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revision 654 by ph10, Tue Aug 2 11:00:40 2011 UTC revision 903 by ph10, Sat Jan 21 16:37:17 2012 UTC
# Line 21  published by O'Reilly, covers regular ex Line 21  published by O'Reilly, covers regular ex
21  description of PCRE's regular expressions is intended as reference material.  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,  there is now also support for UTF-8 strings in the original library, and a
25  PCRE must be built to include UTF-8 support, and you must call  second library that supports 16-bit and UTF-16 character strings. To use these
26  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There  features, PCRE must be built to include appropriate support. When using UTF
27  is also a special sequence that can be given at the start of a pattern:  strings you must either call the compiling function with the PCRE_UTF8 or
28    PCRE_UTF16 option, or the pattern must start with one of these special
29    sequences:
30  .sp  .sp
31    (*UTF8)    (*UTF8)
32      (*UTF16)
33  .sp  .sp
34  Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8  Starting a pattern with such a sequence is equivalent to setting the relevant
35  option. This feature is not Perl-compatible. How setting UTF-8 mode affects  option. This feature is not Perl-compatible. How setting a UTF mode affects
36  pattern matching is mentioned in several places below. There is also a summary  pattern matching is mentioned in several places below. There is also a summary
37  of UTF-8 features in the  of features in the
 .\" HTML <a href="pcre.html#utf8support">  
 .\" </a>  
 section on UTF-8 support  
 .\"  
 in the main  
38  .\" HREF  .\" HREF
39  \fBpcre\fP  \fBpcreunicode\fP
40  .\"  .\"
41  page.  page.
42  .P  .P
43  Another special sequence that may appear at the start of a pattern or in  Another special sequence that may appear at the start of a pattern or in
44  combination with (*UTF8) is:  combination with (*UTF8) or (*UTF16) is:
45  .sp  .sp
46    (*UCP)    (*UCP)
47  .sp  .sp
# Line 58  also some more of these special sequence Line 56  also some more of these special sequence
56  of newlines; they are described below.  of newlines; they are described below.
57  .P  .P
58  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
59  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
60  From release 6.0, PCRE offers a second matching function,  \fBpcre16_exec()\fP (16-bit), is used. PCRE also has alternative matching
61  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  functions, \fBpcre_dfa_exec()\fP and \fBpcre16_dfa_exec()\fP, which match using
62  Perl-compatible. Some of the features discussed below are not available when  a different algorithm that is not Perl-compatible. Some of the features
63  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the  discussed below are not available when DFA matching is used. The advantages and
64  alternative function, and how it differs from the normal function, are  disadvantages of the alternative functions, and how they differ from the normal
65  discussed in the  functions, are discussed in the
66  .\" HREF  .\" HREF
67  \fBpcrematching\fP  \fBpcrematching\fP
68  .\"  .\"
# Line 99  string with one of the following five se Line 97  string with one of the following five se
97    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
98    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
99  .sp  .sp
100  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to the compiling function. For
101  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default  example, on a Unix system where LF is the default newline sequence, the pattern
 newline sequence, the pattern  
102  .sp  .sp
103    (*CR)a.b    (*CR)a.b
104  .sp  .sp
# Line 135  corresponding characters in the subject. Line 132  corresponding characters in the subject.
132  .sp  .sp
133  matches a portion of a subject string that is identical to itself. When  matches a portion of a subject string that is identical to itself. When
134  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
135  independently of case. In UTF-8 mode, PCRE always understands the concept of  independently of case. In a UTF mode, PCRE always understands the concept of
136  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
137  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
138  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
139  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching for characters 128 and above, you must
140  ensure that PCRE is compiled with Unicode property support as well as with  ensure that PCRE is compiled with Unicode property support as well as with
141  UTF-8 support.  UTF support.
142  .P  .P
143  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
144  and repetitions in the pattern. These are encoded in the pattern by the use of  and repetitions in the pattern. These are encoded in the pattern by the use of
# Line 197  otherwise be interpreted as a metacharac Line 194  otherwise be interpreted as a metacharac
194  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
195  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
196  .P  .P
197  In UTF-8 mode, only ASCII numbers and letters have any special meaning after a  In a UTF mode, only ASCII numbers and letters have any special meaning after a
198  backslash. All other characters (in particular, those whose codepoints are  backslash. All other characters (in particular, those whose codepoints are
199  greater than 127) are treated as literals.  greater than 127) are treated as literals.
200  .P  .P
# Line 246  one of the following escape sequences th Line 243  one of the following escape sequences th
243    \et        tab (hex 09)    \et        tab (hex 09)
244    \eddd      character with octal code ddd, or back reference    \eddd      character with octal code ddd, or back reference
245    \exhh      character with hex code hh    \exhh      character with hex code hh
246    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
247      \euhhhh    character with hex code hhhh (JavaScript mode only)
248  .sp  .sp
249  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
250  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
251  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
252  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
253  than 127, a compile-time error occurs. This locks out non-ASCII characters in  than 127, a compile-time error occurs. This locks out non-ASCII characters in
254  both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte  all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. A
255  values are valid. A lower case letter is converted to upper case, and then the  lower case letter is converted to upper case, and then the 0xc0 bits are
256  0xc0 bits are flipped.)  flipped.)
257  .P  .P
258  After \ex, from zero to two hexadecimal digits are read (letters can be in  By default, after \ex, from zero to two hexadecimal digits are read (letters
259  upper or lower case). Any number of hexadecimal digits may appear between \ex{  can be in upper or lower case). Any number of hexadecimal digits may appear
260  and }, but the value of the character code must be less than 256 in non-UTF-8  between \ex{ and }, but the character code is constrained as follows:
261  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in  .sp
262  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code    8-bit non-UTF mode    less than 0x100
263  point, which is 10FFFF.    8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
264      16-bit non-UTF mode   less than 0x10000
265      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
266    .sp
267    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
268    "surrogate" codepoints).
