R,  If the condition is the string (R), and there is no subpattern with the name R,
2040  the condition is satisfied if a recursive call to the pattern or subpattern has  the condition is true if a recursive call to the whole pattern or any
2041  been made. At "top level", the condition is false. This is a PCRE extension.  subpattern has been made. If digits or a name preceded by ampersand follow the
2042  Recursive patterns are described in the next section.  letter R, for example:
2043    .sp
2044      (?(R3)...) or (?(R&name)...)
2045    .sp
2046    the condition is true if the most recent recursion is into a subpattern whose
2047    number or name is given. This condition does not check the entire recursion
2048    stack. If the name used in a condition of this kind is a duplicate, the test is
2049    applied to all subpatterns of the same name, and is true if any one of them is
2050    the most recent recursion.
2051  .P  .P
2052  If the condition is not a sequence of digits or (R), it must be an assertion.  At "top level", all these recursion test conditions are false.
2053    .\" HTML <a href="#recursion">
2054    .\" </a>
2055    The syntax for recursive patterns
2056    .\"
2057    is described below.
2058    .
2059    .SS "Defining subpatterns for use by reference only"
2060    .rs
2061    .sp
2062    If the condition is the string (DEFINE), and there is no subpattern with the
2063    name DEFINE, the condition is always false. In this case, there may be only one
2064    alternative in the subpattern. It is always skipped if control reaches this
2065    point in the pattern; the idea of DEFINE is that it can be used to define
2066    "subroutines" that can be referenced from elsewhere. (The use of
2067    .\" HTML <a href="#subpatternsassubroutines">
2068    .\" </a>
2069    "subroutines"
2070    .\"
2071    is described below.) For example, a pattern to match an IPv4 address could be
2072    written like this (ignore whitespace and line breaks):
2073    .sp
2074      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2075      \eb (?&byte) (\e.(?&byte)){3} \eb
2076    .sp
2077    The first part of the pattern is a DEFINE group inside which a another group
2078    named "byte" is defined. This matches an individual component of an IPv4
2079    address (a number less than 256). When matching takes place, this part of the
2080    pattern is skipped because DEFINE acts like a false condition. The rest of the
2081    pattern uses references to the named group to match the four dot-separated
2082    components of an IPv4 address, insisting on a word boundary at each end.
2083    .
2084    .SS "Assertion conditions"
2085    .rs
2086    .sp
2087    If the condition is not in any of the above formats, it must be an assertion.
2088  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
2089  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
2090  alternatives on the second line:  alternatives on the second line:
# Line 1473  that make up a comment play no part in t Line 2110  that make up a comment play no part in t
2110  .P  .P
2111  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
2112  character class introduces a comment that continues to immediately after the  character class introduces a comment that continues to immediately after the
2113  next newline in the pattern.  next newline character or character sequence in the pattern. Which characters
2114    are interpreted as newlines is controlled by the options passed to
2115    \fBpcre_compile()\fP or by a special sequence at the start of the pattern, as
2116    described in the section entitled
2117    .\" HTML <a href="#recursion">
2118    .\" </a>
2119    "Newline conventions"
2120    .\"
2121    above. Note that end of a comment is a literal newline sequence in the pattern;
2122    escape sequences that happen to represent a newline do not terminate a comment.
2123    For example, consider this pattern when PCRE_EXTENDED is set, and the default
2124    newline convention is in force:
2125    .sp
2126      abc #comment \en still comment
2127    .sp
2128    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2129    a newline in the pattern. The sequence \en is still literal at this stage, so
2130    it does not terminate the comment. Only an actual character with the code value
2131    0x0a does so.
2132  .  .
2133  .  .
2134  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1483  next newline in the pattern. Line 2138  next newline in the pattern.
