Diff for /embedaddon/pcre/doc/html/pcrepattern.html between versions 1.1.1.1 and 1.1.1.4

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Line 14  man page, in case the conversion went wrong. Line 14  man page, in case the conversion went wrong.
 <br>  <br>
 <ul>  <ul>
 <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>  <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
<li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a><li><a name="TOC2" href="#SEC2">SPECIAL START-OF-PATTERN ITEMS</a>
<li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a><li><a name="TOC3" href="#SEC3">EBCDIC CHARACTER CODES</a>
<li><a name="TOC4" href="#SEC4">BACKSLASH</a><li><a name="TOC4" href="#SEC4">CHARACTERS AND METACHARACTERS</a>
<li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a><li><a name="TOC5" href="#SEC5">BACKSLASH</a>
<li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT) AND \N</a><li><a name="TOC6" href="#SEC6">CIRCUMFLEX AND DOLLAR</a>
<li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a><li><a name="TOC7" href="#SEC7">FULL STOP (PERIOD, DOT) AND \N</a>
<li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a><li><a name="TOC8" href="#SEC8">MATCHING A SINGLE DATA UNIT</a>
<li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a><li><a name="TOC9" href="#SEC9">SQUARE BRACKETS AND CHARACTER CLASSES</a>
<li><a name="TOC10" href="#SEC10">VERTICAL BAR</a><li><a name="TOC10" href="#SEC10">POSIX CHARACTER CLASSES</a>
<li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a><li><a name="TOC11" href="#SEC11">VERTICAL BAR</a>
<li><a name="TOC12" href="#SEC12">SUBPATTERNS</a><li><a name="TOC12" href="#SEC12">INTERNAL OPTION SETTING</a>
<li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a><li><a name="TOC13" href="#SEC13">SUBPATTERNS</a>
<li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a><li><a name="TOC14" href="#SEC14">DUPLICATE SUBPATTERN NUMBERS</a>
<li><a name="TOC15" href="#SEC15">REPETITION</a><li><a name="TOC15" href="#SEC15">NAMED SUBPATTERNS</a>
<li><a name="TOC16" href="#SEC16">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><li><a name="TOC16" href="#SEC16">REPETITION</a>
<li><a name="TOC17" href="#SEC17">BACK REFERENCES</a><li><a name="TOC17" href="#SEC17">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
<li><a name="TOC18" href="#SEC18">ASSERTIONS</a><li><a name="TOC18" href="#SEC18">BACK REFERENCES</a>
<li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a><li><a name="TOC19" href="#SEC19">ASSERTIONS</a>
<li><a name="TOC20" href="#SEC20">COMMENTS</a><li><a name="TOC20" href="#SEC20">CONDITIONAL SUBPATTERNS</a>
<li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a><li><a name="TOC21" href="#SEC21">COMMENTS</a>
<li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a><li><a name="TOC22" href="#SEC22">RECURSIVE PATTERNS</a>
<li><a name="TOC23" href="#SEC23">ONIGURUMA SUBROUTINE SYNTAX</a><li><a name="TOC23" href="#SEC23">SUBPATTERNS AS SUBROUTINES</a>
<li><a name="TOC24" href="#SEC24">CALLOUTS</a><li><a name="TOC24" href="#SEC24">ONIGURUMA SUBROUTINE SYNTAX</a>
<li><a name="TOC25" href="#SEC25">BACKTRACKING CONTROL</a><li><a name="TOC25" href="#SEC25">CALLOUTS</a>
<li><a name="TOC26" href="#SEC26">SEE ALSO</a><li><a name="TOC26" href="#SEC26">BACKTRACKING CONTROL</a>
<li><a name="TOC27" href="#SEC27">AUTHOR</a><li><a name="TOC27" href="#SEC27">SEE ALSO</a>
<li><a name="TOC28" href="#SEC28">REVISION</a><li><a name="TOC28" href="#SEC28">AUTHOR</a>
 <li><a name="TOC29" href="#SEC29">REVISION</a>
 </ul>  </ul>
 <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>  <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
 <P>  <P>
Line 60  published by O'Reilly, covers regular expressions in g Line 61  published by O'Reilly, covers regular expressions in g
 description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
 </P>  </P>
 <P>  <P>
   This document discusses the patterns that are supported by PCRE when one its
   main matching functions, <b>pcre_exec()</b> (8-bit) or <b>pcre[16|32]_exec()</b>
   (16- or 32-bit), is used. PCRE also has alternative matching functions,
   <b>pcre_dfa_exec()</b> and <b>pcre[16|32_dfa_exec()</b>, which match using a
   different algorithm that is not Perl-compatible. Some of the features discussed
   below are not available when DFA matching is used. The advantages and
   disadvantages of the alternative functions, and how they differ from the normal
   functions, are discussed in the
   <a href="pcrematching.html"><b>pcrematching</b></a>
   page.
   </P>
   <br><a name="SEC2" href="#TOC1">SPECIAL START-OF-PATTERN ITEMS</a><br>
   <P>
   A number of options that can be passed to <b>pcre_compile()</b> can also be set
   by special items at the start of a pattern. These are not Perl-compatible, but
   are provided to make these options accessible to pattern writers who are not
   able to change the program that processes the pattern. Any number of these
   items may appear, but they must all be together right at the start of the
   pattern string, and the letters must be in upper case.
   </P>
   <br><b>
   UTF support
   </b><br>
   <P>
 The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
there is now also support for UTF-8 character strings. To use this,there is now also support for UTF-8 strings in the original library, an
PCRE must be built to include UTF-8 support, and you must callextra library that supports 16-bit and UTF-16 character strings, and a
<b>pcre_compile()</b> or <b>pcre_compile2()</b> with the PCRE_UTF8 option. Therethird library that supports 32-bit and UTF-32 character strings. To use these
is also a special sequence that can be given at the start of a pattern:features, PCRE must be built to include appropriate support. When using UTF
 strings you must either call the compiling function with the PCRE_UTF8,
 PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
 these special sequences:
 <pre>  <pre>
   (*UTF8)    (*UTF8)
     (*UTF16)
     (*UTF32)
     (*UTF)
 </pre>  </pre>
Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8(*UTF) is a generic sequence that can be used with any of the libraries.
option. This feature is not Perl-compatible. How setting UTF-8 mode affectsStarting a pattern with such a sequence is equivalent to setting the relevant
pattern matching is mentioned in several places below. There is also a summaryoption. How setting a UTF mode affects pattern matching is mentioned in several
of UTF-8 features in theplaces below. There is also a summary of features in the
 <a href="pcreunicode.html"><b>pcreunicode</b></a>  <a href="pcreunicode.html"><b>pcreunicode</b></a>
 page.  page.
 </P>  </P>
 <P>  <P>
Another special sequence that may appear at the start of a pattern or inSome applications that allow their users to supply patterns may wish to
combination with (*UTF8) is:restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
 option is set at compile time, (*UTF) etc. are not allowed, and their
 appearance causes an error.
 </P>
 <br><b>
 Unicode property support
 </b><br>
 <P>
 Another special sequence that may appear at the start of a pattern is
 <pre>  <pre>
   (*UCP)    (*UCP)
 </pre>  </pre>
Line 86  such as \d and \w to use Unicode properties to determi Line 125  such as \d and \w to use Unicode properties to determi
 instead of recognizing only characters with codes less than 128 via a lookup  instead of recognizing only characters with codes less than 128 via a lookup
 table.  table.
 </P>  </P>
   <br><b>
   Disabling start-up optimizations
   </b><br>
 <P>  <P>
 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the  If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
PCRE_NO_START_OPTIMIZE option either at compile or matching time. There arePCRE_NO_START_OPTIMIZE option either at compile or matching time.
also some more of these special sequences that are concerned with the handling 
of newlines; they are described below. 
</P> 
<P> 
The remainder of this document discusses the patterns that are supported by 
PCRE when its main matching function, <b>pcre_exec()</b>, is used. 
From release 6.0, PCRE offers a second matching function, 
<b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not 
Perl-compatible. Some of the features discussed below are not available when 
<b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the 
alternative function, and how it differs from the normal function, are 
discussed in the 
<a href="pcrematching.html"><b>pcrematching</b></a> 
page. 
 <a name="newlines"></a></P>  <a name="newlines"></a></P>
<br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br><br><b>
 Newline conventions
 </b><br>
 <P>  <P>
 PCRE supports five different conventions for indicating line breaks in  PCRE supports five different conventions for indicating line breaks in
 strings: a single CR (carriage return) character, a single LF (linefeed)  strings: a single CR (carriage return) character, a single LF (linefeed)
Line 126  string with one of the following five sequences: Line 156  string with one of the following five sequences:
   (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
   (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
 </pre>  </pre>
These override the default and the options given to <b>pcre_compile()</b> orThese override the default and the options given to the compiling function. For
<b>pcre_compile2()</b>. For example, on a Unix system where LF is the defaultexample, on a Unix system where LF is the default newline sequence, the pattern
newline sequence, the pattern 
 <pre>  <pre>
   (*CR)a.b    (*CR)a.b
 </pre>  </pre>
 changes the convention to CR. That pattern matches "a\nb" because LF is no  changes the convention to CR. That pattern matches "a\nb" because LF is no
longer a newline. Note that these special settings, which are notlonger a newline. If more than one of these settings is present, the last one
Perl-compatible, are recognized only at the very start of a pattern, and that 
they must be in upper case. If more than one of them is present, the last one 
 is used.  is used.
 </P>  </P>
 <P>  <P>
The newline convention affects the interpretation of the dot metacharacter whenThe newline convention affects where the circumflex and dollar assertions are
PCRE_DOTALL is not set, and also the behaviour of \N. However, it does nottrue. It also affects the interpretation of the dot metacharacter when
affect what the \R escape sequence matches. By default, this is any UnicodePCRE_DOTALL is not set, and the behaviour of \N. However, it does not affect
newline sequence, for Perl compatibility. However, this can be changed; see thewhat the \R escape sequence matches. By default, this is any Unicode newline
 sequence, for Perl compatibility. However, this can be changed; see the
 description of \R in the section entitled  description of \R in the section entitled
 <a href="#newlineseq">"Newline sequences"</a>  <a href="#newlineseq">"Newline sequences"</a>
 below. A change of \R setting can be combined with a change of newline  below. A change of \R setting can be combined with a change of newline
 convention.  convention.
