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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  Another special sequence that may appear at the start of a pattern or in
44  combination with (*UTF8) is:  combination with (*UTF8) or (*UTF16) is:
45  .sp  .sp
46    (*UCP)    (*UCP)
47  .sp  .sp
# Line 52  such as \ed and \ew to use Unicode prope Line 50  such as \ed and \ew to use Unicode prope
50  instead of recognizing only characters with codes less than 128 via a lookup  instead of recognizing only characters with codes less than 128 via a lookup
51  table.  table.
52  .P  .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  .\"  .\"
# Line 94  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 130  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 182  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 192  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 211  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.  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 225  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
249    The precise effect of \ecx on ASCII characters is as follows: if x is a lower
250    case letter, it is converted to upper case. Then bit 6 of the character (hex
251    40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
252    but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
253    data item (byte or 16-bit value) following \ec has a value greater than 127, a
254    compile-time error occurs. This locks out non-ASCII characters in all modes.
255    .P
256    The \ec facility was designed for use with ASCII characters, but with the
257    extension to Unicode it is even less useful than it once was. It is, however,
258    recognized when PCRE is compiled in EBCDIC mode, where data items are always
259    bytes. In this mode, all values are valid after \ec. If the next character is a
260    lower case letter, it is converted to upper case. Then the 0xc0 bits of the
261    byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
262    the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
263    characters also generate different values.
264    .P
265    By default, after \ex, from zero to two hexadecimal digits are read (letters
266    can be in upper or lower case). Any number of hexadecimal digits may appear
267    between \ex{ and }, but the character code is constrained as follows:
268    .sp
269      8-bit non-UTF mode    less than 0x100
270      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
271      16-bit non-UTF mode   less than 0x10000
272      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
273  .sp  .sp
274  The precise effect of \ecx is as follows: if x is a lower case letter, it  Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
275  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  "surrogate" codepoints).
 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  
 7B.  
 .P  
 After \ex, from zero to two hexadecimal digits are read (letters can be in  
 upper or lower case). Any number of hexadecimal digits may appear between \ex{  
 and }, but the value of the character code must be less than 256 in non-UTF-8  
 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in  
 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code  
 point, which is 10FFFF.  
276  .P  .P
277  If characters other than hexadecimal digits appear between \ex{ and }, or if  If characters other than hexadecimal digits appear between \ex{ and }, or if
278  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
279  initial \ex will be interpreted as a basic hexadecimal escape, with no  initial \ex will be interpreted as a basic hexadecimal escape, with no
280  following digits, giving a character whose value is zero.  following digits, giving a character whose value is zero.
281  .P  .P
282    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
283    as just described only when it is followed by two hexadecimal digits.
284    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
285    code points greater than 256 is provided by \eu, which must be followed by
286    four hexadecimal digits; otherwise it matches a literal "u" character.
287    Character codes specified by \eu in JavaScript mode are constrained in the same
288    was as those specified by \ex in non-JavaScript mode.
289    .P
290  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
291  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
292  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
293    \eu00dc in JavaScript mode).
294  .P  .P
295  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
296  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 280  parenthesized subpatterns. Line 316  parenthesized subpatterns.
316  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
317  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
318  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
319  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
320  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.
321  to \e777 are permitted. For example:  For example:
322  .sp  .sp
323    \e040   is another way of writing a space    \e040   is another way of writing a space
324  .\" JOIN  .\" JOIN
# Line 299  to \e777 are permitted. For example: Line 335  to \e777 are permitted. For example:
335              character with octal code 113              character with octal code 113
336  .\" JOIN  .\" JOIN
337    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
338              the byte consisting entirely of 1 bits              the value 255 (decimal)
339  .\" JOIN  .\" JOIN
340    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or a binary zero
341              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
# Line 308  Note that octal values of 100 or greater Line 344  Note that octal values of 100 or greater
344  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
345  .P  .P
346  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
347  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
348  sequence \eb is interpreted as the backspace character (hex 08). The sequences  interpreted as the backspace character (hex 08).
349  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other  .P
350  unrecognized escape sequences, they are treated as the literal characters "B",  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
351  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is  inside a character class. Like other unrecognized escape sequences, they are
352  set. Outside a character class, these sequences have different meanings.  treated as the literal characters "B", "R", and "X" by default, but cause an
353    error if the PCRE_EXTRA option is set. Outside a character class, these
354    sequences have different meanings.
355    .
356    .
357    .SS "Unsupported escape sequences"
358    .rs
359    .sp
360    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
361    handler and used to modify the case of following characters. By default, PCRE
362    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
363    option is set, \eU matches a "U" character, and \eu can be used to define a
364    character by code point, as described in the previous section.
365  .  .
366  .  .
367  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 360  Another use of backslash is for specifyi Line 408  Another use of backslash is for specifyi
408  .sp  .sp
409    \ed     any decimal digit    \ed     any decimal digit
410    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
411    \eh     any horizontal whitespace character    \eh     any horizontal white space character
412    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal white space character
413    \es     any whitespace character    \es     any white space character
414    \eS     any character that is not a whitespace character    \eS     any character that is not a white space character
415    \ev     any vertical whitespace character    \ev     any vertical white space character
416    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical white space character
417    \ew     any "word" character    \ew     any "word" character
418    \eW     any "non-word" character    \eW     any "non-word" character
419  .sp  .sp
# Line 375  This is the same as Line 423  This is the same as
423  .\" </a>  .\" </a>
424  the "." metacharacter  the "." metacharacter
425  .\"  .\"
426  when PCRE_DOTALL is not set.  when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
427    PCRE does not support this.
428  .P  .P
429  Each pair of lower and upper case escape sequences partitions the complete set  Each pair of lower and upper case escape sequences partitions the complete set
430  of characters into two disjoint sets. Any given character matches one, and only  of characters into two disjoint sets. Any given character matches one, and only
# Line 407  or "french" in Windows, some character c Line 456  or "french" in Windows, some character c
456  accented letters, and these are then matched by \ew. The use of locales with  accented letters, and these are then matched by \ew. The use of locales with
457  Unicode is discouraged.  Unicode is discouraged.
458  .P  .P
459  By default, in UTF-8 mode, characters with values greater than 128 never match  By default, in a UTF mode, characters with values greater than 128 never match
460  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
461  their original meanings from before UTF-8 support was available, mainly for  their original meanings from before UTF support was available, mainly for
462  efficiency reasons. However, if PCRE is compiled with Unicode property support,  efficiency reasons. However, if PCRE is compiled with Unicode property support,
463  and the PCRE_UCP option is set, the behaviour is changed so that Unicode  and the PCRE_UCP option is set, the behaviour is changed so that Unicode
464  properties are used to determine character types, as follows:  properties are used to determine character types, as follows:
# Line 424  any Unicode letter, and underscore. Note Line 473  any Unicode letter, and underscore. Note
473  \eB because they are defined in terms of \ew and \eW. Matching these sequences  \eB because they are defined in terms of \ew and \eW. Matching these sequences
474  is noticeably slower when PCRE_UCP is set.  is noticeably slower when PCRE_UCP is set.