2138  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2139  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2140  be done is to use a pattern that matches up to some fixed depth of nesting. It  be done is to use a pattern that matches up to some fixed depth of nesting. It
2141  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
2142  that allows regular expressions to recurse (amongst other things). It does this  .P
2143  by interpolating Perl code in the expression at run time, and the code can  For some time, Perl has provided a facility that allows regular expressions to
2144  refer to the expression itself. A Perl pattern to solve the parentheses problem  recurse (amongst other things). It does this by interpolating Perl code in the
2145  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2146    pattern using code interpolation to solve the parentheses problem can be
2147    created like this:
2148  .sp  .sp
2149    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2150  .sp  .sp
2151  The (?p{...}) item interpolates Perl code at run time, and in this case refers  The (?p{...}) item interpolates Perl code at run time, and in this case refers
2152  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2153  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2154  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2155  .P  supports special syntax for recursion of the entire pattern, and also for
2156  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2157  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was subsequently introduced into Perl at release 5.10.
2158  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2159  "subroutine" call, which is described in the next section.) The special item  A special item that consists of (? followed by a number greater than zero and a
2160  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
2161  .P  provided that it occurs inside that subpattern. (If not, it is a
2162  A recursive subpattern call is always treated as an atomic group. That is, once  .\" HTML <a href="#subpatternsassubroutines">
2163  it has matched some of the subject string, it is never re-entered, even if  .\" </a>
2164  it contains untried alternatives and there is a subsequent matching failure.  "subroutine"
2165    .\"
2166    call, which is described in the next section.) The special item (?R) or (?0) is
2167    a recursive call of the entire regular expression.
2168  .P  .P
2169  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2170  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2171  .sp  .sp
2172    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2173  .sp  .sp
2174  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2175  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
2176  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2177  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2178    to avoid backtracking into sequences of non-parentheses.
2179  .P  .P
2180  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
2181  pattern, so instead you could use this:  pattern, so instead you could use this:
2182  .sp  .sp
2183    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2184  .sp  .sp
2185  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
2186  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2187  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2188  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2189  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references (a Perl 5.10 feature).
2190  named parentheses). We could rewrite the above example as follows:  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
2191  .sp  most recently opened parentheses preceding the recursion. In other words, a
2192    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
2193  .sp  it is encountered.
2194  This particular example pattern contains nested unlimited repeats, and so the  .P
2195  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
2196  when applying the pattern to strings that do not match. For example, when this  references such as (?+2). However, these cannot be recursive because the
2197  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
2198    .\" HTML <a href="#subpatternsassubroutines">
2199    .\" </a>
2200    "subroutine"
2201    .\"
2202    calls, as described in the next section.
2203    .P
2204    An alternative approach is to use named parentheses instead. The Perl syntax
2205    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2206    could rewrite the above example as follows:
2207    .sp
2208      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2209    .sp
2210    If there is more than one subpattern with the same name, the earliest one is
2211    used.
2212    .P
2213    This particular example pattern that we have been looking at contains nested
2214    unlimited repeats, and so the use of a possessive quantifier for matching
2215    strings of non-parentheses is important when applying the pattern to strings
2216    that do not match. For example, when this pattern is applied to
2217  .sp  .sp
2218    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2219  .sp  .sp
2220  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,
2221  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
2222  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
2223  before failure can be reported.  before failure can be reported.
2224  .P  .P
2225  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
2226  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
2227  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 the next section and the  
2228  .\" HREF  .\" HREF
2229  \fBpcrecallout\fP  \fBpcrecallout\fP
2230  .\"  .\"
# Line 1553  documentation). If the pattern above is Line 2232  documentation). If the pattern above is
2232  .sp  .sp
2233    (ab(cd)ef)    (ab(cd)ef)
2234  .sp  .sp
2235  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
2236  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
2237  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2238    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2239       ^                        ^  .P
2240       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2241  .sp  obtain extra memory to store data during a recursion, which it does by using
2242  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
2243  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.  
2244  .P  .P
2245  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.
2246  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1578  different alternatives for the recursive Line 2254  different alternatives for the recursive
2254  is the actual recursive call.  is the actual recursive call.
2255  .  .
2256  .  .
