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# Line 18  man page, in case the conversion went wr Line 18  man page, in case the conversion went wr
18  <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>  <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
19  <li><a name="TOC4" href="#SEC4">BACKSLASH</a>  <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
20  <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>  <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
21  <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT)</a>  <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT) AND \N</a>
22  <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a>  <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE DATA UNIT</a>
23  <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a>  <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a>
24  <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>  <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
25  <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>  <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
# Line 61  description of PCRE's regular expression Line 61  description of PCRE's regular expression
61  </P>  </P>
62  <P>  <P>
63  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,
64  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 strings in the original library, and a
65  build PCRE to include UTF-8 support, and then call <b>pcre_compile()</b> with  second library that supports 16-bit and UTF-16 character strings. To use these
66  the PCRE_UTF8 option. There is also a special sequence that can be given at the  features, PCRE must be built to include appropriate support. When using UTF
67  start of a pattern:  strings you must either call the compiling function with the PCRE_UTF8 or
68    PCRE_UTF16 option, or the pattern must start with one of these special
69    sequences:
70  <pre>  <pre>
71    (*UTF8)    (*UTF8)
72      (*UTF16)
73  </pre>  </pre>
74  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
75  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
76  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
77  of UTF-8 features in the  of features in the
78  <a href="pcre.html#utf8support">section on UTF-8 support</a>  <a href="pcreunicode.html"><b>pcreunicode</b></a>
 in the main  
 <a href="pcre.html"><b>pcre</b></a>  
79  page.  page.
80  </P>  </P>
81  <P>  <P>
82    Another special sequence that may appear at the start of a pattern or in
83    combination with (*UTF8) or (*UTF16) is:
84    <pre>
85      (*UCP)
86    </pre>
87    This has the same effect as setting the PCRE_UCP option: it causes sequences
88    such as \d and \w to use Unicode properties to determine character types,
89    instead of recognizing only characters with codes less than 128 via a lookup
90    table.
91    </P>
92    <P>
93    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
94    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
95    also some more of these special sequences that are concerned with the handling
96    of newlines; they are described below.
97    </P>
98    <P>
99  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
100  PCRE when its main matching function, <b>pcre_exec()</b>, is used.  PCRE when one its main matching functions, <b>pcre_exec()</b> (8-bit) or
101  From release 6.0, PCRE offers a second matching function,  <b>pcre16_exec()</b> (16-bit), is used. PCRE also has alternative matching
102  <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not  functions, <b>pcre_dfa_exec()</b> and <b>pcre16_dfa_exec()</b>, which match using
103  Perl-compatible. Some of the features discussed below are not available when  a different algorithm that is not Perl-compatible. Some of the features
104  <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the  discussed below are not available when DFA matching is used. The advantages and
105  alternative function, and how it differs from the normal function, are  disadvantages of the alternative functions, and how they differ from the normal
106  discussed in the  functions, are discussed in the
107  <a href="pcrematching.html"><b>pcrematching</b></a>  <a href="pcrematching.html"><b>pcrematching</b></a>
108  page.  page.
109  </P>  <a name="newlines"></a></P>
110  <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>  <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
111  <P>  <P>
112  PCRE supports five different conventions for indicating line breaks in  PCRE supports five different conventions for indicating line breaks in
# Line 111  string with one of the following five se Line 129  string with one of the following five se
129    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
130    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
131  </pre>  </pre>
132  These override the default and the options given to <b>pcre_compile()</b>. For  These override the default and the options given to the compiling function. For
133  example, on a Unix system where LF is the default newline sequence, the pattern  example, on a Unix system where LF is the default newline sequence, the pattern
134  <pre>  <pre>
135    (*CR)a.b    (*CR)a.b
# Line 123  they must be in upper case. If more than Line 141  they must be in upper case. If more than
141  is used.  is used.
142  </P>  </P>
143  <P>  <P>
144  The newline convention does not affect what the \R escape sequence matches. By  The newline convention affects the interpretation of the dot metacharacter when
145  default, this is any Unicode newline sequence, for Perl compatibility. However,  PCRE_DOTALL is not set, and also the behaviour of \N. However, it does not
146  this can be changed; see the description of \R in the section entitled  affect what the \R escape sequence matches. By default, this is any Unicode
147    newline sequence, for Perl compatibility. However, this can be changed; see the
148    description of \R in the section entitled
149  <a href="#newlineseq">"Newline sequences"</a>  <a href="#newlineseq">"Newline sequences"</a>
150  below. A change of \R setting can be combined with a change of newline  below. A change of \R setting can be combined with a change of newline
151  convention.  convention.
# Line 140  corresponding characters in the subject. Line 160  corresponding characters in the subject.
160  </pre>  </pre>
161  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
162  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
163  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
164  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
165  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
166  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
167  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
168  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
169  UTF-8 support.  UTF support.
170  </P>  </P>
171  <P>  <P>
172  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
# Line 190  The following sections describe the use Line 210  The following sections describe the use
210  <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>  <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
211  <P>  <P>
212  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
213  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
214  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
215  outside character classes.  both inside and outside character classes.
216  </P>  </P>
217  <P>  <P>
218  For example, if you want to match a * character, you write \* in the pattern.  For example, if you want to match a * character, you write \* in the pattern.
# Line 202  non-alphanumeric with backslash to speci Line 222  non-alphanumeric with backslash to speci
222  particular, if you want to match a backslash, you write \\.  particular, if you want to match a backslash, you write \\.
223  </P>  </P>
224  <P>  <P>
225    In a UTF mode, only ASCII numbers and letters have any special meaning after a
226    backslash. All other characters (in particular, those whose codepoints are
227    greater than 127) are treated as literals.
228    </P>
229    <P>
230  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
231  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
232  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
# Line 220  Perl, $ and @ cause variable interpolati Line 245  Perl, $ and @ cause variable interpolati
245    \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz    \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
246  </pre>  </pre>
247  The \Q...\E sequence is recognized both inside and outside character classes.  The \Q...\E sequence is recognized both inside and outside character classes.
248    An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
249    by \E later in the pattern, the literal interpretation continues to the end of
250    the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
251    a character class, this causes an error, because the character class is not
252    terminated.
253  <a name="digitsafterbackslash"></a></P>  <a name="digitsafterbackslash"></a></P>
254  <br><b>  <br><b>
255  Non-printing characters  Non-printing characters
# Line 228  Non-printing characters Line 258  Non-printing characters
258  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
259  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
260  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
261  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is often easier to use
262  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
263  <pre>  <pre>
264    \a        alarm, that is, the BEL character (hex 07)    \a        alarm, that is, the BEL character (hex 07)
265    \cx       "control-x", where x is any character    \cx       "control-x", where x is any ASCII character
266    \e        escape (hex 1B)    \e        escape (hex 1B)
267    \f        formfeed (hex 0C)    \f        formfeed (hex 0C)
268    \n        linefeed (hex 0A)    \n        linefeed (hex 0A)
269    \r        carriage return (hex 0D)    \r        carriage return (hex 0D)
270    \t        tab (hex 09)    \t        tab (hex 09)
271    \ddd      character with octal code ddd, or backreference    \ddd      character with octal code ddd, or back reference
272    \xhh      character with hex code hh    \xhh      character with hex code hh
273    \x{hhh..} character with hex code hhh..    \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
274      \uhhhh    character with hex code hhhh (JavaScript mode only)
275  </pre>  </pre>
276  The precise effect of \cx is as follows: if x is a lower case letter, it  The precise effect of \cx is as follows: if x is a lower case letter, it
277  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
278  Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex  Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ is 7B), while
279  7B.  \c; becomes hex 7B (; is 3B). If the byte following \c has a value greater
280  </P>  than 127, a compile-time error occurs. This locks out non-ASCII characters in
281  <P>  all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. A
282  After \x, from zero to two hexadecimal digits are read (letters can be in  lower case letter is converted to upper case, and then the 0xc0 bits are
283  upper or lower case). Any number of hexadecimal digits may appear between \x{  flipped.)
284  and }, but the value of the character code must be less than 256 in non-UTF-8  </P>
285  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in  <P>
286  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code  By default, after \x, from zero to two hexadecimal digits are read (letters
287  point, which is 10FFFF.  can be in upper or lower case). Any number of hexadecimal digits may appear
288    between \x{ and }, but the character code is constrained as follows:
289    <pre>
290      8-bit non-UTF mode    less than 0x100
291      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
292      16-bit non-UTF mode   less than 0x10000
293      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
294    </pre>
295    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
296    "surrogate" codepoints).
297  </P>  </P>
298  <P>  <P>
299  If characters other than hexadecimal digits appear between \x{ and }, or if  If characters other than hexadecimal digits appear between \x{ and }, or if
# Line 263  initial \x will be interpreted as a basi Line 302  initial \x will be interpreted as a basi
302  following digits, giving a character whose value is zero.  following digits, giving a character whose value is zero.
303  </P>  </P>
304  <P>  <P>
305    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is
306    as just described only when it is followed by two hexadecimal digits.
307    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
308    code points greater than 256 is provided by \u, which must be followed by
309    four hexadecimal digits; otherwise it matches a literal "u" character.
310    </P>
311    <P>
312  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
313  syntaxes for \x. There is no difference in the way they are handled. For  syntaxes for \x (or by \u in JavaScript mode). There is no difference in the
314  example, \xdc is exactly the same as \x{dc}.  way they are handled. For example, \xdc is exactly the same as \x{dc} (or
315    \u00dc in JavaScript mode).
316  </P>  </P>
317  <P>  <P>
318  After \0 up to two further octal digits are read. If there are fewer than two  After \0 up to two further octal digits are read. If there are fewer than two
# Line 288  following the discussion of Line 335  following the discussion of
335  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
336  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
337  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
338  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
339  character specified in octal must be less than \400. In UTF-8 mode, values up  constrained in the same way as characters specified in hexadecimal.
