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revision 394 by ph10, Wed Mar 18 16:38:23 2009 UTC revision 942 by ph10, Tue Feb 28 14:50:31 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, you must  there is now also support for UTF-8 strings in the original library, and a
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  second library that supports 16-bit and UTF-16 character strings. To use these
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  features, PCRE must be built to include appropriate support. When using UTF
27  places below. There is also a summary of UTF-8 features in the  strings you must either call the compiling function with the PCRE_UTF8 or
28  .\" HTML <a href="pcre.html#utf8support">  PCRE_UTF16 option, or the pattern must start with one of these special
29  .\" </a>  sequences:
30  section on UTF-8 support  .sp
31  .\"    (*UTF8)
32  in the main    (*UTF16)
33    .sp
34    Starting a pattern with such a sequence is equivalent to setting the relevant
35    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
37    of features in the
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
44    combination with (*UTF8) or (*UTF16) is:
45    .sp
46      (*UCP)
47    .sp
48    This has the same effect as setting the PCRE_UCP option: it causes sequences
49    such as \ed and \ew to use Unicode properties to determine character types,
50    instead of recognizing only characters with codes less than 128 via a lookup
51    table.
52    .P
53    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
54    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
55    also some more of these special sequences that are concerned with the handling
56    of newlines; they are described below.
57    .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  .\"  .\"
69  page.  page.
70  .  .
71  .  .
72    .\" HTML <a name="newlines"></a>
73  .SH "NEWLINE CONVENTIONS"  .SH "NEWLINE CONVENTIONS"
74  .rs  .rs
75  .sp  .sp
# Line 76  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. For  These override the default and the options given to the compiling function. For
101  example, on a Unix system where LF is the default newline sequence, the pattern  example, on a Unix system where LF is the default newline sequence, the pattern
102  .sp  .sp
103    (*CR)a.b    (*CR)a.b
# Line 87  Perl-compatible, are recognized only at Line 108  Perl-compatible, are recognized only at
108  they must be in upper case. If more than one of them is present, the last one  they must be in upper case. If more than one of them is present, the last one
109  is used.  is used.
110  .P  .P
111  The newline convention does not affect what the \eR escape sequence matches. By  The newline convention affects the interpretation of the dot metacharacter when
112  default, this is any Unicode newline sequence, for Perl compatibility. However,  PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
113  this can be changed; see the description of \eR in the section entitled  affect what the \eR escape sequence matches. By default, this is any Unicode
114    newline sequence, for Perl compatibility. However, this can be changed; see the
115    description of \eR in the section entitled
116  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
117  .\" </a>  .\" </a>
118  "Newline sequences"  "Newline sequences"
# Line 109  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 161  The following sections describe the use Line 184  The following sections describe the use
184  .rs  .rs
185  .sp  .sp
186  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
187  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
188  may have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
189  outside character classes.  both inside and outside character classes.
190  .P  .P
191  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
192  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 171  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 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
199    greater than 127) are treated as literals.
200    .P
201  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
202  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
203  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
# Line 190  Perl, $ and @ cause variable interpolati Line 217  Perl, $ and @ cause variable interpolati
217    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
218  .sp  .sp
219  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
220    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
221    by \eE later in the pattern, the literal interpretation continues to the end of
222    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
223    a character class, this causes an error, because the character class is not
224    terminated.
225  .  .
226  .  .
227  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 199  The \eQ...\eE sequence is recognized bot Line 231  The \eQ...\eE sequence is recognized bot
231  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
232  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
233  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
234  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
235  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:  
236  .sp  .sp
237    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
238    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
239    \ee        escape (hex 1B)    \ee        escape (hex 1B)
240    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
241    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
242    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
243    \et        tab (hex 09)    \et        tab (hex 09)
244    \eddd      character with octal code ddd, or backreference    \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, but \ec{ becomes hex 3B, while \ec; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
252  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
253  .P  than 127, a compile-time error occurs. This locks out non-ASCII characters in
254  After \ex, from zero to two hexadecimal digits are read (letters can be in  all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. A
255  upper or lower case). Any number of hexadecimal digits may appear between \ex{  lower case letter is converted to upper case, and then the 0xc0 bits are
256  and }, but the value of the character code must be less than 256 in non-UTF-8  flipped.)
257  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in  .P
258  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code  By default, after \ex, from zero to two hexadecimal digits are read (letters
259  point, which is 10FFFF.  can be in upper or lower case). Any number of hexadecimal digits may appear
260    between \ex{ and }, but the character code is constrained as follows:
261    .sp
262      8-bit non-UTF mode    less than 0x100
263      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 259  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 278  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 287  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), and the  interpreted as the backspace character (hex 08).
340  sequences \eR and \eX are interpreted as the characters "R" and "X",  .P
341  respectively. Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
342  meanings  inside a character class. Like other unrecognized escape sequences, they are
343  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
344  .\" </a>  error if the PCRE_EXTRA option is set. Outside a character class, these
345  (see below).  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 326  syntax for referencing a subpattern as a Line 383  syntax for referencing a subpattern as a
383  later.  later.
384  .\"  .\"
385  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
386  synonymous. The former is a back reference; the latter is a subroutine call.  synonymous. The former is a back reference; the latter is a
387    .\" HTML <a href="#subpatternsassubroutines">
388    .\" </a>
389    subroutine
390    .\"
391    call.
392  .  .
393  .  .
394    .\" HTML <a name="genericchartypes"></a>
395  .SS "Generic character types"  .SS "Generic character types"
396  .rs  .rs
397  .sp  .sp
398  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
399  .sp  .sp
400    \ed     any decimal digit    \ed     any decimal digit
401    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
# Line 346  following are always recognized: Line 408  following are always recognized:
408    \ew     any "word" character    \ew     any "word" character
409    \eW     any "non-word" character    \eW     any "non-word" character
410  .sp  .sp
411  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.
412  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
413  .P  .\" HTML <a href="#fullstopdot">
414  These character type sequences can appear both inside and outside character  .\" </a>
415    the "." metacharacter
416    .\"
417    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
418    PCRE does not support this.
419    .P
420    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
422    one, of each pair. The sequences can appear both inside and outside character
423  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
424  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
425  there is no character to match.  there is no character to match.
426  .P  .P
427  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).
# Line 360  are HT (9), LF (10), FF (12), CR (13), a Line 430  are HT (9), LF (10), FF (12), CR (13), a
430  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
431  does.  does.
432  .P  .P
433  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  A "word" character is an underscore or any character that is a letter or digit.
434  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  By default, the definition of letters and digits is controlled by PCRE's
435  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
436  meanings from before UTF-8 support was available, mainly for efficiency  place (see
437  reasons. Note that this also affects \eb, because it is defined in terms of \ew  .\" HTML <a href="pcreapi.html#localesupport">
438  and \eW.  .\" </a>
439  .P  "Locale support"
440  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  .\"
441  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  in the
442  The horizontal space characters are:  .\" HREF
443    \fBpcreapi\fP
444    .\"
445    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
446    or "french" in Windows, some character codes greater than 128 are used for
447    accented letters, and these are then matched by \ew. The use of locales with
448    Unicode is discouraged.
449    .P
450    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
452    their original meanings from before UTF support was available, mainly for
453    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
455    properties are used to determine character types, as follows:
456    .sp
457      \ed  any character that \ep{Nd} matches (decimal digit)
458      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
459      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
460    .sp
461    The upper case escapes match the inverse sets of characters. Note that \ed
462    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
463    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
464    \eB because they are defined in terms of \ew and \eW. Matching these sequences
465    is noticeably slower when PCRE_UCP is set.