2257    .\" HTML <a name="recursiondifference"></a>
2258    .SS "Recursion difference from Perl"
2259    .rs
2260    .sp
2261    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2262    treated as an atomic group. That is, once it has matched some of the subject
2263    string, it is never re-entered, even if it contains untried alternatives and
2264    there is a subsequent matching failure. This can be illustrated by the
2265    following pattern, which purports to match a palindromic string that contains
2266    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2267    .sp
2268      ^(.|(.)(?1)\e2)$
2269    .sp
2270    The idea is that it either matches a single character, or two identical
2271    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2272    it does not if the pattern is longer than three characters. Consider the
2273    subject string "abcba":
2274    .P
2275    At the top level, the first character is matched, but as it is not at the end
2276    of the string, the first alternative fails; the second alternative is taken
2277    and the recursion kicks in. The recursive call to subpattern 1 successfully
2278    matches the next character ("b"). (Note that the beginning and end of line
2279    tests are not part of the recursion).
2280    .P
2281    Back at the top level, the next character ("c") is compared with what
2282    subpattern 2 matched, which was "a". This fails. Because the recursion is
2283    treated as an atomic group, there are now no backtracking points, and so the
2284    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2285    try the second alternative.) However, if the pattern is written with the
2286    alternatives in the other order, things are different:
2287    .sp
2288      ^((.)(?1)\e2|.)$
2289    .sp
2290    This time, the recursing alternative is tried first, and continues to recurse
2291    until it runs out of characters, at which point the recursion fails. But this
2292    time we do have another alternative to try at the higher level. That is the big
2293    difference: in the previous case the remaining alternative is at a deeper
2294    recursion level, which PCRE cannot use.
2295    .P
2296    To change the pattern so that matches all palindromic strings, not just those
2297    with an odd number of characters, it is tempting to change the pattern to this:
2298    .sp
2299      ^((.)(?1)\e2|.?)$
2300    .sp
2301    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2302    deeper recursion has matched a single character, it cannot be entered again in
2303    order to match an empty string. The solution is to separate the two cases, and
2304    write out the odd and even cases as alternatives at the higher level:
2305    .sp
2306      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2307    .sp
2308    If you want to match typical palindromic phrases, the pattern has to ignore all
2309    non-word characters, which can be done like this:
2310    .sp
2311      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2312    .sp
2313    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2314    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2315    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2316    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2317    more) to match typical phrases, and Perl takes so long that you think it has
2318    gone into a loop.
2319    .P
2320    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2321    string does not start with a palindrome that is shorter than the entire string.
2322    For example, although "abcba" is correctly matched, if the subject is "ababa",
2323    PCRE finds the palindrome "aba" at the start, then fails at top level because
2324    the end of the string does not follow. Once again, it cannot jump back into the
2325    recursion to try other alternatives, so the entire match fails.
2326    .
2327    .
2328  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2329  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2330  .rs  .rs
2331  .sp  .sp
2332  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern reference (either by number or by
2333  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
2334  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
2335  pattern  before or after the reference. A numbered reference can be absolute or
2336    relative, as in these examples:
2337    .sp
2338      (...(absolute)...)...(?2)...
2339      (...(relative)...)...(?-1)...
2340      (...(?+1)...(relative)...
2341    .sp
2342    An earlier example pointed out that the pattern
2343  .sp  .sp
2344    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2345  .sp  .sp
# Line 1595  matches "sense and sensibility" and "res Line 2349  matches "sense and sensibility" and "res
2349    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2350  .sp  .sp
2351  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
2352  strings. Such references, if given numerically, must follow the subpattern to  strings. Another example is given in the discussion of DEFINE above.
 which they refer. However, named references can refer to later subpatterns.  
2353  .P  .P
2354  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2355  group. That is, once it has matched some of the subject string, it is never  group. That is, once it has matched some of the subject string, it is never
2356  re-entered, even if it contains untried alternatives and there is a subsequent  re-entered, even if it contains untried alternatives and there is a subsequent
2357  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2358    call revert to their previous values afterwards.
2359    .P
2360    When a subpattern is used as a subroutine, processing options such as
2361    case-independence are fixed when the subpattern is defined. They cannot be
2362    changed for different calls. For example, consider this pattern:
2363    .sp
2364      (abc)(?i:(?-1))
2365    .sp
2366    It matches "abcabc". It does not match "abcABC" because the change of
2367    processing option does not affect the called subpattern.