340  to \777 are permitted. For example:  For example:
341  <pre>  <pre>
342    \040   is another way of writing a space    \040   is another way of writing a space
343    \40    is the same, provided there are fewer than 40 previous capturing subpatterns    \40    is the same, provided there are fewer than 40 previous capturing subpatterns
# Line 299  to \777 are permitted. For example: Line 346  to \777 are permitted. For example:
346    \011   is always a tab    \011   is always a tab
347    \0113  is a tab followed by the character "3"    \0113  is a tab followed by the character "3"
348    \113   might be a back reference, otherwise the character with octal code 113    \113   might be a back reference, otherwise the character with octal code 113
349    \377   might be a back reference, otherwise the byte consisting entirely of 1 bits    \377   might be a back reference, otherwise the value 255 (decimal)
350    \81    is either a back reference, or a binary zero followed by the two characters "8" and "1"    \81    is either a back reference, or a binary zero followed by the two characters "8" and "1"
351  </pre>  </pre>
352  Note that octal values of 100 or greater must not be introduced by a leading  Note that octal values of 100 or greater must not be introduced by a leading
# Line 307  zero, because no more than three octal d Line 354  zero, because no more than three octal d
354  </P>  </P>
355  <P>  <P>
356  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
357  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \b is
358  sequence \b is interpreted as the backspace character (hex 08), and the  interpreted as the backspace character (hex 08).
359  sequences \R and \X are interpreted as the characters "R" and "X",  </P>
360  respectively. Outside a character class, these sequences have different  <P>
361  meanings  \N is not allowed in a character class. \B, \R, and \X are not special
362  <a href="#uniextseq">(see below).</a>  inside a character class. Like other unrecognized escape sequences, they are
363    treated as the literal characters "B", "R", and "X" by default, but cause an
364    error if the PCRE_EXTRA option is set. Outside a character class, these
365    sequences have different meanings.
366    </P>
367    <br><b>
368    Unsupported escape sequences
369    </b><br>
370    <P>
371    In Perl, the sequences \l, \L, \u, and \U are recognized by its string
372    handler and used to modify the case of following characters. By default, PCRE
373    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
374    option is set, \U matches a "U" character, and \u can be used to define a
375    character by code point, as described in the previous section.
376  </P>  </P>
377  <br><b>  <br><b>
378  Absolute and relative back references  Absolute and relative back references
# Line 334  a number enclosed either in angle bracke Line 394  a number enclosed either in angle bracke
394  syntax for referencing a subpattern as a "subroutine". Details are discussed  syntax for referencing a subpattern as a "subroutine". Details are discussed
395  <a href="#onigurumasubroutines">later.</a>  <a href="#onigurumasubroutines">later.</a>
396  Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>  Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
397  synonymous. The former is a back reference; the latter is a subroutine call.  synonymous. The former is a back reference; the latter is a
398  </P>  <a href="#subpatternsassubroutines">subroutine</a>
399    call.
400    <a name="genericchartypes"></a></P>
401  <br><b>  <br><b>
402  Generic character types  Generic character types
403  </b><br>  </b><br>
404  <P>  <P>
405  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
406  <pre>  <pre>
407    \d     any decimal digit    \d     any decimal digit
408    \D     any character that is not a decimal digit    \D     any character that is not a decimal digit
# Line 354  following are always recognized: Line 415  following are always recognized:
415    \w     any "word" character    \w     any "word" character
416    \W     any "non-word" character    \W     any "non-word" character
417  </pre>  </pre>
418  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \N, which matches a non-newline character.
419  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
420    <a href="#fullstopdot">the "." metacharacter</a>
421    when PCRE_DOTALL is not set. Perl also uses \N to match characters by name;
422    PCRE does not support this.
423  </P>  </P>
424  <P>  <P>
425  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
426    of characters into two disjoint sets. Any given character matches one, and only
427    one, of each pair. The sequences can appear both inside and outside character
428  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
429  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
430  there is no character to match.  there is no character to match.
431  </P>  </P>
432  <P>  <P>
# Line 371  included in a Perl script, \s may match Line 437  included in a Perl script, \s may match
437  does.  does.
438  </P>  </P>
439  <P>  <P>
440  In UTF-8 mode, characters with values greater than 128 never match \d, \s, or  A "word" character is an underscore or any character that is a letter or digit.
441  \w, and always match \D, \S, and \W. This is true even when Unicode  By default, the definition of letters and digits is controlled by PCRE's
442  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
443  meanings from before UTF-8 support was available, mainly for efficiency  place (see
444  reasons. Note that this also affects \b, because it is defined in terms of \w  <a href="pcreapi.html#localesupport">"Locale support"</a>
445  and \W.  in the
446    <a href="pcreapi.html"><b>pcreapi</b></a>
447    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
448    or "french" in Windows, some character codes greater than 128 are used for
449    accented letters, and these are then matched by \w. The use of locales with
450    Unicode is discouraged.
451  </P>  </P>
452  <P>  <P>
453  The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to the  By default, in a UTF mode, characters with values greater than 128 never match
454  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  \d, \s, or \w, and always match \D, \S, and \W. These sequences retain
455  The horizontal space characters are:  their original meanings from before UTF support was available, mainly for
456    efficiency reasons. However, if PCRE is compiled with Unicode property support,
457    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
458    properties are used to determine character types, as follows:
459    <pre>
460      \d  any character that \p{Nd} matches (decimal digit)
461      \s  any character that \p{Z} matches, plus HT, LF, FF, CR
462      \w  any character that \p{L} or \p{N} matches, plus underscore
463    </pre>
464    The upper case escapes match the inverse sets of characters. Note that \d
465    matches only decimal digits, whereas \w matches any Unicode digit, as well as
466    any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and
467    \B because they are defined in terms of \w and \W. Matching these sequences
468    is noticeably slower when PCRE_UCP is set.
469    </P>
470    <P>
471    The sequences \h, \H, \v, and \V are features that were added to Perl at
472    release 5.10. In contrast to the other sequences, which match only ASCII
473    characters by default, these always match certain high-valued codepoints,
474    whether or not PCRE_UCP is set. The horizontal space characters are:
475  <pre>  <pre>
476    U+0009     Horizontal tab    U+0009     Horizontal tab
477    U+0020     Space    U+0020     Space
# Line 412  The vertical space characters are: Line 502  The vertical space characters are:
502    U+0085     Next line    U+0085     Next line
503    U+2028     Line separator    U+2028     Line separator
504    U+2029     Paragraph separator    U+2029     Paragraph separator
505  </PRE>  </pre>
506  </P>  In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
507  <P>  relevant.
 A "word" character is an underscore or any character less than 256 that is a  
 letter or digit. The definition of letters and digits is controlled by PCRE's  
 low-valued character tables, and may vary if locale-specific matching is taking  
 place (see  
 <a href="pcreapi.html#localesupport">"Locale support"</a>  
 in the  
 <a href="pcreapi.html"><b>pcreapi</b></a>  
 page). For example, in a French locale such as "fr_FR" in Unix-like systems,  
 or "french" in Windows, some character codes greater than 128 are used for  
 accented letters, and these are matched by \w. The use of locales with Unicode  
 is discouraged.  
508  <a name="newlineseq"></a></P>  <a name="newlineseq"></a></P>
509  <br><b>  <br><b>
510  Newline sequences  Newline sequences
511  </b><br>  </b><br>
512  <P>  <P>
513  Outside a character class, by default, the escape sequence \R matches any  Outside a character class, by default, the escape sequence \R matches any
514  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is  Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the
515  equivalent to the following:  following:
516  <pre>  <pre>
517    (?&#62;\r\n|\n|\x0b|\f|\r|\x85)    (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
518  </pre>  </pre>
# Line 446  line, U+0085). The two-character sequenc Line 525  line, U+0085). The two-character sequenc
525  cannot be split.  cannot be split.
526  </P>  </P>
527  <P>  <P>
528  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
529  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).
530  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
531  recognized.  recognized.
# Line 463  one of the following sequences: Line 542  one of the following sequences:
542    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
543    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
544  </pre>  </pre>
545  These override the default and the options given to <b>pcre_compile()</b>, but  These override the default and the options given to the compiling function, but
546  they can be overridden by options given to <b>pcre_exec()</b>. Note that these  they can themselves be overridden by options given to a matching function. Note
547  special settings, which are not Perl-compatible, are recognized only at the  that these special settings, which are not Perl-compatible, are recognized only
548  very start of a pattern, and that they must be in upper case. If more than one  at the very start of a pattern, and that they must be in upper case. If more
549  of them is present, the last one is used. They can be combined with a change of  than one of them is present, the last one is used. They can be combined with a
550  newline convention, for example, a pattern can start with:  change of newline convention; for example, a pattern can start with:
551  <pre>  <pre>
552    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
553  </pre>  </pre>
554  Inside a character class, \R matches the letter "R".  They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
555    sequences. Inside a character class, \R is treated as an unrecognized escape
556    sequence, and so matches the letter "R" by default, but causes an error if
557    PCRE_EXTRA is set.
558  <a name="uniextseq"></a></P>  <a name="uniextseq"></a></P>
559  <br><b>  <br><b>
560  Unicode character properties  Unicode character properties
# Line 480  Unicode character properties Line 562  Unicode character properties
562  <P>  <P>
563  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
564  escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
565  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
566  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.
567  The extra escape sequences are:  The extra escape sequences are:
568  <pre>  <pre>
# Line 489  The extra escape sequences are: Line 571  The extra escape sequences are:
571    \X       an extended Unicode sequence    \X       an extended Unicode sequence
572  </pre>  </pre>
573  The property names represented by <i>xx</i> above are limited to the Unicode  The property names represented by <i>xx</i> above are limited to the Unicode
574  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
575  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
576  not currently supported by PCRE. Note that \P{Any} does not match any  in the
577  characters, so always causes a match failure.  <a href="#extraprops">next section).</a>
578    Other Perl properties such as "InMusicalSymbols" are not currently supported by
579    PCRE. Note that \P{Any} does not match any characters, so always causes a
580    match failure.