466    .P
467    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
469    characters by default, these always match certain high-valued codepoints,
470    whether or not PCRE_UCP is set. The horizontal space characters are:
471  .sp  .sp
472    U+0009     Horizontal tab    U+0009     Horizontal tab
473    U+0020     Space    U+0020     Space
# Line 400  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  .P  .sp
502  A "word" character is an underscore or any character less than 256 that is a  In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
503  letter or digit. The definition of letters and digits is controlled by PCRE's  relevant.
 low-valued character tables, and may vary if locale-specific matching is taking  
 place (see  
 .\" HTML <a href="pcreapi.html#localesupport">  
 .\" </a>  
 "Locale support"  
 .\"  
 in the  
 .\" HREF  
 \fBpcreapi\fP  
 .\"  
 page). For example, in a French locale such as "fr_FR" in Unix-like systems,  
 or "french" in Windows, some character codes greater than 128 are used for  
 accented letters, and these are matched by \ew. The use of locales with Unicode  
 is discouraged.  
504  .  .
505  .  .
506  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 424  is discouraged. Line 508  is discouraged.
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. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is  Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
512  equivalent to the following:  following:
513  .sp  .sp
514    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
515  .sp  .sp
# Line 440  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 456  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, but  These override the default and the options given to the compiling function, but
544  they can be overridden by options given to \fBpcre_exec()\fP. Note that these  they can themselves be overridden by options given to a matching function. Note
545  special settings, which are not Perl-compatible, are recognized only at the  that these special settings, which are not Perl-compatible, are recognized only
546  very start of a pattern, and that they must be in upper case. If more than one  at the very start of a pattern, and that they must be in upper case. If more
547  of them is present, the last one is used. They can be combined with a change of  than one of them is present, the last one is used. They can be combined with a
548  newline convention, for example, a pattern can start with:  change of newline convention; for example, a pattern can start with:
549  .sp  .sp
550    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
551  .sp  .sp
552  Inside a character class, \eR matches the letter "R".  They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
553    sequences. Inside a character class, \eR is treated as an unrecognized escape
554    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 474  Inside a character class, \eR matches th Line 561  Inside a character class, \eR matches th
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 483  The extra escape sequences are: Line 570  The extra escape sequences are:
570    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
571  .sp  .sp
572  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
573  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
574  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
575  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
576  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
577    .\" </a>
578    next section).
579    .\"
580    Other Perl properties such as "InMusicalSymbols" are not currently supported by
581    PCRE. Note that \eP{Any} does not match any characters, so always causes a
582    match failure.
583  .P  .P
584  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
585  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 500  Those that are not part of an identified Line 593  Those that are not part of an identified
593  .P  .P
594  Arabic,  Arabic,
595  Armenian,  Armenian,
596    Avestan,
597  Balinese,  Balinese,
598    Bamum,
599    Batak,
600  Bengali,  Bengali,
601  Bopomofo,  Bopomofo,
602    Brahmi,
603  Braille,  Braille,
604  Buginese,  Buginese,
605  Buhid,  Buhid,
606  Canadian_Aboriginal,  Canadian_Aboriginal,
607    Carian,
608    Chakma,
609    Cham,
610  Cherokee,  Cherokee,
611  Common,  Common,
612  Coptic,  Coptic,
# Line 515  Cypriot, Line 615  Cypriot,
615  Cyrillic,  Cyrillic,
616  Deseret,  Deseret,
617  Devanagari,  Devanagari,
618    Egyptian_Hieroglyphs,
619  Ethiopic,  Ethiopic,
620  Georgian,  Georgian,
621  Glagolitic,  Glagolitic,
# Line 527  Hangul, Line 628  Hangul,
628  Hanunoo,  Hanunoo,
629  Hebrew,  Hebrew,
630  Hiragana,  Hiragana,
631    Imperial_Aramaic,
632  Inherited,  Inherited,
633    Inscriptional_Pahlavi,
634    Inscriptional_Parthian,
635    Javanese,
636    Kaithi,
637  Kannada,  Kannada,
638  Katakana,  Katakana,
639    Kayah_Li,
640  Kharoshthi,  Kharoshthi,
641  Khmer,  Khmer,
642  Lao,  Lao,
643  Latin,  Latin,
644    Lepcha,
645  Limbu,  Limbu,
646  Linear_B,  Linear_B,
647    Lisu,
648    Lycian,
649    Lydian,
650  Malayalam,  Malayalam,
651    Mandaic,
652    Meetei_Mayek,
653    Meroitic_Cursive,
654    Meroitic_Hieroglyphs,
655    Miao,
656  Mongolian,  Mongolian,
657  Myanmar,  Myanmar,
658  New_Tai_Lue,  New_Tai_Lue,
# Line 544  Nko, Line 660  Nko,
660  Ogham,  Ogham,
661  Old_Italic,  Old_Italic,
662  Old_Persian,  Old_Persian,
663    Old_South_Arabian,
664    Old_Turkic,
665    Ol_Chiki,
666  Oriya,  Oriya,
667  Osmanya,  Osmanya,
668  Phags_Pa,  Phags_Pa,
669  Phoenician,  Phoenician,
670    Rejang,
671  Runic,  Runic,
672    Samaritan,
673    Saurashtra,
674    Sharada,
675  Shavian,  Shavian,
676  Sinhala,  Sinhala,
677    Sora_Sompeng,
678    Sundanese,
679  Syloti_Nagri,  Syloti_Nagri,
680  Syriac,  Syriac,
681  Tagalog,  Tagalog,
682  Tagbanwa,  Tagbanwa,
683  Tai_Le,  Tai_Le,
684    Tai_Tham,
685    Tai_Viet,
686    Takri,
687  Tamil,  Tamil,
688  Telugu,  Telugu,
689  Thaana,  Thaana,
# Line 563  Thai, Line 691  Thai,
691  Tibetan,  Tibetan,
692  Tifinagh,  Tifinagh,
693  Ugaritic,  Ugaritic,
694    Vai,
695  Yi.  Yi.
696  .P  .P
697  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
698  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
699  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
700  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
701  .P  .P
702  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
703  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 629  the Lu, Ll, or Lt property, in other wor Line 758  the Lu, Ll, or Lt property, in other wor
758  a modifier or "other".  a modifier or "other".
759  .P  .P
760  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
761  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
762  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
763  (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
764  .\" HREF  .\" HREF
765  \fBpcreapi\fP  \fBpcreapi\fP
766  .\"  .\"
767  page).  page). Perl does not support the Cs property.
768  .P  .P
769  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The long synonyms for property names that Perl supports (such as \ep{Letter})
770  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
771  properties with "Is".  properties with "Is".
772  .P  .P
# Line 662  atomic group Line 791  atomic group
791  .\"  .\"
792  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
793  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
794  non-UTF-8 mode \eX matches any one character.  8-bit non-UTF-8 mode \eX matches any one character.
795    .P
796    Note that recent versions of Perl have changed \eX to match what Unicode calls
797    an "extended grapheme cluster", which has a more complicated definition.
798  .P  .P
799  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
800  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
801  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
802  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
803    PCRE_UCP option or by starting the pattern with (*UCP).
804    .
805    .
806    .\" HTML <a name="extraprops"></a>
807    .SS PCRE's additional properties
808    .rs
809    .sp
810    As well as the standard Unicode properties described in the previous
811    section, PCRE supports four more that make it possible to convert traditional
812    escape sequences such as \ew and \es and POSIX character classes to use Unicode
813    properties. PCRE uses these non-standard, non-Perl properties internally when
814    PCRE_UCP is set. They are:
815    .sp
816      Xan   Any alphanumeric character
817      Xps   Any POSIX space character
818      Xsp   Any Perl space character
819      Xwd   Any Perl "word" character
820    .sp
821    Xan matches characters that have either the L (letter) or the N (number)
822    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
823    carriage return, and any other character that has the Z (separator) property.
824    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
825    same characters as Xan, plus underscore.
826  .  .
827  .  .
828  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
829  .SS "Resetting the match start"  .SS "Resetting the match start"
830  .rs  .rs
831  .sp  .sp
832  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK causes any previously matched characters not to be
833  matched characters not to be included in the final matched sequence. For  included in the final matched sequence. For example, the pattern:
 example, the pattern:  
834  .sp  .sp
835    foo\eKbar    foo\eKbar
836  .sp  .sp
# Line 698  For example, when the pattern Line 852  For example, when the pattern
852    (foo)\eKbar    (foo)\eKbar
853  .sp  .sp
854  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
855    .P
856    Perl documents that the use of \eK within assertions is "not well defined". In
857    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
858    ignored in negative assertions.