269  .P  .P
270  If characters other than hexadecimal digits appear between \ex{ and }, or if  If characters other than hexadecimal digits appear between \ex{ and }, or if
271  there is no terminating }, this form of escape is not recognized. Instead, the  there is no terminating }, this form of escape is not recognized. Instead, the
272  initial \ex will be interpreted as a basic hexadecimal escape, with no  initial \ex will be interpreted as a basic hexadecimal escape, with no
273  following digits, giving a character whose value is zero.  following digits, giving a character whose value is zero.
274  .P  .P
275    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
276    as just described only when it is followed by two hexadecimal digits.
277    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
278    code points greater than 256 is provided by \eu, which must be followed by
279    four hexadecimal digits; otherwise it matches a literal "u" character.
280    .P
281  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
282  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
283  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
284    \eu00dc in JavaScript mode).
285  .P  .P
286  After \e0 up to two further octal digits are read. If there are fewer than two  After \e0 up to two further octal digits are read. If there are fewer than two
287  digits, just those that are present are used. Thus the sequence \e0\ex\e07  digits, just those that are present are used. Thus the sequence \e0\ex\e07
# Line 297  parenthesized subpatterns. Line 307  parenthesized subpatterns.
307  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
308  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
309  digits following the backslash, and uses them to generate a data character. Any  digits following the backslash, and uses them to generate a data character. Any
310  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a  subsequent digits stand for themselves. The value of the character is
311  character specified in octal must be less than \e400. In UTF-8 mode, values up  constrained in the same way as characters specified in hexadecimal.
312  to \e777 are permitted. For example:  For example:
313  .sp  .sp
314    \e040   is another way of writing a space    \e040   is another way of writing a space
315  .\" JOIN  .\" JOIN
# Line 316  to \e777 are permitted. For example: Line 326  to \e777 are permitted. For example:
326              character with octal code 113              character with octal code 113
327  .\" JOIN  .\" JOIN
328    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
329              the byte consisting entirely of 1 bits              the value 255 (decimal)
330  .\" JOIN  .\" JOIN
331    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or a binary zero
332              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
# Line 325  Note that octal values of 100 or greater Line 335  Note that octal values of 100 or greater
335  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
336  .P  .P
337  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
338  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
339  sequence \eb is interpreted as the backspace character (hex 08). The sequences  interpreted as the backspace character (hex 08).
340  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other  .P
341  unrecognized escape sequences, they are treated as the literal characters "B",  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
342  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is  inside a character class. Like other unrecognized escape sequences, they are
343  set. Outside a character class, these sequences have different meanings.  treated as the literal characters "B", "R", and "X" by default, but cause an
344    error if the PCRE_EXTRA option is set. Outside a character class, these
345    sequences have different meanings.
346    .
347    .
348    .SS "Unsupported escape sequences"
349    .rs
350    .sp
351    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
352    handler and used to modify the case of following characters. By default, PCRE
353    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
354    option is set, \eU matches a "U" character, and \eu can be used to define a
355    character by code point, as described in the previous section.
356  .  .
357  .  .
358  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 392  This is the same as Line 414  This is the same as
414  .\" </a>  .\" </a>
415  the "." metacharacter  the "." metacharacter
416  .\"  .\"
417  when PCRE_DOTALL is not set.  when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
418    PCRE does not support this.
419  .P  .P
420  Each pair of lower and upper case escape sequences partitions the complete set  Each pair of lower and upper case escape sequences partitions the complete set
421  of characters into two disjoint sets. Any given character matches one, and only  of characters into two disjoint sets. Any given character matches one, and only
# Line 424  or "french" in Windows, some character c Line 447  or "french" in Windows, some character c
447  accented letters, and these are then matched by \ew. The use of locales with  accented letters, and these are then matched by \ew. The use of locales with
448  Unicode is discouraged.  Unicode is discouraged.
449  .P  .P
450  By default, in UTF-8 mode, characters with values greater than 128 never match  By default, in a UTF mode, characters with values greater than 128 never match
451  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
452  their original meanings from before UTF-8 support was available, mainly for  their original meanings from before UTF support was available, mainly for
453  efficiency reasons. However, if PCRE is compiled with Unicode property support,  efficiency reasons. However, if PCRE is compiled with Unicode property support,
454  and the PCRE_UCP option is set, the behaviour is changed so that Unicode  and the PCRE_UCP option is set, the behaviour is changed so that Unicode
455  properties are used to determine character types, as follows:  properties are used to determine character types, as follows:
# Line 443  is noticeably slower when PCRE_UCP is se Line 466  is noticeably slower when PCRE_UCP is se
466  .P  .P
467  The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at  The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
468  release 5.10. In contrast to the other sequences, which match only ASCII  release 5.10. In contrast to the other sequences, which match only ASCII
469  characters by default, these always match certain high-valued codepoints in  characters by default, these always match certain high-valued codepoints,
470  UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters  whether or not PCRE_UCP is set. The horizontal space characters are:
 are:  
471  .sp  .sp
472    U+0009     Horizontal tab    U+0009     Horizontal tab
473    U+0020     Space    U+0020     Space
# Line 476  The vertical space characters are: Line 498  The vertical space characters are:
498    U+0085     Next line    U+0085     Next line
499    U+2028     Line separator    U+2028     Line separator
500    U+2029     Paragraph separator    U+2029     Paragraph separator
501    .sp
502    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
503    relevant.
504  .  .
505  .  .
506  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 483  The vertical space characters are: Line 508  The vertical space characters are:
508  .rs  .rs
509  .sp  .sp
510  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
511  Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:  Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
512    following:
513  .sp  .sp
514    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
515  .sp  .sp
# Line 498  U+000B), FF (formfeed, U+000C), CR (carr Line 524  U+000B), FF (formfeed, U+000C), CR (carr
524  line, U+0085). The two-character sequence is treated as a single unit that  line, U+0085). The two-character sequence is treated as a single unit that
525  cannot be split.  cannot be split.
526  .P  .P
527  In UTF-8 mode, two additional characters whose codepoints are greater than 255  In other modes, two additional characters whose codepoints are greater than 255
528  are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).  are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
529  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
530  recognized.  recognized.
