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3  <title>pcrepattern specification</title>  <title>pcrepattern specification</title>
4  </head>  </head>
5  <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">  <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6  This HTML document has been generated automatically from the original man page.  <h1>pcrepattern man page</h1>
7  If there is any nonsense in it, please consult the man page, in case the  <p>
8  conversion went wrong.<br>  Return to the <a href="index.html">PCRE index page</a>.
9    </p>
10    <p>
11    This page is part of the PCRE HTML documentation. It was generated automatically
12    from the original man page. If there is any nonsense in it, please consult the
13    man page, in case the conversion went wrong.
14    <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</a>  <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT)</a>
22  <li><a name="TOC7" href="#SEC7">POSIX CHARACTER CLASSES</a>  <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</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">CALLOUTS</a>
39    <li><a name="TOC24" href="#SEC24">BACTRACKING CONTROL</a>
40    <li><a name="TOC25" href="#SEC25">SEE ALSO</a>
41    <li><a name="TOC26" href="#SEC26">AUTHOR</a>
42    <li><a name="TOC27" href="#SEC27">REVISION</a>
43  </ul>  </ul>
44  <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>  <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
45  <P>  <P>
46  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
47  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
48  documentation and in a number of other books, some of which have copious  <a href="pcresyntax.html"><b>pcresyntax</b></a>
49  examples. Jeffrey Friedl's "Mastering Regular Expressions", published by  page. Perl's regular expressions are described in its own documentation, and
50  O'Reilly, covers them in great detail. The description here is intended as  regular expressions in general are covered in a number of books, some of which
51  reference documentation.  have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
52    published by O'Reilly, covers regular expressions in great detail. This
53    description of PCRE's regular expressions is intended as reference material.
54  </P>  </P>
55  <P>  <P>
56  The basic operation of PCRE is on strings of bytes. However, there is also  The original operation of PCRE was on strings of one-byte characters. However,
57  support for UTF-8 character strings. To use this support you must build PCRE to  there is now also support for UTF-8 character strings. To use this, you must
58  include UTF-8 support, and then call <b>pcre_compile()</b> with the PCRE_UTF8  build PCRE to include UTF-8 support, and then call <b>pcre_compile()</b> with
59  option. How this affects the pattern matching is mentioned in several places  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
60  below. There is also a summary of UTF-8 features in the  places below. There is also a summary of UTF-8 features in the
61  <a href="pcre.html#utf8support">section on UTF-8 support</a>  <a href="pcre.html#utf8support">section on UTF-8 support</a>
62  in the main  in the main
63  <a href="pcre.html"><b>pcre</b></a>  <a href="pcre.html"><b>pcre</b></a>
64  page.  page.
65  </P>  </P>
66  <P>  <P>
67    The remainder of this document discusses the patterns that are supported by
68    PCRE when its main matching function, <b>pcre_exec()</b>, is used.
69    From release 6.0, PCRE offers a second matching function,
70    <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
71    Perl-compatible. Some of the features discussed below are not available when
72    <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
73    alternative function, and how it differs from the normal function, are
74    discussed in the
75    <a href="pcrematching.html"><b>pcrematching</b></a>
76    page.
77    </P>
78    <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
79    <P>
80    PCRE supports five different conventions for indicating line breaks in
81    strings: a single CR (carriage return) character, a single LF (linefeed)
82    character, the two-character sequence CRLF, any of the three preceding, or any
83    Unicode newline sequence. The
84    <a href="pcreapi.html"><b>pcreapi</b></a>
85    page has
86    <a href="pcreapi.html#newlines">further discussion</a>
87    about newlines, and shows how to set the newline convention in the
88    <i>options</i> arguments for the compiling and matching functions.
89    </P>
90    <P>
91    It is also possible to specify a newline convention by starting a pattern
92    string with one of the following five sequences:
93    <pre>
94      (*CR)        carriage return
95      (*LF)        linefeed
96      (*CRLF)      carriage return, followed by linefeed
97      (*ANYCRLF)   any of the three above
98      (*ANY)       all Unicode newline sequences
99    </pre>
100    These override the default and the options given to <b>pcre_compile()</b>. For
101    example, on a Unix system where LF is the default newline sequence, the pattern
102    <pre>
103      (*CR)a.b
104    </pre>
105    changes the convention to CR. That pattern matches "a\nb" because LF is no
106    longer a newline. Note that these special settings, which are not
107    Perl-compatible, are recognized only at the very start of a pattern, and that
108    they must be in upper case.
109    </P>
110    <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
111    <P>
112  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
113  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
114  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
 </P>  
 <P>  
115  <pre>  <pre>
116    The quick brown fox    The quick brown fox
117  </PRE>  </pre>
118    matches a portion of a subject string that is identical to itself. When
119    caseless matching is specified (the PCRE_CASELESS option), letters are matched
120    independently of case. In UTF-8 mode, PCRE always understands the concept of
121    case for characters whose values are less than 128, so caseless matching is
122    always possible. For characters with higher values, the concept of case is
123    supported if PCRE is compiled with Unicode property support, but not otherwise.
124    If you want to use caseless matching for characters 128 and above, you must
125    ensure that PCRE is compiled with Unicode property support as well as with
126    UTF-8 support.
127  </P>  </P>
128  <P>  <P>
129  matches a portion of a subject string that is identical to itself. The power of  The power of regular expressions comes from the ability to include alternatives
130  regular expressions comes from the ability to include alternatives and  and repetitions in the pattern. These are encoded in the pattern by the use of
131  repetitions in the pattern. These are encoded in the pattern by the use of  <i>metacharacters</i>, which do not stand for themselves but instead are
 <i>meta-characters</i>, which do not stand for themselves but instead are  
132  interpreted in some special way.  interpreted in some special way.
133  </P>  </P>
134  <P>  <P>
135  There are two different sets of meta-characters: those that are recognized  There are two different sets of metacharacters: those that are recognized
136  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
137  recognized in square brackets. Outside square brackets, the meta-characters are  recognized within square brackets. Outside square brackets, the metacharacters
138  as follows:  are as follows:
 </P>  
 <P>  
139  <pre>  <pre>
140    \      general escape character with several uses    \      general escape character with several uses
141    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
# Line 88  as follows: Line 152  as follows:
152    +      1 or more quantifier    +      1 or more quantifier
153           also "possessive quantifier"           also "possessive quantifier"
154    {      start min/max quantifier    {      start min/max quantifier
155  </PRE>  </pre>
 </P>  
 <P>  
156  Part of a pattern that is in square brackets is called a "character class". In  Part of a pattern that is in square brackets is called a "character class". In
157  a character class the only meta-characters are:  a character class the only metacharacters are:
 </P>  
 <P>  
158  <pre>  <pre>
159    \      general escape character    \      general escape character
160    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
161    -      indicates character range    -      indicates character range
162    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX syntax)
            syntax)  
163    ]      terminates the character class    ]      terminates the character class
164  </PRE>  </pre>
165    The following sections describe the use of each of the metacharacters.
166  </P>  </P>
167  <P>  <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
 The following sections describe the use of each of the meta-characters.  
 </P>  
 <br><a name="SEC2" href="#TOC1">BACKSLASH</a><br>  
168  <P>  <P>
169  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
170  non-alphameric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character
171  have. This use of backslash as an escape character applies both inside and  may have. This use of backslash as an escape character applies both inside and
172  outside character classes.  outside character classes.
173  </P>  </P>
174  <P>  <P>
175  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.
176  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
177  otherwise be interpreted as a meta-character, so it is always safe to precede a  otherwise be interpreted as a metacharacter, so it is always safe to precede a
178  non-alphameric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
179  particular, if you want to match a backslash, you write \\.  particular, if you want to match a backslash, you write \\.
180  </P>  </P>
181  <P>  <P>
182  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
183  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
184  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
185  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.  
186  </P>  </P>
187  <P>  <P>
188  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
189  can do so by putting them between \Q and \E. This is different from Perl in  can do so by putting them between \Q and \E. This is different from Perl in
190  that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in  that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
191  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
 </P>  
 <P>  
192  <pre>  <pre>
193    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
194  </PRE>  
195  </P>    \Qabc$xyz\E        abc$xyz        abc followed by the contents of $xyz
 <P>  
 <pre>  
   \Qabc$xyz\E        abc$xyz        abc followed by the  
                                       contents of $xyz  
196    \Qabc\$xyz\E       abc\$xyz       abc\$xyz    \Qabc\$xyz\E       abc\$xyz       abc\$xyz
197    \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz    \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
198  </PRE>  </pre>
 </P>  
 <P>  
199  The \Q...\E sequence is recognized both inside and outside character classes.  The \Q...\E sequence is recognized both inside and outside character classes.
200  </P>  <a name="digitsafterbackslash"></a></P>
201    <br><b>
202    Non-printing characters
203    </b><br>
204  <P>  <P>
205  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
206  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
# Line 157  non-printing characters, apart from the Line 208  non-printing characters, apart from the
208  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 usually easier to
209  use one of the following escape sequences than the binary character it  use one of the following escape sequences than the binary character it
210  represents:  represents:
 </P>  
 <P>  
211  <pre>  <pre>
212    \a        alarm, that is, the BEL character (hex 07)    \a        alarm, that is, the BEL character (hex 07)
213    \cx       "control-x", where x is any character    \cx       "control-x", where x is any character
214    \e        escape (hex 1B)    \e        escape (hex 1B)
215    \f        formfeed (hex 0C)    \f        formfeed (hex 0C)
216    \n        newline (hex 0A)    \n        linefeed (hex 0A)
217    \r        carriage return (hex 0D)    \r        carriage return (hex 0D)
218    \t        tab (hex 09)    \t        tab (hex 09)
219    \ddd      character with octal code ddd, or backreference    \ddd      character with octal code ddd, or backreference
220    \xhh      character with hex code hh    \xhh      character with hex code hh
221    \x{hhh..} character with hex code hhh... (UTF-8 mode only)    \x{hhh..} character with hex code hhh..
222  </PRE>  </pre>
 </P>  
 <P>  
223  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
224  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.
225  Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex  Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex
# Line 180  Thus \cz becomes hex 1A, but \c{ becomes Line 227  Thus \cz becomes hex 1A, but \c{ becomes
227  </P>  </P>
228  <P>  <P>
229  After \x, from zero to two hexadecimal digits are read (letters can be in  After \x, from zero to two hexadecimal digits are read (letters can be in
230  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  upper or lower case). Any number of hexadecimal digits may appear between \x{
231  appear between \x{ and }, but the value of the character code must be less  and }, but the value of the character code must be less than 256 in non-UTF-8
232  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
233  other than hexadecimal digits appear between \x{ and }, or if there is no  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
234  terminating }, this form of escape is not recognized. Instead, the initial  point, which is 10FFFF.
235  \x will be interpreted as a basic hexadecimal escape, with no following  </P>
236  digits, giving a byte whose value is zero.  <P>
237    If characters other than hexadecimal digits appear between \x{ and }, or if
238    there is no terminating }, this form of escape is not recognized. Instead, the
239    initial \x will be interpreted as a basic hexadecimal escape, with no
240    following digits, giving a character whose value is zero.
241  </P>  </P>
242  <P>  <P>
243  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
244  syntaxes for \x when PCRE is in UTF-8 mode. There is no difference in the  syntaxes for \x. There is no difference in the way they are handled. For
245  way they are handled. For example, \xdc is exactly the same as \x{dc}.  example, \xdc is exactly the same as \x{dc}.
246  </P>  </P>
247  <P>  <P>
248  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
249  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
250  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
251  (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
252  character that follows is itself an octal digit.  follows is itself an octal digit.