581  </P>  </P>
582  <P>  <P>
583  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
# Line 508  Those that are not part of an identified Line 593  Those that are not part of an identified
593  <P>  <P>
594  Arabic,  Arabic,
595  Armenian,  Armenian,
596    Avestan,
597  Balinese,  Balinese,
598    Bamum,
599  Bengali,  Bengali,
600  Bopomofo,  Bopomofo,
601  Braille,  Braille,
602  Buginese,  Buginese,
603  Buhid,  Buhid,
604  Canadian_Aboriginal,  Canadian_Aboriginal,
605    Carian,
606    Cham,
607  Cherokee,  Cherokee,
608  Common,  Common,
609  Coptic,  Coptic,
# Line 523  Cypriot, Line 612  Cypriot,
612  Cyrillic,  Cyrillic,
613  Deseret,  Deseret,
614  Devanagari,  Devanagari,
615    Egyptian_Hieroglyphs,
616  Ethiopic,  Ethiopic,
617  Georgian,  Georgian,
618  Glagolitic,  Glagolitic,
# Line 535  Hangul, Line 625  Hangul,
625  Hanunoo,  Hanunoo,
626  Hebrew,  Hebrew,
627  Hiragana,  Hiragana,
628    Imperial_Aramaic,
629  Inherited,  Inherited,
630    Inscriptional_Pahlavi,
631    Inscriptional_Parthian,
632    Javanese,
633    Kaithi,
634  Kannada,  Kannada,
635  Katakana,  Katakana,
636    Kayah_Li,
637  Kharoshthi,  Kharoshthi,
638  Khmer,  Khmer,
639  Lao,  Lao,
640  Latin,  Latin,
641    Lepcha,
642  Limbu,  Limbu,
643  Linear_B,  Linear_B,
644    Lisu,
645    Lycian,
646    Lydian,
647  Malayalam,  Malayalam,
648    Meetei_Mayek,
649  Mongolian,  Mongolian,
650  Myanmar,  Myanmar,
651  New_Tai_Lue,  New_Tai_Lue,
# Line 552  Nko, Line 653  Nko,
653  Ogham,  Ogham,
654  Old_Italic,  Old_Italic,
655  Old_Persian,  Old_Persian,
656    Old_South_Arabian,
657    Old_Turkic,
658    Ol_Chiki,
659  Oriya,  Oriya,
660  Osmanya,  Osmanya,
661  Phags_Pa,  Phags_Pa,
662  Phoenician,  Phoenician,
663    Rejang,
664  Runic,  Runic,
665    Samaritan,
666    Saurashtra,
667  Shavian,  Shavian,
668  Sinhala,  Sinhala,
669    Sundanese,
670  Syloti_Nagri,  Syloti_Nagri,
671  Syriac,  Syriac,
672  Tagalog,  Tagalog,
673  Tagbanwa,  Tagbanwa,
674  Tai_Le,  Tai_Le,
675    Tai_Tham,
676    Tai_Viet,
677  Tamil,  Tamil,
678  Telugu,  Telugu,
679  Thaana,  Thaana,
# Line 571  Thai, Line 681  Thai,
681  Tibetan,  Tibetan,
682  Tifinagh,  Tifinagh,
683  Ugaritic,  Ugaritic,
684    Vai,
685  Yi.  Yi.
686  </P>  </P>
687  <P>  <P>
688  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
689  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
690  by including a circumflex between the opening brace and the property name. For  specified by including a circumflex between the opening brace and the property
691  example, \p{^Lu} is the same as \P{Lu}.  name. For example, \p{^Lu} is the same as \P{Lu}.
692  </P>  </P>
693  <P>  <P>
694  If only one letter is specified with \p or \P, it includes all the general  If only one letter is specified with \p or \P, it includes all the general
# Line 640  a modifier or "other". Line 751  a modifier or "other".
751  </P>  </P>
752  <P>  <P>
753  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
754  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
755  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
756  (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
757  <a href="pcreapi.html"><b>pcreapi</b></a>  <a href="pcreapi.html"><b>pcreapi</b></a>
758  page).  page). Perl does not support the Cs property.
759  </P>  </P>
760  <P>  <P>
761  The long synonyms for these properties that Perl supports (such as \p{Letter})  The long synonyms for property names that Perl supports (such as \p{Letter})
762  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
763  properties with "Is".  properties with "Is".
764  </P>  </P>
# Line 672  atomic group Line 783  atomic group
783  <a href="#atomicgroup">(see below).</a>  <a href="#atomicgroup">(see below).</a>
784  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
785  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
786  non-UTF-8 mode \X matches any one character.  8-bit non-UTF-8 mode \X matches any one character.
787    </P>
788    <P>
789    Note that recent versions of Perl have changed \X to match what Unicode calls
790    an "extended grapheme cluster", which has a more complicated definition.
791  </P>  </P>
792  <P>  <P>
793  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
794  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
795  why the traditional escape sequences such as \d and \w do not use Unicode  why the traditional escape sequences such as \d and \w do not use Unicode
796  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
797    PCRE_UCP option or by starting the pattern with (*UCP).
798    <a name="extraprops"></a></P>
799    <br><b>
800    PCRE's additional properties
801    </b><br>
802    <P>
803    As well as the standard Unicode properties described in the previous
804    section, PCRE supports four more that make it possible to convert traditional
805    escape sequences such as \w and \s and POSIX character classes to use Unicode
806    properties. PCRE uses these non-standard, non-Perl properties internally when
807    PCRE_UCP is set. They are:
808    <pre>
809      Xan   Any alphanumeric character
810      Xps   Any POSIX space character
811      Xsp   Any Perl space character
812      Xwd   Any Perl "word" character
813    </pre>
814    Xan matches characters that have either the L (letter) or the N (number)
815    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
816    carriage return, and any other character that has the Z (separator) property.
817    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
818    same characters as Xan, plus underscore.
819  <a name="resetmatchstart"></a></P>  <a name="resetmatchstart"></a></P>
820  <br><b>  <br><b>
821  Resetting the match start  Resetting the match start
822  </b><br>  </b><br>
823  <P>  <P>
824  The escape sequence \K, which is a Perl 5.10 feature, causes any previously  The escape sequence \K causes any previously matched characters not to be
825  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:  
826  <pre>  <pre>
827    foo\Kbar    foo\Kbar
828  </pre>  </pre>
# Line 702  For example, when the pattern Line 838  For example, when the pattern
838    (foo)\Kbar    (foo)\Kbar
839  </pre>  </pre>
840  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
841    </P>
842    <P>
843    Perl documents that the use of \K within assertions is "not well defined". In
844    PCRE, \K is acted upon when it occurs inside positive assertions, but is
845    ignored in negative assertions.
846  <a name="smallassertions"></a></P>  <a name="smallassertions"></a></P>
847  <br><b>  <br><b>
848  Simple assertions  Simple assertions
# Line 722  The backslashed assertions are: Line 863  The backslashed assertions are:
863    \z     matches only at the end of the subject    \z     matches only at the end of the subject
864    \G     matches at the first matching position in the subject    \G     matches at the first matching position in the subject
865  </pre>  </pre>
866  These assertions may not appear in character classes (but note that \b has a  Inside a character class, \b has a different meaning; it matches the backspace
867  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
868    default it matches the corresponding literal character (for example, \B
869    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
870    escape sequence" error is generated instead.
871  </P>  </P>
872  <P>  <P>
873  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
874  and the previous character do not both match \w or \W (i.e. one matches  and the previous character do not both match \w or \W (i.e. one matches
875  \w and the other matches \W), or the start or end of the string if the  \w and the other matches \W), or the start or end of the string if the
876  first or last character matches \w, respectively.  first or last character matches \w, respectively. In a UTF mode, the meanings
877    of \w and \W can be changed by setting the PCRE_UCP option. When this is
878    done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
879    of word" or "end of word" metasequence. However, whatever follows \b normally
880    determines which it is. For example, the fragment \ba matches "a" at the start
881    of a word.
882  </P>  </P>
883  <P>  <P>
884  The \A, \Z, and \z assertions differ from the traditional circumflex and  The \A, \Z, and \z assertions differ from the traditional circumflex and
# Line 816  PCRE_DOLLAR_ENDONLY option is ignored if Line 965  PCRE_DOLLAR_ENDONLY option is ignored if
965  Note that the sequences \A, \Z, and \z can be used to match the start and  Note that the sequences \A, \Z, and \z can be used to match the start and
966  end of the subject in both modes, and if all branches of a pattern start with  end of the subject in both modes, and if all branches of a pattern start with
967  \A it is always anchored, whether or not PCRE_MULTILINE is set.  \A it is always anchored, whether or not PCRE_MULTILINE is set.
968  </P>  <a name="fullstopdot"></a></P>
969  <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>  <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
970  <P>  <P>
971  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
972  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
973  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
974  </P>  </P>
975  <P>  <P>
976  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
# Line 842  The handling of dot is entirely independ Line 991  The handling of dot is entirely independ
991  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
992  special meaning in a character class.  special meaning in a character class.
993  </P>  </P>
 <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>  
994  <P>  <P>
995  Outside a character class, the escape sequence \C matches any one byte, both  The escape sequence \N behaves like a dot, except that it is not affected by
996  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  the PCRE_DOTALL option. In other words, it matches any character except one
997  characters. The feature is provided in Perl in order to match individual bytes  that signifies the end of a line. Perl also uses \N to match characters by
998  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  name; PCRE does not support this.
999  what remains in the string may be a malformed UTF-8 string. For this reason,  </P>
1000  the \C escape sequence is best avoided.  <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE DATA UNIT</a><br>
1001    <P>
1002    Outside a character class, the escape sequence \C matches any one data unit,
1003    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1004    byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \C always
1005    matches line-ending characters. The feature is provided in Perl in order to
1006    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1007    used. Because \C breaks up characters into individual data units, matching one
1008    unit with \C in a UTF mode means that the rest of the string may start with a
1009    malformed UTF character. This has undefined results, because PCRE assumes that
1010    it is dealing with valid UTF strings (and by default it checks this at the
1011    start of processing unless the PCRE_NO_UTF8_CHECK option is used).
1012  </P>  </P>
1013  <P>  <P>
1014  PCRE does not allow \C to appear in lookbehind assertions  PCRE does not allow \C to appear in lookbehind assertions
1015  <a href="#lookbehind">(described below),</a>  <a href="#lookbehind">(described below)</a>
1016  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
1017  the lookbehind.  the lookbehind.
1018    </P>
1019    <P>
1020    In general, the \C escape sequence is best avoided. However, one
1021    way of using it that avoids the problem of malformed UTF characters is to use a
1022    lookahead to check the length of the next character, as in this pattern, which
1023    could be used with a UTF-8 string (ignore white space and line breaks):
1024    <pre>
1025      (?| (?=[\x00-\x7f])(\C) |
1026          (?=[\x80-\x{7ff}])(\C)(\C) |
1027          (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
1028          (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
1029    </pre>
1030    A group that starts with (?| resets the capturing parentheses numbers in each
1031    alternative (see
1032    <a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a>
1033    below). The assertions at the start of each branch check the next UTF-8
1034    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1035    character's individual bytes are then captured by the appropriate number of
1036    groups.