859  .  .
860  .  .
861  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 722  The backslashed assertions are: Line 880  The backslashed assertions are:
880    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
881    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
882  .sp  .sp
883  These assertions may not appear in character classes (but note that \eb has a  Inside a character class, \eb has a different meaning; it matches the backspace
884  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
885    default it matches the corresponding literal character (for example, \eB
886    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
887    escape sequence" error is generated instead.
888  .P  .P
889  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
890  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
891  \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
892  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
893    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
894    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
895    of word" or "end of word" metasequence. However, whatever follows \eb normally
896    determines which it is. For example, the fragment \eba matches "a" at the start
897    of a word.
898  .P  .P
899  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
900  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 812  end of the subject in both modes, and if Line 978  end of the subject in both modes, and if
978  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
979  .  .
980  .  .
981  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
982    .SH "FULL STOP (PERIOD, DOT) AND \eN"
983  .rs  .rs
984  .sp  .sp
985  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
986  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
987  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
988  .P  .P
989  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
990  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 834  to match it. Line 1001  to match it.
1001  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
1002  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
1003  special meaning in a character class.  special meaning in a character class.
1004  .  .P
1005  .  The escape sequence \eN behaves like a dot, except that it is not affected by
1006  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
1007  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
1008  .sp  name; PCRE does not support this.
1009  Outside a character class, the escape sequence \eC matches any one byte, both  .
1010  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  .
1011  characters. The feature is provided in Perl in order to match individual bytes  .SH "MATCHING A SINGLE DATA UNIT"
1012  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  .rs
1013  what remains in the string may be a malformed UTF-8 string. For this reason,  .sp
1014  the \eC escape sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1015    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1016    byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always
1017    matches line-ending characters. The feature is provided in Perl in order to
1018    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1019    used. Because \eC breaks up characters into individual data units, matching one
1020    unit with \eC in a UTF mode means that the rest of the string may start with a
1021    malformed UTF character. This has undefined results, because PCRE assumes that
1022    it is dealing with valid UTF strings (and by default it checks this at the
1023    start of processing unless the PCRE_NO_UTF8_CHECK option is used).
1024  .P  .P
1025  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1026  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1027  .\" </a>  .\" </a>
1028  (described below),  (described below)
1029  .\"  .\"
1030  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
1031  the lookbehind.  the lookbehind.
1032    .P
1033    In general, the \eC escape sequence is best avoided. However, one
1034    way of using it that avoids the problem of malformed UTF characters is to use a
1035    lookahead to check the length of the next character, as in this pattern, which
1036    could be used with a UTF-8 string (ignore white space and line breaks):
1037    .sp
1038      (?| (?=[\ex00-\ex7f])(\eC) |
1039          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1040          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1041          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1042    .sp
1043    A group that starts with (?| resets the capturing parentheses numbers in each
1044    alternative (see
1045    .\" HTML <a href="#dupsubpatternnumber">
1046    .\" </a>
1047    "Duplicate Subpattern Numbers"
1048    .\"
1049    below). The assertions at the start of each branch check the next UTF-8
1050    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1051    character's individual bytes are then captured by the appropriate number of
1052    groups.
1053  .  .
1054  .  .
1055  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 860  the lookbehind. Line 1057  the lookbehind.
1057  .rs  .rs
1058  .sp  .sp
1059  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1060  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.
1061  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
1062  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
1063  escaped with a backslash.  a member of the class, it should be the first data character in the class
1064  .P  (after an initial circumflex, if present) or escaped with a backslash.
1065  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1066  character may occupy more than one byte. A matched character must be in the set  A character class matches a single character in the subject. In a UTF mode, the
1067  of characters defined by the class, unless the first character in the class  character may be more than one data unit long. A matched character must be in
1068  definition is a circumflex, in which case the subject character must not be in  the set of characters defined by the class, unless the first character in the
1069  the set defined by the class. If a circumflex is actually required as a member  class definition is a circumflex, in which case the subject character must not
1070  of the class, ensure it is not the first character, or escape it with a  be in the set defined by the class. If a circumflex is actually required as a
1071    member of the class, ensure it is not the first character, or escape it with a
1072  backslash.  backslash.
1073  .P  .P
1074  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1075  [^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
1076  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1077  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
1078  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
1079  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
1080  string.  string.
1081  .P  .P
1082  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
1083  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{
1084    escaping mechanism.
1085  .P  .P
1086  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
1087  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
1088  "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
1089  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
1090  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
1091  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1092  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1093  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in a UTF mode for characters 128 and
1094  ensure that PCRE is compiled with Unicode property support as well as with  above, you must ensure that PCRE is compiled with Unicode property support as
1095  UTF-8 support.  well as with UTF support.
1096  .P  .P
1097  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
1098  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 915  followed by two other characters. The oc Line 1114  followed by two other characters. The oc
1114  "]" can also be used to end a range.  "]" can also be used to end a range.
1115  .P  .P
1116  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
1117  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1118  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}].  
1119  .P  .P
1120  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
1121  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
1122  [][\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
1123  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
1124  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
1125  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
1126  property support.  property support.
1127  .P  .P
1128  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1129  in a character class, and add the characters that they match to the class. For  \eV, \ew, and \eW may appear in a character class, and add the characters that
1130  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1131  conveniently be used with the upper case character types to specify a more  digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1132  restricted set of characters than the matching lower case type. For example,  and their upper case partners, just as it does when they appear outside a
1133  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1134    .\" HTML <a href="#genericchartypes">
1135    .\" </a>
1136    "Generic character types"
1137    .\"
1138    above. The escape sequence \eb has a different meaning inside a character
1139    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1140    are not special inside a character class. Like any other unrecognized escape
1141    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1142    default, but cause an error if the PCRE_EXTRA option is set.
1143    .P
1144    A circumflex can conveniently be used with the upper case character types to
1145    specify a more restricted set of characters than the matching lower case type.
1146    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1147    whereas [\ew] includes underscore. A positive character class should be read as
1148    "something OR something OR ..." and a negative class as "NOT something AND NOT
1149    something AND NOT ...".
1150  .P  .P
1151  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1152  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 952  this notation. For example, Line 1166  this notation. For example,
1166    [01[:alpha:]%]    [01[:alpha:]%]
1167  .sp  .sp
1168  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1169  are  are:
1170  .sp  .sp
1171    alnum    letters and digits    alnum    letters and digits
1172    alpha    letters    alpha    letters
# Line 963  are Line 1177  are
1177    graph    printing characters, excluding space    graph    printing characters, excluding space
1178    lower    lower case letters    lower    lower case letters
1179    print    printing characters, including space    print    printing characters, including space
1180    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1181    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1182    upper    upper case letters    upper    upper case letters
1183    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 984  matches "1", "2", or any non-digit. PCRE Line 1198  matches "1", "2", or any non-digit. PCRE
1198  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
1199  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1200  .P  .P
1201  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF modes, characters with values greater than 128 do not match
1202  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1203    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1204    character properties are used. This is achieved by replacing the POSIX classes
1205    by other sequences, as follows:
1206    .sp
1207      [:alnum:]  becomes  \ep{Xan}
1208      [:alpha:]  becomes  \ep{L}
1209      [:blank:]  becomes  \eh
1210      [:digit:]  becomes  \ep{Nd}
1211      [:lower:]  becomes  \ep{Ll}
1212      [:space:]  becomes  \ep{Xps}
1213      [:upper:]  becomes  \ep{Lu}
1214      [:word:]   becomes  \ep{Xwd}
1215    .sp
1216    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1217    classes are unchanged, and match only characters with code points less than
1218    128.
1219  .  .
1220  .  .