253  </P>  </P>
254  <P>  <P>
255  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 206  Outside a character class, PCRE reads it Line 257  Outside a character class, PCRE reads it
257  number. If the number is less than 10, or if there have been at least that many  number. If the number is less than 10, or if there have been at least that many
258  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
259  taken as a <i>back reference</i>. A description of how this works is given  taken as a <i>back reference</i>. A description of how this works is given
260  later, following the discussion of parenthesized subpatterns.  <a href="#backreferences">later,</a>
261    following the discussion of
262    <a href="#subpattern">parenthesized subpatterns.</a>
263  </P>  </P>
264  <P>  <P>
265  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
266  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
267  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
268  significant 8 bits of the value. Any subsequent digits stand for themselves.  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
269  For example:  character specified in octal must be less than \400. In UTF-8 mode, values up
270  </P>  to \777 are permitted. For example:
 <P>  
271  <pre>  <pre>
272    \040   is another way of writing a space    \040   is another way of writing a space
273    \40    is the same, provided there are fewer than 40    \40    is the same, provided there are fewer than 40 previous capturing subpatterns
             previous capturing subpatterns  
274    \7     is always a back reference    \7     is always a back reference
275    \11    might be a back reference, or another way of    \11    might be a back reference, or another way of writing a tab
             writing a tab  
276    \011   is always a tab    \011   is always a tab
277    \0113  is a tab followed by the character "3"    \0113  is a tab followed by the character "3"
278    \113   might be a back reference, otherwise the    \113   might be a back reference, otherwise the character with octal code 113
279              character with octal code 113    \377   might be a back reference, otherwise the byte consisting entirely of 1 bits
280    \377   might be a back reference, otherwise    \81    is either a back reference, or a binary zero followed by the two characters "8" and "1"
281              the byte consisting entirely of 1 bits  </pre>
   \81    is either a back reference, or a binary zero  
             followed by the two characters "8" and "1"  
 </PRE>  
 </P>  
 <P>  
282  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
283  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
284  </P>  </P>
285  <P>  <P>
286  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
287  (in UTF-8 mode) can be used both inside and outside character classes. In  and outside character classes. In addition, inside a character class, the
288  addition, inside a character class, the sequence \b is interpreted as the  sequence \b is interpreted as the backspace character (hex 08), and the
289  backspace character (hex 08). Outside a character class it has a different  sequences \R and \X are interpreted as the characters "R" and "X",
290  meaning (see below).  respectively. Outside a character class, these sequences have different
291  </P>  meanings
292  <P>  <a href="#uniextseq">(see below).</a>
293  The third use of backslash is for specifying generic character types:  </P>
294  </P>  <br><b>
295    Absolute and relative back references
296    </b><br>
297    <P>
298    The sequence \g followed by an unsigned or a negative number, optionally
299    enclosed in braces, is an absolute or relative back reference. A named back
300    reference can be coded as \g{name}. Back references are discussed
301    <a href="#backreferences">later,</a>
302    following the discussion of
303    <a href="#subpattern">parenthesized subpatterns.</a>
304    </P>
305    <br><b>
306    Generic character types
307    </b><br>
308  <P>  <P>
309    Another use of backslash is for specifying generic character types. The
310    following are always recognized:
311  <pre>  <pre>
312    \d     any decimal digit    \d     any decimal digit
313    \D     any character that is not a decimal digit    \D     any character that is not a decimal digit
314      \h     any horizontal whitespace character
315      \H     any character that is not a horizontal whitespace character
316    \s     any whitespace character    \s     any whitespace character
317    \S     any character that is not a whitespace character    \S     any character that is not a whitespace character
318      \v     any vertical whitespace character
319      \V     any character that is not a vertical whitespace character
320    \w     any "word" character    \w     any "word" character
321    \W     any "non-word" character    \W     any "non-word" character
322  </PRE>  </pre>
 </P>  
 <P>  
323  Each pair of escape sequences partitions the complete set of characters into  Each pair of escape sequences partitions the complete set of characters into
324  two disjoint sets. Any given character matches one, and only one, of each pair.  two disjoint sets. Any given character matches one, and only one, of each pair.
325  </P>  </P>
326  <P>  <P>
327  In UTF-8 mode, characters with values greater than 255 never match \d, \s, or  These character type sequences can appear both inside and outside character
328  \w, and always match \D, \S, and \W.  classes. They each match one character of the appropriate type. If the current
329    matching point is at the end of the subject string, all of them fail, since
330    there is no character to match.
331  </P>  </P>
332  <P>  <P>
333  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).
334  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
335  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
336    included in a Perl script, \s may match the VT character. In PCRE, it never
337    does.
338    </P>
339    <P>
340    In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
341    \w, and always match \D, \S, and \W. This is true even when Unicode
342    character property support is available. These sequences retain their original
343    meanings from before UTF-8 support was available, mainly for efficiency
344    reasons.
345    </P>
346    <P>
347    The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to the
348    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
349    The horizontal space characters are:
350    <pre>
351      U+0009     Horizontal tab
352      U+0020     Space
353      U+00A0     Non-break space
354      U+1680     Ogham space mark
355      U+180E     Mongolian vowel separator
356      U+2000     En quad
357      U+2001     Em quad
358      U+2002     En space
359      U+2003     Em space
360      U+2004     Three-per-em space
361      U+2005     Four-per-em space
362      U+2006     Six-per-em space
363      U+2007     Figure space
364      U+2008     Punctuation space
365      U+2009     Thin space
366      U+200A     Hair space
367      U+202F     Narrow no-break space
368      U+205F     Medium mathematical space
369      U+3000     Ideographic space
370    </pre>
371    The vertical space characters are:
372    <pre>
373      U+000A     Linefeed
374      U+000B     Vertical tab
375      U+000C     Formfeed
376      U+000D     Carriage return
377      U+0085     Next line
378      U+2028     Line separator
379      U+2029     Paragraph separator
380    </PRE>
381  </P>  </P>
382  <P>  <P>
383  A "word" character is any letter or digit or the underscore character, that is,  A "word" character is an underscore or any character less than 256 that is a
384  any character which can be part of a Perl "word". The definition of letters and  letter or digit. The definition of letters and digits is controlled by PCRE's
385  digits is controlled by PCRE's character tables, and may vary if locale-  low-valued character tables, and may vary if locale-specific matching is taking
386  specific matching is taking place (see  place (see
387  <a href="pcreapi.html#localesupport">"Locale support"</a>  <a href="pcreapi.html#localesupport">"Locale support"</a>
388  in the  in the
389  <a href="pcreapi.html"><b>pcreapi</b></a>  <a href="pcreapi.html"><b>pcreapi</b></a>
390  page). For example, in the "fr" (French) locale, some character codes greater  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
391  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
392    accented letters, and these are matched by \w. The use of locales with Unicode
393    is discouraged.
394    </P>
395    <br><b>
396    Newline sequences
397    </b><br>
398    <P>
399    Outside a character class, the escape sequence \R matches any Unicode newline
400    sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is equivalent to
401    the following:
402    <pre>
403      (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
404    </pre>
405    This is an example of an "atomic group", details of which are given
406    <a href="#atomicgroup">below.</a>
407    This particular group matches either the two-character sequence CR followed by
408    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
409    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
410    line, U+0085). The two-character sequence is treated as a single unit that
411    cannot be split.
412    </P>
413    <P>
414    In UTF-8 mode, two additional characters whose codepoints are greater than 255
415    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
416    Unicode character property support is not needed for these characters to be
417    recognized.
418    </P>
419    <P>
420    Inside a character class, \R matches the letter "R".
421    <a name="uniextseq"></a></P>
422    <br><b>
423    Unicode character properties
424    </b><br>
425    <P>
426    When PCRE is built with Unicode character property support, three additional
427    escape sequences that match characters with specific properties are available.
428    When not in UTF-8 mode, these sequences are of course limited to testing
429    characters whose codepoints are less than 256, but they do work in this mode.
430    The extra escape sequences are:
431    <pre>
432      \p{<i>xx</i>}   a character with the <i>xx</i> property
433      \P{<i>xx</i>}   a character without the <i>xx</i> property
434      \X       an extended Unicode sequence
435    </pre>
436    The property names represented by <i>xx</i> above are limited to the Unicode
437    script names, the general category properties, and "Any", which matches any
438    character (including newline). Other properties such as "InMusicalSymbols" are
439    not currently supported by PCRE. Note that \P{Any} does not match any
440    characters, so always causes a match failure.
441    </P>
442    <P>
443    Sets of Unicode characters are defined as belonging to certain scripts. A
444    character from one of these sets can be matched using a script name. For
445    example:
446    <pre>
447      \p{Greek}
448      \P{Han}
449    </pre>
450    Those that are not part of an identified script are lumped together as
451    "Common". The current list of scripts is:
452    </P>
453    <P>
454    Arabic,
455    Armenian,
456    Balinese,
457    Bengali,
458    Bopomofo,
459    Braille,
460    Buginese,
461    Buhid,
462    Canadian_Aboriginal,
463    Cherokee,
464    Common,
465    Coptic,
466    Cuneiform,
467    Cypriot,
468    Cyrillic,
469    Deseret,
470    Devanagari,
471    Ethiopic,
472    Georgian,
473    Glagolitic,
474    Gothic,
475    Greek,
476    Gujarati,
477    Gurmukhi,
478    Han,
479    Hangul,
480    Hanunoo,
481    Hebrew,
482    Hiragana,
483    Inherited,
484    Kannada,
485    Katakana,
486    Kharoshthi,
487    Khmer,
488    Lao,
489    Latin,
490    Limbu,
491    Linear_B,
492    Malayalam,
493    Mongolian,
494    Myanmar,
495    New_Tai_Lue,
496    Nko,
497    Ogham,
498    Old_Italic,
499    Old_Persian,
500    Oriya,
501    Osmanya,
502    Phags_Pa,
503    Phoenician,
504    Runic,
505    Shavian,
506    Sinhala,
507    Syloti_Nagri,
508    Syriac,
509    Tagalog,
510    Tagbanwa,
511    Tai_Le,
512    Tamil,
513    Telugu,
514    Thaana,
515    Thai,
516    Tibetan,
517    Tifinagh,
518    Ugaritic,
519    Yi.
520    </P>
521    <P>
522    Each character has exactly one general category property, specified by a
523    two-letter abbreviation. For compatibility with Perl, negation can be specified
524    by including a circumflex between the opening brace and the property name. For
525    example, \p{^Lu} is the same as \P{Lu}.
526    </P>
527    <P>
528    If only one letter is specified with \p or \P, it includes all the general
529    category properties that start with that letter. In this case, in the absence
530    of negation, the curly brackets in the escape sequence are optional; these two
531    examples have the same effect:
532    <pre>
533      \p{L}
534      \pL
535    </pre>
536    The following general category property codes are supported:
537    <pre>
538      C     Other
539      Cc    Control
540      Cf    Format
541      Cn    Unassigned
542      Co    Private use
543      Cs    Surrogate
544    
545      L     Letter
546      Ll    Lower case letter
547      Lm    Modifier letter
548      Lo    Other letter
549      Lt    Title case letter
550      Lu    Upper case letter
551    
552      M     Mark
553      Mc    Spacing mark
554      Me    Enclosing mark
555      Mn    Non-spacing mark
556    
557      N     Number
558      Nd    Decimal number
559      Nl    Letter number
560      No    Other number
561    
562      P     Punctuation
563      Pc    Connector punctuation
564      Pd    Dash punctuation
565      Pe    Close punctuation
566      Pf    Final punctuation
567      Pi    Initial punctuation
568      Po    Other punctuation
569      Ps    Open punctuation
570    
571      S     Symbol
572      Sc    Currency symbol
573      Sk    Modifier symbol
574      Sm    Mathematical symbol
575      So    Other symbol
576    
577      Z     Separator
578      Zl    Line separator
579      Zp    Paragraph separator
580      Zs    Space separator
581    </pre>
582    The special property L& is also supported: it matches a character that has
583    the Lu, Ll, or Lt property, in other words, a letter that is not classified as
584    a modifier or "other".
585    </P>
586    <P>
587    The Cs (Surrogate) property applies only to characters in the range U+D800 to
588    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
589    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
590    (see the discussion of PCRE_NO_UTF8_CHECK in the
591    <a href="pcreapi.html"><b>pcreapi</b></a>
592    page).