475  .P  .P
476  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
477  other sequences, which match only ASCII characters by default, these always  release 5.10. In contrast to the other sequences, which match only ASCII
478  match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is  characters by default, these always match certain high-valued codepoints,
479  set. The horizontal space characters are:  whether or not PCRE_UCP is set. The horizontal space characters are:
480  .sp  .sp
481    U+0009     Horizontal tab    U+0009     Horizontal tab
482    U+0020     Space    U+0020     Space
# Line 453  The vertical space characters are: Line 502  The vertical space characters are:
502  .sp  .sp
503    U+000A     Linefeed    U+000A     Linefeed
504    U+000B     Vertical tab    U+000B     Vertical tab
505    U+000C     Formfeed    U+000C     Form feed
506    U+000D     Carriage return    U+000D     Carriage return
507    U+0085     Next line    U+0085     Next line
508    U+2028     Line separator    U+2028     Line separator
509    U+2029     Paragraph separator    U+2029     Paragraph separator
510    .sp
511    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
512    relevant.
513  .  .
514  .  .
515  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 465  The vertical space characters are: Line 517  The vertical space characters are:
517  .rs  .rs
518  .sp  .sp
519  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
520  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
521  equivalent to the following:  following:
522  .sp  .sp
523    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
524  .sp  .sp
# Line 477  below. Line 529  below.
529  .\"  .\"
530  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
531  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,
532  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
533  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
534  cannot be split.  cannot be split.
535  .P  .P
536  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
537  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).
538  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
539  recognized.  recognized.
# Line 497  one of the following sequences: Line 549  one of the following sequences:
549    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
550    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
551  .sp  .sp
552  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
553  \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
554  \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
555  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
556  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
557  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:  
558  .sp  .sp
559    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
560  .sp  .sp
561  They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside  They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
562  a character class, \eR is treated as an unrecognized escape sequence, and so  sequences. Inside a character class, \eR is treated as an unrecognized escape
563  matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.  sequence, and so matches the letter "R" by default, but causes an error if
564    PCRE_EXTRA is set.
565  .  .
566  .  .
567  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 518  matches the letter "R" by default, but c Line 570  matches the letter "R" by default, but c
570  .sp  .sp
571  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
572  escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
573  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
574  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.
575  The extra escape sequences are:  The extra escape sequences are:
576  .sp  .sp
577    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
578    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
579    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
580  .sp  .sp
581  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
582  script names, the general category properties, "Any", which matches any  script names, the general category properties, "Any", which matches any
# Line 553  Armenian, Line 605  Armenian,
605  Avestan,  Avestan,
606  Balinese,  Balinese,
607  Bamum,  Bamum,
608    Batak,
609  Bengali,  Bengali,
610  Bopomofo,  Bopomofo,
611    Brahmi,
612  Braille,  Braille,
613  Buginese,  Buginese,
614  Buhid,  Buhid,
615  Canadian_Aboriginal,  Canadian_Aboriginal,
616  Carian,  Carian,
617    Chakma,
618  Cham,  Cham,
619  Cherokee,  Cherokee,
620  Common,  Common,
# Line 602  Lisu, Line 657  Lisu,
657  Lycian,  Lycian,
658  Lydian,  Lydian,
659  Malayalam,  Malayalam,
660    Mandaic,
661  Meetei_Mayek,  Meetei_Mayek,
662    Meroitic_Cursive,
663    Meroitic_Hieroglyphs,
664    Miao,
665  Mongolian,  Mongolian,
666  Myanmar,  Myanmar,
667  New_Tai_Lue,  New_Tai_Lue,
# Line 621  Rejang, Line 680  Rejang,
680  Runic,  Runic,
681  Samaritan,  Samaritan,
682  Saurashtra,  Saurashtra,
683    Sharada,
684  Shavian,  Shavian,
685  Sinhala,  Sinhala,
686    Sora_Sompeng,
687  Sundanese,  Sundanese,
688  Syloti_Nagri,  Syloti_Nagri,
689  Syriac,  Syriac,
# Line 631  Tagbanwa, Line 692  Tagbanwa,
692  Tai_Le,  Tai_Le,
693  Tai_Tham,  Tai_Tham,
694  Tai_Viet,  Tai_Viet,
695    Takri,
696  Tamil,  Tamil,
697  Telugu,  Telugu,
698  Thaana,  Thaana,
# Line 705  the Lu, Ll, or Lt property, in other wor Line 767  the Lu, Ll, or Lt property, in other wor
767  a modifier or "other".  a modifier or "other".
768  .P  .P
769  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
770  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
771  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
772  (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
773  .\" HREF  .\" HREF
774  \fBpcreapi\fP  \fBpcreapi\fP
775  .\"  .\"
# Line 724  Unicode table. Line 786  Unicode table.
786  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
787  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters.
788  .P  .P
789  The \eX escape matches any number of Unicode characters that form an extended  Matching characters by Unicode property is not fast, because PCRE has to do a
790  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
791  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
792    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
793    PCRE_UCP option or by starting the pattern with (*UCP).
794    .
795    .
796    .SS Extended grapheme clusters
797    .rs
798  .sp  .sp
799  That is, it matches a character without the "mark" property, followed by zero  The \eX escape matches any number of Unicode characters that form an "extended
800  or more characters with the "mark" property, and treats the sequence as an  grapheme cluster", and treats the sequence as an atomic group
 atomic group  
801  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
802  .\" </a>  .\" </a>
803  (see below).  (see below).
804  .\"  .\"
805  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
806  preceding character. None of them have codepoints less than 256, so in  that was equivalent to
807  non-UTF-8 mode \eX matches any one character.  .sp
808  .P    (?>\ePM\epM*)
809  Matching characters by Unicode property is not fast, because PCRE has to search  .sp
810  a structure that contains data for over fifteen thousand characters. That is  That is, it matched a character without the "mark" property, followed by zero
811  why the traditional escape sequences such as \ed and \ew do not use Unicode  or more characters with the "mark" property. Characters with the "mark"
812  properties in PCRE by default, though you can make them do so by setting the  property are typically non-spacing accents that affect the preceding character.
