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revision 77 by nigel, Sat Feb 24 21:40:45 2007 UTC revision 954 by ph10, Sat Mar 31 18:09:26 2012 UTC
# Line 14  man page, in case the conversion went wr Line 14  man page, in case the conversion went wr
14  <br>  <br>
15  <ul>  <ul>
16  <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>  <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
17  <li><a name="TOC2" href="#SEC2">BACKSLASH</a>  <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a>
18  <li><a name="TOC3" href="#SEC3">CIRCUMFLEX AND DOLLAR</a>  <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
19  <li><a name="TOC4" href="#SEC4">FULL STOP (PERIOD, DOT)</a>  <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
20  <li><a name="TOC5" href="#SEC5">MATCHING A SINGLE BYTE</a>  <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
21  <li><a name="TOC6" href="#SEC6">SQUARE BRACKETS AND CHARACTER CLASSES</a>  <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT) AND \N</a>
22  <li><a name="TOC7" href="#SEC7">POSIX CHARACTER CLASSES</a>  <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE DATA UNIT</a>
23  <li><a name="TOC8" href="#SEC8">VERTICAL BAR</a>  <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a>
24  <li><a name="TOC9" href="#SEC9">INTERNAL OPTION SETTING</a>  <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
25  <li><a name="TOC10" href="#SEC10">SUBPATTERNS</a>  <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
26  <li><a name="TOC11" href="#SEC11">NAMED SUBPATTERNS</a>  <li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a>
27  <li><a name="TOC12" href="#SEC12">REPETITION</a>  <li><a name="TOC12" href="#SEC12">SUBPATTERNS</a>
28  <li><a name="TOC13" href="#SEC13">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>  <li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a>
29  <li><a name="TOC14" href="#SEC14">BACK REFERENCES</a>  <li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a>
30  <li><a name="TOC15" href="#SEC15">ASSERTIONS</a>  <li><a name="TOC15" href="#SEC15">REPETITION</a>
31  <li><a name="TOC16" href="#SEC16">CONDITIONAL SUBPATTERNS</a>  <li><a name="TOC16" href="#SEC16">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
32  <li><a name="TOC17" href="#SEC17">COMMENTS</a>  <li><a name="TOC17" href="#SEC17">BACK REFERENCES</a>
33  <li><a name="TOC18" href="#SEC18">RECURSIVE PATTERNS</a>  <li><a name="TOC18" href="#SEC18">ASSERTIONS</a>
34  <li><a name="TOC19" href="#SEC19">SUBPATTERNS AS SUBROUTINES</a>  <li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a>
35  <li><a name="TOC20" href="#SEC20">CALLOUTS</a>  <li><a name="TOC20" href="#SEC20">COMMENTS</a>
36    <li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a>
37    <li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a>
38    <li><a name="TOC23" href="#SEC23">ONIGURUMA SUBROUTINE SYNTAX</a>
39    <li><a name="TOC24" href="#SEC24">CALLOUTS</a>
40    <li><a name="TOC25" href="#SEC25">BACKTRACKING CONTROL</a>
41    <li><a name="TOC26" href="#SEC26">SEE ALSO</a>
42    <li><a name="TOC27" href="#SEC27">AUTHOR</a>
43    <li><a name="TOC28" href="#SEC28">REVISION</a>
44  </ul>  </ul>
45  <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>  <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
46  <P>  <P>
47  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
48  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
49  documentation and in a number of books, some of which have copious examples.  <a href="pcresyntax.html"><b>pcresyntax</b></a>
50  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
51  regular expressions in great detail. This description of PCRE's regular  also supports some alternative regular expression syntax (which does not
52  expressions is intended as reference material.  conflict with the Perl syntax) in order to provide some compatibility with
53    regular expressions in Python, .NET, and Oniguruma.
54    </P>
55    <P>
56    Perl's regular expressions are described in its own documentation, and
57    regular expressions in general are covered in a number of books, some of which
58    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
59    published by O'Reilly, covers regular expressions in great detail. This
60    description of PCRE's regular expressions is intended as reference material.
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. How this affects pattern matching is mentioned in several  features, PCRE must be built to include appropriate support. When using UTF
67  places below. There is also a summary of UTF-8 features in the  strings you must either call the compiling function with the PCRE_UTF8 or
68  <a href="pcre.html#utf8support">section on UTF-8 support</a>  PCRE_UTF16 option, or the pattern must start with one of these special
69  in the main  sequences:
70  <a href="pcre.html"><b>pcre</b></a>  <pre>
71      (*UTF8)
72      (*UTF16)
73    </pre>
74    Starting a pattern with such a sequence is equivalent to setting the relevant
75    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
77    of features in the
78    <a href="pcreunicode.html"><b>pcreunicode</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. The advantages and disadvantages of the alternative function,  a different algorithm that is not Perl-compatible. Some of the features
104  and how it differs from the normal function, are discussed in the  discussed below are not available when DFA matching is used. The advantages and
105    disadvantages of the alternative functions, and how they differ from the normal
106    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    <a name="newlines"></a></P>
110    <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
111    <P>
112    PCRE supports five different conventions for indicating line breaks in
113    strings: a single CR (carriage return) character, a single LF (linefeed)
114    character, the two-character sequence CRLF, any of the three preceding, or any
115    Unicode newline sequence. The
116    <a href="pcreapi.html"><b>pcreapi</b></a>
117    page has
118    <a href="pcreapi.html#newlines">further discussion</a>
119    about newlines, and shows how to set the newline convention in the
120    <i>options</i> arguments for the compiling and matching functions.
121    </P>
122    <P>
123    It is also possible to specify a newline convention by starting a pattern
124    string with one of the following five sequences:
125    <pre>
126      (*CR)        carriage return
127      (*LF)        linefeed
128      (*CRLF)      carriage return, followed by linefeed
129      (*ANYCRLF)   any of the three above
130      (*ANY)       all Unicode newline sequences
131    </pre>
132    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
134    <pre>
135      (*CR)a.b
136    </pre>
137    changes the convention to CR. That pattern matches "a\nb" because LF is no
138    longer a newline. Note that these special settings, which are not
139    Perl-compatible, are recognized only at the very start of a pattern, and that
140    they must be in upper case. If more than one of them is present, the last one
141    is used.
142    </P>
143    <P>
144    The newline convention affects the interpretation of the dot metacharacter when
145    PCRE_DOTALL is not set, and also the behaviour of \N. However, it does not
146    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>
150    below. A change of \R setting can be combined with a change of newline
151    convention.
152  </P>  </P>
153    <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
154  <P>  <P>
155  A regular expression is a pattern that is matched against a subject string from  A regular expression is a pattern that is matched against a subject string from
156  left to right. Most characters stand for themselves in a pattern, and match the  left to right. Most characters stand for themselves in a pattern, and match the
# Line 73  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 90  interpreted in some special way. Line 177  interpreted in some special way.
177  <P>  <P>
178  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
179  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
180  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
181  as follows:  are as follows:
182  <pre>  <pre>
183    \      general escape character with several uses    \      general escape character with several uses
184    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
# Line 120  a character class the only metacharacter Line 207  a character class the only metacharacter
207  </pre>  </pre>
208  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
209  </P>  </P>
210  <br><a name="SEC2" 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 may  character that is not a number or a letter, it takes away any special meaning
214  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 135  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 character are ignored. An escaping  a character class and the next newline are ignored. An escaping backslash can
233  backslash can be used to include a whitespace or # character as part of the  be used to include a whitespace or # character as part of the pattern.
 pattern.  
234  </P>  </P>
235  <P>  <P>
236  If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
# Line 154  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 162  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        newline (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... (UTF-8 mode only)    \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). In UTF-8 mode, any number of hexadecimal digits may  flipped.)
284  appear between \x{ and }, but the value of the character code must be less  </P>
285  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  <P>
286  other than hexadecimal digits appear between \x{ and }, or if there is no  By default, after \x, from zero to two hexadecimal digits are read (letters
287  terminating }, this form of escape is not recognized. Instead, the initial  can be in upper or lower case). Any number of hexadecimal digits may appear
288  \x will be interpreted as a basic hexadecimal escape, with no following  between \x{ and }, but the character code is constrained as follows:
289  digits, giving a character whose value is zero.  <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>
298    <P>
299    If characters other than hexadecimal digits appear between \x{ and }, or if
300    there is no terminating }, this form of escape is not recognized. Instead, the
301    initial \x will be interpreted as a basic hexadecimal escape, with no
302    following digits, giving a character whose value is zero.
303    </P>
304    <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>  </P>
311  <P>  <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 when PCRE is in UTF-8 mode. There is no difference in the  syntaxes for \x (or by \u in JavaScript mode). There is no difference in the
314  way they are handled. For 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. In both cases, if there  After \0 up to two further octal digits are read. If there are fewer than two
319  are fewer than two digits, just those that are present are used. Thus the  digits, just those that are present are used. Thus the sequence \0\x\07
320  sequence \0\x\07 specifies two binary zeros followed by a BEL character  specifies two binary zeros followed by a BEL character (code value 7). Make
321  (code value 7). Make sure you supply two digits after the initial zero if the  sure you supply two digits after the initial zero if the pattern character that
322  pattern character that follows is itself an octal digit.  follows is itself an octal digit.
323  </P>  </P>
324  <P>  <P>
325  The handling of a backslash followed by a digit other than 0 is complicated.  The handling of a backslash followed by a digit other than 0 is complicated.
# Line 217  following the discussion of Line 334  following the discussion of
334  <P>  <P>
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 generates a single byte from the least  digits following the backslash, and uses them to generate a data character. Any
338  significant 8 bits of the value. Any subsequent digits stand for themselves.  subsequent digits stand for themselves. The value of the character is
339    constrained in the same way as characters specified in hexadecimal.
340  For example:  For example:
341  <pre>  <pre>
342    \040   is another way of writing a space    \040   is another way of writing a space
# Line 228  For example: Line 346  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
353  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
354  </P>  </P>
355  <P>  <P>
356  All the sequences that define a single byte value or a single UTF-8 character  All the sequences that define a single character value can be used both inside
357  (in UTF-8 mode) can be used both inside and outside character classes. In  and outside character classes. In addition, inside a character class, \b is
358  addition, inside a character class, the sequence \b is interpreted as the  interpreted as the backspace character (hex 08).
359  backspace character (hex 08), and the sequence \X is interpreted as the  </P>
360  character "X". 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>
377    <br><b>
378    Absolute and relative back references
379    </b><br>
380    <P>
381    The sequence \g followed by an unsigned or a negative number, optionally
382    enclosed in braces, is an absolute or relative back reference. A named back
383    reference can be coded as \g{name}. Back references are discussed
384    <a href="#backreferences">later,</a>
385    following the discussion of
386    <a href="#subpattern">parenthesized subpatterns.</a>
387  </P>  </P>
388  <br><b>  <br><b>
389    Absolute and relative subroutine calls
390    </b><br>
391    <P>
392    For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
393    a number enclosed either in angle brackets or single quotes, is an alternative
394    syntax for referencing a subpattern as a "subroutine". Details are discussed
395    <a href="#onigurumasubroutines">later.</a>
396    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
398    <a href="#subpatternsassubroutines">subroutine</a>
399    call.
400    <a name="genericchartypes"></a></P>
401    <br><b>
402  Generic character types  Generic character types
403  </b><br>  </b><br>
404  <P>  <P>
405  The third 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
409      \h     any horizontal whitespace character
410      \H     any character that is not a horizontal whitespace character
411    \s     any whitespace character    \s     any whitespace character
412    \S     any character that is not a whitespace character    \S     any character that is not a whitespace character
413      \v     any vertical whitespace character
414      \V     any character that is not a vertical whitespace character
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>
433  For compatibility with Perl, \s does not match the VT character (code 11).  For compatibility with Perl, \s does not match the VT character (code 11).
434  This makes it different from the the POSIX "space" class. The \s characters  This makes it different from the the POSIX "space" class. The \s characters
435  are HT (9), LF (10), FF (12), CR (13), and space (32).  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
436    included in a Perl script, \s may match the VT character. In PCRE, it never
437    does.
438  </P>  </P>
439  <P>  <P>
440  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character that is a letter or digit.