2368    .
2369    .
2370    .\" HTML <a name="onigurumasubroutines"></a>
2371    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2372    .rs
2373    .sp
2374    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2375    a number enclosed either in angle brackets or single quotes, is an alternative
2376    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2377    are two of the examples used above, rewritten using this syntax:
2378    .sp
2379      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2380      (sens|respons)e and \eg'1'ibility
2381    .sp
2382    PCRE supports an extension to Oniguruma: if a number is preceded by a
2383    plus or a minus sign it is taken as a relative reference. For example:
2384    .sp
2385      (abc)(?i:\eg<-1>)
2386    .sp
2387    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2388    synonymous. The former is a back reference; the latter is a subroutine call.
2389  .  .
2390  .  .
2391  .SH CALLOUTS  .SH CALLOUTS
# Line 1622  function is to be called. If you want to Line 2406  function is to be called. If you want to
2406  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
2407  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2408  .sp  .sp
2409    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2410  .sp  .sp
2411  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2412  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
# Line 1638  description of the interface to the call Line 2422  description of the interface to the call
2422  \fBpcrecallout\fP  \fBpcrecallout\fP
2423  .\"  .\"
2424  documentation.  documentation.
2425    .
2426    .
2427    .\" HTML <a name="backtrackcontrol"></a>
2428    .SH "BACKTRACKING CONTROL"
2429    .rs
2430    .sp
2431    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2432    are described in the Perl documentation as "experimental and subject to change
2433    or removal in a future version of Perl". It goes on to say: "Their usage in
2434    production code should be noted to avoid problems during upgrades." The same
2435    remarks apply to the PCRE features described in this section.
2436    .P
2437    Since these verbs are specifically related to backtracking, most of them can be
2438    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2439    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2440    failing negative assertion, they cause an error if encountered by
2441    \fBpcre_dfa_exec()\fP.
2442    .P
2443    If any of these verbs are used in an assertion or subroutine subpattern
2444    (including recursive subpatterns), their effect is confined to that subpattern;
2445    it does not extend to the surrounding pattern. Note that such subpatterns are
2446    processed as anchored at the point where they are tested.
2447    .P
2448    The new verbs make use of what was previously invalid syntax: an opening
2449    parenthesis followed by an asterisk. They are generally of the form
2450    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2451    depending on whether or not an argument is present. An name is a sequence of
2452    letters, digits, and underscores. If the name is empty, that is, if the closing
2453    parenthesis immediately follows the colon, the effect is as if the colon were
2454    not there. Any number of these verbs may occur in a pattern.
2455  .P  .P
2456  .in 0  PCRE contains some optimizations that are used to speed up matching by running
2457  Last updated: 06 June 2006  some checks at the start of each match attempt. For example, it may know the
2458  .br  minimum length of matching subject, or that a particular character must be
2459  Copyright (c) 1997-2006 University of Cambridge.  present. When one of these optimizations suppresses the running of a match, any
2460    included backtracking verbs will not, of course, be processed. You can suppress
2461    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2462    when calling \fBpcre_exec()\fP.
2463    .
2464    .
2465    .SS "Verbs that act immediately"
2466    .rs
2467    .sp
2468    The following verbs act as soon as they are encountered. They may not be
2469    followed by a name.
2470    .sp
2471       (*ACCEPT)
2472    .sp
2473    This verb causes the match to end successfully, skipping the remainder of the
2474    pattern. When inside a recursion, only the innermost pattern is ended
2475    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2476    captured. (This feature was added to PCRE at release 8.00.) For example:
2477    .sp
2478      A((?:A|B(*ACCEPT)|C)D)
2479    .sp
2480    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2481    the outer parentheses.
2482    .sp
2483      (*FAIL) or (*F)
2484    .sp
2485    This verb causes the match to fail, forcing backtracking to occur. It is
2486    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2487    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2488    Perl features that are not present in PCRE. The nearest equivalent is the
2489    callout feature, as for example in this pattern:
2490    .sp
2491      a+(?C)(*FAIL)
2492    .sp
2493    A match with the string "aaaa" always fails, but the callout is taken before
2494    each backtrack happens (in this example, 10 times).