813  PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with  .P
814  (*UCP).  This simple definition was extended in Unicode to include more complicated
815    kinds of composite character by giving each character a grapheme breaking
816    property, and creating rules that use these properties to define the boundaries
817    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
818    one of these clusters.
819    .P
820    \eX always matches at least one character. Then it decides whether to add
821    additional characters according to the following rules for ending a cluster:
822    .P
823    1. End at the end of the subject string.
824    .P
825    2. Do not end between CR and LF; otherwise end after any control character.
826    .P
827    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
828    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
829    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
830    character; an LVT or T character may be follwed only by a T character.
831    .P
832    4. Do not end before extending characters or spacing marks. Characters with
833    the "mark" property always have the "extend" grapheme breaking property.
834    .P
835    5. Do not end after prepend characters.
836    .P
837    6. Otherwise, end the cluster.
838  .  .
839  .  .
840  .\" HTML <a name="extraprops"></a>  .\" HTML <a name="extraprops"></a>
841  .SS PCRE's additional properties  .SS PCRE's additional properties
842  .rs  .rs
843  .sp  .sp
844  As well as the standard Unicode properties described in the previous  As well as the standard Unicode properties described above, PCRE supports four
845  section, PCRE supports four more that make it possible to convert traditional  more that make it possible to convert traditional escape sequences such as \ew
846  escape sequences such as \ew and \es and POSIX character classes to use Unicode  and \es and POSIX character classes to use Unicode properties. PCRE uses these
847  properties. PCRE uses these non-standard, non-Perl properties internally when  non-standard, non-Perl properties internally when PCRE_UCP is set. They are:
 PCRE_UCP is set. They are:  
848  .sp  .sp
849    Xan   Any alphanumeric character    Xan   Any alphanumeric character
850    Xps   Any POSIX space character    Xps   Any POSIX space character
# Line 764  PCRE_UCP is set. They are: Line 852  PCRE_UCP is set. They are:
852    Xwd   Any Perl "word" character    Xwd   Any Perl "word" character
853  .sp  .sp
854  Xan matches characters that have either the L (letter) or the N (number)  Xan matches characters that have either the L (letter) or the N (number)
855  property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or  property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
856  carriage return, and any other character that has the Z (separator) property.  carriage return, and any other character that has the Z (separator) property.
857  Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the  Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
858  same characters as Xan, plus underscore.  same characters as Xan, plus underscore.
# Line 774  same characters as Xan, plus underscore. Line 862  same characters as Xan, plus underscore.
862  .SS "Resetting the match start"  .SS "Resetting the match start"
863  .rs  .rs
864  .sp  .sp
865  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
866  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:  
867  .sp  .sp
868    foo\eKbar    foo\eKbar
869  .sp  .sp
# Line 835  escape sequence" error is generated inst Line 922  escape sequence" error is generated inst
922  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
923  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
924  \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
925  first or last character matches \ew, respectively. In UTF-8 mode, the meanings  first or last character matches \ew, respectively. In a UTF mode, the meanings
926  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
927  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
928  of word" or "end of word" metasequence. However, whatever follows \eb normally  of word" or "end of word" metasequence. However, whatever follows \eb normally
# Line 930  end of the subject in both modes, and if Line 1017  end of the subject in both modes, and if
1017  .sp  .sp
1018  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
1019  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
1020  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
1021  .P  .P
1022  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
1023  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 948  The handling of dot is entirely independ Line 1035  The handling of dot is entirely independ
1035  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
1036  special meaning in a character class.  special meaning in a character class.
1037  .P  .P
1038  The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not  The escape sequence \eN behaves like a dot, except that it is not affected by
1039  set. In other words, it matches any one character except one that signifies the  the PCRE_DOTALL option. In other words, it matches any character except one
1040  end of a line.  that signifies the end of a line. Perl also uses \eN to match characters by
1041  .  name; PCRE does not support this.
1042  .  .
1043  .SH "MATCHING A SINGLE BYTE"  .
1044  .rs  .SH "MATCHING A SINGLE DATA UNIT"
1045  .sp  .rs
1046  Outside a character class, the escape sequence \eC matches any one byte, both  .sp
1047  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  Outside a character class, the escape sequence \eC matches any one data unit,
1048  characters. The feature is provided in Perl in order to match individual bytes  whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1049  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always
1050  what remains in the string may be a malformed UTF-8 string. For this reason,  matches line-ending characters. The feature is provided in Perl in order to
1051  the \eC escape sequence is best avoided.  match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1052    used. Because \eC breaks up characters into individual data units, matching one
1053    unit with \eC in a UTF mode means that the rest of the string may start with a
1054    malformed UTF character. This has undefined results, because PCRE assumes that
1055    it is dealing with valid UTF strings (and by default it checks this at the
1056    start of processing unless the PCRE_NO_UTF8_CHECK or PCRE_NO_UTF16_CHECK option
1057    is used).
1058  .P  .P
1059  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1060  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1061  .\" </a>  .\" </a>
1062  (described below),  (described below)
1063  .\"  .\"
1064  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
1065  the lookbehind.  the lookbehind.
1066    .P
1067    In general, the \eC escape sequence is best avoided. However, one
1068    way of using it that avoids the problem of malformed UTF characters is to use a
1069    lookahead to check the length of the next character, as in this pattern, which
1070    could be used with a UTF-8 string (ignore white space and line breaks):
1071    .sp
1072      (?| (?=[\ex00-\ex7f])(\eC) |
1073          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1074          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1075          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1076    .sp
1077    A group that starts with (?| resets the capturing parentheses numbers in each
1078    alternative (see
1079    .\" HTML <a href="#dupsubpatternnumber">
1080    .\" </a>
1081    "Duplicate Subpattern Numbers"
1082    .\"
1083    below). The assertions at the start of each branch check the next UTF-8
1084    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1085    character's individual bytes are then captured by the appropriate number of
1086    groups.
1087  .  .
1088  .  .
1089  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 983  bracket causes a compile-time error. If Line 1097  bracket causes a compile-time error. If
1097  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
1098  (after an initial circumflex, if present) or escaped with a backslash.  (after an initial circumflex, if present) or escaped with a backslash.
1099  .P  .P
1100  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
1101  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
1102  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
1103  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
1104  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
1105  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
1106  backslash.  backslash.
1107  .P  .P
1108  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 999  circumflex is not an assertion; it still Line 1113  circumflex is not an assertion; it still
1113  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
1114  string.  string.
