Diff for /embedaddon/pcre/doc/pcrepattern.3 between versions 1.1.1.3 and 1.1.1.4

version 1.1.1.3, 2012/10/09 09:19:17 version 1.1.1.4, 2013/07/22 08:25:57
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.TH PCREPATTERN 3 "04 May 2012" "PCRE 8.31".TH PCREPATTERN 3 "26 April 2013" "PCRE 8.33"
 .SH NAME  .SH NAME
 PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
 .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
Line 20  have copious examples. Jeffrey Friedl's "Mastering Reg Line 20  have copious examples. Jeffrey Friedl's "Mastering Reg
 published by O'Reilly, covers regular expressions in great detail. This  published by O'Reilly, covers regular expressions in great detail. This
 description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
 .P  .P
   This document discusses the patterns that are supported by PCRE when one its
   main matching functions, \fBpcre_exec()\fP (8-bit) or \fBpcre[16|32]_exec()\fP
   (16- or 32-bit), is used. PCRE also has alternative matching functions,
   \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP, 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
   .\" HREF
   \fBpcrematching\fP
   .\"
   page.
   .
   .
   .SH "SPECIAL START-OF-PATTERN ITEMS"
   .rs
   .sp
   A number of options that can be passed to \fBpcre_compile()\fP 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.
   .
   .
   .SS "UTF support"
   .rs
   .sp
 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 strings in the original library, and athere is now also support for UTF-8 strings in the original library, an
second library that supports 16-bit and UTF-16 character strings. To use theseextra library that supports 16-bit and UTF-16 character strings, and a
 third library that supports 32-bit and UTF-32 character strings. To use these
 features, PCRE must be built to include appropriate support. When using UTF  features, PCRE must be built to include appropriate support. When using UTF
strings you must either call the compiling function with the PCRE_UTF8 orstrings you must either call the compiling function with the PCRE_UTF8,
PCRE_UTF16 option, or the pattern must start with one of these specialPCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
sequences:these special sequences:
 .sp  .sp
   (*UTF8)    (*UTF8)
   (*UTF16)    (*UTF16)
     (*UTF32)
     (*UTF)
 .sp  .sp
   (*UTF) is a generic sequence that can be used with any of the libraries.
 Starting a pattern with such a sequence is equivalent to setting the relevant  Starting a pattern with such a sequence is equivalent to setting the relevant
option. This feature is not Perl-compatible. How setting a UTF mode affectsoption. How setting a UTF mode affects pattern matching is mentioned in several
pattern matching is mentioned in several places below. There is also a summaryplaces below. There is also a summary of features in the
of features in the 
 .\" HREF  .\" HREF
 \fBpcreunicode\fP  \fBpcreunicode\fP
 .\"  .\"
 page.  page.
 .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) or (*UTF16) 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.
 .
 .
 .SS "Unicode property support"
 .rs
 .sp  .sp
   Another special sequence that may appear at the start of a pattern is
   .sp
   (*UCP)    (*UCP)
 .sp  .sp
 This has the same effect as setting the PCRE_UCP option: it causes sequences  This has the same effect as setting the PCRE_UCP option: it causes sequences
 such as \ed and \ew to use Unicode properties to determine character types,  such as \ed and \ew to use Unicode properties to determine character types,
 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.
 .
 .SS "Disabling start-up optimizations"
 .rs
 .sp
 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 
The remainder of this document discusses the patterns that are supported by 
PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or 
\fBpcre16_exec()\fP (16-bit), is used. PCRE also has alternative matching 
functions, \fBpcre_dfa_exec()\fP and \fBpcre16_dfa_exec()\fP, 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 
.\" HREF 
\fBpcrematching\fP 
.\" 
page. 
 .  .
 .  .
 .\" HTML <a name="newlines"></a>  .\" HTML <a name="newlines"></a>
.SH "NEWLINE CONVENTIONS".SS "Newline conventions"
 .rs  .rs
 .sp  .sp
 PCRE supports five different conventions for indicating line breaks in  PCRE supports five different conventions for indicating line breaks in
Line 103  example, on a Unix system where LF is the default newl Line 131  example, on a Unix system where LF is the default newl
   (*CR)a.b    (*CR)a.b
 .sp  .sp
 changes the convention to CR. That pattern matches "a\enb" because LF is no  changes the convention to CR. That pattern matches "a\enb" 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
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 \eN. However, it does nottrue. It also affects the interpretation of the dot metacharacter when
affect what the \eR escape sequence matches. By default, this is any UnicodePCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
newline sequence, for Perl compatibility. However, this can be changed; see thewhat the \eR escape sequence matches. By default, this is any Unicode newline
 sequence, for Perl compatibility. However, this can be changed; see the
 description of \eR in the section entitled  description of \eR in the section entitled
 .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
 .\" </a>  .\" </a>
Line 121  below. A change of \eR setting can be combined with a  Line 148  below. A change of \eR setting can be combined with a 
 convention.  convention.