593  </P>  </P>
594  <P>  <P>
595  These character type sequences can appear both inside and outside character  The long synonyms for these properties that Perl supports (such as \p{Letter})
596  classes. They each match one character of the appropriate type. If the current  are not supported by PCRE, nor is it permitted to prefix any of these
597  matching point is at the end of the subject string, all of them fail, since  properties with "Is".
598  there is no character to match.  </P>
599  </P>  <P>
600    No character that is in the Unicode table has the Cn (unassigned) property.
601    Instead, this property is assumed for any code point that is not in the
602    Unicode table.
603    </P>
604    <P>
605    Specifying caseless matching does not affect these escape sequences. For
606    example, \p{Lu} always matches only upper case letters.
607    </P>
608    <P>
609    The \X escape matches any number of Unicode characters that form an extended
610    Unicode sequence. \X is equivalent to
611    <pre>
612      (?&#62;\PM\pM*)
613    </pre>
614    That is, it matches a character without the "mark" property, followed by zero
615    or more characters with the "mark" property, and treats the sequence as an
616    atomic group
617    <a href="#atomicgroup">(see below).</a>
618    Characters with the "mark" property are typically accents that affect the
619    preceding character. None of them have codepoints less than 256, so in
620    non-UTF-8 mode \X matches any one character.
621    </P>
622    <P>
623    Matching characters by Unicode property is not fast, because PCRE has to search
624    a structure that contains data for over fifteen thousand characters. That is
625    why the traditional escape sequences such as \d and \w do not use Unicode
626    properties in PCRE.
627    <a name="resetmatchstart"></a></P>
628    <br><b>
629    Resetting the match start
630    </b><br>
631    <P>
632    The escape sequence \K, which is a Perl 5.10 feature, causes any previously
633    matched characters not to be included in the final matched sequence. For
634    example, the pattern:
635    <pre>
636      foo\Kbar
637    </pre>
638    matches "foobar", but reports that it has matched "bar". This feature is
639    similar to a lookbehind assertion
640    <a href="#lookbehind">(described below).</a>
641    However, in this case, the part of the subject before the real match does not
642    have to be of fixed length, as lookbehind assertions do. The use of \K does
643    not interfere with the setting of
644    <a href="#subpattern">captured substrings.</a>
645    For example, when the pattern
646    <pre>
647      (foo)\Kbar
648    </pre>
649    matches "foobar", the first substring is still set to "foo".
650    <a name="smallassertions"></a></P>
651    <br><b>
652    Simple assertions
653    </b><br>
654  <P>  <P>
655  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
656  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,
657  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
658  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
659  assertions are  <a href="#bigassertions">below.</a>
660  </P>  The backslashed assertions are:
 <P>  
661  <pre>  <pre>
662    \b     matches at a word boundary    \b     matches at a word boundary
663    \B     matches when not at a word boundary    \B     matches when not at a word boundary
664    \A     matches at start of subject    \A     matches at the start of the subject
665    \Z     matches at end of subject or before newline at end    \Z     matches at the end of the subject
666    \z     matches at end of subject            also matches before a newline at the end of the subject
667    \G     matches at first matching position in subject    \z     matches only at the end of the subject
668  </PRE>    \G     matches at the first matching position in the subject
669  </P>  </pre>
 <P>  
670  These assertions may not appear in character classes (but note that \b has a  These assertions may not appear in character classes (but note that \b has a
671  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
672  </P>  </P>
# Line 316  first or last character matches \w, resp Line 678  first or last character matches \w, resp
678  </P>  </P>
679  <P>  <P>
680  The \A, \Z, and \z assertions differ from the traditional circumflex and  The \A, \Z, and \z assertions differ from the traditional circumflex and
681  dollar (described below) in that they only ever match at the very start and end  dollar (described in the next section) in that they only ever match at the very
682  of the subject string, whatever options are set. Thus, they are independent of  start and end of the subject string, whatever options are set. Thus, they are
683  multiline mode.  independent of multiline mode. These three assertions are not affected by the
684  </P>  PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
685  <P>  circumflex and dollar metacharacters. However, if the <i>startoffset</i>
686  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
687  <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero, indicating that  at a point other than the beginning of the subject, \A can never match. The
688  matching is to start at a point other than the beginning of the subject, \A  difference between \Z and \z is that \Z matches before a newline at the end
689  can never match. The difference between \Z and \z is that \Z matches before  of the string as well as at the very end, whereas \z matches only at the end.
 a newline that is the last character of the string as well as at the end of the  
 string, whereas \z matches only at the end.  
690  </P>  </P>
691  <P>  <P>
692  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 348  If all the alternatives of a pattern beg Line 708  If all the alternatives of a pattern beg
708  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
709  regular expression.  regular expression.
710  </P>  </P>
711  <br><a name="SEC3" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>  <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
712  <P>  <P>
713  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
714  character is an assertion which is true only if the current matching point is  character is an assertion that is true only if the current matching point is
715  at the start of the subject string. If the <i>startoffset</i> argument of  at the start of the subject string. If the <i>startoffset</i> argument of
716  <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE  <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
717  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
718  meaning (see below).  meaning
719    <a href="#characterclass">(see below).</a>
720  </P>  </P>
721  <P>  <P>
722  Circumflex need not be the first character of the pattern if a number of  Circumflex need not be the first character of the pattern if a number of
# Line 367  constrained to match only at the start o Line 728  constrained to match only at the start o
728  to be anchored.)  to be anchored.)
729  </P>  </P>
730  <P>  <P>
731  A dollar character is an assertion which is true only if the current matching  A dollar character is an assertion that is true only if the current matching
732  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
733  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
734  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
735  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
736  Dollar has no special meaning in a character class.  character class.
737  </P>  </P>
738  <P>  <P>
739  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 381  does not affect the \Z assertion. Line 742  does not affect the \Z assertion.
742  </P>  </P>
743  <P>  <P>
744  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
745  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
746  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
747  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
748  the pattern /^abc$/ matches the subject string "def\nabc" in multiline mode,  matches before any newlines in the string, as well as at the very end, when
749  but not otherwise. Consequently, patterns that are anchored in single line mode  PCRE_MULTILINE is set. When newline is specified as the two-character
750  because all branches start with ^ are not anchored in multiline mode, and a  sequence CRLF, isolated CR and LF characters do not indicate newlines.
751  match for circumflex is possible when the <i>startoffset</i> argument of  </P>
752  <b>pcre_exec()</b> is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if  <P>
753  PCRE_MULTILINE is set.  For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
754    \n represents a newline) in multiline mode, but not otherwise. Consequently,
755    patterns that are anchored in single line mode because all branches start with
756    ^ are not anchored in multiline mode, and a match for circumflex is possible
757    when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
758    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
759  </P>  </P>
760  <P>  <P>
761  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
762  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
763  \A it is always anchored, whether PCRE_MULTILINE is set or not.  \A it is always anchored, whether or not PCRE_MULTILINE is set.
764  </P>  </P>
765  <br><a name="SEC4" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>  <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>
766  <P>  <P>
767  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
768  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
769  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  line. In UTF-8 mode, the matched character may be more than one byte long.
 byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  
 dots match newlines as well. The handling of dot is entirely independent of the  
 handling of circumflex and dollar, the only relationship being that they both  
 involve newline characters. Dot has no special meaning in a character class.  
770  </P>  </P>
 <br><a name="SEC5" href="#TOC1">MATCHING A SINGLE BYTE</a><br>  
771  <P>  <P>
772  Outside a character class, the escape sequence \C matches any one byte, both  When a line ending is defined as a single character, dot never matches that
773  in and out of UTF-8 mode. Unlike a dot, it always matches a newline. The  character; when the two-character sequence CRLF is used, dot does not match CR
774  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  if it is immediately followed by LF, but otherwise it matches all characters
775  Because it breaks up UTF-8 characters into individual bytes, what remains in  (including isolated CRs and LFs). When any Unicode line endings are being
776  the string may be a malformed UTF-8 string. For this reason it is best avoided.  recognized, dot does not match CR or LF or any of the other line ending
777    characters.
778    </P>
779    <P>
780    The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
781    option is set, a dot matches any one character, without exception. If the
782    two-character sequence CRLF is present in the subject string, it takes two dots
783    to match it.
784    </P>
785    <P>
786    The handling of dot is entirely independent of the handling of circumflex and
787    dollar, the only relationship being that they both involve newlines. Dot has no
788    special meaning in a character class.
789  </P>  </P>
790    <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
791  <P>  <P>
792  PCRE does not allow \C to appear in lookbehind assertions (see below), because  Outside a character class, the escape sequence \C matches any one byte, both
793  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
794    characters. The feature is provided in Perl in order to match individual bytes
795    in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
796    what remains in the string may be a malformed UTF-8 string. For this reason,
797    the \C escape sequence is best avoided.
798  </P>  </P>
799  <br><a name="SEC6" href="#TOC1">SQUARE BRACKETS</a><br>  <P>
800    PCRE does not allow \C to appear in lookbehind assertions
801    <a href="#lookbehind">(described below),</a>
802    because in UTF-8 mode this would make it impossible to calculate the length of
803    the lookbehind.
804    <a name="characterclass"></a></P>
805    <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
806  <P>  <P>
807  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
808  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. If a
# Line 438  backslash. Line 822  backslash.
822  <P>  <P>
823  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
824  [^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
825  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
826  are in the class by enumerating those that are not. It is not an assertion: it  are in the class by enumerating those that are not. A class that starts with a
827  still consumes a character from the subject string, and fails if the current  circumflex is not an assertion: it still consumes a character from the subject
828  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
829    string.
830  </P>  </P>
831  <P>  <P>
832  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 mode, characters with values greater than 255 can be included in a
# Line 451  class as a literal string of bytes, or b Line 836  class as a literal string of bytes, or b
836  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
837  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
838  "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
839  caseful version would. PCRE does not support the concept of case for characters  caseful version would. In UTF-8 mode, PCRE always understands the concept of
840  with values greater than 255.  case for characters whose values are less than 128, so caseless matching is
841    always possible. For characters with higher values, the concept of case is
842    supported if PCRE is compiled with Unicode property support, but not otherwise.
843    If you want to use caseless matching for characters 128 and above, you must
844    ensure that PCRE is compiled with Unicode property support as well as with
845    UTF-8 support.
846  </P>  </P>
847  <P>  <P>
848  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
849  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
850  such as [^a] will always match a newline.  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
851    such as [^a] always matches one of these characters.
852  </P>  </P>
853  <P>  <P>
854  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 471  It is not possible to have the literal c Line 862  It is not possible to have the literal c
862  range. A pattern such as [W-]46] is interpreted as a class of two characters  range. A pattern such as [W-]46] is interpreted as a class of two characters
863  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
864  "-46]". However, if the "]" is escaped with a backslash it is interpreted as  "-46]". However, if the "]" is escaped with a backslash it is interpreted as
865  the end of range, so [W-\]46] is interpreted as a single class containing a  the end of range, so [W-\]46] is interpreted as a class containing a range
866  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
867  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
868  </P>  </P>
869  <P>  <P>
870  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
# Line 484  example [\x{100}-\x{2ff}]. Line 875  example [\x{100}-\x{2ff}].
875  <P>  <P>
876  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
877  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
878  [][\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
879  locale are in use, [\xc8-\xcb] matches accented E characters in both cases.  tables for a French locale are in use, [\xc8-\xcb] matches accented E
880    characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
881    characters with values greater than 128 only when it is compiled with Unicode
882    property support.
883  </P>  </P>
884  <P>  <P>
885  The character types \d, \D, \s, \S, \w, and \W may also appear in a  The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
886  character class, and add the characters that they match to the class. For  in a character class, and add the characters that they match to the class. For
887  example, [\dABCDEF] matches any hexadecimal digit. A circumflex can  example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
888  conveniently be used with the upper case character types to specify a more  conveniently be used with the upper case character types to specify a more
889  restricted set of characters than the matching lower case type. For example,  restricted set of characters than the matching lower case type. For example,
890  the class [^\W_] matches any letter or digit, but not underscore.  the class [^\W_] matches any letter or digit, but not underscore.