1115  .P  .P
1116  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
1117  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{
1118    escaping mechanism.
1119  .P  .P
1120  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
1121  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
1122  "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
1123  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
1124  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
1125  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1126  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1127  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
1128  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
1129  with UTF-8 support.  well as with UTF support.
1130  .P  .P
1131  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
1132  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 1033  followed by two other characters. The oc Line 1148  followed by two other characters. The oc
1148  "]" can also be used to end a range.  "]" can also be used to end a range.
1149  .P  .P
1150  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
1151  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1152  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}].  
1153  .P  .P
1154  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
1155  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
1156  [][\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
1157  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
1158  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
1159  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
1160  property support.  property support.
1161  .P  .P
1162  The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1163  \eW may also appear in a character class, and add the characters that they  \eV, \ew, and \eW may appear in a character class, and add the characters that
1164  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1165  circumflex can conveniently be used with the upper case character types to  digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1166    and their upper case partners, just as it does when they appear outside a
1167    character class, as described in the section entitled
1168    .\" HTML <a href="#genericchartypes">
1169    .\" </a>
1170    "Generic character types"
1171    .\"
1172    above. The escape sequence \eb has a different meaning inside a character
1173    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1174    are not special inside a character class. Like any other unrecognized escape
1175    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1176    default, but cause an error if the PCRE_EXTRA option is set.
1177    .P
1178    A circumflex can conveniently be used with the upper case character types to
1179  specify a more restricted set of characters than the matching lower case type.  specify a more restricted set of characters than the matching lower case type.
1180  For example, the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore,
1181    whereas [\ew] includes underscore. A positive character class should be read as
1182    "something OR something OR ..." and a negative class as "NOT something AND NOT
1183    something AND NOT ...".
1184  .P  .P
1185  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1186  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 1102  matches "1", "2", or any non-digit. PCRE Line 1232  matches "1", "2", or any non-digit. PCRE
1232  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
1233  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1234  .P  .P
1235  By default, in UTF-8 mode, characters with values greater than 128 do not match  By default, in UTF modes, characters with values greater than 128 do not match
1236  any of the POSIX character classes. However, if the PCRE_UCP option is passed  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1237  to \fBpcre_compile()\fP, some of the classes are changed so that Unicode  to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1238  character properties are used. This is achieved by replacing the POSIX classes  character properties are used. This is achieved by replacing the POSIX classes
# Line 1173  extracts it into the global options (and Line 1303  extracts it into the global options (and
1303  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1304  .P  .P
1305  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1306  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
1307  .sp  .sp
1308    (a(?i)b)c    (a(?i)b)c
1309  .sp  .sp
# Line 1190  option settings happen at compile time. Line 1320  option settings happen at compile time.
1320  behaviour otherwise.  behaviour otherwise.
1321  .P  .P
1322  \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
1323  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
1324  pattern can contain special leading sequences such as (*CRLF) to override what  the pattern can contain special leading sequences such as (*CRLF) to override
1325  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
1326  section entitled  the section entitled
1327  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
1328  .\" </a>  .\" </a>
1329  "Newline sequences"  "Newline sequences"
1330  .\"  .\"
1331  above. There are also the (*UTF8) and (*UCP) leading sequences that can be used  above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1332  to set UTF-8 and Unicode property modes; they are equivalent to setting the  can be used to set UTF and Unicode property modes; they are equivalent to
1333  PCRE_UTF8 and the PCRE_UCP options, respectively.  setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1334  .  .
1335  .  .
1336  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1214  Turning part of a pattern into a subpatt Line 1344  Turning part of a pattern into a subpatt
1344  .sp  .sp
1345    cat(aract|erpillar|)    cat(aract|erpillar|)
1346  .sp  .sp
1347  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1348  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1349  .sp  .sp
1350  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
1351  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
1352  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
1353  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1354  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1355  .P  .P
1356  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
1357    numbers for the capturing subpatterns. For example, if the string "the red
1358    king" is matched against the pattern
1359  .sp  .sp
1360    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1361  .sp  .sp
# Line 1272  at captured substring number one, whiche Line 1404  at captured substring number one, whiche
1404  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
1405  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1406  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
1407  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
1408  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1409  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.  
1410  .sp  .sp
1411    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1412    / ( 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 1287  or "defdef": Line 1418  or "defdef":
1418  .sp  .sp
1419    /(?|(abc)|(def))\e1/    /(?|(abc)|(def))\e1/
1420  .sp  .sp
1421  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
1422  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
1423  pattern matches "abcabc" or "defabc":  "abcabc" or "defabc":
1424  .sp  .sp
1425    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1426  .sp  .sp
# Line 1400  items: Line 1531  items:
1531    a literal data character    a literal data character
1532    the dot metacharacter    the dot metacharacter
1533    the \eC escape sequence    the \eC escape sequence
1534    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1535    the \eR escape sequence    the \eR escape sequence
1536    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1537    a character class    a character class
1538    a back reference (see next section)    a back reference (see next section)
1539    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1540    a recursive or "subroutine" call to a subpattern    a subroutine call to a subpattern (recursive or otherwise)
1541  .sp  .sp
1542  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1543  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1431  where a quantifier is not allowed, or on Line 1562  where a quantifier is not allowed, or on
1562  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
1563  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1564  .P  .P
1565  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
1566  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
1567  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,
1568  support is available, \eX{3} matches three Unicode extended sequences, each of  \eX{3} matches three Unicode extended grapheme clusters, each of which may be
1569  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1570  .P  .P
1571  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
1572  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 1444  subpatterns that are referenced as Line 1575  subpatterns that are referenced as
1575  .\" </a>  .\" </a>
1576  subroutines  subroutines
1577  .\"  .\"
1578  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1579  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1580    .\" </a>
1581    "Defining subpatterns for use by reference only"
1582    .\"
1583    below). Items other than subpatterns that have a {0} quantifier are omitted
1584    from the compiled pattern.
1585  .P  .P
1586  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1587  abbreviations:  abbreviations:
# Line 1516  In cases where it is known that the subj Line 1652  In cases where it is known that the subj
1652  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1653  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1654  .P  .P
1655  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1656  is inside capturing parentheses that are the subject of a back reference  is inside capturing parentheses that are the subject of a back reference
1657  elsewhere in the pattern, a match at the start may fail where a later one  elsewhere in the pattern, a match at the start may fail where a later one
1658  succeeds. Consider, for example:  succeeds. Consider, for example:
# Line 1526  succeeds. Consider, for example: Line 1662  succeeds. Consider, for example:
1662  If the subject is "xyz123abc123" the match point is the fourth character. For  If the subject is "xyz123abc123" the match point is the fourth character. For
1663  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1664  .P  .P
1665    Another case where implicit anchoring is not applied is when the leading .* is
1666    inside an atomic group. Once again, a match at the start may fail where a later
1667    one succeeds. Consider this pattern:
1668    .sp
1669      (?>.*?a)b
1670    .sp
1671    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1672    (*PRUNE) and (*SKIP) also disable this optimization.
