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1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "04 May 2012" "PCRE 8.31"
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
# Line 21  published by O'Reilly, covers regular ex Line 21  published by O'Reilly, covers regular ex
21  description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this,  there is now also support for UTF-8 strings in the original library, and a
25  PCRE must be built to include UTF-8 support, and you must call  second library that supports 16-bit and UTF-16 character strings. To use these
26  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There  features, PCRE must be built to include appropriate support. When using UTF
27  is also a special sequence that can be given at the start of a pattern:  strings you must either call the compiling function with the PCRE_UTF8 or
28    PCRE_UTF16 option, or the pattern must start with one of these special
29    sequences:
30  .sp  .sp
31    (*UTF8)    (*UTF8)
32      (*UTF16)
33  .sp  .sp
34  Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8  Starting a pattern with such a sequence is equivalent to setting the relevant
35  option. This feature is not Perl-compatible. How setting UTF-8 mode affects  option. This feature is not Perl-compatible. How setting a UTF mode affects
36  pattern matching is mentioned in several places below. There is also a summary  pattern matching is mentioned in several places below. There is also a summary
37  of UTF-8 features in the  of features in the
 .\" HTML <a href="pcre.html#utf8support">  
 .\" </a>  
 section on UTF-8 support  
 .\"  
 in the main  
38  .\" HREF  .\" HREF
39  \fBpcre\fP  \fBpcreunicode\fP
40  .\"  .\"
41  page.  page.
42  .P  .P
43    Another special sequence that may appear at the start of a pattern or in
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 83  string with one of the following five se Line 97  string with one of the following five se
97    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
98    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
99  .sp  .sp
100  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to the compiling function. For
101  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default  example, on a Unix system where LF is the default newline sequence, the pattern
 newline sequence, the pattern  
102  .sp  .sp
103    (*CR)a.b    (*CR)a.b
104  .sp  .sp
# Line 95  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 117  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 169  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 179  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  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  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, white space 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
204  be used to include a whitespace or # character as part of the pattern.  be used to include a white space or # character as part of the pattern.
205  .P  .P
206  If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
207  can do so by putting them between \eQ and \eE. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
# Line 198  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 211  but when a pattern is being prepared by Line 235  but when a pattern is being prepared by
235  one of the following escape sequences than the binary character it represents:  one of the following escape sequences than the binary character it 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        form feed (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 back reference    \eddd      character with octal code ddd, or back reference
245    \exhh      character with hex code hh    \exhh      character with hex code hh
246    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
247      \euhhhh    character with hex code hhhh (JavaScript mode only)
248  .sp  .sp
249  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
250  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
251  Thus \ecz becomes hex 1A, 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 266  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 285  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 294  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 341  subroutine Line 391  subroutine
391  call.  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
402    \eh     any horizontal whitespace character    \eh     any horizontal white space character
403    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal white space character
404    \es     any whitespace character    \es     any white space character
405    \eS     any character that is not a whitespace character    \eS     any character that is not a white space character
406    \ev     any vertical whitespace character    \ev     any vertical white space character
407    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical white space character
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 372  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 407  The vertical space characters are: Line 493  The vertical space characters are:
493  .sp  .sp
494    U+000A     Linefeed    U+000A     Linefeed
495    U+000B     Vertical tab    U+000B     Vertical tab
496    U+000C     Formfeed    U+000C     Form feed
497    U+000D     Carriage return    U+000D     Carriage return
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 436  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 448  below. Line 520  below.
520  .\"  .\"
521  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
522  LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,  LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
523  U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next  U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
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 468  one of the following sequences: Line 540  one of the following sequences:
540    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
541    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
542  .sp  .sp
543  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to the compiling function, but
544  \fBpcre_compile2()\fP, but they can be overridden by options given to  they can themselves be overridden by options given to a matching function. Note
545  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,  that these special settings, which are not Perl-compatible, are recognized only
546  which are not Perl-compatible, are recognized only at the very start of a  at the very start of a pattern, and that they must be in upper case. If more
547  pattern, and that they must be in upper case. If more than one of them is  than one of them is present, the last one is used. They can be combined with a
548  present, the last one is used. They can be combined with a change of newline  change of newline convention; for example, a pattern can start with:
 convention, for example, a pattern can start with:  
549  .sp  .sp
550    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
551  .sp  .sp
552  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 487  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 496  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 516  Armenian, Line 596  Armenian,
596  Avestan,  Avestan,
597  Balinese,  Balinese,
598  Bamum,  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,  Carian,
608    Chakma,
609  Cham,  Cham,
610  Cherokee,  Cherokee,
611  Common,  Common,
# Line 565  Lisu, Line 648  Lisu,
648  Lycian,  Lycian,
649  Lydian,  Lydian,
650  Malayalam,  Malayalam,
651    Mandaic,
652  Meetei_Mayek,  Meetei_Mayek,
653    Meroitic_Cursive,
654    Meroitic_Hieroglyphs,
655    Miao,
656  Mongolian,  Mongolian,
657  Myanmar,  Myanmar,
658  New_Tai_Lue,  New_Tai_Lue,
# Line 584  Rejang, Line 671  Rejang,
671  Runic,  Runic,
672  Samaritan,  Samaritan,
673  Saurashtra,  Saurashtra,
674    Sharada,
675  Shavian,  Shavian,
676  Sinhala,  Sinhala,
677    Sora_Sompeng,
678  Sundanese,  Sundanese,
679  Syloti_Nagri,  Syloti_Nagri,
680  Syriac,  Syriac,
# Line 594  Tagbanwa, Line 683  Tagbanwa,
683  Tai_Le,  Tai_Le,
684  Tai_Tham,  Tai_Tham,
685  Tai_Viet,  Tai_Viet,
686    Takri,
687  Tamil,  Tamil,
688  Telugu,  Telugu,
689  Thaana,  Thaana,
# Line 604  Ugaritic, Line 694  Ugaritic,
694  Vai,  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 668  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  .\"  .\"
# Line 701  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, form feed, 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 737  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 761  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. Neither PCRE nor Perl has a  first or last character matches \ew, respectively. In a UTF mode, the meanings
893  separte "start of word" or "end of word" metasequence. However, whatever  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
894  follows \eb normally determines which it is. For example, the fragment  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
895  \eba matches "a" at the start of a word.  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 854  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 876  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 or PCRE_NO_UTF16_CHECK option
1024    is used).