1037  <a name="characterclass"></a></P>  <a name="characterclass"></a></P>
1038  <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>  <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
1039  <P>  <P>
1040  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1041  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special by default.
1042  closing square bracket is required as a member of the class, it should be the  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1043  first data character in the class (after an initial circumflex, if present) or  bracket causes a compile-time error. If a closing square bracket is required as
1044  escaped with a backslash.  a member of the class, it should be the first data character in the class
1045    (after an initial circumflex, if present) or escaped with a backslash.
1046  </P>  </P>
1047  <P>  <P>
1048  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
1049  character may occupy more than one byte. A matched character must be in the set  character may be more than one data unit long. A matched character must be in
1050  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
1051  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
1052  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
1053  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
1054  backslash.  backslash.
1055  </P>  </P>
1056  <P>  <P>
# Line 879  For example, the character class [aeiou] Line 1058  For example, the character class [aeiou]
1058  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
1059  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1060  are in the class by enumerating those that are not. A class that starts with a  are in the class by enumerating those that are not. A class that starts with a
1061  circumflex is not an assertion: it still consumes a character from the subject  circumflex is not an assertion; it still consumes a character from the subject
1062  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
1063  string.  string.
1064  </P>  </P>
1065  <P>  <P>
1066  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
1067  class as a literal string of bytes, or by using the \x{ escaping mechanism.  included in a class as a literal string of data units, or by using the \x{
1068    escaping mechanism.
1069  </P>  </P>
1070  <P>  <P>
1071  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
1072  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
1073  "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
1074  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
1075  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
1076  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1077  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1078  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in a UTF mode for characters 128 and
1079  ensure that PCRE is compiled with Unicode property support as well as with  above, you must ensure that PCRE is compiled with Unicode property support as
1080  UTF-8 support.  well as with UTF support.
1081  </P>  </P>
1082  <P>  <P>
1083  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
# Line 923  followed by two other characters. The oc Line 1103  followed by two other characters. The oc
1103  </P>  </P>
1104  <P>  <P>
1105  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
1106  used for characters specified numerically, for example [\000-\037]. In UTF-8  used for characters specified numerically, for example [\000-\037]. Ranges
1107  mode, ranges can include characters whose values are greater than 255, for  can include any characters that are valid for the current mode.
 example [\x{100}-\x{2ff}].  
1108  </P>  </P>
1109  <P>  <P>
1110  If a range that includes letters is used when caseless matching is set, it  If a range that includes letters is used when caseless matching is set, it
1111  matches the letters in either case. For example, [W-c] is equivalent to  matches the letters in either case. For example, [W-c] is equivalent to
1112  [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1113  tables for a French locale are in use, [\xc8-\xcb] matches accented E  tables for a French locale are in use, [\xc8-\xcb] matches accented E
1114  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF modes, PCRE supports the concept of case for
1115  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1116  property support.  property support.
1117  </P>  </P>
1118  <P>  <P>
1119  The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear  The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
1120  in a character class, and add the characters that they match to the class. For  \V, \w, and \W may appear in a character class, and add the characters that
1121  example, [\dABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\dABCDEF] matches any hexadecimal
1122  conveniently be used with the upper case character types to specify a more  digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w
1123  restricted set of characters than the matching lower case type. For example,  and their upper case partners, just as it does when they appear outside a
1124  the class [^\W_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1125    <a href="#genericchartypes">"Generic character types"</a>
1126    above. The escape sequence \b has a different meaning inside a character
1127    class; it matches the backspace character. The sequences \B, \N, \R, and \X
1128    are not special inside a character class. Like any other unrecognized escape
1129    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1130    default, but cause an error if the PCRE_EXTRA option is set.
1131    </P>
1132    <P>
1133    A circumflex can conveniently be used with the upper case character types to
1134    specify a more restricted set of characters than the matching lower case type.
1135    For example, the class [^\W_] matches any letter or digit, but not underscore,
1136    whereas [\w] includes underscore. A positive character class should be read as
1137    "something OR something OR ..." and a negative class as "NOT something AND NOT
1138    something AND NOT ...".
1139  </P>  </P>
1140  <P>  <P>
1141  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
# Line 961  this notation. For example, Line 1154  this notation. For example,
1154    [01[:alpha:]%]    [01[:alpha:]%]
1155  </pre>  </pre>
1156  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1157  are  are:
1158  <pre>  <pre>
1159    alnum    letters and digits    alnum    letters and digits
1160    alpha    letters    alpha    letters
# Line 972  are Line 1165  are
1165    graph    printing characters, excluding space    graph    printing characters, excluding space
1166    lower    lower case letters    lower    lower case letters
1167    print    printing characters, including space    print    printing characters, including space
1168    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1169    space    white space (not quite the same as \s)    space    white space (not quite the same as \s)
1170    upper    upper case letters    upper    upper case letters
1171    word     "word" characters (same as \w)    word     "word" characters (same as \w)
# Line 995  syntax [.ch.] and [=ch=] where "ch" is a Line 1188  syntax [.ch.] and [=ch=] where "ch" is a
1188  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1189  </P>  </P>
1190  <P>  <P>
1191  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF modes, characters with values greater than 128 do not match
1192  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1193    to <b>pcre_compile()</b>, some of the classes are changed so that Unicode
1194    character properties are used. This is achieved by replacing the POSIX classes
1195    by other sequences, as follows:
1196    <pre>
1197      [:alnum:]  becomes  \p{Xan}
1198      [:alpha:]  becomes  \p{L}
1199      [:blank:]  becomes  \h
1200      [:digit:]  becomes  \p{Nd}
1201      [:lower:]  becomes  \p{Ll}
1202      [:space:]  becomes  \p{Xps}
1203      [:upper:]  becomes  \p{Lu}
1204      [:word:]   becomes  \p{Xwd}
1205    </pre>
1206    Negated versions, such as [:^alpha:] use \P instead of \p. The other POSIX
1207    classes are unchanged, and match only characters with code points less than
1208    128.
1209  </P>  </P>
1210  <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>  <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
1211  <P>  <P>
# Line 1046  extracted by the pcre_fullinfo() Line 1255  extracted by the pcre_fullinfo()
1255  </P>  </P>
1256  <P>  <P>
1257  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1258  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
1259  <pre>  <pre>
1260    (a(?i)b)c    (a(?i)b)c
1261  </pre>  </pre>
# Line 1064  behaviour otherwise. Line 1273  behaviour otherwise.
1273  </P>  </P>
1274  <P>  <P>
1275  <b>Note:</b> There are other PCRE-specific options that can be set by the  <b>Note:</b> There are other PCRE-specific options that can be set by the
1276  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
1277  pattern can contain special leading sequences such as (*CRLF) to override what  the pattern can contain special leading sequences such as (*CRLF) to override
1278  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
1279  section entitled  the section entitled
1280  <a href="#newlineseq">"Newline sequences"</a>  <a href="#newlineseq">"Newline sequences"</a>
1281  above. There is also the (*UTF8) leading sequence that can be used to set UTF-8  above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1282  mode; this is equivalent to setting the PCRE_UTF8 option.  can be used to set UTF and Unicode property modes; they are equivalent to
1283    setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1284  <a name="subpattern"></a></P>  <a name="subpattern"></a></P>
1285  <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>  <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
1286  <P>  <P>
# Line 1082  Turning part of a pattern into a subpatt Line 1292  Turning part of a pattern into a subpatt
1292  <pre>  <pre>
1293    cat(aract|erpillar|)    cat(aract|erpillar|)
1294  </pre>  </pre>
1295  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1296  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1297  <br>  <br>
1298  <br>  <br>
1299  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
1300  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
1301  subpattern is passed back to the caller via the <i>ovector</i> argument of  subpattern is passed back to the caller via the <i>ovector</i> argument of the
1302  <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1303  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1304  </P>  </P>
1305  <P>  <P>
1306  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
1307    numbers for the capturing subpatterns. For example, if the string "the red
1308    king" is matched against the pattern
1309  <pre>  <pre>
1310    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1311  </pre>  </pre>
# Line 1125  match exactly the same set of strings. B Line 1337  match exactly the same set of strings. B
1337  from left to right, and options are not reset until the end of the subpattern  from left to right, and options are not reset until the end of the subpattern
1338  is reached, an option setting in one branch does affect subsequent branches, so  is reached, an option setting in one branch does affect subsequent branches, so
1339  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1340  </P>  <a name="dupsubpatternnumber"></a></P>
1341  <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>  <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1342  <P>  <P>
1343  Perl 5.10 introduced a feature whereby each alternative in a subpattern uses  Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
# Line 1141  at captured substring number one, whiche Line 1353  at captured substring number one, whiche
1353  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
1354  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1355  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
1356  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
1357  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1358  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.  
1359  <pre>  <pre>
1360    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1361    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1362    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1363  </pre>  </pre>
1364  A backreference or a recursive call to a numbered subpattern always refers to  A back reference to a numbered subpattern uses the most recent value that is
1365  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1366    or "defdef":
1367    <pre>
1368      /(?|(abc)|(def))\1/
1369    </pre>
1370    In contrast, a subroutine call to a numbered subpattern always refers to the
1371    first one in the pattern with the given number. The following pattern matches
1372    "abcabc" or "defabc":
1373    <pre>
1374      /(?|(abc)|(def))(?1)/
1375    </pre>
1376    If a
1377    <a href="#conditions">condition test</a>
1378    for a subpattern's having matched refers to a non-unique number, the test is
1379    true if any of the subpatterns of that number have matched.
1380  </P>  </P>
1381  <P>  <P>
1382  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
# Line 1165  if an expression is modified, the number Line 1390  if an expression is modified, the number
1390  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1391  added to Perl until release 5.10. Python had the feature earlier, and PCRE  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1392  introduced it at release 4.0, using the Python syntax. PCRE now supports both  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1393  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1394    have different names, but PCRE does not.