 </P>  </P>
<br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br><br><b>
 Setting match and recursion limits
 </b><br>
 <P>  <P>
   The caller of <b>pcre_exec()</b> can set a limit on the number of times the
   internal <b>match()</b> function is called and on the maximum depth of
   recursive calls. These facilities are provided to catch runaway matches that
   are provoked by patterns with huge matching trees (a typical example is a
   pattern with nested unlimited repeats) and to avoid running out of system stack
   by too much recursion. When one of these limits is reached, <b>pcre_exec()</b>
   gives an error return. The limits can also be set by items at the start of the
   pattern of the form
   <pre>
     (*LIMIT_MATCH=d)
     (*LIMIT_RECURSION=d)
   </pre>
   where d is any number of decimal digits. However, the value of the setting must
   be less than the value set by the caller of <b>pcre_exec()</b> for it to have
   any effect. In other words, the pattern writer can lower the limit set by the
   programmer, but not raise it. If there is more than one setting of one of these
   limits, the lower value is used.
   </P>
   <br><a name="SEC3" href="#TOC1">EBCDIC CHARACTER CODES</a><br>
   <P>
   PCRE can be compiled to run in an environment that uses EBCDIC as its character
   code rather than ASCII or Unicode (typically a mainframe system). In the
   sections below, character code values are ASCII or Unicode; in an EBCDIC
   environment these characters may have different code values, and there are no
   code points greater than 255.
   </P>
   <br><a name="SEC4" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
   <P>
 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
 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
 corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
Line 158  corresponding characters in the subject. As a trivial  Line 216  corresponding characters in the subject. As a trivial 
 </pre>  </pre>
 matches a portion of a subject string that is identical to itself. When  matches a portion of a subject string that is identical to itself. When
 caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
independently of case. In UTF-8 mode, PCRE always understands the concept ofindependently of case. In a UTF mode, PCRE always understands the concept of
 case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
 always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
 supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
 If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching for characters 128 and above, you must
 ensure that PCRE is compiled with Unicode property support as well as with  ensure that PCRE is compiled with Unicode property support as well as with
UTF-8 support.UTF support.
 </P>  </P>
 <P>  <P>
 The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
Line 205  a character class the only metacharacters are: Line 263  a character class the only metacharacters are:
 </pre>  </pre>
 The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
 </P>  </P>
<br><a name="SEC4" href="#TOC1">BACKSLASH</a><br><br><a name="SEC5" href="#TOC1">BACKSLASH</a><br>
 <P>  <P>
 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
 character that is not a number or a letter, it takes away any special meaning  character that is not a number or a letter, it takes away any special meaning
Line 220  non-alphanumeric with backslash to specify that it sta Line 278  non-alphanumeric with backslash to specify that it sta
 particular, if you want to match a backslash, you write \\.  particular, if you want to match a backslash, you write \\.
 </P>  </P>
 <P>  <P>
In UTF-8 mode, only ASCII numbers and letters have any special meaning after aIn a UTF mode, only ASCII numbers and letters have any special meaning after a
 backslash. All other characters (in particular, those whose codepoints are  backslash. All other characters (in particular, those whose codepoints are
 greater than 127) are treated as literals.  greater than 127) are treated as literals.
 </P>  </P>
 <P>  <P>
If a pattern is compiled with the PCRE_EXTENDED option, whitespace in theIf a pattern is compiled with the PCRE_EXTENDED option, white space in the
 pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
 a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
be used to include a whitespace or # character as part of the pattern.be used to include a white space or # character as part of the pattern.
 </P>  </P>
 <P>  <P>
 If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
Line 262  one of the following escape sequences than the binary  Line 320  one of the following escape sequences than the binary 
   \a        alarm, that is, the BEL character (hex 07)    \a        alarm, that is, the BEL character (hex 07)
   \cx       "control-x", where x is any ASCII character    \cx       "control-x", where x is any ASCII character
   \e        escape (hex 1B)    \e        escape (hex 1B)
  \f        formfeed (hex 0C)  \f        form feed (hex 0C)
   \n        linefeed (hex 0A)    \n        linefeed (hex 0A)
   \r        carriage return (hex 0D)    \r        carriage return (hex 0D)
   \t        tab (hex 09)    \t        tab (hex 09)
Line 271  one of the following escape sequences than the binary  Line 329  one of the following escape sequences than the binary 
   \x{hhh..} character with hex code hhh.. (non-JavaScript mode)    \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
   \uhhhh    character with hex code hhhh (JavaScript mode only)    \uhhhh    character with hex code hhhh (JavaScript mode only)
 </pre>  </pre>
The precise effect of \cx is as follows: if x is a lower case letter, itThe precise effect of \cx on ASCII characters is as follows: if x is a lower
is converted to upper case. Then bit 6 of the character (hex 40) is inverted.case letter, it is converted to upper case. Then bit 6 of the character (hex
Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ is 7B), while40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A (A is 41, Z is 5A),
\c; becomes hex 7B (; is 3B). If the byte following \c has a value greaterbut \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the
than 127, a compile-time error occurs. This locks out non-ASCII characters indata item (byte or 16-bit value) following \c has a value greater than 127, a
both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all bytecompile-time error occurs. This locks out non-ASCII characters in all modes.
values are valid. A lower case letter is converted to upper case, and then the 
0xc0 bits are flipped.) 
 </P>  </P>
 <P>  <P>
   The \c facility was designed for use with ASCII characters, but with the
   extension to Unicode it is even less useful than it once was. It is, however,
   recognized when PCRE is compiled in EBCDIC mode, where data items are always
   bytes. In this mode, all values are valid after \c. If the next character is a
   lower case letter, it is converted to upper case. Then the 0xc0 bits of the
   byte are inverted. Thus \cA becomes hex 01, as in ASCII (A is C1), but because
   the EBCDIC letters are disjoint, \cZ becomes hex 29 (Z is E9), and other
   characters also generate different values.
   </P>
   <P>
 By default, after \x, from zero to two hexadecimal digits are read (letters  By default, after \x, from zero to two hexadecimal digits are read (letters
 can be in upper or lower case). Any number of hexadecimal digits may appear  can be in upper or lower case). Any number of hexadecimal digits may appear
between \x{ and }, but the value of the character code must be less than 256between \x{ and }, but the character code is constrained as follows:
in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, the maximum<pre>
value in hexadecimal is 7FFFFFFF. Note that this is bigger than the largest  8-bit non-UTF mode    less than 0x100
Unicode code point, which is 10FFFF.  8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
   16-bit non-UTF mode   less than 0x10000
   16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
   32-bit non-UTF mode   less than 0x80000000
   32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
 </pre>
 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
 "surrogate" codepoints), and 0xffef.
 </P>  </P>
 <P>  <P>
 If characters other than hexadecimal digits appear between \x{ and }, or if  If characters other than hexadecimal digits appear between \x{ and }, or if
Line 300  as just described only when it is followed by two hexa Line 373  as just described only when it is followed by two hexa
 Otherwise, it matches a literal "x" character. In JavaScript mode, support for  Otherwise, it matches a literal "x" character. In JavaScript mode, support for
 code points greater than 256 is provided by \u, which must be followed by  code points greater than 256 is provided by \u, which must be followed by
 four hexadecimal digits; otherwise it matches a literal "u" character.  four hexadecimal digits; otherwise it matches a literal "u" character.
   Character codes specified by \u in JavaScript mode are constrained in the same
   was as those specified by \x in non-JavaScript mode.
 </P>  </P>
 <P>  <P>
 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
Line 328  following the discussion of Line 403  following the discussion of
 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
 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
 digits following the backslash, and uses them to generate a data character. Any  digits following the backslash, and uses them to generate a data character. Any
subsequent digits stand for themselves. In non-UTF-8 mode, the value of asubsequent digits stand for themselves. The value of the character is
character specified in octal must be less than \400. In UTF-8 mode, values upconstrained in the same way as characters specified in hexadecimal.
to \777 are permitted. For example:For example:
 <pre>  <pre>
  \040   is another way of writing a space  \040   is another way of writing an ASCII space
   \40    is the same, provided there are fewer than 40 previous capturing subpatterns    \40    is the same, provided there are fewer than 40 previous capturing subpatterns
   \7     is always a back reference    \7     is always a back reference
   \11    might be a back reference, or another way of writing a tab    \11    might be a back reference, or another way of writing a tab
   \011   is always a tab    \011   is always a tab
   \0113  is a tab followed by the character "3"    \0113  is a tab followed by the character "3"
   \113   might be a back reference, otherwise the character with octal code 113    \113   might be a back reference, otherwise the character with octal code 113
  \377   might be a back reference, otherwise the byte consisting entirely of 1 bits  \377   might be a back reference, otherwise the value 255 (decimal)
   \81    is either a back reference, or a binary zero followed by the two characters "8" and "1"    \81    is either a back reference, or a binary zero followed by the two characters "8" and "1"
 </pre>  </pre>
 Note that octal values of 100 or greater must not be introduced by a leading  Note that octal values of 100 or greater must not be introduced by a leading
Line 399  Another use of backslash is for specifying generic cha Line 474  Another use of backslash is for specifying generic cha
 <pre>  <pre>
   \d     any decimal digit    \d     any decimal digit
   \D     any character that is not a decimal digit    \D     any character that is not a decimal digit
  \h     any horizontal whitespace character  \h     any horizontal white space character
  \H     any character that is not a horizontal whitespace character  \H     any character that is not a horizontal white space character
  \s     any whitespace character  \s     any white space character
  \S     any character that is not a whitespace character  \S     any character that is not a white space character
  \v     any vertical whitespace character  \v     any vertical white space character
  \V     any character that is not a vertical whitespace character  \V     any character that is not a vertical white space character
   \w     any "word" character    \w     any "word" character
   \W     any "non-word" character    \W     any "non-word" character
 </pre>  </pre>
Line 443  accented letters, and these are then matched by \w. Th Line 518  accented letters, and these are then matched by \w. Th
 Unicode is discouraged.  Unicode is discouraged.