 .  .
 .  .
   .SS "Setting match and recursion limits"
   .rs
   .sp
   The caller of \fBpcre_exec()\fP can set a limit on the number of times the
   internal \fBmatch()\fP 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, \fBpcre_exec()\fP
   gives an error return. The limits can also be set by items at the start of the
   pattern of the form
   .sp
     (*LIMIT_MATCH=d)
     (*LIMIT_RECURSION=d)
   .sp
   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 \fBpcre_exec()\fP 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.
   .
   .
   .SH "EBCDIC CHARACTER CODES"
   .rs
   .sp
   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.
   .
   .
 .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
 .rs  .rs
 .sp  .sp
Line 246  one of the following escape sequences than the binary  Line 305  one of the following escape sequences than the binary 
   \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
   \euhhhh    character with hex code hhhh (JavaScript mode only)    \euhhhh    character with hex code hhhh (JavaScript mode only)
 .sp  .sp
The precise effect of \ecx is as follows: if x is a lower case letter, itThe precise effect of \ecx 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 \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
\ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greaterbut \ec{ becomes hex 3B ({ is 7B), and \ec; 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 \ec has a value greater than 127, a
all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. Acompile-time error occurs. This locks out non-ASCII characters in all modes.
lower case letter is converted to upper case, and then the 0xc0 bits are 
flipped.) 
 .P  .P
   The \ec 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 \ec. 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 \ecA becomes hex 01, as in ASCII (A is C1), but because
   the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
   characters also generate different values.
   .P
 By default, after \ex, from zero to two hexadecimal digits are read (letters  By default, after \ex, from zero to two hexadecimal digits are read (letters
 can be in upper or lower case). Any number of hexadecimal digits may appear  can be in upper or lower case). Any number of hexadecimal digits may appear
 between \ex{ and }, but the character code is constrained as follows:  between \ex{ and }, but the character code is constrained as follows:
Line 263  between \ex{ and }, but the character code is constrai Line 329  between \ex{ and }, but the character code is constrai
   8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint    8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
   16-bit non-UTF mode   less than 0x10000    16-bit non-UTF mode   less than 0x10000
   16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint    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
 .sp  .sp
 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called  Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
"surrogate" codepoints)."surrogate" codepoints), and 0xffef.
 .P  .P
 If characters other than hexadecimal digits appear between \ex{ and }, or if  If characters other than hexadecimal digits appear between \ex{ and }, or if
 there is no terminating }, this form of escape is not recognized. Instead, the  there is no terminating }, this form of escape is not recognized. Instead, the
Line 313  subsequent digits stand for themselves. The value of t Line 381  subsequent digits stand for themselves. The value of t
 constrained in the same way as characters specified in hexadecimal.  constrained in the same way as characters specified in hexadecimal.
 For example:  For example:
 .sp  .sp
  \e040   is another way of writing a space  \e040   is another way of writing an ASCII space
 .\" JOIN  .\" JOIN
   \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
             previous capturing subpatterns              previous capturing subpatterns
Line 471  release 5.10. In contrast to the other sequences, whic Line 539  release 5.10. In contrast to the other sequences, whic
 characters by default, these always match certain high-valued codepoints,  characters by default, these always match certain high-valued codepoints,
 whether or not PCRE_UCP is set. The horizontal space characters are:  whether or not PCRE_UCP is set. The horizontal space characters are:
 .sp  .sp
  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 493  whether or not PCRE_UCP is set. The horizontal space c Line 561  whether or not PCRE_UCP is set. The horizontal space c
 .sp  .sp
 The vertical space characters are:  The vertical space characters are:
 .sp  .sp
  U+000A     Linefeed  U+000A     Linefeed (LF)
  U+000B     Vertical tab  U+000B     Vertical tab (VT)
  U+000C     Form feed  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
 .sp  .sp
Line 551  change of newline convention; for example, a pattern c Line 619  change of newline convention; for example, a pattern c
 .sp  .sp
   (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
 .sp  .sp
They can also be combined with the (*UTF8), (*UTF16), or (*UCP) specialThey can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
sequences. Inside a character class, \eR is treated as an unrecognized escape(*UCP) special sequences. Inside a character class, \eR is treated as an
sequence, and so matches the letter "R" by default, but causes an error ifunrecognized escape sequence, and so matches the letter "R" by default, but
PCRE_EXTRA is set.causes an error if PCRE_EXTRA is set.