1025  .P  .P
1026  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1027  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1028  .\" </a>  .\" </a>
1029  (described below),  (described below)
1030  .\"  .\"
1031  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
1032  the lookbehind.  the lookbehind.
1033    .P
1034    In general, the \eC escape sequence is best avoided. However, one
1035    way of using it that avoids the problem of malformed UTF characters is to use a
1036    lookahead to check the length of the next character, as in this pattern, which
1037    could be used with a UTF-8 string (ignore white space and line breaks):
1038    .sp
1039      (?| (?=[\ex00-\ex7f])(\eC) |
1040          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1041          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1042          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1043    .sp
1044    A group that starts with (?| resets the capturing parentheses numbers in each
1045    alternative (see
1046    .\" HTML <a href="#dupsubpatternnumber">
1047    .\" </a>
1048    "Duplicate Subpattern Numbers"
1049    .\"
1050    below). The assertions at the start of each branch check the next UTF-8
1051    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1052    character's individual bytes are then captured by the appropriate number of
1053    groups.
1054  .  .
1055  .  .
1056  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 908  bracket causes a compile-time error. If Line 1064  bracket causes a compile-time error. If
1064  a member of the class, it should be the first data character in the class  a member of the class, it should be the first data character in the class
1065  (after an initial circumflex, if present) or escaped with a backslash.  (after an initial circumflex, if present) or escaped with a backslash.
1066  .P  .P
1067  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In a UTF mode, the
1068  character may be more than one byte long. A matched character must be in the  character may be more than one data unit long. A matched character must be in
1069  set of characters defined by the class, unless the first character in the class  the set of characters defined by the class, unless the first character in the
1070  definition is a circumflex, in which case the subject character must not be in  class definition is a circumflex, in which case the subject character must not
1071  the set defined by the class. If a circumflex is actually required as a member  be in the set defined by the class. If a circumflex is actually required as a
1072  of the class, ensure it is not the first character, or escape it with a  member of the class, ensure it is not the first character, or escape it with a
1073  backslash.  backslash.
1074  .P  .P
1075  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
# Line 924  circumflex is not an assertion; it still Line 1080  circumflex is not an assertion; it still
1080  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
1081  string.  string.
1082  .P  .P
1083  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
1084  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{
1085    escaping mechanism.
1086  .P  .P
1087  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
1088  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
1089  "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
1090  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
1091  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
1092  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1093  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1094  If you want to use caseless matching in UTF8-mode for characters 128 and above,  If you want to use caseless matching in a UTF mode for characters 128 and
1095  you must ensure that PCRE is compiled with Unicode property support as well as  above, you must ensure that PCRE is compiled with Unicode property support as
1096  with UTF-8 support.  well as with UTF support.
1097  .P  .P
1098  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
1099  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 958  followed by two other characters. The oc Line 1115  followed by two other characters. The oc
1115  "]" can also be used to end a range.  "]" can also be used to end a range.
1116  .P  .P
1117  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
1118  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1119  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}].  
1120  .P  .P
1121  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
1122  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
1123  [][\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
1124  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
1125  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
1126  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
1127  property support.  property support.
1128  .P  .P
1129  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,
1130  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
1131  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1132  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
1133  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
1134  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1135    .\" HTML <a href="#genericchartypes">
1136    .\" </a>
1137    "Generic character types"
1138    .\"
1139    above. The escape sequence \eb has a different meaning inside a character
1140    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1141    are not special inside a character class. Like any other unrecognized escape
1142    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1143    default, but cause an error if the PCRE_EXTRA option is set.
1144    .P
1145    A circumflex can conveniently be used with the upper case character types to
1146    specify a more restricted set of characters than the matching lower case type.
1147    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1148    whereas [\ew] includes underscore. A positive character class should be read as
1149    "something OR something OR ..." and a negative class as "NOT something AND NOT
1150    something AND NOT ...".