 </P>  </P>
 <P>  <P>
By default, in UTF-8 mode, characters with values greater than 128 never matchBy default, in a UTF mode, characters with values greater than 128 never match
 \d, \s, or \w, and always match \D, \S, and \W. These sequences retain  \d, \s, or \w, and always match \D, \S, and \W. These sequences retain
their original meanings from before UTF-8 support was available, mainly fortheir original meanings from before UTF support was available, mainly for
 efficiency reasons. However, if PCRE is compiled with Unicode property support,  efficiency reasons. However, if PCRE is compiled with Unicode property support,
 and the PCRE_UCP option is set, the behaviour is changed so that Unicode  and the PCRE_UCP option is set, the behaviour is changed so that Unicode
 properties are used to determine character types, as follows:  properties are used to determine character types, as follows:
Line 463  is noticeably slower when PCRE_UCP is set. Line 538  is noticeably slower when PCRE_UCP is set.
 <P>  <P>
 The sequences \h, \H, \v, and \V are features that were added to Perl at  The sequences \h, \H, \v, and \V are features that were added to Perl at
 release 5.10. In contrast to the other sequences, which match only ASCII  release 5.10. In contrast to the other sequences, which match only ASCII
characters by default, these always match certain high-valued codepoints incharacters by default, these always match certain high-valued codepoints,
UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characterswhether or not PCRE_UCP is set. The horizontal space characters are:
are: 
 <pre>  <pre>
  U+0009     Horizontal tab  U+0009     Horizontal tab (HT)
   U+0020     Space    U+0020     Space
   U+00A0     Non-break space    U+00A0     Non-break space
   U+1680     Ogham space mark    U+1680     Ogham space mark
Line 489  are: Line 563  are:
 </pre>  </pre>
 The vertical space characters are:  The vertical space characters are:
 <pre>  <pre>
  U+000A     Linefeed  U+000A     Linefeed (LF)
  U+000B     Vertical tab  U+000B     Vertical tab (VT)
  U+000C     Formfeed  U+000C     Form feed (FF)
  U+000D     Carriage return  U+000D     Carriage return (CR)
  U+0085     Next line  U+0085     Next line (NEL)
   U+2028     Line separator    U+2028     Line separator
   U+2029     Paragraph separator    U+2029     Paragraph separator
<a name="newlineseq"></a></PRE></pre>
</P>In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
 relevant.
 <a name="newlineseq"></a></P>
 <br><b>  <br><b>
 Newline sequences  Newline sequences
 </b><br>  </b><br>
 <P>  <P>
 Outside a character class, by default, the escape sequence \R matches any  Outside a character class, by default, the escape sequence \R matches any
Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the following:Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the
 following:
 <pre>  <pre>
   (?&#62;\r\n|\n|\x0b|\f|\r|\x85)    (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
 </pre>  </pre>
Line 511  This is an example of an "atomic group", details of wh Line 588  This is an example of an "atomic group", details of wh
 <a href="#atomicgroup">below.</a>  <a href="#atomicgroup">below.</a>
 This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,  LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (nextU+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
 line, U+0085). The two-character sequence is treated as a single unit that  line, U+0085). The two-character sequence is treated as a single unit that
 cannot be split.  cannot be split.
 </P>  </P>
 <P>  <P>
In UTF-8 mode, two additional characters whose codepoints are greater than 255In other modes, two additional characters whose codepoints are greater than 255
 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).  are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
 Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
 recognized.  recognized.
Line 533  one of the following sequences: Line 610  one of the following sequences:
   (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
   (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
 </pre>  </pre>
These override the default and the options given to <b>pcre_compile()</b> orThese override the default and the options given to the compiling function, but
<b>pcre_compile2()</b>, but they can be overridden by options given tothey can themselves be overridden by options given to a matching function. Note
<b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. Note that these special settings,that these special settings, which are not Perl-compatible, are recognized only
which are not Perl-compatible, are recognized only at the very start of aat the very start of a pattern, and that they must be in upper case. If more
pattern, and that they must be in upper case. If more than one of them isthan one of them is present, the last one is used. They can be combined with a
present, the last one is used. They can be combined with a change of newlinechange of newline convention; for example, a pattern can start with:
convention; for example, a pattern can start with: 
 <pre>  <pre>
   (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
 </pre>  </pre>
They can also be combined with the (*UTF8) or (*UCP) special sequences. InsideThey can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
a character class, \R is treated as an unrecognized escape sequence, and so(*UCP) special sequences. Inside a character class, \R is treated as an
matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.unrecognized escape sequence, and so matches the letter "R" by default, but
 causes an error if PCRE_EXTRA is set.
 <a name="uniextseq"></a></P>  <a name="uniextseq"></a></P>
 <br><b>  <br><b>
 Unicode character properties  Unicode character properties
Line 553  Unicode character properties Line 630  Unicode character properties
 <P>  <P>
 When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
 escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
When not in UTF-8 mode, these sequences are of course limited to testingWhen in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
 characters whose codepoints are less than 256, but they do work in this mode.  characters whose codepoints are less than 256, but they do work in this mode.
 The extra escape sequences are:  The extra escape sequences are:
 <pre>  <pre>
   \p{<i>xx</i>}   a character with the <i>xx</i> property    \p{<i>xx</i>}   a character with the <i>xx</i> property
   \P{<i>xx</i>}   a character without the <i>xx</i> property    \P{<i>xx</i>}   a character without the <i>xx</i> property
  \X       an extended Unicode sequence  \X       a Unicode extended grapheme cluster
 </pre>  </pre>
 The property names represented by <i>xx</i> above are limited to the Unicode  The property names represented by <i>xx</i> above are limited to the Unicode
 script names, the general category properties, "Any", which matches any  script names, the general category properties, "Any", which matches any
Line 587  Armenian, Line 664  Armenian,
 Avestan,  Avestan,
 Balinese,  Balinese,
 Bamum,  Bamum,
   Batak,
 Bengali,  Bengali,
 Bopomofo,  Bopomofo,
   Brahmi,
 Braille,  Braille,
 Buginese,  Buginese,
 Buhid,  Buhid,
 Canadian_Aboriginal,  Canadian_Aboriginal,
 Carian,  Carian,
   Chakma,
 Cham,  Cham,
 Cherokee,  Cherokee,
 Common,  Common,
Line 636  Lisu, Line 716  Lisu,
 Lycian,  Lycian,
 Lydian,  Lydian,
 Malayalam,  Malayalam,
   Mandaic,
 Meetei_Mayek,  Meetei_Mayek,
   Meroitic_Cursive,
   Meroitic_Hieroglyphs,
   Miao,
 Mongolian,  Mongolian,
 Myanmar,  Myanmar,
 New_Tai_Lue,  New_Tai_Lue,
Line 655  Rejang, Line 739  Rejang,
 Runic,  Runic,
 Samaritan,  Samaritan,
 Saurashtra,  Saurashtra,
   Sharada,
 Shavian,  Shavian,
 Sinhala,  Sinhala,
   Sora_Sompeng,
 Sundanese,  Sundanese,
 Syloti_Nagri,  Syloti_Nagri,
 Syriac,  Syriac,
Line 665  Tagbanwa, Line 751  Tagbanwa,
 Tai_Le,  Tai_Le,
 Tai_Tham,  Tai_Tham,
 Tai_Viet,  Tai_Viet,
   Takri,
 Tamil,  Tamil,
 Telugu,  Telugu,
 Thaana,  Thaana,
Line 742  a modifier or "other". Line 829  a modifier or "other".
 </P>  </P>
 <P>  <P>
 The Cs (Surrogate) property applies only to characters in the range U+D800 to  The Cs (Surrogate) property applies only to characters in the range U+D800 to
U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and soU+DFFF. Such characters are not valid in Unicode strings and so
cannot be tested by PCRE, unless UTF-8 validity checking has been turned offcannot be tested by PCRE, unless UTF validity checking has been turned off
(see the discussion of PCRE_NO_UTF8_CHECK in the(see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
 PCRE_NO_UTF32_CHECK in the
 <a href="pcreapi.html"><b>pcreapi</b></a>  <a href="pcreapi.html"><b>pcreapi</b></a>
 page). Perl does not support the Cs property.  page). Perl does not support the Cs property.
 </P>  </P>
Line 760  Unicode table. Line 848  Unicode table.
 </P>  </P>
 <P>  <P>
 Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
example, \p{Lu} always matches only upper case letters.example, \p{Lu} always matches only upper case letters. This is different from
 the behaviour of current versions of Perl.
 </P>  </P>
 <P>  <P>
The \X escape matches any number of Unicode characters that form an extendedMatching characters by Unicode property is not fast, because PCRE has to do a
Unicode sequence. \X is equivalent tomultistage table lookup in order to find a character's property. That is why
 the traditional escape sequences such as \d and \w do not use Unicode
 properties in PCRE by default, though you can make them do so by setting the
 PCRE_UCP option or by starting the pattern with (*UCP).
 </P>
 <br><b>
 Extended grapheme clusters
 </b><br>
 <P>
 The \X escape matches any number of Unicode characters that form an "extended
 grapheme cluster", and treats the sequence as an atomic group
 <a href="#atomicgroup">(see below).</a>
 Up to and including release 8.31, PCRE matched an earlier, simpler definition
 that was equivalent to
 <pre>  <pre>
   (?&#62;\PM\pM*)    (?&#62;\PM\pM*)
 </pre>  </pre>
That is, it matches a character without the "mark" property, followed by zeroThat is, it matched a character without the "mark" property, followed by zero
or more characters with the "mark" property, and treats the sequence as anor more characters with the "mark" property. Characters with the "mark"
atomic groupproperty are typically non-spacing accents that affect the preceding character.
<a href="#atomicgroup">(see below).</a> 
Characters with the "mark" property are typically accents that affect the 
preceding character. None of them have codepoints less than 256, so in 
non-UTF-8 mode \X matches any one character. 
 </P>  </P>
 <P>  <P>
Note that recent versions of Perl have changed \X to match what Unicode callsThis simple definition was extended in Unicode to include more complicated
an "extended grapheme cluster", which has a more complicated definition.kinds of composite character by giving each character a grapheme breaking
 property, and creating rules that use these properties to define the boundaries
 of extended grapheme clusters. In releases of PCRE later than 8.31, \X matches
 one of these clusters.