441  letter or digit. The definition of letters and digits is controlled by PCRE's  By default, the definition of letters and digits is controlled by PCRE's
442  low-valued character tables, and may vary if locale-specific matching is taking  low-valued character tables, and may vary if locale-specific matching is taking
443  place (see  place (see
444  <a href="pcreapi.html#localesupport">"Locale support"</a>  <a href="pcreapi.html#localesupport">"Locale support"</a>
445  in the  in the
446  <a href="pcreapi.html"><b>pcreapi</b></a>  <a href="pcreapi.html"><b>pcreapi</b></a>
447  page). For example, in the "fr_FR" (French) locale, some character codes  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
448  greater than 128 are used for accented letters, and these are matched by \w.  or "french" in Windows, some character codes greater than 128 are used for
449  </P>  accented letters, and these are then matched by \w. The use of locales with
450    Unicode is discouraged.
451    </P>
452    <P>
453    By default, in a UTF mode, characters with values greater than 128 never match
454    \d, \s, or \w, and always match \D, \S, and \W. These sequences retain
455    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>
476      U+0009     Horizontal tab
477      U+0020     Space
478      U+00A0     Non-break space
479      U+1680     Ogham space mark
480      U+180E     Mongolian vowel separator
481      U+2000     En quad
482      U+2001     Em quad
483      U+2002     En space
484      U+2003     Em space
485      U+2004     Three-per-em space
486      U+2005     Four-per-em space
487      U+2006     Six-per-em space
488      U+2007     Figure space
489      U+2008     Punctuation space
490      U+2009     Thin space
491      U+200A     Hair space
492      U+202F     Narrow no-break space
493      U+205F     Medium mathematical space
494      U+3000     Ideographic space
495    </pre>
496    The vertical space characters are:
497    <pre>
498      U+000A     Linefeed
499      U+000B     Vertical tab
500      U+000C     Formfeed
501      U+000D     Carriage return
502      U+0085     Next line
503      U+2028     Line separator
504      U+2029     Paragraph separator
505    </pre>
506    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
507    relevant.
508    <a name="newlineseq"></a></P>
509    <br><b>
510    Newline sequences
511    </b><br>
512  <P>  <P>
513  In UTF-8 mode, characters with values greater than 128 never match \d, \s, or  Outside a character class, by default, the escape sequence \R matches any
514  \w, and always match \D, \S, and \W. This is true even when Unicode  Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the
515  character property support is available.  following:
516    <pre>
517      (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
518    </pre>
519    This is an example of an "atomic group", details of which are given
520    <a href="#atomicgroup">below.</a>
521    This particular group matches either the two-character sequence CR followed by
522    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
523    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
524    line, U+0085). The two-character sequence is treated as a single unit that
525    cannot be split.
526    </P>
527    <P>
528    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).
530    Unicode character property support is not needed for these characters to be
531    recognized.
532    </P>
533    <P>
534    It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
535    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
536    either at compile time or when the pattern is matched. (BSR is an abbrevation
537    for "backslash R".) This can be made the default when PCRE is built; if this is
538    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
539    It is also possible to specify these settings by starting a pattern string with
540    one of the following sequences:
541    <pre>
542      (*BSR_ANYCRLF)   CR, LF, or CRLF only
543      (*BSR_UNICODE)   any Unicode newline sequence
544    </pre>
545    These override the default and the options given to the compiling function, but
546    they can themselves be overridden by options given to a matching function. Note
547    that these special settings, which are not Perl-compatible, are recognized only
548    at the very start of a pattern, and that they must be in upper case. If more
549    than one of them is present, the last one is used. They can be combined with a
550    change of newline convention; for example, a pattern can start with:
551    <pre>
552      (*ANY)(*BSR_ANYCRLF)
553    </pre>
554    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
561  </b><br>  </b><br>
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 to match generic character types are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
565  is selected. They are:  When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
566  <pre>  characters whose codepoints are less than 256, but they do work in this mode.
567   \p{<i>xx</i>}   a character with the <i>xx</i> property  The extra escape sequences are:
568   \P{<i>xx</i>}   a character without the <i>xx</i> property  <pre>
569   \X       an extended Unicode sequence    \p{<i>xx</i>}   a character with the <i>xx</i> property
570  </pre>    \P{<i>xx</i>}   a character without the <i>xx</i> property
571  The property names represented by <i>xx</i> above are limited to the    \X       an extended Unicode sequence
572  Unicode general category properties. Each character has exactly one such  </pre>
573  property, specified by a two-letter abbreviation. For compatibility with Perl,  The property names represented by <i>xx</i> above are limited to the Unicode
574  negation can be specified by including a circumflex between the opening brace  script names, the general category properties, "Any", which matches any
575  and the property name. For example, \p{^Lu} is the same as \P{Lu}.  character (including newline), and some special PCRE properties (described
576  </P>  in the
577  <P>  <a href="#extraprops">next section).</a>
578  If only one letter is specified with \p or \P, it includes all the properties  Other Perl properties such as "InMusicalSymbols" are not currently supported by
579  that start with that letter. In this case, in the absence of negation, the  PCRE. Note that \P{Any} does not match any characters, so always causes a
580  curly brackets in the escape sequence are optional; these two examples have  match failure.
581  the same effect:  </P>
582    <P>
583    Sets of Unicode characters are defined as belonging to certain scripts. A
584    character from one of these sets can be matched using a script name. For
585    example:
586    <pre>
587      \p{Greek}
588      \P{Han}
589    </pre>
590    Those that are not part of an identified script are lumped together as
591    "Common". The current list of scripts is:
592    </P>
593    <P>
594    Arabic,
595    Armenian,
596    Avestan,
597    Balinese,
598    Bamum,
599    Batak,
600    Bengali,
601    Bopomofo,
602    Brahmi,
603    Braille,
604    Buginese,
605    Buhid,
606    Canadian_Aboriginal,
607    Carian,
608    Chakma,
609    Cham,
610    Cherokee,
611    Common,
612    Coptic,
613    Cuneiform,
614    Cypriot,
615    Cyrillic,
616    Deseret,
617    Devanagari,
618    Egyptian_Hieroglyphs,
619    Ethiopic,
620    Georgian,
621    Glagolitic,
622    Gothic,
623    Greek,
624    Gujarati,
625    Gurmukhi,
626    Han,
627    Hangul,
628    Hanunoo,
629    Hebrew,
630    Hiragana,
631    Imperial_Aramaic,
632    Inherited,
633    Inscriptional_Pahlavi,
634    Inscriptional_Parthian,
635    Javanese,
636    Kaithi,
637    Kannada,
638    Katakana,
639    Kayah_Li,
640    Kharoshthi,
641    Khmer,
642    Lao,
643    Latin,
644    Lepcha,
645    Limbu,
646    Linear_B,
647    Lisu,
648    Lycian,
649    Lydian,
650    Malayalam,
651    Mandaic,
652    Meetei_Mayek,
653    Meroitic_Cursive,
654    Meroitic_Hieroglyphs,
655    Miao,
656    Mongolian,
657    Myanmar,
658    New_Tai_Lue,
659    Nko,
660    Ogham,
661    Old_Italic,
662    Old_Persian,
663    Old_South_Arabian,
664    Old_Turkic,
665    Ol_Chiki,
666    Oriya,
667    Osmanya,
668    Phags_Pa,
669    Phoenician,
670    Rejang,
671    Runic,
672    Samaritan,
673    Saurashtra,
674    Sharada,
675    Shavian,
676    Sinhala,
677    Sora_Sompeng,
678    Sundanese,
679    Syloti_Nagri,
680    Syriac,
681    Tagalog,
682    Tagbanwa,
683    Tai_Le,
684    Tai_Tham,
685    Tai_Viet,
686    Takri,
687    Tamil,
688    Telugu,
689    Thaana,
690    Thai,
691    Tibetan,
692    Tifinagh,
693    Ugaritic,
694    Vai,
695    Yi.
696    </P>
697    <P>
698    Each character has exactly one Unicode general category property, specified by
699    a two-letter abbreviation. For compatibility with Perl, negation can be
700    specified by including a circumflex between the opening brace and the property
701    name. For example, \p{^Lu} is the same as \P{Lu}.
702    </P>
703    <P>
704    If only one letter is specified with \p or \P, it includes all the general
705    category properties that start with that letter. In this case, in the absence
706    of negation, the curly brackets in the escape sequence are optional; these two
707    examples have the same effect:
708  <pre>  <pre>
709    \p{L}    \p{L}
710    \pL    \pL
711  </pre>  </pre>
712  The following property codes are supported:  The following general category property codes are supported:
713  <pre>  <pre>
714    C     Other    C     Other
715    Cc    Control    Cc    Control
# Line 360  The following property codes are support Line 755  The following property codes are support
755    Zp    Paragraph separator    Zp    Paragraph separator
756    Zs    Space separator    Zs    Space separator
757  </pre>  </pre>
758  Extended properties such as "Greek" or "InMusicalSymbols" are not supported by  The special property L& is also supported: it matches a character that has
759  PCRE.  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
760    a modifier or "other".
761    </P>
762    <P>
763    The Cs (Surrogate) property applies only to characters in the range U+D800 to
764    U+DFFF. Such characters are not valid in Unicode strings and so
765    cannot be tested by PCRE, unless UTF validity checking has been turned off
766    (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the
767    <a href="pcreapi.html"><b>pcreapi</b></a>
768    page). Perl does not support the Cs property.
769    </P>
770    <P>
771    The long synonyms for property names that Perl supports (such as \p{Letter})
772    are not supported by PCRE, nor is it permitted to prefix any of these
773    properties with "Is".
774    </P>
775    <P>
776    No character that is in the Unicode table has the Cn (unassigned) property.
777    Instead, this property is assumed for any code point that is not in the
778    Unicode table.
779  </P>  </P>
780  <P>  <P>
781  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
# Line 378  or more characters with the "mark" prope Line 792  or more characters with the "mark" prope
792  atomic group  atomic group
793  <a href="#atomicgroup">(see below).</a>  <a href="#atomicgroup">(see below).</a>
794  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
795  preceding character.  preceding character. None of them have codepoints less than 256, so in
796    8-bit non-UTF-8 mode \X matches any one character.
797    </P>
798    <P>
799    Note that recent versions of Perl have changed \X to match what Unicode calls
800    an "extended grapheme cluster", which has a more complicated definition.
801  </P>  </P>
802  <P>  <P>
803  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
804  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
805  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
806  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
807    PCRE_UCP option or by starting the pattern with (*UCP).
808    <a name="extraprops"></a></P>
809    <br><b>
810    PCRE's additional properties
811    </b><br>
812    <P>
813    As well as the standard Unicode properties described in the previous
814    section, PCRE supports four more that make it possible to convert traditional
815    escape sequences such as \w and \s and POSIX character classes to use Unicode
816    properties. PCRE uses these non-standard, non-Perl properties internally when
817    PCRE_UCP is set. They are:
818    <pre>
819      Xan   Any alphanumeric character
820      Xps   Any POSIX space character
821      Xsp   Any Perl space character
822      Xwd   Any Perl "word" character
823    </pre>
824    Xan matches characters that have either the L (letter) or the N (number)
825    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
826    carriage return, and any other character that has the Z (separator) property.
827    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
828    same characters as Xan, plus underscore.
829    <a name="resetmatchstart"></a></P>
830    <br><b>
831    Resetting the match start
832    </b><br>
833    <P>
834    The escape sequence \K causes any previously matched characters not to be
835    included in the final matched sequence. For example, the pattern:
836    <pre>
837      foo\Kbar
838    </pre>
839    matches "foobar", but reports that it has matched "bar". This feature is
840    similar to a lookbehind assertion
841    <a href="#lookbehind">(described below).</a>
842    However, in this case, the part of the subject before the real match does not
843    have to be of fixed length, as lookbehind assertions do. The use of \K does
844    not interfere with the setting of
845    <a href="#subpattern">captured substrings.</a>
846    For example, when the pattern
847    <pre>
848      (foo)\Kbar
849    </pre>
850    matches "foobar", the first substring is still set to "foo".
851    </P>
852    <P>
853    Perl documents that the use of \K within assertions is "not well defined". In
854    PCRE, \K is acted upon when it occurs inside positive assertions, but is
855    ignored in negative assertions.
