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

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-revision 87 by nigel,
Sat Feb 24 21:41:21 2007 UTC
+revision 469 by ph10,
Mon Oct 19 14:38:48 2009 UTC
 Line 4 
 PCRE - Perl-compatible regular expressio
  .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 character strings. To use this,
- build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
+ PCRE must be built to include UTF-8 support, and you must call
- the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
+ \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
- places below. There is also a summary of UTF-8 features in the
+ is also a special sequence that can be given at the start of a pattern:
+ .sp
+   (*UTF8)
+ .sp
+ Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
+ option. This feature is not Perl-compatible. How setting UTF-8 mode affects
+ pattern matching is mentioned in several places below. There is also a summary
+ of UTF-8 features in the
  .\" HTML <a href="pcre.html#utf8support">
  .\" </a>
  section on UTF-8 support
-Line 30 
 The remainder of this document discusses
+Line 46 
 The remainder of this document discusses
  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
  From release 6.0, PCRE offers a second matching function,
  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
- Perl-compatible. The advantages and disadvantages of the alternative function,
+ Perl-compatible. Some of the features discussed below are not available when
- and how it differs from the normal function, are discussed in the
+ \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
+ alternative function, and how it differs from the normal function, are
+ discussed in the
  .\" HREF
  \fBpcrematching\fP
  .\"
  page.
+ .
+ .
+ .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 \fBpcre_compile()\fP or
+ \fBpcre_compile2()\fP. 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 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 60 
 interpreted in some special way.
+Line 132 
 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 164 
 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
+ non-alphanumeric character, it takes away any special meaning that character
- have. This use of backslash as an escape character applies both inside and
+ may have. This use of backslash as an escape character applies both inside and
  outside character classes.
  .P
  For example, if you want to match a * character, you write \e* in the pattern.
-Line 108 
 particular, if you want to match a backs
+Line 181 
 particular, if you want to match a backs
  .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 135 
 The \eQ...\eE sequence is recognized bot
+Line 207 
 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
    \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
-Line 158 
 Thus \ecz becomes hex 1A, but \ec{ becom
+Line 229 
 Thus \ecz becomes hex 1A, but \ec{ becom
  After \ex, from zero to two hexadecimal digits are read (letters can be in
  upper or lower case). Any number of hexadecimal digits may appear between \ex{
  and }, but the value of the character code must be less than 256 in non-UTF-8
- mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value
+ mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
- is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{
+ hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
- and }, or if there is no terminating }, this form of escape is not recognized.
+ point, which is 10FFFF.
- Instead, the initial \ex will be interpreted as a basic hexadecimal escape,
+ .P
- with no following digits, giving a character whose value is zero.
+ 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
  Characters whose value is less than 256 can be defined by either of the two
  syntaxes for \ex. There is no difference in the way they are handled. For
  example, \exdc is exactly the same as \ex{dc}.
  .P
- After \e0 up to two further octal digits are read. In both cases, if there
+ After \e0 up to two further octal digits are read. If there are fewer than two
- are fewer than two digits, just those that are present are used. Thus the
+ digits, just those that are present are used. Thus the sequence \e0\ex\e07
- sequence \e0\ex\e07 specifies two binary zeros followed by a BEL character
+ specifies two binary zeros followed by a BEL character (code value 7). Make
- (code value 7). Make sure you supply two digits after the initial zero if the
+ sure you supply two digits after the initial zero if the pattern character that
- pattern character that follows is itself an octal digit.
+ 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 265 
 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. In non-UTF-8 mode, the value of a
- For example:
+ character specified in octal must be less than \e400. In UTF-8 mode, values up
+ to \e777 are permitted. For example:
  .sp
    \e040   is another way of writing a space
  .\" JOIN
-Line 218 
 For example:
+Line 293 
 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, the
- addition, inside a character class, the sequence \eb is interpreted as the
+ sequence \eb is interpreted as the backspace character (hex 08), and the
- backspace character (hex 08), and the sequence \eX is interpreted as the
+ sequences \eR and \eX are interpreted as the characters "R" and "X",
- character "X". Outside a character class, these sequences have different
+ respectively. Outside a character class, these sequences have different
  meanings
  .\" HTML <a href="#uniextseq">
  .\" </a>
-Line 230 
 meanings
+Line 305 
 meanings
  .\"
  .