 .  .
 .  .
 .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
Line 569  The extra escape sequences are: Line 637  The extra escape sequences are:
 .sp  .sp
   \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
   \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
  \eX       an extended Unicode sequence  \eX       a Unicode extended grapheme cluster
 .sp  .sp
 The property names represented by \fIxx\fP above are limited to the Unicode  The property names represented by \fIxx\fP above are limited to the Unicode
 script names, the general category properties, "Any", which matches any  script names, the general category properties, "Any", which matches any
Line 762  a modifier or "other". Line 830  a modifier or "other".
 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 Unicode strings and so  U+DFFF. Such characters are not valid in Unicode strings and so
 cannot be tested by PCRE, unless UTF validity checking has been turned off  cannot be tested by PCRE, unless UTF validity checking has been turned off
(see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the(see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
 PCRE_NO_UTF32_CHECK in the
 .\" HREF  .\" HREF
 \fBpcreapi\fP  \fBpcreapi\fP
 .\"  .\"
Line 777  Instead, this property is assumed for any code point t Line 846  Instead, this property is assumed for any code point t
 Unicode table.  Unicode table.
 .P  .P
 Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
example, \ep{Lu} always matches only upper case letters.example, \ep{Lu} always matches only upper case letters. This is different from
 the behaviour of current versions of Perl.
 .P  .P
The \eX 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. \eX is equivalent tomultistage table lookup in order to find a character's property. That is why
 the traditional escape sequences such as \ed and \ew 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).
 .
 .
 .SS Extended grapheme clusters
 .rs
 .sp  .sp
  (?>\ePM\epM*)The \eX escape matches any number of Unicode characters that form an "extended
.spgrapheme cluster", and treats the sequence as an atomic group
That is, it matches a character without the "mark" property, followed by zero 
or more characters with the "mark" property, and treats the sequence as an 
atomic group 
 .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
 .\" </a>  .\" </a>
 (see below).  (see below).
 .\"  .\"
Characters with the "mark" property are typically accents that affect theUp to and including release 8.31, PCRE matched an earlier, simpler definition
preceding character. None of them have codepoints less than 256, so inthat was equivalent to
8-bit non-UTF-8 mode \eX matches any one character..sp
   (?>\ePM\epM*)
 .sp
 That is, it matched a character without the "mark" property, followed by zero
 or more characters with the "mark" property. Characters with the "mark"
 property are typically non-spacing accents that affect the preceding character.
 .P  .P
Note that recent versions of Perl have changed \eX 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, \eX matches
 one of these clusters.
 .P  .P
Matching characters by Unicode property is not fast, because PCRE has to search\eX 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 \ed and \ew do not use Unicode.P
properties in PCRE by default, though you can make them do so by setting the1. End at the end of the subject string.
PCRE_UCP option or by starting the pattern with (*UCP)..P
 2. Do not end between CR and LF; otherwise end after any control character.
 .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
 4. Do not end before extending characters or spacing marks. Characters with
 the "mark" property always have the "extend" grapheme breaking property.
 .P
 5. Do not end after prepend characters.
 .P
 6. Otherwise, end the cluster.
 .  .
 .  .
 .\" HTML <a name="extraprops"></a>  .\" HTML <a name="extraprops"></a>
 .SS PCRE's additional properties  .SS PCRE's additional properties
 .rs  .rs
 .sp  .sp
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 \ew
escape sequences such as \ew and \es and POSIX character classes to use Unicodeand \es 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:
 .sp  .sp
   Xan   Any alphanumeric character    Xan   Any alphanumeric character
   Xps   Any POSIX space character    Xps   Any POSIX space character
Line 825  property. Xps matches the characters tab, linefeed, ve Line 920  property. Xps matches the characters tab, linefeed, ve
 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
   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 \euHHHH or \eUHHHHHHHH
   where H is a hexadecimal digit. Note that the Xuc property does not match these
   sequences but the characters that they represent.)