1151  .P  .P
1152  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1153  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 995  this notation. For example, Line 1167  this notation. For example,
1167    [01[:alpha:]%]    [01[:alpha:]%]
1168  .sp  .sp
1169  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1170  are  are:
1171  .sp  .sp
1172    alnum    letters and digits    alnum    letters and digits
1173    alpha    letters    alpha    letters
# Line 1006  are Line 1178  are
1178    graph    printing characters, excluding space    graph    printing characters, excluding space
1179    lower    lower case letters    lower    lower case letters
1180    print    printing characters, including space    print    printing characters, including space
1181    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1182    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1183    upper    upper case letters    upper    upper case letters
1184    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 1027  matches "1", "2", or any non-digit. PCRE Line 1199  matches "1", "2", or any non-digit. PCRE
1199  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
1200  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1201  .P  .P
1202  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
1203  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1204    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1205    character properties are used. This is achieved by replacing the POSIX classes
1206    by other sequences, as follows:
1207    .sp
1208      [:alnum:]  becomes  \ep{Xan}
1209      [:alpha:]  becomes  \ep{L}
1210      [:blank:]  becomes  \eh
1211      [:digit:]  becomes  \ep{Nd}
1212      [:lower:]  becomes  \ep{Ll}
1213      [:space:]  becomes  \ep{Xps}
1214      [:upper:]  becomes  \ep{Lu}
1215      [:word:]   becomes  \ep{Xwd}
1216    .sp
1217    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1218    classes are unchanged, and match only characters with code points less than
1219    128.
1220  .  .
1221  .  .
1222  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1082  extracts it into the global options (and Line 1270  extracts it into the global options (and
1270  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1271  .P  .P
1272  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1273  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
1274  .sp  .sp
1275    (a(?i)b)c    (a(?i)b)c
1276  .sp  .sp
# Line 1099  option settings happen at compile time. Line 1287  option settings happen at compile time.
1287  behaviour otherwise.  behaviour otherwise.
1288  .P  .P
1289  \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
1290  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
1291  pattern can contain special leading sequences such as (*CRLF) to override what  the pattern can contain special leading sequences such as (*CRLF) to override
1292  the application has set or what has been defaulted. Details are given in the  what the application has set or what has been defaulted. Details are given in
1293  section entitled  the section entitled
1294  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
1295  .\" </a>  .\" </a>
1296  "Newline sequences"  "Newline sequences"
1297  .\"  .\"
1298  above. There is also the (*UTF8) leading sequence that can be used to set UTF-8  above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1299  mode; this is equivalent to setting the PCRE_UTF8 option.  can be used to set UTF and Unicode property modes; they are equivalent to
1300    setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1301  .  .
1302  .  .
1303  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1122  Turning part of a pattern into a subpatt Line 1311  Turning part of a pattern into a subpatt
1311  .sp  .sp
1312    cat(aract|erpillar|)    cat(aract|erpillar|)
1313  .sp  .sp
1314  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1315  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1316  .sp  .sp
1317  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
1318  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
1319  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
1320  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1321  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1322  .P  .P
1323  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
1324    numbers for the capturing subpatterns. For example, if the string "the red
1325    king" is matched against the pattern
1326  .sp  .sp
1327    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1328  .sp  .sp
# Line 1180  at captured substring number one, whiche Line 1371  at captured substring number one, whiche
1371  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
1372  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1373  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
1374  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
1375  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1376  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.  
1377  .sp  .sp
1378    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1379    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# Line 1195  or "defdef": Line 1385  or "defdef":
1385  .sp  .sp
1386    /(?|(abc)|(def))\e1/    /(?|(abc)|(def))\e1/
1387  .sp  .sp
1388  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a subroutine call to a numbered subpattern always refers to the
1389  refers to the first one in the pattern with the given number. The following  first one in the pattern with the given number. The following pattern matches
1390  pattern matches "abcabc" or "defabc":  "abcabc" or "defabc":
1391  .sp  .sp
1392    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1393  .sp  .sp
# Line 1308  items: Line 1498  items:
1498    a literal data character    a literal data character
1499    the dot metacharacter    the dot metacharacter
1500    the \eC escape sequence    the \eC escape sequence
1501    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1502    the \eR escape sequence    the \eR escape sequence
1503    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1504    a character class    a character class
1505    a back reference (see next section)    a back reference (see next section)
1506    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1507    a recursive or "subroutine" call to a subpattern    a subroutine call to a subpattern (recursive or otherwise)
1508  .sp  .sp
1509  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1510  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1339  where a quantifier is not allowed, or on Line 1529  where a quantifier is not allowed, or on
1529  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
1530  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1531  .P  .P
1532  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
1533  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
1534  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,
1535  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
1536  which may be several bytes long (and they may be of different lengths).  data units long (and they may be of different lengths).
1537  .P  .P
1538  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
1539  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 1352  subpatterns that are referenced as Line 1542  subpatterns that are referenced as
1542  .\" </a>  .\" </a>
1543  subroutines  subroutines
1544  .\"  .\"
1545  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1546  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1547    .\" </a>
1548    "Defining subpatterns for use by reference only"
1549    .\"
1550    below). Items other than subpatterns that have a {0} quantifier are omitted
1551    from the compiled pattern.