 </P>  </P>
 <P>  <P>
Matching characters by Unicode property is not fast, because PCRE has to search\X always matches at least one character. Then it decides whether to add
a structure that contains data for over fifteen thousand characters. That isadditional characters according to the following rules for ending a cluster:
why the traditional escape sequences such as \d and \w do not use Unicode</P>
properties in PCRE by default, though you can make them do so by setting the<P>
PCRE_UCP option for <b>pcre_compile()</b> or by starting the pattern with1. End at the end of the subject string.
(*UCP).</P>
 <P>
 2. Do not end between CR and LF; otherwise end after any control character.
 </P>
 <P>
 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
 are of five types: L, V, T, LV, and LVT. An L character may be followed by an
 L, V, LV, or LVT character; an LV or V character may be followed by a V or T
 character; an LVT or T character may be follwed only by a T character.
 </P>
 <P>
 4. Do not end before extending characters or spacing marks. Characters with
 the "mark" property always have the "extend" grapheme breaking property.
 </P>
 <P>
 5. Do not end after prepend characters.
 </P>
 <P>
 6. Otherwise, end the cluster.
 <a name="extraprops"></a></P>  <a name="extraprops"></a></P>
 <br><b>  <br><b>
 PCRE's additional properties  PCRE's additional properties
 </b><br>  </b><br>
 <P>  <P>
As well as the standard Unicode properties described in the previousAs well as the standard Unicode properties described above, PCRE supports four
section, PCRE supports four more that make it possible to convert traditionalmore that make it possible to convert traditional escape sequences such as \w
escape sequences such as \w and \s and POSIX character classes to use Unicodeand \s and POSIX character classes to use Unicode properties. PCRE uses these
properties. PCRE uses these non-standard, non-Perl properties internally whennon-standard, non-Perl properties internally when PCRE_UCP is set. However,
PCRE_UCP is set. They are:they may also be used explicitly. These properties are:
 <pre>  <pre>
   Xan   Any alphanumeric character    Xan   Any alphanumeric character
   Xps   Any POSIX space character    Xps   Any POSIX space character
Line 804  PCRE_UCP is set. They are: Line 923  PCRE_UCP is set. They are:
   Xwd   Any Perl "word" character    Xwd   Any Perl "word" character
 </pre>  </pre>
 Xan matches characters that have either the L (letter) or the N (number)  Xan matches characters that have either the L (letter) or the N (number)
property. Xps matches the characters tab, linefeed, vertical tab, formfeed, orproperty. Xps matches the characters tab, linefeed, vertical tab, form feed, or
 carriage return, and any other character that has the Z (separator) property.  carriage return, and any other character that has the Z (separator) property.
 Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the  Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
 same characters as Xan, plus underscore.  same characters as Xan, plus underscore.
   </P>
   <P>
   There is another non-standard property, Xuc, which matches any character that
   can be represented by a Universal Character Name in C++ and other programming
   languages. These are the characters $, @, ` (grave accent), and all characters
   with Unicode code points greater than or equal to U+00A0, except for the
   surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
   excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH
   where H is a hexadecimal digit. Note that the Xuc property does not match these
   sequences but the characters that they represent.)
 <a name="resetmatchstart"></a></P>  <a name="resetmatchstart"></a></P>
 <br><b>  <br><b>
 Resetting the match start  Resetting the match start
Line 865  escape sequence" error is generated instead. Line 994  escape sequence" error is generated instead.
 A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
 and the previous character do not both match \w or \W (i.e. one matches  and the previous character do not both match \w or \W (i.e. one matches
 \w and the other matches \W), or the start or end of the string if the  \w and the other matches \W), or the start or end of the string if the
first or last character matches \w, respectively. In UTF-8 mode, the meaningsfirst or last character matches \w, respectively. In a UTF mode, the meanings
 of \w and \W can be changed by setting the PCRE_UCP option. When this is  of \w and \W can be changed by setting the PCRE_UCP option. When this is
 done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start  done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
 of word" or "end of word" metasequence. However, whatever follows \b normally  of word" or "end of word" metasequence. However, whatever follows \b normally
Line 904  If all the alternatives of a pattern begin with \G, th Line 1033  If all the alternatives of a pattern begin with \G, th
 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
 regular expression.  regular expression.
 </P>  </P>
<br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br><br><a name="SEC6" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
 <P>  <P>
   The circumflex and dollar metacharacters are zero-width assertions. That is,
   they test for a particular condition being true without consuming any
   characters from the subject string.
   </P>
   <P>
 Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
character is an assertion that is true only if the current matching point ischaracter is an assertion that is true only if the current matching point is at
at the start of the subject string. If the <i>startoffset</i> argument ofthe start of the subject string. If the <i>startoffset</i> argument of
 <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
 option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
 meaning  meaning
Line 924  constrained to match only at the start of the subject, Line 1058  constrained to match only at the start of the subject,
 to be anchored.)  to be anchored.)
 </P>  </P>
 <P>  <P>
A dollar character is an assertion that is true only if the current matchingThe dollar character is an assertion that is true only if the current matching
point is at the end of the subject string, or immediately before a newlinepoint is at the end of the subject string, or immediately before a newline at
at the end of the string (by default). Dollar need not be the last character ofthe end of the string (by default). Note, however, that it does not actually
the pattern if a number of alternatives are involved, but it should be the lastmatch the newline. Dollar need not be the last character of the pattern if a
item in any branch in which it appears. Dollar has no special meaning in anumber of alternatives are involved, but it should be the last item in any
character class.branch in which it appears. Dollar has no special meaning in a character class.
 </P>  </P>
 <P>  <P>
 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 958  Note that the sequences \A, \Z, and \z can be used to  Line 1092  Note that the sequences \A, \Z, and \z can be used to 
 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
 \A it is always anchored, whether or not PCRE_MULTILINE is set.  \A it is always anchored, whether or not PCRE_MULTILINE is set.
 <a name="fullstopdot"></a></P>  <a name="fullstopdot"></a></P>
<br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br><br><a name="SEC7" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
 <P>  <P>
 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
 the subject string except (by default) a character that signifies the end of a  the subject string except (by default) a character that signifies the end of a
line. In UTF-8 mode, the matched character may be more than one byte long.line.
 </P>  </P>
 <P>  <P>
 When a line ending is defined as a single character, dot never matches that  When a line ending is defined as a single character, dot never matches that
Line 989  the PCRE_DOTALL option. In other words, it matches any Line 1123  the PCRE_DOTALL option. In other words, it matches any
 that signifies the end of a line. Perl also uses \N to match characters by  that signifies the end of a line. Perl also uses \N to match characters by
 name; PCRE does not support this.  name; PCRE does not support this.
 </P>  </P>
<br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br><br><a name="SEC8" href="#TOC1">MATCHING A SINGLE DATA UNIT</a><br>
 <P>  <P>
Outside a character class, the escape sequence \C matches any one byte, bothOutside a character class, the escape sequence \C matches any one data unit,
in and out of UTF-8 mode. Unlike a dot, it always matches line-endingwhether or not a UTF mode is set. In the 8-bit library, one data unit is one
characters. The feature is provided in Perl in order to match individual bytesbyte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
in UTF-8 mode, but it is unclear how it can usefully be used. Because \Ca 32-bit unit. Unlike a dot, \C always
breaks up characters into individual bytes, matching one byte with \C in UTF-8matches line-ending characters. The feature is provided in Perl in order to
mode means that the rest of the string may start with a malformed UTF-8match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
character. This has undefined results, because PCRE assumes that it is dealingused. Because \C breaks up characters into individual data units, matching one
with valid UTF-8 strings (and by default it checks this at the start ofunit with \C in a UTF mode means that the rest of the string may start with a
processing unless the PCRE_NO_UTF8_CHECK option is used).malformed UTF character. This has undefined results, because PCRE assumes that
 it is dealing with valid UTF strings (and by default it checks this at the
 start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
 PCRE_NO_UTF32_CHECK option is used).
 </P>  </P>
 <P>  <P>
 PCRE does not allow \C to appear in lookbehind assertions  PCRE does not allow \C to appear in lookbehind assertions
 <a href="#lookbehind">(described below)</a>  <a href="#lookbehind">(described below)</a>
in UTF-8 mode, because this would make it impossible to calculate the length ofin a UTF mode, because this would make it impossible to calculate the length of
 the lookbehind.  the lookbehind.
 </P>  </P>
 <P>  <P>
In general, the \C escape sequence is best avoided in UTF-8 mode. However, oneIn general, the \C escape sequence is best avoided. However, one
way of using it that avoids the problem of malformed UTF-8 characters is toway of using it that avoids the problem of malformed UTF characters is to use a
use a lookahead to check the length of the next character, as in this patternlookahead to check the length of the next character, as in this pattern, which
(ignore white space and line breaks):could be used with a UTF-8 string (ignore white space and line breaks):
 <pre>  <pre>
   (?| (?=[\x00-\x7f])(\C) |    (?| (?=[\x00-\x7f])(\C) |
       (?=[\x80-\x{7ff}])(\C)(\C) |        (?=[\x80-\x{7ff}])(\C)(\C) |
Line 1026  character for values whose encoding uses 1, 2, 3, or 4 Line 1163  character for values whose encoding uses 1, 2, 3, or 4
 character's individual bytes are then captured by the appropriate number of  character's individual bytes are then captured by the appropriate number of
 groups.  groups.
 <a name="characterclass"></a></P>  <a name="characterclass"></a></P>
<br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br><br><a name="SEC9" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
 <P>  <P>
 An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
 square bracket. A closing square bracket on its own is not special by default.  square bracket. A closing square bracket on its own is not special by default.
Line 1036  a member of the class, it should be the first data cha Line 1173  a member of the class, it should be the first data cha
 (after an initial circumflex, if present) or escaped with a backslash.  (after an initial circumflex, if present) or escaped with a backslash.
