/[pcre]/code/trunk/doc/pcrepattern.3

Diff of /code/trunk/doc/pcrepattern.3

Parent Directory | Revision Log | View Patch Patch

-revision 79 by nigel,
Sat Feb 24 21:40:52 2007 UTC
+revision 964 by ph10,
Fri May  4 13:03:39 2012 UTC
 Line 1
- .TH PCREPATTERN 3
+ .TH PCREPATTERN 3 "04 May 2012" "PCRE 8.31"
  .SH NAME
  PCRE - Perl-compatible regular expressions
  .SH "PCRE REGULAR EXPRESSION DETAILS"
  .rs
  .sp
- The syntax and semantics of the regular expressions supported by PCRE are
+ The syntax and semantics of the regular expressions that are supported by PCRE
- described below. Regular expressions are also described in the Perl
+ are described in detail below. There is a quick-reference syntax summary in the
- documentation and in a number of books, some of which have copious examples.
+ .\" HREF
- Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
+ \fBpcresyntax\fP
- regular expressions in great detail. This description of PCRE's regular
+ .\"
- expressions is intended as reference material.
+ page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
+ also supports some alternative regular expression syntax (which does not
+ conflict with the Perl syntax) in order to provide some compatibility with
+ regular expressions in Python, .NET, and Oniguruma.
+ .P
+ Perl's regular expressions are described in its own documentation, and
+ regular expressions in general are covered in a number of books, some of which
+ have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
+ published by O'Reilly, covers regular expressions in great detail. This
+ description of PCRE's regular expressions is intended as reference material.
  .P
  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, you must
+ there is now also support for UTF-8 strings in the original library, and a
- build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
+ second library that supports 16-bit and UTF-16 character strings. To use these
- the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
+ features, PCRE must be built to include appropriate support. When using UTF
- places below. There is also a summary of UTF-8 features in the
+ strings you must either call the compiling function with the PCRE_UTF8 or
- .\" HTML <a href="pcre.html#utf8support">
+ PCRE_UTF16 option, or the pattern must start with one of these special
- .\" </a>
+ sequences:
- section on UTF-8 support
+ .sp
- .\"
+   (*UTF8)
- in the main
+   (*UTF16)
+ .sp
+ 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 affects
+ pattern matching is mentioned in several places below. There is also a summary
+ of features in the
  .\" HREF
- \fBpcre\fP
+ \fBpcreunicode\fP
  .\"
  page.
  .P
+ Another special sequence that may appear at the start of a pattern or in
+ combination with (*UTF8) or (*UTF16) is:
+ .sp
+   (*UCP)
+ .sp
+ 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,
+ instead of recognizing only characters with codes less than 128 via a lookup
+ table.
+ .P
+ 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 are
+ 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 its main matching function, \fBpcre_exec()\fP, is used.
+ PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
- From release 6.0, PCRE offers a second matching function,
+ \fBpcre16_exec()\fP (16-bit), is used. PCRE also has alternative matching
- \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
+ functions, \fBpcre_dfa_exec()\fP and \fBpcre16_dfa_exec()\fP, which match using
- Perl-compatible. The advantages and disadvantages of the alternative function,
+ a different algorithm that is not Perl-compatible. Some of the features
- and how it differs from the normal function, are discussed in the
+ discussed below are not available when DFA matching is used. The advantages and
+ 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>
+ .SH "NEWLINE CONVENTIONS"
+ .rs
+ .sp
+ PCRE supports five different conventions for indicating line breaks in
+ strings: a single CR (carriage return) character, a single LF (linefeed)
+ character, the two-character sequence CRLF, any of the three preceding, or any
+ Unicode newline sequence. The
+ .\" HREF
+ \fBpcreapi\fP
+ .\"
+ page has
+ .\" HTML <a href="pcreapi.html#newlines">
+ .\" </a>
+ further discussion
+ .\"
+ about newlines, and shows how to set the newline convention in the
+ \fIoptions\fP arguments for the compiling and matching functions.
  .P
+ It is also possible to specify a newline convention by starting a pattern
+ string with one of the following five sequences:
+ .sp
+   (*CR)        carriage return
+   (*LF)        linefeed
+   (*CRLF)      carriage return, followed by linefeed
+   (*ANYCRLF)   any of the three above
+   (*ANY)       all Unicode newline sequences
+ .sp
+ These override the default and the options given to the compiling function. For
+ example, on a Unix system where LF is the default newline sequence, the pattern
+ .sp
+   (*CR)a.b
+ .sp
+ changes the convention to CR. That pattern matches "a\enb" because LF is no
+ longer a newline. Note that these special settings, which are not
+ 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.
+ .P
+ The newline convention affects the interpretation of the dot metacharacter when
+ PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
+ affect what 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
+ .\" HTML <a href="#newlineseq">
+ .\" </a>
+ "Newline sequences"
+ .\"
+ below. A change of \eR setting can be combined with a change of newline
+ convention.
+ .
+ .
+ .SH "CHARACTERS AND METACHARACTERS"
+ .rs
+ .sp
  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
  corresponding characters in the subject. As a trivial example, the pattern
-Line 45 
 corresponding characters in the subject.
+Line 132 
 corresponding characters in the subject.
  .sp
  matches a portion of a subject string that is identical to itself. When
  caseless matching is specified (the PCRE_CASELESS option), letters are matched
- independently of case. In UTF-8 mode, PCRE always understands the concept of
+ independently of case. In a UTF mode, PCRE always understands the concept of
  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
  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
  ensure that PCRE is compiled with Unicode property support as well as with
- UTF-8 support.
+ UTF support.
  .P
  The power of regular expressions comes from the ability to include alternatives
  and repetitions in the pattern. These are encoded in the pattern by the use of
-Line 60 
 interpreted in some special way.
+Line 147 
 interpreted in some special way.
  .P
  There are two different sets of metacharacters: those that are recognized
  anywhere in the pattern except within square brackets, and those that are
- recognized in square brackets. Outside square brackets, the metacharacters are
+ recognized within square brackets. Outside square brackets, the metacharacters
- as follows:
+ are as follows:
  .sp
    \e      general escape character with several uses
    ^      assert start of string (or line, in multiline mode)
-Line 92 
 a character class the only metacharacter
+Line 179 
 a character class the only metacharacter
  .sp
  The following sections describe the use of each of the metacharacters.
  .
+ .
  .SH BACKSLASH
  .rs
  .sp
  The backslash character has several uses. Firstly, if it is followed by a
- non-alphanumeric character, it takes away any special meaning that character may
+ character that is not a number or a letter, it takes away any special meaning
- have. This use of backslash as an escape character applies both inside and
+ that character may have. This use of backslash as an escape character applies
- outside character classes.
+ both inside and outside character classes.
  .P
  For example, if you want to match a * character, you write \e* in the pattern.
  This escaping action applies whether or not the following character would
-Line 106 
 otherwise be interpreted as a metacharac
+Line 194 
 otherwise be interpreted as a metacharac
  non-alphanumeric with backslash to specify that it stands for itself. In
  particular, if you want to match a backslash, you write \e\e.
  .P
+ In a UTF mode, only ASCII numbers and letters have any special meaning after a
+ backslash. All other characters (in particular, those whose codepoints are
+ greater than 127) are treated as literals.
+ .P
  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
  pattern (other than in a character class) and characters between a # outside
- a character class and the next newline character are ignored. An escaping
+ a character class and the next newline are ignored. An escaping backslash can
- backslash can be used to include a whitespace or # character as part of the
+ be used to include a whitespace or # character as part of the pattern.
- pattern.
  .P
  If you want to remove the special meaning from a sequence of characters, you
  can do so by putting them between \eQ and \eE. This is different from Perl in
-Line 126 
 Perl, $ and @ cause variable interpolati
+Line 217 
 Perl, $ and @ cause variable interpolati
    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
  .sp
  The \eQ...\eE sequence is recognized both inside and outside character classes.
+ An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
+ by \eE later in the pattern, the literal interpretation continues to the end of
+ the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
+ a character class, this causes an error, because the character class is not
+ terminated.
  .
  .
  .\" HTML <a name="digitsafterbackslash"></a>
-Line 135 
 The \eQ...\eE sequence is recognized bot
+Line 231 
 The \eQ...\eE sequence is recognized bot
  A second use of backslash provides a way of encoding non-printing characters
  in patterns in a visible manner. There is no restriction on the appearance of
  non-printing characters, apart from the binary zero that terminates a pattern,
- but when a pattern is being prepared by text editing, it is usually easier to
+ but when a pattern is being prepared by text editing, it is often easier to use
- use one of the following escape sequences than the binary character it
+ one of the following escape sequences than the binary character it represents:
- represents:
  .sp
    \ea        alarm, that is, the BEL character (hex 07)
-   \ecx       "control-x", where x is any character
+   \ecx       "control-x", where x is any ASCII character
    \ee        escape (hex 1B)
    \ef        formfeed (hex 0C)
-   \en        newline (hex 0A)
+   \en        linefeed (hex 0A)
    \er        carriage return (hex 0D)
    \et        tab (hex 09)
-   \eddd      character with octal code ddd, or backreference
+   \eddd      character with octal code ddd, or back reference
    \exhh      character with hex code hh
-   \ex{hhh..} character with hex code hhh... (UTF-8 mode only)
+   \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
+   \euhhhh    character with hex code hhhh (JavaScript mode only)
  .sp
  The precise effect of \ecx is as follows: if x is a lower case letter, it
  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
- Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
+ Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
-B.
+ \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
- .P
+ than 127, a compile-time error occurs. This locks out non-ASCII characters in
- After \ex, from zero to two hexadecimal digits are read (letters can be in
+ all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. A
- upper or lower case). In UTF-8 mode, any number of hexadecimal digits may
+ lower case letter is converted to upper case, and then the 0xc0 bits are
- appear between \ex{ and }, but the value of the character code must be less
+ flipped.)
- than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters
+ .P
- other than hexadecimal digits appear between \ex{ and }, or if there is no
+ By default, after \ex, from zero to two hexadecimal digits are read (letters
- terminating }, this form of escape is not recognized. Instead, the initial
+ can be in upper or lower case). Any number of hexadecimal digits may appear
- \ex will be interpreted as a basic hexadecimal escape, with no following
+ between \ex{ and }, but the character code is constrained as follows:
- digits, giving a character whose value is zero.
+ .sp
+-bit non-UTF mode    less than 0x100
+-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
+-bit non-UTF mode   less than 0x10000
+-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
+ .sp
+ Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
+ "surrogate" codepoints).
+ .P
+ 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
+ initial \ex will be interpreted as a basic hexadecimal escape, with no
+ following digits, giving a character whose value is zero.
+ .P
+ If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
+ as just described only when it is followed by two hexadecimal digits.
+ Otherwise, it matches a literal "x" character. In JavaScript mode, support for
+ code points greater than 256 is provided by \eu, which must be followed by
+ four hexadecimal digits; otherwise it matches a literal "u" character.
  .P
  Characters whose value is less than 256 can be defined by either of the two
- syntaxes for \ex when PCRE is in UTF-8 mode. There is no difference in the
+ syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
- way they are handled. For example, \exdc is exactly the same as \ex{dc}.
+ way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
- .P
+ \eu00dc in JavaScript mode).
- After \e0 up to two further octal digits are read. In both cases, if there
+ .P
- are fewer than two digits, just those that are present are used. Thus the
+ After \e0 up to two further octal digits are read. If there are fewer than two
- sequence \e0\ex\e07 specifies two binary zeros followed by a BEL character
+ digits, just those that are present are used. Thus the sequence \e0\ex\e07
- (code value 7). Make sure you supply two digits after the initial zero if the
+ specifies two binary zeros followed by a BEL character (code value 7). Make
- pattern character that follows is itself an octal digit.
+ sure you supply two digits after the initial zero if the pattern character that
+ follows is itself an octal digit.
  .P
  The handling of a backslash followed by a digit other than 0 is complicated.
  Outside a character class, PCRE reads it and any following digits as a decimal
-Line 191 
 parenthesized subpatterns.
+Line 306 
 parenthesized subpatterns.
  .P
  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
- digits following the backslash, and generates a single byte from the least
+ digits following the backslash, and uses them to generate a data character. Any
- significant 8 bits of the value. Any subsequent digits stand for themselves.
+ subsequent digits stand for themselves. The value of the character is
+ constrained in the same way as characters specified in hexadecimal.
  For example:
  .sp
    \e040   is another way of writing a space
-Line 210 
 For example:
+Line 326 
 For example:
              character with octal code 113
  .\" JOIN
    \e377   might be a back reference, otherwise
-             the byte consisting entirely of 1 bits
+             the value 255 (decimal)
  .\" JOIN
    \e81    is either a back reference, or a binary zero
              followed by the two characters "8" and "1"
-Line 218 
 For example:
+Line 334 
 For example:
  Note that octal values of 100 or greater must not be introduced by a leading
  zero, because no more than three octal digits are ever read.
  .P
- All the sequences that define a single byte value or a single UTF-8 character
+ All the sequences that define a single character value can be used both inside
- (in UTF-8 mode) can be used both inside and outside character classes. In
+ and outside character classes. In addition, inside a character class, \eb is
- addition, inside a character class, the sequence \eb is interpreted as the
+ interpreted as the backspace character (hex 08).