1552  .P  .P
1553  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1554  abbreviations:  abbreviations:
# Line 1578  no such problem when named parentheses a Line 1773  no such problem when named parentheses a
1773  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1774  .P  .P
1775  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
1776  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
1777  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
1778  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1779  .sp  .sp
1780    (ring), \e1    (ring), \e1
1781    (ring), \eg1    (ring), \eg1
# Line 1594  example: Line 1789  example:
1789    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1790  .sp  .sp
1791  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
1792  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.
1793  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
1794  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
1795  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1796  .P  .P
1797  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1798  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1648  Because there may be many capturing pare Line 1843  Because there may be many capturing pare
1843  following a backslash are taken as part of a potential back reference number.  following a backslash are taken as part of a potential back reference number.
1844  If the pattern continues with a digit character, some delimiter must be used to  If the pattern continues with a digit character, some delimiter must be used to
1845  terminate the back reference. If the PCRE_EXTENDED option is set, this can be  terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1846  whitespace. Otherwise, the \eg{ syntax or an empty comment (see  white space. Otherwise, the \eg{ syntax or an empty comment (see
1847  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1848  .\" </a>  .\" </a>
1849  "Comments"  "Comments"
# Line 1699  those that look ahead of the current pos Line 1894  those that look ahead of the current pos
1894  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,
1895  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.
1896  .P  .P
1897  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1898  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
1899  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1900  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1901  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1902  because it does not make sense for negative assertions.  .P
1903    For compatibility with Perl, assertion subpatterns may be repeated; though
1904    it makes no sense to assert the same thing several times, the side effect of
1905    capturing parentheses may occasionally be useful. In practice, there only three
1906    cases:
1907    .sp
1908    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1909    However, it may contain internal capturing parenthesized groups that are called
1910    from elsewhere via the
1911    .\" HTML <a href="#subpatternsassubroutines">
1912    .\" </a>
1913    subroutine mechanism.
1914    .\"
1915    .sp
1916    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1917    were {0,1}. At run time, the rest of the pattern match is tried with and
1918    without the assertion, the order depending on the greediness of the quantifier.
1919    .sp
1920    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1921    The assertion is obeyed just once when encountered during matching.
1922  .  .
1923  .  .
1924  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1733  lookbehind assertion is needed to achiev Line 1947  lookbehind assertion is needed to achiev
1947  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
1948  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
1949  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.
1950  The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a  The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
 synonym for (?!).  
1951  .  .
1952  .  .
1953  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1759  is permitted, but Line 1972  is permitted, but
1972  .sp  .sp
1973  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1974  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
1975  extension compared with Perl (5.8 and 5.10), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1976  match the same length of string. An assertion such as  length of string. An assertion such as
1977  .sp  .sp
1978    (?<=ab(c|de))    (?<=ab(c|de))
1979  .sp  .sp
# Line 1770  branches: Line 1983  branches:
1983  .sp  .sp
1984    (?<=abc|abde)    (?<=abc|abde)
1985  .sp  .sp
1986  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1987  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1988  .\" </a>  .\" </a>
1989  (see above)  (see above)
# Line 1783  temporarily move the current position ba Line 1996  temporarily move the current position ba
1996  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1997  assertion fails.  assertion fails.
1998  .P  .P
1999  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
2000  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
2001  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
2002  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2003    permitted.
2004  .P  .P
2005  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2006  .\" </a>  .\" </a>
# Line 1874  already been matched. The two possible f Line 2088  already been matched. The two possible f
2088  .sp  .sp
2089  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2090  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
2091  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2092    itself contain nested subpatterns of any form, including conditional
2093    subpatterns; the restriction to two alternatives applies only at the level of
2094    the condition. This pattern fragment is an example where the alternatives are
2095    complex:
2096    .sp
2097      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2098    .sp
2099  .P  .P
2100  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2101  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1891  matched. If there is more than one captu Line 2112  matched. If there is more than one captu
2112  .\" </a>  .\" </a>
2113  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2114  .\"  .\"
2115  the condition is true if any of them have been set. An alternative notation is  the condition is true if any of them have matched. An alternative notation is
2116  to precede the digits with a plus or minus sign. In this case, the subpattern  to precede the digits with a plus or minus sign. In this case, the subpattern
2117  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2118  can be referenced by (?(-1), the next most recent by (?(-2), and so on. In  can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2119  looping constructs it can also make sense to refer to subsequent groups with  loops it can also make sense to refer to subsequent groups. The next
2120  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2121    zero in any of these forms is not used; it provokes a compile-time error.)
2122  .P  .P
2123  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2124  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 1907  three parts for ease of discussion: Line 2129  three parts for ease of discussion:
2129  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2130  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
2131  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
2132  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2133  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,
2134  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
2135  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2136  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 1964  The syntax for recursive patterns Line 2186  The syntax for recursive patterns
2186  .\"  .\"
2187  is described below.  is described below.
2188  .  .