 .  .
 .  .
 .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
Line 930  regular expression. Line 1034  regular expression.
 .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
 .rs  .rs
 .sp  .sp
   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
 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 \fIstartoffset\fP argument ofthe start of the subject string. If the \fIstartoffset\fP argument of
 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE  \fBpcre_exec()\fP 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 949  constrained to match only at the start of the subject, Line 1057  constrained to match only at the start of the subject,
 "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
 to be anchored.)  to be anchored.)
 .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
 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
 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
Line 1015  name; PCRE does not support this. Line 1123  name; PCRE does not support this.
 .sp  .sp
 Outside a character class, the escape sequence \eC matches any one data unit,  Outside a character class, the escape sequence \eC matches any one data unit,
 whether or not a UTF mode is set. In the 8-bit library, one data unit is one  whether or not a UTF mode is set. In the 8-bit library, one data unit is one
byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC alwaysbyte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
 a 32-bit unit. Unlike a dot, \eC always
 matches line-ending characters. The feature is provided in Perl in order to  matches line-ending characters. The feature is provided in Perl in order to
 match individual bytes in UTF-8 mode, but it is unclear how it can usefully be  match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
 used. Because \eC breaks up characters into individual data units, matching one  used. Because \eC breaks up characters into individual data units, matching one
 unit with \eC in a UTF mode means that the rest of the string may start with a  unit with \eC in a UTF mode means that the rest of the string may start with a
 malformed UTF character. This has undefined results, because PCRE assumes that  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  it is dealing with valid UTF strings (and by default it checks this at the
start of processing unless the PCRE_NO_UTF8_CHECK or PCRE_NO_UTF16_CHECK optionstart of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
is used).PCRE_NO_UTF32_CHECK option is used).
 .P  .P
 PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
 .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
Line 1082  circumflex is not an assertion; it still consumes a ch Line 1191  circumflex is not an assertion; it still consumes a ch
 string, and therefore it fails if the current pointer is at the end of the  string, and therefore it fails if the current pointer is at the end of the
 string.  string.
 .P  .P
In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can beIn UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
included in a class as a literal string of data units, or by using the \ex{can be included in a class as a literal string of data units, or by using the
escaping mechanism.\ex{ escaping mechanism.
 .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
Line 1297  the section entitled Line 1406  the section entitled
 .\" </a>  .\" </a>
 "Newline sequences"  "Newline sequences"
 .\"  .\"
above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences thatabove. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
can be used to set UTF and Unicode property modes; they are equivalent tosequences that can be used to set UTF and Unicode property modes; they are
setting the PCRE_UTF8, PCRE_UTF16, 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.
 .  .
 .  .
 .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
Line 1534  quantifier, but a literal string of four characters. Line 1646  quantifier, but a literal string of four characters.
 In UTF modes, quantifiers apply to characters rather than to individual data  In UTF modes, quantifiers apply to characters rather than to individual data
 units. Thus, for example, \ex{100}{2} matches two characters, each of  units. Thus, for example, \ex{100}{2} matches two characters, each of
 which is represented by a two-byte sequence in a UTF-8 string. Similarly,  which is represented by a two-byte sequence in a UTF-8 string. Similarly,
\eX{3} matches three Unicode extended sequences, each of which may be several\eX{3} matches three Unicode extended grapheme clusters, each of which may be
data units long (and they may be of different lengths).several data units long (and they may be of different lengths).
 .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
 previous item and the quantifier were not present. This may be useful for  previous item and the quantifier were not present. This may be useful for
Line 1621  In cases where it is known that the subject string con Line 1733  In cases where it is known that the subject string con
 worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
 alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
 .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 1631  succeeds. Consider, for example: Line 1743  succeeds. Consider, for example:
 If the subject is "xyz123abc123" the match point is the fourth character. For  If the subject is "xyz123abc123" the match point is the fourth character. For
 this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
 .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:
   .sp
     (?>.*?a)b
   .sp
   It matches "ab" in the subject "aab". The use of the backtracking control verbs
   (*PRUNE) and (*SKIP) also disable this optimization.