891  </P>  </P>
892  <P>  <P>
893  All non-alphameric characters other than \, -, ^ (at the start) and the  The only metacharacters that are recognized in character classes are backslash,
894  terminating ] are non-special in character classes, but it does no harm if they  hyphen (only where it can be interpreted as specifying a range), circumflex
895  are escaped.  (only at the start), opening square bracket (only when it can be interpreted as
896    introducing a POSIX class name - see the next section), and the terminating
897    closing square bracket. However, escaping other non-alphanumeric characters
898    does no harm.
899  </P>  </P>
900  <br><a name="SEC7" href="#TOC1">POSIX CHARACTER CLASSES</a><br>  <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
901  <P>  <P>
902  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
903  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
904  this notation. For example,  this notation. For example,
 </P>  
 <P>  
905  <pre>  <pre>
906    [01[:alpha:]%]    [01[:alpha:]%]
907  </PRE>  </pre>
 </P>  
 <P>  
908  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
909  are  are
 </P>  
 <P>  
910  <pre>  <pre>
911    alnum    letters and digits    alnum    letters and digits
912    alpha    letters    alpha    letters
# Line 531  are Line 922  are
922    upper    upper case letters    upper    upper case letters
923    word     "word" characters (same as \w)    word     "word" characters (same as \w)
924    xdigit   hexadecimal digits    xdigit   hexadecimal digits
925  </PRE>  </pre>
 </P>  
 <P>  
926  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
927  space (32). Notice that this list includes the VT character (code 11). This  space (32). Notice that this list includes the VT character (code 11). This
928  makes "space" different to \s, which does not include VT (for Perl  makes "space" different to \s, which does not include VT (for Perl
# Line 543  compatibility). Line 932  compatibility).
932  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
933  5.8. Another Perl extension is negation, which is indicated by a ^ character  5.8. Another Perl extension is negation, which is indicated by a ^ character
934  after the colon. For example,  after the colon. For example,
 </P>  
 <P>  
935  <pre>  <pre>
936    [12[:^digit:]]    [12[:^digit:]]
937  </PRE>  </pre>
 </P>  
 <P>  
938  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
939  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
940  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
941  </P>  </P>
942  <P>  <P>
943  In UTF-8 mode, characters with values greater than 255 do not match any of  In UTF-8 mode, characters with values greater than 128 do not match any of
944  the POSIX character classes.  the POSIX character classes.
945  </P>  </P>
946  <br><a name="SEC8" href="#TOC1">VERTICAL BAR</a><br>  <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
947  <P>  <P>
948  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
949  the pattern  the pattern
 </P>  
 <P>  
950  <pre>  <pre>
951    gilbert|sullivan    gilbert|sullivan
952  </PRE>  </pre>
 </P>  
 <P>  
953  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
954  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
955  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
956  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
957  subpattern (defined below), "succeeds" means matching the rest of the main  <a href="#subpattern">(defined below),</a>
958  pattern as well as the alternative in the subpattern.  "succeeds" means matching the rest of the main pattern as well as the
959    alternative in the subpattern.
960  </P>  </P>
961  <br><a name="SEC9" href="#TOC1">INTERNAL OPTION SETTING</a><br>  <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
962  <P>  <P>
963  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
964  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options can be changed from within the pattern by a sequence of
965  Perl option letters enclosed between "(?" and ")". The option letters are  Perl option letters enclosed between "(?" and ")". The option letters are
 </P>  
 <P>  
966  <pre>  <pre>
967    i  for PCRE_CASELESS    i  for PCRE_CASELESS
968    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
969    s  for PCRE_DOTALL    s  for PCRE_DOTALL
970    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
971  </PRE>  </pre>
 </P>  
 <P>  
972  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
973  unset these options by preceding the letter with a hyphen, and a combined  unset these options by preceding the letter with a hyphen, and a combined
974  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
# Line 606  the global options (and it will therefor Line 984  the global options (and it will therefor
984  <b>pcre_fullinfo()</b> function).  <b>pcre_fullinfo()</b> function).
985  </P>  </P>
986  <P>  <P>
987  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
988  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
 </P>  
 <P>  
989  <pre>  <pre>
990    (a(?i)b)c    (a(?i)b)c
991  </PRE>  </pre>
 </P>  
 <P>  
992  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
993  By this means, options can be made to have different settings in different  By this means, options can be made to have different settings in different
994  parts of the pattern. Any changes made in one alternative do carry on  parts of the pattern. Any changes made in one alternative do carry on
995  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
 </P>  
 <P>  
996  <pre>  <pre>
997    (a(?i)b|c)    (a(?i)b|c)
998  </PRE>  </pre>
 </P>  
 <P>  
999  matches "ab", "aB", "c", and "C", even though when matching "C" the first  matches "ab", "aB", "c", and "C", even though when matching "C" the first
1000  branch is abandoned before the option setting. This is because the effects of  branch is abandoned before the option setting. This is because the effects of
1001  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1002  behaviour otherwise.  behaviour otherwise.
1003  </P>  </P>
1004  <P>  <P>
1005  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1006  same way as the Perl-compatible options by using the characters U and X  changed in the same way as the Perl-compatible options by using the characters
1007  respectively. The (?X) flag setting is special in that it must always occur  J, U and X respectively.
1008  earlier in the pattern than any of the additional features it turns on, even  <a name="subpattern"></a></P>
1009  when it is at top level. It is best put at the start.  <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
 </P>  
 <br><a name="SEC10" href="#TOC1">SUBPATTERNS</a><br>  
1010  <P>  <P>
1011  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
1012  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
1013  </P>  <br>
1014  <P>  <br>
1015  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
 </P>  
 <P>  
1016  <pre>  <pre>
1017    cat(aract|erpillar|)    cat(aract|erpillar|)
1018  </PRE>  </pre>
 </P>  
 <P>  
1019  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1020  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1021  </P>  <br>
1022  <P>  <br>
1023  2. It sets up the subpattern as a capturing subpattern (as defined above).  2. It sets up the subpattern as a capturing subpattern. This means that, when
1024  When the whole pattern matches, that portion of the subject string that matched  the whole pattern matches, that portion of the subject string that matched the
1025  the 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
1026  <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting  <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
1027  from 1) to obtain the numbers of the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns.
1028  </P>  </P>
1029  <P>  <P>
1030  For example, if the string "the red king" is matched against the pattern  For example, if the string "the red king" is matched against the pattern
 </P>  
 <P>  
1031  <pre>  <pre>
1032    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1033  </PRE>  </pre>
 </P>  
 <P>  
1034  the captured substrings are "red king", "red", and "king", and are numbered 1,  the captured substrings are "red king", "red", and "king", and are numbered 1,
1035  2, and 3, respectively.  2, and 3, respectively.
1036  </P>  </P>
# Line 681  capturing requirement. If an opening par Line 1041  capturing requirement. If an opening par
1041  and a colon, the subpattern does not do any capturing, and is not counted when  and a colon, the subpattern does not do any capturing, and is not counted when
1042  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
1043  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
 </P>  
 <P>  
1044  <pre>  <pre>
1045    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1046  </PRE>  </pre>
 </P>  
 <P>  
1047  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
1048  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.  
1049  </P>  </P>
1050  <P>  <P>
1051  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
1052  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
1053  the ":". Thus the two patterns  the ":". Thus the two patterns
 </P>  
 <P>  
1054  <pre>  <pre>
1055    (?i:saturday|sunday)    (?i:saturday|sunday)
1056    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
1057  </PRE>  </pre>
 </P>  
 <P>  
1058  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
1059  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
1060  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
1061  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1062  </P>  </P>
1063  <br><a name="SEC11" href="#TOC1">NAMED SUBPATTERNS</a><br>  <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1064  <P>  <P>
1065  Identifying capturing parentheses by number is simple, but it can be very hard  Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1066  to keep track of the numbers in complicated regular expressions. Furthermore,  the same numbers for its capturing parentheses. Such a subpattern starts with
1067  if an expression is modified, the numbers may change. To help with the  (?| and is itself a non-capturing subpattern. For example, consider this
1068  difficulty, PCRE supports the naming of subpatterns, something that Perl does  pattern:
1069  not provide. The Python syntax (?P&#60;name&#62;...) is used. Names consist of  <pre>
1070  alphanumeric characters and underscores, and must be unique within a pattern.    (?|(Sat)ur|(Sun))day
1071    </pre>
1072    Because the two alternatives are inside a (?| group, both sets of capturing
1073    parentheses are numbered one. Thus, when the pattern matches, you can look
1074    at captured substring number one, whichever alternative matched. This construct
1075    is useful when you want to capture part, but not all, of one of a number of
1076    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1077    number is reset at the start of each branch. The numbers of any capturing
1078    buffers that follow the subpattern start after the highest number used in any
1079    branch. The following example is taken from the Perl documentation.
1080    The numbers underneath show in which buffer the captured content will be
1081    stored.
1082    <pre>
1083      # before  ---------------branch-reset----------- after
1084      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1085      # 1            2         2  3        2     3     4
1086    </pre>
1087    A backreference or a recursive call to a numbered subpattern always refers to
1088    the first one in the pattern with the given number.
1089    </P>
1090    <P>
1091    An alternative approach to using this "branch reset" feature is to use
1092    duplicate named subpatterns, as described in the next section.
1093  </P>  </P>
1094    <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
1095  <P>  <P>
1096  Named capturing parentheses are still allocated numbers as well as names. The  Identifying capturing parentheses by number is simple, but it can be very hard
1097  PCRE API provides function calls for extracting the name-to-number translation  to keep track of the numbers in complicated regular expressions. Furthermore,
1098  table from a compiled pattern. For further details see the  if an expression is modified, the numbers may change. To help with this
1099    difficulty, PCRE supports the naming of subpatterns. This feature was not
1100    added to Perl until release 5.10. Python had the feature earlier, and PCRE
1101    introduced it at release 4.0, using the Python syntax. PCRE now supports both
1102    the Perl and the Python syntax.
1103    </P>
1104    <P>
1105    In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1106    (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1107    parentheses from other parts of the pattern, such as
1108    <a href="#backreferences">backreferences,</a>
1109    <a href="#recursion">recursion,</a>
1110    and
1111    <a href="#conditions">conditions,</a>
1112    can be made by name as well as by number.
1113    </P>
1114    <P>
1115    Names consist of up to 32 alphanumeric characters and underscores. Named
1116    capturing parentheses are still allocated numbers as well as names, exactly as
1117    if the names were not present. The PCRE API provides function calls for
1118    extracting the name-to-number translation table from a compiled pattern. There
1119    is also a convenience function for extracting a captured substring by name.
1120    </P>
1121    <P>
1122    By default, a name must be unique within a pattern, but it is possible to relax
1123    this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1124    be useful for patterns where only one instance of the named parentheses can
1125    match. Suppose you want to match the name of a weekday, either as a 3-letter
1126    abbreviation or as the full name, and in both cases you want to extract the
1127    abbreviation. This pattern (ignoring the line breaks) does the job:
1128    <pre>
1129      (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1130      (?&#60;DN&#62;Tue)(?:sday)?|
1131      (?&#60;DN&#62;Wed)(?:nesday)?|
1132      (?&#60;DN&#62;Thu)(?:rsday)?|
1133      (?&#60;DN&#62;Sat)(?:urday)?
1134    </pre>
1135    There are five capturing substrings, but only one is ever set after a match.
1136    (An alternative way of solving this problem is to use a "branch reset"
1137    subpattern, as described in the previous section.)
1138    </P>
1139    <P>
1140    The convenience function for extracting the data by name returns the substring
1141    for the first (and in this example, the only) subpattern of that name that
1142    matched. This saves searching to find which numbered subpattern it was. If you
1143    make a reference to a non-unique named subpattern from elsewhere in the
1144    pattern, the one that corresponds to the lowest number is used. For further
1145    details of the interfaces for handling named subpatterns, see the
1146  <a href="pcreapi.html"><b>pcreapi</b></a>  <a href="pcreapi.html"><b>pcreapi</b></a>
1147  documentation.  documentation.