1673    .P
1674  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1675  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1676  .sp  .sp
# Line 1670  no such problem when named parentheses a Line 1815  no such problem when named parentheses a
1815  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1816  .P  .P
1817  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
1818  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
1819  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
1820  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1821  .sp  .sp
1822    (ring), \e1    (ring), \e1
1823    (ring), \eg1    (ring), \eg1
# Line 1686  example: Line 1831  example:
1831    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1832  .sp  .sp
1833  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
1834  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.
1835  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
1836  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
1837  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1838  .P  .P
1839  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1840  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1740  Because there may be many capturing pare Line 1885  Because there may be many capturing pare
1885  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.
1886  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
1887  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
1888  whitespace. Otherwise, the \eg{ syntax or an empty comment (see  white space. Otherwise, the \eg{ syntax or an empty comment (see
1889  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1890  .\" </a>  .\" </a>
1891  "Comments"  "Comments"
# Line 1791  those that look ahead of the current pos Line 1936  those that look ahead of the current pos
1936  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,
1937  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.
1938  .P  .P
1939  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1940  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
1941  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1942  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1943  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1944  because it does not make sense for negative assertions.  .P
1945    For compatibility with Perl, assertion subpatterns may be repeated; though
1946    it makes no sense to assert the same thing several times, the side effect of
1947    capturing parentheses may occasionally be useful. In practice, there only three
1948    cases:
1949    .sp
1950    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1951    However, it may contain internal capturing parenthesized groups that are called
1952    from elsewhere via the
1953    .\" HTML <a href="#subpatternsassubroutines">
1954    .\" </a>
1955    subroutine mechanism.
1956    .\"
1957    .sp
1958    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1959    were {0,1}. At run time, the rest of the pattern match is tried with and
1960    without the assertion, the order depending on the greediness of the quantifier.
1961    .sp
1962    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1963    The assertion is obeyed just once when encountered during matching.
1964  .  .
1965  .  .
1966  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1825  lookbehind assertion is needed to achiev Line 1989  lookbehind assertion is needed to achiev
1989  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
1990  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
1991  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.
1992  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 (?!).  
1993  .  .
1994  .  .
1995  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1851  is permitted, but Line 2014  is permitted, but
2014  .sp  .sp
2015  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2016  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
2017  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
2018  match the same length of string. An assertion such as  length of string. An assertion such as
2019  .sp  .sp
2020    (?<=ab(c|de))    (?<=ab(c|de))
2021  .sp  .sp
# Line 1862  branches: Line 2025  branches:
2025  .sp  .sp
2026    (?<=abc|abde)    (?<=abc|abde)
2027  .sp  .sp
2028  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
2029  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
2030  .\" </a>  .\" </a>
2031  (see above)  (see above)
# Line 1875  temporarily move the current position ba Line 2038  temporarily move the current position ba
2038  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2039  assertion fails.  assertion fails.
2040  .P  .P
2041  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
2042  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
2043  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
2044  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2045    permitted.
2046  .P  .P
2047  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2048  .\" </a>  .\" </a>
# Line 1967  already been matched. The two possible f Line 2131  already been matched. The two possible f
2131  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2132  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
2133  subpattern, a compile-time error occurs. Each of the two alternatives may  subpattern, a compile-time error occurs. Each of the two alternatives may
2134  itself contain nested subpatterns of any form, including conditional  itself contain nested subpatterns of any form, including conditional
2135  subpatterns; the restriction to two alternatives applies only at the level of  subpatterns; the restriction to two alternatives applies only at the level of
2136  the condition. This pattern fragment is an example where the alternatives are  the condition. This pattern fragment is an example where the alternatives are
2137  complex:  complex:
2138  .sp  .sp
2139    (?(1) (A|B|C) | (D | (?(2)E|F) | E) )    (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
# Line 1990  matched. If there is more than one captu Line 2154  matched. If there is more than one captu
2154  .\" </a>  .\" </a>
2155  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2156  .\"  .\"
2157  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
2158  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
2159  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2160  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
2161  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
2162  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2163    zero in any of these forms is not used; it provokes a compile-time error.)
2164  .P  .P
2165  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2166  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 2006  three parts for ease of discussion: Line 2171  three parts for ease of discussion:
2171  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2172  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
2173  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
2174  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2175  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,
2176  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
2177  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2178  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 2063  The syntax for recursive patterns Line 2228  The syntax for recursive patterns
2228  .\"  .\"
2229  is described below.  is described below.
2230  .  .
2231    .\" HTML <a name="subdefine"></a>
2232  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2233  .rs  .rs
2234  .sp  .sp
# Line 2070  If the condition is the string (DEFINE), Line 2236  If the condition is the string (DEFINE),
2236  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
2237  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2238  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
2239  "subroutines" that can be referenced from elsewhere. (The use of  subroutines that can be referenced from elsewhere. (The use of
2240  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2241  .\" </a>  .\" </a>
2242  "subroutines"  subroutines
2243  .\"  .\"
2244  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
2245  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore white space and line
2246    breaks):
2247  .sp  .sp
2248    (?(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) )
2249    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 2111  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2278  dd-aaa-dd or dd-dd-dd, where aaa are let
2278  .SH COMMENTS  .SH COMMENTS
2279  .rs  .rs
2280  .sp  .sp
2281  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
2282  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,
2283  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
2284    subpattern name or number. The characters that make up a comment play no part
2285    in the pattern matching.