  .
+ .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>
+ 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.
+ .
+ .
  .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. The
  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
-Line 253 
 there is no character to match.
+Line 368 
 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
+ In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
+ \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
+ character property support is available. These sequences retain their original
+ meanings from before UTF-8 support was available, mainly for efficiency
+ reasons. Note that this also affects \eb, because it is defined in terms of \ew
+ and \eW.
+ .P
+ The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
+ other sequences, these do match certain high-valued codepoints in UTF-8 mode.
+ 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
  .P
  A "word" character is an underscore or any character less than 256 that is a
  letter or digit. The definition of letters and digits is controlled by PCRE's
-Line 267 
 in the
+Line 425 
 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 matched by \ew. The use of locales with Unicode
- In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
+ is discouraged.
- \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
+ .
- character property support is available. The use of locales with Unicode is
+ .
- discouraged.
+ .\" 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. This is a Perl 5.10 feature. In 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 UTF-8 mode, 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 \fBpcre_compile()\fP or
+ \fBpcre_compile2()\fP, but they can be overridden by options given to
+ \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. 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
+ Inside a character class, \eR matches the letter "R".
  .
  .
  .\" HTML <a name="uniextseq"></a>
-Line 281 
 discouraged.
+Line 486 
 discouraged.
  .rs
  .sp
  When PCRE is built with Unicode character property support, three additional
- escape sequences to match character properties are available when UTF-8 mode
+ escape sequences that match characters with specific properties are available.
- is selected. They are:
+ When not in UTF-8 mode, these sequences are of course limited to testing
+ characters whose codepoints are less than 256, but they do work in this mode.
+ The extra escape sequences are:
  .sp
    \ep{\fIxx\fP}   a character with the \fIxx\fP property
    \eP{\fIxx\fP}   a character without the \fIxx\fP property
-Line 306 
 Those that are not part of an identified
+Line 513 
 Those that are not part of an identified
  .P
  Arabic,
  Armenian,
+ Balinese,
  Bengali,
  Bopomofo,
  Braille,
-Line 315 
 Canadian_Aboriginal,
+Line 523 
 Canadian_Aboriginal,
  Cherokee,
  Common,
  Coptic,
+ Cuneiform,
  Cypriot,
  Cyrillic,
  Deseret,
-Line 344 
 Malayalam,
+Line 553 
 Malayalam,
  Mongolian,
  Myanmar,
  New_Tai_Lue,
+ Nko,
  Ogham,
  Old_Italic,
  Old_Persian,
  Oriya,
  Osmanya,
+ Phags_Pa,
+ Phoenician,
  Runic,
  Shavian,
  Sinhala,
-Line 429 
 The special property L& is also supporte
+Line 641 
 The special property L& is also supporte
  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
  a modifier or "other".
  .P
- The long synonyms for these properties that Perl supports (such as \ep{Letter})
+ The Cs (Surrogate) property applies only to characters in the range U+D800 to
- are not supported by PCRE. Nor is is permitted to prefix any of these
+ U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
+ cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
+ (see the discussion of PCRE_NO_UTF8_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.
-Line 453 
 atomic group
+Line 674 
 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
+ non-UTF-8 mode \eX matches any one character.
  .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
-Line 461 
 why the traditional escape sequences suc
+Line 683 
 why the traditional escape sequences suc
  properties in PCRE.
  .
  .
+ .\" HTML <a name="resetmatchstart"></a>
+ .SS "Resetting the match start"
+ .rs
+ .sp
+ The escape sequence \eK, which is a Perl 5.10 feature, 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".
+ .
+ .
  .\" 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 473 
 subpatterns for more complicated asserti
+Line 725 
 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
+   \eG     matches at the first matching position in the subject
  .sp
  These assertions may not appear in character classes (but note that \eb has a
  different meaning, namely the backspace character, inside a character class).