2189    .\" HTML <a name="subdefine"></a>
2190  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2191  .rs  .rs
2192  .sp  .sp
# Line 1971  If the condition is the string (DEFINE), Line 2194  If the condition is the string (DEFINE),
2194  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
2195  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2196  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
2197  "subroutines" that can be referenced from elsewhere. (The use of  subroutines that can be referenced from elsewhere. (The use of
2198  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2199  .\" </a>  .\" </a>
2200  "subroutines"  subroutines
2201  .\"  .\"
2202  is described below.) For example, a pattern to match an IPv4 address could be  is described below.) For example, a pattern to match an IPv4 address such as
2203  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore white space and line
2204    breaks):
2205  .sp  .sp
2206    (?(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) )
2207    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 2012  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2236  dd-aaa-dd or dd-dd-dd, where aaa are let
2236  .SH COMMENTS  .SH COMMENTS
2237  .rs  .rs
2238  .sp  .sp
2239  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
2240  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,
2241  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
2242    subpattern name or number. The characters that make up a comment play no part
2243    in the pattern matching.
2244  .P  .P
2245  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
2246  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2247  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2248    this case continues to immediately after the next newline character or
2249    character sequence in the pattern. Which characters are interpreted as newlines
2250    is controlled by the options passed to a compiling function or by a special
2251    sequence at the start of the pattern, as described in the section entitled
2252    .\" HTML <a href="#newlines">
2253    .\" </a>
2254    "Newline conventions"
2255    .\"
2256    above. Note that the end of this type of comment is a literal newline sequence
2257    in the pattern; escape sequences that happen to represent a newline do not
2258    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2259    default newline convention is in force:
2260    .sp
2261      abc #comment \en still comment
2262    .sp
2263    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2264    a newline in the pattern. The sequence \en is still literal at this stage, so
2265    it does not terminate the comment. Only an actual character with the code value
2266    0x0a (the default newline) does so.
2267  .  .
2268  .  .
2269  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 2047  individual subpattern recursion. After i Line 2292  individual subpattern recursion. After i
2292  this kind of recursion was subsequently introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2293  .P  .P
2294  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
2295  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
2296  provided that it occurs inside that subpattern. (If not, it is a  given number, provided that it occurs inside that subpattern. (If not, it is a
2297  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2298  .\" </a>  .\" </a>
2299  "subroutine"  non-recursive subroutine
2300  .\"  .\"
2301  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
2302  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
# Line 2076  We have put the pattern into parentheses Line 2321  We have put the pattern into parentheses
2321  them instead of the whole pattern.  them instead of the whole pattern.
2322  .P  .P
2323  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
2324  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
2325  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
2326  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2327  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.  
2328  .P  .P
2329  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2330  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2331  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2332  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2333  .\" </a>  .\" </a>
2334  "subroutine"  non-recursive subroutine
2335  .\"  .\"
2336  calls, as described in the next section.  calls, as described in the next section.
2337  .P  .P
# Line 2124  documentation). If the pattern above is Line 2368  documentation). If the pattern above is
2368  .sp  .sp
2369  the value for the inner capturing parentheses (numbered 2) is "ef", which is  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2370  the last value taken on at the top level. If a capturing subpattern is not  the last value taken on at the top level. If a capturing subpattern is not
2371  matched at the top level, its final value is unset, even if it is (temporarily)  matched at the top level, its final captured value is unset, even if it was
2372  set at a deeper level.  (temporarily) set at a deeper level during the matching process.
2373  .P  .P
2374  If there are more than 15 capturing parentheses in a pattern, PCRE has to  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2375  obtain extra memory to store data during a recursion, which it does by using  obtain extra memory to store data during a recursion, which it does by using
# Line 2145  is the actual recursive call. Line 2389  is the actual recursive call.
2389  .  .
2390  .  .
2391  .\" HTML <a name="recursiondifference"></a>  .\" HTML <a name="recursiondifference"></a>
2392  .SS "Recursion difference from Perl"  .SS "Differences in recursion processing between PCRE and Perl"
2393  .rs  .rs
2394  .sp  .sp
2395  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2396  treated as an atomic group. That is, once it has matched some of the subject  (like Python, but unlike Perl), a recursive subpattern call is always treated
2397  string, it is never re-entered, even if it contains untried alternatives and  as an atomic group. That is, once it has matched some of the subject string, it
2398  there is a subsequent matching failure. This can be illustrated by the  is never re-entered, even if it contains untried alternatives and there is a
2399  following pattern, which purports to match a palindromic string that contains  subsequent matching failure. This can be illustrated by the following pattern,
2400  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  which purports to match a palindromic string that contains an odd number of
2401    characters (for example, "a", "aba", "abcba", "abcdcba"):
2402  .sp  .sp
2403    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2404  .sp  .sp
# Line 2183  time we do have another alternative to t Line 2428  time we do have another alternative to t
2428  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2429  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2430  .P  .P
2431  To change the pattern so that matches all palindromic strings, not just those  To change the pattern so that it matches all palindromic strings, not just
2432  with an odd number of characters, it is tempting to change the pattern to this:  those with an odd number of characters, it is tempting to change the pattern to
2433    this:
2434  .sp  .sp
2435    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2436  .sp  .sp
# Line 2213  For example, although "abcba" is correct Line 2459  For example, although "abcba" is correct
2459  PCRE finds the palindrome "aba" at the start, then fails at top level because  PCRE finds the palindrome "aba" at the start, then fails at top level because
2460  the end of the string does not follow. Once again, it cannot jump back into the  the end of the string does not follow. Once again, it cannot jump back into the
2461  recursion to try other alternatives, so the entire match fails.  recursion to try other alternatives, so the entire match fails.