- backspace character (hex 08), and the sequence \eX is interpreted as the
+ .P
- character "X". Outside a character class, these sequences have different
+ \eN is not allowed in a character class. \eB, \eR, and \eX are not special
- meanings
+ inside a character class. Like other unrecognized escape sequences, they are
- .\" HTML <a href="#uniextseq">
+ treated as the literal characters "B", "R", and "X" by default, but cause an
+ error if the PCRE_EXTRA option is set. Outside a character class, these
+ sequences have different meanings.
+ .
+ .
+ .SS "Unsupported escape sequences"
+ .rs
+ .sp
+ In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
+ handler and used to modify the case of following characters. By default, PCRE
+ does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
+ option is set, \eU matches a "U" character, and \eu can be used to define a
+ character by code point, as described in the previous section.
+ .
+ .
+ .SS "Absolute and relative back references"
+ .rs
+ .sp
+ The sequence \eg followed by an unsigned or a negative number, optionally
+ enclosed in braces, is an absolute or relative back reference. A named back
+ reference can be coded as \eg{name}. Back references are discussed
+ .\" HTML <a href="#backreferences">
  .\" </a>
- (see below).
+ later,
+ .\"
+ following the discussion of
+ .\" HTML <a href="#subpattern">
+ .\" </a>
+ parenthesized subpatterns.
  .\"
  .
  .
+ .SS "Absolute and relative subroutine calls"
+ .rs
+ .sp
+ For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
+ a number enclosed either in angle brackets or single quotes, is an alternative
+ syntax for referencing a subpattern as a "subroutine". Details are discussed
+ .\" HTML <a href="#onigurumasubroutines">
+ .\" </a>
+ later.
+ .\"
+ Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
+ synonymous. The former is a back reference; the latter is a
+ .\" HTML <a href="#subpatternsassubroutines">
+ .\" </a>
+ subroutine
+ .\"
+ call.
+ .
+ .
+ .\" HTML <a name="genericchartypes"></a>
  .SS "Generic character types"
  .rs
  .sp
- The third use of backslash is for specifying generic character types. The
+ Another use of backslash is for specifying generic character types:
- following are always recognized:
  .sp
    \ed     any decimal digit
    \eD     any character that is not a decimal digit
+   \eh     any horizontal whitespace character
+   \eH     any character that is not a horizontal whitespace character
    \es     any whitespace character
    \eS     any character that is not a whitespace character
+   \ev     any vertical whitespace character
+   \eV     any character that is not a vertical whitespace character
    \ew     any "word" character
    \eW     any "non-word" character
  .sp
- Each pair of escape sequences partitions the complete set of characters into
+ There is also the single sequence \eN, which matches a non-newline character.
- two disjoint sets. Any given character matches one, and only one, of each pair.
+ This is the same as
+ .\" HTML <a href="#fullstopdot">
+ .\" </a>
+ the "." metacharacter
+ .\"
+ when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
+ PCRE does not support this.
  .P
- These character type sequences can appear both inside and outside character
+ Each pair of lower and upper case escape sequences partitions the complete set
+ of characters into two disjoint sets. Any given character matches one, and only
+ one, of each pair. The sequences can appear both inside and outside character
  classes. They each match one character of the appropriate type. If the current
- matching point is at the end of the subject string, all of them fail, since
+ matching point is at the end of the subject string, all of them fail, because
  there is no character to match.
  .P
  For compatibility with Perl, \es does not match the VT character (code 11).
  This makes it different from the the POSIX "space" class. The \es characters
- are HT (9), LF (10), FF (12), CR (13), and space (32).
+ are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
+ included in a Perl script, \es may match the VT character. In PCRE, it never
+ does.
  .P
- A "word" character is an underscore or any character less than 256 that is a
+ A "word" character is an underscore or any character that is a letter or digit.
- letter or digit. The definition of letters and digits is controlled by PCRE's
+ By default, the definition of letters and digits is controlled by PCRE's
  low-valued character tables, and may vary if locale-specific matching is taking
  place (see
  .\" HTML <a href="pcreapi.html#localesupport">
-Line 267 
 in the
+Line 442 
 in the
  .\" HREF
  \fBpcreapi\fP
  .\"
- page). For example, in the "fr_FR" (French) locale, some character codes
+ page). For example, in a French locale such as "fr_FR" in Unix-like systems,
- greater than 128 are used for accented letters, and these are matched by \ew.
+ or "french" in Windows, some character codes greater than 128 are used for
- .P
+ accented letters, and these are then matched by \ew. The use of locales with
- In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
+ Unicode is discouraged.
- \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
+ .P
- character property support is available.
+ By default, in a UTF mode, characters with values greater than 128 never match
+ \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
+ their original meanings from before UTF support was available, mainly for
+ 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
+ properties are used to determine character types, as follows:
+ .sp
+   \ed  any character that \ep{Nd} matches (decimal digit)
+   \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
+   \ew  any character that \ep{L} or \ep{N} matches, plus underscore
+ .sp
+ The upper case escapes match the inverse sets of characters. Note that \ed
+ matches only decimal digits, whereas \ew matches any Unicode digit, as well as
+ any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
+ \eB because they are defined in terms of \ew and \eW. Matching these sequences
+ is noticeably slower when PCRE_UCP is set.
+ .P
+ The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
+ release 5.10. In contrast to the other sequences, which match only ASCII
+ characters by default, these always match certain high-valued codepoints,
+ whether or not PCRE_UCP is set. The horizontal space characters are:
+ .sp
+   U+0009     Horizontal tab
+   U+0020     Space
+   U+00A0     Non-break space
+   U+1680     Ogham space mark
+   U+180E     Mongolian vowel separator
+   U+2000     En quad
+   U+2001     Em quad
+   U+2002     En space
+   U+2003     Em space
+   U+2004     Three-per-em space
+   U+2005     Four-per-em space
+   U+2006     Six-per-em space
+   U+2007     Figure space
+   U+2008     Punctuation space
+   U+2009     Thin space
+   U+200A     Hair space
+   U+202F     Narrow no-break space
+   U+205F     Medium mathematical space
+   U+3000     Ideographic space
+ .sp
+ The vertical space characters are:
+ .sp
+   U+000A     Linefeed
+   U+000B     Vertical tab
+   U+000C     Formfeed
+   U+000D     Carriage return
+   U+0085     Next line
+   U+2028     Line separator
+   U+2029     Paragraph separator
+ .sp
+ In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
+ relevant.
+ .
+ .
+ .\" HTML <a name="newlineseq"></a>
+ .SS "Newline sequences"
+ .rs
+ .sp
+ Outside a character class, by default, the escape sequence \eR matches any
+ Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
+ following:
+ .sp
+   (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
+ .sp
+ This is an example of an "atomic group", details of which are given
+ .\" HTML <a href="#atomicgroup">
+ .\" </a>
+ below.
+ .\"
+ 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,
+ U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
+ line, U+0085). The two-character sequence is treated as a single unit that
+ cannot be split.
+ .P
+ In other modes, two additional characters whose codepoints are greater than 255
+ 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
+ recognized.
+ .P
+ It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
+ complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
+ either at compile time or when the pattern is matched. (BSR is an abbrevation
+ for "backslash R".) This can be made the default when PCRE is built; if this is
+ the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
+ It is also possible to specify these settings by starting a pattern string with
+ one of the following sequences:
+ .sp
+   (*BSR_ANYCRLF)   CR, LF, or CRLF only
+   (*BSR_UNICODE)   any Unicode newline sequence
+ .sp
+ These override the default and the options given to the compiling function, but
+ they can themselves be overridden by options given to a matching function. Note
+ that these special settings, which are not 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. They can be combined with a
+ change of newline convention; for example, a pattern can start with:
+ .sp
+   (*ANY)(*BSR_ANYCRLF)
+ .sp
+ They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
+ sequences. Inside a character class, \eR is treated as an unrecognized escape
+ sequence, and so matches the letter "R" by default, but causes an error if
+ PCRE_EXTRA is set.
  .
  .
  .\" HTML <a name="uniextseq"></a>
-Line 280 
 character property support is available.
+Line 560 
 character property support is available.
  .rs
  .sp
  When PCRE is built with Unicode character property support, three additional
- escape sequences to match generic character types are available when UTF-8 mode
+ escape sequences that match characters with specific properties are available.
- is selected. They are:
+ When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
- .sp
+ characters whose codepoints are less than 256, but they do work in this mode.
-  \ep{\fIxx\fP}   a character with the \fIxx\fP property
+ The extra escape sequences are:
-  \eP{\fIxx\fP}   a character without the \fIxx\fP property
+ .sp
-  \eX       an extended Unicode sequence
+   \ep{\fIxx\fP}   a character with the \fIxx\fP property
- .sp
+   \eP{\fIxx\fP}   a character without the \fIxx\fP property
- The property names represented by \fIxx\fP above are limited to the
+   \eX       an extended Unicode sequence
- Unicode general category properties. Each character has exactly one such
+ .sp
- property, specified by a two-letter abbreviation. For compatibility with Perl,
+ The property names represented by \fIxx\fP above are limited to the Unicode
- negation can be specified by including a circumflex between the opening brace
+ script names, the general category properties, "Any", which matches any
- and the property name. For example, \ep{^Lu} is the same as \eP{Lu}.
+ character (including newline), and some special PCRE properties (described
- .P
+ in the
- If only one letter is specified with \ep or \eP, it includes all the properties
+ .\" HTML <a href="#extraprops">
- that start with that letter. In this case, in the absence of negation, the
+ .\" </a>
- curly brackets in the escape sequence are optional; these two examples have
+ next section).
- the same effect:
+ .\"
+ Other Perl properties such as "InMusicalSymbols" are not currently supported by
+ PCRE. Note that \eP{Any} does not match any characters, so always causes a
+ match failure.
+ .P
+ Sets of Unicode characters are defined as belonging to certain scripts. A
+ character from one of these sets can be matched using a script name. For
+ example:
+ .sp
+   \ep{Greek}
+   \eP{Han}
+ .sp
+ Those that are not part of an identified script are lumped together as
+ "Common". The current list of scripts is:
+ .P
+ Arabic,
+ Armenian,
+ Avestan,
+ Balinese,
+ Bamum,
+ Batak,
+ Bengali,
+ Bopomofo,
+ Brahmi,
+ Braille,
+ Buginese,
+ Buhid,
+ Canadian_Aboriginal,
+ Carian,
+ Chakma,
+ Cham,
+ Cherokee,
+ Common,
+ Coptic,
+ Cuneiform,
+ Cypriot,
+ Cyrillic,
+ Deseret,
+ Devanagari,
+ Egyptian_Hieroglyphs,
+ Ethiopic,
+ Georgian,
+ Glagolitic,
+ Gothic,
+ Greek,
+ Gujarati,
+ Gurmukhi,
+ Han,
+ Hangul,
+ Hanunoo,
+ Hebrew,
+ Hiragana,
+ Imperial_Aramaic,
+ Inherited,
+ Inscriptional_Pahlavi,
+ Inscriptional_Parthian,
+ Javanese,
+ Kaithi,
+ Kannada,
+ Katakana,
+ Kayah_Li,
+ Kharoshthi,
+ Khmer,
+ Lao,
+ Latin,
+ Lepcha,
+ Limbu,
+ Linear_B,
+ Lisu,
+ Lycian,
+ Lydian,
+ Malayalam,
+ Mandaic,
+ Meetei_Mayek,
+ Meroitic_Cursive,
+ Meroitic_Hieroglyphs,
+ Miao,
+ Mongolian,
+ Myanmar,
+ New_Tai_Lue,
+ Nko,
+ Ogham,
+ Old_Italic,
+ Old_Persian,
+ Old_South_Arabian,
+ Old_Turkic,
+ Ol_Chiki,
+ Oriya,
+ Osmanya,
+ Phags_Pa,
+ Phoenician,
+ Rejang,
+ Runic,
+ Samaritan,
+ Saurashtra,
+ Sharada,
+ Shavian,
+ Sinhala,
+ Sora_Sompeng,
+ Sundanese,
+ Syloti_Nagri,
+ Syriac,
+ Tagalog,
+ Tagbanwa,
+ Tai_Le,
+ Tai_Tham,
+ Tai_Viet,
+ Takri,
+ Tamil,
+ Telugu,
+ Thaana,
+ Thai,
+ Tibetan,
+ Tifinagh,
+ Ugaritic,
+ Vai,
+ Yi.
+ .P
+ Each character has exactly one Unicode general category property, specified by
+ a two-letter abbreviation. For compatibility with Perl, negation can be
+ specified by including a circumflex between the opening brace and the property
+ name. For example, \ep{^Lu} is the same as \eP{Lu}.
+ .P
+ If only one letter is specified with \ep or \eP, it includes all the general
+ category properties that start with that letter. In this case, in the absence
+ of negation, the curly brackets in the escape sequence are optional; these two
+ examples have the same effect:
  .sp
    \ep{L}
    \epL
  .sp
- The following property codes are supported:
+ The following general category property codes are supported:
  .sp
    C     Other
    Cc    Control
-Line 347 
 The following property codes are support
+Line 753 
 The following property codes are support
    Zp    Paragraph separator
    Zs    Space separator
  .sp
- Extended properties such as "Greek" or "InMusicalSymbols" are not supported by
+ The special property L& is also supported: it matches a character that has
- PCRE.
+ the Lu, Ll, or Lt property, in other words, a letter that is not classified as
+ a modifier or "other".
+ .P
+ 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
+ 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
+ .\" HREF
+ \fBpcreapi\fP
+ .\"
+ page). Perl does not support the Cs property.