856  <a name="smallassertions"></a></P>  <a name="smallassertions"></a></P>
857  <br><b>  <br><b>
858  Simple assertions  Simple assertions
859  </b><br>  </b><br>
860  <P>  <P>
861  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
862  specifies a condition that has to be met at a particular point in a match,  specifies a condition that has to be met at a particular point in a match,
863  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
864  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
865  <a href="#bigassertions">below.</a>  <a href="#bigassertions">below.</a>
866  The backslashed  The backslashed assertions are:
 assertions are:  
867  <pre>  <pre>
868    \b     matches at a word boundary    \b     matches at a word boundary
869    \B     matches when not at a word boundary    \B     matches when not at a word boundary
870    \A     matches at start of subject    \A     matches at the start of the subject
871    \Z     matches at end of subject or before newline at end    \Z     matches at the end of the subject
872    \z     matches at end of subject            also matches before a newline at the end of the subject
873    \G     matches at first matching position in subject    \z     matches only at the end of the subject
874  </pre>    \G     matches at the first matching position in the subject
875  These assertions may not appear in character classes (but note that \b has a  </pre>
876  different meaning, namely the backspace character, inside a character class).  Inside a character class, \b has a different meaning; it matches the backspace
877    character. If any other of these assertions appears in a character class, by
878    default it matches the corresponding literal character (for example, \B
879    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
880    escape sequence" error is generated instead.
881  </P>  </P>
882  <P>  <P>
883  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
884  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
885  \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
886  first or last character matches \w, respectively.  first or last character matches \w, respectively. In a UTF mode, the meanings
887    of \w and \W can be changed by setting the PCRE_UCP option. When this is
888    done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
889    of word" or "end of word" metasequence. However, whatever follows \b normally
890    determines which it is. For example, the fragment \ba matches "a" at the start
891    of a word.
892  </P>  </P>
893  <P>  <P>
894  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 423  PCRE_NOTBOL or PCRE_NOTEOL options, whic Line 899  PCRE_NOTBOL or PCRE_NOTEOL options, whic
899  circumflex and dollar metacharacters. However, if the <i>startoffset</i>  circumflex and dollar metacharacters. However, if the <i>startoffset</i>
900  argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start  argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
901  at a point other than the beginning of the subject, \A can never match. The  at a point other than the beginning of the subject, \A can never match. The
902  difference between \Z and \z is that \Z matches before a newline that is the  difference between \Z and \z is that \Z matches before a newline at the end
903  last character of the string as well as at the end of the string, whereas \z  of the string as well as at the very end, whereas \z matches only at the end.
 matches only at the end.  
904  </P>  </P>
905  <P>  <P>
906  The \G assertion is true only when the current matching position is at the  The \G assertion is true only when the current matching position is at the
# Line 447  If all the alternatives of a pattern beg Line 922  If all the alternatives of a pattern beg
922  to the starting match position, and the "anchored" flag is set in the compiled  to the starting match position, and the "anchored" flag is set in the compiled
923  regular expression.  regular expression.
924  </P>  </P>
925  <br><a name="SEC3" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>  <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
926  <P>  <P>
927  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
928  character is an assertion that is true only if the current matching point is  character is an assertion that is true only if the current matching point is
# Line 469  to be anchored.) Line 944  to be anchored.)
944  <P>  <P>
945  A dollar character is an assertion that is true only if the current matching  A dollar character is an assertion that is true only if the current matching
946  point is at the end of the subject string, or immediately before a newline  point is at the end of the subject string, or immediately before a newline
947  character that is the last character in the string (by default). Dollar need  at the end of the string (by default). Dollar need not be the last character of
948  not be the last character of the pattern if a number of alternatives are  the pattern if a number of alternatives are involved, but it should be the last
949  involved, but it should be the last item in any branch in which it appears.  item in any branch in which it appears. Dollar has no special meaning in a
950  Dollar has no special meaning in a character class.  character class.
951  </P>  </P>
952  <P>  <P>
953  The meaning of dollar can be changed so that it matches only at the very end of  The meaning of dollar can be changed so that it matches only at the very end of
# Line 481  does not affect the \Z assertion. Line 956  does not affect the \Z assertion.
956  </P>  </P>
957  <P>  <P>
958  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
959  PCRE_MULTILINE option is set. When this is the case, they match immediately  PCRE_MULTILINE option is set. When this is the case, a circumflex matches
960  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
961  addition to matching at the start and end of the subject string. For example,  string. It does not match after a newline that ends the string. A dollar
962  the pattern /^abc$/ matches the subject string "def\nabc" (where \n  matches before any newlines in the string, as well as at the very end, when
963  represents a newline character) in multiline mode, but not otherwise.  PCRE_MULTILINE is set. When newline is specified as the two-character
964  Consequently, patterns that are anchored in single line mode because all  sequence CRLF, isolated CR and LF characters do not indicate newlines.
965  branches start with ^ are not anchored in multiline mode, and a match for  </P>
966  circumflex is possible when the <i>startoffset</i> argument of <b>pcre_exec()</b>  <P>
967  is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is  For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
968  set.  \n represents a newline) in multiline mode, but not otherwise. Consequently,
969    patterns that are anchored in single line mode because all branches start with
970    ^ are not anchored in multiline mode, and a match for circumflex is possible
971    when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
972    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
973  </P>  </P>
974  <P>  <P>
975  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
976  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
977  \A it is always anchored, whether PCRE_MULTILINE is set or not.  \A it is always anchored, whether or not PCRE_MULTILINE is set.
978  </P>  <a name="fullstopdot"></a></P>
979  <br><a name="SEC4" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>  <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
980  <P>  <P>
981  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
982  the subject, including a non-printing character, but not (by default) newline.  the subject string except (by default) a character that signifies the end of a
983  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  line.
984  byte long, except (by default) newline. If the PCRE_DOTALL option is set,  </P>
985  dots match newlines as well. The handling of dot is entirely independent of the  <P>
986  handling of circumflex and dollar, the only relationship being that they both  When a line ending is defined as a single character, dot never matches that
987  involve newline characters. Dot has no special meaning in a character class.  character; when the two-character sequence CRLF is used, dot does not match CR
988  </P>  if it is immediately followed by LF, but otherwise it matches all characters
989  <br><a name="SEC5" href="#TOC1">MATCHING A SINGLE BYTE</a><br>  (including isolated CRs and LFs). When any Unicode line endings are being
990  <P>  recognized, dot does not match CR or LF or any of the other line ending
991  Outside a character class, the escape sequence \C matches any one byte, both  characters.
992  in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature is  </P>
993  provided in Perl in order to match individual bytes in UTF-8 mode. Because it  <P>
994  breaks up UTF-8 characters into individual bytes, what remains in the string  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
995  may be a malformed UTF-8 string. For this reason, the \C escape sequence is  option is set, a dot matches any one character, without exception. If the
996  best avoided.  two-character sequence CRLF is present in the subject string, it takes two dots
997    to match it.
998    </P>
999    <P>
1000    The handling of dot is entirely independent of the handling of circumflex and
1001    dollar, the only relationship being that they both involve newlines. Dot has no
1002    special meaning in a character class.
1003    </P>
1004    <P>
1005    The escape sequence \N behaves like a dot, except that it is not affected by
1006    the PCRE_DOTALL option. In other words, it matches any character except one
1007    that signifies the end of a line. Perl also uses \N to match characters by
1008    name; PCRE does not support this.
1009    </P>
1010    <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE DATA UNIT</a><br>
1011    <P>
1012    Outside a character class, the escape sequence \C matches any one data unit,
1013    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1014    byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \C always
1015    matches line-ending characters. The feature is provided in Perl in order to
1016    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1017    used. Because \C breaks up characters into individual data units, matching one
1018    unit with \C in a UTF mode means that the rest of the string may start with a
1019    malformed UTF character. This has undefined results, because PCRE assumes that
1020    it is dealing with valid UTF strings (and by default it checks this at the
1021    start of processing unless the PCRE_NO_UTF8_CHECK option is used).
1022  </P>  </P>
1023  <P>  <P>
1024  PCRE does not allow \C to appear in lookbehind assertions  PCRE does not allow \C to appear in lookbehind assertions
1025  <a href="#lookbehind">(described below),</a>  <a href="#lookbehind">(described below)</a>
1026  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
1027  the lookbehind.  the lookbehind.
1028    </P>
1029    <P>
1030    In general, the \C escape sequence is best avoided. However, one
1031    way of using it that avoids the problem of malformed UTF characters is to use a
1032    lookahead to check the length of the next character, as in this pattern, which
1033    could be used with a UTF-8 string (ignore white space and line breaks):
1034    <pre>
1035      (?| (?=[\x00-\x7f])(\C) |
1036          (?=[\x80-\x{7ff}])(\C)(\C) |
1037          (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
1038          (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
1039    </pre>
1040    A group that starts with (?| resets the capturing parentheses numbers in each
1041    alternative (see
1042    <a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a>
1043    below). The assertions at the start of each branch check the next UTF-8
1044    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1045    character's individual bytes are then captured by the appropriate number of
1046    groups.
1047  <a name="characterclass"></a></P>  <a name="characterclass"></a></P>
1048  <br><a name="SEC6" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>  <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
1049  <P>  <P>
1050  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1051  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.
1052  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
1053  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
1054  escaped with a backslash.  a member of the class, it should be the first data character in the class
1055    (after an initial circumflex, if present) or escaped with a backslash.
1056  </P>  </P>
1057  <P>  <P>
1058  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
1059  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
1060  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
1061  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
1062  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
1063  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
1064  backslash.  backslash.
1065  </P>  </P>
1066  <P>  <P>
# Line 544  For example, the character class [aeiou] Line 1068  For example, the character class [aeiou]
1068  [^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
1069  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1070  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
1071  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
1072  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
1073  string.  string.
1074  </P>  </P>
1075  <P>  <P>
1076  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
1077  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{
1078    escaping mechanism.
1079  </P>  </P>
1080  <P>  <P>
1081  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
1082  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
1083  "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
1084  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
1085  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
1086  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1087  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1088  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
1089  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
1090  UTF-8 support.  well as with UTF support.
1091  </P>  </P>
1092  <P>  <P>
1093  The newline character is never treated in any special way in character classes,  Characters that might indicate line breaks are never treated in any special way
1094  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  when matching character classes, whatever line-ending sequence is in use, and
1095  such as [^a] will always match a newline.  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1096    such as [^a] always matches one of these characters.
1097  </P>  </P>
1098  <P>  <P>
1099  The minus (hyphen) character can be used to specify a range of characters in a  The minus (hyphen) character can be used to specify a range of characters in a
# Line 587  followed by two other characters. The oc Line 1113  followed by two other characters. The oc
1113  </P>  </P>
1114  <P>  <P>
1115  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
1116  used for characters specified numerically, for example [\000-\037]. In UTF-8  used for characters specified numerically, for example [\000-\037]. Ranges
1117  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}].  
1118  </P>  </P>
1119  <P>  <P>
1120  If a range that includes letters is used when caseless matching is set, it  If a range that includes letters is used when caseless matching is set, it
1121  matches the letters in either case. For example, [W-c] is equivalent to  matches the letters in either case. For example, [W-c] is equivalent to
1122  [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1123  tables for the "fr_FR" locale are in use, [\xc8-\xcb] matches accented E  tables for a French locale are in use, [\xc8-\xcb] matches accented E
1124  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF modes, PCRE supports the concept of case for
1125  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1126  property support.  property support.
1127  </P>  </P>
1128  <P>  <P>
1129  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,
1130  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
1131  example, [\dABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\dABCDEF] matches any hexadecimal
1132  conveniently be used with the upper case character types to specify a more  digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w
1133  restricted set of characters than the matching lower case type. For example,  and their upper case partners, just as it does when they appear outside a
1134  the class [^\W_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1135    <a href="#genericchartypes">"Generic character types"</a>
1136    above. The escape sequence \b has a different meaning inside a character
1137    class; it matches the backspace character. The sequences \B, \N, \R, and \X
1138    are not special inside a character class. Like any other unrecognized escape
1139    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1140    default, but cause an error if the PCRE_EXTRA option is set.
1141    </P>
1142    <P>
1143    A circumflex can conveniently be used with the upper case character types to
1144    specify a more restricted set of characters than the matching lower case type.
1145    For example, the class [^\W_] matches any letter or digit, but not underscore,
1146    whereas [\w] includes underscore. A positive character class should be read as
1147    "something OR something OR ..." and a negative class as "NOT something AND NOT
1148    something AND NOT ...".
1149  </P>  </P>
1150  <P>  <P>
1151  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
# Line 616  introducing a POSIX class name - see the Line 1155  introducing a POSIX class name - see the
1155  closing square bracket. However, escaping other non-alphanumeric characters  closing square bracket. However, escaping other non-alphanumeric characters
1156  does no harm.  does no harm.