 </P>  </P>
 <P>  <P>
A character class matches a single character in the subject. In UTF-8 mode, theA character class matches a single character in the subject. In a UTF mode, the
character may be more than one byte long. A matched character must be in thecharacter may be more than one data unit long. A matched character must be in
set of characters defined by the class, unless the first character in the classthe set of characters defined by the class, unless the first character in the
definition is a circumflex, in which case the subject character must not be inclass definition is a circumflex, in which case the subject character must not
the set defined by the class. If a circumflex is actually required as a memberbe in the set defined by the class. If a circumflex is actually required as a
of the class, ensure it is not the first character, or escape it with amember of the class, ensure it is not the first character, or escape it with a
 backslash.  backslash.
 </P>  </P>
 <P>  <P>
Line 1054  string, and therefore it fails if the current pointer  Line 1191  string, and therefore it fails if the current pointer 
 string.  string.
 </P>  </P>
 <P>  <P>
In UTF-8 mode, characters with values greater than 255 can be included in aIn UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
class as a literal string of bytes, or by using the \x{ escaping mechanism.can be included in a class as a literal string of data units, or by using the
 \x{ escaping mechanism.
 </P>  </P>
 <P>  <P>
 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
 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
 "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
caseful version would. In UTF-8 mode, PCRE always understands the concept ofcaseful version would. In a UTF mode, PCRE always understands the concept of
 case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
 always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
 supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
If you want to use caseless matching in UTF8-mode for characters 128 and above,If you want to use caseless matching in a UTF mode for characters 128 and
you must ensure that PCRE is compiled with Unicode property support as well asabove, you must ensure that PCRE is compiled with Unicode property support as
with UTF-8 support.well as with UTF support.
 </P>  </P>
 <P>  <P>
 Characters that might indicate line breaks are never treated in any special way  Characters that might indicate line breaks are never treated in any special way
Line 1093  followed by two other characters. The octal or hexadec Line 1231  followed by two other characters. The octal or hexadec
 </P>  </P>
 <P>  <P>
 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
used for characters specified numerically, for example [\000-\037]. In UTF-8used for characters specified numerically, for example [\000-\037]. Ranges
mode, ranges can include characters whose values are greater than 255, forcan include any characters that are valid for the current mode.
example [\x{100}-\x{2ff}]. 
 </P>  </P>
 <P>  <P>
 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
 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
[][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character[][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
 tables for a French locale are in use, [\xc8-\xcb] matches accented E  tables for a French locale are in use, [\xc8-\xcb] matches accented E
characters in both cases. In UTF-8 mode, PCRE supports the concept of case forcharacters in both cases. In UTF modes, PCRE supports the concept of case for
 characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
 property support.  property support.
 </P>  </P>
Line 1110  property support. Line 1247  property support.
 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,  The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
 \V, \w, and \W may appear in a character class, and add the characters that  \V, \w, and \W may appear in a character class, and add the characters that
 they match to the class. For example, [\dABCDEF] matches any hexadecimal  they match to the class. For example, [\dABCDEF] matches any hexadecimal
digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \d, \s, \wdigit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w
 and their upper case partners, just as it does when they appear outside a  and their upper case partners, just as it does when they appear outside a
 character class, as described in the section entitled  character class, as described in the section entitled
 <a href="#genericchartypes">"Generic character types"</a>  <a href="#genericchartypes">"Generic character types"</a>
Line 1136  introducing a POSIX class name - see the next section) Line 1273  introducing a POSIX class name - see the next section)
 closing square bracket. However, escaping other non-alphanumeric characters  closing square bracket. However, escaping other non-alphanumeric characters
 does no harm.  does no harm.
 </P>  </P>
<br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br><br><a name="SEC10" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
 <P>  <P>
 Perl supports the POSIX notation for character classes. This uses names  Perl supports the POSIX notation for character classes. This uses names
 enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
Line 1179  syntax [.ch.] and [=ch=] where "ch" is a "collating el Line 1316  syntax [.ch.] and [=ch=] where "ch" is a "collating el
 supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
 </P>  </P>
 <P>  <P>
By default, in UTF-8 mode, characters with values greater than 128 do not matchBy default, in UTF modes, characters with values greater than 128 do not match
 any of the POSIX character classes. However, if the PCRE_UCP option is passed  any of the POSIX character classes. However, if the PCRE_UCP option is passed
 to <b>pcre_compile()</b>, some of the classes are changed so that Unicode  to <b>pcre_compile()</b>, some of the classes are changed so that Unicode
 character properties are used. This is achieved by replacing the POSIX classes  character properties are used. This is achieved by replacing the POSIX classes
Line 1198  Negated versions, such as [:^alpha:] use \P instead of Line 1335  Negated versions, such as [:^alpha:] use \P instead of
 classes are unchanged, and match only characters with code points less than  classes are unchanged, and match only characters with code points less than
 128.  128.
 </P>  </P>
<br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br><br><a name="SEC11" href="#TOC1">VERTICAL BAR</a><br>
 <P>  <P>
 Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
 the pattern  the pattern
Line 1213  that succeeds is used. If the alternatives are within  Line 1350  that succeeds is used. If the alternatives are within 
 "succeeds" means matching the rest of the main pattern as well as the  "succeeds" means matching the rest of the main pattern as well as the
 alternative in the subpattern.  alternative in the subpattern.
 </P>  </P>
<br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br><br><a name="SEC12" href="#TOC1">INTERNAL OPTION SETTING</a><br>
 <P>  <P>
 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
Line 1264  behaviour otherwise. Line 1401  behaviour otherwise.
 </P>  </P>
 <P>  <P>
 <b>Note:</b> There are other PCRE-specific options that can be set by the  <b>Note:</b> There are other PCRE-specific options that can be set by the
application when the compile or match functions are called. In some cases theapplication when the compiling or matching functions are called. In some cases
pattern can contain special leading sequences such as (*CRLF) to override whatthe pattern can contain special leading sequences such as (*CRLF) to override
the application has set or what has been defaulted. Details are given in thewhat the application has set or what has been defaulted. Details are given in
section entitledthe section entitled
 <a href="#newlineseq">"Newline sequences"</a>  <a href="#newlineseq">"Newline sequences"</a>
above. There are also the (*UTF8) and (*UCP) leading sequences that can be usedabove. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
to set UTF-8 and Unicode property modes; they are equivalent to setting thesequences that can be used to set UTF and Unicode property modes; they are
PCRE_UTF8 and the PCRE_UCP options, respectively.equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
 options, respectively. The (*UTF) sequence is a generic version that can be
 used with any of the libraries. However, the application can set the
 PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
 <a name="subpattern"></a></P>  <a name="subpattern"></a></P>
<br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br><br><a name="SEC13" href="#TOC1">SUBPATTERNS</a><br>
 <P>  <P>
 Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
 Turning part of a pattern into a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
Line 1289  match "cataract", "erpillar" or an empty string. Line 1429  match "cataract", "erpillar" or an empty string.
 <br>  <br>
 2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
 the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
subpattern is passed back to the caller via the <i>ovector</i> argument ofsubpattern is passed back to the caller via the <i>ovector</i> argument of the
<b>pcre_exec()</b>. Opening parentheses are counted from left to right (startingmatching function. (This applies only to the traditional matching functions;
from 1) to obtain numbers for the capturing subpatterns. For example, if thethe DFA matching functions do not support capturing.)
string "the red king" is matched against the pattern</P>
 <P>
 Opening parentheses are counted from left to right (starting from 1) to obtain
 numbers for the capturing subpatterns. For example, if the string "the red
 king" is matched against the pattern
 <pre>  <pre>
   the ((red|white) (king|queen))    the ((red|white) (king|queen))
 </pre>  </pre>
Line 1325  from left to right, and options are not reset until th Line 1469  from left to right, and options are not reset until th
 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
 the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
 <a name="dupsubpatternnumber"></a></P>  <a name="dupsubpatternnumber"></a></P>
<br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br><br><a name="SEC14" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
 <P>  <P>
 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses  Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
 the same numbers for its capturing parentheses. Such a subpattern starts with  the same numbers for its capturing parentheses. Such a subpattern starts with
Line 1369  true if any of the subpatterns of that number have mat Line 1513  true if any of the subpatterns of that number have mat
 An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
 duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
 </P>  </P>
<br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br><br><a name="SEC15" href="#TOC1">NAMED SUBPATTERNS</a><br>
 <P>  <P>
 Identifying capturing parentheses by number is simple, but it can be very hard  Identifying capturing parentheses by number is simple, but it can be very hard
 to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
Line 1444  matching. For this reason, an error is given at compil Line 1588  matching. For this reason, an error is given at compil
 are given to subpatterns with the same number. However, you can give the same  are given to subpatterns with the same number. However, you can give the same
 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.  name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
 </P>  </P>
<br><a name="SEC15" href="#TOC1">REPETITION</a><br><br><a name="SEC16" href="#TOC1">REPETITION</a><br>
 <P>  <P>
 Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
 items:  items:
Line 1452  items: Line 1596  items:
   a literal data character    a literal data character
   the dot metacharacter    the dot metacharacter
   the \C escape sequence    the \C escape sequence
  the \X escape sequence (in UTF-8 mode with Unicode properties)  the \X escape sequence
   the \R escape sequence    the \R escape sequence
   an escape such as \d or \pL that matches a single character    an escape such as \d or \pL that matches a single character
   a character class    a character class
Line 1484  quantifier, is taken as a literal character. For examp Line 1628  quantifier, is taken as a literal character. For examp
 quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
 </P>  </P>
 <P>  <P>
In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individualIn UTF modes, quantifiers apply to characters rather than to individual data
bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each ofunits. Thus, for example, \x{100}{2} matches two characters, each of
which is represented by a two-byte sequence. Similarly, when Unicode propertywhich is represented by a two-byte sequence in a UTF-8 string. Similarly,
support is available, \X{3} matches three Unicode extended sequences, each of\X{3} matches three Unicode extended grapheme clusters, each of which may be
which may be several bytes long (and they may be of different lengths).several data units long (and they may be of different lengths).