1221  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1032  The PCRE-specific options PCRE_DUPNAMES, Line 1262  The PCRE-specific options PCRE_DUPNAMES,
1262  changed in the same way as the Perl-compatible options by using the characters  changed in the same way as the Perl-compatible options by using the characters
1263  J, U and X respectively.  J, U and X respectively.
1264  .P  .P
1265  When an option change occurs at top level (that is, not inside subpattern  When one of these option changes occurs at top level (that is, not inside
1266  parentheses), the change applies to the remainder of the pattern that follows.  subpattern parentheses), the change applies to the remainder of the pattern
1267  If the change is placed right at the start of a pattern, PCRE extracts it into  that follows. If the change is placed right at the start of a pattern, PCRE
1268  the global options (and it will therefore show up in data extracted by the  extracts it into the global options (and it will therefore show up in data
1269  \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1270  .P  .P
1271  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1272  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1273  .sp  .sp
1274    (a(?i)b)c    (a(?i)b)c
1275  .sp  .sp
# Line 1056  option settings happen at compile time. Line 1286  option settings happen at compile time.
1286  behaviour otherwise.  behaviour otherwise.
1287  .P  .P
1288  \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
1289  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
1290  pattern can contain special leading sequences to override what the application  the pattern can contain special leading sequences such as (*CRLF) to override
1291  has set or what has been defaulted. Details are given in the section entitled  what the application has set or what has been defaulted. Details are given in
1292    the section entitled
1293  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
1294  .\" </a>  .\" </a>
1295  "Newline sequences"  "Newline sequences"
1296  .\"  .\"
1297  above.  above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1298    can be used to set UTF and Unicode property modes; they are equivalent to
1299    setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1300  .  .
1301  .  .
1302  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1077  Turning part of a pattern into a subpatt Line 1310  Turning part of a pattern into a subpatt
1310  .sp  .sp
1311    cat(aract|erpillar|)    cat(aract|erpillar|)
1312  .sp  .sp
1313  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1314  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1315  .sp  .sp
1316  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
1317  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
1318  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
1319  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1320  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1321  .P  .P
1322  For example, if the string "the red king" is matched against the pattern  Opening parentheses are counted from left to right (starting from 1) to obtain
1323    numbers for the capturing subpatterns. For example, if the string "the red
1324    king" is matched against the pattern
1325  .sp  .sp
1326    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1327  .sp  .sp
# Line 1118  is reached, an option setting in one bra Line 1353  is reached, an option setting in one bra
1353  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1354  .  .
1355  .  .
1356    .\" HTML <a name="dupsubpatternnumber"></a>
1357  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1358  .rs  .rs
1359  .sp  .sp
# Line 1134  at captured substring number one, whiche Line 1370  at captured substring number one, whiche
1370  is useful when you want to capture part, but not all, of one of a number of  is useful when you want to capture part, but not all, of one of a number of
1371  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1372  number is reset at the start of each branch. The numbers of any capturing  number is reset at the start of each branch. The numbers of any capturing
1373  buffers that follow the subpattern start after the highest number used in any  parentheses that follow the subpattern start after the highest number used in
1374  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1375  The numbers underneath show in which buffer the captured content will be  numbers underneath show in which buffer the captured content will be stored.
 stored.  
1376  .sp  .sp
1377    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1378    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1379    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1380  .sp  .sp
1381  A backreference or a recursive call to a numbered subpattern always refers to  A back reference to a numbered subpattern uses the most recent value that is
1382  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1383    or "defdef":
1384    .sp
1385      /(?|(abc)|(def))\e1/
1386    .sp
1387    In contrast, a subroutine call to a numbered subpattern always refers to the
1388    first one in the pattern with the given number. The following pattern matches
1389    "abcabc" or "defabc":
1390    .sp
1391      /(?|(abc)|(def))(?1)/
1392    .sp
1393    If a
1394    .\" HTML <a href="#conditions">
1395    .\" </a>
1396    condition test
1397    .\"
1398    for a subpattern's having matched refers to a non-unique number, the test is
1399    true if any of the subpatterns of that number have matched.
1400  .P  .P
1401  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1402  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
# Line 1159  if an expression is modified, the number Line 1411  if an expression is modified, the number
1411  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1412  added to Perl until release 5.10. Python had the feature earlier, and PCRE  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1413  introduced it at release 4.0, using the Python syntax. PCRE now supports both  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1414  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1415    have different names, but PCRE does not.
1416  .P  .P
1417  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1418  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1419  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1420  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1421  .\" </a>  .\" </a>
1422  backreferences,  back references,
1423  .\"  .\"
1424  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1425  .\" </a>  .\" </a>
# Line 1186  extracting the name-to-number translatio Line 1439  extracting the name-to-number translatio
1439  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1440  .P  .P
1441  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
1442  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
1443  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
1444  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
1445  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
1446  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
1447    name, and in both cases you want to extract the abbreviation. This pattern
1448    (ignoring the line breaks) does the job:
1449  .sp  .sp
1450    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1451    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1204  subpattern, as described in the previous Line 1459  subpattern, as described in the previous
1459  .P  .P
1460  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1461  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
1462  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1463  make a reference to a non-unique named subpattern from elsewhere in the  .P
1464  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
1465  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1466    used. In the absence of duplicate numbers (see the previous section) this is
1467    the one with the lowest number. If you use a named reference in a condition
1468    test (see the
1469    .\"
1470    .\" HTML <a href="#conditions">
1471    .\" </a>
1472    section about conditions
1473    .\"
1474    below), either to check whether a subpattern has matched, or to check for
1475    recursion, all subpatterns with the same name are tested. If the condition is
1476    true for any one of them, the overall condition is true. This is the same
1477    behaviour as testing by number. For further details of the interfaces for
1478    handling named subpatterns, see the
1479  .\" HREF  .\" HREF
1480  \fBpcreapi\fP  \fBpcreapi\fP
1481  .\"  .\"
1482  documentation.  documentation.
1483  .P  .P
1484  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1485  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1486  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1487    are given to subpatterns with the same number. However, you can give the same
1488    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1489  .  .
1490  .  .
1491  .SH REPETITION  .SH REPETITION
# Line 1227  items: Line 1497  items:
1497    a literal data character    a literal data character
1498    the dot metacharacter    the dot metacharacter
1499    the \eC escape sequence    the \eC escape sequence
1500    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1501    the \eR escape sequence    the \eR escape sequence
1502    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1503    a character class    a character class
1504    a back reference (see next section)    a back reference (see next section)
1505    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1506      a subroutine call to a subpattern (recursive or otherwise)
1507  .sp  .sp
1508  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1509  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1257  where a quantifier is not allowed, or on Line 1528  where a quantifier is not allowed, or on
1528  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
1529  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1530  .P  .P
1531  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
1532  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
1533  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,
1534  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
1535  which may be several bytes long (and they may be of different lengths).  data units long (and they may be of different lengths).
1536  .P  .P
1537  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
1538  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 1270  subpatterns that are referenced as Line 1541  subpatterns that are referenced as
1541  .\" </a>  .\" </a>
1542  subroutines  subroutines
1543  .\"  .\"
1544  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1545  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1546    .\" </a>
1547    "Defining subpatterns for use by reference only"
1548    .\"
1549    below). Items other than subpatterns that have a {0} quantifier are omitted
1550    from the compiled pattern.
1551  .P  .P
1552  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1553  abbreviations:  abbreviations:
# Line 1343  worth setting PCRE_DOTALL in order to ob Line 1619  worth setting PCRE_DOTALL in order to ob
1619  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1620  .P  .P
1621  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1622  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1623  elsewhere in the pattern, a match at the start may fail where a later one  elsewhere in the pattern, a match at the start may fail where a later one
1624  succeeds. Consider, for example:  succeeds. Consider, for example:
1625  .sp  .sp
# Line 1496  no such problem when named parentheses a Line 1772  no such problem when named parentheses a
1772  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1773  .P  .P
1774  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1775  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence. This escape must be followed by an
1776  Perl 5.10. This escape must be followed by an unsigned number or a negative  unsigned number or a negative number, optionally enclosed in braces. These
1777  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1778  .sp  .sp
1779    (ring), \e1    (ring), \e1
1780    (ring), \eg1    (ring), \eg1
# Line 1512  example: Line 1788  example:
1788    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1789  .sp  .sp
1790  The sequence \eg{-1} is a reference to the most recently started capturing  The sequence \eg{-1} is a reference to the most recently started capturing
1791  subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}  subpattern before \eg, that is, is it equivalent to \e2 in this example.