1157  </P>  </P>
1158  <br><a name="SEC7" href="#TOC1">POSIX CHARACTER CLASSES</a><br>  <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
1159  <P>  <P>
1160  Perl supports the POSIX notation for character classes. This uses names  Perl supports the POSIX notation for character classes. This uses names
1161  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
# Line 625  this notation. For example, Line 1164  this notation. For example,
1164    [01[:alpha:]%]    [01[:alpha:]%]
1165  </pre>  </pre>
1166  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1167  are  are:
1168  <pre>  <pre>
1169    alnum    letters and digits    alnum    letters and digits
1170    alpha    letters    alpha    letters
# Line 636  are Line 1175  are
1175    graph    printing characters, excluding space    graph    printing characters, excluding space
1176    lower    lower case letters    lower    lower case letters
1177    print    printing characters, including space    print    printing characters, including space
1178    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1179    space    white space (not quite the same as \s)    space    white space (not quite the same as \s)
1180    upper    upper case letters    upper    upper case letters
1181    word     "word" characters (same as \w)    word     "word" characters (same as \w)
# Line 659  syntax [.ch.] and [=ch=] where "ch" is a Line 1198  syntax [.ch.] and [=ch=] where "ch" is a
1198  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1199  </P>  </P>
1200  <P>  <P>
1201  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF modes, characters with values greater than 128 do not match
1202  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1203    to <b>pcre_compile()</b>, some of the classes are changed so that Unicode
1204    character properties are used. This is achieved by replacing the POSIX classes
1205    by other sequences, as follows:
1206    <pre>
1207      [:alnum:]  becomes  \p{Xan}
1208      [:alpha:]  becomes  \p{L}
1209      [:blank:]  becomes  \h
1210      [:digit:]  becomes  \p{Nd}
1211      [:lower:]  becomes  \p{Ll}
1212      [:space:]  becomes  \p{Xps}
1213      [:upper:]  becomes  \p{Lu}
1214      [:word:]   becomes  \p{Xwd}
1215    </pre>
1216    Negated versions, such as [:^alpha:] use \P instead of \p. The other POSIX
1217    classes are unchanged, and match only characters with code points less than
1218    128.
1219  </P>  </P>
1220  <br><a name="SEC8" href="#TOC1">VERTICAL BAR</a><br>  <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
1221  <P>  <P>
1222  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
1223  the pattern  the pattern
# Line 670  the pattern Line 1225  the pattern
1225    gilbert|sullivan    gilbert|sullivan
1226  </pre>  </pre>
1227  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1228  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
1229  The matching process tries each alternative in turn, from left to right,  process tries each alternative in turn, from left to right, and the first one
1230  and the first one that succeeds is used. If the alternatives are within a  that succeeds is used. If the alternatives are within a subpattern
 subpattern  
1231  <a href="#subpattern">(defined below),</a>  <a href="#subpattern">(defined below),</a>
1232  "succeeds" means matching the rest of the main pattern as well as the  "succeeds" means matching the rest of the main pattern as well as the
1233  alternative in the subpattern.  alternative in the subpattern.
1234  </P>  </P>
1235  <br><a name="SEC9" href="#TOC1">INTERNAL OPTION SETTING</a><br>  <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
1236  <P>  <P>
1237  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1238  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1239  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1240    The option letters are
1241  <pre>  <pre>
1242    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1243    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 697  permitted. If a letter appears both befo Line 1252  permitted. If a letter appears both befo
1252  unset.  unset.
1253  </P>  </P>
1254  <P>  <P>
1255  When an option change occurs at top level (that is, not inside subpattern  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1256  parentheses), the change applies to the remainder of the pattern that follows.  changed in the same way as the Perl-compatible options by using the characters
1257  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
 the global options (and it will therefore show up in data extracted by the  
 <b>pcre_fullinfo()</b> function).  
1258  </P>  </P>
1259  <P>  <P>
1260  An option change within a subpattern affects only that part of the current  When one of these option changes occurs at top level (that is, not inside
1261  pattern that follows it, so  subpattern parentheses), the change applies to the remainder of the pattern
1262    that follows. If the change is placed right at the start of a pattern, PCRE
1263    extracts it into the global options (and it will therefore show up in data
1264    extracted by the <b>pcre_fullinfo()</b> function).
1265    </P>
1266    <P>
1267    An option change within a subpattern (see below for a description of
1268    subpatterns) affects only that part of the subpattern that follows it, so
1269  <pre>  <pre>
1270    (a(?i)b)c    (a(?i)b)c
1271  </pre>  </pre>
# Line 722  option settings happen at compile time. Line 1282  option settings happen at compile time.
1282  behaviour otherwise.  behaviour otherwise.
1283  </P>  </P>
1284  <P>  <P>
1285  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  <b>Note:</b> There are other PCRE-specific options that can be set by the
1286  same way as the Perl-compatible options by using the characters U and X  application when the compiling or matching functions are called. In some cases
1287  respectively. The (?X) flag setting is special in that it must always occur  the pattern can contain special leading sequences such as (*CRLF) to override
1288  earlier in the pattern than any of the additional features it turns on, even  what the application has set or what has been defaulted. Details are given in
1289  when it is at top level. It is best to put it at the start.  the section entitled
1290    <a href="#newlineseq">"Newline sequences"</a>
1291    above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1292    can be used to set UTF and Unicode property modes; they are equivalent to
1293    setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1294  <a name="subpattern"></a></P>  <a name="subpattern"></a></P>
1295  <br><a name="SEC10" href="#TOC1">SUBPATTERNS</a><br>  <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
1296  <P>  <P>
1297  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
1298  Turning part of a pattern into a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
# Line 738  Turning part of a pattern into a subpatt Line 1302  Turning part of a pattern into a subpatt
1302  <pre>  <pre>
1303    cat(aract|erpillar|)    cat(aract|erpillar|)
1304  </pre>  </pre>
1305  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1306  parentheses, it would match "cataract", "erpillar" or the empty string.  match "cataract", "erpillar" or an empty string.
1307  <br>  <br>
1308  <br>  <br>
1309  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
1310  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
1311  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
1312  <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1313  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1314  </P>  </P>
1315  <P>  <P>
1316  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
1317    numbers for the capturing subpatterns. For example, if the string "the red
1318    king" is matched against the pattern
1319  <pre>  <pre>
1320    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1321  </pre>  </pre>
# Line 767  the string "the white queen" is matched Line 1333  the string "the white queen" is matched
1333    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1334  </pre>  </pre>
1335  the captured substrings are "white queen" and "queen", and are numbered 1 and  the captured substrings are "white queen" and "queen", and are numbered 1 and
1336  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
 of nesting of all subpatterns, both capturing and non-capturing, is 200.  
1337  </P>  </P>
1338  <P>  <P>
1339  As a convenient shorthand, if any option settings are required at the start of  As a convenient shorthand, if any option settings are required at the start of
# Line 782  match exactly the same set of strings. B Line 1347  match exactly the same set of strings. B
1347  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
1348  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
1349  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1350    <a name="dupsubpatternnumber"></a></P>
1351    <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1352    <P>
1353    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1354    the same numbers for its capturing parentheses. Such a subpattern starts with
1355    (?| and is itself a non-capturing subpattern. For example, consider this
1356    pattern:
1357    <pre>
1358      (?|(Sat)ur|(Sun))day
1359    </pre>
1360    Because the two alternatives are inside a (?| group, both sets of capturing
1361    parentheses are numbered one. Thus, when the pattern matches, you can look
1362    at captured substring number one, whichever alternative matched. This construct
1363    is useful when you want to capture part, but not all, of one of a number of
1364    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1365    number is reset at the start of each branch. The numbers of any capturing
1366    parentheses that follow the subpattern start after the highest number used in
1367    any branch. The following example is taken from the Perl documentation. The
1368    numbers underneath show in which buffer the captured content will be stored.
1369    <pre>
1370      # before  ---------------branch-reset----------- after
1371      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1372      # 1            2         2  3        2     3     4
1373    </pre>
1374    A back reference to a numbered subpattern uses the most recent value that is
1375    set for that number by any subpattern. The following pattern matches "abcabc"
1376    or "defdef":
1377    <pre>
1378      /(?|(abc)|(def))\1/
1379    </pre>
1380    In contrast, a subroutine call to a numbered subpattern always refers to the
1381    first one in the pattern with the given number. The following pattern matches
1382    "abcabc" or "defabc":
1383    <pre>
1384      /(?|(abc)|(def))(?1)/
1385    </pre>
1386    If a
1387    <a href="#conditions">condition test</a>
1388    for a subpattern's having matched refers to a non-unique number, the test is
1389    true if any of the subpatterns of that number have matched.
1390    </P>
1391    <P>
1392    An alternative approach to using this "branch reset" feature is to use
1393    duplicate named subpatterns, as described in the next section.
1394  </P>  </P>
1395  <br><a name="SEC11" href="#TOC1">NAMED SUBPATTERNS</a><br>  <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
1396  <P>  <P>
1397  Identifying capturing parentheses by number is simple, but it can be very hard  Identifying capturing parentheses by number is simple, but it can be very hard
1398  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1399  if an expression is modified, the numbers may change. To help with this  if an expression is modified, the numbers may change. To help with this
1400  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1401  not provide. The Python syntax (?P&#60;name&#62;...) is used. Names consist of  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1402  alphanumeric characters and underscores, and must be unique within a pattern.  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1403  </P>  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1404  <P>  have different names, but PCRE does not.
1405  Named capturing parentheses are still allocated numbers as well as names. The  </P>
1406  PCRE API provides function calls for extracting the name-to-number translation  <P>
1407  table from a compiled pattern. There is also a convenience function for  In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1408  extracting a captured substring by name. For further details see the  (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1409    parentheses from other parts of the pattern, such as
1410    <a href="#backreferences">back references,</a>
1411    <a href="#recursion">recursion,</a>
1412    and
1413    <a href="#conditions">conditions,</a>
1414    can be made by name as well as by number.
1415    </P>
1416    <P>
1417    Names consist of up to 32 alphanumeric characters and underscores. Named
1418    capturing parentheses are still allocated numbers as well as names, exactly as
1419    if the names were not present. The PCRE API provides function calls for
1420    extracting the name-to-number translation table from a compiled pattern. There
1421    is also a convenience function for extracting a captured substring by name.
1422    </P>
1423    <P>
1424    By default, a name must be unique within a pattern, but it is possible to relax
1425    this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1426    names are also always permitted for subpatterns with the same number, set up as
1427    described in the previous section.) Duplicate names can be useful for patterns
1428    where only one instance of the named parentheses can match. Suppose you want to
1429    match the name of a weekday, either as a 3-letter abbreviation or as the full
1430    name, and in both cases you want to extract the abbreviation. This pattern
1431    (ignoring the line breaks) does the job:
1432    <pre>
1433      (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1434      (?&#60;DN&#62;Tue)(?:sday)?|
1435      (?&#60;DN&#62;Wed)(?:nesday)?|
1436      (?&#60;DN&#62;Thu)(?:rsday)?|
1437      (?&#60;DN&#62;Sat)(?:urday)?
1438    </pre>
1439    There are five capturing substrings, but only one is ever set after a match.
1440    (An alternative way of solving this problem is to use a "branch reset"
1441    subpattern, as described in the previous section.)
1442    </P>
1443    <P>
1444    The convenience function for extracting the data by name returns the substring
1445    for the first (and in this example, the only) subpattern of that name that
1446    matched. This saves searching to find which numbered subpattern it was.
1447    </P>
1448    <P>
1449    If you make a back reference to a non-unique named subpattern from elsewhere in
1450    the pattern, the one that corresponds to the first occurrence of the name is
1451    used. In the absence of duplicate numbers (see the previous section) this is
1452    the one with the lowest number. If you use a named reference in a condition
1453    test (see the
1454    <a href="#conditions">section about conditions</a>
1455    below), either to check whether a subpattern has matched, or to check for
1456    recursion, all subpatterns with the same name are tested. If the condition is
1457    true for any one of them, the overall condition is true. This is the same
1458    behaviour as testing by number. For further details of the interfaces for
1459    handling named subpatterns, see the
1460  <a href="pcreapi.html"><b>pcreapi</b></a>  <a href="pcreapi.html"><b>pcreapi</b></a>
1461  documentation.  documentation.