+ .P
+ The long synonyms for property names that Perl supports (such as \ep{Letter})
+ are not supported by PCRE, nor is it permitted to prefix any of these
+ properties with "Is".
+ .P
+ No character that is in the Unicode table has the Cn (unassigned) property.
+ Instead, this property is assumed for any code point that is not in the
+ Unicode table.
  .P
  Specifying caseless matching does not affect these escape sequences. For
  example, \ep{Lu} always matches only upper case letters.
-Line 366 
 atomic group
+Line 790 
 atomic group
  (see below).
  .\"
  Characters with the "mark" property are typically accents that affect the
- preceding character.
+ preceding character. None of them have codepoints less than 256, so in
+-bit non-UTF-8 mode \eX matches any one character.
+ .P
+ Note that recent versions of Perl have changed \eX to match what Unicode calls
+ an "extended grapheme cluster", which has a more complicated definition.
  .P
  Matching characters by Unicode property is not fast, because PCRE has to search
  a structure that contains data for over fifteen thousand characters. That is
  why the traditional escape sequences such as \ed and \ew do not use Unicode
- properties in PCRE.
+ 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).
+ .
+ .
+ .\" HTML <a name="extraprops"></a>
+ .SS PCRE's additional properties
+ .rs
+ .sp
+ As well as the standard Unicode properties described in the previous
+ section, PCRE supports four more that make it possible to convert traditional
+ escape sequences such as \ew and \es and POSIX character classes to use Unicode
+ properties. PCRE uses these non-standard, non-Perl properties internally when
+ PCRE_UCP is set. They are:
+ .sp
+   Xan   Any alphanumeric character
+   Xps   Any POSIX space character
+   Xsp   Any Perl space character
+   Xwd   Any Perl "word" character
+ .sp
+ Xan matches characters that have either the L (letter) or the N (number)
+ property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
+ 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
+ same characters as Xan, plus underscore.
+ .
+ .
+ .\" HTML <a name="resetmatchstart"></a>
+ .SS "Resetting the match start"
+ .rs
+ .sp
+ The escape sequence \eK causes any previously matched characters not to be
+ included in the final matched sequence. For example, the pattern:
+ .sp
+   foo\eKbar
+ .sp
+ matches "foobar", but reports that it has matched "bar". This feature is
+ similar to a lookbehind assertion
+ .\" HTML <a href="#lookbehind">
+ .\" </a>
+ (described below).
+ .\"
+ However, in this case, the part of the subject before the real match does not
+ have to be of fixed length, as lookbehind assertions do. The use of \eK does
+ not interfere with the setting of
+ .\" HTML <a href="#subpattern">
+ .\" </a>
+ captured substrings.
+ .\"
+ For example, when the pattern
+ .sp
+   (foo)\eKbar
+ .sp
+ matches "foobar", the first substring is still set to "foo".
+ .P
+ Perl documents that the use of \eK within assertions is "not well defined". In
+ PCRE, \eK is acted upon when it occurs inside positive assertions, but is
+ ignored in negative assertions.
  .
  .
  .\" HTML <a name="smallassertions"></a>
  .SS "Simple assertions"
  .rs
  .sp
- The fourth use of backslash is for certain simple assertions. An assertion
+ The final use of backslash is for certain simple assertions. An assertion
  specifies a condition that has to be met at a particular point in a match,
  without consuming any characters from the subject string. The use of
  subpatterns for more complicated assertions is described
-Line 386 
 subpatterns for more complicated asserti
+Line 870 
 subpatterns for more complicated asserti
  .\" </a>
  below.
  .\"
- The backslashed
+ The backslashed assertions are:
- assertions are:
  .sp
    \eb     matches at a word boundary
    \eB     matches when not at a word boundary
-   \eA     matches at start of subject
+   \eA     matches at the start of the subject
-   \eZ     matches at end of subject or before newline at end
+   \eZ     matches at the end of the subject
-   \ez     matches at end of subject
+           also matches before a newline at the end of the subject
-   \eG     matches at first matching position in subject
+   \ez     matches only at the end of the subject
- .sp
+   \eG     matches at the first matching position in the subject
- These assertions may not appear in character classes (but note that \eb has a
+ .sp
- different meaning, namely the backspace character, inside a character class).
+ Inside a character class, \eb has a different meaning; it matches the backspace
+ character. If any other of these assertions appears in a character class, by
+ default it matches the corresponding literal character (for example, \eB
+ matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
+ escape sequence" error is generated instead.
  .P
  A word boundary is a position in the subject string where the current character
  and the previous character do not both match \ew or \eW (i.e. one matches
  \ew and the other matches \eW), or the start or end of the string if the
- first or last character matches \ew, respectively.
+ first or last character matches \ew, respectively. In a UTF mode, the meanings
+ of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
+ done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
+ of word" or "end of word" metasequence. However, whatever follows \eb normally
+ determines which it is. For example, the fragment \eba matches "a" at the start
+ of a word.
  .P
  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
  dollar (described in the next section) in that they only ever match at the very
-Line 412 
 PCRE_NOTBOL or PCRE_NOTEOL options, whic
+Line 904 
 PCRE_NOTBOL or PCRE_NOTEOL options, whic
  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
  at a point other than the beginning of the subject, \eA can never match. The
- difference between \eZ and \ez is that \eZ matches before a newline that is the
+ difference between \eZ and \ez is that \eZ matches before a newline at the end
- last character of the string as well as at the end of the string, whereas \ez
+ of the string as well as at the very end, whereas \ez matches only at the end.
- matches only at the end.
  .P
  The \eG assertion is true only when the current matching position is at the
  start point of the match, as specified by the \fIstartoffset\fP argument of
-Line 458 
 to be anchored.)
+Line 949 
 to be anchored.)
  .P
  A 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 newline
- character that is the last character in the string (by default). Dollar need
+ at the end of the string (by default). Dollar need not be the last character of
- not be the last character of the pattern if a number of alternatives are
+ the pattern if a number of alternatives are involved, but it should be the last
- involved, but it should be the last item in any branch in which it appears.
+ item in any branch in which it appears. Dollar has no special meaning in a
- Dollar has no special meaning in a character class.
+ character class.
  .P
  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
  does not affect the \eZ assertion.
  .P
  The meanings of the circumflex and dollar characters are changed if the
- PCRE_MULTILINE option is set. When this is the case, they match immediately
+ PCRE_MULTILINE option is set. When this is the case, a circumflex matches
- after and immediately before an internal newline character, respectively, in
+ immediately after internal newlines as well as at the start of the subject
- addition to matching at the start and end of the subject string. For example,
+ string. It does not match after a newline that ends the string. A dollar
- the pattern /^abc$/ matches the subject string "def\enabc" (where \en
+ matches before any newlines in the string, as well as at the very end, when
- represents a newline character) in multiline mode, but not otherwise.
+ PCRE_MULTILINE is set. When newline is specified as the two-character
- Consequently, patterns that are anchored in single line mode because all
+ sequence CRLF, isolated CR and LF characters do not indicate newlines.
- branches start with ^ are not anchored in multiline mode, and a match for
+ .P
- circumflex is possible when the \fIstartoffset\fP argument of \fBpcre_exec()\fP
+ For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
- is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
+ \en represents a newline) in multiline mode, but not otherwise. Consequently,
- set.
+ patterns that are anchored in single line mode because all branches start with
+ ^ are not anchored in multiline mode, and a match for circumflex is possible
+ when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
+ PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
  .P
  Note that the sequences \eA, \eZ, and \ez can be used to match the start and
  end of the subject in both modes, and if all branches of a pattern start with
- \eA it is always anchored, whether PCRE_MULTILINE is set or not.
+ \eA it is always anchored, whether or not PCRE_MULTILINE is set.
  .
  .
- .SH "FULL STOP (PERIOD, DOT)"
+ .\" HTML <a name="fullstopdot"></a>
+ .SH "FULL STOP (PERIOD, DOT) AND \eN"
  .rs
  .sp
  Outside a character class, a dot in the pattern matches any one character in
- the subject, including a non-printing character, but not (by default) newline.
+ the subject string except (by default) a character that signifies the end of a
- In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one
+ line.
- byte long, except (by default) newline. If the PCRE_DOTALL option is set,
+ .P
- dots match newlines as well. The handling of dot is entirely independent of the
+ When a line ending is defined as a single character, dot never matches that
- handling of circumflex and dollar, the only relationship being that they both
+ character; when the two-character sequence CRLF is used, dot does not match CR
- involve newline characters. Dot has no special meaning in a character class.
+ if it is immediately followed by LF, but otherwise it matches all characters
- .
+ (including isolated CRs and LFs). When any Unicode line endings are being
- .
+ recognized, dot does not match CR or LF or any of the other line ending
- .SH "MATCHING A SINGLE BYTE"
+ characters.
- .rs
+ .P
- .sp
+ The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
- Outside a character class, the escape sequence \eC matches any one byte, both
+ option is set, a dot matches any one character, without exception. If the
- in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature is
+ two-character sequence CRLF is present in the subject string, it takes two dots
- provided in Perl in order to match individual bytes in UTF-8 mode. Because it
+ to match it.
- breaks up UTF-8 characters into individual bytes, what remains in the string
+ .P
- may be a malformed UTF-8 string. For this reason, the \eC escape sequence is
+ The handling of dot is entirely independent of the handling of circumflex and
- best avoided.
+ dollar, the only relationship being that they both involve newlines. Dot has no
+ special meaning in a character class.
+ .P
+ The escape sequence \eN behaves like a dot, except that it is not affected by
+ the PCRE_DOTALL option. In other words, it matches any character except one
+ that signifies the end of a line. Perl also uses \eN to match characters by
+ name; PCRE does not support this.
+ .
+ .
+ .SH "MATCHING A SINGLE DATA UNIT"
+ .rs
+ .sp
+ 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
+ byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always
+ 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
+ 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
+ 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 or PCRE_NO_UTF16_CHECK option
+ is used).
  .P
  PCRE does not allow \eC to appear in lookbehind assertions
  .\" HTML <a href="#lookbehind">
  .\" </a>
- (described below),
+ (described below)
  .\"
- because in UTF-8 mode this would make it impossible to calculate the length of
+ in a UTF mode, because this would make it impossible to calculate the length of
  the lookbehind.
+ .P
+ In general, the \eC escape sequence is best avoided. However, one
+ way of using it that avoids the problem of malformed UTF characters is to use a
+ lookahead to check the length of the next character, as in this pattern, which
+ could be used with a UTF-8 string (ignore white space and line breaks):
+ .sp
+   (?| (?=[\ex00-\ex7f])(\eC) |
+       (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
+       (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
+       (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
+ .sp
+ A group that starts with (?| resets the capturing parentheses numbers in each
+ alternative (see
+ .\" HTML <a href="#dupsubpatternnumber">
+ .\" </a>
+ "Duplicate Subpattern Numbers"
+ .\"
+ below). The assertions at the start of each branch check the next UTF-8
+ character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
+ character's individual bytes are then captured by the appropriate number of
+ groups.
  .
  .
  .\" HTML <a name="characterclass"></a>
-Line 520 
 the lookbehind.
+Line 1058 
 the lookbehind.
  .rs
  .sp
  An opening square bracket introduces a character class, terminated by a closing
- square bracket. A closing square bracket on its own is not special. If a
+ square bracket. A closing square bracket on its own is not special by default.
- closing square bracket is required as a member of the class, it should be the
+ However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
- first data character in the class (after an initial circumflex, if present) or
+ bracket causes a compile-time error. If a closing square bracket is required as
- escaped with a backslash.
+ a member of the class, it should be the first data character in the class
- .P
+ (after an initial circumflex, if present) or escaped with a backslash.
- A character class matches a single character in the subject. In UTF-8 mode, the
+ .P
- character may occupy more than one byte. A matched character must be in the set
+ A character class matches a single character in the subject. In a UTF mode, the
- of characters defined by the class, unless the first character in the class
+ character may be more than one data unit long. A matched character must be in
- definition is a circumflex, in which case the subject character must not be in
+ the set of characters defined by the class, unless the first character in the
- the set defined by the class. If a circumflex is actually required as a member
+ class definition is a circumflex, in which case the subject character must not
- of the class, ensure it is not the first character, or escape it with a
+ be in the set defined by the class. If a circumflex is actually required as a
+ member of the class, ensure it is not the first character, or escape it with a
  backslash.
  .P
  For example, the character class [aeiou] matches any lower case vowel, while
  [^aeiou] matches any character that is not a lower case vowel. Note that a
  circumflex is just a convenient notation for specifying the characters that
  are in the class by enumerating those that are not. A class that starts with a
- circumflex is not an assertion: it still consumes a character from the subject
+ circumflex is not an assertion; it still consumes a character from the subject
  string, and therefore it fails if the current pointer is at the end of the
  string.
  .P
- In UTF-8 mode, characters with values greater than 255 can be included in a
+ In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be
- class as a literal string of bytes, or by using the \ex{ escaping mechanism.
+ included in a class as a literal string of data units, or by using the \ex{
+ escaping mechanism.
  .P
  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
  "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 of
+ caseful 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
  always possible. For characters with higher values, the concept of case is
  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 in a UTF mode for characters 128 and
- ensure that PCRE is compiled with Unicode property support as well as with
+ above, you must ensure that PCRE is compiled with Unicode property support as
- UTF-8 support.
+ well as with UTF support.