 </P>  </P>
 <P>  <P>
 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
Line 1577  worth setting PCRE_DOTALL in order to obtain this opti Line 1721  worth setting PCRE_DOTALL in order to obtain this opti
 alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
 </P>  </P>
 <P>  <P>
However, there is one situation where the optimization cannot be used. When .*However, there are some cases where the optimization cannot be used. When .*
 is inside capturing parentheses that are the subject of a back reference  is inside capturing parentheses that are the subject of a back reference
 elsewhere in the pattern, a match at the start may fail where a later one  elsewhere in the pattern, a match at the start may fail where a later one
 succeeds. Consider, for example:  succeeds. Consider, for example:
Line 1588  If the subject is "xyz123abc123" the match point is th Line 1732  If the subject is "xyz123abc123" the match point is th
 this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
 </P>  </P>
 <P>  <P>
   Another case where implicit anchoring is not applied is when the leading .* is
   inside an atomic group. Once again, a match at the start may fail where a later
   one succeeds. Consider this pattern:
   <pre>
     (?&#62;.*?a)b
   </pre>
   It matches "ab" in the subject "aab". The use of the backtracking control verbs
   (*PRUNE) and (*SKIP) also disable this optimization.
   </P>
   <P>
 When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
 that matched the final iteration. For example, after  that matched the final iteration. For example, after
 <pre>  <pre>
Line 1602  example, after Line 1756  example, after
 </pre>  </pre>
 matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
 <a name="atomicgroup"></a></P>  <a name="atomicgroup"></a></P>
<br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br><br><a name="SEC17" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
 <P>  <P>
 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
 repetition, failure of what follows normally causes the repeated item to be  repetition, failure of what follows normally causes the repeated item to be
Line 1706  an atomic group, like this: Line 1860  an atomic group, like this:
 </pre>  </pre>
 sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
 <a name="backreferences"></a></P>  <a name="backreferences"></a></P>
<br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br><br><a name="SEC18" href="#TOC1">BACK REFERENCES</a><br>
 <P>  <P>
 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
 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 1805  Because there may be many capturing parentheses in a p Line 1959  Because there may be many capturing parentheses in a p
 following a backslash are taken as part of a potential back reference number.  following a backslash are taken as part of a potential back reference number.
 If the pattern continues with a digit character, some delimiter must be used to  If the pattern continues with a digit character, some delimiter must be used to
 terminate the back reference. If the PCRE_EXTENDED option is set, this can be  terminate the back reference. If the PCRE_EXTENDED option is set, this can be
whitespace. Otherwise, the \g{ syntax or an empty comment (seewhite space. Otherwise, the \g{ syntax or an empty comment (see
 <a href="#comments">"Comments"</a>  <a href="#comments">"Comments"</a>
 below) can be used.  below) can be used.
 </P>  </P>
Line 1834  as an Line 1988  as an
 Once the whole group has been matched, a subsequent matching failure cannot  Once the whole group has been matched, a subsequent matching failure cannot
 cause backtracking into the middle of the group.  cause backtracking into the middle of the group.
 <a name="bigassertions"></a></P>  <a name="bigassertions"></a></P>
<br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br><br><a name="SEC19" href="#TOC1">ASSERTIONS</a><br>
 <P>  <P>
 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
 matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
Line 1851  except that it does not cause the current matching pos Line 2005  except that it does not cause the current matching pos
 Assertion subpatterns are not capturing subpatterns. If such an assertion  Assertion subpatterns are not capturing subpatterns. If such an assertion
 contains capturing subpatterns within it, these are counted for the purposes of  contains capturing subpatterns within it, these are counted for the purposes of
 numbering the capturing subpatterns in the whole pattern. However, substring  numbering the capturing subpatterns in the whole pattern. However, substring
capturing is carried out only for positive assertions, because it does not makecapturing is carried out only for positive assertions. (Perl sometimes, but not
sense for negative assertions.always, does do capturing in negative assertions.)
 </P>  </P>
 <P>  <P>
 For compatibility with Perl, assertion subpatterns may be repeated; though  For compatibility with Perl, assertion subpatterns may be repeated; though
Line 1950  match. If there are insufficient characters before the Line 2104  match. If there are insufficient characters before the
 assertion fails.  assertion fails.
 </P>  </P>
 <P>  <P>
In UTF-8 mode, PCRE does not allow the \C escape (which matches a single byte,In a UTF mode, PCRE does not allow the \C escape (which matches a single data
even in UTF-8 mode) to appear in lookbehind assertions, because it makes itunit even in a UTF mode) to appear in lookbehind assertions, because it makes
impossible to calculate the length of the lookbehind. The \X and \R escapes,it impossible to calculate the length of the lookbehind. The \X and \R
which can match different numbers of bytes, are also not permitted.escapes, which can match different numbers of data units, are also not
 permitted.
 </P>  </P>
 <P>  <P>
 <a href="#subpatternsassubroutines">"Subroutine"</a>  <a href="#subpatternsassubroutines">"Subroutine"</a>
Line 2023  preceded by "foo", while Line 2178  preceded by "foo", while
 is another pattern that matches "foo" preceded by three digits and any three  is another pattern that matches "foo" preceded by three digits and any three
 characters that are not "999".  characters that are not "999".
 <a name="conditions"></a></P>  <a name="conditions"></a></P>
<br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br><br><a name="SEC20" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
 <P>  <P>
 It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
 conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
Line 2146  point in the pattern; the idea of DEFINE is that it ca Line 2301  point in the pattern; the idea of DEFINE is that it ca
 subroutines that can be referenced from elsewhere. (The use of  subroutines that can be referenced from elsewhere. (The use of
 <a href="#subpatternsassubroutines">subroutines</a>  <a href="#subpatternsassubroutines">subroutines</a>
 is described below.) For example, a pattern to match an IPv4 address such as  is described below.) For example, a pattern to match an IPv4 address such as
"192.168.23.245" could be written like this (ignore whitespace and line"192.168.23.245" could be written like this (ignore white space and line
 breaks):  breaks):
 <pre>  <pre>
   (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )    (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
Line 2178  subject is matched against the first alternative; othe Line 2333  subject is matched against the first alternative; othe
 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
 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.
 <a name="comments"></a></P>  <a name="comments"></a></P>
<br><a name="SEC20" href="#TOC1">COMMENTS</a><br><br><a name="SEC21" href="#TOC1">COMMENTS</a><br>
 <P>  <P>
 There are two ways of including comments in patterns that are processed by  There are two ways of including comments in patterns that are processed by
 PCRE. In both cases, the start of the comment must not be in a character class,  PCRE. In both cases, the start of the comment must not be in a character class,
Line 2192  closing parenthesis. Nested parentheses are not permit Line 2347  closing parenthesis. Nested parentheses are not permit
 option is set, an unescaped # character also introduces a comment, which in  option is set, an unescaped # character also introduces a comment, which in
 this case continues to immediately after the next newline character or  this case continues to immediately after the next newline character or
 character sequence in the pattern. Which characters are interpreted as newlines  character sequence in the pattern. Which characters are interpreted as newlines
is controlled by the options passed to <b>pcre_compile()</b> or by a specialis controlled by the options passed to a compiling function or by a special
 sequence at the start of the pattern, as described in the section entitled  sequence at the start of the pattern, as described in the section entitled
 <a href="#newlines">"Newline conventions"</a>  <a href="#newlines">"Newline conventions"</a>
 above. Note that the end of this type of comment is a literal newline sequence  above. Note that the end of this type of comment is a literal newline sequence
Line 2207  a newline in the pattern. The sequence \n is still lit Line 2362  a newline in the pattern. The sequence \n is still lit
 it does not terminate the comment. Only an actual character with the code value  it does not terminate the comment. Only an actual character with the code value
 0x0a (the default newline) does so.  0x0a (the default newline) does so.
 <a name="recursion"></a></P>  <a name="recursion"></a></P>
<br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br><br><a name="SEC22" href="#TOC1">RECURSIVE PATTERNS</a><br>
 <P>  <P>
 Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
 unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
Line 2422  now match "b" and so the whole match succeeds. In Perl Line 2577  now match "b" and so the whole match succeeds. In Perl
 match because inside the recursive call \1 cannot access the externally set  match because inside the recursive call \1 cannot access the externally set
 value.  value.
 <a name="subpatternsassubroutines"></a></P>  <a name="subpatternsassubroutines"></a></P>
<br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br><br><a name="SEC23" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
 <P>  <P>
 If the syntax for a recursive subpattern call (either by number or by  If the syntax for a recursive subpattern call (either by number or by
 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
Line 2463  different calls. For example, consider this pattern: Line 2618  different calls. For example, consider this pattern:
 It matches "abcabc". It does not match "abcABC" because the change of  It matches "abcabc". It does not match "abcABC" because the change of
 processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
 <a name="onigurumasubroutines"></a></P>  <a name="onigurumasubroutines"></a></P>
<br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br><br><a name="SEC24" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
 <P>  <P>
 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or  For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
 a number enclosed either in angle brackets or single quotes, is an alternative  a number enclosed either in angle brackets or single quotes, is an alternative
Line 2481  plus or a minus sign it is taken as a relative referen Line 2636  plus or a minus sign it is taken as a relative referen
 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>  Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
 synonymous. The former is a back reference; the latter is a subroutine call.  synonymous. The former is a back reference; the latter is a subroutine call.
 </P>  </P>
<br><a name="SEC24" href="#TOC1">CALLOUTS</a><br><br><a name="SEC25" href="#TOC1">CALLOUTS</a><br>
 <P>  <P>
 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl  Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
 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 2491  same pair of parentheses when there is a repetition. Line 2646  same pair of parentheses when there is a repetition.
 <P>  <P>
 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
 code. The feature is called "callout". The caller of PCRE provides an external  code. The feature is called "callout". The caller of PCRE provides an external
function by putting its entry point in the global variable <i>pcre_callout</i>.function by putting its entry point in the global variable <i>pcre_callout</i>
 (8-bit library) or <i>pcre[16|32]_callout</i> (16-bit or 32-bit library).
 By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
 </P>  </P>
 <P>  <P>
Line 2502  For example, this pattern has two callout points: Line 2658  For example, this pattern has two callout points:
 <pre>  <pre>
   (?C1)abc(?C2)def    (?C1)abc(?C2)def
 </pre>  </pre>
If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts areIf the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
 automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
255.255. If there is a conditional group in the pattern whose condition is an
 assertion, an additional callout is inserted just before the condition. An
 explicit callout may also be set at this position, as in this example:
 <pre>
   (?(?C9)(?=a)abc|def)
 </pre>
 Note that this applies only to assertion conditions, not to other types of
 condition.