# Line 514  one of the following sequences: Line 540  one of the following sequences:
540    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
541    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
542  .sp  .sp
543  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to the compiling function, but
544  \fBpcre_compile2()\fP, but they can be overridden by options given to  they can themselves be overridden by options given to a matching function. Note
545  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,  that these special settings, which are not Perl-compatible, are recognized only
546  which are not Perl-compatible, are recognized only at the very start of a  at the very start of a pattern, and that they must be in upper case. If more
547  pattern, and that they must be in upper case. If more than one of them is  than one of them is present, the last one is used. They can be combined with a
548  present, the last one is used. They can be combined with a change of newline  change of newline convention; for example, a pattern can start with:
 convention; for example, a pattern can start with:  
549  .sp  .sp
550    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
551  .sp  .sp
552  They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside  They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
553  a character class, \eR is treated as an unrecognized escape sequence, and so  sequences. Inside a character class, \eR is treated as an unrecognized escape
554  matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.  sequence, and so matches the letter "R" by default, but causes an error if
555    PCRE_EXTRA is set.
556  .  .
557  .  .
558  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 535  matches the letter "R" by default, but c Line 561  matches the letter "R" by default, but c
561  .sp  .sp
562  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
563  escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
564  When not in UTF-8 mode, these sequences are of course limited to testing  When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
565  characters whose codepoints are less than 256, but they do work in this mode.  characters whose codepoints are less than 256, but they do work in this mode.
566  The extra escape sequences are:  The extra escape sequences are:
567  .sp  .sp
# Line 722  the Lu, Ll, or Lt property, in other wor Line 748  the Lu, Ll, or Lt property, in other wor
748  a modifier or "other".  a modifier or "other".
749  .P  .P
750  The Cs (Surrogate) property applies only to characters in the range U+D800 to  The Cs (Surrogate) property applies only to characters in the range U+D800 to
751  U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so  U+DFFF. Such characters are not valid in Unicode strings and so
752  cannot be tested by PCRE, unless UTF-8 validity checking has been turned off  cannot be tested by PCRE, unless UTF validity checking has been turned off
753  (see the discussion of PCRE_NO_UTF8_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the
754  .\" HREF  .\" HREF
755  \fBpcreapi\fP  \fBpcreapi\fP
756  .\"  .\"
# Line 755  atomic group Line 781  atomic group
781  .\"  .\"
782  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
783  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
784  non-UTF-8 mode \eX matches any one character.  8-bit non-UTF-8 mode \eX matches any one character.
785  .P  .P
786  Note that recent versions of Perl have changed \eX to match what Unicode calls  Note that recent versions of Perl have changed \eX to match what Unicode calls
787  an "extended grapheme cluster", which has a more complicated definition.  an "extended grapheme cluster", which has a more complicated definition.
# Line 764  Matching characters by Unicode property Line 790  Matching characters by Unicode property
790  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
791  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
792  properties in PCRE by default, though you can make them do so by setting the  properties in PCRE by default, though you can make them do so by setting the
793  PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with  PCRE_UCP option or by starting the pattern with (*UCP).
 (*UCP).  
794  .  .
795  .  .
796  .\" HTML <a name="extraprops"></a>  .\" HTML <a name="extraprops"></a>
# Line 854  escape sequence" error is generated inst Line 879  escape sequence" error is generated inst
879  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
880  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
881  \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
882  first or last character matches \ew, respectively. In UTF-8 mode, the meanings  first or last character matches \ew, respectively. In a UTF mode, the meanings
883  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
884  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
885  of word" or "end of word" metasequence. However, whatever follows \eb normally  of word" or "end of word" metasequence. However, whatever follows \eb normally
# Line 949  end of the subject in both modes, and if Line 974  end of the subject in both modes, and if
974  .sp  .sp
975  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
976  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
977  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
978  .P  .P
979  When a line ending is defined as a single character, dot never matches that  When a line ending is defined as a single character, dot never matches that
980  character; when the two-character sequence CRLF is used, dot does not match CR  character; when the two-character sequence CRLF is used, dot does not match CR
# Line 969  special meaning in a character class. Line 994  special meaning in a character class.
994  .P  .P
995  The escape sequence \eN behaves like a dot, except that it is not affected by  The escape sequence \eN behaves like a dot, except that it is not affected by
996  the PCRE_DOTALL option. In other words, it matches any character except one  the PCRE_DOTALL option. In other words, it matches any character except one
997  that signifies the end of a line.  that signifies the end of a line. Perl also uses \eN to match characters by
998    name; PCRE does not support this.
999  .  .
1000  .  .
1001  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE DATA UNIT"
1002  .rs  .rs
1003  .sp  .sp
1004  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one data unit,
1005  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1006  characters. The feature is provided in Perl in order to match individual bytes  byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always
1007  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the  matches line-ending characters. The feature is provided in Perl in order to
1008  rest of the string may start with a malformed UTF-8 character. For this reason,  match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1009  the \eC escape sequence is best avoided.  used. Because \eC breaks up characters into individual data units, matching one
1010    unit with \eC in a UTF mode means that the rest of the string may start with a
1011    malformed UTF character. This has undefined results, because PCRE assumes that
1012    it is dealing with valid UTF strings (and by default it checks this at the
1013    start of processing unless the PCRE_NO_UTF8_CHECK option is used).
1014  .P  .P
1015  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1016  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1017  .\" </a>  .\" </a>
1018  (described below),  (described below)
1019  .\"  .\"
1020  because in UTF-8 mode this would make it impossible to calculate the length of  in a UTF mode, because this would make it impossible to calculate the length of
1021  the lookbehind.  the lookbehind.
1022    .P
1023    In general, the \eC escape sequence is best avoided. However, one
1024    way of using it that avoids the problem of malformed UTF characters is to use a
1025    lookahead to check the length of the next character, as in this pattern, which
1026    could be used with a UTF-8 string (ignore white space and line breaks):
1027    .sp
1028      (?| (?=[\ex00-\ex7f])(\eC) |
1029          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1030          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1031          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1032    .sp
1033    A group that starts with (?| resets the capturing parentheses numbers in each
1034    alternative (see
1035    .\" HTML <a href="#dupsubpatternnumber">
1036    .\" </a>
1037    "Duplicate Subpattern Numbers"
1038    .\"
1039    below). The assertions at the start of each branch check the next UTF-8
1040    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1041    character's individual bytes are then captured by the appropriate number of
1042    groups.