  .P
- The newline character is never treated in any special way in character classes,
+ Characters that might indicate line breaks are never treated in any special way
- whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
+ when matching character classes, whatever line-ending sequence is in use, and
- such as [^a] will always match a newline.
+ whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
+ such as [^a] always matches one of these characters.
  .P
  The minus (hyphen) character can be used to specify a range of characters in a
  character class. For example, [d-m] matches any letter between d and m,
-Line 574 
 followed by two other characters. The oc
+Line 1115 
 followed by two other characters. The oc
  "]" can also be used to end a range.
  .P
  Ranges operate in the collating sequence of character values. They can also be
- used for characters specified numerically, for example [\e000-\e037]. In UTF-8
+ used for characters specified numerically, for example [\e000-\e037]. Ranges
- mode, ranges can include characters whose values are greater than 255, for
+ can include any characters that are valid for the current mode.
- example [\ex{100}-\ex{2ff}].
  .P
  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
- [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
+ [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
- tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E
+ tables for a French locale are in use, [\exc8-\excb] matches accented E
- characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
+ characters in both cases. In UTF modes, PCRE supports the concept of case for
  characters with values greater than 128 only when it is compiled with Unicode
  property support.
  .P
- The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
+ The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
- in a character class, and add the characters that they match to the class. For
+ \eV, \ew, and \eW may appear in a character class, and add the characters that
- example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
+ they match to the class. For example, [\edABCDEF] matches any hexadecimal
- conveniently be used with the upper case character types to specify a more
+ digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
- restricted set of characters than the matching lower case type. For example,
+ and their upper case partners, just as it does when they appear outside a
- the class [^\eW_] matches any letter or digit, but not underscore.
+ character class, as described in the section entitled
+ .\" HTML <a href="#genericchartypes">
+ .\" </a>
+ "Generic character types"
+ .\"
+ above. The escape sequence \eb has a different meaning inside a character
+ class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
+ are not special inside a character class. Like any other unrecognized escape
+ sequences, they are treated as the literal characters "B", "N", "R", and "X" by
+ default, but cause an error if the PCRE_EXTRA option is set.
+ .P
+ A circumflex can conveniently be used with the upper case character types to
+ specify a more restricted set of characters than the matching lower case type.
+ For example, the class [^\eW_] matches any letter or digit, but not underscore,
+ whereas [\ew] includes underscore. A positive character class should be read as
+ "something OR something OR ..." and a negative class as "NOT something AND NOT
+ something AND NOT ...".
  .P
  The only metacharacters that are recognized in character classes are backslash,
  hyphen (only where it can be interpreted as specifying a range), circumflex
-Line 611 
 this notation. For example,
+Line 1167 
 this notation. For example,
    [01[:alpha:]%]
  .sp
  matches "0", "1", any alphabetic character, or "%". The supported class names
- are
+ are:
  .sp
    alnum    letters and digits
    alpha    letters
-Line 622 
 are
+Line 1178 
 are
    graph    printing characters, excluding space
    lower    lower case letters
    print    printing characters, including space
-   punct    printing characters, excluding letters and digits
+   punct    printing characters, excluding letters and digits and space
    space    white space (not quite the same as \es)
    upper    upper case letters
    word     "word" characters (same as \ew)
-Line 643 
 matches "1", "2", or any non-digit. PCRE
+Line 1199 
 matches "1", "2", or any non-digit. PCRE
  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
  supported, and an error is given if they are encountered.
  .P
- In UTF-8 mode, characters with values greater than 128 do not match any of
+ By default, in UTF modes, characters with values greater than 128 do not match
- the POSIX character classes.
+ any of the POSIX character classes. However, if the PCRE_UCP option is passed
+ to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
+ character properties are used. This is achieved by replacing the POSIX classes
+ by other sequences, as follows:
+ .sp
+   [:alnum:]  becomes  \ep{Xan}
+   [:alpha:]  becomes  \ep{L}
+   [:blank:]  becomes  \eh
+   [:digit:]  becomes  \ep{Nd}
+   [:lower:]  becomes  \ep{Ll}
+   [:space:]  becomes  \ep{Xps}
+   [:upper:]  becomes  \ep{Lu}
+   [:word:]   becomes  \ep{Xwd}
+ .sp
+ Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
+ classes are unchanged, and match only characters with code points less than
+.
  .
  .
  .SH "VERTICAL BAR"
-Line 656 
 the pattern
+Line 1228 
 the pattern
    gilbert|sullivan
  .sp
  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
- and an empty alternative is permitted (matching the empty string).
+ and an empty alternative is permitted (matching the empty string). The matching
- The matching process tries each alternative in turn, from left to right,
+ process tries each alternative in turn, from left to right, and the first one
- and the first one that succeeds is used. If the alternatives are within a
+ that succeeds is used. If the alternatives are within a subpattern
- subpattern
  .\" HTML <a href="#subpattern">
  .\" </a>
  (defined below),
-Line 672 
 alternative in the subpattern.
+Line 1243 
 alternative in the subpattern.
  .rs
  .sp
  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
- PCRE_EXTENDED options can be changed from within the pattern by a sequence of
+ PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
- Perl option letters enclosed between "(?" and ")". The option letters are
+ the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
+ The option letters are
  .sp
    i  for PCRE_CASELESS
    m  for PCRE_MULTILINE
-Line 687 
 PCRE_MULTILINE while unsetting PCRE_DOTA
+Line 1259 
 PCRE_MULTILINE while unsetting PCRE_DOTA
  permitted. If a letter appears both before and after the hyphen, the option is
  unset.
  .P
- When an option change occurs at top level (that is, not inside subpattern
+ The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
- parentheses), the change applies to the remainder of the pattern that follows.
+ changed in the same way as the Perl-compatible options by using the characters
- If the change is placed right at the start of a pattern, PCRE extracts it into
+ J, U and X respectively.
- the global options (and it will therefore show up in data extracted by the
+ .P
- \fBpcre_fullinfo()\fP function).
+ When one of these option changes occurs at top level (that is, not inside
+ subpattern parentheses), the change applies to the remainder of the pattern
+ that follows. If the change is placed right at the start of a pattern, PCRE
+ extracts it into the global options (and it will therefore show up in data
+ extracted by the \fBpcre_fullinfo()\fP function).
  .P
- An option change within a subpattern affects only that part of the current
+ An option change within a subpattern (see below for a description of
- pattern that follows it, so
+ subpatterns) affects only that part of the subpattern that follows it, so
  .sp
    (a(?i)b)c
  .sp
-Line 710 
 branch is abandoned before the option se
+Line 1286 
 branch is abandoned before the option se
  option settings happen at compile time. There would be some very weird
  behaviour otherwise.
  .P
- The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
+ \fBNote:\fP There are other PCRE-specific options that can be set by the
- same way as the Perl-compatible options by using the characters U and X
+ application when the compiling or matching functions are called. In some cases
- respectively. The (?X) flag setting is special in that it must always occur
+ the pattern can contain special leading sequences such as (*CRLF) to override
- earlier in the pattern than any of the additional features it turns on, even
+ what the application has set or what has been defaulted. Details are given in
- when it is at top level. It is best to put it at the start.
+ the section entitled
+ .\" HTML <a href="#newlineseq">
+ .\" </a>
+ "Newline sequences"
+ .\"
+ above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
+ can be used to set UTF and Unicode property modes; they are equivalent to
+ setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
  .
  .
  .\" HTML <a name="subpattern"></a>
-Line 728 
 Turning part of a pattern into a subpatt
+Line 1311 
 Turning part of a pattern into a subpatt
  .sp
    cat(aract|erpillar|)
  .sp
- matches one of the words "cat", "cataract", or "caterpillar". Without the
+ matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
- parentheses, it would match "cataract", "erpillar" or the empty string.
+ match "cataract", "erpillar" or an empty string.
  .sp
 . 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
- subpattern is passed back to the caller via the \fIovector\fP argument of
+ subpattern is passed back to the caller via the \fIovector\fP argument of the
- \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
+ matching function. (This applies only to the traditional matching functions;
- from 1) to obtain numbers for the capturing subpatterns.
+ the DFA matching functions do not support capturing.)
  .P
- For example, if the string "the red king" is matched against the pattern
+ Opening parentheses are counted from left to right (starting from 1) to obtain
+ numbers for the capturing subpatterns. For example, if the string "the red
+ king" is matched against the pattern
  .sp
    the ((red|white) (king|queen))
  .sp
-Line 754 
 the string "the white queen" is matched
+Line 1339 
 the string "the white queen" is matched
    the ((?:red|white) (king|queen))
  .sp
  the captured substrings are "white queen" and "queen", and are numbered 1 and
-. The maximum number of capturing subpatterns is 65535, and the maximum depth
+. The maximum number of capturing subpatterns is 65535.
- of nesting of all subpatterns, both capturing and non-capturing, is 200.
  .P
  As a convenient shorthand, if any option settings are required at the start of
  a non-capturing subpattern, the option letters may appear between the "?" and
-Line 770 
 is reached, an option setting in one bra
+Line 1354 
 is reached, an option setting in one bra
  the above patterns match "SUNDAY" as well as "Saturday".
  .
  .
+ .\" HTML <a name="dupsubpatternnumber"></a>
+ .SH "DUPLICATE SUBPATTERN NUMBERS"
+ .rs
+ .sp
+ 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
+ (?| and is itself a non-capturing subpattern. For example, consider this
+ pattern:
+ .sp
+   (?|(Sat)ur|(Sun))day
+ .sp
+ Because the two alternatives are inside a (?| group, both sets of capturing
+ parentheses are numbered one. Thus, when the pattern matches, you can look
+ at captured substring number one, whichever alternative matched. This construct
+ is useful when you want to capture part, but not all, of one of a number of
+ alternatives. Inside a (?| group, parentheses are numbered as usual, but the
+ number is reset at the start of each branch. The numbers of any capturing
+ parentheses that follow the subpattern start after the highest number used in
+ any branch. The following example is taken from the Perl documentation. The
+ numbers underneath show in which buffer the captured content will be stored.
+ .sp
+   # before  ---------------branch-reset----------- after
+   / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
+   # 1            2         2  3        2     3     4
+ .sp
+ A back reference to a numbered subpattern uses the most recent value that is
+ set for that number by any subpattern. The following pattern matches "abcabc"
+ or "defdef":
+ .sp
+   /(?|(abc)|(def))\e1/
+ .sp
+ In contrast, a subroutine call to a numbered subpattern always refers to the
+ first one in the pattern with the given number. The following pattern matches
+ "abcabc" or "defabc":
+ .sp
+   /(?|(abc)|(def))(?1)/
+ .sp
+ If a
+ .\" HTML <a href="#conditions">
+ .\" </a>
+ condition test
+ .\"
+ for a subpattern's having matched refers to a non-unique number, the test is
+ true if any of the subpatterns of that number have matched.
+ .P
+ An alternative approach to using this "branch reset" feature is to use
+ duplicate named subpatterns, as described in the next section.
+ .
+ .
  .SH "NAMED SUBPATTERNS"
  .rs
  .sp
  Identifying capturing parentheses by number is simple, but it can be very hard
  to keep track of the numbers in complicated regular expressions. Furthermore,
  if an expression is modified, the numbers may change. To help with this
- difficulty, PCRE supports the naming of subpatterns, something that Perl does
+ difficulty, PCRE supports the naming of subpatterns. This feature was not
- not provide. The Python syntax (?P<name>...) is used. Names consist of
+ added to Perl until release 5.10. Python had the feature earlier, and PCRE
- alphanumeric characters and underscores, and must be unique within a pattern.
+ introduced it at release 4.0, using the Python syntax. PCRE now supports both
- .P
+ the Perl and the Python syntax. Perl allows identically numbered subpatterns to
- Named capturing parentheses are still allocated numbers as well as names. The
+ have different names, but PCRE does not.
- PCRE API provides function calls for extracting the name-to-number translation
+ .P
- table from a compiled pattern. There is also a convenience function for
+ In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
- extracting a captured substring by name. For further details see the
+ (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
+ parentheses from other parts of the pattern, such as
+ .\" HTML <a href="#backreferences">
+ .\" </a>
+ back references,
+ .\"
+ .\" HTML <a href="#recursion">
+ .\" </a>
+ recursion,
+ .\"
+ and
+ .\" HTML <a href="#conditions">
+ .\" </a>
+ conditions,
+ .\"
+ can be made by name as well as by number.
+ .P
+ Names consist of up to 32 alphanumeric characters and underscores. Named
+ capturing parentheses are still allocated numbers as well as names, exactly as
+ if the names were not present. The PCRE API provides function calls for
+ extracting the name-to-number translation table from a compiled pattern. There
+ is also a convenience function for extracting a captured substring by name.
+ .P
+ By default, a name must be unique within a pattern, but it is possible to relax
+ this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
+ names are also always permitted for subpatterns with the same number, set up as
+ described in the previous section.) Duplicate names can be useful for patterns
+ where only one instance of the named parentheses can match. Suppose you want to
+ match the name of a weekday, either as a 3-letter abbreviation or as the full
+ name, and in both cases you want to extract the abbreviation. This pattern
+ (ignoring the line breaks) does the job:
+ .sp
+   (?<DN>Mon|Fri|Sun)(?:day)?|
+   (?<DN>Tue)(?:sday)?|
+   (?<DN>Wed)(?:nesday)?|
+   (?<DN>Thu)(?:rsday)?|
+   (?<DN>Sat)(?:urday)?