 </P>  </P>
 <P>  <P>
During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> isDuring matching, when PCRE reaches a callout point, the external function is
set), the external function is called. It is provided with the number of thecalled. It is provided with the number of the callout, the position in the
callout, the position in the pattern, and, optionally, one item of datapattern, and, optionally, one item of data originally supplied by the caller of
originally supplied by the caller of <b>pcre_exec()</b>. The callout functionthe matching function. The callout function may cause matching to proceed, to
may cause matching to proceed, to backtrack, or to fail altogether. A completebacktrack, or to fail altogether. A complete description of the interface to
description of the interface to the callout function is given in thethe callout function is given in the
 <a href="pcrecallout.html"><b>pcrecallout</b></a>  <a href="pcrecallout.html"><b>pcrecallout</b></a>
 documentation.  documentation.
 <a name="backtrackcontrol"></a></P>  <a name="backtrackcontrol"></a></P>
<br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br><br><a name="SEC26" href="#TOC1">BACKTRACKING CONTROL</a><br>
 <P>  <P>
 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
are described in the Perl documentation as "experimental and subject to changeare still described in the Perl documentation as "experimental and subject to
or removal in a future version of Perl". It goes on to say: "Their usage inchange or removal in a future version of Perl". It goes on to say: "Their usage
production code should be noted to avoid problems during upgrades." The samein production code should be noted to avoid problems during upgrades." The same
 remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
 </P>  </P>
 <P>  <P>
Since these verbs are specifically related to backtracking, most of them can beThe new verbs make use of what was previously invalid syntax: an opening
used only when the pattern is to be matched using <b>pcre_exec()</b>, which usesparenthesis followed by an asterisk. They are generally of the form
a backtracking algorithm. With the exception of (*FAIL), which behaves like a(*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
failing negative assertion, they cause an error if encountered bydifferently depending on whether or not a name is present. A name is any
<b>pcre_dfa_exec()</b>.sequence of characters that does not include a closing parenthesis. The maximum
 length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
 libraries. If the name is empty, that is, if the closing parenthesis
 immediately follows the colon, the effect is as if the colon were not there.
 Any number of these verbs may occur in a pattern.
 </P>  </P>
 <P>  <P>
If any of these verbs are used in an assertion or in a subpattern that isSince these verbs are specifically related to backtracking, most of them can be
called as a subroutine (whether or not recursively), their effect is confinedused only when the pattern is to be matched using one of the traditional
to that subpattern; it does not extend to the surrounding pattern, with onematching functions, because these use a backtracking algorithm. With the
exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered inexception of (*FAIL), which behaves like a failing negative assertion, the
a successful positive assertion <i>is</i> passed back when a match succeedsbacktracking control verbs cause an error if encountered by a DFA matching
(compare capturing parentheses in assertions). Note that such subpatterns arefunction.
processed as anchored at the point where they are tested. Note also that Perl's 
treatment of subroutines is different in some cases. 
 </P>  </P>
 <P>  <P>
The new verbs make use of what was previously invalid syntax: an openingThe behaviour of these verbs in
parenthesis followed by an asterisk. They are generally of the form<a href="#btrepeat">repeated groups,</a>
(*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,<a href="#btassert">assertions,</a>
depending on whether or not an argument is present. A name is any sequence ofand in
characters that does not include a closing parenthesis. If the name is empty,<a href="#btsub">subpatterns called as subroutines</a>
that is, if the closing parenthesis immediately follows the colon, the effect(whether or not recursively) is documented below.
is as if the colon were not there. Any number of these verbs may occur in a<a name="nooptimize"></a></P>
pattern.<br><b>
</P>Optimizations that affect backtracking verbs
 </b><br>
 <P>  <P>
 PCRE contains some optimizations that are used to speed up matching by running  PCRE contains some optimizations that are used to speed up matching by running
 some checks at the start of each match attempt. For example, it may know the  some checks at the start of each match attempt. For example, it may know the
 minimum length of matching subject, or that a particular character must be  minimum length of matching subject, or that a particular character must be
present. When one of these optimizations suppresses the running of a match, anypresent. When one of these optimizations bypasses the running of a match, any
 included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
 when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the  when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
pattern with (*NO_START_OPT).pattern with (*NO_START_OPT). There is more discussion of this option in the
 section entitled
 <a href="pcreapi.html#execoptions">"Option bits for <b>pcre_exec()</b>"</a>
 in the
 <a href="pcreapi.html"><b>pcreapi</b></a>
 documentation.
 </P>  </P>
 <P>  <P>
 Experiments with Perl suggest that it too has similar optimizations, sometimes  Experiments with Perl suggest that it too has similar optimizations, sometimes
Line 2577  followed by a name. Line 2748  followed by a name.
 This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
 pattern. However, when it is inside a subpattern that is called as a  pattern. However, when it is inside a subpattern that is called as a
 subroutine, only that subpattern is ended successfully. Matching then continues  subroutine, only that subpattern is ended successfully. Matching then continues
at the outer level. If (*ACCEPT) is inside capturing parentheses, the data soat the outer level. If (*ACCEPT) in triggered in a positive assertion, the
far is captured. For example:assertion succeeds; in a negative assertion, the assertion fails.
 </P>
 <P>
 If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
 example:
 <pre>  <pre>
   A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
 </pre>  </pre>
Line 2612  A name is always required with this verb. There may be Line 2787  A name is always required with this verb. There may be
 (*MARK) as you like in a pattern, and their names do not have to be unique.  (*MARK) as you like in a pattern, and their names do not have to be unique.
 </P>  </P>
 <P>  <P>
When a match succeeds, the name of the last-encountered (*MARK) on the matchingWhen a match succeeds, the name of the last-encountered (*MARK:NAME),
path is passed back to the caller via the <i>pcre_extra</i> data structure, as(*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
described in thecaller as described in the section entitled
<a href="pcreapi.html#extradata">section on <i>pcre_extra</i></a><a href="pcreapi.html#extradata">"Extra data for <b>pcre_exec()</b>"</a>
 in the  in the
 <a href="pcreapi.html"><b>pcreapi</b></a>  <a href="pcreapi.html"><b>pcreapi</b></a>
 documentation. Here is an example of <b>pcretest</b> output, where the /K  documentation. Here is an example of <b>pcretest</b> output, where the /K
Line 2635  of obtaining this information than putting each altern Line 2810  of obtaining this information than putting each altern
 capturing parentheses.  capturing parentheses.
 </P>  </P>
 <P>  <P>
If (*MARK) is encountered in a positive assertion, its name is recorded andIf a verb with a name is encountered in a positive assertion that is true, the
passed back if it is the last-encountered. This does not happen for negativename is recorded and passed back if it is the last-encountered. This does not
assertions.happen for negative assertions or failing positive assertions.
 </P>  </P>
 <P>  <P>
After a partial match or a failed match, the name of the last encounteredAfter a partial match or a failed match, the last encountered name in the
(*MARK) in the entire match process is returned. For example:entire match process is returned. For example:
 <pre>  <pre>
     re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K      re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K
   data&#62; XP    data&#62; XP
   No match, mark = B    No match, mark = B
 </pre>  </pre>
 Note that in this unanchored example the mark is retained from the match  Note that in this unanchored example the mark is retained from the match
attempt that started at the letter "X". Subsequent match attempts starting atattempt that started at the letter "X" in the subject. Subsequent match
"P" and then with an empty string do not get as far as the (*MARK) item, butattempts starting at "P" and then with an empty string do not get as far as the
nevertheless do not reset it.(*MARK) item, but nevertheless do not reset it.
 </P>  </P>
   <P>
   If you are interested in (*MARK) values after failed matches, you should
   probably set the PCRE_NO_START_OPTIMIZE option
   <a href="#nooptimize">(see above)</a>
   to ensure that the match is always attempted.
   </P>
 <br><b>  <br><b>
 Verbs that act after backtracking  Verbs that act after backtracking
 </b><br>  </b><br>
Line 2659  Verbs that act after backtracking Line 2840  Verbs that act after backtracking
 The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
 with what follows, but if there is no subsequent match, causing a backtrack to  with what follows, but if there is no subsequent match, causing a backtrack to
 the verb, a failure is forced. That is, backtracking cannot pass to the left of  the verb, a failure is forced. That is, backtracking cannot pass to the left of
the verb. However, when one of these verbs appears inside an atomic group, itsthe verb. However, when one of these verbs appears inside an atomic group or an
effect is confined to that group, because once the group has been matched,assertion that is true, its effect is confined to that group, because once the
there is never any backtracking into it. In this situation, backtracking cangroup has been matched, there is never any backtracking into it. In this
"jump back" to the left of the entire atomic group. (Remember also, as statedsituation, backtracking can "jump back" to the left of the entire atomic group
above, that this localization also applies in subroutine calls and assertions.)or assertion. (Remember also, as stated above, that this localization also
 applies in subroutine calls.)
 </P>  </P>
 <P>  <P>
 These verbs differ in exactly what kind of failure occurs when backtracking  These verbs differ in exactly what kind of failure occurs when backtracking
reaches them.reaches them. The behaviour described below is what happens when the verb is
 not in a subroutine or an assertion. Subsequent sections cover these special
 cases.
 <pre>  <pre>
   (*COMMIT)    (*COMMIT)
 </pre>  </pre>
 This verb, which may not be followed by a name, causes the whole match to fail  This verb, which may not be followed by a name, causes the whole match to fail
outright if the rest of the pattern does not match. Even if the pattern isoutright if there is a later matching failure that causes backtracking to reach
unanchored, no further attempts to find a match by advancing the starting pointit. Even if the pattern is unanchored, no further attempts to find a match by
take place. Once (*COMMIT) has been passed, <b>pcre_exec()</b> is committed toadvancing the starting point take place. If (*COMMIT) is the only backtracking
finding a match at the current starting point, or not at all. For example:verb that is encountered, once it has been passed <b>pcre_exec()</b> is
 committed to finding a match at the current starting point, or not at all. For
 example:
 <pre>  <pre>
   a+(*COMMIT)b    a+(*COMMIT)b
 </pre>  </pre>
Line 2685  recently passed (*MARK) in the path is passed back whe Line 2871  recently passed (*MARK) in the path is passed back whe
 match failure.  match failure.