1043  .  .
1044  .  .
1045  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 1002  bracket causes a compile-time error. If Line 1053  bracket causes a compile-time error. If
1053  a member of the class, it should be the first data character in the class  a member of the class, it should be the first data character in the class
1054  (after an initial circumflex, if present) or escaped with a backslash.  (after an initial circumflex, if present) or escaped with a backslash.
1055  .P  .P
1056  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 a UTF mode, the
1057  character may be more than one byte long. A matched character must be in the  character may be more than one data unit long. A matched character must be in
1058  set of characters defined by the class, unless the first character in the class  the set of characters defined by the class, unless the first character in the
1059  definition is a circumflex, in which case the subject character must not be in  class definition is a circumflex, in which case the subject character must not
1060  the set defined by the class. If a circumflex is actually required as a member  be in the set defined by the class. If a circumflex is actually required as a
1061  of the class, ensure it is not the first character, or escape it with a  member of the class, ensure it is not the first character, or escape it with a
1062  backslash.  backslash.
1063  .P  .P
1064  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
# Line 1018  circumflex is not an assertion; it still Line 1069  circumflex is not an assertion; it still
1069  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
1070  string.  string.
1071  .P  .P
1072  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be
1073  class as a literal string of bytes, or by using the \ex{ escaping mechanism.  included in a class as a literal string of data units, or by using the \ex{
1074    escaping mechanism.
1075  .P  .P
1076  When caseless matching is set, any letters in a class represent both their  When caseless matching is set, any letters in a class represent both their
1077  upper case and lower case versions, so for example, a caseless [aeiou] matches  upper case and lower case versions, so for example, a caseless [aeiou] matches
1078  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1079  caseful version would. In UTF-8 mode, PCRE always understands the concept of  caseful version would. In a UTF mode, PCRE always understands the concept of
1080  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
1081  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1082  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1083  If you want to use caseless matching in UTF8-mode for characters 128 and above,  If you want to use caseless matching in a UTF mode for characters 128 and
1084  you must ensure that PCRE is compiled with Unicode property support as well as  above, you must ensure that PCRE is compiled with Unicode property support as
1085  with UTF-8 support.  well as with UTF support.
1086  .P  .P
1087  Characters that might indicate line breaks are never treated in any special way  Characters that might indicate line breaks are never treated in any special way
1088  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 1052  followed by two other characters. The oc Line 1104  followed by two other characters. The oc
1104  "]" can also be used to end a range.  "]" can also be used to end a range.
1105  .P  .P
1106  Ranges operate in the collating sequence of character values. They can also be  Ranges operate in the collating sequence of character values. They can also be
1107  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1108  mode, ranges can include characters whose values are greater than 255, for  can include any characters that are valid for the current mode.
 example [\ex{100}-\ex{2ff}].  
1109  .P  .P
1110  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
1111  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
1112  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1113  tables for a French locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
1114  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF modes, PCRE supports the concept of case for
1115  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
1116  property support.  property support.
1117  .P  .P
1118  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1119  \eV, \ew, and \eW may appear in a character class, and add the characters that  \eV, \ew, and \eW may appear in a character class, and add the characters that
1120  they match to the class. For example, [\edABCDEF] matches any hexadecimal  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1121  digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew  digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1122  and their upper case partners, just as it does when they appear outside a  and their upper case partners, just as it does when they appear outside a
1123  character class, as described in the section entitled  character class, as described in the section entitled
1124  .\" HTML <a href="#genericchartypes">  .\" HTML <a href="#genericchartypes">
# Line 1137  matches "1", "2", or any non-digit. PCRE Line 1188  matches "1", "2", or any non-digit. PCRE
1188  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
1189  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1190  .P  .P
1191  By default, in UTF-8 mode, characters with values greater than 128 do not match  By default, in UTF modes, characters with values greater than 128 do not match
1192  any of the POSIX character classes. However, if the PCRE_UCP option is passed  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1193  to \fBpcre_compile()\fP, some of the classes are changed so that Unicode  to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1194  character properties are used. This is achieved by replacing the POSIX classes  character properties are used. This is achieved by replacing the POSIX classes
# Line 1225  option settings happen at compile time. Line 1276  option settings happen at compile time.
1276  behaviour otherwise.  behaviour otherwise.
1277  .P  .P
1278  \fBNote:\fP There are other PCRE-specific options that can be set by the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1279  application when the compile or match functions are called. In some cases the  application when the compiling or matching functions are called. In some cases
1280  pattern can contain special leading sequences such as (*CRLF) to override what  the pattern can contain special leading sequences such as (*CRLF) to override
1281  the application has set or what has been defaulted. Details are given in the  what the application has set or what has been defaulted. Details are given in
1282  section entitled  the section entitled
1283  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
1284  .\" </a>  .\" </a>
1285  "Newline sequences"  "Newline sequences"
1286  .\"  .\"
1287  above. There are also the (*UTF8) and (*UCP) leading sequences that can be used  above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1288  to set UTF-8 and Unicode property modes; they are equivalent to setting the  can be used to set UTF and Unicode property modes; they are equivalent to
1289  PCRE_UTF8 and the PCRE_UCP options, respectively.  setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1290  .  .
1291  .  .
1292  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1254  match "cataract", "erpillar" or an empty Line 1305  match "cataract", "erpillar" or an empty
1305  .sp  .sp
1306  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
1307  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
1308  subpattern is passed back to the caller via the \fIovector\fP argument of  subpattern is passed back to the caller via the \fIovector\fP argument of the
1309  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1310  from 1) to obtain numbers for the capturing subpatterns. For example, if the  the DFA matching functions do not support capturing.)