+ .sp
+ There are five capturing substrings, but only one is ever set after a match.
+ (An alternative way of solving this problem is to use a "branch reset"
+ subpattern, as described in the previous section.)
+ .P
+ The convenience function for extracting the data by name returns the substring
+ for the first (and in this example, the only) subpattern of that name that
+ matched. This saves searching to find which numbered subpattern it was.
+ .P
+ If you make a back reference to a non-unique named subpattern from elsewhere in
+ the pattern, the one that corresponds to the first occurrence of the name is
+ used. In the absence of duplicate numbers (see the previous section) this is
+ the one with the lowest number. If you use a named reference in a condition
+ test (see the
+ .\"
+ .\" HTML <a href="#conditions">
+ .\" </a>
+ section about conditions
+ .\"
+ below), either to check whether a subpattern has matched, or to check for
+ recursion, all subpatterns with the same name are tested. If the condition is
+ true for any one of them, the overall condition is true. This is the same
+ behaviour as testing by number. For further details of the interfaces for
+ handling named subpatterns, see the
  .\" HREF
  \fBpcreapi\fP
  .\"
  documentation.
+ .P
+ \fBWarning:\fP You cannot use different names to distinguish between two
+ subpatterns with the same number because PCRE uses only the numbers when
+ matching. For this reason, an error is given at compile time if different names
+ 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.
  .
  .
  .SH REPETITION
-Line 797 
 Repetition is specified by quantifiers,
+Line 1496 
 Repetition is specified by quantifiers,
  items:
  .sp
    a literal data character
-   the . metacharacter
+   the dot metacharacter
    the \eC escape sequence
-   the \eX escape sequence (in UTF-8 mode with Unicode properties)
+   the \eX escape sequence
-   an escape such as \ed that matches a single character
+   the \eR escape sequence
+   an escape such as \ed or \epL that matches a single character
    a character class
    a back reference (see next section)
-   a parenthesized subpattern (unless it is an assertion)
+   a parenthesized subpattern (including assertions)
+   a subroutine call to a subpattern (recursive or otherwise)
  .sp
  The general repetition quantifier specifies a minimum and maximum number of
  permitted matches, by giving the two numbers in curly brackets (braces),
-Line 828 
 where a quantifier is not allowed, or on
+Line 1529 
 where a quantifier is not allowed, or on
  quantifier, is taken as a literal character. For example, {,6} is not a
  quantifier, but a literal string of four characters.
  .P
- In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
+ In UTF modes, quantifiers apply to characters rather than to individual data
- bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
+ units. Thus, for example, \ex{100}{2} matches two characters, each of
- which is represented by a two-byte sequence. Similarly, when Unicode property
+ which is represented by a two-byte sequence in a UTF-8 string. Similarly,
- support is available, \eX{3} matches three Unicode extended sequences, each of
+ \eX{3} matches three Unicode extended sequences, each of which may be several
- which may be several bytes long (and they may be of different lengths).
+ data units long (and they may be of different lengths).
  .P
  The quantifier {0} is permitted, causing the expression to behave as if the
- previous item and the quantifier were not present.
+ previous item and the quantifier were not present. This may be useful for
+ subpatterns that are referenced as
+ .\" HTML <a href="#subpatternsassubroutines">
+ .\" </a>
+ subroutines
+ .\"
+ from elsewhere in the pattern (but see also the section entitled
+ .\" HTML <a href="#subdefine">
+ .\" </a>
+ "Defining subpatterns for use by reference only"
+ .\"
+ below). Items other than subpatterns that have a {0} quantifier are omitted
+ from the compiled pattern.
  .P
- For convenience (and historical compatibility) the three most common
+ For convenience, the three most common quantifiers have single-character
- quantifiers have single-character abbreviations:
+ abbreviations:
  .sp
    *    is equivalent to {0,}
    +    is equivalent to {1,}
-Line 886 
 own right. Because it has two uses, it c
+Line 1599 
 own right. Because it has two uses, it c
  which matches one digit by preference, but can match two if that is the only
  way the rest of the pattern matches.
  .P
- If the PCRE_UNGREEDY option is set (an option which is not available in Perl),
+ If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
  the quantifiers are not greedy by default, but individual ones can be made
  greedy by following them with a question mark. In other words, it inverts the
  default behaviour.
-Line 896 
 is greater than 1 or with a limited maxi
+Line 1609 
 is greater than 1 or with a limited maxi
  compiled pattern, in proportion to the size of the minimum or maximum.
  .P
  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
- to Perl's /s) is set, thus allowing the . to match newlines, the pattern is
+ to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
  implicitly anchored, because whatever follows will be tried against every
  character position in the subject string, so there is no point in retrying the
  overall match at any position after the first. PCRE normally treats such a
-Line 907 
 worth setting PCRE_DOTALL in order to ob
+Line 1620 
 worth setting PCRE_DOTALL in order to ob
  alternatively using ^ to indicate anchoring explicitly.
  .P
  However, there is one situation where the optimization cannot be used. When .*
- is inside capturing parentheses that are the subject of a backreference
+ is inside capturing parentheses that are the subject of a back reference
- elsewhere in the pattern, a match at the start may fail, and a later one
+ elsewhere in the pattern, a match at the start may fail where a later one
- succeed. Consider, for example:
+ succeeds. Consider, for example:
  .sp
    (.*)abc\e1
  .sp
-Line 935 
 matches "aba" the value of the second ca
+Line 1648 
 matches "aba" the value of the second ca
  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
  .rs
  .sp
- With both maximizing and minimizing repetition, failure of what follows
+ With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
- normally causes the repeated item to be re-evaluated to see if a different
+ repetition, failure of what follows normally causes the repeated item to be
- number of repeats allows the rest of the pattern to match. Sometimes it is
+ re-evaluated to see if a different number of repeats allows the rest of the
- useful to prevent this, either to change the nature of the match, or to cause
+ pattern to match. Sometimes it is useful to prevent this, either to change the
- it fail earlier than it otherwise might, when the author of the pattern knows
+ nature of the match, or to cause it fail earlier than it otherwise might, when
- there is no point in carrying on.
+ the author of the pattern knows there is no point in carrying on.
  .P
  Consider, for example, the pattern \ed+foo when applied to the subject line
  .sp
-Line 952 
 item, and then with 4, and so on, before
+Line 1665 
 item, and then with 4, and so on, before
  (a term taken from Jeffrey Friedl's book) provides the means for specifying
  that once a subpattern has matched, it is not to be re-evaluated in this way.
  .P
- If we use atomic grouping for the previous example, the matcher would give up
+ If we use atomic grouping for the previous example, the matcher gives up
  immediately on failing to match "foo" the first time. The notation is a kind of
  special parenthesis, starting with (?> as in this example:
  .sp
-Line 982 
 previous example can be rewritten as
+Line 1695 
 previous example can be rewritten as
  .sp
    \ed++foo
  .sp
+ Note that a possessive quantifier can be used with an entire group, for
+ example:
+ .sp
+   (abc|xyz){2,3}+
+ .sp
  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
  option is ignored. They are a convenient notation for the simpler forms of
- atomic group. However, there is no difference in the meaning or processing of a
+ atomic group. However, there is no difference in the meaning of a possessive
- possessive quantifier and the equivalent atomic group.
+ quantifier and the equivalent atomic group, though there may be a performance
- .P
+ difference; possessive quantifiers should be slightly faster.
- The possessive quantifier syntax is an extension to the Perl syntax. It
+ .P
- originates in Sun's Java package.
+ The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
+ Jeffrey Friedl originated the idea (and the name) in the first edition of his
+ book. Mike McCloskey liked it, so implemented it when he built Sun's Java
+ package, and PCRE copied it from there. It ultimately found its way into Perl
+ at release 5.10.
+ .P
+ PCRE has an optimization that automatically "possessifies" certain simple
+ pattern constructs. For example, the sequence A+B is treated as A++B because
+ there is no point in backtracking into a sequence of A's when B must follow.
  .P
  When a pattern contains an unlimited repeat inside a subpattern that can itself
  be repeated an unlimited number of times, the use of an atomic group is the
-Line 1030 
 However, if the decimal number following
+Line 1756 
 However, if the decimal number following
  always taken as a back reference, and causes an error only if there are not
  that many capturing left parentheses in the entire pattern. In other words, the
  parentheses that are referenced need not be to the left of the reference for
- numbers less than 10. See the subsection entitled "Non-printing characters"
+ numbers less than 10. A "forward back reference" of this type can make sense
+ when a repetition is involved and the subpattern to the right has participated
+ in an earlier iteration.
+ .P
+ It is not possible to have a numerical "forward back reference" to a subpattern
+ whose number is 10 or more using this syntax because a sequence such as \e50 is
+ interpreted as a character defined in octal. See the subsection entitled
+ "Non-printing characters"
  .\" HTML <a href="#digitsafterbackslash">
  .\" </a>
  above
  .\"
- for further details of the handling of digits following a backslash.
+ for further details of the handling of digits following a backslash. There is
+ no such problem when named parentheses are used. A back reference to any
+ subpattern is possible using named parentheses (see below).
+ .P
+ Another way of avoiding the ambiguity inherent in the use of digits following a
+ backslash is to use the \eg escape sequence. This escape must be followed by an
+ unsigned number or a negative number, optionally enclosed in braces. These
+ examples are all identical:
+ .sp
+   (ring), \e1
+   (ring), \eg1
+   (ring), \eg{1}
+ .sp
+ An unsigned number specifies an absolute reference without the ambiguity that
+ is present in the older syntax. It is also useful when literal digits follow
+ the reference. A negative number is a relative reference. Consider this
+ example:
+ .sp
+   (abc(def)ghi)\eg{-1}
+ .sp
+ The sequence \eg{-1} is a reference to the most recently started capturing
+ subpattern before \eg, that is, is it equivalent to \e2 in this example.
+ Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
+ can be helpful in long patterns, and also in patterns that are created by
+ joining together fragments that contain references within themselves.
  .P
  A back reference matches whatever actually matched the capturing subpattern in
  the current subject string, rather than anything matching the subpattern
-Line 1057 
 back reference, the case of letters is r
+Line 1814 
 back reference, the case of letters is r
  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
  capturing subpattern is matched caselessly.
  .P
- Back references to named subpatterns use the Python syntax (?P=name). We could
+ There are several different ways of writing back references to named
- rewrite the above example as follows:
+ subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
- .sp
+ \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
-   (?<p1>(?i)rah)\es+(?P=p1)
+ back reference syntax, in which \eg can be used for both numeric and named
+ references, is also supported. We could rewrite the above example in any of
+ the following ways:
+ .sp
+   (?<p1>(?i)rah)\es+\ek<p1>
+   (?'p1'(?i)rah)\es+\ek{p1}
+   (?P<p1>(?i)rah)\es+(?P=p1)
+   (?<p1>(?i)rah)\es+\eg{p1}
  .sp
+ A subpattern that is referenced by name may appear in the pattern before or
+ after the reference.
+ .P
  There may be more than one back reference to the same subpattern. If a
  subpattern has not actually been used in a particular match, any back
- references to it always fail. For example, the pattern
+ references to it always fail by default. For example, the pattern
  .sp
    (a|(bc))\e2
  .sp
- always fails if it starts to match "a" rather than "bc". Because there may be
+ always fails if it starts to match "a" rather than "bc". However, if the
- many capturing parentheses in a pattern, all digits following the backslash are
+ PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
- taken as part of a potential back reference number. If the pattern continues
+ unset value matches an empty string.
- with a digit character, some delimiter must be used to terminate the back
+ .P
- reference. If the PCRE_EXTENDED option is set, this can be whitespace.
+ Because there may be many capturing parentheses in a pattern, all digits
- Otherwise an empty comment (see
+ 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
+ terminate the back reference. If the PCRE_EXTENDED option is set, this can be
+ whitespace. Otherwise, the \eg{ syntax or an empty comment (see
  .\" HTML <a href="#comments">
  .\" </a>
  "Comments"
  .\"
  below) can be used.
- .P
+ .
+ .SS "Recursive back references"
+ .rs
+ .sp
  A back reference that occurs inside the parentheses to which it refers fails
  when the subpattern is first used, so, for example, (a\e1) never matches.
  However, such references can be useful inside repeated subpatterns. For
-Line 1093 
 to the previous iteration. In order for
+Line 1866 
 to the previous iteration. In order for
  that the first iteration does not need to match the back reference. This can be
  done using alternation, as in the example above, or by a quantifier with a
  minimum of zero.
+ .P
+ Back references of this type cause the group that they reference to be treated
+ as an
+ .\" HTML <a href="#atomicgroup">
+ .\" </a>
+ atomic group.
+ .\"
+ Once the whole group has been matched, a subsequent matching failure cannot
+ cause backtracking into the middle of the group.
  .
  .
  .\" HTML <a name="bigassertions"></a>
-Line 1112 
 those that look ahead of the current pos
+Line 1894 
 those that look ahead of the current pos
  that look behind it. An assertion subpattern is matched in the normal way,
  except that it does not cause the current matching position to be changed.
  .P
- Assertion subpatterns are not capturing subpatterns, and may not be repeated,
+ Assertion subpatterns are not capturing subpatterns. If such an assertion
- because it makes no sense to assert the same thing several times. If any kind
+ contains capturing subpatterns within it, these are counted for the purposes of
- of assertion contains capturing subpatterns within it, these are counted for
+ numbering the capturing subpatterns in the whole pattern. However, substring
- the purposes of numbering the capturing subpatterns in the whole pattern.