1148  </P>  </P>
1149  <br><a name="SEC12" href="#TOC1">REPETITION</a><br>  <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1150  <P>  <P>
1151  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
1152  items:  items:
 </P>  
 <P>  
1153  <pre>  <pre>
1154    a literal data character    a literal data character
1155    the . metacharacter    the dot metacharacter
1156    the \C escape sequence    the \C escape sequence
1157    escapes such as \d that match single characters    the \X escape sequence (in UTF-8 mode with Unicode properties)
1158      the \R escape sequence
1159      an escape such as \d that matches a single character
1160    a character class    a character class
1161    a back reference (see next section)    a back reference (see next section)
1162    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1163  </PRE>  </pre>
 </P>  
 <P>  
1164  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1165  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
1166  separated by a comma. The numbers must be less than 65536, and the first must  separated by a comma. The numbers must be less than 65536, and the first must
1167  be less than or equal to the second. For example:  be less than or equal to the second. For example:
 </P>  
 <P>  
1168  <pre>  <pre>
1169    z{2,4}    z{2,4}
1170  </PRE>  </pre>
 </P>  
 <P>  
1171  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1172  character. If the second number is omitted, but the comma is present, there is  character. If the second number is omitted, but the comma is present, there is
1173  no upper limit; if the second number and the comma are both omitted, the  no upper limit; if the second number and the comma are both omitted, the
1174  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
 </P>  
 <P>  
1175  <pre>  <pre>
1176    [aeiou]{3,}    [aeiou]{3,}
1177  </PRE>  </pre>
 </P>  
 <P>  
1178  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
 </P>  
 <P>  
1179  <pre>  <pre>
1180    \d{8}    \d{8}
1181  </PRE>  </pre>
 </P>  
 <P>  
1182  matches exactly 8 digits. An opening curly bracket that appears in a position  matches exactly 8 digits. An opening curly bracket that appears in a position
1183  where a quantifier is not allowed, or one that does not match the syntax of a  where a quantifier is not allowed, or one that does not match the syntax of a
1184  quantifier, is taken as a literal character. For example, {,6} is not a  quantifier, is taken as a literal character. For example, {,6} is not a
# Line 780  quantifier, but a literal string of four Line 1187  quantifier, but a literal string of four
1187  <P>  <P>
1188  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1189  bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of  bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
1190  which is represented by a two-byte sequence.  which is represented by a two-byte sequence. Similarly, when Unicode property
1191    support is available, \X{3} matches three Unicode extended sequences, each of
1192    which may be several bytes long (and they may be of different lengths).
1193  </P>  </P>
1194  <P>  <P>
1195  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
1196  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1197  </P>  </P>
1198  <P>  <P>
1199  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1200  quantifiers have single-character abbreviations:  abbreviations:
 </P>  
 <P>  
1201  <pre>  <pre>
1202    *    is equivalent to {0,}    *    is equivalent to {0,}
1203    +    is equivalent to {1,}    +    is equivalent to {1,}
1204    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
1205  </PRE>  </pre>
 </P>  
 <P>  
1206  It is possible to construct infinite loops by following a subpattern that can  It is possible to construct infinite loops by following a subpattern that can
1207  match no characters with a quantifier that has no upper limit, for example:  match no characters with a quantifier that has no upper limit, for example:
 </P>  
 <P>  
1208  <pre>  <pre>
1209    (a?)*    (a?)*
1210  </PRE>  </pre>
 </P>  
 <P>  
1211  Earlier versions of Perl and PCRE used to give an error at compile time for  Earlier versions of Perl and PCRE used to give an error at compile time for
1212  such patterns. However, because there are cases where this can be useful, such  such patterns. However, because there are cases where this can be useful, such
1213  patterns are now accepted, but if any repetition of the subpattern does in fact  patterns are now accepted, but if any repetition of the subpattern does in fact
# Line 816  match no characters, the loop is forcibl Line 1217  match no characters, the loop is forcibl
1217  By default, the quantifiers are "greedy", that is, they match as much as  By default, the quantifiers are "greedy", that is, they match as much as
1218  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
1219  rest of the pattern to fail. The classic example of where this gives problems  rest of the pattern to fail. The classic example of where this gives problems
1220  is in trying to match comments in C programs. These appear between the  is in trying to match comments in C programs. These appear between /* and */
1221  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
1222  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
 </P>  
 <P>  
1223  <pre>  <pre>
1224    /\*.*\*/    /\*.*\*/
1225  </PRE>  </pre>
 </P>  
 <P>  
1226  to the string  to the string
 </P>  
 <P>  
1227  <pre>  <pre>
1228    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
1229  </PRE>  </pre>
 </P>  
 <P>  
1230  fails, because it matches the entire string owing to the greediness of the .*  fails, because it matches the entire string owing to the greediness of the .*
1231  item.  item.
1232  </P>  </P>
# Line 841  item. Line 1234  item.
1234  However, if a quantifier is followed by a question mark, it ceases to be  However, if a quantifier is followed by a question mark, it ceases to be
1235  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
1236  pattern  pattern
 </P>  
 <P>  
1237  <pre>  <pre>
1238    /\*.*?\*/    /\*.*?\*/
1239  </PRE>  </pre>
 </P>  
 <P>  
1240  does the right thing with the C comments. The meaning of the various  does the right thing with the C comments. The meaning of the various
1241  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
1242  Do not confuse this use of question mark with its use as a quantifier in its  Do not confuse this use of question mark with its use as a quantifier in its
1243  own right. Because it has two uses, it can sometimes appear doubled, as in  own right. Because it has two uses, it can sometimes appear doubled, as in
 </P>  
 <P>  
1244  <pre>  <pre>
1245    \d??\d    \d??\d
1246  </PRE>  </pre>
 </P>  
 <P>  
1247  which matches one digit by preference, but can match two if that is the only  which matches one digit by preference, but can match two if that is the only
1248  way the rest of the pattern matches.  way the rest of the pattern matches.
1249  </P>  </P>
1250  <P>  <P>
1251  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),
1252  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
1253  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
1254  default behaviour.  default behaviour.
1255  </P>  </P>
1256  <P>  <P>
1257  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
1258  is greater than 1 or with a limited maximum, more store is required for the  is greater than 1 or with a limited maximum, more memory is required for the
1259  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1260  </P>  </P>
1261  <P>  <P>
1262  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
1263  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
1264  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1265  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
1266  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 889  alternatively using ^ to indicate anchor Line 1274  alternatively using ^ to indicate anchor
1274  <P>  <P>
1275  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1276  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1277  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
1278  succeed. Consider, for example:  succeeds. Consider, for example:
 </P>  
 <P>  
1279  <pre>  <pre>
1280    (.*)abc\1    (.*)abc\1
1281  </PRE>  </pre>
 </P>  
 <P>  
1282  If the subject is "xyz123abc123" the match point is the fourth character. For  If the subject is "xyz123abc123" the match point is the fourth character. For
1283  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1284  </P>  </P>
1285  <P>  <P>
1286  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1287  that matched the final iteration. For example, after  that matched the final iteration. For example, after
 </P>  
 <P>  
1288  <pre>  <pre>
1289    (tweedle[dume]{3}\s*)+    (tweedle[dume]{3}\s*)+
1290  </PRE>  </pre>
 </P>  
 <P>  
1291  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
1292  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
1293  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
1294  example, after  example, after
 </P>  
 <P>  
1295  <pre>  <pre>
1296    /(a|(b))+/    /(a|(b))+/
1297  </PRE>  </pre>
 </P>  
 <P>  
1298  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1299  </P>  <a name="atomicgroup"></a></P>
1300  <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>
1301  <P>  <P>
1302  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1303  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
1304  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
1305  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
1306  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
1307  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1308  </P>  </P>
1309  <P>  <P>
1310  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
 </P>  
 <P>  
1311  <pre>  <pre>
1312    123456bar    123456bar
1313  </PRE>  </pre>
 </P>  
 <P>  
1314  After matching all 6 digits and then failing to match "foo", the normal  After matching all 6 digits and then failing to match "foo", the normal
1315  action of the matcher is to try again with only 5 digits matching the \d+  action of the matcher is to try again with only 5 digits matching the \d+
1316  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
# Line 949  item, and then with 4, and so on, before Line 1318  item, and then with 4, and so on, before
1318  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.
1319  </P>  </P>
1320  <P>  <P>
1321  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
1322  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
1323  special parenthesis, starting with (?&#62; as in this example:  special parenthesis, starting with (?&#62; as in this example:
 </P>  
 <P>  
1324  <pre>  <pre>
1325    (?&#62;\d+)bar    (?&#62;\d+)foo
1326  </PRE>  </pre>
 </P>  
 <P>  
1327  This kind of parenthesis "locks up" the  part of the pattern it contains once  This kind of parenthesis "locks up" the  part of the pattern it contains once
1328  it has matched, and a failure further into the pattern is prevented from  it has matched, and a failure further into the pattern is prevented from
1329  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
# Line 983  group is just a single repeated item, as Line 1348  group is just a single repeated item, as
1348  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
1349  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
1350  previous example can be rewritten as  previous example can be rewritten as
 </P>  
 <P>  
1351  <pre>  <pre>
1352    \d++bar    \d++foo
1353  </PRE>  </pre>
1354  </P>  Note that a possessive quantifier can be used with an entire group, for
1355  <P>  example:
1356    <pre>
1357      (abc|xyz){2,3}+
1358    </pre>
1359  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1360  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
1361  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
1362  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1363    difference; possessive quantifiers should be slightly faster.
1364    </P>
1365    <P>
1366    The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1367    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1368    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1369    package, and PCRE copied it from there. It ultimately found its way into Perl
1370    at release 5.10.
1371  </P>  </P>
1372  <P>  <P>
1373  The possessive quantifier syntax is an extension to the Perl syntax. It  PCRE has an optimization that automatically "possessifies" certain simple
1374  originates in Sun's Java package.  pattern constructs. For example, the sequence A+B is treated as A++B because
1375    there is no point in backtracking into a sequence of A's when B must follow.
1376  </P>  </P>
1377  <P>  <P>
1378  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
1379  be repeated an unlimited number of times, the use of an atomic group is the  be repeated an unlimited number of times, the use of an atomic group is the
1380  only way to avoid some failing matches taking a very long time indeed. The  only way to avoid some failing matches taking a very long time indeed. The
1381  pattern  pattern
 </P>  
 <P>  
1382  <pre>  <pre>
1383    (\D+|&#60;\d+&#62;)*[!?]    (\D+|&#60;\d+&#62;)*[!?]
1384  </PRE>  </pre>
 </P>  
 <P>  
1385  matches an unlimited number of substrings that either consist of non-digits, or  matches an unlimited number of substrings that either consist of non-digits, or
1386  digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs  digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1387  quickly. However, if it is applied to  quickly. However, if it is applied to
 </P>  
 <P>  
1388  <pre>  <pre>
1389    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1390  </PRE>  </pre>
 </P>  
 <P>  
1391  it takes a long time before reporting failure. This is because the string can  it takes a long time before reporting failure. This is because the string can
1392  be divided between the two repeats in a large number of ways, and all have to  be divided between the internal \D+ repeat and the external * repeat in a
1393  be tried. (The example used [!?] rather than a single character at the end,  large number of ways, and all have to be tried. (The example uses [!?] rather
1394  because both PCRE and Perl have an optimization that allows for fast failure  than a single character at the end, because both PCRE and Perl have an
1395  when a single character is used. They remember the last single character that  optimization that allows for fast failure when a single character is used. They
1396  is required for a match, and fail early if it is not present in the string.)  remember the last single character that is required for a match, and fail early
1397  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1398  </P>  an atomic group, like this:
 <P>  
1399  <pre>  <pre>
1400    ((?&#62;\D+)|&#60;\d+&#62;)*[!?]    ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
1401  </PRE>  </pre>
 </P>  
 <P>  
1402  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1403  </P>  <a name="backreferences"></a></P>
1404  <br><a name="SEC14" href="#TOC1">BACK REFERENCES</a><br>  <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
1405  <P>  <P>
1406  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
1407  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 1049  However, if the decimal number following Line 1413  However, if the decimal number following
1413  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
1414  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
1415  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
1416  numbers less than 10. See the section entitled "Backslash" above for further  numbers less than 10. A "forward back reference" of this type can make sense
1417  details of the handling of digits following a backslash.  when a repetition is involved and the subpattern to the right has participated
1418    in an earlier iteration.