1462  </P>  </P>
1463  <br><a name="SEC12" href="#TOC1">REPETITION</a><br>  <P>
1464    <b>Warning:</b> You cannot use different names to distinguish between two
1465    subpatterns with the same number because PCRE uses only the numbers when
1466    matching. For this reason, an error is given at compile time if different names
1467    are given to subpatterns with the same number. However, you can give the same
1468    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1469    </P>
1470    <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1471  <P>  <P>
1472  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
1473  items:  items:
1474  <pre>  <pre>
1475    a literal data character    a literal data character
1476    the . metacharacter    the dot metacharacter
1477    the \C escape sequence    the \C escape sequence
1478    the \X escape sequence (in UTF-8 mode with Unicode properties)    the \X escape sequence
1479    an escape such as \d that matches a single character    the \R escape sequence
1480      an escape such as \d or \pL that matches a single character
1481    a character class    a character class
1482    a back reference (see next section)    a back reference (see next section)
1483    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1484      a subroutine call to a subpattern (recursive or otherwise)
1485  </pre>  </pre>
1486  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1487  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 838  quantifier, is taken as a literal charac Line 1507  quantifier, is taken as a literal charac
1507  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1508  </P>  </P>
1509  <P>  <P>
1510  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
1511  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
1512  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,
1513  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
1514  which may be several bytes long (and they may be of different lengths).  data units long (and they may be of different lengths).
1515  </P>  </P>
1516  <P>  <P>
1517  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
1518  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1519    subpatterns that are referenced as
1520    <a href="#subpatternsassubroutines">subroutines</a>
1521    from elsewhere in the pattern (but see also the section entitled
1522    <a href="#subdefine">"Defining subpatterns for use by reference only"</a>
1523    below). Items other than subpatterns that have a {0} quantifier are omitted
1524    from the compiled pattern.
1525  </P>  </P>
1526  <P>  <P>
1527  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1528  quantifiers have single-character abbreviations:  abbreviations:
1529  <pre>  <pre>
1530    *    is equivalent to {0,}    *    is equivalent to {0,}
1531    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 901  which matches one digit by preference, b Line 1576  which matches one digit by preference, b
1576  way the rest of the pattern matches.  way the rest of the pattern matches.
1577  </P>  </P>
1578  <P>  <P>
1579  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1580  the quantifiers are not greedy by default, but individual ones can be made  the quantifiers are not greedy by default, but individual ones can be made
1581  greedy by following them with a question mark. In other words, it inverts the  greedy by following them with a question mark. In other words, it inverts the
1582  default behaviour.  default behaviour.
# Line 913  compiled pattern, in proportion to the s Line 1588  compiled pattern, in proportion to the s
1588  </P>  </P>
1589  <P>  <P>
1590  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1591  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1592  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1593  character position in the subject string, so there is no point in retrying the  character position in the subject string, so there is no point in retrying the
1594  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
# Line 926  alternatively using ^ to indicate anchor Line 1601  alternatively using ^ to indicate anchor
1601  </P>  </P>
1602  <P>  <P>
1603  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1604  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1605  elsewhere in the pattern, a match at the start may fail, and a later one  elsewhere in the pattern, a match at the start may fail where a later one
1606  succeed. Consider, for example:  succeeds. Consider, for example:
1607  <pre>  <pre>
1608    (.*)abc\1    (.*)abc\1
1609  </pre>  </pre>
# Line 950  example, after Line 1625  example, after
1625  </pre>  </pre>
1626  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1627  <a name="atomicgroup"></a></P>  <a name="atomicgroup"></a></P>
1628  <br><a name="SEC13" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>  <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1629  <P>  <P>
1630  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1631  normally causes the repeated item to be re-evaluated to see if a different  repetition, failure of what follows normally causes the repeated item to be
1632  number of repeats allows the rest of the pattern to match. Sometimes it is  re-evaluated to see if a different number of repeats allows the rest of the
1633  useful to prevent this, either to change the nature of the match, or to cause  pattern to match. Sometimes it is useful to prevent this, either to change the
1634  it fail earlier than it otherwise might, when the author of the pattern knows  nature of the match, or to cause it fail earlier than it otherwise might, when
1635  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1636  </P>  </P>
1637  <P>  <P>
1638  Consider, for example, the pattern \d+foo when applied to the subject line  Consider, for example, the pattern \d+foo when applied to the subject line
# Line 971  item, and then with 4, and so on, before Line 1646  item, and then with 4, and so on, before
1646  that once a subpattern has matched, it is not to be re-evaluated in this way.  that once a subpattern has matched, it is not to be re-evaluated in this way.
1647  </P>  </P>
1648  <P>  <P>
1649  If we use atomic grouping for the previous example, the matcher would give up  If we use atomic grouping for the previous example, the matcher gives up
1650  immediately on failing to match "foo" the first time. The notation is a kind of  immediately on failing to match "foo" the first time. The notation is a kind of
1651  special parenthesis, starting with (?&#62; as in this example:  special parenthesis, starting with (?&#62; as in this example:
1652  <pre>  <pre>
# Line 1004  previous example can be rewritten as Line 1679  previous example can be rewritten as
1679  <pre>  <pre>
1680    \d++foo    \d++foo
1681  </pre>  </pre>
1682    Note that a possessive quantifier can be used with an entire group, for
1683    example:
1684    <pre>
1685      (abc|xyz){2,3}+
1686    </pre>
1687  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1688  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1689  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning of a possessive
1690  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1691    difference; possessive quantifiers should be slightly faster.
1692  </P>  </P>
1693  <P>  <P>
1694  The possessive quantifier syntax is an extension to the Perl syntax. It  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1695  originates in Sun's Java package.  Jeffrey Friedl originated the idea (and the name) in the first edition of his
1696    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1697    package, and PCRE copied it from there. It ultimately found its way into Perl
1698    at release 5.10.
1699    </P>
1700    <P>
1701    PCRE has an optimization that automatically "possessifies" certain simple
1702    pattern constructs. For example, the sequence A+B is treated as A++B because
1703    there is no point in backtracking into a sequence of A's when B must follow.
1704  </P>  </P>
1705  <P>  <P>
1706  When a pattern contains an unlimited repeat inside a subpattern that can itself  When a pattern contains an unlimited repeat inside a subpattern that can itself
# Line 1040  an atomic group, like this: Line 1729  an atomic group, like this:
1729  </pre>  </pre>
1730  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1731  <a name="backreferences"></a></P>  <a name="backreferences"></a></P>
1732  <br><a name="SEC14" href="#TOC1">BACK REFERENCES</a><br>  <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
1733  <P>  <P>
1734  Outside a character class, a backslash followed by a digit greater than 0 (and  Outside a character class, a backslash followed by a digit greater than 0 (and
1735  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
# Line 1052  However, if the decimal number following Line 1741  However, if the decimal number following
1741  always taken as a back reference, and causes an error only if there are not  always taken as a back reference, and causes an error only if there are not
1742  that many capturing left parentheses in the entire pattern. In other words, the  that many capturing left parentheses in the entire pattern. In other words, the
1743  parentheses that are referenced need not be to the left of the reference for  parentheses that are referenced need not be to the left of the reference for
1744  numbers less than 10. See the subsection entitled "Non-printing characters"  numbers less than 10. A "forward back reference" of this type can make sense
1745    when a repetition is involved and the subpattern to the right has participated
1746    in an earlier iteration.
1747    </P>
1748    <P>
1749    It is not possible to have a numerical "forward back reference" to a subpattern
1750    whose number is 10 or more using this syntax because a sequence such as \50 is
1751    interpreted as a character defined in octal. See the subsection entitled
1752    "Non-printing characters"
1753  <a href="#digitsafterbackslash">above</a>  <a href="#digitsafterbackslash">above</a>
1754  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1755    no such problem when named parentheses are used. A back reference to any
1756    subpattern is possible using named parentheses (see below).
1757    </P>
1758    <P>
1759    Another way of avoiding the ambiguity inherent in the use of digits following a
1760    backslash is to use the \g escape sequence. This escape must be followed by an
1761    unsigned number or a negative number, optionally enclosed in braces. These
1762    examples are all identical:
1763    <pre>
1764      (ring), \1
1765      (ring), \g1
1766      (ring), \g{1}
1767    </pre>
1768    An unsigned number specifies an absolute reference without the ambiguity that
1769    is present in the older syntax. It is also useful when literal digits follow
1770    the reference. A negative number is a relative reference. Consider this
1771    example:
1772    <pre>
1773      (abc(def)ghi)\g{-1}
1774    </pre>
1775    The sequence \g{-1} is a reference to the most recently started capturing
1776    subpattern before \g, that is, is it equivalent to \2 in this example.
1777    Similarly, \g{-2} would be equivalent to \1. The use of relative references
1778    can be helpful in long patterns, and also in patterns that are created by
1779    joining together fragments that contain references within themselves.
1780  </P>  </P>
1781  <P>  <P>
1782  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
# Line 1075  matches "rah rah" and "RAH RAH", but not Line 1797  matches "rah rah" and "RAH RAH", but not
1797  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1798  </P>  </P>
1799  <P>  <P>
1800  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1801  rewrite the above example as follows:  subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
1802  <pre>  \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1803    (?&#60;p1&#62;(?i)rah)\s+(?P=p1)  back reference syntax, in which \g can be used for both numeric and named
1804    references, is also supported. We could rewrite the above example in any of
1805    the following ways:
1806    <pre>
1807      (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
1808      (?'p1'(?i)rah)\s+\k{p1}
1809      (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
1810      (?&#60;p1&#62;(?i)rah)\s+\g{p1}
1811  </pre>  </pre>
1812    A subpattern that is referenced by name may appear in the pattern before or
1813    after the reference.
1814    </P>
1815    <P>
1816  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
1817  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1818  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1819  <pre>  <pre>
1820    (a|(bc))\2    (a|(bc))\2
1821  </pre>  </pre>
1822  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
1823  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
1824  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1825  with a digit character, some delimiter must be used to terminate the back  </P>
1826  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  <P>
1827  Otherwise an empty comment (see  Because there may be many capturing parentheses in a pattern, all digits
1828    following a backslash are taken as part of a potential back reference number.
1829    If the pattern continues with a digit character, some delimiter must be used to
1830    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1831    whitespace. Otherwise, the \g{ syntax or an empty comment (see
1832  <a href="#comments">"Comments"</a>  <a href="#comments">"Comments"</a>
1833  below) can be used.  below) can be used.
1834  </P>  </P>
1835    <br><b>
1836    Recursive back references
1837    </b><br>
1838  <P>  <P>
1839  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
1840  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 1109  to the previous iteration. In order for Line 1849  to the previous iteration. In order for
1849  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
1850  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
1851  minimum of zero.  minimum of zero.
1852    </P>
1853    <P>
1854    Back references of this type cause the group that they reference to be treated
1855    as an
1856    <a href="#atomicgroup">atomic group.</a>
1857    Once the whole group has been matched, a subsequent matching failure cannot
1858    cause backtracking into the middle of the group.
1859  <a name="bigassertions"></a></P>  <a name="bigassertions"></a></P>
1860  <br><a name="SEC15" href="#TOC1">ASSERTIONS</a><br>  <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1861  <P>  <P>
1862  An assertion is a test on the characters following or preceding the current  An assertion is a test on the characters following or preceding the current
1863  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
# Line 1124  that look behind it. An assertion subpat Line 1871  that look behind it. An assertion subpat
1871  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.
1872  </P>  </P>
1873  <P>  <P>
1874  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1875  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
1876  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1877  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1878  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1879  because it does not make sense for negative assertions.  </P>
1880    <P>
1881    For compatibility with Perl, assertion subpatterns may be repeated; though
1882    it makes no sense to assert the same thing several times, the side effect of
1883    capturing parentheses may occasionally be useful. In practice, there only three
1884    cases:
1885    <br>
1886    <br>
1887    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1888    However, it may contain internal capturing parenthesized groups that are called
1889    from elsewhere via the
1890    <a href="#subpatternsassubroutines">subroutine mechanism.</a>
1891    <br>
1892    <br>
1893    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1894    were {0,1}. At run time, the rest of the pattern match is tried with and
1895    without the assertion, the order depending on the greediness of the quantifier.
1896    <br>
1897    <br>
1898    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1899    The assertion is obeyed just once when encountered during matching.
1900  </P>  </P>
1901  <br><b>  <br><b>
1902  Lookahead assertions  Lookahead assertions
1903  </b><br>  </b><br>
1904  <P>  <P>
1905  Lookahead assertions start  Lookahead assertions start with (?= for positive assertions and (?! for
1906  with (?= for positive assertions and (?! for negative assertions. For example,  negative assertions. For example,
1907  <pre>  <pre>
1908    \w+(?=;)    \w+(?=;)
1909  </pre>  </pre>
# Line 1159  lookbehind assertion is needed to achiev Line 1926  lookbehind assertion is needed to achiev
1926  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
1927  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
1928  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.