+ capturing is carried out only for positive assertions, because it does not make
- However, substring capturing is carried out only for positive assertions,
+ sense for negative assertions.
- because it does not make sense for negative assertions.
+ .P
+ 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
+ capturing parentheses may occasionally be useful. In practice, there only three
+ cases:
+ .sp
+ (1) If the quantifier is {0}, the assertion is never obeyed during matching.
+ However, it may contain internal capturing parenthesized groups that are called
+ from elsewhere via the
+ .\" HTML <a href="#subpatternsassubroutines">
+ .\" </a>
+ subroutine mechanism.
+ .\"
+ .sp
+ (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
+ were {0,1}. At run time, the rest of the pattern match is tried with and
+ without the assertion, the order depending on the greediness of the quantifier.
+ .sp
+ (3) If the minimum repetition is greater than zero, the quantifier is ignored.
+ The assertion is obeyed just once when encountered during matching.
  .
  .
  .SS "Lookahead assertions"
  .rs
  .sp
- Lookahead assertions start
+ Lookahead assertions start with (?= for positive assertions and (?! for
- with (?= for positive assertions and (?! for negative assertions. For example,
+ negative assertions. For example,
  .sp
    \ew+(?=;)
  .sp
-Line 1146 
 lookbehind assertion is needed to achiev
+Line 1947 
 lookbehind assertion is needed to achiev
  If you want to force a matching failure at some point in a pattern, the most
  convenient way to do it is with (?!) because an empty string always matches, so
  an assertion that requires there not to be an empty string must always fail.
+ The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
  .
  .
  .\" HTML <a name="lookbehind"></a>
-Line 1159 
 negative assertions. For example,
+Line 1961 
 negative assertions. For example,
  .sp
  does find an occurrence of "bar" that is not preceded by "foo". The contents of
  a lookbehind assertion are restricted such that all the strings it matches must
- have a fixed length. However, if there are several alternatives, they do not
+ have a fixed length. However, if there are several top-level alternatives, they
- all have to have the same fixed length. Thus
+ do not all have to have the same fixed length. Thus
  .sp
    (?<=bullock|donkey)
  .sp
-Line 1170 
 is permitted, but
+Line 1972 
 is permitted, but
  .sp
  causes an error at compile time. Branches that match different length strings
  are permitted only at the top level of a lookbehind assertion. This is an
- extension compared with Perl (at least for 5.8), which requires all branches to
+ extension compared with Perl, which requires all branches to match the same
- match the same length of string. An assertion such as
+ length of string. An assertion such as
  .sp
    (?<=ab(c|de))
  .sp
  is not permitted, because its single top-level branch can match two different
- lengths, but it is acceptable if rewritten to use two top-level branches:
+ lengths, but it is acceptable to PCRE if rewritten to use two top-level
+ branches:
  .sp
    (?<=abc|abde)
  .sp
+ In some cases, the escape sequence \eK
+ .\" HTML <a href="#resetmatchstart">
+ .\" </a>
+ (see above)
+ .\"
+ can be used instead of a lookbehind assertion to get round the fixed-length
+ restriction.
+ .P
  The implementation of lookbehind assertions is, for each alternative, to
- temporarily move the current position back by the fixed width and then try to
+ temporarily move the current position back by the fixed length and then try to
  match. If there are insufficient characters before the current position, the
- match is deemed to fail.
+ assertion fails.
+ .P
+ In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
+ unit even in a UTF mode) to appear in lookbehind assertions, because it makes
+ it impossible to calculate the length of the lookbehind. The \eX and \eR
+ escapes, which can match different numbers of data units, are also not
+ permitted.
+ .P
+ .\" HTML <a href="#subpatternsassubroutines">
+ .\" </a>
+ "Subroutine"
+ .\"
+ calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
+ as the subpattern matches a fixed-length string.
+ .\" HTML <a href="#recursion">
+ .\" </a>
+ Recursion,
+ .\"
+ however, is not supported.
  .P
- PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
+ Possessive quantifiers can be used in conjunction with lookbehind assertions to
- to appear in lookbehind assertions, because it makes it impossible to calculate
+ specify efficient matching of fixed-length strings at the end of subject
- the length of the lookbehind. The \eX escape, which can match different numbers
+ strings. Consider a simple pattern such as
- of bytes, is also not permitted.
- .P
- Atomic groups can be used in conjunction with lookbehind assertions to specify
- efficient matching at the end of the subject string. Consider a simple pattern
- such as
  .sp
    abcd$
  .sp
-Line 1208 
 then all but the last two characters, an
+Line 2032 
 then all but the last two characters, an
  covers the entire string, from right to left, so we are no better off. However,
  if the pattern is written as
  .sp
-   ^(?>.*)(?<=abcd)
- .sp
- or, equivalently, using the possessive quantifier syntax,
- .sp
    ^.*+(?<=abcd)
  .sp
- there can be no backtracking for the .* item; it can match only the entire
+ there can be no backtracking for the .*+ item; it can match only the entire
  string. The subsequent lookbehind assertion does a single test on the last four
  characters. If it fails, the match fails immediately. For long strings, this
  approach makes a significant difference to the processing time.
-Line 1254 
 is another pattern that matches "foo" pr
+Line 2074 
 is another pattern that matches "foo" pr
  characters that are not "999".
  .
  .
+ .\" HTML <a name="conditions"></a>
  .SH "CONDITIONAL SUBPATTERNS"
  .rs
  .sp
  It is possible to cause the matching process to obey a subpattern
  conditionally or to choose between two alternative subpatterns, depending on
- the result of an assertion, or whether a previous capturing subpattern matched
+ the result of an assertion, or whether a specific capturing subpattern has
- or not. The two possible forms of conditional subpattern are
+ already been matched. The two possible forms of conditional subpattern are:
  .sp
    (?(condition)yes-pattern)
    (?(condition)yes-pattern|no-pattern)
  .sp
  If the condition is satisfied, the yes-pattern is used; otherwise the
  no-pattern (if present) is used. If there are more than two alternatives in the
- subpattern, a compile-time error occurs.
+ subpattern, a compile-time error occurs. Each of the two alternatives may
+ itself contain nested subpatterns of any form, including conditional
+ subpatterns; the restriction to two alternatives applies only at the level of
+ the condition. This pattern fragment is an example where the alternatives are
+ complex:
+ .sp
+   (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
+ .sp
  .P
- There are three kinds of condition. If the text between the parentheses
+ There are four kinds of condition: references to subpatterns, references to
- consists of a sequence of digits, the condition is satisfied if the capturing
+ recursion, a pseudo-condition called DEFINE, and assertions.
- subpattern of that number has previously matched. The number must be greater
+ .
- than zero. Consider the following pattern, which contains non-significant white
+ .SS "Checking for a used subpattern by number"
- space to make it more readable (assume the PCRE_EXTENDED option) and to divide
+ .rs
- it into three parts for ease of discussion:
+ .sp
+ If the text between the parentheses consists of a sequence of digits, the
+ condition is true if a capturing subpattern of that number has previously
+ matched. If there is more than one capturing subpattern with the same number
+ (see the earlier
+ .\"
+ .\" HTML <a href="#recursion">
+ .\" </a>
+ section about duplicate subpattern numbers),
+ .\"
+ the condition is true if any of them have matched. An alternative notation is
+ to precede the digits with a plus or minus sign. In this case, the subpattern
+ number is relative rather than absolute. The most recently opened parentheses
+ can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
+ loops it can also make sense to refer to subsequent groups. The next
+ parentheses to be opened can be referenced as (?(+1), and so on. (The value
+ zero in any of these forms is not used; it provokes a compile-time error.)
+ .P
+ Consider the following pattern, which contains non-significant white space to
+ make it more readable (assume the PCRE_EXTENDED option) and to divide it into
+ three parts for ease of discussion:
  .sp
    ( \e( )?    [^()]+    (?(1) \e) )
  .sp
  The first part matches an optional opening parenthesis, and if that
  character is present, sets it as the first captured substring. The second part
  matches one or more characters that are not parentheses. The third part is a
- conditional subpattern that tests whether the first set of parentheses matched
+ conditional subpattern that tests whether or not the first set of parentheses
- or not. If they did, that is, if subject started with an opening parenthesis,
+ matched. If they did, that is, if subject started with an opening parenthesis,
  the condition is true, and so the yes-pattern is executed and a closing
  parenthesis is required. Otherwise, since no-pattern is not present, the
  subpattern matches nothing. In other words, this pattern matches a sequence of
  non-parentheses, optionally enclosed in parentheses.
  .P
- If the condition is the string (R), it is satisfied if a recursive call to the
+ If you were embedding this pattern in a larger one, you could use a relative
- pattern or subpattern has been made. At "top level", the condition is false.
+ reference:
- This is a PCRE extension. Recursive patterns are described in the next section.
+ .sp
+   ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
+ .sp
+ This makes the fragment independent of the parentheses in the larger pattern.
+ .
+ .SS "Checking for a used subpattern by name"
+ .rs
+ .sp
+ Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
+ subpattern by name. For compatibility with earlier versions of PCRE, which had
+ this facility before Perl, the syntax (?(name)...) is also recognized. However,
+ there is a possible ambiguity with this syntax, because subpattern names may
+ consist entirely of digits. PCRE looks first for a named subpattern; if it
+ cannot find one and the name consists entirely of digits, PCRE looks for a
+ subpattern of that number, which must be greater than zero. Using subpattern
+ names that consist entirely of digits is not recommended.
  .P
- If the condition is not a sequence of digits or (R), it must be an assertion.
+ Rewriting the above example to use a named subpattern gives this:
+ .sp
+   (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
+ .sp
+ If the name used in a condition of this kind is a duplicate, the test is
+ applied to all subpatterns of the same name, and is true if any one of them has
+ matched.
+ .
+ .SS "Checking for pattern recursion"
+ .rs
+ .sp
+ If the condition is the string (R), and there is no subpattern with the name R,
+ the condition is true if a recursive call to the whole pattern or any
+ subpattern has been made. If digits or a name preceded by ampersand follow the
+ letter R, for example:
+ .sp
+   (?(R3)...) or (?(R&name)...)
+ .sp
+ the condition is true if the most recent recursion is into a subpattern whose
+ number or name is given. This condition does not check the entire recursion
+ stack. If the name used in a condition of this kind is a duplicate, the test is
+ applied to all subpatterns of the same name, and is true if any one of them is
+ the most recent recursion.
+ .P
+ At "top level", all these recursion test conditions are false.
+ .\" HTML <a href="#recursion">
+ .\" </a>
+ The syntax for recursive patterns
+ .\"
+ is described below.
+ .
+ .\" HTML <a name="subdefine"></a>
+ .SS "Defining subpatterns for use by reference only"
+ .rs
+ .sp
+ If the condition is the string (DEFINE), and there is no subpattern with the
+ name DEFINE, the condition is always false. In this case, there may be only one
+ alternative in the subpattern. It is always skipped if control reaches this
+ point in the pattern; the idea of DEFINE is that it can be used to define
+ subroutines that can be referenced from elsewhere. (The use of
+ .\" HTML <a href="#subpatternsassubroutines">
+ .\" </a>
+ subroutines
+ .\"
+ 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
+ breaks):
+ .sp
+   (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
+   \eb (?&byte) (\e.(?&byte)){3} \eb
+ .sp
+ The first part of the pattern is a DEFINE group inside which a another group
+ named "byte" is defined. This matches an individual component of an IPv4
+ address (a number less than 256). When matching takes place, this part of the
+ pattern is skipped because DEFINE acts like a false condition. The rest of the
+ pattern uses references to the named group to match the four dot-separated
+ components of an IPv4 address, insisting on a word boundary at each end.
+ .
+ .SS "Assertion conditions"
+ .rs
+ .sp
+ If the condition is not in any of the above formats, it must be an assertion.
  This may be a positive or negative lookahead or lookbehind assertion. Consider
  this pattern, again containing non-significant white space, and with the two
  alternatives on the second line:
-Line 1312 
 dd-aaa-dd or dd-dd-dd, where aaa are let
+Line 2236 
 dd-aaa-dd or dd-dd-dd, where aaa are let
  .SH COMMENTS
  .rs
  .sp
- The sequence (?# marks the start of a comment that continues up to the next
+ There are two ways of including comments in patterns that are processed by
- closing parenthesis. Nested parentheses are not permitted. The characters
+ PCRE. In both cases, the start of the comment must not be in a character class,
- that make up a comment play no part in the pattern matching at all.
+ nor in the middle of any other sequence of related characters such as (?: or a
+ subpattern name or number. The characters that make up a comment play no part
+ in the pattern matching.
  .P
- If the PCRE_EXTENDED option is set, an unescaped # character outside a
+ The sequence (?# marks the start of a comment that continues up to the next
- character class introduces a comment that continues up to the next newline
+ closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
- character in the pattern.
+ option is set, an unescaped # character also introduces a comment, which in
+ this case continues to immediately after the next newline character or
+ character sequence in the pattern. Which characters are interpreted as newlines
+ is 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
+ .\" HTML <a href="#newlines">
+ .\" </a>
+ "Newline conventions"
+ .\"
+ above. Note that the end of this type of comment is a literal newline sequence
+ in the pattern; escape sequences that happen to represent a newline do not
+ count. For example, consider this pattern when PCRE_EXTENDED is set, and the
+ default newline convention is in force:
+ .sp
+   abc #comment \en still comment
+ .sp
+ On encountering the # character, \fBpcre_compile()\fP skips along, looking for
+ a newline in the pattern. The sequence \en is still literal at this stage, so
+ it does not terminate the comment. Only an actual character with the code value
+x0a (the default newline) does so.