2495    .
2496    .
2497    .SS "Recording which path was taken"
2498    .rs
2499    .sp
2500    There is one verb whose main purpose is to track how a match was arrived at,
2501    though it also has a secondary use in conjunction with advancing the match
2502    starting point (see (*SKIP) below).
2503    .sp
2504      (*MARK:NAME) or (*:NAME)
2505    .sp
2506    A name is always required with this verb. There may be as many instances of
2507    (*MARK) as you like in a pattern, and their names do not have to be unique.
2508    .P
2509    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2510    to the caller via the \fIpcre_extra\fP data structure, as described in the
2511    .\" HTML <a href="pcreapi.html#extradata">
2512    .\" </a>
2513    section on \fIpcre_extra\fP
2514    .\"
2515    in the
2516    .\" HREF
2517    \fBpcreapi\fP
2518    .\"
2519    documentation. No data is returned for a partial match. Here is an example of
2520    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2521    outputting of (*MARK) data:
2522    .sp
2523      /X(*MARK:A)Y|X(*MARK:B)Z/K
2524      XY
2525       0: XY
2526      MK: A
2527      XZ
2528       0: XZ
2529      MK: B
2530    .sp
2531    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2532    indicates which of the two alternatives matched. This is a more efficient way
2533    of obtaining this information than putting each alternative in its own
2534    capturing parentheses.
2535    .P
2536    A name may also be returned after a failed match if the final path through the
2537    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2538    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2539    starting point for matching is advanced, the final check is often with an empty
2540    string, causing a failure before (*MARK) is reached. For example:
2541    .sp
2542      /X(*MARK:A)Y|X(*MARK:B)Z/K
2543      XP
2544      No match
2545    .sp
2546    There are three potential starting points for this match (starting with X,
2547    starting with P, and with an empty string). If the pattern is anchored, the
2548    result is different:
2549    .sp
2550      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2551      XP
2552      No match, mark = B
2553    .sp
2554    PCRE's start-of-match optimizations can also interfere with this. For example,
2555    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2556    subject length for a match, a shorter subject will not be scanned at all.
2557    .P
2558    Note that similar anomalies (though different in detail) exist in Perl, no
2559    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2560    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2561    .
2562    .
2563    .SS "Verbs that act after backtracking"
2564    .rs
2565    .sp
2566    The following verbs do nothing when they are encountered. Matching continues
2567    with what follows, but if there is no subsequent match, causing a backtrack to
2568    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2569    the verb. However, when one of these verbs appears inside an atomic group, its
2570    effect is confined to that group, because once the group has been matched,
2571    there is never any backtracking into it. In this situation, backtracking can
2572    "jump back" to the left of the entire atomic group. (Remember also, as stated
2573    above, that this localization also applies in subroutine calls and assertions.)
2574    .P
2575    These verbs differ in exactly what kind of failure occurs when backtracking
2576    reaches them.
2577    .sp
2578      (*COMMIT)
2579    .sp
2580    This verb, which may not be followed by a name, causes the whole match to fail
2581    outright if the rest of the pattern does not match. Even if the pattern is
2582    unanchored, no further attempts to find a match by advancing the starting point
2583    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2584    finding a match at the current starting point, or not at all. For example:
2585    .sp
2586      a+(*COMMIT)b
2587    .sp
2588    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2589    dynamic anchor, or "I've started, so I must finish." The name of the most
2590    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2591    match failure.
2592    .P
2593    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2594    unless PCRE's start-of-match optimizations are turned off, as shown in this
2595    \fBpcretest\fP example:
2596    .sp
2597      /(*COMMIT)abc/
2598      xyzabc
2599       0: abc
2600      xyzabc\eY
2601      No match
2602    .sp
2603    PCRE knows that any match must start with "a", so the optimization skips along
2604    the subject to "a" before running the first match attempt, which succeeds. When
2605    the optimization is disabled by the \eY escape in the second subject, the match
2606    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2607    starting points.