1311  string "the red king" is matched against the pattern  .P
1312    Opening parentheses are counted from left to right (starting from 1) to obtain
1313    numbers for the capturing subpatterns. For example, if the string "the red
1314    king" is matched against the pattern
1315  .sp  .sp
1316    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1317  .sp  .sp
# Line 1320  or "defdef": Line 1374  or "defdef":
1374  .sp  .sp
1375    /(?|(abc)|(def))\e1/    /(?|(abc)|(def))\e1/
1376  .sp  .sp
1377  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a subroutine call to a numbered subpattern always refers to the
1378  refers to the first one in the pattern with the given number. The following  first one in the pattern with the given number. The following pattern matches
1379  pattern matches "abcabc" or "defabc":  "abcabc" or "defabc":
1380  .sp  .sp
1381    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1382  .sp  .sp
# Line 1433  items: Line 1487  items:
1487    a literal data character    a literal data character
1488    the dot metacharacter    the dot metacharacter
1489    the \eC escape sequence    the \eC escape sequence
1490    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1491    the \eR escape sequence    the \eR escape sequence
1492    an escape such as \ed or \epL that matches a single character    an escape such as \ed or \epL that matches a single character
1493    a character class    a character class
1494    a back reference (see next section)    a back reference (see next section)
1495    a parenthesized subpattern (including assertions)    a parenthesized subpattern (including assertions)
1496    a recursive or "subroutine" call to a subpattern    a subroutine call to a subpattern (recursive or otherwise)
1497  .sp  .sp
1498  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1499  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1464  where a quantifier is not allowed, or on Line 1518  where a quantifier is not allowed, or on
1518  quantifier, is taken as a literal character. For example, {,6} is not a  quantifier, is taken as a literal character. For example, {,6} is not a
1519  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1520  .P  .P
1521  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual  In UTF modes, quantifiers apply to characters rather than to individual data
1522  bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of  units. Thus, for example, \ex{100}{2} matches two characters, each of
1523  which is represented by a two-byte sequence. Similarly, when Unicode property  which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1524  support is available, \eX{3} matches three Unicode extended sequences, each of  \eX{3} matches three Unicode extended sequences, each of which may be several
1525  which may be several bytes long (and they may be of different lengths).  data units long (and they may be of different lengths).
1526  .P  .P
1527  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
1528  previous item and the quantifier were not present. This may be useful for  previous item and the quantifier were not present. This may be useful for
# Line 1931  temporarily move the current position ba Line 1985  temporarily move the current position ba
1985  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1986  assertion fails.  assertion fails.
1987  .P  .P
1988  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
1989  to appear in lookbehind assertions, because it makes it impossible to calculate  unit even in a UTF mode) to appear in lookbehind assertions, because it makes
1990  the length of the lookbehind. The \eX and \eR escapes, which can match  it impossible to calculate the length of the lookbehind. The \eX and \eR
1991  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
1992    permitted.
1993  .P  .P
1994  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
1995  .\" </a>  .\" </a>
# Line 2128  If the condition is the string (DEFINE), Line 2183  If the condition is the string (DEFINE),
2183  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
2184  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2185  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
2186  "subroutines" that can be referenced from elsewhere. (The use of  subroutines that can be referenced from elsewhere. (The use of
2187  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2188  .\" </a>  .\" </a>
2189  "subroutines"  subroutines
2190  .\"  .\"
2191  is described below.) For example, a pattern to match an IPv4 address such as  is described below.) For example, a pattern to match an IPv4 address such as
2192  "192.168.23.245" could be written like this (ignore whitespace and line  "192.168.23.245" could be written like this (ignore whitespace and line
# Line 2181  closing parenthesis. Nested parentheses Line 2236  closing parenthesis. Nested parentheses
2236  option is set, an unescaped # character also introduces a comment, which in  option is set, an unescaped # character also introduces a comment, which in
2237  this case continues to immediately after the next newline character or  this case continues to immediately after the next newline character or
2238  character sequence in the pattern. Which characters are interpreted as newlines  character sequence in the pattern. Which characters are interpreted as newlines
2239  is controlled by the options passed to \fBpcre_compile()\fP or by a special  is controlled by the options passed to a compiling function or by a special
2240  sequence at the start of the pattern, as described in the section entitled  sequence at the start of the pattern, as described in the section entitled
2241  .\" HTML <a href="#newlines">  .\" HTML <a href="#newlines">
2242  .\" </a>  .\" </a>
# Line 2226  individual subpattern recursion. After i Line 2281  individual subpattern recursion. After i
2281  this kind of recursion was subsequently introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2282  .P  .P
2283  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
2284  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive subroutine call of the subpattern of the
2285  provided that it occurs inside that subpattern. (If not, it is a  given number, provided that it occurs inside that subpattern. (If not, it is a
2286  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2287  .\" </a>  .\" </a>
2288  "subroutine"  non-recursive subroutine
2289  .\"  .\"
2290  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
2291  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
# Line 2265  references such as (?+2). However, these Line 2320  references such as (?+2). However, these
2320  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2321  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2322  .\" </a>  .\" </a>
2323  "subroutine"  non-recursive subroutine
2324  .\"  .\"
2325  calls, as described in the next section.  calls, as described in the next section.
2326  .P  .P
# Line 2302  documentation). If the pattern above is Line 2357  documentation). If the pattern above is
2357  .sp  .sp
2358  the value for the inner capturing parentheses (numbered 2) is "ef", which is  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2359  the last value taken on at the top level. If a capturing subpattern is not  the last value taken on at the top level. If a capturing subpattern is not
2360  matched at the top level, its final value is unset, even if it is (temporarily)  matched at the top level, its final captured value is unset, even if it was
2361  set at a deeper level.  (temporarily) set at a deeper level during the matching process.
2362  .P  .P
2363  If there are more than 15 capturing parentheses in a pattern, PCRE has to  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2364  obtain extra memory to store data during a recursion, which it does by using  obtain extra memory to store data during a recursion, which it does by using
# Line 2323  is the actual recursive call. Line 2378  is the actual recursive call.
2378  .  .
2379  .  .