1929    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1930  <a name="lookbehind"></a></P>  <a name="lookbehind"></a></P>
1931  <br><b>  <br><b>
1932  Lookbehind assertions  Lookbehind assertions
# Line 1171  negative assertions. For example, Line 1939  negative assertions. For example,
1939  </pre>  </pre>
1940  does find an occurrence of "bar" that is not preceded by "foo". The contents of  does find an occurrence of "bar" that is not preceded by "foo". The contents of
1941  a lookbehind assertion are restricted such that all the strings it matches must  a lookbehind assertion are restricted such that all the strings it matches must
1942  have a fixed length. However, if there are several alternatives, they do not  have a fixed length. However, if there are several top-level alternatives, they
1943  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1944  <pre>  <pre>
1945    (?&#60;=bullock|donkey)    (?&#60;=bullock|donkey)
1946  </pre>  </pre>
# Line 1182  is permitted, but Line 1950  is permitted, but
1950  </pre>  </pre>
1951  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1952  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
1953  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
1954  match the same length of string. An assertion such as  length of string. An assertion such as
1955  <pre>  <pre>
1956    (?&#60;=ab(c|de))    (?&#60;=ab(c|de))
1957  </pre>  </pre>
1958  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
1959  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
1960    branches:
1961  <pre>  <pre>
1962    (?&#60;=abc|abde)    (?&#60;=abc|abde)
1963  </pre>  </pre>
1964    In some cases, the escape sequence \K
1965    <a href="#resetmatchstart">(see above)</a>
1966    can be used instead of a lookbehind assertion to get round the fixed-length
1967    restriction.
1968    </P>
1969    <P>
1970  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1971  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed length and then try to
1972  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1973  match is deemed to fail.  assertion fails.
1974  </P>  </P>
1975  <P>  <P>
1976  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
1977  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
1978  the length of the lookbehind. The \X escape, which can match different numbers  it impossible to calculate the length of the lookbehind. The \X and \R
1979  of bytes, is also not permitted.  escapes, which can match different numbers of data units, are also not
1980    permitted.
1981  </P>  </P>
1982  <P>  <P>
1983  Atomic groups can be used in conjunction with lookbehind assertions to specify  <a href="#subpatternsassubroutines">"Subroutine"</a>
1984  efficient matching at the end of the subject string. Consider a simple pattern  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1985  such as  as the subpattern matches a fixed-length string.
1986    <a href="#recursion">Recursion,</a>
1987    however, is not supported.
1988    </P>
1989    <P>
1990    Possessive quantifiers can be used in conjunction with lookbehind assertions to
1991    specify efficient matching of fixed-length strings at the end of subject
1992    strings. Consider a simple pattern such as
1993  <pre>  <pre>
1994    abcd$    abcd$
1995  </pre>  </pre>
# Line 1222  then all but the last two characters, an Line 2005  then all but the last two characters, an
2005  covers the entire string, from right to left, so we are no better off. However,  covers the entire string, from right to left, so we are no better off. However,
2006  if the pattern is written as  if the pattern is written as
2007  <pre>  <pre>
   ^(?&#62;.*)(?&#60;=abcd)  
 </pre>  
 or, equivalently, using the possessive quantifier syntax,  
 <pre>  
2008    ^.*+(?&#60;=abcd)    ^.*+(?&#60;=abcd)
2009  </pre>  </pre>
2010  there can be no backtracking for the .* item; it can match only the entire  there can be no backtracking for the .*+ item; it can match only the entire
2011  string. The subsequent lookbehind assertion does a single test on the last four  string. The subsequent lookbehind assertion does a single test on the last four
2012  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
2013  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1267  preceded by "foo", while Line 2046  preceded by "foo", while
2046  </pre>  </pre>
2047  is another pattern that matches "foo" preceded by three digits and any three  is another pattern that matches "foo" preceded by three digits and any three
2048  characters that are not "999".  characters that are not "999".
2049  </P>  <a name="conditions"></a></P>
2050  <br><a name="SEC16" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>  <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
2051  <P>  <P>
2052  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2053  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2054  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2055  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2056  <pre>  <pre>
2057    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2058    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2059  </pre>  </pre>
2060  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2061  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
2062  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2063    itself contain nested subpatterns of any form, including conditional
2064    subpatterns; the restriction to two alternatives applies only at the level of
2065    the condition. This pattern fragment is an example where the alternatives are
2066    complex:
2067    <pre>
2068      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2069    
2070    </PRE>
2071  </P>  </P>
2072  <P>  <P>
2073  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
2074  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
2075  subpattern of that number has previously matched. The number must be greater  </P>
2076  than zero. Consider the following pattern, which contains non-significant white  <br><b>
2077  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  Checking for a used subpattern by number
2078  it into three parts for ease of discussion:  </b><br>
2079    <P>
2080    If the text between the parentheses consists of a sequence of digits, the
2081    condition is true if a capturing subpattern of that number has previously
2082    matched. If there is more than one capturing subpattern with the same number
2083    (see the earlier
2084    <a href="#recursion">section about duplicate subpattern numbers),</a>
2085    the condition is true if any of them have matched. An alternative notation is
2086    to precede the digits with a plus or minus sign. In this case, the subpattern
2087    number is relative rather than absolute. The most recently opened parentheses
2088    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2089    loops it can also make sense to refer to subsequent groups. The next
2090    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2091    zero in any of these forms is not used; it provokes a compile-time error.)
2092    </P>
2093    <P>
2094    Consider the following pattern, which contains non-significant white space to
2095    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2096    three parts for ease of discussion:
2097  <pre>  <pre>
2098    ( \( )?    [^()]+    (?(1) \) )    ( \( )?    [^()]+    (?(1) \) )
2099  </pre>  </pre>
2100  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2101  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
2102  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
2103  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2104  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,
2105  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
2106  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2107  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2108  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
2109  </P>  </P>
2110  <P>  <P>
2111  If the condition is the string (R), it is satisfied if a recursive call to the  If you were embedding this pattern in a larger one, you could use a relative
2112  pattern or subpattern has been made. At "top level", the condition is false.  reference:
2113  This is a PCRE extension. Recursive patterns are described in the next section.  <pre>
2114      ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...
2115    </pre>
2116    This makes the fragment independent of the parentheses in the larger pattern.
2117  </P>  </P>
2118    <br><b>
2119    Checking for a used subpattern by name
2120    </b><br>
2121  <P>  <P>
2122  If the condition is not a sequence of digits or (R), it must be an assertion.  Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
2123    subpattern by name. For compatibility with earlier versions of PCRE, which had
2124    this facility before Perl, the syntax (?(name)...) is also recognized. However,
2125    there is a possible ambiguity with this syntax, because subpattern names may
2126    consist entirely of digits. PCRE looks first for a named subpattern; if it
2127    cannot find one and the name consists entirely of digits, PCRE looks for a
2128    subpattern of that number, which must be greater than zero. Using subpattern
2129    names that consist entirely of digits is not recommended.
2130    </P>
2131    <P>
2132    Rewriting the above example to use a named subpattern gives this:
2133    <pre>
2134      (?&#60;OPEN&#62; \( )?    [^()]+    (?(&#60;OPEN&#62;) \) )
2135    </pre>
2136    If the name used in a condition of this kind is a duplicate, the test is
2137    applied to all subpatterns of the same name, and is true if any one of them has
2138    matched.
2139    </P>
2140    <br><b>
2141    Checking for pattern recursion
2142    </b><br>
2143    <P>
2144    If the condition is the string (R), and there is no subpattern with the name R,
2145    the condition is true if a recursive call to the whole pattern or any
2146    subpattern has been made. If digits or a name preceded by ampersand follow the
2147    letter R, for example:
2148    <pre>
2149      (?(R3)...) or (?(R&name)...)
2150    </pre>
2151    the condition is true if the most recent recursion is into a subpattern whose
2152    number or name is given. This condition does not check the entire recursion
2153    stack. If the name used in a condition of this kind is a duplicate, the test is
2154    applied to all subpatterns of the same name, and is true if any one of them is
2155    the most recent recursion.
2156    </P>
2157    <P>
2158    At "top level", all these recursion test conditions are false.
2159    <a href="#recursion">The syntax for recursive patterns</a>
2160    is described below.
2161    <a name="subdefine"></a></P>
2162    <br><b>
2163    Defining subpatterns for use by reference only
2164    </b><br>
2165    <P>
2166    If the condition is the string (DEFINE), and there is no subpattern with the
2167    name DEFINE, the condition is always false. In this case, there may be only one
2168    alternative in the subpattern. It is always skipped if control reaches this
2169    point in the pattern; the idea of DEFINE is that it can be used to define
2170    subroutines that can be referenced from elsewhere. (The use of
2171    <a href="#subpatternsassubroutines">subroutines</a>
2172    is described below.) For example, a pattern to match an IPv4 address such as
2173    "192.168.23.245" could be written like this (ignore whitespace and line
2174    breaks):
2175    <pre>
2176      (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
2177      \b (?&byte) (\.(?&byte)){3} \b
2178    </pre>
2179    The first part of the pattern is a DEFINE group inside which a another group
2180    named "byte" is defined. This matches an individual component of an IPv4
2181    address (a number less than 256). When matching takes place, this part of the
2182    pattern is skipped because DEFINE acts like a false condition. The rest of the
2183    pattern uses references to the named group to match the four dot-separated
2184    components of an IPv4 address, insisting on a word boundary at each end.
2185    </P>
2186    <br><b>
2187    Assertion conditions
2188    </b><br>
2189    <P>
2190    If the condition is not in any of the above formats, it must be an assertion.
2191  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
2192  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
2193  alternatives on the second line:  alternatives on the second line:
# Line 1323  subject is matched against the first alt Line 2202  subject is matched against the first alt
2202  against the second. This pattern matches strings in one of the two forms  against the second. This pattern matches strings in one of the two forms
2203  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2204  <a name="comments"></a></P>  <a name="comments"></a></P>
2205  <br><a name="SEC17" href="#TOC1">COMMENTS</a><br>  <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
2206  <P>  <P>
2207  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
2208  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,
2209  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
2210    subpattern name or number. The characters that make up a comment play no part
2211    in the pattern matching.
2212  </P>  </P>
2213  <P>  <P>
2214  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
2215  character class introduces a comment that continues up to the next newline  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2216  character in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2217  </P>  this case continues to immediately after the next newline character or
2218  <br><a name="SEC18" href="#TOC1">RECURSIVE PATTERNS</a><br>  character sequence in the pattern. Which characters are interpreted as newlines
2219    is controlled by the options passed to a compiling function or by a special
2220    sequence at the start of the pattern, as described in the section entitled
2221    <a href="#newlines">"Newline conventions"</a>
2222    above. Note that the end of this type of comment is a literal newline sequence
2223    in the pattern; escape sequences that happen to represent a newline do not
2224    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2225    default newline convention is in force:
2226    <pre>
2227      abc #comment \n still comment
2228    </pre>
2229    On encountering the # character, <b>pcre_compile()</b> skips along, looking for
2230    a newline in the pattern. The sequence \n is still literal at this stage, so
2231    it does not terminate the comment. Only an actual character with the code value
2232    0x0a (the default newline) does so.
2233    <a name="recursion"></a></P>
2234    <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
2235  <P>  <P>
2236  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2237  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2238  be done is to use a pattern that matches up to some fixed depth of nesting. It  be done is to use a pattern that matches up to some fixed depth of nesting. It
2239  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
2240  that allows regular expressions to recurse (amongst other things). It does this  </P>
2241  by interpolating Perl code in the expression at run time, and the code can  <P>
2242  refer to the expression itself. A Perl pattern to solve the parentheses problem  For some time, Perl has provided a facility that allows regular expressions to
2243  can be created like this:  recurse (amongst other things). It does this by interpolating Perl code in the
2244    expression at run time, and the code can refer to the expression itself. A Perl
2245    pattern using code interpolation to solve the parentheses problem can be
2246    created like this:
2247  <pre>  <pre>
2248    $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;    $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
2249  </pre>  </pre>
2250  The (?p{...}) item interpolates Perl code at run time, and in this case refers  The (?p{...}) item interpolates Perl code at run time, and in this case refers
2251  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2252  the interpolation of Perl code. Instead, it supports some special syntax for  </P>
2253  recursion of the entire pattern, and also for individual subpattern recursion.  <P>
2254    Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2255    supports special syntax for recursion of the entire pattern, and also for
2256    individual subpattern recursion. After its introduction in PCRE and Python,
2257    this kind of recursion was subsequently introduced into Perl at release 5.10.