1419    </P>
1420    <P>
1421    It is not possible to have a numerical "forward back reference" to a subpattern
1422    whose number is 10 or more using this syntax because a sequence such as \50 is
1423    interpreted as a character defined in octal. See the subsection entitled
1424    "Non-printing characters"
1425    <a href="#digitsafterbackslash">above</a>
1426    for further details of the handling of digits following a backslash. There is
1427    no such problem when named parentheses are used. A back reference to any
1428    subpattern is possible using named parentheses (see below).
1429    </P>
1430    <P>
1431    Another way of avoiding the ambiguity inherent in the use of digits following a
1432    backslash is to use the \g escape sequence, which is a feature introduced in
1433    Perl 5.10. This escape must be followed by an unsigned number or a negative
1434    number, optionally enclosed in braces. These examples are all identical:
1435    <pre>
1436      (ring), \1
1437      (ring), \g1
1438      (ring), \g{1}
1439    </pre>
1440    An unsigned number specifies an absolute reference without the ambiguity that
1441    is present in the older syntax. It is also useful when literal digits follow
1442    the reference. A negative number is a relative reference. Consider this
1443    example:
1444    <pre>
1445      (abc(def)ghi)\g{-1}
1446    </pre>
1447    The sequence \g{-1} is a reference to the most recently started capturing
1448    subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2}
1449    would be equivalent to \1. The use of relative references can be helpful in
1450    long patterns, and also in patterns that are created by joining together
1451    fragments that contain references within themselves.
1452  </P>  </P>
1453  <P>  <P>
1454  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
# Line 1058  the current subject string, rather than Line 1456  the current subject string, rather than
1456  itself (see  itself (see
1457  <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>  <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1458  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
 </P>  
 <P>  
1459  <pre>  <pre>
1460    (sens|respons)e and \1ibility    (sens|respons)e and \1ibility
1461  </PRE>  </pre>
 </P>  
 <P>  
1462  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1463  "sense and responsibility". If caseful matching is in force at the time of the  "sense and responsibility". If caseful matching is in force at the time of the
1464  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
 </P>  
 <P>  
1465  <pre>  <pre>
1466    ((?i)rah)\s+\1    ((?i)rah)\s+\1
1467  </PRE>  </pre>
 </P>  
 <P>  
1468  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1469  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1470  </P>  </P>
1471  <P>  <P>
1472  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1473  rewrite the above example as follows:  subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
1474  </P>  \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1475  <P>  back reference syntax, in which \g can be used for both numeric and named
1476  <pre>  references, is also supported. We could rewrite the above example in any of
1477    (?&#60;p1&#62;(?i)rah)\s+(?P=p1)  the following ways:
1478  </PRE>  <pre>
1479      (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
1480      (?'p1'(?i)rah)\s+\k{p1}
1481      (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
1482      (?&#60;p1&#62;(?i)rah)\s+\g{p1}
1483    </pre>
1484    A subpattern that is referenced by name may appear in the pattern before or
1485    after the reference.
1486  </P>  </P>
1487  <P>  <P>
1488  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
1489  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1490  references to it always fail. For example, the pattern  references to it always fail. For example, the pattern
 </P>  
 <P>  
1491  <pre>  <pre>
1492    (a|(bc))\2    (a|(bc))\2
1493  </PRE>  </pre>
 </P>  
 <P>  
1494  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". Because there may be
1495  many capturing parentheses in a pattern, all digits following the backslash are  many capturing parentheses in a pattern, all digits following the backslash are
1496  taken as part of a potential back reference number. If the pattern continues  taken as part of a potential back reference number. If the pattern continues
1497  with a digit character, some delimiter must be used to terminate the back  with a digit character, some delimiter must be used to terminate the back
1498  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1499  Otherwise an empty comment can be used.  Otherwise an empty comment (see
1500    <a href="#comments">"Comments"</a>
1501    below) can be used.
1502  </P>  </P>
1503  <P>  <P>
1504  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
1505  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.
1506  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1507  example, the pattern  example, the pattern
 </P>  
 <P>  
1508  <pre>  <pre>
1509    (a|b\1)+    (a|b\1)+
1510  </PRE>  </pre>
 </P>  
 <P>  
1511  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1512  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1513  to the previous iteration. In order for this to work, the pattern must be such  to the previous iteration. In order for this to work, the pattern must be such
1514  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
1515  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
1516  minimum of zero.  minimum of zero.
1517  </P>  <a name="bigassertions"></a></P>
1518  <br><a name="SEC15" href="#TOC1">ASSERTIONS</a><br>  <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1519  <P>  <P>
1520  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
1521  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1522  assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described above.  assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1523    <a href="#smallassertions">above.</a>
1524    </P>
1525    <P>
1526  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1527  those that look ahead of the current position in the subject string, and those  those that look ahead of the current position in the subject string, and those
1528  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1529    except that it does not cause the current matching position to be changed.
1530  </P>  </P>
1531  <P>  <P>
1532  An assertion subpattern is matched in the normal way, except that it does not  Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1533  cause the current matching position to be changed. Lookahead assertions start  because it makes no sense to assert the same thing several times. If any kind
1534  with (?= for positive assertions and (?! for negative assertions. For example,  of assertion contains capturing subpatterns within it, these are counted for
1535    the purposes of numbering the capturing subpatterns in the whole pattern.
1536    However, substring capturing is carried out only for positive assertions,
1537    because it does not make sense for negative assertions.
1538  </P>  </P>
1539    <br><b>
1540    Lookahead assertions
1541    </b><br>
1542  <P>  <P>
1543    Lookahead assertions start with (?= for positive assertions and (?! for
1544    negative assertions. For example,
1545  <pre>  <pre>
1546    \w+(?=;)    \w+(?=;)
1547  </PRE>  </pre>
 </P>  
 <P>  
1548  matches a word followed by a semicolon, but does not include the semicolon in  matches a word followed by a semicolon, but does not include the semicolon in
1549  the match, and  the match, and
 </P>  
 <P>  
1550  <pre>  <pre>
1551    foo(?!bar)    foo(?!bar)
1552  </PRE>  </pre>
 </P>  
 <P>  
1553  matches any occurrence of "foo" that is not followed by "bar". Note that the  matches any occurrence of "foo" that is not followed by "bar". Note that the
1554  apparently similar pattern  apparently similar pattern
 </P>  
 <P>  
1555  <pre>  <pre>
1556    (?!foo)bar    (?!foo)bar
1557  </PRE>  </pre>
 </P>  
 <P>  
1558  does not find an occurrence of "bar" that is preceded by something other than  does not find an occurrence of "bar" that is preceded by something other than
1559  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1560  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1561  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1562  </P>  </P>
1563  <P>  <P>
1564  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
1565  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
1566  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.
1567  </P>  <a name="lookbehind"></a></P>
1568    <br><b>
1569    Lookbehind assertions
1570    </b><br>
1571  <P>  <P>
1572  Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for  Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
1573  negative assertions. For example,  negative assertions. For example,
 </P>  
 <P>  
1574  <pre>  <pre>
1575    (?&#60;!foo)bar    (?&#60;!foo)bar
1576  </PRE>  </pre>
 </P>  
 <P>  
1577  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
1578  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
1579  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
1580  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
 </P>  
 <P>  
1581  <pre>  <pre>
1582    (?&#60;=bullock|donkey)    (?&#60;=bullock|donkey)
1583  </PRE>  </pre>
 </P>  
 <P>  
1584  is permitted, but  is permitted, but
 </P>  
 <P>  
1585  <pre>  <pre>
1586    (?&#60;!dogs?|cats?)    (?&#60;!dogs?|cats?)
1587  </PRE>  </pre>
 </P>  
 <P>  
1588  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1589  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
1590  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (at least for 5.8), which requires all branches to
1591  match the same length of string. An assertion such as  match the same length of string. An assertion such as
 </P>  
 <P>  
1592  <pre>  <pre>
1593    (?&#60;=ab(c|de))    (?&#60;=ab(c|de))
1594  </PRE>  </pre>
 </P>  
 <P>  
1595  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
1596  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable if rewritten to use two top-level branches:
 </P>  
 <P>  
1597  <pre>  <pre>
1598    (?&#60;=abc|abde)    (?&#60;=abc|abde)
1599  </PRE>  </pre>
1600    In some cases, the Perl 5.10 escape sequence \K
1601    <a href="#resetmatchstart">(see above)</a>
1602    can be used instead of a lookbehind assertion; this is not restricted to a
1603    fixed-length.
1604  </P>  </P>
1605  <P>  <P>
1606  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1607  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
1608  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1609  match is deemed to fail.  assertion fails.
1610  </P>  </P>
1611  <P>  <P>
1612  PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)  PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
1613  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1614  the length of the lookbehind.  the length of the lookbehind. The \X and \R escapes, which can match
1615  </P>  different numbers of bytes, are also not permitted.
 <P>  
 Atomic groups can be used in conjunction with lookbehind assertions to specify  
 efficient matching at the end of the subject string. Consider a simple pattern  
 such as  
1616  </P>  </P>
1617  <P>  <P>
1618    Possessive quantifiers can be used in conjunction with lookbehind assertions to
1619    specify efficient matching at the end of the subject string. Consider a simple
1620    pattern such as
1621  <pre>  <pre>
1622    abcd$    abcd$
1623  </PRE>  </pre>
 </P>  
 <P>  
1624  when applied to a long string that does not match. Because matching proceeds  when applied to a long string that does not match. Because matching proceeds
1625  from left to right, PCRE will look for each "a" in the subject and then see if  from left to right, PCRE will look for each "a" in the subject and then see if
1626  what follows matches the rest of the pattern. If the pattern is specified as  what follows matches the rest of the pattern. If the pattern is specified as
 </P>  
 <P>  
1627  <pre>  <pre>
1628    ^.*abcd$    ^.*abcd$
1629  </PRE>  </pre>
 </P>  
 <P>  
1630  the initial .* matches the entire string at first, but when this fails (because  the initial .* matches the entire string at first, but when this fails (because
1631  there is no following "a"), it backtracks to match all but the last character,  there is no following "a"), it backtracks to match all but the last character,
1632  then all but the last two characters, and so on. Once again the search for "a"  then all but the last two characters, and so on. Once again the search for "a"
1633  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,
1634  if the pattern is written as  if the pattern is written as
 </P>  
 <P>  
 <pre>  
   ^(?&#62;.*)(?&#60;=abcd)  
 </PRE>  
 </P>  
 <P>  
 or, equivalently,  
 </P>  
 <P>  
1635  <pre>  <pre>
1636    ^.*+(?&#60;=abcd)    ^.*+(?&#60;=abcd)
1637  </PRE>  </pre>
1638  </P>  there can be no backtracking for the .*+ item; it can match only the entire
 <P>  
 there can be no backtracking for the .* item; it can match only the entire  
1639  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
1640  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1641  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
1642  </P>  </P>
1643    <br><b>
1644    Using multiple assertions
1645    </b><br>
1646  <P>  <P>
1647  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
 </P>  
 <P>  
1648  <pre>  <pre>
1649    (?&#60;=\d{3})(?&#60;!999)foo    (?&#60;=\d{3})(?&#60;!999)foo
1650  </PRE>  </pre>
 </P>  
 <P>  
1651  matches "foo" preceded by three digits that are not "999". Notice that each of  matches "foo" preceded by three digits that are not "999". Notice that each of
1652  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
1653  string. First there is a check that the previous three characters are all  string. First there is a check that the previous three characters are all
# Line 1293  digits, and then there is a check that t Line 1655  digits, and then there is a check that t
1655  This pattern does <i>not</i> match "foo" preceded by six characters, the first  This pattern does <i>not</i> match "foo" preceded by six characters, the first
1656  of which are digits and the last three of which are not "999". For example, it  of which are digits and the last three of which are not "999". For example, it
1657  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
 </P>  
 <P>  
1658  <pre>  <pre>
1659    (?&#60;=\d{3}...)(?&#60;!999)foo    (?&#60;=\d{3}...)(?&#60;!999)foo
1660  </PRE>  </pre>
 </P>  
 <P>  
1661  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
1662  that the first three are digits, and then the second assertion checks that the  that the first three are digits, and then the second assertion checks that the
1663  preceding three characters are not "999".  preceding three characters are not "999".