   .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
 .sp  .sp
Line 1899  except that it does not cause the current matching pos Line 2020  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
 For compatibility with Perl, assertion subpatterns may be repeated; though  For compatibility with Perl, assertion subpatterns may be repeated; though
 it makes no sense to assert the same thing several times, the side effect of  it makes no sense to assert the same thing several times, the side effect of
Line 2552  same pair of parentheses when there is a repetition. Line 2673  same pair of parentheses when there is a repetition.
 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 \fIpcre_callout\fP  function by putting its entry point in the global variable \fIpcre_callout\fP
(8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this(8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
variable contains NULL, which disables all calling out.By default, this variable contains NULL, which disables all calling out.
 .P  .P
 Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
 function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
Line 2564  For example, this pattern has two callout points: Line 2685  For example, this pattern has two callout points:
 .sp  .sp
 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are  If 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:
 .sp
   (?(?C9)(?=a)abc|def)
 .sp
 Note that this applies only to assertion conditions, not to other types of
 condition.
 .P  .P
 During matching, when PCRE reaches a callout point, the external function is  During matching, when PCRE reaches a callout point, the external function is
 called. It is provided with the number of the callout, the position in the  called. It is provided with the number of the callout, the position in the
Line 2583  documentation. Line 2711  documentation.
 .rs  .rs
 .sp  .sp
 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
   The new verbs make use of what was previously invalid syntax: an opening
   parenthesis followed by an asterisk. They are generally of the form
   (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
   differently depending on whether or not a name is present. A name is any
   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
 Since these verbs are specifically related to backtracking, most of them can be  Since these verbs are specifically related to backtracking, most of them can be
 used only when the pattern is to be matched using one of the traditional  used only when the pattern is to be matched using one of the traditional
matching functions, which use a backtracking algorithm. With the exception ofmatching functions, because these use a backtracking algorithm. With the
(*FAIL), which behaves like a failing negative assertion, they cause an errorexception of (*FAIL), which behaves like a failing negative assertion, the
if encountered by a DFA matching function.backtracking control verbs cause an error if encountered by a DFA matching
 function.
 .P  .P
If any of these verbs are used in an assertion or in a subpattern that isThe behaviour of these verbs in
called as a subroutine (whether or not recursively), their effect is confined.\" HTML <a href="#btrepeat">
to that subpattern; it does not extend to the surrounding pattern, with one.\" </a>
exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered inrepeated groups,
a successful positive assertion \fIis\fP passed back when a match succeeds.\"
(compare capturing parentheses in assertions). Note that such subpatterns are.\" HTML <a href="#btassert">
processed as anchored at the point where they are tested. Note also that Perl's.\" </a>
treatment of subroutines and assertions is different in some cases.assertions,
.P.\"
The new verbs make use of what was previously invalid syntax: an openingand in
parenthesis followed by an asterisk. They are generally of the form.\" HTML <a href="#btsub">
(*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,.\" </a>
depending on whether or not an argument is present. A name is any sequence ofsubpatterns called as subroutines
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 library. If the name(whether or not recursively) is documented below.
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. 
 .  .
 .  .
 .\" HTML <a name="nooptimize"></a>  .\" HTML <a name="nooptimize"></a>
Line 2621  occur in a pattern. Line 2757  occur in a pattern.
 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 \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
Line 2652  followed by a name. Line 2788  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
 If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
 example:
 .sp  .sp
   A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
 .sp  .sp
Line 2686  starting point (see (*SKIP) below). Line 2825  starting point (see (*SKIP) below).
 A name is always required with this verb. There may be as many instances of  A name is always required with this verb. There may be as many instances of
 (*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
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 as described in the section entitled(*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
 caller as described in the section entitled
 .\" HTML <a href="pcreapi.html#extradata">  .\" HTML <a href="pcreapi.html#extradata">
 .\" </a>  .\" </a>
 "Extra data for \fBpcre_exec()\fP"  "Extra data for \fBpcre_exec()\fP"
Line 2712  indicates which of the two alternatives matched. This  Line 2852  indicates which of the two alternatives matched. This 
 of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
 capturing parentheses.  capturing parentheses.
 .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
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:
 .sp  .sp
     re> /X(*MARK:A)Y|X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
   data> XP    data> XP
Line 2743  to ensure that the match is always attempted. Line 2883  to ensure that the match is always attempted.
 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
 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.