2258  </P>  </P>
2259  <P>  <P>
2260  The special item that consists of (? followed by a number greater than zero and  A special item that consists of (? followed by a number greater than zero and a
2261  a closing parenthesis is a recursive call of the subpattern of the given  closing parenthesis is a recursive subroutine call of the subpattern of the
2262  number, provided that it occurs inside that subpattern. (If not, it is a  given number, provided that it occurs inside that subpattern. (If not, it is a
2263  "subroutine" call, which is described in the next section.) The special item  <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2264  (?R) is a recursive call of the entire regular expression.  call, which is described in the next section.) The special item (?R) or (?0) is
2265    a recursive call of the entire regular expression.
2266  </P>  </P>
2267  <P>  <P>
2268  For example, this PCRE pattern solves the nested parentheses problem (assume  This PCRE pattern solves the nested parentheses problem (assume the
2269  the PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2270  <pre>  <pre>
2271    \( ( (?&#62;[^()]+) | (?R) )* \)    \( ( [^()]++ | (?R) )* \)
2272  </pre>  </pre>
2273  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2274  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
2275  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2276  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2277    to avoid backtracking into sequences of non-parentheses.
2278  </P>  </P>
2279  <P>  <P>
2280  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
2281  pattern, so instead you could use this:  pattern, so instead you could use this:
2282  <pre>  <pre>
2283    ( \( ( (?&#62;[^()]+) | (?1) )* \) )    ( \( ( [^()]++ | (?1) )* \) )
2284  </pre>  </pre>
2285  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
2286  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2287  parenthesis numbers can be tricky. It may be more convenient to use named  </P>
2288  parentheses instead. For this, PCRE uses (?P&#62;name), which is an extension to  <P>
2289  the Python syntax that PCRE uses for named parentheses (Perl does not provide  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2290  named parentheses). We could rewrite the above example as follows:  is made easier by the use of relative references. Instead of (?1) in the
2291  <pre>  pattern above you can write (?-2) to refer to the second most recently opened
2292    (?P&#60;pn&#62; \( ( (?&#62;[^()]+) | (?P&#62;pn) )* \) )  parentheses preceding the recursion. In other words, a negative number counts
2293  </pre>  capturing parentheses leftwards from the point at which it is encountered.
2294  This particular example pattern contains nested unlimited repeats, and so the  </P>
2295  use of atomic grouping for matching strings of non-parentheses is important  <P>
2296  when applying the pattern to strings that do not match. For example, when this  It is also possible to refer to subsequently opened parentheses, by writing
2297  pattern is applied to  references such as (?+2). However, these cannot be recursive because the
2298    reference is not inside the parentheses that are referenced. They are always
2299    <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2300    calls, as described in the next section.
2301    </P>
2302    <P>
2303    An alternative approach is to use named parentheses instead. The Perl syntax
2304    for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
2305    could rewrite the above example as follows:
2306    <pre>
2307      (?&#60;pn&#62; \( ( [^()]++ | (?&pn) )* \) )
2308    </pre>
2309    If there is more than one subpattern with the same name, the earliest one is
2310    used.
2311    </P>
2312    <P>
2313    This particular example pattern that we have been looking at contains nested
2314    unlimited repeats, and so the use of a possessive quantifier for matching
2315    strings of non-parentheses is important when applying the pattern to strings
2316    that do not match. For example, when this pattern is applied to
2317  <pre>  <pre>
2318    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2319  </pre>  </pre>
2320  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,
2321  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
2322  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
2323  before failure can be reported.  before failure can be reported.
2324  </P>  </P>
2325  <P>  <P>
2326  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
2327  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
2328  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 the next section and the  
2329  <a href="pcrecallout.html"><b>pcrecallout</b></a>  <a href="pcrecallout.html"><b>pcrecallout</b></a>
2330  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
2331  <pre>  <pre>
2332    (ab(cd)ef)    (ab(cd)ef)
2333  </pre>  </pre>
2334  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
2335  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
2336  <pre>  matched at the top level, its final captured value is unset, even if it was
2337    \( ( ( (?&#62;[^()]+) | (?R) )* ) \)  (temporarily) set at a deeper level during the matching process.
2338       ^                        ^  </P>
2339       ^                        ^  <P>
2340  </pre>  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2341  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
2342  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
2343  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.  
2344  </P>  </P>
2345  <P>  <P>
2346  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 1431  recursing), whereas any characters are p Line 2353  recursing), whereas any characters are p
2353  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
2354  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
2355  is the actual recursive call.  is the actual recursive call.
2356    <a name="recursiondifference"></a></P>
2357    <br><b>
2358    Differences in recursion processing between PCRE and Perl
2359    </b><br>
2360    <P>
2361    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2362    (like Python, but unlike Perl), a recursive subpattern call is always treated
2363    as an atomic group. That is, once it has matched some of the subject string, it
2364    is never re-entered, even if it contains untried alternatives and there is a
2365    subsequent matching failure. This can be illustrated by the following pattern,
2366    which purports to match a palindromic string that contains an odd number of
2367    characters (for example, "a", "aba", "abcba", "abcdcba"):
2368    <pre>
2369      ^(.|(.)(?1)\2)$
2370    </pre>
2371    The idea is that it either matches a single character, or two identical
2372    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2373    it does not if the pattern is longer than three characters. Consider the
2374    subject string "abcba":
2375    </P>
2376    <P>
2377    At the top level, the first character is matched, but as it is not at the end
2378    of the string, the first alternative fails; the second alternative is taken
2379    and the recursion kicks in. The recursive call to subpattern 1 successfully
2380    matches the next character ("b"). (Note that the beginning and end of line
2381    tests are not part of the recursion).
2382    </P>
2383    <P>
2384    Back at the top level, the next character ("c") is compared with what
2385    subpattern 2 matched, which was "a". This fails. Because the recursion is
2386    treated as an atomic group, there are now no backtracking points, and so the
2387    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2388    try the second alternative.) However, if the pattern is written with the
2389    alternatives in the other order, things are different:
2390    <pre>
2391      ^((.)(?1)\2|.)$
2392    </pre>
2393    This time, the recursing alternative is tried first, and continues to recurse
2394    until it runs out of characters, at which point the recursion fails. But this
2395    time we do have another alternative to try at the higher level. That is the big
2396    difference: in the previous case the remaining alternative is at a deeper
2397    recursion level, which PCRE cannot use.
2398    </P>
2399    <P>
2400    To change the pattern so that it matches all palindromic strings, not just
2401    those with an odd number of characters, it is tempting to change the pattern to
2402    this:
2403    <pre>
2404      ^((.)(?1)\2|.?)$
2405    </pre>
2406    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2407    deeper recursion has matched a single character, it cannot be entered again in
2408    order to match an empty string. The solution is to separate the two cases, and
2409    write out the odd and even cases as alternatives at the higher level:
2410    <pre>
2411      ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2412    </pre>
2413    If you want to match typical palindromic phrases, the pattern has to ignore all
2414    non-word characters, which can be done like this:
2415    <pre>
2416      ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2417    </pre>
2418    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2419    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2420    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2421    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2422    more) to match typical phrases, and Perl takes so long that you think it has
2423    gone into a loop.
2424    </P>
2425    <P>
2426    <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
2427    string does not start with a palindrome that is shorter than the entire string.
2428    For example, although "abcba" is correctly matched, if the subject is "ababa",
2429    PCRE finds the palindrome "aba" at the start, then fails at top level because
2430    the end of the string does not follow. Once again, it cannot jump back into the
2431    recursion to try other alternatives, so the entire match fails.
2432    </P>
2433    <P>
2434    The second way in which PCRE and Perl differ in their recursion processing is
2435    in the handling of captured values. In Perl, when a subpattern is called
2436    recursively or as a subpattern (see the next section), it has no access to any
2437    values that were captured outside the recursion, whereas in PCRE these values
2438    can be referenced. Consider this pattern:
2439    <pre>
2440      ^(.)(\1|a(?2))
2441    </pre>
2442    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2443    then in the second group, when the back reference \1 fails to match "b", the
2444    second alternative matches "a" and then recurses. In the recursion, \1 does
2445    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2446    match because inside the recursive call \1 cannot access the externally set
2447    value.
2448  <a name="subpatternsassubroutines"></a></P>  <a name="subpatternsassubroutines"></a></P>
2449  <br><a name="SEC19" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>  <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
2450  <P>  <P>
2451  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
2452  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
2453  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The called subpattern may be defined
2454  pattern  before or after the reference. A numbered reference can be absolute or
2455    relative, as in these examples:
2456    <pre>
2457      (...(absolute)...)...(?2)...
2458      (...(relative)...)...(?-1)...
2459      (...(?+1)...(relative)...
2460    </pre>
2461    An earlier example pointed out that the pattern
2462  <pre>  <pre>
2463    (sens|respons)e and \1ibility    (sens|respons)e and \1ibility
2464  </pre>  </pre>
# Line 1447  matches "sense and sensibility" and "res Line 2468  matches "sense and sensibility" and "res
2468    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2469  </pre>  </pre>
2470  is used, it does match "sense and responsibility" as well as the other two  is used, it does match "sense and responsibility" as well as the other two
2471  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
2472  refer.  </P>
2473    <P>
2474    All subroutine calls, whether recursive or not, are always treated as atomic
2475    groups. That is, once a subroutine has matched some of the subject string, it
2476    is never re-entered, even if it contains untried alternatives and there is a
2477    subsequent matching failure. Any capturing parentheses that are set during the
2478    subroutine call revert to their previous values afterwards.
2479    </P>
2480    <P>
2481    Processing options such as case-independence are fixed when a subpattern is
2482    defined, so if it is used as a subroutine, such options cannot be changed for
2483    different calls. For example, consider this pattern:
2484    <pre>
2485      (abc)(?i:(?-1))
2486    </pre>
2487    It matches "abcabc". It does not match "abcABC" because the change of
2488    processing option does not affect the called subpattern.
2489    <a name="onigurumasubroutines"></a></P>
2490    <br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
2491    <P>
2492    For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
2493    a number enclosed either in angle brackets or single quotes, is an alternative
2494    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2495    are two of the examples used above, rewritten using this syntax:
2496    <pre>
2497      (?&#60;pn&#62; \( ( (?&#62;[^()]+) | \g&#60;pn&#62; )* \) )
2498      (sens|respons)e and \g'1'ibility
2499    </pre>
2500    PCRE supports an extension to Oniguruma: if a number is preceded by a
2501    plus or a minus sign it is taken as a relative reference. For example:
2502    <pre>
2503      (abc)(?i:\g&#60;-1&#62;)
2504    </pre>
2505    Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
2506    synonymous. The former is a back reference; the latter is a subroutine call.
2507  </P>  </P>
2508  <br><a name="SEC20" href="#TOC1">CALLOUTS</a><br>  <br><a name="SEC24" href="#TOC1">CALLOUTS</a><br>
2509  <P>  <P>
2510  Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl  Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2511  code to be obeyed in the middle of matching a regular expression. This makes it  code to be obeyed in the middle of matching a regular expression. This makes it
# Line 1460  same pair of parentheses when there is a Line 2515  same pair of parentheses when there is a
2515  <P>  <P>
2516  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
2517  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
2518  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>
2519  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
2520    variable contains NULL, which disables all calling out.
2521  </P>  </P>
2522  <P>  <P>
2523  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 1469  function is to be called. If you want to Line 2525  function is to be called. If you want to
2525  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
2526  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2527  <pre>  <pre>
2528    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2529  </pre>  </pre>
2530  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
2531  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2532  255.  255.
2533  </P>  </P>
2534  <P>  <P>
2535  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
2536  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
2537  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
2538  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
2539  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
2540  description of the interface to the callout function is given in the  the callout function is given in the
2541  <a href="pcrecallout.html"><b>pcrecallout</b></a>  <a href="pcrecallout.html"><b>pcrecallout</b></a>
2542  documentation.  documentation.
2543    <a name="backtrackcontrol"></a></P>
2544    <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
2545    <P>
2546    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2547    are described in the Perl documentation as "experimental and subject to change
2548    or removal in a future version of Perl". It goes on to say: "Their usage in
2549    production code should be noted to avoid problems during upgrades." The same
2550    remarks apply to the PCRE features described in this section.
2551    </P>
2552    <P>
2553    Since these verbs are specifically related to backtracking, most of them can be
2554    used only when the pattern is to be matched using one of the traditional
2555    matching functions, which use a backtracking algorithm. With the exception of
2556    (*FAIL), which behaves like a failing negative assertion, they cause an error
2557    if encountered by a DFA matching function.