2380  .\" HTML <a name="recursiondifference"></a>  .\" HTML <a name="recursiondifference"></a>
2381  .SS "Recursion difference from Perl"  .SS "Differences in recursion processing between PCRE and Perl"
2382  .rs  .rs
2383  .sp  .sp
2384  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2385  treated as an atomic group. That is, once it has matched some of the subject  (like Python, but unlike Perl), a recursive subpattern call is always treated
2386  string, it is never re-entered, even if it contains untried alternatives and  as an atomic group. That is, once it has matched some of the subject string, it
2387  there is a subsequent matching failure. This can be illustrated by the  is never re-entered, even if it contains untried alternatives and there is a
2388  following pattern, which purports to match a palindromic string that contains  subsequent matching failure. This can be illustrated by the following pattern,
2389  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  which purports to match a palindromic string that contains an odd number of
2390    characters (for example, "a", "aba", "abcba", "abcdcba"):
2391  .sp  .sp
2392    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2393  .sp  .sp
# Line 2392  For example, although "abcba" is correct Line 2448  For example, although "abcba" is correct
2448  PCRE finds the palindrome "aba" at the start, then fails at top level because  PCRE finds the palindrome "aba" at the start, then fails at top level because
2449  the end of the string does not follow. Once again, it cannot jump back into the  the end of the string does not follow. Once again, it cannot jump back into the
2450  recursion to try other alternatives, so the entire match fails.  recursion to try other alternatives, so the entire match fails.
2451    .P
2452    The second way in which PCRE and Perl differ in their recursion processing is
2453    in the handling of captured values. In Perl, when a subpattern is called
2454    recursively or as a subpattern (see the next section), it has no access to any
2455    values that were captured outside the recursion, whereas in PCRE these values
2456    can be referenced. Consider this pattern:
2457    .sp
2458      ^(.)(\e1|a(?2))
2459    .sp
2460    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2461    then in the second group, when the back reference \e1 fails to match "b", the
2462    second alternative matches "a" and then recurses. In the recursion, \e1 does
2463    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2464    match because inside the recursive call \e1 cannot access the externally set
2465    value.
2466  .  .
2467  .  .
2468  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2469  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2470  .rs  .rs
2471  .sp  .sp
2472  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern call (either by number or by
2473  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
2474  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2475  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2476  relative, as in these examples:  relative, as in these examples:
2477  .sp  .sp
# Line 2420  matches "sense and sensibility" and "res Line 2491  matches "sense and sensibility" and "res
2491  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
2492  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2493  .P  .P
2494  Like recursive subpatterns, a subroutine call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2495  group. That is, once it has matched some of the subject string, it is never  groups. That is, once a subroutine has matched some of the subject string, it
2496  re-entered, even if it contains untried alternatives and there is a subsequent  is never re-entered, even if it contains untried alternatives and there is a
2497  matching failure. Any capturing parentheses that are set during the subroutine  subsequent matching failure. Any capturing parentheses that are set during the
2498  call revert to their previous values afterwards.  subroutine call revert to their previous values afterwards.
2499  .P  .P
2500  When a subpattern is used as a subroutine, processing options such as  Processing options such as case-independence are fixed when a subpattern is
2501  case-independence are fixed when the subpattern is defined. They cannot be  defined, so if it is used as a subroutine, such options cannot be changed for
2502  changed for different calls. For example, consider this pattern:  different calls. For example, consider this pattern:
2503  .sp  .sp
2504    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2505  .sp  .sp
# Line 2467  same pair of parentheses when there is a Line 2538  same pair of parentheses when there is a
2538  .P  .P
2539  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2540  code. The feature is called "callout". The caller of PCRE provides an external  code. The feature is called "callout". The caller of PCRE provides an external
2541  function by putting its entry point in the global variable \fIpcre_callout\fP.  function by putting its entry point in the global variable \fIpcre_callout\fP
2542  By default, this variable contains NULL, which disables all calling out.  (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this
2543    variable contains NULL, which disables all calling out.
2544  .P  .P
2545  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2546  function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
# Line 2477  For example, this pattern has two callou Line 2549  For example, this pattern has two callou
2549  .sp  .sp
2550    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2551  .sp  .sp
2552  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2553  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2554  255.  255.
2555  .P  .P
2556  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is  During matching, when PCRE reaches a callout point, the external function is
2557  set), the external function is called. It is provided with the number of the  called. It is provided with the number of the callout, the position in the
2558  callout, the position in the pattern, and, optionally, one item of data  pattern, and, optionally, one item of data originally supplied by the caller of
2559  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  the matching function. The callout function may cause matching to proceed, to
2560  may cause matching to proceed, to backtrack, or to fail altogether. A complete  backtrack, or to fail altogether. A complete description of the interface to
2561  description of the interface to the callout function is given in the  the callout function is given in the
2562  .\" HREF  .\" HREF
2563  \fBpcrecallout\fP  \fBpcrecallout\fP
2564  .\"  .\"
# Line 2504  production code should be noted to avoid Line 2576  production code should be noted to avoid
2576  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2577  .P  .P
2578  Since these verbs are specifically related to backtracking, most of them can be  Since these verbs are specifically related to backtracking, most of them can be
2579  used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses  used only when the pattern is to be matched using one of the traditional
2580  a backtracking algorithm. With the exception of (*FAIL), which behaves like a  matching functions, which use a backtracking algorithm. With the exception of
2581  failing negative assertion, they cause an error if encountered by  (*FAIL), which behaves like a failing negative assertion, they cause an error
2582  \fBpcre_dfa_exec()\fP.  if encountered by a DFA matching function.
2583  .P  .P
2584  If any of these verbs are used in an assertion or subroutine subpattern  If any of these verbs are used in an assertion or in a subpattern that is
2585  (including recursive subpatterns), their effect is confined to that subpattern;  called as a subroutine (whether or not recursively), their effect is confined
2586  it does not extend to the surrounding pattern, with one exception: a *MARK that  to that subpattern; it does not extend to the surrounding pattern, with one
2587  is encountered in a positive assertion \fIis\fP passed back (compare capturing  exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2588  parentheses in assertions). Note that such subpatterns are processed as  a successful positive assertion \fIis\fP passed back when a match succeeds
2589  anchored at the point where they are tested.  (compare capturing parentheses in assertions). Note that such subpatterns are
2590    processed as anchored at the point where they are tested. Note also that Perl's
2591    treatment of subroutines is different in some cases.