1792  would be equivalent to \e1. The use of relative references can be helpful in  Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1793  long patterns, and also in patterns that are created by joining together  can be helpful in long patterns, and also in patterns that are created by
1794  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1795  .P  .P
1796  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1797  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1554  after the reference. Line 1830  after the reference.
1830  .P  .P
1831  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
1832  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1833  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1834  .sp  .sp
1835    (a|(bc))\e2    (a|(bc))\e2
1836  .sp  .sp
1837  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
1838  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
1839  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1840  with a digit character, some delimiter must be used to terminate the back  .P
1841  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1842  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1843    If the pattern continues with a digit character, some delimiter must be used to
1844    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1845    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1846  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1847  .\" </a>  .\" </a>
1848  "Comments"  "Comments"
1849  .\"  .\"
1850  below) can be used.  below) can be used.
1851  .P  .
1852    .SS "Recursive back references"
1853    .rs
1854    .sp
1855  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
1856  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.
1857  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1583  to the previous iteration. In order for Line 1865  to the previous iteration. In order for
1865  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
1866  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
1867  minimum of zero.  minimum of zero.
1868    .P
1869    Back references of this type cause the group that they reference to be treated
1870    as an
1871    .\" HTML <a href="#atomicgroup">
1872    .\" </a>
1873    atomic group.
1874    .\"
1875    Once the whole group has been matched, a subsequent matching failure cannot
1876    cause backtracking into the middle of the group.
1877  .  .
1878  .  .
1879  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1602  those that look ahead of the current pos Line 1893  those that look ahead of the current pos
1893  that look behind it. An assertion subpattern is matched in the normal way,  that look behind it. An assertion subpattern is matched in the normal way,
1894  except that it does not cause the current matching position to be changed.  except that it does not cause the current matching position to be changed.
1895  .P  .P
1896  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1897  because it makes no sense to assert the same thing several times. If any kind  contains capturing subpatterns within it, these are counted for the purposes of
1898  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1899  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1900  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1901  because it does not make sense for negative assertions.  .P
1902    For compatibility with Perl, assertion subpatterns may be repeated; though
1903    it makes no sense to assert the same thing several times, the side effect of
1904    capturing parentheses may occasionally be useful. In practice, there only three
1905    cases:
1906    .sp
1907    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1908    However, it may contain internal capturing parenthesized groups that are called
1909    from elsewhere via the
1910    .\" HTML <a href="#subpatternsassubroutines">
1911    .\" </a>
1912    subroutine mechanism.
1913    .\"
1914    .sp
1915    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1916    were {0,1}. At run time, the rest of the pattern match is tried with and
1917    without the assertion, the order depending on the greediness of the quantifier.
1918    .sp
1919    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1920    The assertion is obeyed just once when encountered during matching.
1921  .  .
1922  .  .
1923  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1636  lookbehind assertion is needed to achiev Line 1946  lookbehind assertion is needed to achiev
1946  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
1947  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
1948  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.
1949    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1950  .  .
1951  .  .
1952  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1660  is permitted, but Line 1971  is permitted, but
1971  .sp  .sp
1972  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1973  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
1974  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1975  match the same length of string. An assertion such as  length of string. An assertion such as
1976  .sp  .sp
1977    (?<=ab(c|de))    (?<=ab(c|de))
1978  .sp  .sp
1979  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
1980  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
1981    branches:
1982  .sp  .sp
1983    (?<=abc|abde)    (?<=abc|abde)
1984  .sp  .sp
1985  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1986  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1987  .\" </a>  .\" </a>
1988  (see above)  (see above)
1989  .\"  .\"
1990  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
1991  fixed-length.  restriction.
1992  .P  .P
1993  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1994  temporarily move the current position back by the fixed length and then try to  temporarily move the current position back by the fixed length and then try to
1995  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1996  assertion fails.  assertion fails.
1997  .P  .P
1998  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
1999  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
2000  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
2001  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2002    permitted.
2003    .P
2004    .\" HTML <a href="#subpatternsassubroutines">
2005    .\" </a>
2006    "Subroutine"
2007    .\"
2008    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2009    as the subpattern matches a fixed-length string.
2010    .\" HTML <a href="#recursion">
2011    .\" </a>
2012    Recursion,
2013    .\"
2014    however, is not supported.
2015  .P  .P
2016  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
2017  specify efficient matching at the end of the subject string. Consider a simple  specify efficient matching of fixed-length strings at the end of subject
2018  pattern such as  strings. Consider a simple pattern such as
2019  .sp  .sp
2020    abcd$    abcd$
2021  .sp  .sp
# Line 1754  characters that are not "999". Line 2079  characters that are not "999".
2079  .sp  .sp
2080  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2081  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2082  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2083  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2084  .sp  .sp
2085    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2086    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2087  .sp  .sp
2088  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2089  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
2090  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2091    itself contain nested subpatterns of any form, including conditional
2092    subpatterns; the restriction to two alternatives applies only at the level of
2093    the condition. This pattern fragment is an example where the alternatives are
2094    complex:
2095    .sp
2096      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2097    .sp
2098  .P  .P
2099  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2100  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1771  recursion, a pseudo-condition called DEF Line 2103  recursion, a pseudo-condition called DEF
2103  .rs  .rs
2104  .sp  .sp
2105  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
2106  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2107  matched. An alternative notation is to precede the digits with a plus or minus  matched. If there is more than one capturing subpattern with the same number
2108  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2109  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2110  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2111  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2112    section about duplicate subpattern numbers),
2113    .\"
2114    the condition is true if any of them have matched. An alternative notation is
2115    to precede the digits with a plus or minus sign. In this case, the subpattern
2116    number is relative rather than absolute. The most recently opened parentheses
2117    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2118    loops it can also make sense to refer to subsequent groups. The next
2119    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2120    zero in any of these forms is not used; it provokes a compile-time error.)
2121  .P  .P
2122  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2123  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 1787  three parts for ease of discussion: Line 2128  three parts for ease of discussion:
2128  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2129  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
2130  matches one or more characters that are not parentheses. The third part is a  matches one or more characters that are not parentheses. The third part is a
2131  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2132  or not. If they did, that is, if subject started with an opening parenthesis,  matched. If they did, that is, if subject started with an opening parenthesis,
2133  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
2134  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2135  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
# Line 1817  Rewriting the above example to use a nam Line 2158  Rewriting the above example to use a nam
2158  .sp  .sp
2159    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2160  .sp  .sp
2161    If the name used in a condition of this kind is a duplicate, the test is
2162    applied to all subpatterns of the same name, and is true if any one of them has
2163    matched.
2164  .  .
2165  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2166  .rs  .rs
# Line 1828  letter R, for example: Line 2172  letter R, for example:
2172  .sp  .sp
2173    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2174  .sp  .sp
2175  the condition is true if the most recent recursion is into the subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
2176  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
2177  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2178    applied to all subpatterns of the same name, and is true if any one of them is
2179    the most recent recursion.
2180  .P  .P
2181  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2182  patterns are described below.  .\" HTML <a href="#recursion">
2183    .\" </a>
2184    The syntax for recursive patterns
2185    .\"
2186    is described below.
2187  .  .
2188    .\" HTML <a name="subdefine"></a>
2189  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2190  .rs  .rs
2191  .sp  .sp
# Line 1842  If the condition is the string (DEFINE), Line 2193  If the condition is the string (DEFINE),
2193  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
2194  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2195  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
2196  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2197  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2198  written like this (ignore whitespace and line breaks):  .\" </a>
2199    subroutines
2200    .\"
2201    is described below.) For example, a pattern to match an IPv4 address such as
2202    "192.168.23.245" could be written like this (ignore whitespace and line
2203    breaks):
2204  .sp  .sp
2205    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2206    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1852  written like this (ignore whitespace and Line 2208  written like this (ignore whitespace and
2208  The first part of the pattern is a DEFINE group inside which a another group  The first part of the pattern is a DEFINE group inside which a another group
2209  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2210  address (a number less than 256). When matching takes place, this part of the  address (a number less than 256). When matching takes place, this part of the
2211  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2212  .P  pattern uses references to the named group to match the four dot-separated
2213  The rest of the pattern uses references to the named group to match the four  components of an IPv4 address, insisting on a word boundary at each end.
 dot-separated components of an IPv4 address, insisting on a word boundary at  
 each end.  