 .sp  .sp
   (*COMMIT)    (*COMMIT)
 .sp  .sp
 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, \fBpcre_exec()\fP 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 \fBpcre_exec()\fP is
 committed to finding a match at the current starting point, or not at all. For
 example:
 .sp  .sp
   a+(*COMMIT)b    a+(*COMMIT)b
 .sp  .sp
Line 2767  dynamic anchor, or "I've started, so I must finish." T Line 2912  dynamic anchor, or "I've started, so I must finish." T
 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
 match failure.  match failure.
 .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
 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
 \fBpcretest\fP example:  \fBpcretest\fP example:
Line 2786  starting points. Line 2935  starting points.
   (*PRUNE) or (*PRUNE:NAME)    (*PRUNE) or (*PRUNE:NAME)
 .sp  .sp
 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
 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).
 .sp  .sp
   (*SKIP)    (*SKIP)
 .sp  .sp
Line 2815  instead of skipping on to "c". Line 2968  instead of skipping on to "c".
 .sp  .sp
   (*SKIP:NAME)    (*SKIP:NAME)
 .sp  .sp
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
 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
 names that are set by (*PRUNE:NAME) or (*THEN:NAME).
 .sp  .sp
   (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
 .sp  .sp
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:
 .sp  .sp
   ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
 .sp  .sp
 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
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
 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 2857  in C, matching moves to (*FAIL), which causes the whol Line 3018  in C, matching moves to (*FAIL), which causes the whol
 because there are no more alternatives to try. In this case, matching does now  because there are no more alternatives to try. In this case, matching does now
 backtrack into A.  backtrack into A.
 .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 2879  starting position, but allowing an advance to the next Line 3040  starting position, but allowing an advance to the next
 unanchored pattern). (*SKIP) is similar, except that the advance may be more  unanchored pattern). (*SKIP) is similar, except that the advance may be more
 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.
If more than one such verb is present in a pattern, the "strongest" one wins..
For example, consider this pattern, where A, B, etc. are complex pattern.SS "More than one backtracking verb"
fragments:.rs
 .sp  .sp
  (A(*COMMIT)B(*THEN)C|D)If more than one backtracking verb is present in a pattern, the one that is
 backtracked onto first acts. For example, consider this pattern, where A, B,
 etc. are complex pattern fragments:
 .sp  .sp
Once A has matched, PCRE is committed to this match, at the current starting  (A(*COMMIT)B(*THEN)C|ABD)
position. If subsequently B matches, but C does not, the normal (*THEN) action.sp
of trying the next alternative (that is, D) does not happen because (*COMMIT)If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
overrides.fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
 the next alternative (ABD) to be tried. This behaviour is consistent, but is
 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:
 .sp
   ...(*COMMIT)(*PRUNE)...
 .sp
 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).
 .  .
 .  .
   .\" HTML <a name="btrepeat"></a>
   .SS "Backtracking verbs in repeated groups"
   .rs
   .sp
   PCRE differs from Perl in its handling of backtracking verbs in repeated
   groups. For example, consider:
   .sp
     /(a(*COMMIT)b)+ac/
   .sp
   If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
   the second repeat of the group acts.
   .
   .
   .\" HTML <a name="btassert"></a>
   .SS "Backtracking verbs in assertions"
   .rs
   .sp
   (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
   .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
   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
   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).
   .
   .
   .\" HTML <a name="btsub"></a>
   .SS "Backtracking verbs in subroutines"
   .rs
   .sp
   These behaviours occur whether or not the subpattern is called recursively.
   Perl's treatment of subroutines is different in some cases.
   .P
   (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
   an immediate backtrack.
   .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
   (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
   the subroutine match to fail.
   .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.
   .
   .
 .SH "SEE ALSO"  .SH "SEE ALSO"
 .rs  .rs
 .sp  .sp
 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
\fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.\fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
 .  .
 .  .
 .SH AUTHOR  .SH AUTHOR
Line 2913  Cambridge CB2 3QH, England. Line 3145  Cambridge CB2 3QH, England.
 .rs  .rs
 .sp  .sp
 .nf  .nf
Last updated: 17 June 2012Last updated: 26 April 2013
Copyright (c) 1997-2012 University of Cambridge.Copyright (c) 1997-2013 University of Cambridge.
 .fi  .fi

Removed from v.1.1.1.3  
changed lines
  Added in v.1.1.1.4


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