2592  .P  .P
2593  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2594  parenthesis followed by an asterisk. They are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2595  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2596  depending on whether or not an argument is present. An name is a sequence of  depending on whether or not an argument is present. A name is any sequence of
2597  letters, digits, and underscores. If the name is empty, that is, if the closing  characters that does not include a closing parenthesis. If the name is empty,
2598  parenthesis immediately follows the colon, the effect is as if the colon were  that is, if the closing parenthesis immediately follows the colon, the effect
2599  not there. Any number of these verbs may occur in a pattern.  is as if the colon were not there. Any number of these verbs may occur in a
2600    pattern.
2601  .P  .P
2602  PCRE contains some optimizations that are used to speed up matching by running  PCRE contains some optimizations that are used to speed up matching by running
2603  some checks at the start of each match attempt. For example, it may know the  some checks at the start of each match attempt. For example, it may know the
# Line 2532  included backtracking verbs will not, of Line 2607  included backtracking verbs will not, of
2607  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2608  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2609  pattern with (*NO_START_OPT).  pattern with (*NO_START_OPT).
2610    .P
2611    Experiments with Perl suggest that it too has similar optimizations, sometimes
2612    leading to anomalous results.
2613  .  .
2614  .  .
2615  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
# Line 2543  followed by a name. Line 2621  followed by a name.
2621     (*ACCEPT)     (*ACCEPT)
2622  .sp  .sp
2623  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2624  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2625  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is  subroutine, only that subpattern is ended successfully. Matching then continues
2626  captured. (This feature was added to PCRE at release 8.00.) For example:  at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2627    far is captured. For example:
2628  .sp  .sp
2629    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2630  .sp  .sp
# Line 2554  the outer parentheses. Line 2633  the outer parentheses.
2633  .sp  .sp
2634    (*FAIL) or (*F)    (*FAIL) or (*F)
2635  .sp  .sp
2636  This verb causes the match to fail, forcing backtracking to occur. It is  This verb causes a matching failure, forcing backtracking to occur. It is
2637  equivalent to (?!) but easier to read. The Perl documentation notes that it is  equivalent to (?!) but easier to read. The Perl documentation notes that it is
2638  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2639  Perl features that are not present in PCRE. The nearest equivalent is the  Perl features that are not present in PCRE. The nearest equivalent is the
# Line 2578  starting point (see (*SKIP) below). Line 2657  starting point (see (*SKIP) below).
2657  A name is always required with this verb. There may be as many instances of  A name is always required with this verb. There may be as many instances of
2658  (*MARK) as you like in a pattern, and their names do not have to be unique.  (*MARK) as you like in a pattern, and their names do not have to be unique.
2659  .P  .P
2660  When a match succeeds, the name of the last-encountered (*MARK) is passed back  When a match succeeds, the name of the last-encountered (*MARK) on the matching
2661  to the caller via the \fIpcre_extra\fP data structure, as described in the  path is passed back to the caller as described in the section entitled
2662  .\" HTML <a href="pcreapi.html#extradata">  .\" HTML <a href="pcreapi.html#extradata">
2663  .\" </a>  .\" </a>
2664  section on \fIpcre_extra\fP  "Extra data for \fBpcre_exec()\fP"
2665  .\"  .\"
2666  in the  in the
2667  .\" HREF  .\" HREF
2668  \fBpcreapi\fP  \fBpcreapi\fP
2669  .\"  .\"
2670  documentation. No data is returned for a partial match. Here is an example of  documentation. Here is an example of \fBpcretest\fP output, where the /K
2671  \fBpcretest\fP output, where the /K modifier requests the retrieval and  modifier requests the retrieval and outputting of (*MARK) data:
 outputting of (*MARK) data:  
2672  .sp  .sp
2673    /X(*MARK:A)Y|X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2674    XY    data> XY
2675     0: XY     0: XY
2676    MK: A    MK: A
2677    XZ    XZ
# Line 2607  capturing parentheses. Line 2685  capturing parentheses.
2685  .P  .P
2686  If (*MARK) is encountered in a positive assertion, its name is recorded and  If (*MARK) is encountered in a positive assertion, its name is recorded and
2687  passed back if it is the last-encountered. This does not happen for negative  passed back if it is the last-encountered. This does not happen for negative
2688  assetions.  assertions.
2689  .P  .P
2690  A name may also be returned after a failed match if the final path through the  After a partial match or a failed match, the name of the last encountered
2691  pattern involves (*MARK). However, unless (*MARK) used in conjunction with  (*MARK) in the entire match process is returned. For example:
 (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the  
 starting point for matching is advanced, the final check is often with an empty  
 string, causing a failure before (*MARK) is reached. For example:  
2692  .sp  .sp
2693    /X(*MARK:A)Y|X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2694    XP    data> XP
   No match  
 .sp  
 There are three potential starting points for this match (starting with X,  
 starting with P, and with an empty string). If the pattern is anchored, the  
 result is different:  
 .sp  
   /^X(*MARK:A)Y|^X(*MARK:B)Z/K  
   XP  
2695    No match, mark = B    No match, mark = B
2696  .sp  .sp
2697  PCRE's start-of-match optimizations can also interfere with this. For example,  Note that in this unanchored example the mark is retained from the match
2698  if, as a result of a call to \fBpcre_study()\fP, it knows the minimum  attempt that started at the letter "X". Subsequent match attempts starting at
2699  subject length for a match, a shorter subject will not be scanned at all.  "P" and then with an empty string do not get as far as the (*MARK) item, but
2700  .P  nevertheless do not reset it.
 Note that similar anomalies (though different in detail) exist in Perl, no  
 doubt for the same reasons. The use of (*MARK) data after a failed match of an  
 unanchored pattern is not recommended, unless (*COMMIT) is involved.  
2701  .  .
2702  .  .