1395  </P>  </P>
1396  <P>  <P>
1397  In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or  In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1398  (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing  (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1399  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1400  <a href="#backreferences">backreferences,</a>  <a href="#backreferences">back references,</a>
1401  <a href="#recursion">recursion,</a>  <a href="#recursion">recursion,</a>
1402  and  and
1403  <a href="#conditions">conditions,</a>  <a href="#conditions">conditions,</a>
# Line 1186  is also a convenience function for extra Line 1412  is also a convenience function for extra
1412  </P>  </P>
1413  <P>  <P>
1414  By default, a name must be unique within a pattern, but it is possible to relax  By default, a name must be unique within a pattern, but it is possible to relax
1415  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1416  be useful for patterns where only one instance of the named parentheses can  names are also always permitted for subpatterns with the same number, set up as
1417  match. Suppose you want to match the name of a weekday, either as a 3-letter  described in the previous section.) Duplicate names can be useful for patterns
1418  abbreviation or as the full name, and in both cases you want to extract the  where only one instance of the named parentheses can match. Suppose you want to
1419  abbreviation. This pattern (ignoring the line breaks) does the job:  match the name of a weekday, either as a 3-letter abbreviation or as the full
1420    name, and in both cases you want to extract the abbreviation. This pattern
1421    (ignoring the line breaks) does the job:
1422  <pre>  <pre>
1423    (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|    (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1424    (?&#60;DN&#62;Tue)(?:sday)?|    (?&#60;DN&#62;Tue)(?:sday)?|
# Line 1205  subpattern, as described in the previous Line 1433  subpattern, as described in the previous
1433  <P>  <P>
1434  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1435  for the first (and in this example, the only) subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1436  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1437  make a reference to a non-unique named subpattern from elsewhere in the  </P>
1438  pattern, the one that corresponds to the lowest number is used. For further  <P>
1439  details of the interfaces for handling named subpatterns, see the  If you make a back reference to a non-unique named subpattern from elsewhere in
1440    the pattern, the one that corresponds to the first occurrence of the name is
1441    used. In the absence of duplicate numbers (see the previous section) this is
1442    the one with the lowest number. If you use a named reference in a condition
1443    test (see the
1444    <a href="#conditions">section about conditions</a>
1445    below), either to check whether a subpattern has matched, or to check for
1446    recursion, all subpatterns with the same name are tested. If the condition is
1447    true for any one of them, the overall condition is true. This is the same
1448    behaviour as testing by number. For further details of the interfaces for
1449    handling named subpatterns, see the
1450  <a href="pcreapi.html"><b>pcreapi</b></a>  <a href="pcreapi.html"><b>pcreapi</b></a>
1451  documentation.  documentation.
1452  </P>  </P>
1453  <P>  <P>
1454  <b>Warning:</b> You cannot use different names to distinguish between two  <b>Warning:</b> You cannot use different names to distinguish between two
1455  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1456  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1457    are given to subpatterns with the same number. However, you can give the same
1458    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1459  </P>  </P>
1460  <br><a name="SEC15" href="#TOC1">REPETITION</a><br>  <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1461  <P>  <P>
# Line 1225  items: Line 1465  items:
1465    a literal data character    a literal data character
1466    the dot metacharacter    the dot metacharacter
1467    the \C escape sequence    the \C escape sequence
1468    the \X escape sequence (in UTF-8 mode with Unicode properties)    the \X escape sequence
1469    the \R escape sequence    the \R escape sequence
1470    an escape such as \d that matches a single character    an escape such as \d or \pL that matches a single character
1471    a character class    a character class
1472    a back reference (see next section)    a back reference (see next section)
1473    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1474      a subroutine call to a subpattern (recursive or otherwise)
1475  </pre>  </pre>
1476  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1477  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1256  quantifier, is taken as a literal charac Line 1497  quantifier, is taken as a literal charac
1497  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1498  </P>  </P>
1499  <P>  <P>
1500  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
1501  bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of  units. Thus, for example, \x{100}{2} matches two characters, each of
1502  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,
1503  support is available, \X{3} matches three Unicode extended sequences, each of  \X{3} matches three Unicode extended sequences, each of which may be several
1504  which may be several bytes long (and they may be of different lengths).  data units long (and they may be of different lengths).
1505  </P>  </P>
1506  <P>  <P>
1507  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
1508  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
1509  subpatterns that are referenced as  subpatterns that are referenced as
1510  <a href="#subpatternsassubroutines">subroutines</a>  <a href="#subpatternsassubroutines">subroutines</a>
1511  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1512  quantifier are omitted from the compiled pattern.  <a href="#subdefine">"Defining subpatterns for use by reference only"</a>
1513    below). Items other than subpatterns that have a {0} quantifier are omitted
1514    from the compiled pattern.
1515  </P>  </P>
1516  <P>  <P>
1517  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
# Line 1348  alternatively using ^ to indicate anchor Line 1591  alternatively using ^ to indicate anchor
1591  </P>  </P>
1592  <P>  <P>
1593  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1594  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1595  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
1596  succeeds. Consider, for example:  succeeds. Consider, for example:
1597  <pre>  <pre>
# Line 1504  subpattern is possible using named paren Line 1747  subpattern is possible using named paren
1747  </P>  </P>
1748  <P>  <P>
1749  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
1750  backslash is to use the \g escape sequence, which is a feature introduced in  backslash is to use the \g escape sequence. This escape must be followed by an
1751  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
1752  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1753  <pre>  <pre>
1754    (ring), \1    (ring), \1
1755    (ring), \g1    (ring), \g1
# Line 1520  example: Line 1763  example:
1763    (abc(def)ghi)\g{-1}    (abc(def)ghi)\g{-1}
1764  </pre>  </pre>
1765  The sequence \g{-1} is a reference to the most recently started capturing  The sequence \g{-1} is a reference to the most recently started capturing
1766  subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2}  subpattern before \g, that is, is it equivalent to \2 in this example.
1767  would be equivalent to \1. The use of relative references can be helpful in  Similarly, \g{-2} would be equivalent to \1. The use of relative references
1768  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
1769  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1770  </P>  </P>
1771  <P>  <P>
1772  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
# Line 1562  after the reference. Line 1805  after the reference.
1805  <P>  <P>
1806  There may be more than one back reference to the same subpattern. If a  There may be more than one back reference to the same subpattern. If a
1807  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1808  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1809  <pre>  <pre>
1810    (a|(bc))\2    (a|(bc))\2
1811  </pre>  </pre>
1812  always fails if it starts to match "a" rather than "bc". Because there may be  always fails if it starts to match "a" rather than "bc". However, if the
1813  many capturing parentheses in a pattern, all digits following the backslash are  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1814  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1815  with a digit character, some delimiter must be used to terminate the back  </P>
1816  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  <P>
1817  Otherwise an empty comment (see  Because there may be many capturing parentheses in a pattern, all digits
1818    following a backslash are taken as part of a potential back reference number.
1819    If the pattern continues with a digit character, some delimiter must be used to
1820    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1821    whitespace. Otherwise, the \g{ syntax or an empty comment (see
1822  <a href="#comments">"Comments"</a>  <a href="#comments">"Comments"</a>
1823  below) can be used.  below) can be used.
1824  </P>  </P>
1825    <br><b>
1826    Recursive back references
1827    </b><br>
1828  <P>  <P>
1829  A back reference that occurs inside the parentheses to which it refers fails  A back reference that occurs inside the parentheses to which it refers fails
1830  when the subpattern is first used, so, for example, (a\1) never matches.  when the subpattern is first used, so, for example, (a\1) never matches.
# Line 1589  to the previous iteration. In order for Line 1839  to the previous iteration. In order for
1839  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
1840  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1841  minimum of zero.  minimum of zero.
1842    </P>
1843    <P>
1844    Back references of this type cause the group that they reference to be treated
1845    as an
1846    <a href="#atomicgroup">atomic group.</a>
1847    Once the whole group has been matched, a subsequent matching failure cannot
1848    cause backtracking into the middle of the group.
1849  <a name="bigassertions"></a></P>  <a name="bigassertions"></a></P>
1850  <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>  <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1851  <P>  <P>
# Line 1604  that look behind it. An assertion subpat Line 1861  that look behind it. An assertion subpat
1861  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.
1862  </P>  </P>
1863  <P>  <P>
1864  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1865  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
1866  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1867  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1868  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1869  because it does not make sense for negative assertions.  </P>
1870    <P>
1871    For compatibility with Perl, assertion subpatterns may be repeated; though
1872    it makes no sense to assert the same thing several times, the side effect of
1873    capturing parentheses may occasionally be useful. In practice, there only three
1874    cases:
1875    <br>
1876    <br>
1877    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1878    However, it may contain internal capturing parenthesized groups that are called
1879    from elsewhere via the
1880    <a href="#subpatternsassubroutines">subroutine mechanism.</a>
1881    <br>
1882    <br>
1883    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1884    were {0,1}. At run time, the rest of the pattern match is tried with and
1885    without the assertion, the order depending on the greediness of the quantifier.
1886    <br>
1887    <br>
1888    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1889    The assertion is obeyed just once when encountered during matching.
1890  </P>  </P>
1891  <br><b>  <br><b>
1892  Lookahead assertions  Lookahead assertions
# Line 1639  lookbehind assertion is needed to achiev Line 1916  lookbehind assertion is needed to achiev
1916  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
1917  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
1918  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.
1919    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1920  <a name="lookbehind"></a></P>  <a name="lookbehind"></a></P>
1921  <br><b>  <br><b>
1922  Lookbehind assertions  Lookbehind assertions
# Line 1662  is permitted, but Line 1940  is permitted, but
1940  </pre>  </pre>
1941  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1942  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
1943  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1944  match the same length of string. An assertion such as  length of string. An assertion such as
1945  <pre>  <pre>
1946    (?&#60;=ab(c|de))    (?&#60;=ab(c|de))
1947  </pre>  </pre>
1948  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
1949  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
1950    branches:
1951  <pre>  <pre>
1952    (?&#60;=abc|abde)    (?&#60;=abc|abde)
1953  </pre>  </pre>
1954  In some cases, the Perl 5.10 escape sequence \K  In some cases, the escape sequence \K
1955  <a href="#resetmatchstart">(see above)</a>  <a href="#resetmatchstart">(see above)</a>
1956  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
1957  fixed-length.  restriction.
1958  </P>  </P>
1959  <P>  <P>
1960  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
# Line 1684  match. If there are insufficient charact Line 1963  match. If there are insufficient charact
1963  assertion fails.  assertion fails.