2214  .  .
2215  .SS "Assertion conditions"  .SS "Assertion conditions"
2216  .rs  .rs
# Line 1881  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2235  dd-aaa-dd or dd-dd-dd, where aaa are let
2235  .SH COMMENTS  .SH COMMENTS
2236  .rs  .rs
2237  .sp  .sp
2238  The sequence (?# marks the start of a comment that continues up to the next  There are two ways of including comments in patterns that are processed by
2239  closing parenthesis. Nested parentheses are not permitted. The characters  PCRE. In both cases, the start of the comment must not be in a character class,
2240  that make up a comment play no part in the pattern matching at all.  nor in the middle of any other sequence of related characters such as (?: or a
2241    subpattern name or number. The characters that make up a comment play no part
2242    in the pattern matching.
2243  .P  .P
2244  If the PCRE_EXTENDED option is set, an unescaped # character outside a  The sequence (?# marks the start of a comment that continues up to the next
2245  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2246  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2247    this case continues to immediately after the next newline character or
2248    character sequence in the pattern. Which characters are interpreted as newlines
2249    is controlled by the options passed to a compiling function or by a special
2250    sequence at the start of the pattern, as described in the section entitled
2251    .\" HTML <a href="#newlines">
2252    .\" </a>
2253    "Newline conventions"
2254    .\"
2255    above. Note that the end of this type of comment is a literal newline sequence
2256    in the pattern; escape sequences that happen to represent a newline do not
2257    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2258    default newline convention is in force:
2259    .sp
2260      abc #comment \en still comment
2261    .sp
2262    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2263    a newline in the pattern. The sequence \en is still literal at this stage, so
2264    it does not terminate the comment. Only an actual character with the code value
2265    0x0a (the default newline) does so.
2266  .  .
2267  .  .
2268  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1913  recursively to the pattern in which it a Line 2288  recursively to the pattern in which it a
2288  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2289  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2290  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2291  this kind of recursion was introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2292  .P  .P
2293  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
2294  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
2295  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  given number, provided that it occurs inside that subpattern. (If not, it is a
2296    .\" HTML <a href="#subpatternsassubroutines">
2297    .\" </a>
2298    non-recursive subroutine
2299    .\"
2300  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
2301  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2302  .P  .P
 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  
 treated as an atomic group. That is, once it has matched some of the subject  
 string, it is never re-entered, even if it contains untried alternatives and  
 there is a subsequent matching failure.  
 .P  
2303  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2304  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2305  .sp  .sp
2306    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2307  .sp  .sp
2308  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2309  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
2310  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2311  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2312    to avoid backtracking into sequences of non-parentheses.
2313  .P  .P
2314  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
2315  pattern, so instead you could use this:  pattern, so instead you could use this:
2316  .sp  .sp
2317    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2318  .sp  .sp
2319  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
2320  them instead of the whole pattern.  them instead of the whole pattern.
2321  .P  .P
2322  In a larger pattern, keeping track of parenthesis numbers can be tricky. This  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2323  is made easier by the use of relative references. (A Perl 5.10 feature.)  is made easier by the use of relative references. Instead of (?1) in the
2324  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  pattern above you can write (?-2) to refer to the second most recently opened
2325  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2326  negative number counts capturing parentheses leftwards from the point at which  capturing parentheses leftwards from the point at which it is encountered.
 it is encountered.  
2327  .P  .P
2328  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2329  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2330  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2331  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2332    .\" </a>
2333    non-recursive subroutine
2334    .\"
2335    calls, as described in the next section.
2336  .P  .P
2337  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2338  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2339  could rewrite the above example as follows:  could rewrite the above example as follows:
2340  .sp  .sp
2341    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2342  .sp  .sp
2343  If there is more than one subpattern with the same name, the earliest one is  If there is more than one subpattern with the same name, the earliest one is
2344  used.  used.
2345  .P  .P
2346  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2347  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of a possessive quantifier for matching
2348  non-parentheses is important when applying the pattern to strings that do not  strings of non-parentheses is important when applying the pattern to strings
2349  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2350  .sp  .sp
2351    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2352  .sp  .sp
2353  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,
2354  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
2355  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
2356  before failure can be reported.  before failure can be reported.
2357  .P  .P
2358  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
2359  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
2360  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2361  .\" HREF  .\" HREF
2362  \fBpcrecallout\fP  \fBpcrecallout\fP
2363  .\"  .\"
# Line 1988  documentation). If the pattern above is Line 2365  documentation). If the pattern above is
2365  .sp  .sp
2366    (ab(cd)ef)    (ab(cd)ef)
2367  .sp  .sp
2368  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
2369  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
2370  .sp  matched at the top level, its final captured value is unset, even if it was
2371    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2372       ^                        ^  .P
2373       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2374  .sp  obtain extra memory to store data during a recursion, which it does by using
2375  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
2376  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.  
2377  .P  .P
2378  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.
2379  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 2013  different alternatives for the recursive Line 2387  different alternatives for the recursive
2387  is the actual recursive call.  is the actual recursive call.
2388  .  .
2389  .  .
2390    .\" HTML <a name="recursiondifference"></a>
2391    .SS "Differences in recursion processing between PCRE and Perl"
2392    .rs
2393    .sp
2394    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2395    (like Python, but unlike Perl), a recursive subpattern call is always treated
2396    as an atomic group. That is, once it has matched some of the subject string, it
2397    is never re-entered, even if it contains untried alternatives and there is a
2398    subsequent matching failure. This can be illustrated by the following pattern,
2399    which purports to match a palindromic string that contains an odd number of
2400    characters (for example, "a", "aba", "abcba", "abcdcba"):
2401    .sp
2402      ^(.|(.)(?1)\e2)$
2403    .sp
2404    The idea is that it either matches a single character, or two identical
2405    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2406    it does not if the pattern is longer than three characters. Consider the
2407    subject string "abcba":
2408    .P
2409    At the top level, the first character is matched, but as it is not at the end
2410    of the string, the first alternative fails; the second alternative is taken
2411    and the recursion kicks in. The recursive call to subpattern 1 successfully
2412    matches the next character ("b"). (Note that the beginning and end of line
2413    tests are not part of the recursion).
2414    .P
2415    Back at the top level, the next character ("c") is compared with what
2416    subpattern 2 matched, which was "a". This fails. Because the recursion is
2417    treated as an atomic group, there are now no backtracking points, and so the
2418    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2419    try the second alternative.) However, if the pattern is written with the
2420    alternatives in the other order, things are different:
2421    .sp
2422      ^((.)(?1)\e2|.)$
2423    .sp
2424    This time, the recursing alternative is tried first, and continues to recurse
2425    until it runs out of characters, at which point the recursion fails. But this
2426    time we do have another alternative to try at the higher level. That is the big
2427    difference: in the previous case the remaining alternative is at a deeper
2428    recursion level, which PCRE cannot use.
2429    .P
2430    To change the pattern so that it matches all palindromic strings, not just
2431    those with an odd number of characters, it is tempting to change the pattern to
2432    this:
2433    .sp
2434      ^((.)(?1)\e2|.?)$
2435    .sp
2436    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2437    deeper recursion has matched a single character, it cannot be entered again in
2438    order to match an empty string. The solution is to separate the two cases, and
2439    write out the odd and even cases as alternatives at the higher level:
2440    .sp
2441      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2442    .sp
2443    If you want to match typical palindromic phrases, the pattern has to ignore all
2444    non-word characters, which can be done like this:
2445    .sp
2446      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2447    .sp
2448    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2449    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2450    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2451    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2452    more) to match typical phrases, and Perl takes so long that you think it has
2453    gone into a loop.