2286  .P  .P
2287  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
2288  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2289  next newline character or character sequence in the pattern. Which characters  option is set, an unescaped # character also introduces a comment, which in
2290  are interpreted as newlines is controlled by the options passed to  this case continues to immediately after the next newline character or
2291  \fBpcre_compile()\fP or by a special sequence at the start of the pattern, as  character sequence in the pattern. Which characters are interpreted as newlines
2292  described in the section entitled  is controlled by the options passed to a compiling function or by a special
2293  .\" HTML <a href="#recursion">  sequence at the start of the pattern, as described in the section entitled
2294    .\" HTML <a href="#newlines">
2295  .\" </a>  .\" </a>
2296  "Newline conventions"  "Newline conventions"
2297  .\"  .\"
2298  above. Note that end of a comment is a literal newline sequence in the pattern;  above. Note that the end of this type of comment is a literal newline sequence
2299  escape sequences that happen to represent a newline do not terminate a comment.  in the pattern; escape sequences that happen to represent a newline do not
2300  For example, consider this pattern when PCRE_EXTENDED is set, and the default  count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2301  newline convention is in force:  default newline convention is in force:
2302  .sp  .sp
2303    abc #comment \en still comment    abc #comment \en still comment
2304  .sp  .sp
2305  On encountering the # character, \fBpcre_compile()\fP skips along, looking for  On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2306  a newline in the pattern. The sequence \en is still literal at this stage, so  a newline in the pattern. The sequence \en is still literal at this stage, so
2307  it does not terminate the comment. Only an actual character with the code value  it does not terminate the comment. Only an actual character with the code value
2308  0x0a does so.  0x0a (the default newline) does so.
2309  .  .
2310  .  .
2311  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 2164  individual subpattern recursion. After i Line 2334  individual subpattern recursion. After i
2334  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.
2335  .P  .P
2336  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
2337  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
2338  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
2339  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2340  .\" </a>  .\" </a>
2341  "subroutine"  non-recursive subroutine
2342  .\"  .\"
2343  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
2344  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
# Line 2193  We have put the pattern into parentheses Line 2363  We have put the pattern into parentheses
2363  them instead of the whole pattern.  them instead of the whole pattern.
2364  .P  .P
2365  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
2366  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
2367  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
2368  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2369  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.  
2370  .P  .P
2371  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2372  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2373  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2374  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2375  .\" </a>  .\" </a>
2376  "subroutine"  non-recursive subroutine
2377  .\"  .\"
2378  calls, as described in the next section.  calls, as described in the next section.
2379  .P  .P
# Line 2241  documentation). If the pattern above is Line 2410  documentation). If the pattern above is
2410  .sp  .sp
2411  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
2412  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
2413  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
2414  set at a deeper level.  (temporarily) set at a deeper level during the matching process.
2415  .P  .P
2416  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
2417  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 2262  is the actual recursive call. Line 2431  is the actual recursive call.
2431  .  .
2432  .  .
2433  .\" HTML <a name="recursiondifference"></a>  .\" HTML <a name="recursiondifference"></a>
2434  .SS "Recursion difference from Perl"  .SS "Differences in recursion processing between PCRE and Perl"
2435  .rs  .rs
2436  .sp  .sp
2437  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
2438  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
2439  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
2440  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
2441  following pattern, which purports to match a palindromic string that contains  subsequent matching failure. This can be illustrated by the following pattern,
2442  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  which purports to match a palindromic string that contains an odd number of
2443    characters (for example, "a", "aba", "abcba", "abcdcba"):
2444  .sp  .sp
2445    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2446  .sp  .sp
# Line 2300  time we do have another alternative to t Line 2470  time we do have another alternative to t
2470  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2471  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2472  .P  .P
2473  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
2474  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
2475    this:
2476  .sp  .sp
2477    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2478  .sp  .sp
# Line 2330  For example, although "abcba" is correct Line 2501  For example, although "abcba" is correct
2501  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
2502  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
2503  recursion to try other alternatives, so the entire match fails.  recursion to try other alternatives, so the entire match fails.
2504    .P
2505    The second way in which PCRE and Perl differ in their recursion processing is
2506    in the handling of captured values. In Perl, when a subpattern is called
2507    recursively or as a subpattern (see the next section), it has no access to any
2508    values that were captured outside the recursion, whereas in PCRE these values
2509    can be referenced. Consider this pattern:
2510    .sp
2511      ^(.)(\e1|a(?2))
2512    .sp
2513    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2514    then in the second group, when the back reference \e1 fails to match "b", the
2515    second alternative matches "a" and then recurses. In the recursion, \e1 does
2516    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2517    match because inside the recursive call \e1 cannot access the externally set
2518    value.
2519  .  .
2520  .  .
2521  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2522  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2523  .rs  .rs
2524  .sp  .sp
2525  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
2526  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
2527  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2528  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2529  relative, as in these examples:  relative, as in these examples:
2530  .sp  .sp
# Line 2358  matches "sense and sensibility" and "res Line 2544  matches "sense and sensibility" and "res
2544  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
2545  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2546  .P  .P
2547  Like recursive subpatterns, a subroutine call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2548  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
2549  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
2550  matching failure. Any capturing parentheses that are set during the subroutine  subsequent matching failure. Any capturing parentheses that are set during the
2551  call revert to their previous values afterwards.  subroutine call revert to their previous values afterwards.
2552  .P  .P
2553  When a subpattern is used as a subroutine, processing options such as  Processing options such as case-independence are fixed when a subpattern is
2554  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
2555  changed for different calls. For example, consider this pattern:  different calls. For example, consider this pattern:
2556  .sp  .sp
2557    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2558  .sp  .sp
# Line 2405  same pair of parentheses when there is a Line 2591  same pair of parentheses when there is a
2591  .P  .P
2592  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
2593  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
2594  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
2595  By default, this variable contains NULL, which disables all calling out.  (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this
2596    variable contains NULL, which disables all calling out.
2597  .P  .P
2598  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2599  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 2415  For example, this pattern has two callou Line 2602  For example, this pattern has two callou
2602  .sp  .sp
2603    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2604  .sp  .sp
2605  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
2606  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2607  255.  255.
2608  .P  .P
2609  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
2610  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
2611  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
2612  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  the matching function. The callout function may cause matching to proceed, to
2613  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
2614  description of the interface to the callout function is given in the  the callout function is given in the
2615  .\" HREF  .\" HREF
2616  \fBpcrecallout\fP  \fBpcrecallout\fP
2617  .\"  .\"
# Line 2442  production code should be noted to avoid Line 2629  production code should be noted to avoid
2629  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2630  .P  .P
2631  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
2632  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
2633  a backtracking algorithm. With the exception of (*FAIL), which behaves like a  matching functions, which use a backtracking algorithm. With the exception of
2634  failing negative assertion, they cause an error if encountered by  (*FAIL), which behaves like a failing negative assertion, they cause an error
2635  \fBpcre_dfa_exec()\fP.  if encountered by a DFA matching function.
2636  .P  .P
2637  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
2638  (including recursive subpatterns), their effect is confined to that subpattern;  called as a subroutine (whether or not recursively), their effect is confined
2639  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
2640  processed as anchored at the point where they are tested.  exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2641    a successful positive assertion \fIis\fP passed back when a match succeeds
2642    (compare capturing parentheses in assertions). Note that such subpatterns are
2643    processed as anchored at the point where they are tested. Note also that Perl's
2644    treatment of subroutines and assertions is different in some cases.