2462    .P
2463    The second way in which PCRE and Perl differ in their recursion processing is
2464    in the handling of captured values. In Perl, when a subpattern is called
2465    recursively or as a subpattern (see the next section), it has no access to any
2466    values that were captured outside the recursion, whereas in PCRE these values
2467    can be referenced. Consider this pattern:
2468    .sp
2469      ^(.)(\e1|a(?2))
2470    .sp
2471    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2472    then in the second group, when the back reference \e1 fails to match "b", the
2473    second alternative matches "a" and then recurses. In the recursion, \e1 does
2474    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2475    match because inside the recursive call \e1 cannot access the externally set
2476    value.
2477  .  .
2478  .  .
2479  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2480  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2481  .rs  .rs
2482  .sp  .sp
2483  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
2484  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
2485  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2486  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2487  relative, as in these examples:  relative, as in these examples:
2488  .sp  .sp
# Line 2241  matches "sense and sensibility" and "res Line 2502  matches "sense and sensibility" and "res
2502  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
2503  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2504  .P  .P
2505  Like recursive subpatterns, a subroutine call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2506  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
2507  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
2508  matching failure. Any capturing parentheses that are set during the subroutine  subsequent matching failure. Any capturing parentheses that are set during the
2509  call revert to their previous values afterwards.  subroutine call revert to their previous values afterwards.
2510  .P  .P
2511  When a subpattern is used as a subroutine, processing options such as  Processing options such as case-independence are fixed when a subpattern is
2512  case-independence are fixed when the subpattern is defined. They cannot be  defined, so if it is used as a subroutine, such options cannot be changed for
2513  changed for different calls. For example, consider this pattern:  different calls. For example, consider this pattern:
2514  .sp  .sp
2515    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2516  .sp  .sp
# Line 2288  same pair of parentheses when there is a Line 2549  same pair of parentheses when there is a
2549  .P  .P
2550  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
2551  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
2552  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
2553  By default, this variable contains NULL, which disables all calling out.  (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this
2554    variable contains NULL, which disables all calling out.
2555  .P  .P
2556  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2557  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 2298  For example, this pattern has two callou Line 2560  For example, this pattern has two callou
2560  .sp  .sp
2561    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2562  .sp  .sp
2563  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
2564  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2565  255.  255.
2566  .P  .P
2567  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
2568  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
2569  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
2570  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  the matching function. The callout function may cause matching to proceed, to
2571  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
2572  description of the interface to the callout function is given in the  the callout function is given in the
2573  .\" HREF  .\" HREF
2574  \fBpcrecallout\fP  \fBpcrecallout\fP
2575  .\"  .\"
2576  documentation.  documentation.
2577  .  .
2578  .  .
2579    .\" HTML <a name="backtrackcontrol"></a>
2580  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2581  .rs  .rs
2582  .sp  .sp
# Line 2324  production code should be noted to avoid Line 2587  production code should be noted to avoid
2587  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2588  .P  .P
2589  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
2590  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
2591  a backtracking algorithm. With the exception of (*FAIL), which behaves like a  matching functions, which use a backtracking algorithm. With the exception of
2592  failing negative assertion, they cause an error if encountered by  (*FAIL), which behaves like a failing negative assertion, they cause an error
2593  \fBpcre_dfa_exec()\fP.  if encountered by a DFA matching function.
2594  .P  .P
2595  If any of these verbs are used in an assertion or subroutine subpattern  If any of these verbs are used in an assertion or in a subpattern that is
2596  (including recursive subpatterns), their effect is confined to that subpattern;  called as a subroutine (whether or not recursively), their effect is confined
2597  it does not extend to the surrounding pattern. Note that such subpatterns are  to that subpattern; it does not extend to the surrounding pattern, with one
2598  processed as anchored at the point where they are tested.  exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2599    a successful positive assertion \fIis\fP passed back when a match succeeds
2600    (compare capturing parentheses in assertions). Note that such subpatterns are
2601    processed as anchored at the point where they are tested. Note also that Perl's
2602    treatment of subroutines and assertions is different in some cases.
2603  .P  .P
2604  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
2605  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2606  (*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,
2607  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
2608  are two kinds:  characters that does not include a closing parenthesis. The maximum length of
2609    name is 255 in the 8-bit library and 65535 in the 16-bit library. If the name
2610    is empty, that is, if the closing parenthesis immediately follows the colon,
2611    the effect is as if the colon were not there. Any number of these verbs may
2612    occur in a pattern.
2613    .
2614    .