1964  </P>  </P>
1965  <P>  <P>
1966  PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)  In a UTF mode, PCRE does not allow the \C escape (which matches a single data
1967  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
1968  the length of the lookbehind. The \X and \R escapes, which can match  it impossible to calculate the length of the lookbehind. The \X and \R
1969  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
1970    permitted.
1971    </P>
1972    <P>
1973    <a href="#subpatternsassubroutines">"Subroutine"</a>
1974    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1975    as the subpattern matches a fixed-length string.
1976    <a href="#recursion">Recursion,</a>
1977    however, is not supported.
1978  </P>  </P>
1979  <P>  <P>
1980  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1981  specify efficient matching at the end of the subject string. Consider a simple  specify efficient matching of fixed-length strings at the end of subject
1982  pattern such as  strings. Consider a simple pattern such as
1983  <pre>  <pre>
1984    abcd$    abcd$
1985  </pre>  </pre>
# Line 1754  characters that are not "999". Line 2041  characters that are not "999".
2041  <P>  <P>
2042  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2043  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2044  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2045  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2046  <pre>  <pre>
2047    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2048    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2049  </pre>  </pre>
2050  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2051  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
2052  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2053    itself contain nested subpatterns of any form, including conditional
2054    subpatterns; the restriction to two alternatives applies only at the level of
2055    the condition. This pattern fragment is an example where the alternatives are
2056    complex:
2057    <pre>
2058      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2059    
2060    </PRE>
2061  </P>  </P>
2062  <P>  <P>
2063  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
# Line 1773  Checking for a used subpattern by number Line 2068  Checking for a used subpattern by number
2068  </b><br>  </b><br>
2069  <P>  <P>
2070  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
2071  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2072  matched. An alternative notation is to precede the digits with a plus or minus  matched. If there is more than one capturing subpattern with the same number
2073  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2074  The most recently opened parentheses can be referenced by (?(-1), the next most  <a href="#recursion">section about duplicate subpattern numbers),</a>
2075  recent by (?(-2), and so on. In looping constructs it can also make sense to  the condition is true if any of them have matched. An alternative notation is
2076  refer to subsequent groups with constructs such as (?(+2).  to precede the digits with a plus or minus sign. In this case, the subpattern
2077    number is relative rather than absolute. The most recently opened parentheses
2078    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2079    loops it can also make sense to refer to subsequent groups. The next
2080    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2081    zero in any of these forms is not used; it provokes a compile-time error.)
2082  </P>  </P>
2083  <P>  <P>
2084  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
# Line 1790  three parts for ease of discussion: Line 2090  three parts for ease of discussion:
2090  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2091  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
2092  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
2093  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2094  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,
2095  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
2096  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2097  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 1822  names that consist entirely of digits is Line 2122  names that consist entirely of digits is
2122  Rewriting the above example to use a named subpattern gives this:  Rewriting the above example to use a named subpattern gives this:
2123  <pre>  <pre>
2124    (?&#60;OPEN&#62; \( )?    [^()]+    (?(&#60;OPEN&#62;) \) )    (?&#60;OPEN&#62; \( )?    [^()]+    (?(&#60;OPEN&#62;) \) )
2125    </pre>
2126  </PRE>  If the name used in a condition of this kind is a duplicate, the test is
2127    applied to all subpatterns of the same name, and is true if any one of them has
2128    matched.
2129  </P>  </P>
2130  <br><b>  <br><b>
2131  Checking for pattern recursion  Checking for pattern recursion
# Line 1836  letter R, for example: Line 2138  letter R, for example:
2138  <pre>  <pre>
2139    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2140  </pre>  </pre>
2141  the condition is true if the most recent recursion is into the subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
2142  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
2143  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2144    applied to all subpatterns of the same name, and is true if any one of them is
2145    the most recent recursion.
2146  </P>  </P>
2147  <P>  <P>
2148  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2149  patterns are described below.  <a href="#recursion">The syntax for recursive patterns</a>
2150  </P>  is described below.
2151    <a name="subdefine"></a></P>
2152  <br><b>  <br><b>
2153  Defining subpatterns for use by reference only  Defining subpatterns for use by reference only
2154  </b><br>  </b><br>
# Line 1852  If the condition is the string (DEFINE), Line 2157  If the condition is the string (DEFINE),
2157  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
2158  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2159  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
2160  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2161  is described below.) For example, a pattern to match an IPv4 address could be  <a href="#subpatternsassubroutines">subroutines</a>
2162  written like this (ignore whitespace and line breaks):  is described below.) For example, a pattern to match an IPv4 address such as
2163    "192.168.23.245" could be written like this (ignore whitespace and line
2164    breaks):
2165  <pre>  <pre>
2166    (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )    (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
2167    \b (?&byte) (\.(?&byte)){3} \b    \b (?&byte) (\.(?&byte)){3} \b
# Line 1862  written like this (ignore whitespace and Line 2169  written like this (ignore whitespace and
2169  The first part of the pattern is a DEFINE group inside which a another group  The first part of the pattern is a DEFINE group inside which a another group
2170  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2171  address (a number less than 256). When matching takes place, this part of the  address (a number less than 256). When matching takes place, this part of the
2172  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2173  </P>  pattern uses references to the named group to match the four dot-separated
2174  <P>  components of an IPv4 address, insisting on a word boundary at each end.
 The rest of the pattern uses references to the named group to match the four  
 dot-separated components of an IPv4 address, insisting on a word boundary at  
 each end.  
2175  </P>  </P>
2176  <br><b>  <br><b>
2177  Assertion conditions  Assertion conditions
# Line 1890  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2194  dd-aaa-dd or dd-dd-dd, where aaa are let
2194  <a name="comments"></a></P>  <a name="comments"></a></P>
2195  <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>  <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
2196  <P>  <P>
2197  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
2198  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,
2199  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
2200    subpattern name or number. The characters that make up a comment play no part
2201    in the pattern matching.
2202  </P>  </P>
2203  <P>  <P>
2204  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
2205  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2206  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2207    this case continues to immediately after the next newline character or
2208    character sequence in the pattern. Which characters are interpreted as newlines
2209    is controlled by the options passed to a compiling function or by a special
2210    sequence at the start of the pattern, as described in the section entitled
2211    <a href="#newlines">"Newline conventions"</a>
2212    above. Note that the end of this type of comment is a literal newline sequence
2213    in the pattern; escape sequences that happen to represent a newline do not
2214    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2215    default newline convention is in force:
2216    <pre>
2217      abc #comment \n still comment
2218    </pre>
2219    On encountering the # character, <b>pcre_compile()</b> skips along, looking for
2220    a newline in the pattern. The sequence \n is still literal at this stage, so
2221    it does not terminate the comment. Only an actual character with the code value
2222    0x0a (the default newline) does so.
2223  <a name="recursion"></a></P>  <a name="recursion"></a></P>
2224  <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>  <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
2225  <P>  <P>
# Line 1922  recursively to the pattern in which it a Line 2244  recursively to the pattern in which it a
2244  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2245  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2246  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2247  this kind of recursion was introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2248  </P>  </P>
2249  <P>  <P>
2250  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
2251  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
2252  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  given number, provided that it occurs inside that subpattern. (If not, it is a
2253    <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2254  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
2255  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2256  </P>  </P>
2257  <P>  <P>
 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  
 treated as an atomic group. That is, once it has matched some of the subject  
 string, it is never re-entered, even if it contains untried alternatives and  
 there is a subsequent matching failure.  
 </P>  
 <P>  
2258  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2259  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2260  <pre>  <pre>
2261    \( ( (?&#62;[^()]+) | (?R) )* \)    \( ( [^()]++ | (?R) )* \)
2262  </pre>  </pre>
2263  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2264  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
2265  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2266  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2267    to avoid backtracking into sequences of non-parentheses.
2268  </P>  </P>
2269  <P>  <P>
2270  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
2271  pattern, so instead you could use this:  pattern, so instead you could use this:
2272  <pre>  <pre>
2273    ( \( ( (?&#62;[^()]+) | (?1) )* \) )    ( \( ( [^()]++ | (?1) )* \) )
2274  </pre>  </pre>
2275  We have put the pattern into parentheses, and caused the recursion to refer to  We have put the pattern into parentheses, and caused the recursion to refer to
2276  them instead of the whole pattern.  them instead of the whole pattern.
2277  </P>  </P>
2278  <P>  <P>
2279  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
2280  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
2281  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
2282  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2283  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.  
2284  </P>  </P>
2285  <P>  <P>
2286  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2287  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2288  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2289  "subroutine" calls, as described in the next section.  <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2290    calls, as described in the next section.
2291  </P>  </P>
2292  <P>  <P>
2293  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2294  for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We  for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
2295  could rewrite the above example as follows:  could rewrite the above example as follows:
2296  <pre>  <pre>
2297    (?&#60;pn&#62; \( ( (?&#62;[^()]+) | (?&pn) )* \) )    (?&#60;pn&#62; \( ( [^()]++ | (?&pn) )* \) )
2298  </pre>  </pre>
2299  If there is more than one subpattern with the same name, the earliest one is  If there is more than one subpattern with the same name, the earliest one is
2300  used.  used.
2301  </P>  </P>
2302  <P>  <P>
2303  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2304  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of a possessive quantifier for matching
2305  non-parentheses is important when applying the pattern to strings that do not  strings of non-parentheses is important when applying the pattern to strings
2306  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2307  <pre>  <pre>
2308    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2309  </pre>  </pre>
2310  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if a possessive quantifier is not used,
2311  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
2312  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
2313  before failure can be reported.  before failure can be reported.
2314  </P>  </P>
2315  <P>  <P>
2316  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values of capturing parentheses are those from
2317  from the outermost level of the recursion at which the subpattern value is set.  the outermost level. If you want to obtain intermediate values, a callout
2318  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2319  <a href="pcrecallout.html"><b>pcrecallout</b></a>  <a href="pcrecallout.html"><b>pcrecallout</b></a>
2320  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
2321  <pre>  <pre>
2322    (ab(cd)ef)    (ab(cd)ef)
2323  </pre>  </pre>
2324  the value for the capturing parentheses is "ef", which is the last value taken  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2325  on at the top level. If additional parentheses are added, giving  the last value taken on at the top level. If a capturing subpattern is not
2326  <pre>  matched at the top level, its final captured value is unset, even if it was
2327    \( ( ( (?&#62;[^()]+) | (?R) )* ) \)  (temporarily) set at a deeper level during the matching process.