2645  .P  .P
2646  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
2647  parenthesis followed by an asterisk. They are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2648  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2649  depending on whether or not an argument is present. An name is a sequence of  depending on whether or not an argument is present. A name is any sequence of
2650  letters, digits, and underscores. If the name is empty, that is, if the closing  characters that does not include a closing parenthesis. The maximum length of
2651  parenthesis immediately follows the colon, the effect is as if the colon were  name is 255 in the 8-bit library and 65535 in the 16-bit library. If the name
2652  not there. Any number of these verbs may occur in a pattern.  is empty, that is, if the closing parenthesis immediately follows the colon,
2653  .P  the effect is as if the colon were not there. Any number of these verbs may
2654    occur in a pattern.
2655    .
2656    .
2657    .\" HTML <a name="nooptimize"></a>
2658    .SS "Optimizations that affect backtracking verbs"
2659    .rs
2660    .sp
2661  PCRE contains some optimizations that are used to speed up matching by running  PCRE contains some optimizations that are used to speed up matching by running
2662  some checks at the start of each match attempt. For example, it may know the  some checks at the start of each match attempt. For example, it may know the
2663  minimum length of matching subject, or that a particular character must be  minimum length of matching subject, or that a particular character must be
2664  present. When one of these optimizations suppresses the running of a match, any  present. When one of these optimizations suppresses the running of a match, any
2665  included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
2666  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2667  when calling \fBpcre_exec()\fP.  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2668    pattern with (*NO_START_OPT). There is more discussion of this option in the
2669    section entitled
2670    .\" HTML <a href="pcreapi.html#execoptions">
2671    .\" </a>
2672    "Option bits for \fBpcre_exec()\fP"
2673    .\"
2674    in the
2675    .\" HREF
2676    \fBpcreapi\fP
2677    .\"
2678    documentation.
2679    .P
2680    Experiments with Perl suggest that it too has similar optimizations, sometimes
2681    leading to anomalous results.
2682  .  .
2683  .  .
2684  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
# Line 2478  followed by a name. Line 2690  followed by a name.
2690     (*ACCEPT)     (*ACCEPT)
2691  .sp  .sp
2692  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
2693  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2694  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is  subroutine, only that subpattern is ended successfully. Matching then continues
2695  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
2696    far is captured. For example:
2697  .sp  .sp
2698    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2699  .sp  .sp
# Line 2489  the outer parentheses. Line 2702  the outer parentheses.
2702  .sp  .sp
2703    (*FAIL) or (*F)    (*FAIL) or (*F)
2704  .sp  .sp
2705  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
2706  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
2707  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2708  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 2513  starting point (see (*SKIP) below). Line 2726  starting point (see (*SKIP) below).
2726  A name is always required with this verb. There may be as many instances of  A name is always required with this verb. There may be as many instances of
2727  (*MARK) as you like in a pattern, and their names do not have to be unique.  (*MARK) as you like in a pattern, and their names do not have to be unique.
2728  .P  .P
2729  When a match succeeds, the name of the last-encountered (*MARK) is passed back  When a match succeeds, the name of the last-encountered (*MARK) on the matching
2730  to the caller via the \fIpcre_extra\fP data structure, as described in the  path is passed back to the caller as described in the section entitled
2731  .\" HTML <a href="pcreapi.html#extradata">  .\" HTML <a href="pcreapi.html#extradata">
2732  .\" </a>  .\" </a>
2733  section on \fIpcre_extra\fP  "Extra data for \fBpcre_exec()\fP"
2734  .\"  .\"
2735  in the  in the
2736  .\" HREF  .\" HREF
2737  \fBpcreapi\fP  \fBpcreapi\fP
2738  .\"  .\"
2739  documentation. No data is returned for a partial match. Here is an example of  documentation. Here is an example of \fBpcretest\fP output, where the /K
2740  \fBpcretest\fP output, where the /K modifier requests the retrieval and  modifier requests the retrieval and outputting of (*MARK) data:
 outputting of (*MARK) data:  
2741  .sp  .sp
2742    /X(*MARK:A)Y|X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2743    XY    data> XY
2744     0: XY     0: XY
2745    MK: A    MK: A
2746    XZ    XZ
# Line 2540  indicates which of the two alternatives Line 2752  indicates which of the two alternatives
2752  of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
2753  capturing parentheses.  capturing parentheses.
2754  .P  .P
2755  A name may also be returned after a failed match if the final path through the  If (*MARK) is encountered in a positive assertion, its name is recorded and
2756  pattern involves (*MARK). However, unless (*MARK) used in conjunction with  passed back if it is the last-encountered. This does not happen for negative
2757  (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the  assertions.
2758  starting point for matching is advanced, the final check is often with an empty  .P
2759  string, causing a failure before (*MARK) is reached. For example:  After a partial match or a failed match, the name of the last encountered
2760  .sp  (*MARK) in the entire match process is returned. For example:
   /X(*MARK:A)Y|X(*MARK:B)Z/K  
   XP  
   No match  
 .sp  
 There are three potential starting points for this match (starting with X,  
 starting with P, and with an empty string). If the pattern is anchored, the  
 result is different:  
2761  .sp  .sp
2762    /^X(*MARK:A)Y|^X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2763    XP    data> XP
2764    No match, mark = B    No match, mark = B
2765  .sp  .sp
2766  PCRE's start-of-match optimizations can also interfere with this. For example,  Note that in this unanchored example the mark is retained from the match
2767  if, as a result of a call to \fBpcre_study()\fP, it knows the minimum  attempt that started at the letter "X" in the subject. Subsequent match
2768  subject length for a match, a shorter subject will not be scanned at all.  attempts starting at "P" and then with an empty string do not get as far as the
2769  .P  (*MARK) item, but nevertheless do not reset it.
2770  Note that similar anomalies (though different in detail) exist in Perl, no  .P
2771  doubt for the same reasons. The use of (*MARK) data after a failed match of an  If you are interested in (*MARK) values after failed matches, you should
2772  unanchored pattern is not recommended, unless (*COMMIT) is involved.  probably set the PCRE_NO_START_OPTIMIZE option
2773    .\" HTML <a href="#nooptimize">
2774    .\" </a>
2775    (see above)
2776    .\"
2777    to ensure that the match is always attempted.
2778  .  .
2779  .  .