2703  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
# Line 2670  Note that (*COMMIT) at the start of a pa Line 2734  Note that (*COMMIT) at the start of a pa
2734  unless PCRE's start-of-match optimizations are turned off, as shown in this  unless PCRE's start-of-match optimizations are turned off, as shown in this
2735  \fBpcretest\fP example:  \fBpcretest\fP example:
2736  .sp  .sp
2737    /(*COMMIT)abc/      re> /(*COMMIT)abc/
2738    xyzabc    data> xyzabc
2739     0: abc     0: abc
2740    xyzabc\eY    xyzabc\eY
2741    No match    No match
# Line 2692  reached, or when matching to the right o Line 2756  reached, or when matching to the right o
2756  the right, backtracking cannot cross (*PRUNE). In simple cases, the use of  the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2757  (*PRUNE) is just an alternative to an atomic group or possessive quantifier,  (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2758  but there are some uses of (*PRUNE) that cannot be expressed in any other way.  but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2759  The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the  The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2760  match fails completely; the name is passed back if this is the final attempt.  anchored pattern (*PRUNE) has the same effect as (*COMMIT).
 (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored  
 pattern (*PRUNE) has the same effect as (*COMMIT).  
2761  .sp  .sp
2762    (*SKIP)    (*SKIP)
2763  .sp  .sp
# Line 2721  following pattern fails to match, the pr Line 2783  following pattern fails to match, the pr
2783  searched for the most recent (*MARK) that has the same name. If one is found,  searched for the most recent (*MARK) that has the same name. If one is found,
2784  the "bumpalong" advance is to the subject position that corresponds to that  the "bumpalong" advance is to the subject position that corresponds to that
2785  (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a  (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2786  matching name is found, normal "bumpalong" of one character happens (the  matching name is found, the (*SKIP) is ignored.
 (*SKIP) is ignored).  
2787  .sp  .sp
2788    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
2789  .sp  .sp
2790  This verb causes a skip to the next alternation in the innermost enclosing  This verb causes a skip to the next innermost alternative if the rest of the
2791  group if the rest of the pattern does not match. That is, it cancels pending  pattern does not match. That is, it cancels pending backtracking, but only
2792  backtracking, but only within the current alternation. Its name comes from the  within the current alternative. Its name comes from the observation that it can
2793  observation that it can be used for a pattern-based if-then-else block:  be used for a pattern-based if-then-else block:
2794  .sp  .sp
2795    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2796  .sp  .sp
2797  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
2798  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure, the matcher skips to the
2799  second alternative and tries COND2, without backtracking into COND1. The  second alternative and tries COND2, without backtracking into COND1. The
2800  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2801  overall match fails. If (*THEN) is not directly inside an alternation, it acts  If (*THEN) is not inside an alternation, it acts like (*PRUNE).
 like (*PRUNE).  
 .  
 .P  
 The above verbs provide four different "strengths" of control when subsequent  
 matching fails. (*THEN) is the weakest, carrying on the match at the next  
 alternation. (*PRUNE) comes next, failing the match at the current starting  
 position, but allowing an advance to the next character (for an unanchored  
 pattern). (*SKIP) is similar, except that the advance may be more than one  
 character. (*COMMIT) is the strongest, causing the entire match to fail.  
2802  .P  .P
2803  If more than one is present in a pattern, the "stongest" one wins. For example,  Note that a subpattern that does not contain a | character is just a part of
2804  consider this pattern, where A, B, etc. are complex pattern fragments:  the enclosing alternative; it is not a nested alternation with only one
2805    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2806    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2807    pattern fragments that do not contain any | characters at this level:
2808    .sp
2809      A (B(*THEN)C) | D
2810    .sp
2811    If A and B are matched, but there is a failure in C, matching does not
2812    backtrack into A; instead it moves to the next alternative, that is, D.
2813    However, if the subpattern containing (*THEN) is given an alternative, it
2814    behaves differently:
2815    .sp
2816      A (B(*THEN)C | (*FAIL)) | D
2817    .sp
2818    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2819    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2820    because there are no more alternatives to try. In this case, matching does now
2821    backtrack into A.
2822    .P
2823    Note also that a conditional subpattern is not considered as having two
2824    alternatives, because only one is ever used. In other words, the | character in
2825    a conditional subpattern has a different meaning. Ignoring white space,
2826    consider:
2827    .sp
2828      ^.*? (?(?=a) a | b(*THEN)c )
2829    .sp
2830    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2831    it initially matches zero characters. The condition (?=a) then fails, the
2832    character "b" is matched, but "c" is not. At this point, matching does not
2833    backtrack to .*? as might perhaps be expected from the presence of the |
2834    character. The conditional subpattern is part of the single alternative that
2835    comprises the whole pattern, and so the match fails. (If there was a backtrack
2836    into .*?, allowing it to match "b", the match would succeed.)
2837    .P
2838    The verbs just described provide four different "strengths" of control when
2839    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2840    next alternative. (*PRUNE) comes next, failing the match at the current
2841    starting position, but allowing an advance to the next character (for an
2842    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2843    than one character. (*COMMIT) is the strongest, causing the entire match to
2844    fail.
2845    .P
2846    If more than one such verb is present in a pattern, the "strongest" one wins.
2847    For example, consider this pattern, where A, B, etc. are complex pattern
2848    fragments:
2849  .sp  .sp
2850    (A(*COMMIT)B(*THEN)C|D)    (A(*COMMIT)B(*THEN)C|D)
2851  .sp  .sp
2852  Once A has matched, PCRE is committed to this match, at the current starting  Once A has matched, PCRE is committed to this match, at the current starting
2853  position. If subsequently B matches, but C does not, the normal (*THEN) action  position. If subsequently B matches, but C does not, the normal (*THEN) action
2854  of trying the next alternation (that is, D) does not happen because (*COMMIT)  of trying the next alternative (that is, D) does not happen because (*COMMIT)
2855  overrides.  overrides.
2856  .  .
2857  .  .
# Line 2763  overrides. Line 2859  overrides.
2859  .rs  .rs
2860  .sp  .sp
2861  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2862  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.
2863  .  .
2864  .  .
2865  .SH AUTHOR  .SH AUTHOR
# Line 2780  Cambridge CB2 3QH, England. Line 2876  Cambridge CB2 3QH, England.
2876  .rs  .rs
2877  .sp  .sp
2878  .nf  .nf
2879  Last updated: 24 July 2011  Last updated: 09 January 2012
2880  Copyright (c) 1997-2011 University of Cambridge.  Copyright (c) 1997-2012 University of Cambridge.
2881  .fi  .fi

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