2328       ^                        ^  </P>
2329       ^                        ^  <P>
2330  </pre>  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2331  the string they capture is "ab(cd)ef", the contents of the top level  obtain extra memory to store data during a recursion, which it does by using
2332  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
2333  has to obtain extra memory to store data during a recursion, which it does by  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
 using <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
2334  </P>  </P>
2335  <P>  <P>
2336  Do not confuse the (?R) item with the condition (R), which tests for recursion.  Do not confuse the (?R) item with the condition (R), which tests for recursion.
# Line 2028  recursing), whereas any characters are p Line 2343  recursing), whereas any characters are p
2343  In this pattern, (?(R) is the start of a conditional subpattern, with two  In this pattern, (?(R) is the start of a conditional subpattern, with two
2344  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
2345  is the actual recursive call.  is the actual recursive call.
2346    <a name="recursiondifference"></a></P>
2347    <br><b>
2348    Differences in recursion processing between PCRE and Perl
2349    </b><br>
2350    <P>
2351    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2352    (like Python, but unlike Perl), a recursive subpattern call is always treated
2353    as an atomic group. That is, once it has matched some of the subject string, it
2354    is never re-entered, even if it contains untried alternatives and there is a
2355    subsequent matching failure. This can be illustrated by the following pattern,
2356    which purports to match a palindromic string that contains an odd number of
2357    characters (for example, "a", "aba", "abcba", "abcdcba"):
2358    <pre>
2359      ^(.|(.)(?1)\2)$
2360    </pre>
2361    The idea is that it either matches a single character, or two identical
2362    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2363    it does not if the pattern is longer than three characters. Consider the
2364    subject string "abcba":
2365    </P>
2366    <P>
2367    At the top level, the first character is matched, but as it is not at the end
2368    of the string, the first alternative fails; the second alternative is taken
2369    and the recursion kicks in. The recursive call to subpattern 1 successfully
2370    matches the next character ("b"). (Note that the beginning and end of line
2371    tests are not part of the recursion).
2372    </P>
2373    <P>
2374    Back at the top level, the next character ("c") is compared with what
2375    subpattern 2 matched, which was "a". This fails. Because the recursion is
2376    treated as an atomic group, there are now no backtracking points, and so the
2377    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2378    try the second alternative.) However, if the pattern is written with the
2379    alternatives in the other order, things are different:
2380    <pre>
2381      ^((.)(?1)\2|.)$
2382    </pre>
2383    This time, the recursing alternative is tried first, and continues to recurse
2384    until it runs out of characters, at which point the recursion fails. But this
2385    time we do have another alternative to try at the higher level. That is the big
2386    difference: in the previous case the remaining alternative is at a deeper
2387    recursion level, which PCRE cannot use.
2388    </P>
2389    <P>
2390    To change the pattern so that it matches all palindromic strings, not just
2391    those with an odd number of characters, it is tempting to change the pattern to
2392    this:
2393    <pre>
2394      ^((.)(?1)\2|.?)$
2395    </pre>
2396    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2397    deeper recursion has matched a single character, it cannot be entered again in
2398    order to match an empty string. The solution is to separate the two cases, and
2399    write out the odd and even cases as alternatives at the higher level:
2400    <pre>
2401      ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2402    </pre>
2403    If you want to match typical palindromic phrases, the pattern has to ignore all
2404    non-word characters, which can be done like this:
2405    <pre>
2406      ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2407    </pre>
2408    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2409    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2410    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2411    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2412    more) to match typical phrases, and Perl takes so long that you think it has
2413    gone into a loop.
2414    </P>
2415    <P>
2416    <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
2417    string does not start with a palindrome that is shorter than the entire string.
2418    For example, although "abcba" is correctly matched, if the subject is "ababa",
2419    PCRE finds the palindrome "aba" at the start, then fails at top level because
2420    the end of the string does not follow. Once again, it cannot jump back into the
2421    recursion to try other alternatives, so the entire match fails.
2422    </P>
2423    <P>
2424    The second way in which PCRE and Perl differ in their recursion processing is
2425    in the handling of captured values. In Perl, when a subpattern is called
2426    recursively or as a subpattern (see the next section), it has no access to any
2427    values that were captured outside the recursion, whereas in PCRE these values
2428    can be referenced. Consider this pattern:
2429    <pre>
2430      ^(.)(\1|a(?2))
2431    </pre>
2432    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2433    then in the second group, when the back reference \1 fails to match "b", the
2434    second alternative matches "a" and then recurses. In the recursion, \1 does
2435    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2436    match because inside the recursive call \1 cannot access the externally set
2437    value.
2438  <a name="subpatternsassubroutines"></a></P>  <a name="subpatternsassubroutines"></a></P>
2439  <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>  <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
2440  <P>  <P>
2441  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
2442  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
2443  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2444  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2445  relative, as in these examples:  relative, as in these examples:
2446  <pre>  <pre>
# Line 2054  is used, it does match "sense and respon Line 2461  is used, it does match "sense and respon
2461  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2462  </P>  </P>
2463  <P>  <P>
2464  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2465  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
2466  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
2467  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2468    subroutine call revert to their previous values afterwards.
2469  </P>  </P>
2470  <P>  <P>
2471  When a subpattern is used as a subroutine, processing options such as  Processing options such as case-independence are fixed when a subpattern is
2472  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
2473  changed for different calls. For example, consider this pattern:  different calls. For example, consider this pattern:
2474  <pre>  <pre>
2475    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2476  </pre>  </pre>
# Line 2097  same pair of parentheses when there is a Line 2505  same pair of parentheses when there is a
2505  <P>  <P>
2506  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
2507  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
2508  function by putting its entry point in the global variable <i>pcre_callout</i>.  function by putting its entry point in the global variable <i>pcre_callout</i>
2509  By default, this variable contains NULL, which disables all calling out.  (8-bit library) or <i>pcre16_callout</i> (16-bit library). By default, this
2510    variable contains NULL, which disables all calling out.
2511  </P>  </P>
2512  <P>  <P>
2513  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
# Line 2108  For example, this pattern has two callou Line 2517  For example, this pattern has two callou
2517  <pre>  <pre>
2518    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2519  </pre>  </pre>
2520  If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2521  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2522  255.  255.
2523  </P>  </P>
2524  <P>  <P>
2525  During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is  During matching, when PCRE reaches a callout point, the external function is
2526  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
2527  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
2528  originally supplied by the caller of <b>pcre_exec()</b>. The callout function  the matching function. The callout function may cause matching to proceed, to
2529  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
2530  description of the interface to the callout function is given in the  the callout function is given in the
2531  <a href="pcrecallout.html"><b>pcrecallout</b></a>  <a href="pcrecallout.html"><b>pcrecallout</b></a>
2532  documentation.  documentation.
2533  </P>  <a name="backtrackcontrol"></a></P>
2534  <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>  <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
2535  <P>  <P>
2536  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
# Line 2132  remarks apply to the PCRE features descr Line 2541  remarks apply to the PCRE features descr
2541  </P>  </P>
2542  <P>  <P>
2543  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
2544  used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses  used only when the pattern is to be matched using one of the traditional
2545  a backtracking algorithm. With the exception of (*FAIL), which behaves like a  matching functions, which use a backtracking algorithm. With the exception of
2546  failing negative assertion, they cause an error if encountered by  (*FAIL), which behaves like a failing negative assertion, they cause an error
2547  <b>pcre_dfa_exec()</b>.  if encountered by a DFA matching function.
2548    </P>
2549    <P>
2550    If any of these verbs are used in an assertion or in a subpattern that is
2551    called as a subroutine (whether or not recursively), their effect is confined
2552    to that subpattern; it does not extend to the surrounding pattern, with one
2553    exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2554    a successful positive assertion <i>is</i> passed back when a match succeeds
2555    (compare capturing parentheses in assertions). Note that such subpatterns are
2556    processed as anchored at the point where they are tested. Note also that Perl's
2557    treatment of subroutines is different in some cases.
2558  </P>  </P>
2559  <P>  <P>
2560  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
2561  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2562  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2563  form is just (*VERB). Any number of these verbs may occur in a pattern. There  depending on whether or not an argument is present. A name is any sequence of
2564  are two kinds:  characters that does not include a closing parenthesis. If the name is empty,
2565    that is, if the closing parenthesis immediately follows the colon, the effect
2566    is as if the colon were not there. Any number of these verbs may occur in a
2567    pattern.
2568    </P>
2569    <P>
2570    PCRE contains some optimizations that are used to speed up matching by running
2571    some checks at the start of each match attempt. For example, it may know the
2572    minimum length of matching subject, or that a particular character must be
2573    present. When one of these optimizations suppresses the running of a match, any
2574    included backtracking verbs will not, of course, be processed. You can suppress
2575    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2576    when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
2577    pattern with (*NO_START_OPT).
2578    </P>
2579    <P>
2580    Experiments with Perl suggest that it too has similar optimizations, sometimes
2581    leading to anomalous results.
2582  </P>  </P>
2583  <br><b>  <br><b>
2584  Verbs that act immediately  Verbs that act immediately
2585  </b><br>  </b><br>
2586  <P>  <P>
2587  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2588    followed by a name.
2589  <pre>  <pre>
2590     (*ACCEPT)     (*ACCEPT)
2591  </pre>  </pre>
2592  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
2593  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2594  immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside  subroutine, only that subpattern is ended successfully. Matching then continues
2595  capturing parentheses. In Perl, the data so far is captured: in PCRE no data is  at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2596  captured. For example:  far is captured. For example:
2597  <pre>  <pre>
2598    A(A|B(*ACCEPT)|C)D    A((?:A|B(*ACCEPT)|C)D)
2599  </pre>  </pre>
2600  This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2601  captured.  the outer parentheses.
2602  <pre>  <pre>
2603    (*FAIL) or (*F)    (*FAIL) or (*F)
2604  </pre>  </pre>
2605  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
2606  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
2607  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2608  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 2177  A match with the string "aaaa" always fa Line 2614  A match with the string "aaaa" always fa
2614  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2615  </P>  </P>
2616  <br><b>  <br><b>
2617    Recording which path was taken
2618    </b><br>
2619    <P>
2620    There is one verb whose main purpose is to track how a match was arrived at,
2621    though it also has a secondary use in conjunction with advancing the match
2622    starting point (see (*SKIP) below).