2608    .sp
2609      (*PRUNE) or (*PRUNE:NAME)
2610    .sp
2611    This verb causes the match to fail at the current starting position in the
2612    subject if the rest of the pattern does not match. If the pattern is
2613    unanchored, the normal "bumpalong" advance to the next starting character then
2614    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2615    reached, or when matching to the right of (*PRUNE), but if there is no match to
2616    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2617    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2618    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2619    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2620    match fails completely; the name is passed back if this is the final attempt.
2621    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2622    pattern (*PRUNE) has the same effect as (*COMMIT).
2623    .sp
2624      (*SKIP)
2625    .sp
2626    This verb, when given without a name, is like (*PRUNE), except that if the
2627    pattern is unanchored, the "bumpalong" advance is not to the next character,
2628    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2629    signifies that whatever text was matched leading up to it cannot be part of a
2630    successful match. Consider:
2631    .sp
2632      a+(*SKIP)b
2633    .sp
2634    If the subject is "aaaac...", after the first match attempt fails (starting at
2635    the first character in the string), the starting point skips on to start the
2636    next attempt at "c". Note that a possessive quantifer does not have the same
2637    effect as this example; although it would suppress backtracking during the
2638    first match attempt, the second attempt would start at the second character
2639    instead of skipping on to "c".
2640    .sp
2641      (*SKIP:NAME)
2642    .sp
2643    When (*SKIP) has an associated name, its behaviour is modified. If the
2644    following pattern fails to match, the previous path through the pattern is
2645    searched for the most recent (*MARK) that has the same name. If one is found,
2646    the "bumpalong" advance is to the subject position that corresponds to that
2647    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2648    matching name is found, normal "bumpalong" of one character happens (the
2649    (*SKIP) is ignored).
2650    .sp
2651      (*THEN) or (*THEN:NAME)
2652    .sp
2653    This verb causes a skip to the next alternation in the innermost enclosing
2654    group if the rest of the pattern does not match. That is, it cancels pending
2655    backtracking, but only within the current alternation. Its name comes from the
2656    observation that it can be used for a pattern-based if-then-else block:
2657    .sp
2658      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2659    .sp
2660    If the COND1 pattern matches, FOO is tried (and possibly further items after
2661    the end of the group if FOO succeeds); on failure the matcher skips to the
2662    second alternative and tries COND2, without backtracking into COND1. The
2663    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2664    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2665    like (*PRUNE).
2666    .
2667    .P
2668    The above verbs provide four different "strengths" of control when subsequent
2669    matching fails. (*THEN) is the weakest, carrying on the match at the next
2670    alternation. (*PRUNE) comes next, failing the match at the current starting
2671    position, but allowing an advance to the next character (for an unanchored
2672    pattern). (*SKIP) is similar, except that the advance may be more than one
2673    character. (*COMMIT) is the strongest, causing the entire match to fail.
2674    .P
2675    If more than one is present in a pattern, the "stongest" one wins. For example,
2676    consider this pattern, where A, B, etc. are complex pattern fragments:
2677    .sp
2678      (A(*COMMIT)B(*THEN)C|D)
2679    .sp
2680    Once A has matched, PCRE is committed to this match, at the current starting
2681    position. If subsequently B matches, but C does not, the normal (*THEN) action
2682    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2683    overrides.
2684    .
2685    .
2686    .SH "SEE ALSO"
2687    .rs
2688    .sp
2689    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2690    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2691    .
2692    .
2693    .SH AUTHOR
2694    .rs
2695    .sp
2696    .nf
2697    Philip Hazel
2698    University Computing Service
2699    Cambridge CB2 3QH, England.
2700    .fi
2701    .
2702    .
2703    .SH REVISION
2704    .rs
2705    .sp
2706    .nf
2707    Last updated: 26 October 2010
2708    Copyright (c) 1997-2010 University of Cambridge.
2709    .fi

Legend:
Removed from v.91  
changed lines
  Added in v.556

  ViewVC Help
Powered by ViewVC 1.1.5