1664  </P>  </P>
1665  <P>  <P>
1666  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
 </P>  
 <P>  
1667  <pre>  <pre>
1668    (?&#60;=(?&#60;!foo)bar)baz    (?&#60;=(?&#60;!foo)bar)baz
1669  </PRE>  </pre>
 </P>  
 <P>  
1670  matches an occurrence of "baz" that is preceded by "bar" which in turn is not  matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1671  preceded by "foo", while  preceded by "foo", while
 </P>  
 <P>  
1672  <pre>  <pre>
1673    (?&#60;=\d{3}(?!999)...)foo    (?&#60;=\d{3}(?!999)...)foo
1674  </PRE>  </pre>
1675  </P>  is another pattern that matches "foo" preceded by three digits and any three
 <P>  
 is another pattern which matches "foo" preceded by three digits and any three  
1676  characters that are not "999".  characters that are not "999".
1677  </P>  <a name="conditions"></a></P>
1678  <P>  <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
 Assertion subpatterns are not capturing subpatterns, and may not be repeated,  
 because it makes no sense to assert the same thing several times. If any kind  
 of assertion contains capturing subpatterns within it, these are counted for  
 the purposes of numbering the capturing subpatterns in the whole pattern.  
 However, substring capturing is carried out only for positive assertions,  
 because it does not make sense for negative assertions.  
 </P>  
 <br><a name="SEC16" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>  
1679  <P>  <P>
1680  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1681  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1682  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a previous capturing subpattern matched
1683  or not. The two possible forms of conditional subpattern are  or not. The two possible forms of conditional subpattern are
 </P>  
 <P>  
1684  <pre>  <pre>
1685    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1686    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1687  </PRE>  </pre>
 </P>  
 <P>  
1688  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1689  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
1690  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1691  </P>  </P>
1692  <P>  <P>
1693  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1694  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
 subpattern of that number has previously matched. The number must be greater  
 than zero. Consider the following pattern, which contains non-significant white  
 space to make it more readable (assume the PCRE_EXTENDED option) and to divide  
 it into three parts for ease of discussion:  
1695  </P>  </P>
1696  <P>  <br><b>
1697    Checking for a used subpattern by number
1698    </b><br>
1699    <P>
1700    If the text between the parentheses consists of a sequence of digits, the
1701    condition is true if the capturing subpattern of that number has previously
1702    matched. An alternative notation is to precede the digits with a plus or minus
1703    sign. In this case, the subpattern number is relative rather than absolute.
1704    The most recently opened parentheses can be referenced by (?(-1), the next most
1705    recent by (?(-2), and so on. In looping constructs it can also make sense to
1706    refer to subsequent groups with constructs such as (?(+2).
1707    </P>
1708    <P>
1709    Consider the following pattern, which contains non-significant white space to
1710    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1711    three parts for ease of discussion:
1712  <pre>  <pre>
1713    ( \( )?    [^()]+    (?(1) \) )    ( \( )?    [^()]+    (?(1) \) )
1714  </PRE>  </pre>
 </P>  
 <P>  
1715  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
1716  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
1717  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
# Line 1376  subpattern matches nothing. In other wor Line 1723  subpattern matches nothing. In other wor
1723  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1724  </P>  </P>
1725  <P>  <P>
1726  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
1727  pattern or subpattern has been made. At "top level", the condition is false.  reference:
1728  This is a PCRE extension. Recursive patterns are described in the next section.  <pre>
1729  </P>    ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...
1730    </pre>
1731    This makes the fragment independent of the parentheses in the larger pattern.
1732    </P>
1733    <br><b>
1734    Checking for a used subpattern by name
1735    </b><br>
1736    <P>
1737    Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
1738    subpattern by name. For compatibility with earlier versions of PCRE, which had
1739    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1740    there is a possible ambiguity with this syntax, because subpattern names may
1741    consist entirely of digits. PCRE looks first for a named subpattern; if it
1742    cannot find one and the name consists entirely of digits, PCRE looks for a
1743    subpattern of that number, which must be greater than zero. Using subpattern
1744    names that consist entirely of digits is not recommended.
1745    </P>
1746    <P>
1747    Rewriting the above example to use a named subpattern gives this:
1748    <pre>
1749      (?&#60;OPEN&#62; \( )?    [^()]+    (?(&#60;OPEN&#62;) \) )
1750    
1751    </PRE>
1752    </P>
1753    <br><b>
1754    Checking for pattern recursion
1755    </b><br>
1756    <P>
1757    If the condition is the string (R), and there is no subpattern with the name R,
1758    the condition is true if a recursive call to the whole pattern or any
1759    subpattern has been made. If digits or a name preceded by ampersand follow the
1760    letter R, for example:
1761    <pre>
1762      (?(R3)...) or (?(R&name)...)
1763    </pre>
1764    the condition is true if the most recent recursion is into the subpattern whose
1765    number or name is given. This condition does not check the entire recursion
1766    stack.
1767    </P>
1768    <P>
1769    At "top level", all these recursion test conditions are false. Recursive
1770    patterns are described below.
1771    </P>
1772    <br><b>
1773    Defining subpatterns for use by reference only
1774    </b><br>
1775    <P>
1776    If the condition is the string (DEFINE), and there is no subpattern with the
1777    name DEFINE, the condition is always false. In this case, there may be only one
1778    alternative in the subpattern. It is always skipped if control reaches this
1779    point in the pattern; the idea of DEFINE is that it can be used to define
1780    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1781    is described below.) For example, a pattern to match an IPv4 address could be
1782    written like this (ignore whitespace and line breaks):
1783    <pre>
1784      (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
1785      \b (?&byte) (\.(?&byte)){3} \b
1786    </pre>
1787    The first part of the pattern is a DEFINE group inside which a another group
1788    named "byte" is defined. This matches an individual component of an IPv4
1789    address (a number less than 256). When matching takes place, this part of the
1790    pattern is skipped because DEFINE acts like a false condition.
1791    </P>
1792    <P>
1793    The rest of the pattern uses references to the named group to match the four
1794    dot-separated components of an IPv4 address, insisting on a word boundary at
1795    each end.
1796    </P>
1797    <br><b>
1798    Assertion conditions
1799    </b><br>
1800  <P>  <P>
1801  If the condition is not a sequence of digits or (R), it must be an assertion.  If the condition is not in any of the above formats, it must be an assertion.
1802  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1803  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1804  alternatives on the second line:  alternatives on the second line:
 </P>  
 <P>  
1805  <pre>  <pre>
1806    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
1807    \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )    \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )
1808  </PRE>  </pre>
 </P>  
 <P>  
1809  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
1810  sequence of non-letters followed by a letter. In other words, it tests for the  sequence of non-letters followed by a letter. In other words, it tests for the
1811  presence of at least one letter in the subject. If a letter is found, the  presence of at least one letter in the subject. If a letter is found, the
1812  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
1813  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
1814  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.
1815  </P>  <a name="comments"></a></P>
1816  <br><a name="SEC17" href="#TOC1">COMMENTS</a><br>  <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
1817  <P>  <P>
1818  The sequence (?# marks the start of a comment which continues up to the next  The sequence (?# marks the start of a comment that continues up to the next
1819  closing parenthesis. Nested parentheses are not permitted. The characters  closing parenthesis. Nested parentheses are not permitted. The characters
1820  that make up a comment play no part in the pattern matching at all.  that make up a comment play no part in the pattern matching at all.
1821  </P>  </P>
1822  <P>  <P>
1823  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1824  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1825  character in the pattern.  next newline in the pattern.
1826  </P>  <a name="recursion"></a></P>
1827  <br><a name="SEC18" href="#TOC1">RECURSIVE PATTERNS</a><br>  <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
1828  <P>  <P>
1829  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1830  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1831  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
1832  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth.
 experimental facility that allows regular expressions to recurse (amongst other  
 things). It does this by interpolating Perl code in the expression at run time,  
 and the code can refer to the expression itself. A Perl pattern to solve the  
 parentheses problem can be created like this:  
1833  </P>  </P>
1834  <P>  <P>
1835    For some time, Perl has provided a facility that allows regular expressions to
1836    recurse (amongst other things). It does this by interpolating Perl code in the
1837    expression at run time, and the code can refer to the expression itself. A Perl
1838    pattern using code interpolation to solve the parentheses problem can be
1839    created like this:
1840  <pre>  <pre>
1841    $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;    $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
1842  </PRE>  </pre>
1843    The (?p{...}) item interpolates Perl code at run time, and in this case refers
1844    recursively to the pattern in which it appears.
1845  </P>  </P>
1846  <P>  <P>
1847  The (?p{...}) item interpolates Perl code at run time, and in this case refers  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1848  recursively to the pattern in which it appears. Obviously, PCRE cannot support  supports special syntax for recursion of the entire pattern, and also for
1849  the interpolation of Perl code. Instead, it supports some special syntax for  individual subpattern recursion. After its introduction in PCRE and Python,
1850  recursion of the entire pattern, and also for individual subpattern recursion.  this kind of recursion was introduced into Perl at release 5.10.
1851  </P>  </P>
1852  <P>  <P>
1853  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
1854  a closing parenthesis is a recursive call of the subpattern of the given  closing parenthesis is a recursive call of the subpattern of the given number,
1855  number, provided that it occurs inside that subpattern. (If not, it is a  provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1856  "subroutine" call, which is described in the next section.) The special item  call, which is described in the next section.) The special item (?R) or (?0) is
1857  (?R) is a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
1858  </P>  </P>
1859  <P>  <P>
1860  For example, this PCRE pattern solves the nested parentheses problem (assume  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1861  the PCRE_EXTENDED option is set so that white space is ignored):  treated as an atomic group. That is, once it has matched some of the subject
1862    string, it is never re-entered, even if it contains untried alternatives and
1863    there is a subsequent matching failure.
1864  </P>  </P>
1865  <P>  <P>
1866    This PCRE pattern solves the nested parentheses problem (assume the
1867    PCRE_EXTENDED option is set so that white space is ignored):
1868  <pre>  <pre>
1869    \( ( (?&#62;[^()]+) | (?R) )* \)    \( ( (?&#62;[^()]+) | (?R) )* \)
1870  </PRE>  </pre>
 </P>  
 <P>  
1871  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1872  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
1873  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
1874  Finally there is a closing parenthesis.  Finally there is a closing parenthesis.
1875  </P>  </P>
1876  <P>  <P>
1877  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
1878  pattern, so instead you could use this:  pattern, so instead you could use this:
 </P>  
 <P>  
1879  <pre>  <pre>
1880    ( \( ( (?&#62;[^()]+) | (?1) )* \) )    ( \( ( (?&#62;[^()]+) | (?1) )* \) )
1881  </PRE>  </pre>
 </P>  
 <P>  
1882  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
1883  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
 parenthesis numbers can be tricky. It may be more convenient to use named  
 parentheses instead. For this, PCRE uses (?P&#62;name), which is an extension to  
 the Python syntax that PCRE uses for named parentheses (Perl does not provide  
 named parentheses). We could rewrite the above example as follows:  
1884  </P>  </P>
1885  <P>  <P>
1886  <pre>  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1887    (?&#60;pn&#62; \( ( (?&#62;[^()]+) | (?P&#62;pn) )* \) )  is made easier by the use of relative references. (A Perl 5.10 feature.)