2615    .\" HTML <a name="nooptimize"></a>
2616    .SS "Optimizations that affect backtracking verbs"
2617    .rs
2618    .sp
2619    PCRE contains some optimizations that are used to speed up matching by running
2620    some checks at the start of each match attempt. For example, it may know the
2621    minimum length of matching subject, or that a particular character must be
2622    present. When one of these optimizations suppresses the running of a match, any
2623    included backtracking verbs will not, of course, be processed. You can suppress
2624    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2625    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2626    pattern with (*NO_START_OPT). There is more discussion of this option in the
2627    section entitled
2628    .\" HTML <a href="pcreapi.html#execoptions">
2629    .\" </a>
2630    "Option bits for \fBpcre_exec()\fP"
2631    .\"
2632    in the
2633    .\" HREF
2634    \fBpcreapi\fP
2635    .\"
2636    documentation.
2637    .P
2638    Experiments with Perl suggest that it too has similar optimizations, sometimes
2639    leading to anomalous results.
2640    .
2641  .  .
2642  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2643  .rs  .rs
2644  .sp  .sp
2645  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2646    followed by a name.
2647  .sp  .sp
2648     (*ACCEPT)     (*ACCEPT)
2649  .sp  .sp
2650  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
2651  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2652  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is  subroutine, only that subpattern is ended successfully. Matching then continues
2653  captured. (This feature was added to PCRE at release 8.00.) For example:  at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2654    far is captured. For example:
2655  .sp  .sp
2656    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2657  .sp  .sp
# Line 2359  the outer parentheses. Line 2660  the outer parentheses.
2660  .sp  .sp
2661    (*FAIL) or (*F)    (*FAIL) or (*F)
2662  .sp  .sp
2663  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
2664  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
2665  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2666  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 2370  callout feature, as for example in this Line 2671  callout feature, as for example in this
2671  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
2672  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2673  .  .
2674    .
2675    .SS "Recording which path was taken"
2676    .rs
2677    .sp
2678    There is one verb whose main purpose is to track how a match was arrived at,
2679    though it also has a secondary use in conjunction with advancing the match
2680    starting point (see (*SKIP) below).
2681    .sp
2682      (*MARK:NAME) or (*:NAME)
2683    .sp
2684    A name is always required with this verb. There may be as many instances of
2685    (*MARK) as you like in a pattern, and their names do not have to be unique.
2686    .P
2687    When a match succeeds, the name of the last-encountered (*MARK) on the matching
2688    path is passed back to the caller as described in the section entitled
2689    .\" HTML <a href="pcreapi.html#extradata">
2690    .\" </a>
2691    "Extra data for \fBpcre_exec()\fP"
2692    .\"
2693    in the
2694    .\" HREF
2695    \fBpcreapi\fP
2696    .\"
2697    documentation. Here is an example of \fBpcretest\fP output, where the /K
2698    modifier requests the retrieval and outputting of (*MARK) data:
2699    .sp
2700        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2701      data> XY
2702       0: XY
2703      MK: A
2704      XZ
2705       0: XZ
2706      MK: B
2707    .sp
2708    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2709    indicates which of the two alternatives matched. This is a more efficient way
2710    of obtaining this information than putting each alternative in its own
2711    capturing parentheses.
2712    .P
2713    If (*MARK) is encountered in a positive assertion, its name is recorded and
2714    passed back if it is the last-encountered. This does not happen for negative
2715    assertions.
2716    .P
2717    After a partial match or a failed match, the name of the last encountered
2718    (*MARK) in the entire match process is returned. For example:
2719    .sp
2720        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2721      data> XP
2722      No match, mark = B
2723    .sp
2724    Note that in this unanchored example the mark is retained from the match
2725    attempt that started at the letter "X" in the subject. Subsequent match
2726    attempts starting at "P" and then with an empty string do not get as far as the
2727    (*MARK) item, but nevertheless do not reset it.
2728    .P
2729    If you are interested in (*MARK) values after failed matches, you should
2730    probably set the PCRE_NO_START_OPTIMIZE option
2731    .\" HTML <a href="#nooptimize">
2732    .\" </a>
2733    (see above)
2734    .\"
2735    to ensure that the match is always attempted.
2736    .
2737    .
2738  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2739  .rs  .rs
2740  .sp  .sp
2741  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2742  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
2743  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
2744    the verb. However, when one of these verbs appears inside an atomic group, its
2745    effect is confined to that group, because once the group has been matched,
2746    there is never any backtracking into it. In this situation, backtracking can
2747    "jump back" to the left of the entire atomic group. (Remember also, as stated
2748    above, that this localization also applies in subroutine calls and assertions.)
2749    .P
2750    These verbs differ in exactly what kind of failure occurs when backtracking
2751    reaches them.
2752  .sp  .sp
2753    (*COMMIT)    (*COMMIT)
2754  .sp  .sp
2755  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
2756  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
2757  a match by advancing the starting point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2758  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
2759  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2760  .sp  .sp
2761    a+(*COMMIT)b    a+(*COMMIT)b
2762  .sp  .sp
2763  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
2764  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
2765  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2766    (*PRUNE)  match failure.