  .
  .
+ .\" HTML <a name="recursion"></a>
  .SH "RECURSIVE PATTERNS"
  .rs
  .sp
  Consider the problem of matching a string in parentheses, allowing for
  unlimited nested parentheses. Without the use of recursion, the best that can
  be done is to use a pattern that matches up to some fixed depth of nesting. It
- is not possible to handle an arbitrary nesting depth. Perl provides a facility
+ is not possible to handle an arbitrary nesting depth.
- that allows regular expressions to recurse (amongst other things). It does this
+ .P
- by interpolating Perl code in the expression at run time, and the code can
+ For some time, Perl has provided a facility that allows regular expressions to
- refer to the expression itself. A Perl pattern to solve the parentheses problem
+ recurse (amongst other things). It does this by interpolating Perl code in the
- can be created like this:
+ expression at run time, and the code can refer to the expression itself. A Perl
+ pattern using code interpolation to solve the parentheses problem can be
+ created like this:
  .sp
    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
  .sp
  The (?p{...}) item interpolates Perl code at run time, and in this case refers
- recursively to the pattern in which it appears. Obviously, PCRE cannot support
+ recursively to the pattern in which it appears.
- the interpolation of Perl code. Instead, it supports some special syntax for
- recursion of the entire pattern, and also for individual subpattern recursion.
- .P
- The special item that consists of (? followed by a number greater than zero and
- a closing parenthesis is a recursive call of the subpattern of the given
- number, provided that it occurs inside that subpattern. (If not, it is a
- "subroutine" call, which is described in the next section.) The special item
- (?R) is a recursive call of the entire regular expression.
  .P
- For example, this PCRE pattern solves the nested parentheses problem (assume
+ Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
- the PCRE_EXTENDED option is set so that white space is ignored):
+ supports special syntax for recursion of the entire pattern, and also for
+ individual subpattern recursion. After its introduction in PCRE and Python,
+ this kind of recursion was subsequently introduced into Perl at release 5.10.
+ .P
+ A special item that consists of (? followed by a number greater than zero and a
+ closing parenthesis is a recursive subroutine call of the subpattern of the
+ given number, provided that it occurs inside that subpattern. (If not, it is a
+ .\" HTML <a href="#subpatternsassubroutines">
+ .\" </a>
+ non-recursive subroutine
+ .\"
+ call, which is described in the next section.) The special item (?R) or (?0) is
+ a recursive call of the entire regular expression.
+ .P
+ This PCRE pattern solves the nested parentheses problem (assume the
+ PCRE_EXTENDED option is set so that white space is ignored):
  .sp
-   \e( ( (?>[^()]+) | (?R) )* \e)
+   \e( ( [^()]++ | (?R) )* \e)
  .sp
  First it matches an opening parenthesis. Then it matches any number of
  substrings which can either be a sequence of non-parentheses, or a recursive
- match of the pattern itself (that is a correctly parenthesized substring).
+ match of the pattern itself (that is, a correctly parenthesized substring).
- Finally there is a closing parenthesis.
+ Finally there is a closing parenthesis. Note the use of a possessive quantifier
+ to avoid backtracking into sequences of non-parentheses.
  .P
  If this were part of a larger pattern, you would not want to recurse the entire
  pattern, so instead you could use this:
  .sp
-   ( \e( ( (?>[^()]+) | (?1) )* \e) )
+   ( \e( ( [^()]++ | (?1) )* \e) )
  .sp
  We have put the pattern into parentheses, and caused the recursion to refer to
- them instead of the whole pattern. In a larger pattern, keeping track of
+ them instead of the whole pattern.
- parenthesis numbers can be tricky. It may be more convenient to use named
+ .P
- parentheses instead. For this, PCRE uses (?P>name), which is an extension to
+ In a larger pattern, keeping track of parenthesis numbers can be tricky. This
- the Python syntax that PCRE uses for named parentheses (Perl does not provide
+ is made easier by the use of relative references. Instead of (?1) in the
- named parentheses). We could rewrite the above example as follows:
+ pattern above you can write (?-2) to refer to the second most recently opened
- .sp
+ parentheses preceding the recursion. In other words, a negative number counts
-   (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )
+ capturing parentheses leftwards from the point at which it is encountered.
- .sp
+ .P
- This particular example pattern contains nested unlimited repeats, and so the
+ It is also possible to refer to subsequently opened parentheses, by writing
- use of atomic grouping for matching strings of non-parentheses is important
+ references such as (?+2). However, these cannot be recursive because the
- when applying the pattern to strings that do not match. For example, when this
+ reference is not inside the parentheses that are referenced. They are always
- pattern is applied to
+ .\" HTML <a href="#subpatternsassubroutines">
+ .\" </a>
+ non-recursive subroutine
+ .\"
+ calls, as described in the next section.
+ .P
+ An alternative approach is to use named parentheses instead. The Perl syntax
+ for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
+ could rewrite the above example as follows:
+ .sp
+   (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
+ .sp
+ If there is more than one subpattern with the same name, the earliest one is
+ used.
+ .P
+ This particular example pattern that we have been looking at contains nested
+ unlimited repeats, and so the use of a possessive quantifier for matching
+ strings of non-parentheses is important when applying the pattern to strings
+ that do not match. For example, when this pattern is applied to
  .sp
    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
  .sp
- it yields "no match" quickly. However, if atomic grouping is not used,
+ it yields "no match" quickly. However, if a possessive quantifier is not used,
  the match runs for a very long time indeed because there are so many different
  ways the + and * repeats can carve up the subject, and all have to be tested
  before failure can be reported.
  .P
- At the end of a match, the values set for any capturing subpatterns are those
+ At the end of a match, the values of capturing parentheses are those from
- from the outermost level of the recursion at which the subpattern value is set.
+ the outermost level. If you want to obtain intermediate values, a callout
- If you want to obtain intermediate values, a callout function can be used (see
+ function can be used (see below and the
- the next section and the
  .\" HREF
  \fBpcrecallout\fP
  .\"
-Line 1393 
 documentation). If the pattern above is
+Line 2366 
 documentation). If the pattern above is
  .sp
    (ab(cd)ef)
  .sp
- the value for the capturing parentheses is "ef", which is the last value taken
+ the value for the inner capturing parentheses (numbered 2) is "ef", which is
- on at the top level. If additional parentheses are added, giving
+ the last value taken on at the top level. If a capturing subpattern is not
- .sp
+ matched at the top level, its final captured value is unset, even if it was
-   \e( ( ( (?>[^()]+) | (?R) )* ) \e)
+ (temporarily) set at a deeper level during the matching process.
-      ^                        ^
+ .P
-      ^                        ^
+ If there are more than 15 capturing parentheses in a pattern, PCRE has to
- .sp
+ obtain extra memory to store data during a recursion, which it does by using
- the string they capture is "ab(cd)ef", the contents of the top level
+ \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
- parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
+ be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
- has to obtain extra memory to store data during a recursion, which it does by
- using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
- memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
  .P
  Do not confuse the (?R) item with the condition (R), which tests for recursion.
  Consider this pattern, which matches text in angle brackets, allowing for
-Line 1418 
 different alternatives for the recursive
+Line 2388 
 different alternatives for the recursive
  is the actual recursive call.
  .
  .
+ .\" HTML <a name="recursiondifference"></a>
+ .SS "Differences in recursion processing between PCRE and Perl"
+ .rs
+ .sp
+ Recursion processing in PCRE differs from Perl in two important ways. In PCRE
+ (like Python, but unlike Perl), a recursive subpattern call is always treated
+ as an atomic group. That is, once it has matched some of the subject string, it
+ is never re-entered, even if it contains untried alternatives and there is a
+ subsequent matching failure. This can be illustrated by the following pattern,
+ which purports to match a palindromic string that contains an odd number of
+ characters (for example, "a", "aba", "abcba", "abcdcba"):
+ .sp
+   ^(.|(.)(?1)\e2)$
+ .sp
+ The idea is that it either matches a single character, or two identical
+ characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
+ it does not if the pattern is longer than three characters. Consider the
+ subject string "abcba":
+ .P
+ At the top level, the first character is matched, but as it is not at the end
+ of the string, the first alternative fails; the second alternative is taken
+ and the recursion kicks in. The recursive call to subpattern 1 successfully
+ matches the next character ("b"). (Note that the beginning and end of line
+ tests are not part of the recursion).
+ .P
+ Back at the top level, the next character ("c") is compared with what
+ subpattern 2 matched, which was "a". This fails. Because the recursion is
+ treated as an atomic group, there are now no backtracking points, and so the
+ entire match fails. (Perl is able, at this point, to re-enter the recursion and
+ try the second alternative.) However, if the pattern is written with the
+ alternatives in the other order, things are different:
+ .sp
+   ^((.)(?1)\e2|.)$
+ .sp
+ This time, the recursing alternative is tried first, and continues to recurse
+ until it runs out of characters, at which point the recursion fails. But this
+ time we do have another alternative to try at the higher level. That is the big
+ difference: in the previous case the remaining alternative is at a deeper
+ recursion level, which PCRE cannot use.
+ .P
+ To change the pattern so that it matches all palindromic strings, not just
+ those with an odd number of characters, it is tempting to change the pattern to
+ this:
+ .sp
+   ^((.)(?1)\e2|.?)$
+ .sp
+ Again, this works in Perl, but not in PCRE, and for the same reason. When a
+ deeper recursion has matched a single character, it cannot be entered again in
+ order to match an empty string. The solution is to separate the two cases, and
+ write out the odd and even cases as alternatives at the higher level:
+ .sp
+   ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
+ .sp
+ If you want to match typical palindromic phrases, the pattern has to ignore all
+ non-word characters, which can be done like this:
+ .sp
+   ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
+ .sp
+ If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
+ man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
+ the use of the possessive quantifier *+ to avoid backtracking into sequences of
+ non-word characters. Without this, PCRE takes a great deal longer (ten times or
+ more) to match typical phrases, and Perl takes so long that you think it has
+ gone into a loop.
+ .P
+ \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
+ string does not start with a palindrome that is shorter than the entire string.
+ For example, although "abcba" is correctly matched, if the subject is "ababa",
+ PCRE finds the palindrome "aba" at the start, then fails at top level because
+ the end of the string does not follow. Once again, it cannot jump back into the
+ recursion to try other alternatives, so the entire match fails.
+ .P
+ The second way in which PCRE and Perl differ in their recursion processing is
+ in the handling of captured values. In Perl, when a subpattern is called
+ recursively or as a subpattern (see the next section), it has no access to any
+ values that were captured outside the recursion, whereas in PCRE these values
+ can be referenced. Consider this pattern:
+ .sp
+   ^(.)(\e1|a(?2))
+ .sp
+ In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
+ then in the second group, when the back reference \e1 fails to match "b", the
+ second alternative matches "a" and then recurses. In the recursion, \e1 does
+ now match "b" and so the whole match succeeds. In Perl, the pattern fails to
+ match because inside the recursive call \e1 cannot access the externally set
+ value.
+ .
+ .
  .\" HTML <a name="subpatternsassubroutines"></a>
  .SH "SUBPATTERNS AS SUBROUTINES"
  .rs
  .sp
- If the syntax for a recursive subpattern reference (either by number or by
+ If the syntax for a recursive subpattern call (either by number or by
  name) is used outside the parentheses to which it refers, it operates like a
- subroutine in a programming language. An earlier example pointed out that the
+ subroutine in a programming language. The called subpattern may be defined
- pattern
+ before or after the reference. A numbered reference can be absolute or
+ relative, as in these examples:
+ .sp
+   (...(absolute)...)...(?2)...
+   (...(relative)...)...(?-1)...
+   (...(?+1)...(relative)...
+ .sp
+ An earlier example pointed out that the pattern
  .sp
    (sens|respons)e and \e1ibility
  .sp
-Line 1435 
 matches "sense and sensibility" and "res
+Line 2500 
 matches "sense and sensibility" and "res
    (sens|respons)e and (?1)ibility
  .sp
  is used, it does match "sense and responsibility" as well as the other two
- strings. Such references must, however, follow the subpattern to which they
+ strings. Another example is given in the discussion of DEFINE above.
- refer.
+ .P
+ All subroutine calls, whether recursive or not, are always treated as atomic
+ groups. That is, once a subroutine has matched some of the subject string, it
+ is never re-entered, even if it contains untried alternatives and there is a
+ subsequent matching failure. Any capturing parentheses that are set during the
+ subroutine call revert to their previous values afterwards.
+ .P
+ Processing options such as case-independence are fixed when a subpattern is
+ defined, so if it is used as a subroutine, such options cannot be changed for
+ different calls. For example, consider this pattern:
+ .sp
+   (abc)(?i:(?-1))
+ .sp
+ It matches "abcabc". It does not match "abcABC" because the change of
+ processing option does not affect the called subpattern.
+ .
+ .