2780  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
# Line 2601  Note that (*COMMIT) at the start of a pa Line 2811  Note that (*COMMIT) at the start of a pa
2811  unless PCRE's start-of-match optimizations are turned off, as shown in this  unless PCRE's start-of-match optimizations are turned off, as shown in this
2812  \fBpcretest\fP example:  \fBpcretest\fP example:
2813  .sp  .sp
2814    /(*COMMIT)abc/      re> /(*COMMIT)abc/
2815    xyzabc    data> xyzabc
2816     0: abc     0: abc
2817    xyzabc\eY    xyzabc\eY
2818    No match    No match
# Line 2623  reached, or when matching to the right o Line 2833  reached, or when matching to the right o
2833  the right, backtracking cannot cross (*PRUNE). In simple cases, the use of  the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2834  (*PRUNE) is just an alternative to an atomic group or possessive quantifier,  (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2835  but there are some uses of (*PRUNE) that cannot be expressed in any other way.  but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2836  The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the  The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2837  match fails completely; the name is passed back if this is the final attempt.  anchored pattern (*PRUNE) has the same effect as (*COMMIT).
 (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored  
 pattern (*PRUNE) has the same effect as (*COMMIT).  
2838  .sp  .sp
2839    (*SKIP)    (*SKIP)
2840  .sp  .sp
# Line 2652  following pattern fails to match, the pr Line 2860  following pattern fails to match, the pr
2860  searched for the most recent (*MARK) that has the same name. If one is found,  searched for the most recent (*MARK) that has the same name. If one is found,
2861  the "bumpalong" advance is to the subject position that corresponds to that  the "bumpalong" advance is to the subject position that corresponds to that
2862  (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a  (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2863  matching name is found, normal "bumpalong" of one character happens (the  matching name is found, the (*SKIP) is ignored.
 (*SKIP) is ignored).  
2864  .sp  .sp
2865    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
2866  .sp  .sp
2867  This verb causes a skip to the next alternation in the innermost enclosing  This verb causes a skip to the next innermost alternative if the rest of the
2868  group if the rest of the pattern does not match. That is, it cancels pending  pattern does not match. That is, it cancels pending backtracking, but only
2869  backtracking, but only within the current alternation. Its name comes from the  within the current alternative. Its name comes from the observation that it can
2870  observation that it can be used for a pattern-based if-then-else block:  be used for a pattern-based if-then-else block:
2871  .sp  .sp
2872    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2873  .sp  .sp
2874  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
2875  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
2876  second alternative and tries COND2, without backtracking into COND1. The  second alternative and tries COND2, without backtracking into COND1. The
2877  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2878  overall match fails. If (*THEN) is not directly inside an alternation, it acts  If (*THEN) is not inside an alternation, it acts like (*PRUNE).
 like (*PRUNE).  
 .  
 .P  
 The above verbs provide four different "strengths" of control when subsequent  
 matching fails. (*THEN) is the weakest, carrying on the match at the next  
 alternation. (*PRUNE) comes next, failing the match at the current starting  
 position, but allowing an advance to the next character (for an unanchored  
 pattern). (*SKIP) is similar, except that the advance may be more than one  
 character. (*COMMIT) is the strongest, causing the entire match to fail.  
2879  .P  .P
2880  If more than one is present in a pattern, the "stongest" one wins. For example,  Note that a subpattern that does not contain a | character is just a part of
2881  consider this pattern, where A, B, etc. are complex pattern fragments:  the enclosing alternative; it is not a nested alternation with only one
2882    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2883    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2884    pattern fragments that do not contain any | characters at this level:
2885    .sp
2886      A (B(*THEN)C) | D
2887    .sp
2888    If A and B are matched, but there is a failure in C, matching does not
2889    backtrack into A; instead it moves to the next alternative, that is, D.
2890    However, if the subpattern containing (*THEN) is given an alternative, it
2891    behaves differently:
2892    .sp
2893      A (B(*THEN)C | (*FAIL)) | D
2894    .sp
2895    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2896    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2897    because there are no more alternatives to try. In this case, matching does now
2898    backtrack into A.
2899    .P
2900    Note also that a conditional subpattern is not considered as having two
2901    alternatives, because only one is ever used. In other words, the | character in
2902    a conditional subpattern has a different meaning. Ignoring white space,
2903    consider:
2904    .sp
2905      ^.*? (?(?=a) a | b(*THEN)c )
2906    .sp
2907    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2908    it initially matches zero characters. The condition (?=a) then fails, the
2909    character "b" is matched, but "c" is not. At this point, matching does not
2910    backtrack to .*? as might perhaps be expected from the presence of the |
2911    character. The conditional subpattern is part of the single alternative that
2912    comprises the whole pattern, and so the match fails. (If there was a backtrack
2913    into .*?, allowing it to match "b", the match would succeed.)
2914    .P
2915    The verbs just described provide four different "strengths" of control when
2916    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2917    next alternative. (*PRUNE) comes next, failing the match at the current
2918    starting position, but allowing an advance to the next character (for an
2919    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2920    than one character. (*COMMIT) is the strongest, causing the entire match to
2921    fail.
2922    .P
2923    If more than one such verb is present in a pattern, the "strongest" one wins.
2924    For example, consider this pattern, where A, B, etc. are complex pattern
2925    fragments:
2926  .sp  .sp
2927    (A(*COMMIT)B(*THEN)C|D)    (A(*COMMIT)B(*THEN)C|D)
2928  .sp  .sp
2929  Once A has matched, PCRE is committed to this match, at the current starting  Once A has matched, PCRE is committed to this match, at the current starting
2930  position. If subsequently B matches, but C does not, the normal (*THEN) action  position. If subsequently B matches, but C does not, the normal (*THEN) action
2931  of trying the next alternation (that is, D) does not happen because (*COMMIT)  of trying the next alternative (that is, D) does not happen because (*COMMIT)
2932  overrides.  overrides.
2933  .  .
2934  .  .
# Line 2694  overrides. Line 2936  overrides.
2936  .rs  .rs
2937  .sp  .sp
2938  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2939  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.
2940  .  .
2941  .  .
2942  .SH AUTHOR  .SH AUTHOR
# Line 2711  Cambridge CB2 3QH, England. Line 2953  Cambridge CB2 3QH, England.
2953  .rs  .rs
2954  .sp  .sp
2955  .nf  .nf
2956  Last updated: 26 October 2010  Last updated: 25 August 2012
2957  Copyright (c) 1997-2010 University of Cambridge.  Copyright (c) 1997-2012 University of Cambridge.
2958  .fi  .fi

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