2767  .sp  .P
2768  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,
2769  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
2770  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2771  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2772  if there is no match to the right, backtracking cannot cross (*PRUNE).      re> /(*COMMIT)abc/
2773  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    data> xyzabc
2774  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2775  be expressed in any other way.    xyzabc\eY
2776      No match
2777    .sp
2778    PCRE knows that any match must start with "a", so the optimization skips along
2779    the subject to "a" before running the first match attempt, which succeeds. When
2780    the optimization is disabled by the \eY escape in the second subject, the match
2781    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2782    starting points.
2783    .sp
2784      (*PRUNE) or (*PRUNE:NAME)
2785    .sp
2786    This verb causes the match to fail at the current starting position in the
2787    subject if the rest of the pattern does not match. If the pattern is
2788    unanchored, the normal "bumpalong" advance to the next starting character then
2789    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2790    reached, or when matching to the right of (*PRUNE), but if there is no match to
2791    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2792    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2793    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2794    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2795    anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2796  .sp  .sp
2797    (*SKIP)    (*SKIP)
2798  .sp  .sp
2799  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
2800  "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,
2801  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2802  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
2803    successful match. Consider:
2804  .sp  .sp
2805    a+(*SKIP)b    a+(*SKIP)b
2806  .sp  .sp
# Line 2417  effect as this example; although it woul Line 2811  effect as this example; although it woul
2811  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2812  instead of skipping on to "c".  instead of skipping on to "c".
2813  .sp  .sp
2814    (*THEN)    (*SKIP:NAME)
2815  .sp  .sp
2816  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
2817  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2818  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,
2819  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2820    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2821    matching name is found, the (*SKIP) is ignored.
2822    .sp
2823      (*THEN) or (*THEN:NAME)
2824    .sp
2825    This verb causes a skip to the next innermost alternative if the rest of the
2826    pattern does not match. That is, it cancels pending backtracking, but only
2827    within the current alternative. Its name comes from the observation that it can
2828    be used for a pattern-based if-then-else block:
2829  .sp  .sp
2830    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2831  .sp  .sp
2832  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
2833  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
2834  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2835  is used outside of any alternation, it acts exactly like (*PRUNE).  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2836    If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2837    .P
2838    Note that a subpattern that does not contain a | character is just a part of
2839    the enclosing alternative; it is not a nested alternation with only one
2840    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2841    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2842    pattern fragments that do not contain any | characters at this level:
2843    .sp
2844      A (B(*THEN)C) | D
2845    .sp
2846    If A and B are matched, but there is a failure in C, matching does not
2847    backtrack into A; instead it moves to the next alternative, that is, D.
2848    However, if the subpattern containing (*THEN) is given an alternative, it
2849    behaves differently:
2850    .sp
2851      A (B(*THEN)C | (*FAIL)) | D
2852    .sp
2853    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2854    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2855    because there are no more alternatives to try. In this case, matching does now
2856    backtrack into A.
2857    .P
2858    Note also that a conditional subpattern is not considered as having two
2859    alternatives, because only one is ever used. In other words, the | character in
2860    a conditional subpattern has a different meaning. Ignoring white space,
2861    consider:
2862    .sp
2863      ^.*? (?(?=a) a | b(*THEN)c )
2864    .sp
2865    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2866    it initially matches zero characters. The condition (?=a) then fails, the
2867    character "b" is matched, but "c" is not. At this point, matching does not
2868    backtrack to .*? as might perhaps be expected from the presence of the |
2869    character. The conditional subpattern is part of the single alternative that
2870    comprises the whole pattern, and so the match fails. (If there was a backtrack
2871    into .*?, allowing it to match "b", the match would succeed.)
2872    .P
2873    The verbs just described provide four different "strengths" of control when
2874    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2875    next alternative. (*PRUNE) comes next, failing the match at the current
2876    starting position, but allowing an advance to the next character (for an
2877    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2878    than one character. (*COMMIT) is the strongest, causing the entire match to
2879    fail.
2880    .P
2881    If more than one such verb is present in a pattern, the "strongest" one wins.
2882    For example, consider this pattern, where A, B, etc. are complex pattern
2883    fragments:
2884    .sp
2885      (A(*COMMIT)B(*THEN)C|D)
2886    .sp
2887    Once A has matched, PCRE is committed to this match, at the current starting
2888    position. If subsequently B matches, but C does not, the normal (*THEN) action
2889    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2890    overrides.
2891  .  .
2892  .  .
2893  .SH "SEE ALSO"  .SH "SEE ALSO"
2894  .rs  .rs
2895  .sp  .sp
2896  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2897  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.
2898  .  .
2899  .  .
2900  .SH AUTHOR  .SH AUTHOR
# Line 2453  Cambridge CB2 3QH, England. Line 2911  Cambridge CB2 3QH, England.
2911  .rs  .rs
2912  .sp  .sp
2913  .nf  .nf
2914  Last updated: 01 March 2010  Last updated: 01 June 2012
2915  Copyright (c) 1997-2010 University of Cambridge.  Copyright (c) 1997-2012 University of Cambridge.
2916  .fi  .fi

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