2623    <pre>
2624      (*MARK:NAME) or (*:NAME)
2625    </pre>
2626    A name is always required with this verb. There may be as many instances of
2627    (*MARK) as you like in a pattern, and their names do not have to be unique.
2628    </P>
2629    <P>
2630    When a match succeeds, the name of the last-encountered (*MARK) on the matching
2631    path is passed back to the caller as described in the section entitled
2632    <a href="pcreapi.html#extradata">"Extra data for <b>pcre_exec()</b>"</a>
2633    in the
2634    <a href="pcreapi.html"><b>pcreapi</b></a>
2635    documentation. Here is an example of <b>pcretest</b> output, where the /K
2636    modifier requests the retrieval and outputting of (*MARK) data:
2637    <pre>
2638        re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K
2639      data&#62; XY
2640       0: XY
2641      MK: A
2642      XZ
2643       0: XZ
2644      MK: B
2645    </pre>
2646    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2647    indicates which of the two alternatives matched. This is a more efficient way
2648    of obtaining this information than putting each alternative in its own
2649    capturing parentheses.
2650    </P>
2651    <P>
2652    If (*MARK) is encountered in a positive assertion, its name is recorded and
2653    passed back if it is the last-encountered. This does not happen for negative
2654    assertions.
2655    </P>
2656    <P>
2657    After a partial match or a failed match, the name of the last encountered
2658    (*MARK) in the entire match process is returned. For example:
2659    <pre>
2660        re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K
2661      data&#62; XP
2662      No match, mark = B
2663    </pre>
2664    Note that in this unanchored example the mark is retained from the match
2665    attempt that started at the letter "X". Subsequent match attempts starting at
2666    "P" and then with an empty string do not get as far as the (*MARK) item, but
2667    nevertheless do not reset it.
2668    </P>
2669    <br><b>
2670  Verbs that act after backtracking  Verbs that act after backtracking
2671  </b><br>  </b><br>
2672  <P>  <P>
2673  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2674  with what follows, but if there is no subsequent match, a failure is forced.  with what follows, but if there is no subsequent match, causing a backtrack to
2675  The verbs differ in exactly what kind of failure occurs.  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2676    the verb. However, when one of these verbs appears inside an atomic group, its
2677    effect is confined to that group, because once the group has been matched,
2678    there is never any backtracking into it. In this situation, backtracking can
2679    "jump back" to the left of the entire atomic group. (Remember also, as stated
2680    above, that this localization also applies in subroutine calls and assertions.)
2681    </P>
2682    <P>
2683    These verbs differ in exactly what kind of failure occurs when backtracking
2684    reaches them.
2685  <pre>  <pre>
2686    (*COMMIT)    (*COMMIT)
2687  </pre>  </pre>
2688  This verb causes the whole match to fail outright if the rest of the pattern  This verb, which may not be followed by a name, causes the whole match to fail
2689  does not match. Even if the pattern is unanchored, no further attempts to find  outright if the rest of the pattern does not match. Even if the pattern is
2690  a match by advancing the start point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2691  passed, <b>pcre_exec()</b> is committed to finding a match at the current  take place. Once (*COMMIT) has been passed, <b>pcre_exec()</b> is committed to
2692  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2693  <pre>  <pre>
2694    a+(*COMMIT)b    a+(*COMMIT)b
2695  </pre>  </pre>
2696  This matches "xxaab" but not "aacaab". It can be thought of as a kind of  This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2697  dynamic anchor, or "I've started, so I must finish."  dynamic anchor, or "I've started, so I must finish." The name of the most
2698  <pre>  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2699    (*PRUNE)  match failure.
2700  </pre>  </P>
2701  This verb causes the match to fail at the current position if the rest of the  <P>
2702  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2703  advance to the next starting character then happens. Backtracking can occur as  unless PCRE's start-of-match optimizations are turned off, as shown in this
2704  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  <b>pcretest</b> example:
2705  if there is no match to the right, backtracking cannot cross (*PRUNE).  <pre>
2706  In simple cases, the use of (*PRUNE) is just an alternative to an atomic      re&#62; /(*COMMIT)abc/
2707  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot    data&#62; xyzabc
2708  be expressed in any other way.     0: abc
2709      xyzabc\Y
2710      No match
2711    </pre>
2712    PCRE knows that any match must start with "a", so the optimization skips along
2713    the subject to "a" before running the first match attempt, which succeeds. When
2714    the optimization is disabled by the \Y escape in the second subject, the match
2715    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2716    starting points.
2717    <pre>
2718      (*PRUNE) or (*PRUNE:NAME)
2719    </pre>
2720    This verb causes the match to fail at the current starting position in the
2721    subject if the rest of the pattern does not match. If the pattern is
2722    unanchored, the normal "bumpalong" advance to the next starting character then
2723    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2724    reached, or when matching to the right of (*PRUNE), but if there is no match to
2725    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2726    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2727    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2728    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2729    anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2730  <pre>  <pre>
2731    (*SKIP)    (*SKIP)
2732  </pre>  </pre>
2733  This verb is like (*PRUNE), except that if the pattern is unanchored, the  This verb, when given without a name, is like (*PRUNE), except that if the
2734  "bumpalong" advance is not to the next character, but to the position in the  pattern is unanchored, the "bumpalong" advance is not to the next character,
2735  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2736  was matched leading up to it cannot be part of a successful match. Consider:  signifies that whatever text was matched leading up to it cannot be part of a
2737    successful match. Consider:
2738  <pre>  <pre>
2739    a+(*SKIP)b    a+(*SKIP)b
2740  </pre>  </pre>
2741  If the subject is "aaaac...", after the first match attempt fails (starting at  If the subject is "aaaac...", after the first match attempt fails (starting at
2742  the first character in the string), the starting point skips on to start the  the first character in the string), the starting point skips on to start the
2743  next attempt at "c". Note that a possessive quantifer does not have the same  next attempt at "c". Note that a possessive quantifer does not have the same
2744  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
2745  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2746  instead of skipping on to "c".  instead of skipping on to "c".
2747  <pre>  <pre>
2748    (*THEN)    (*SKIP:NAME)
2749  </pre>  </pre>
2750  This verb causes a skip to the next alternation if the rest of the pattern does  When (*SKIP) has an associated name, its behaviour is modified. If the
2751  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2752  current alternation. Its name comes from the observation that it can be used  searched for the most recent (*MARK) that has the same name. If one is found,
2753  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2754    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2755    matching name is found, the (*SKIP) is ignored.
2756    <pre>
2757      (*THEN) or (*THEN:NAME)
2758    </pre>
2759    This verb causes a skip to the next innermost alternative if the rest of the
2760    pattern does not match. That is, it cancels pending backtracking, but only
2761    within the current alternative. Its name comes from the observation that it can
2762    be used for a pattern-based if-then-else block:
2763  <pre>  <pre>
2764    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2765  </pre>  </pre>
2766  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
2767  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
2768  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2769  is used outside of any alternation, it acts exactly like (*PRUNE).  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2770    If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2771    </P>
2772    <P>
2773    Note that a subpattern that does not contain a | character is just a part of
2774    the enclosing alternative; it is not a nested alternation with only one
2775    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2776    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2777    pattern fragments that do not contain any | characters at this level:
2778    <pre>
2779      A (B(*THEN)C) | D
2780    </pre>
2781    If A and B are matched, but there is a failure in C, matching does not
2782    backtrack into A; instead it moves to the next alternative, that is, D.
2783    However, if the subpattern containing (*THEN) is given an alternative, it
2784    behaves differently:
2785    <pre>
2786      A (B(*THEN)C | (*FAIL)) | D
2787    </pre>
2788    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2789    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2790    because there are no more alternatives to try. In this case, matching does now
2791    backtrack into A.
2792    </P>
2793    <P>
2794    Note also that a conditional subpattern is not considered as having two
2795    alternatives, because only one is ever used. In other words, the | character in
2796    a conditional subpattern has a different meaning. Ignoring white space,
2797    consider:
2798    <pre>
2799      ^.*? (?(?=a) a | b(*THEN)c )
2800    </pre>
2801    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2802    it initially matches zero characters. The condition (?=a) then fails, the
2803    character "b" is matched, but "c" is not. At this point, matching does not
2804    backtrack to .*? as might perhaps be expected from the presence of the |
2805    character. The conditional subpattern is part of the single alternative that
2806    comprises the whole pattern, and so the match fails. (If there was a backtrack
2807    into .*?, allowing it to match "b", the match would succeed.)
2808    </P>
2809    <P>
2810    The verbs just described provide four different "strengths" of control when
2811    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2812    next alternative. (*PRUNE) comes next, failing the match at the current
2813    starting position, but allowing an advance to the next character (for an
2814    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2815    than one character. (*COMMIT) is the strongest, causing the entire match to
2816    fail.
2817    </P>
2818    <P>
2819    If more than one such verb is present in a pattern, the "strongest" one wins.
2820    For example, consider this pattern, where A, B, etc. are complex pattern
2821    fragments:
2822    <pre>
2823      (A(*COMMIT)B(*THEN)C|D)
2824    </pre>
2825    Once A has matched, PCRE is committed to this match, at the current starting
2826    position. If subsequently B matches, but C does not, the normal (*THEN) action
2827    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2828    overrides.
2829  </P>  </P>
2830  <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>  <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
2831  <P>  <P>
2832  <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), <b>pcre</b>(3).  <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
2833    <b>pcresyntax</b>(3), <b>pcre</b>(3), <b>pcre16(3)</b>.
2834  </P>  </P>
2835  <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>  <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
2836  <P>  <P>
# Line 2253  Cambridge CB2 3QH, England. Line 2843  Cambridge CB2 3QH, England.
2843  </P>  </P>
2844  <br><a name="SEC28" href="#TOC1">REVISION</a><br>  <br><a name="SEC28" href="#TOC1">REVISION</a><br>
2845  <P>  <P>
2846  Last updated: 11 April 2009  Last updated: 09 January 2012
2847  <br>  <br>
2848  Copyright &copy; 1997-2009 University of Cambridge.  Copyright &copy; 1997-2012 University of Cambridge.
2849  <br>  <br>
2850  <p>  <p>
2851  Return to the <a href="index.html">PCRE index page</a>.  Return to the <a href="index.html">PCRE index page</a>.

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