2558    </P>
2559    <P>
2560    If any of these verbs are used in an assertion or in a subpattern that is
2561    called as a subroutine (whether or not recursively), their effect is confined
2562    to that subpattern; it does not extend to the surrounding pattern, with one
2563    exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2564    a successful positive assertion <i>is</i> passed back when a match succeeds
2565    (compare capturing parentheses in assertions). Note that such subpatterns are
2566    processed as anchored at the point where they are tested. Note also that Perl's
2567    treatment of subroutines is different in some cases.
2568    </P>
2569    <P>
2570    The new verbs make use of what was previously invalid syntax: an opening
2571    parenthesis followed by an asterisk. They are generally of the form
2572    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2573    depending on whether or not an argument is present. A name is any sequence of
2574    characters that does not include a closing parenthesis. If the name is empty,
2575    that is, if the closing parenthesis immediately follows the colon, the effect
2576    is as if the colon were not there. Any number of these verbs may occur in a
2577    pattern.
2578    <a name="nooptimize"></a></P>
2579    <br><b>
2580    Optimizations that affect backtracking verbs
2581    </b><br>
2582    <P>
2583    PCRE contains some optimizations that are used to speed up matching by running
2584    some checks at the start of each match attempt. For example, it may know the
2585    minimum length of matching subject, or that a particular character must be
2586    present. When one of these optimizations suppresses the running of a match, any
2587    included backtracking verbs will not, of course, be processed. You can suppress
2588    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2589    when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
2590    pattern with (*NO_START_OPT). There is more discussion of this option in the
2591    section entitled
2592    <a href="pcreapi.html#execoptions">"Option bits for <b>pcre_exec()</b>"</a>
2593    in the
2594    <a href="pcreapi.html"><b>pcreapi</b></a>
2595    documentation.
2596    </P>
2597    <P>
2598    Experiments with Perl suggest that it too has similar optimizations, sometimes
2599    leading to anomalous results.
2600    </P>
2601    <br><b>
2602    Verbs that act immediately
2603    </b><br>
2604    <P>
2605    The following verbs act as soon as they are encountered. They may not be
2606    followed by a name.
2607    <pre>
2608       (*ACCEPT)
2609    </pre>
2610    This verb causes the match to end successfully, skipping the remainder of the
2611    pattern. However, when it is inside a subpattern that is called as a
2612    subroutine, only that subpattern is ended successfully. Matching then continues
2613    at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2614    far is captured. For example:
2615    <pre>
2616      A((?:A|B(*ACCEPT)|C)D)
2617    </pre>
2618    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2619    the outer parentheses.
2620    <pre>
2621      (*FAIL) or (*F)
2622    </pre>
2623    This verb causes a matching failure, forcing backtracking to occur. It is
2624    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2625    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2626    Perl features that are not present in PCRE. The nearest equivalent is the
2627    callout feature, as for example in this pattern:
2628    <pre>
2629      a+(?C)(*FAIL)
2630    </pre>
2631    A match with the string "aaaa" always fails, but the callout is taken before
2632    each backtrack happens (in this example, 10 times).
2633    </P>
2634    <br><b>
2635    Recording which path was taken
2636    </b><br>
2637    <P>
2638    There is one verb whose main purpose is to track how a match was arrived at,
2639    though it also has a secondary use in conjunction with advancing the match
2640    starting point (see (*SKIP) below).
2641    <pre>
2642      (*MARK:NAME) or (*:NAME)
2643    </pre>
2644    A name is always required with this verb. There may be as many instances of
2645    (*MARK) as you like in a pattern, and their names do not have to be unique.
2646    </P>
2647    <P>
2648    When a match succeeds, the name of the last-encountered (*MARK) on the matching
2649    path is passed back to the caller as described in the section entitled
2650    <a href="pcreapi.html#extradata">"Extra data for <b>pcre_exec()</b>"</a>
2651    in the
2652    <a href="pcreapi.html"><b>pcreapi</b></a>
2653    documentation. Here is an example of <b>pcretest</b> output, where the /K
2654    modifier requests the retrieval and outputting of (*MARK) data:
2655    <pre>
2656        re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K
2657      data&#62; XY
2658       0: XY
2659      MK: A
2660      XZ
2661       0: XZ
2662      MK: B
2663    </pre>
2664    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2665    indicates which of the two alternatives matched. This is a more efficient way
2666    of obtaining this information than putting each alternative in its own
2667    capturing parentheses.
2668    </P>
2669    <P>
2670    If (*MARK) is encountered in a positive assertion, its name is recorded and
2671    passed back if it is the last-encountered. This does not happen for negative
2672    assertions.
2673    </P>
2674    <P>
2675    After a partial match or a failed match, the name of the last encountered
2676    (*MARK) in the entire match process is returned. For example:
2677    <pre>
2678        re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K
2679      data&#62; XP
2680      No match, mark = B
2681    </pre>
2682    Note that in this unanchored example the mark is retained from the match
2683    attempt that started at the letter "X" in the subject. Subsequent match
2684    attempts starting at "P" and then with an empty string do not get as far as the
2685    (*MARK) item, but nevertheless do not reset it.
2686    </P>
2687    <P>
2688    If you are interested in (*MARK) values after failed matches, you should
2689    probably set the PCRE_NO_START_OPTIMIZE option
2690    <a href="#nooptimize">(see above)</a>
2691    to ensure that the match is always attempted.
2692    </P>
2693    <br><b>
2694    Verbs that act after backtracking
2695    </b><br>
2696    <P>
2697    The following verbs do nothing when they are encountered. Matching continues
2698    with what follows, but if there is no subsequent match, causing a backtrack to
2699    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2700    the verb. However, when one of these verbs appears inside an atomic group, its
2701    effect is confined to that group, because once the group has been matched,
2702    there is never any backtracking into it. In this situation, backtracking can
2703    "jump back" to the left of the entire atomic group. (Remember also, as stated
2704    above, that this localization also applies in subroutine calls and assertions.)
2705    </P>
2706    <P>
2707    These verbs differ in exactly what kind of failure occurs when backtracking
2708    reaches them.
2709    <pre>
2710      (*COMMIT)
2711    </pre>
2712    This verb, which may not be followed by a name, causes the whole match to fail
2713    outright if the rest of the pattern does not match. Even if the pattern is
2714    unanchored, no further attempts to find a match by advancing the starting point
2715    take place. Once (*COMMIT) has been passed, <b>pcre_exec()</b> is committed to
2716    finding a match at the current starting point, or not at all. For example:
2717    <pre>
2718      a+(*COMMIT)b
2719    </pre>
2720    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2721    dynamic anchor, or "I've started, so I must finish." The name of the most
2722    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2723    match failure.
2724  </P>  </P>
2725  <P>  <P>
2726  Last updated: 28 February 2005  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2727    unless PCRE's start-of-match optimizations are turned off, as shown in this
2728    <b>pcretest</b> example:
2729    <pre>
2730        re&#62; /(*COMMIT)abc/
2731      data&#62; xyzabc
2732       0: abc
2733      xyzabc\Y
2734      No match
2735    </pre>
2736    PCRE knows that any match must start with "a", so the optimization skips along
2737    the subject to "a" before running the first match attempt, which succeeds. When
2738    the optimization is disabled by the \Y escape in the second subject, the match
2739    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2740    starting points.
2741    <pre>
2742      (*PRUNE) or (*PRUNE:NAME)
2743    </pre>
2744    This verb causes the match to fail at the current starting position in the
2745    subject if the rest of the pattern does not match. If the pattern is
2746    unanchored, the normal "bumpalong" advance to the next starting character then
2747    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2748    reached, or when matching to the right of (*PRUNE), but if there is no match to
2749    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2750    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2751    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2752    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2753    anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2754    <pre>
2755      (*SKIP)
2756    </pre>
2757    This verb, when given without a name, is like (*PRUNE), except that if the
2758    pattern is unanchored, the "bumpalong" advance is not to the next character,
2759    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2760    signifies that whatever text was matched leading up to it cannot be part of a
2761    successful match. Consider:
2762    <pre>
2763      a+(*SKIP)b
2764    </pre>
2765    If the subject is "aaaac...", after the first match attempt fails (starting at
2766    the first character in the string), the starting point skips on to start the
2767    next attempt at "c". Note that a possessive quantifer does not have the same
2768    effect as this example; although it would suppress backtracking during the
2769    first match attempt, the second attempt would start at the second character
2770    instead of skipping on to "c".
2771    <pre>
2772      (*SKIP:NAME)
2773    </pre>
2774    When (*SKIP) has an associated name, its behaviour is modified. If the
2775    following pattern fails to match, the previous path through the pattern is
2776    searched for the most recent (*MARK) that has the same name. If one is found,
2777    the "bumpalong" advance is to the subject position that corresponds to that
2778    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2779    matching name is found, the (*SKIP) is ignored.
2780    <pre>
2781      (*THEN) or (*THEN:NAME)
2782    </pre>
2783    This verb causes a skip to the next innermost alternative if the rest of the
2784    pattern does not match. That is, it cancels pending backtracking, but only
2785    within the current alternative. Its name comes from the observation that it can
2786    be used for a pattern-based if-then-else block:
2787    <pre>
2788      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2789    </pre>
2790    If the COND1 pattern matches, FOO is tried (and possibly further items after
2791    the end of the group if FOO succeeds); on failure, the matcher skips to the
2792    second alternative and tries COND2, without backtracking into COND1. The
2793    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2794    If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2795    </P>
2796    <P>
2797    Note that a subpattern that does not contain a | character is just a part of
2798    the enclosing alternative; it is not a nested alternation with only one
2799    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2800    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2801    pattern fragments that do not contain any | characters at this level:
2802    <pre>
2803      A (B(*THEN)C) | D
2804    </pre>
2805    If A and B are matched, but there is a failure in C, matching does not
2806    backtrack into A; instead it moves to the next alternative, that is, D.
2807    However, if the subpattern containing (*THEN) is given an alternative, it
2808    behaves differently:
2809    <pre>
2810      A (B(*THEN)C | (*FAIL)) | D
2811    </pre>
2812    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2813    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2814    because there are no more alternatives to try. In this case, matching does now
2815    backtrack into A.
2816    </P>
2817    <P>
2818    Note also that a conditional subpattern is not considered as having two
2819    alternatives, because only one is ever used. In other words, the | character in
2820    a conditional subpattern has a different meaning. Ignoring white space,
2821    consider:
2822    <pre>
2823      ^.*? (?(?=a) a | b(*THEN)c )
2824    </pre>
2825    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2826    it initially matches zero characters. The condition (?=a) then fails, the
2827    character "b" is matched, but "c" is not. At this point, matching does not
2828    backtrack to .*? as might perhaps be expected from the presence of the |
2829    character. The conditional subpattern is part of the single alternative that
2830    comprises the whole pattern, and so the match fails. (If there was a backtrack
2831    into .*?, allowing it to match "b", the match would succeed.)
2832    </P>
2833    <P>
2834    The verbs just described provide four different "strengths" of control when
2835    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2836    next alternative. (*PRUNE) comes next, failing the match at the current
2837    starting position, but allowing an advance to the next character (for an
2838    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2839    than one character. (*COMMIT) is the strongest, causing the entire match to
2840    fail.
2841    </P>
2842    <P>
2843    If more than one such verb is present in a pattern, the "strongest" one wins.
2844    For example, consider this pattern, where A, B, etc. are complex pattern
2845    fragments:
2846    <pre>
2847      (A(*COMMIT)B(*THEN)C|D)
2848    </pre>
2849    Once A has matched, PCRE is committed to this match, at the current starting
2850    position. If subsequently B matches, but C does not, the normal (*THEN) action
2851    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2852    overrides.
2853    </P>
2854    <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
2855    <P>
2856    <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
2857    <b>pcresyntax</b>(3), <b>pcre</b>(3), <b>pcre16(3)</b>.
2858    </P>
2859    <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
2860    <P>
2861    Philip Hazel
2862    <br>
2863    University Computing Service
2864    <br>
2865    Cambridge CB2 3QH, England.
2866    <br>
2867    </P>
2868    <br><a name="SEC28" href="#TOC1">REVISION</a><br>
2869    <P>
2870    Last updated: 24 February 2012
2871    <br>
2872    Copyright &copy; 1997-2012 University of Cambridge.
2873  <br>  <br>
 Copyright &copy; 1997-2005 University of Cambridge.  
2874  <p>  <p>
2875  Return to the <a href="index.html">PCRE index page</a>.  Return to the <a href="index.html">PCRE index page</a>.
2876  </p>  </p>

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