1888  </PRE>  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1889    most recently opened parentheses preceding the recursion. In other words, a
1890    negative number counts capturing parentheses leftwards from the point at which
1891    it is encountered.
1892  </P>  </P>
1893  <P>  <P>
1894  This particular example pattern contains nested unlimited repeats, and so the  It is also possible to refer to subsequently opened parentheses, by writing
1895  use of atomic grouping for matching strings of non-parentheses is important  references such as (?+2). However, these cannot be recursive because the
1896  when applying the pattern to strings that do not match. For example, when this  reference is not inside the parentheses that are referenced. They are always
1897  pattern is applied to  "subroutine" calls, as described in the next section.
1898  </P>  </P>
1899  <P>  <P>
1900    An alternative approach is to use named parentheses instead. The Perl syntax
1901    for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
1902    could rewrite the above example as follows:
1903  <pre>  <pre>
1904    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (?&#60;pn&#62; \( ( (?&#62;[^()]+) | (?&pn) )* \) )
1905  </PRE>  </pre>
1906    If there is more than one subpattern with the same name, the earliest one is
1907    used.
1908  </P>  </P>
1909  <P>  <P>
1910    This particular example pattern that we have been looking at contains nested
1911    unlimited repeats, and so the use of atomic grouping for matching strings of
1912    non-parentheses is important when applying the pattern to strings that do not
1913    match. For example, when this pattern is applied to
1914    <pre>
1915      (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1916    </pre>
1917  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if atomic grouping is not used,
1918  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
1919  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
# Line 1501  If you want to obtain intermediate value Line 1926  If you want to obtain intermediate value
1926  below and the  below and the
1927  <a href="pcrecallout.html"><b>pcrecallout</b></a>  <a href="pcrecallout.html"><b>pcrecallout</b></a>
1928  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
 </P>  
 <P>  
1929  <pre>  <pre>
1930    (ab(cd)ef)    (ab(cd)ef)
1931  </PRE>  </pre>
 </P>  
 <P>  
1932  the value for the capturing parentheses is "ef", which is the last value taken  the value for the capturing parentheses is "ef", which is the last value taken
1933  on at the top level. If additional parentheses are added, giving  on at the top level. If additional parentheses are added, giving
 </P>  
 <P>  
1934  <pre>  <pre>
1935    \( ( ( (?&#62;[^()]+) | (?R) )* ) \)    \( ( ( (?&#62;[^()]+) | (?R) )* ) \)
1936       ^                        ^       ^                        ^
1937       ^                        ^       ^                        ^
1938  </PRE>  </pre>
 </P>  
 <P>  
1939  the string they capture is "ab(cd)ef", the contents of the top level  the string they capture is "ab(cd)ef", the contents of the top level
1940  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1941  has to obtain extra memory to store data during a recursion, which it does by  has to obtain extra memory to store data during a recursion, which it does by
# Line 1530  Do not confuse the (?R) item with the co Line 1947  Do not confuse the (?R) item with the co
1947  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
1948  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1949  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
 </P>  
 <P>  
1950  <pre>  <pre>
1951    &#60; (?: (?(R) \d++  | [^&#60;&#62;]*+) | (?R)) * &#62;    &#60; (?: (?(R) \d++  | [^&#60;&#62;]*+) | (?R)) * &#62;
1952  </PRE>  </pre>
 </P>  
 <P>  
1953  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
1954  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
1955  is the actual recursive call.  is the actual recursive call.
1956  </P>  <a name="subpatternsassubroutines"></a></P>
1957  <a name="subpatternsassubroutines"></a><br><a name="SEC19" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>  <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
1958  <P>  <P>
1959  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern reference (either by number or by
1960  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
1961  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1962  pattern  before or after the reference. A numbered reference can be absolute or
1963  </P>  relative, as in these examples:
1964  <P>  <pre>
1965      (...(absolute)...)...(?2)...
1966      (...(relative)...)...(?-1)...
1967      (...(?+1)...(relative)...
1968    </pre>
1969    An earlier example pointed out that the pattern
1970  <pre>  <pre>
1971    (sens|respons)e and \1ibility    (sens|respons)e and \1ibility
1972  </PRE>  </pre>
 </P>  
 <P>  
1973  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1974  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
 </P>  
 <P>  
1975  <pre>  <pre>
1976    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1977  </PRE>  </pre>
1978    is used, it does match "sense and responsibility" as well as the other two
1979    strings. Another example is given in the discussion of DEFINE above.
1980  </P>  </P>
1981  <P>  <P>
1982  is used, it does match "sense and responsibility" as well as the other two  Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1983  strings. Such references must, however, follow the subpattern to which they  group. That is, once it has matched some of the subject string, it is never
1984  refer.  re-entered, even if it contains untried alternatives and there is a subsequent
1985    matching failure.
1986    </P>
1987    <P>
1988    When a subpattern is used as a subroutine, processing options such as
1989    case-independence are fixed when the subpattern is defined. They cannot be
1990    changed for different calls. For example, consider this pattern:
1991    <pre>
1992      (abc)(?i:(?-1))
1993    </pre>
1994    It matches "abcabc". It does not match "abcABC" because the change of
1995    processing option does not affect the called subpattern.
1996  </P>  </P>
1997  <br><a name="SEC20" href="#TOC1">CALLOUTS</a><br>  <br><a name="SEC23" href="#TOC1">CALLOUTS</a><br>
1998  <P>  <P>
1999  Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl  Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2000  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 1585  Within a regular expression, (?C) indica Line 2012  Within a regular expression, (?C) indica
2012  function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
2013  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.
2014  For example, this pattern has two callout points:  For example, this pattern has two callout points:
 </P>  
 <P>  
2015  <pre>  <pre>
2016    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2017  </PRE>  </pre>
2018    If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
2019    automatically installed before each item in the pattern. They are all numbered
2020    255.
2021  </P>  </P>
2022  <P>  <P>
2023  During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is  During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
2024  set), the external function is called. It is provided with the number of the  set), the external function is called. It is provided with the number of the
2025  callout, and, optionally, one item of data originally supplied by the caller of  callout, the position in the pattern, and, optionally, one item of data
2026  <b>pcre_exec()</b>. The callout function may cause matching to backtrack, or to  originally supplied by the caller of <b>pcre_exec()</b>. The callout function
2027  fail altogether. A complete description of the interface to the callout  may cause matching to proceed, to backtrack, or to fail altogether. A complete
2028  function is given in the  description of the interface to the callout function is given in the
2029  <a href="pcrecallout.html"><b>pcrecallout</b></a>  <a href="pcrecallout.html"><b>pcrecallout</b></a>
2030  documentation.  documentation.
2031  </P>  </P>
2032    <br><a name="SEC24" href="#TOC1">BACTRACKING CONTROL</a><br>
2033    <P>
2034    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2035    are described in the Perl documentation as "experimental and subject to change
2036    or removal in a future version of Perl". It goes on to say: "Their usage in
2037    production code should be noted to avoid problems during upgrades." The same
2038    remarks apply to the PCRE features described in this section.
2039    </P>
2040    <P>
2041    Since these verbs are specifically related to backtracking, they can be used
2042    only when the pattern is to be matched using <b>pcre_exec()</b>, which uses a
2043    backtracking algorithm. They cause an error if encountered by
2044    <b>pcre_dfa_exec()</b>.
2045    </P>
2046    <P>
2047    The new verbs make use of what was previously invalid syntax: an opening
2048    parenthesis followed by an asterisk. In Perl, they are generally of the form
2049    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2050    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2051    are two kinds:
2052    </P>
2053    <br><b>
2054    Verbs that act immediately
2055    </b><br>
2056    <P>
2057    The following verbs act as soon as they are encountered:
2058    <pre>
2059       (*ACCEPT)
2060    </pre>
2061    This verb causes the match to end successfully, skipping the remainder of the
2062    pattern. When inside a recursion, only the innermost pattern is ended
2063    immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2064    capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2065    captured. For example:
2066    <pre>
2067      A(A|B(*ACCEPT)|C)D
2068    </pre>
2069    This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2070    captured.
2071    <pre>
2072      (*FAIL) or (*F)
2073    </pre>
2074    This verb causes the match to fail, forcing backtracking to occur. It is
2075    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2076    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2077    Perl features that are not present in PCRE. The nearest equivalent is the
2078    callout feature, as for example in this pattern:
2079    <pre>
2080      a+(?C)(*FAIL)
2081    </pre>
2082    A match with the string "aaaa" always fails, but the callout is taken before
2083    each backtrack happens (in this example, 10 times).
2084    </P>
2085    <br><b>
2086    Verbs that act after backtracking
2087    </b><br>
2088    <P>
2089    The following verbs do nothing when they are encountered. Matching continues
2090    with what follows, but if there is no subsequent match, a failure is forced.
2091    The verbs differ in exactly what kind of failure occurs.
2092    <pre>
2093      (*COMMIT)
2094    </pre>
2095    This verb causes the whole match to fail outright if the rest of the pattern
2096    does not match. Even if the pattern is unanchored, no further attempts to find
2097    a match by advancing the start point take place. Once (*COMMIT) has been
2098    passed, <b>pcre_exec()</b> is committed to finding a match at the current
2099    starting point, or not at all. For example:
2100    <pre>
2101      a+(*COMMIT)b
2102    </pre>
2103    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2104    dynamic anchor, or "I've started, so I must finish."
2105    <pre>
2106      (*PRUNE)
2107    </pre>
2108    This verb causes the match to fail at the current position if the rest of the
2109    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2110    advance to the next starting character then happens. Backtracking can occur as
2111    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2112    if there is no match to the right, backtracking cannot cross (*PRUNE).
2113    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2114    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2115    be expressed in any other way.
2116    <pre>
2117      (*SKIP)
2118    </pre>
2119    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2120    "bumpalong" advance is not to the next character, but to the position in the
2121    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2122    was matched leading up to it cannot be part of a successful match. Consider:
2123    <pre>
2124      a+(*SKIP)b
2125    </pre>
2126    If the subject is "aaaac...", after the first match attempt fails (starting at
2127    the first character in the string), the starting point skips on to start the
2128    next attempt at "c". Note that a possessive quantifer does not have the same
2129    effect in this example; although it would suppress backtracking during the
2130    first match attempt, the second attempt would start at the second character
2131    instead of skipping on to "c".
2132    <pre>
2133      (*THEN)
2134    </pre>
2135    This verb causes a skip to the next alternation if the rest of the pattern does
2136    not match. That is, it cancels pending backtracking, but only within the
2137    current alternation. Its name comes from the observation that it can be used
2138    for a pattern-based if-then-else block:
2139    <pre>
2140      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2141    </pre>
2142    If the COND1 pattern matches, FOO is tried (and possibly further items after
2143    the end of the group if FOO succeeds); on failure the matcher skips to the
2144    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2145    is used outside of any alternation, it acts exactly like (*PRUNE).
2146    </P>
2147    <br><a name="SEC25" href="#TOC1">SEE ALSO</a><br>
2148    <P>
2149    <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), <b>pcre</b>(3).
2150    </P>
2151    <br><a name="SEC26" href="#TOC1">AUTHOR</a><br>
2152  <P>  <P>
2153  Last updated: 03 February 2003  Philip Hazel
2154    <br>
2155    University Computing Service
2156    <br>
2157    Cambridge CB2 3QH, England.
2158    <br>
2159    </P>
2160    <br><a name="SEC27" href="#TOC1">REVISION</a><br>
2161    <P>
2162    Last updated: 21 August 2007
2163    <br>
2164    Copyright &copy; 1997-2007 University of Cambridge.
2165  <br>  <br>
2166  Copyright &copy; 1997-2003 University of Cambridge.  <p>
2167    Return to the <a href="index.html">PCRE index page</a>.
2168    </p>

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