2454    .P
2455    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2456    string does not start with a palindrome that is shorter than the entire string.
2457    For example, although "abcba" is correctly matched, if the subject is "ababa",
2458    PCRE finds the palindrome "aba" at the start, then fails at top level because
2459    the end of the string does not follow. Once again, it cannot jump back into the
2460    recursion to try other alternatives, so the entire match fails.
2461    .P
2462    The second way in which PCRE and Perl differ in their recursion processing is
2463    in the handling of captured values. In Perl, when a subpattern is called
2464    recursively or as a subpattern (see the next section), it has no access to any
2465    values that were captured outside the recursion, whereas in PCRE these values
2466    can be referenced. Consider this pattern:
2467    .sp
2468      ^(.)(\e1|a(?2))
2469    .sp
2470    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2471    then in the second group, when the back reference \e1 fails to match "b", the
2472    second alternative matches "a" and then recurses. In the recursion, \e1 does
2473    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2474    match because inside the recursive call \e1 cannot access the externally set
2475    value.
2476    .
2477    .
2478  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2479  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2480  .rs  .rs
2481  .sp  .sp
2482  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
2483  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
2484  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2485  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2486  relative, as in these examples:  relative, as in these examples:
2487  .sp  .sp
# Line 2039  matches "sense and sensibility" and "res Line 2501  matches "sense and sensibility" and "res
2501  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
2502  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2503  .P  .P
2504  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2505  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
2506  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
2507  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2508  .P  subroutine call revert to their previous values afterwards.
2509  When a subpattern is used as a subroutine, processing options such as  .P
2510  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2511  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2512    different calls. For example, consider this pattern:
2513  .sp  .sp
2514    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2515  .sp  .sp
# Line 2085  same pair of parentheses when there is a Line 2548  same pair of parentheses when there is a
2548  .P  .P
2549  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
2550  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
2551  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
2552  By default, this variable contains NULL, which disables all calling out.  (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this
2553    variable contains NULL, which disables all calling out.
2554  .P  .P
2555  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2556  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 2095  For example, this pattern has two callou Line 2559  For example, this pattern has two callou
2559  .sp  .sp
2560    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2561  .sp  .sp
2562  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
2563  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2564  255.  255.
2565  .P  .P
2566  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
2567  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
2568  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
2569  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  the matching function. The callout function may cause matching to proceed, to
2570  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
2571  description of the interface to the callout function is given in the  the callout function is given in the
2572  .\" HREF  .\" HREF
2573  \fBpcrecallout\fP  \fBpcrecallout\fP
2574  .\"  .\"
2575  documentation.  documentation.
2576  .  .
2577  .  .
2578    .\" HTML <a name="backtrackcontrol"></a>
2579  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2580  .rs  .rs
2581  .sp  .sp
# Line 2121  production code should be noted to avoid Line 2586  production code should be noted to avoid
2586  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2587  .P  .P
2588  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
2589  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
2590  a backtracking algorithm. With the exception of (*FAIL), which behaves like a  matching functions, which use a backtracking algorithm. With the exception of
2591  failing negative assertion, they cause an error if encountered by  (*FAIL), which behaves like a failing negative assertion, they cause an error
2592  \fBpcre_dfa_exec()\fP.  if encountered by a DFA matching function.
2593    .P
2594    If any of these verbs are used in an assertion or in a subpattern that is
2595    called as a subroutine (whether or not recursively), their effect is confined
2596    to that subpattern; it does not extend to the surrounding pattern, with one
2597    exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2598    a successful positive assertion \fIis\fP passed back when a match succeeds
2599    (compare capturing parentheses in assertions). Note that such subpatterns are
2600    processed as anchored at the point where they are tested. Note also that Perl's
2601    treatment of subroutines is different in some cases.
2602  .P  .P
2603  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
2604  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2605  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2606  form is just (*VERB). Any number of these verbs may occur in a pattern. There  depending on whether or not an argument is present. A name is any sequence of
2607  are two kinds:  characters that does not include a closing parenthesis. If the name is empty,
2608    that is, if the closing parenthesis immediately follows the colon, the effect
2609    is as if the colon were not there. Any number of these verbs may occur in a
2610    pattern.
2611    .
2612    .
2613    .\" HTML <a name="nooptimize"></a>
2614    .SS "Optimizations that affect backtracking verbs"
2615    .rs
2616    .sp
2617    PCRE contains some optimizations that are used to speed up matching by running
2618    some checks at the start of each match attempt. For example, it may know the
2619    minimum length of matching subject, or that a particular character must be
2620    present. When one of these optimizations suppresses the running of a match, any
2621    included backtracking verbs will not, of course, be processed. You can suppress
2622    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2623    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2624    pattern with (*NO_START_OPT). There is more discussion of this option in the
2625    section entitled
2626    .\" HTML <a href="pcreapi.html#execoptions">
2627    .\" </a>
2628    "Option bits for \fBpcre_exec()\fP"
2629    .\"
2630    in the
2631    .\" HREF
2632    \fBpcreapi\fP
2633    .\"
2634    documentation.
2635    .P
2636    Experiments with Perl suggest that it too has similar optimizations, sometimes
2637    leading to anomalous results.
2638    .
2639  .  .
2640  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2641  .rs  .rs
2642  .sp  .sp
2643  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2644    followed by a name.
2645  .sp  .sp
2646     (*ACCEPT)     (*ACCEPT)
2647  .sp  .sp
2648  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
2649  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2650  immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside  subroutine, only that subpattern is ended successfully. Matching then continues
2651  capturing parentheses. In Perl, the data so far is captured: in PCRE no data is  at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2652  captured. For example:  far is captured. For example:
2653  .sp  .sp
2654    A(A|B(*ACCEPT)|C)D    A((?:A|B(*ACCEPT)|C)D)
2655  .sp  .sp
2656  This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2657  captured.  the outer parentheses.
2658  .sp  .sp
2659    (*FAIL) or (*F)    (*FAIL) or (*F)
2660  .sp  .sp
2661  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
2662  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
2663  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2664  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 2163  callout feature, as for example in this Line 2669  callout feature, as for example in this
2669  A match with the string "aaaa" always fails, but the callout is taken before  A match with the string "aaaa" always fails, but the callout is taken before
2670  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2671  .  .
2672    .
2673    .SS "Recording which path was taken"
2674    .rs
2675    .sp
2676    There is one verb whose main purpose is to track how a match was arrived at,
2677    though it also has a secondary use in conjunction with advancing the match
2678    starting point (see (*SKIP) below).
2679    .sp
2680      (*MARK:NAME) or (*:NAME)
2681    .sp
2682    A name is always required with this verb. There may be as many instances of
2683    (*MARK) as you like in a pattern, and their names do not have to be unique.
2684    .P
2685    When a match succeeds, the name of the last-encountered (*MARK) on the matching
2686    path is passed back to the caller as described in the section entitled
2687    .\" HTML <a href="pcreapi.html#extradata">
2688    .\" </a>
2689    "Extra data for \fBpcre_exec()\fP"
2690    .\"
2691    in the
2692    .\" HREF
2693    \fBpcreapi\fP
2694    .\"
2695    documentation. Here is an example of \fBpcretest\fP output, where the /K
2696    modifier requests the retrieval and outputting of (*MARK) data:
2697    .sp
2698        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2699      data> XY
2700       0: XY
2701      MK: A
2702      XZ
2703       0: XZ
2704      MK: B
2705    .sp
2706    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2707    indicates which of the two alternatives matched. This is a more efficient way
2708    of obtaining this information than putting each alternative in its own
2709    capturing parentheses.
2710    .P
2711    If (*MARK) is encountered in a positive assertion, its name is recorded and
2712    passed back if it is the last-encountered. This does not happen for negative
2713    assertions.
2714    .P
2715    After a partial match or a failed match, the name of the last encountered
2716    (*MARK) in the entire match process is returned. For example:
2717    .sp
2718        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2719      data> XP
2720      No match, mark = B
2721    .sp
2722    Note that in this unanchored example the mark is retained from the match
2723    attempt that started at the letter "X" in the subject. Subsequent match
2724    attempts starting at "P" and then with an empty string do not get as far as the
2725    (*MARK) item, but nevertheless do not reset it.