+ .\" HTML <a name="onigurumasubroutines"></a>
+ .SH "ONIGURUMA SUBROUTINE SYNTAX"
+ .rs
+ .sp
+ For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
+ a number enclosed either in angle brackets or single quotes, is an alternative
+ syntax for referencing a subpattern as a subroutine, possibly recursively. Here
+ are two of the examples used above, rewritten using this syntax:
+ .sp
+   (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
+   (sens|respons)e and \eg'1'ibility
+ .sp
+ PCRE supports an extension to Oniguruma: if a number is preceded by a
+ plus or a minus sign it is taken as a relative reference. For example:
+ .sp
+   (abc)(?i:\eg<-1>)
+ .sp
+ Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
+ synonymous. The former is a back reference; the latter is a subroutine call.
  .
  .
  .SH CALLOUTS
-Line 1449 
 same pair of parentheses when there is a
+Line 2549 
 same pair of parentheses when there is a
  .P
  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
- 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
- By default, this variable contains NULL, which disables all calling out.
+ (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this
+ variable contains NULL, which disables all calling out.
  .P
  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
  can put a number less than 256 after the letter C. The default value is zero.
  For example, this pattern has two callout points:
  .sp
-   (?C1)\dabc(?C2)def
+   (?C1)abc(?C2)def
  .sp
- If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, 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
 .
  .P
- During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
+ During matching, when PCRE reaches a callout point, the external function is
- set), the external function is called. It is provided with the number of the
+ called. It is provided with the number of the callout, the position in the
- callout, the position in the pattern, and, optionally, one item of data
+ pattern, and, optionally, one item of data originally supplied by the caller of
- originally supplied by the caller of \fBpcre_exec()\fP. The callout function
+ the matching function. The callout function may cause matching to proceed, to
- may cause matching to proceed, to backtrack, or to fail altogether. A complete
+ backtrack, or to fail altogether. A complete description of the interface to
- description of the interface to the callout function is given in the
+ the callout function is given in the
  .\" HREF
  \fBpcrecallout\fP
  .\"
  documentation.
+ .
+ .
+ .\" HTML <a name="backtrackcontrol"></a>
+ .SH "BACKTRACKING CONTROL"
+ .rs
+ .sp
+ Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
+ are described in the Perl documentation as "experimental and subject to change
+ or removal in a future version of Perl". It goes on to say: "Their usage in
+ production code should be noted to avoid problems during upgrades." The same
+ remarks apply to the PCRE features described in this section.
+ .P
+ 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
+ matching functions, which use a backtracking algorithm. With the exception of
+ (*FAIL), which behaves like a failing negative assertion, they cause an error
+ if encountered by a DFA matching function.
+ .P
+ If any of these verbs are used in an assertion or in a subpattern that is
+ called as a subroutine (whether or not recursively), their effect is confined
+ to that subpattern; it does not extend to the surrounding pattern, with one
+ exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
+ a successful positive assertion \fIis\fP passed back when a match succeeds
+ (compare capturing parentheses in assertions). Note that such subpatterns are
+ processed as anchored at the point where they are tested. Note also that Perl's
+ treatment of subroutines is different in some cases.
+ .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, with differing behaviour,
+ depending on whether or not an argument 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 library. 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.
+ .
+ .
+ .\" HTML <a name="nooptimize"></a>
+ .SS "Optimizations that affect backtracking verbs"
+ .rs
+ .sp
+ 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
+ minimum length of matching subject, or that a particular character must be
+ present. When one of these optimizations suppresses the running of a match, any
+ 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
+ when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
+ pattern with (*NO_START_OPT). There is more discussion of this option in the
+ section entitled
+ .\" HTML <a href="pcreapi.html#execoptions">
+ .\" </a>
+ "Option bits for \fBpcre_exec()\fP"
+ .\"
+ in the
+ .\" HREF
+ \fBpcreapi\fP
+ .\"
+ documentation.
  .P
- .in 0
+ Experiments with Perl suggest that it too has similar optimizations, sometimes
- Last updated: 28 February 2005
+ leading to anomalous results.
- .br
+ .
- Copyright (c) 1997-2005 University of Cambridge.
+ .
+ .SS "Verbs that act immediately"
+ .rs
+ .sp
+ The following verbs act as soon as they are encountered. They may not be
+ followed by a name.
+ .sp
+    (*ACCEPT)
+ .sp
+ 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
+ subroutine, only that subpattern is ended successfully. Matching then continues
+ at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
+ far is captured. For example:
+ .sp
+   A((?:A|B(*ACCEPT)|C)D)
+ .sp
+ This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
+ the outer parentheses.
+ .sp
+   (*FAIL) or (*F)
+ .sp
+ This verb causes a matching failure, forcing backtracking to occur. It is
+ equivalent to (?!) but easier to read. The Perl documentation notes that it is
+ probably useful only when combined with (?{}) or (??{}). Those are, of course,
+ Perl features that are not present in PCRE. The nearest equivalent is the
+ callout feature, as for example in this pattern:
+ .sp
+   a+(?C)(*FAIL)
+ .sp
+ A match with the string "aaaa" always fails, but the callout is taken before
+ each backtrack happens (in this example, 10 times).
+ .
+ .
+ .SS "Recording which path was taken"
+ .rs
+ .sp
+ There is one verb whose main purpose is to track how a match was arrived at,
+ though it also has a secondary use in conjunction with advancing the match
+ starting point (see (*SKIP) below).
+ .sp
+   (*MARK:NAME) or (*:NAME)
+ .sp
+ 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.
+ .P
+ When a match succeeds, the name of the last-encountered (*MARK) on the matching
+ path is passed back to the caller as described in the section entitled
+ .\" HTML <a href="pcreapi.html#extradata">
+ .\" </a>
+ "Extra data for \fBpcre_exec()\fP"
+ .\"
+ in the
+ .\" HREF
+ \fBpcreapi\fP
+ .\"
+ documentation. Here is an example of \fBpcretest\fP output, where the /K
+ modifier requests the retrieval and outputting of (*MARK) data:
+ .sp
+     re> /X(*MARK:A)Y|X(*MARK:B)Z/K
+   data> XY
+: XY
+   MK: A
+   XZ
+: XZ
+   MK: B
+ .sp
+ The (*MARK) name is tagged with "MK:" in this output, and in this example it
+ indicates which of the two alternatives matched. This is a more efficient way
+ of obtaining this information than putting each alternative in its own
+ capturing parentheses.
+ .P
+ If (*MARK) is encountered in a positive assertion, its name is recorded and
+ passed back if it is the last-encountered. This does not happen for negative
+ assertions.
+ .P
+ After a partial match or a failed match, the name of the last encountered
+ (*MARK) in the entire match process is returned. For example:
+ .sp
+     re> /X(*MARK:A)Y|X(*MARK:B)Z/K
+   data> XP
+   No match, mark = B
+ .sp
+ Note that in this unanchored example the mark is retained from the match
+ attempt that started at the letter "X" in the subject. Subsequent match
+ attempts starting at "P" and then with an empty string do not get as far as the
+ (*MARK) item, but nevertheless do not reset it.
+ .P
+ If you are interested in (*MARK) values after failed matches, you should
+ probably set the PCRE_NO_START_OPTIMIZE option
+ .\" HTML <a href="#nooptimize">
+ .\" </a>
+ (see above)
+ .\"
+ to ensure that the match is always attempted.
+ .
+ .
+ .SS "Verbs that act after backtracking"
+ .rs
+ .sp
+ 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
+ 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, its
+ effect is confined to that group, because once the group has been matched,
+ there is never any backtracking into it. In this situation, backtracking can
+ "jump back" to the left of the entire atomic group. (Remember also, as stated
+ above, that this localization also applies in subroutine calls and assertions.)
+ .P
+ These verbs differ in exactly what kind of failure occurs when backtracking
+ reaches them.
+ .sp
+   (*COMMIT)
+ .sp
+ 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 is
+ unanchored, no further attempts to find a match by advancing the starting point
+ take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
+ finding a match at the current starting point, or not at all. For example:
+ .sp
+   a+(*COMMIT)b
+ .sp
+ This matches "xxaab" but not "aacaab". It can be thought of as a kind of
+ dynamic anchor, or "I've started, so I must finish." The name of the most
+ recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
+ match failure.
+ .P
+ 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
+ \fBpcretest\fP example:
+ .sp
+     re> /(*COMMIT)abc/
+   data> xyzabc
+: abc
+   xyzabc\eY
+   No match
+ .sp
+ PCRE knows that any match must start with "a", so the optimization skips along
+ the subject to "a" before running the first match attempt, which succeeds. When
+ the optimization is disabled by the \eY escape in the second subject, the match
+ starts at "x" and so the (*COMMIT) causes it to fail without trying any other
+ starting points.
+ .sp
+   (*PRUNE) or (*PRUNE:NAME)
+ .sp
+ 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 is
+ unanchored, the normal "bumpalong" advance to the next starting character then
+ happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
+ reached, or when matching to the right of (*PRUNE), but if there is no match to
+ the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
+ (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
+ but there are some uses of (*PRUNE) that cannot be expressed in any other way.
+ The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
+ anchored pattern (*PRUNE) has the same effect as (*COMMIT).
+ .sp
+   (*SKIP)
+ .sp
+ This verb, when given without a name, is like (*PRUNE), except that if the
+ pattern is unanchored, the "bumpalong" advance is not to the next character,
+ but to the position in the subject where (*SKIP) was encountered. (*SKIP)
+ signifies that whatever text was matched leading up to it cannot be part of a
+ successful match. Consider:
+ .sp
+   a+(*SKIP)b
+ .sp
+ If the subject is "aaaac...", after the first match attempt fails (starting at
+ the first character in the string), the starting point skips on to start the
+ next attempt at "c". Note that a possessive quantifer does not have the same
+ effect as this example; although it would suppress backtracking during the
+ first match attempt, the second attempt would start at the second character
+ instead of skipping on to "c".
+ .sp
+   (*SKIP:NAME)
+ .sp
+ When (*SKIP) has an associated name, its behaviour is modified. If the
+ following pattern fails to match, the previous path through the pattern is
+ searched for the most recent (*MARK) that has the same name. If one is found,
+ the "bumpalong" advance is to the subject position that corresponds to that
+ (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
+ matching name is found, the (*SKIP) is ignored.
+ .sp
+   (*THEN) or (*THEN:NAME)
+ .sp
+ This verb causes a skip to the next innermost alternative if the rest of the
+ pattern does not match. That is, it cancels pending backtracking, but only
+ within the current alternative. Its name comes from the observation that it can
+ be used for a pattern-based if-then-else block:
+ .sp
+   ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
+ .sp
+ 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
+ second alternative and tries COND2, without backtracking into COND1. The
+ behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
+ If (*THEN) is not inside an alternation, it acts like (*PRUNE).
+ .P
+ Note that 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
+ enclosing alternative. Consider this pattern, where A, B, etc. are complex
+ pattern fragments that do not contain any | characters at this level:
+ .sp
+   A (B(*THEN)C) | D
+ .sp
+ If A and B are matched, but there is a failure in C, matching does not
+ backtrack into A; instead it moves to the next alternative, that is, D.
+ However, if the subpattern containing (*THEN) is given an alternative, it
+ behaves differently:
+ .sp
+   A (B(*THEN)C | (*FAIL)) | D
+ .sp
+ The effect of (*THEN) is now confined to the inner subpattern. After a failure
+ in C, matching moves to (*FAIL), which causes the whole subpattern to fail
+ because there are no more alternatives to try. In this case, matching does now
+ backtrack into A.
+ .P
+ Note also that a conditional subpattern is not considered as having two
+ alternatives, because only one is ever used. In other words, the | character in
+ a conditional subpattern has a different meaning. Ignoring white space,
+ consider:
+ .sp
+   ^.*? (?(?=a) a | b(*THEN)c )
+ .sp
+ If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
+ it initially matches zero characters. The condition (?=a) then fails, the
+ character "b" is matched, but "c" is not. At this point, matching does not
+ backtrack to .*? as might perhaps be expected from the presence of the |
+ character. The conditional subpattern is part of the single alternative that
+ comprises the whole pattern, and so the match fails. (If there was a backtrack
+ into .*?, allowing it to match "b", the match would succeed.)
+ .P
+ The verbs just described provide four different "strengths" of control when
+ subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
+ next alternative. (*PRUNE) comes next, failing the match at the current
+ starting position, but allowing an advance to the next character (for an
+ unanchored pattern). (*SKIP) is similar, except that the advance may be more
+ than one character. (*COMMIT) is the strongest, causing the entire match to
+ 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
+ fragments:
+ .sp
+   (A(*COMMIT)B(*THEN)C|D)
+ .sp
+ Once A has matched, PCRE is committed to this match, at the current starting
+ position. If subsequently B matches, but C does not, the normal (*THEN) action
+ of trying the next alternative (that is, D) does not happen because (*COMMIT)
+ overrides.
+ .
+ .
+ .SH "SEE ALSO"
+ .rs
+ .sp
+ \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
+ \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.
+ .
+ .
+ .SH AUTHOR
+ .rs
+ .sp
+ .nf
+ Philip Hazel
+ University Computing Service
+ Cambridge CB2 3QH, England.
+ .fi
+ .
+ .
+ .SH REVISION
+ .rs
+ .sp
+ .nf
+ Last updated: 04 May 2012
+ Copyright (c) 1997-2012 University of Cambridge.
+ .fi

 Legend:



Removed from v.79
 


changed lines


 
Added in v.964
 Legend:



Removed from v.79
 


changed lines


 
Added in v.964
-Removed from v.79
+Added in v.964

webmaster@exim.org	ViewVC Help
Powered by ViewVC 1.1.12