 </P>  </P>
 <P>  <P>
   If there is more than one backtracking verb in a pattern, a different one that
   follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
   match does not always guarantee that a match must be at this starting point.
   </P>
   <P>
 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
 unless PCRE's start-of-match optimizations are turned off, as shown in this  unless PCRE's start-of-match optimizations are turned off, as shown in this
 <b>pcretest</b> example:  <b>pcretest</b> example:
Line 2704  starting points. Line 2895  starting points.
   (*PRUNE) or (*PRUNE:NAME)    (*PRUNE) or (*PRUNE:NAME)
 </pre>  </pre>
 This verb causes the match to fail at the current starting position in the  This verb causes the match to fail at the current starting position in the
subject if the rest of the pattern does not match. If the pattern issubject if there is a later matching failure that causes backtracking to reach
unanchored, the normal "bumpalong" advance to the next starting character thenit. If the pattern is unanchored, the normal "bumpalong" advance to the next
happens. Backtracking can occur as usual to the left of (*PRUNE), before it isstarting character then happens. Backtracking can occur as usual to the left of
reached, or when matching to the right of (*PRUNE), but if there is no match to(*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
the right, backtracking cannot cross (*PRUNE). In simple cases, the use ofif there is no match to the right, backtracking cannot cross (*PRUNE). In
(*PRUNE) is just an alternative to an atomic group or possessive quantifier,simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
but there are some uses of (*PRUNE) that cannot be expressed in any other way.possessive quantifier, but there are some uses of (*PRUNE) that cannot be
The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In anexpressed in any other way. In an anchored pattern (*PRUNE) has the same effect
anchored pattern (*PRUNE) has the same effect as (*COMMIT).as (*COMMIT).
 </P>
 <P>
 The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
 It is like (*MARK:NAME) in that the name is remembered for passing back to the
 caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
 <pre>  <pre>
   (*SKIP)    (*SKIP)
 </pre>  </pre>
Line 2733  instead of skipping on to "c". Line 2929  instead of skipping on to "c".
 <pre>  <pre>
   (*SKIP:NAME)    (*SKIP:NAME)
 </pre>  </pre>
When (*SKIP) has an associated name, its behaviour is modified. If theWhen (*SKIP) has an associated name, its behaviour is modified. When it is
following pattern fails to match, the previous path through the pattern istriggered, the previous path through the pattern is searched for the most
searched for the most recent (*MARK) that has the same name. If one is found,recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
the "bumpalong" advance is to the subject position that corresponds to thatis to the subject position that corresponds to that (*MARK) instead of to where
(*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a(*SKIP) was encountered. If no (*MARK) with a matching name is found, the
matching name is found, the (*SKIP) is ignored.(*SKIP) is ignored.
 </P>
 <P>
 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
 names that are set by (*PRUNE:NAME) or (*THEN:NAME).
 <pre>  <pre>
   (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
 </pre>  </pre>
This verb causes a skip to the next innermost alternative if the rest of theThis verb causes a skip to the next innermost alternative when backtracking
pattern does not match. That is, it cancels pending backtracking, but onlyreaches it. That is, it cancels any further backtracking within the current
within the current alternative. Its name comes from the observation that it canalternative. Its name comes from the observation that it can be used for a
be used for a pattern-based if-then-else block:pattern-based if-then-else block:
 <pre>  <pre>
   ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
 </pre>  </pre>
 If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
 the end of the group if FOO succeeds); on failure, the matcher skips to the  the end of the group if FOO succeeds); on failure, the matcher skips to the
second alternative and tries COND2, without backtracking into COND1. Thesecond alternative and tries COND2, without backtracking into COND1. If that
behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
If (*THEN) is not inside an alternation, it acts like (*PRUNE).more alternatives, so there is a backtrack to whatever came before the entire
 group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
 </P>  </P>
 <P>  <P>
Note that a subpattern that does not contain a | character is just a part ofThe behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
the enclosing alternative; it is not a nested alternation with only oneIt is like (*MARK:NAME) in that the name is remembered for passing back to the
 caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
 </P>
 <P>
 A subpattern that does not contain a | character is just a part of the
 enclosing alternative; it is not a nested alternation with only one
 alternative. The effect of (*THEN) extends beyond such a subpattern to the  alternative. The effect of (*THEN) extends beyond such a subpattern to the
 enclosing alternative. Consider this pattern, where A, B, etc. are complex  enclosing alternative. Consider this pattern, where A, B, etc. are complex
 pattern fragments that do not contain any | characters at this level:  pattern fragments that do not contain any | characters at this level:
Line 2777  because there are no more alternatives to try. In this Line 2983  because there are no more alternatives to try. In this
 backtrack into A.  backtrack into A.
 </P>  </P>
 <P>  <P>
Note also that a conditional subpattern is not considered as having twoNote that a conditional subpattern is not considered as having two
 alternatives, because only one is ever used. In other words, the | character in  alternatives, because only one is ever used. In other words, the | character in
 a conditional subpattern has a different meaning. Ignoring white space,  a conditional subpattern has a different meaning. Ignoring white space,
 consider:  consider:
Line 2801  unanchored pattern). (*SKIP) is similar, except that t Line 3007  unanchored pattern). (*SKIP) is similar, except that t
 than one character. (*COMMIT) is the strongest, causing the entire match to  than one character. (*COMMIT) is the strongest, causing the entire match to
 fail.  fail.
 </P>  </P>
   <br><b>
   More than one backtracking verb
   </b><br>
 <P>  <P>
If more than one such verb is present in a pattern, the "strongest" one wins.If more than one backtracking verb is present in a pattern, the one that is
For example, consider this pattern, where A, B, etc. are complex patternbacktracked onto first acts. For example, consider this pattern, where A, B,
fragments:etc. are complex pattern fragments:
 <pre>  <pre>
  (A(*COMMIT)B(*THEN)C|D)  (A(*COMMIT)B(*THEN)C|ABD)
 </pre>  </pre>
Once A has matched, PCRE is committed to this match, at the current startingIf A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
position. If subsequently B matches, but C does not, the normal (*THEN) actionfail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
of trying the next alternative (that is, D) does not happen because (*COMMIT)the next alternative (ABD) to be tried. This behaviour is consistent, but is
overrides.not always the same as Perl's. It means that if two or more backtracking verbs
 appear in succession, all the the last of them has no effect. Consider this
 example:
 <pre>
   ...(*COMMIT)(*PRUNE)...
 </pre>
 If there is a matching failure to the right, backtracking onto (*PRUNE) cases
 it to be triggered, and its action is taken. There can never be a backtrack
 onto (*COMMIT).
 <a name="btrepeat"></a></P>
 <br><b>
 Backtracking verbs in repeated groups
 </b><br>
 <P>
 PCRE differs from Perl in its handling of backtracking verbs in repeated
 groups. For example, consider:
 <pre>
   /(a(*COMMIT)b)+ac/
 </pre>
 If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
 the second repeat of the group acts.
 <a name="btassert"></a></P>
 <br><b>
 Backtracking verbs in assertions
 </b><br>
 <P>
 (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
 </P>  </P>
 <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>  
 <P>  <P>
   (*ACCEPT) in a positive assertion causes the assertion to succeed without any
   further processing. In a negative assertion, (*ACCEPT) causes the assertion to
   fail without any further processing.
   </P>
   <P>
   The other backtracking verbs are not treated specially if they appear in a
   positive assertion. In particular, (*THEN) skips to the next alternative in the
   innermost enclosing group that has alternations, whether or not this is within
   the assertion.
   </P>
   <P>
   Negative assertions are, however, different, in order to ensure that changing a
   positive assertion into a negative assertion changes its result. Backtracking
   into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
   without considering any further alternative branches in the assertion.
   Backtracking into (*THEN) causes it to skip to the next enclosing alternative
   within the assertion (the normal behaviour), but if the assertion does not have
   such an alternative, (*THEN) behaves like (*PRUNE).
   <a name="btsub"></a></P>
   <br><b>
   Backtracking verbs in subroutines
   </b><br>
   <P>
   These behaviours occur whether or not the subpattern is called recursively.
   Perl's treatment of subroutines is different in some cases.
   </P>
   <P>
   (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
   an immediate backtrack.
   </P>
   <P>
   (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
   succeed without any further processing. Matching then continues after the
   subroutine call.
   </P>
   <P>
   (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
   the subroutine match to fail.
   </P>
   <P>
   (*THEN) skips to the next alternative in the innermost enclosing group within
   the subpattern that has alternatives. If there is no such group within the
   subpattern, (*THEN) causes the subroutine match to fail.
   </P>
   <br><a name="SEC27" href="#TOC1">SEE ALSO</a><br>
   <P>
 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),  <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
<b>pcresyntax</b>(3), <b>pcre</b>(3).<b>pcresyntax</b>(3), <b>pcre</b>(3), <b>pcre16(3)</b>, <b>pcre32(3)</b>.
 </P>  </P>
<br><a name="SEC27" href="#TOC1">AUTHOR</a><br><br><a name="SEC28" href="#TOC1">AUTHOR</a><br>
 <P>  <P>
 Philip Hazel  Philip Hazel
 <br>  <br>
Line 2827  University Computing Service Line 3107  University Computing Service
 Cambridge CB2 3QH, England.  Cambridge CB2 3QH, England.
 <br>  <br>
 </P>  </P>
<br><a name="SEC28" href="#TOC1">REVISION</a><br><br><a name="SEC29" href="#TOC1">REVISION</a><br>
 <P>  <P>
Last updated: 29 November 2011Last updated: 26 April 2013
 <br>  <br>
Copyright &copy; 1997-2011 University of Cambridge.Copyright &copy; 1997-2013 University of Cambridge.
 <br>  <br>
 <p>  <p>
 Return to the <a href="index.html">PCRE index page</a>.  Return to the <a href="index.html">PCRE index page</a>.

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