2726    .P
2727    If you are interested in (*MARK) values after failed matches, you should
2728    probably set the PCRE_NO_START_OPTIMIZE option
2729    .\" HTML <a href="#nooptimize">
2730    .\" </a>
2731    (see above)
2732    .\"
2733    to ensure that the match is always attempted.
2734    .
2735    .
2736  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2737  .rs  .rs
2738  .sp  .sp
2739  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2740  with what follows, but if there is no subsequent match, a failure is forced.  with what follows, but if there is no subsequent match, causing a backtrack to
2741  The verbs differ in exactly what kind of failure occurs.  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2742    the verb. However, when one of these verbs appears inside an atomic group, its
2743    effect is confined to that group, because once the group has been matched,
2744    there is never any backtracking into it. In this situation, backtracking can
2745    "jump back" to the left of the entire atomic group. (Remember also, as stated
2746    above, that this localization also applies in subroutine calls and assertions.)
2747    .P
2748    These verbs differ in exactly what kind of failure occurs when backtracking
2749    reaches them.
2750  .sp  .sp
2751    (*COMMIT)    (*COMMIT)
2752  .sp  .sp
2753  This verb causes the whole match to fail outright if the rest of the pattern  This verb, which may not be followed by a name, causes the whole match to fail
2754  does not match. Even if the pattern is unanchored, no further attempts to find  outright if the rest of the pattern does not match. Even if the pattern is
2755  a match by advancing the start point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2756  passed, \fBpcre_exec()\fP is committed to finding a match at the current  take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2757  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2758  .sp  .sp
2759    a+(*COMMIT)b    a+(*COMMIT)b
2760  .sp  .sp
2761  This matches "xxaab" but not "aacaab". It can be thought of as a kind of  This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2762  dynamic anchor, or "I've started, so I must finish."  dynamic anchor, or "I've started, so I must finish." The name of the most
2763  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2764    (*PRUNE)  match failure.
2765  .sp  .P
2766  This verb causes the match to fail at the current position if the rest of the  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2767  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  unless PCRE's start-of-match optimizations are turned off, as shown in this
2768  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2769  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2770  if there is no match to the right, backtracking cannot cross (*PRUNE).      re> /(*COMMIT)abc/
2771  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    data> xyzabc
2772  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2773  be expressed in any other way.    xyzabc\eY
2774      No match
2775    .sp
2776    PCRE knows that any match must start with "a", so the optimization skips along
2777    the subject to "a" before running the first match attempt, which succeeds. When
2778    the optimization is disabled by the \eY escape in the second subject, the match
2779    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2780    starting points.
2781    .sp
2782      (*PRUNE) or (*PRUNE:NAME)
2783    .sp
2784    This verb causes the match to fail at the current starting position in the
2785    subject if the rest of the pattern does not match. If the pattern is
2786    unanchored, the normal "bumpalong" advance to the next starting character then
2787    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2788    reached, or when matching to the right of (*PRUNE), but if there is no match to
2789    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2790    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2791    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2792    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2793    anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2794  .sp  .sp
2795    (*SKIP)    (*SKIP)
2796  .sp  .sp
2797  This verb is like (*PRUNE), except that if the pattern is unanchored, the  This verb, when given without a name, is like (*PRUNE), except that if the
2798  "bumpalong" advance is not to the next character, but to the position in the  pattern is unanchored, the "bumpalong" advance is not to the next character,
2799  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2800  was matched leading up to it cannot be part of a successful match. Consider:  signifies that whatever text was matched leading up to it cannot be part of a
2801    successful match. Consider:
2802  .sp  .sp
2803    a+(*SKIP)b    a+(*SKIP)b
2804  .sp  .sp
2805  If the subject is "aaaac...", after the first match attempt fails (starting at  If the subject is "aaaac...", after the first match attempt fails (starting at
2806  the first character in the string), the starting point skips on to start the  the first character in the string), the starting point skips on to start the
2807  next attempt at "c". Note that a possessive quantifer does not have the same  next attempt at "c". Note that a possessive quantifer does not have the same
2808  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
2809  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2810  instead of skipping on to "c".  instead of skipping on to "c".
2811  .sp  .sp
2812    (*THEN)    (*SKIP:NAME)
2813  .sp  .sp
2814  This verb causes a skip to the next alternation if the rest of the pattern does  When (*SKIP) has an associated name, its behaviour is modified. If the
2815  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2816  current alternation. Its name comes from the observation that it can be used  searched for the most recent (*MARK) that has the same name. If one is found,
2817  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2818    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2819    matching name is found, the (*SKIP) is ignored.
2820    .sp
2821      (*THEN) or (*THEN:NAME)
2822    .sp
2823    This verb causes a skip to the next innermost alternative if the rest of the
2824    pattern does not match. That is, it cancels pending backtracking, but only
2825    within the current alternative. Its name comes from the observation that it can
2826    be used for a pattern-based if-then-else block:
2827  .sp  .sp
2828    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2829  .sp  .sp
2830  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
2831  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
2832  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2833  is used outside of any alternation, it acts exactly like (*PRUNE).  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2834    If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2835    .P
2836    Note that a subpattern that does not contain a | character is just a part of
2837    the enclosing alternative; it is not a nested alternation with only one
2838    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2839    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2840    pattern fragments that do not contain any | characters at this level:
2841    .sp
2842      A (B(*THEN)C) | D
2843    .sp
2844    If A and B are matched, but there is a failure in C, matching does not
2845    backtrack into A; instead it moves to the next alternative, that is, D.
2846    However, if the subpattern containing (*THEN) is given an alternative, it
2847    behaves differently:
2848    .sp
2849      A (B(*THEN)C | (*FAIL)) | D
2850    .sp
2851    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2852    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2853    because there are no more alternatives to try. In this case, matching does now
2854    backtrack into A.
2855    .P
2856    Note also that a conditional subpattern is not considered as having two
2857    alternatives, because only one is ever used. In other words, the | character in
2858    a conditional subpattern has a different meaning. Ignoring white space,
2859    consider:
2860    .sp
2861      ^.*? (?(?=a) a | b(*THEN)c )
2862    .sp
2863    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2864    it initially matches zero characters. The condition (?=a) then fails, the
2865    character "b" is matched, but "c" is not. At this point, matching does not
2866    backtrack to .*? as might perhaps be expected from the presence of the |
2867    character. The conditional subpattern is part of the single alternative that
2868    comprises the whole pattern, and so the match fails. (If there was a backtrack
2869    into .*?, allowing it to match "b", the match would succeed.)
2870    .P
2871    The verbs just described provide four different "strengths" of control when
2872    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2873    next alternative. (*PRUNE) comes next, failing the match at the current
2874    starting position, but allowing an advance to the next character (for an
2875    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2876    than one character. (*COMMIT) is the strongest, causing the entire match to
2877    fail.
2878    .P
2879    If more than one such verb is present in a pattern, the "strongest" one wins.
2880    For example, consider this pattern, where A, B, etc. are complex pattern
2881    fragments:
2882    .sp
2883      (A(*COMMIT)B(*THEN)C|D)
2884    .sp
2885    Once A has matched, PCRE is committed to this match, at the current starting
2886    position. If subsequently B matches, but C does not, the normal (*THEN) action
2887    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2888    overrides.
2889  .  .
2890  .  .
2891  .SH "SEE ALSO"  .SH "SEE ALSO"
2892  .rs  .rs
2893  .sp  .sp
2894  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2895    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.
2896  .  .
2897  .  .
2898  .SH AUTHOR  .SH AUTHOR
# Line 2245  Cambridge CB2 3QH, England. Line 2909  Cambridge CB2 3QH, England.
2909  .rs  .rs
2910  .sp  .sp
2911  .nf  .nf
2912  Last updated: 18 March 2009  Last updated: 24 February 2012
2913  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2012 University of Cambridge.
2914  .fi  .fi

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