-Line 489 
 different meaning, namely the backspace
+Line 741 
 different meaning, namely the backspace
  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. Neither PCRE nor Perl has a
+ separte "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 499 
 PCRE_NOTBOL or PCRE_NOTEOL options, whic
+Line 754 
 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 545 
 to be anchored.)
+Line 799 
 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)"
  .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. In UTF-8 mode, the matched character may be more than one byte long.
- 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
+ characters.
+ .P
+ The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
+ option is set, a dot matches any one character, without exception. If the
+ two-character sequence CRLF is present in the subject string, it takes two dots
+ to match it.
+ .P
+ The handling of dot is entirely independent of the handling of circumflex and
+ dollar, the only relationship being that they both involve newlines. Dot has no
+ special meaning in a character class.
  .
  .
  .SH "MATCHING A SINGLE BYTE"
  .rs
  .sp
  Outside a character class, the escape sequence \eC matches any one byte, both
- in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature is
+ in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
- provided in Perl in order to match individual bytes in UTF-8 mode. Because it
+ characters. The feature is provided in Perl in order to match individual bytes
- breaks up UTF-8 characters into individual bytes, what remains in the string
+ in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
- may be a malformed UTF-8 string. For this reason, the \eC escape sequence is
+ what remains in the string may be a malformed UTF-8 string. For this reason,
- best avoided.
+ the \eC escape sequence is best avoided.
  .P
  PCRE does not allow \eC to appear in lookbehind assertions
  .\" HTML <a href="#lookbehind">
-Line 607 
 the lookbehind.
+Line 876 
 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
+ (after an initial circumflex, if present) or escaped with a backslash.
  .P
  A character class matches a single character in the subject. In UTF-8 mode, the
- character may occupy more than one byte. A matched character must be in the set
+ character may be more than one byte long. A matched character must be in the
- of characters defined by the class, unless the first character in the class
+ set of characters defined by the class, unless the first character in the class
  definition is a circumflex, in which case the subject character must not 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
-Line 624 
 For example, the character class [aeiou]
+Line 894 
 For example, the character class [aeiou]
  [^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
-Line 638 
 caseful version would. In UTF-8 mode, PC
+Line 908 
 caseful version would. In UTF-8 mode, PC
  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 UTF8-mode for characters 128 and above,
- ensure that PCRE is compiled with Unicode property support as well as with
+ you must ensure that PCRE is compiled with Unicode property support as well as
- UTF-8 support.
+ with UTF-8 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 668 
 example [\ex{100}-\ex{2ff}].
+Line 939 
 example [\ex{100}-\ex{2ff}].
  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
- 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 with values greater than 128 only when it is compiled with Unicode
  property support.
-Line 743 
 the pattern
+Line 1014 
 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 759 
 alternative in the subpattern.
+Line 1029 
 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 774 
 PCRE_MULTILINE while unsetting PCRE_DOTA
+Line 1045 
 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 current pattern that follows it, so
  .sp
    (a(?i)b)c
  .sp
-Line 797 
 branch is abandoned before the option se
+Line 1072 
 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 compile or match functions are called. In some cases the
- respectively. The (?X) flag setting is special in that it must always occur
+ pattern can contain special leading sequences such as (*CRLF) to override what
- earlier in the pattern than any of the additional features it turns on, even
+ the application has set or what has been defaulted. Details are given in the
- when it is at top level. It is best to put it at the start.
+ section entitled
+ .\" HTML <a href="#newlineseq">
+ .\" </a>
+ "Newline sequences"
+ .\"
+ above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
+ mode; this is equivalent to setting the PCRE_UTF8 option.
  .
  .
  .\" HTML <a name="subpattern"></a>
-Line 816 
 Turning part of a pattern into a subpatt
+Line 1097 
 Turning part of a pattern into a subpatt
    cat(aract|erpillar|)
  .sp
  matches one of the words "cat", "cataract", or "caterpillar". Without the
- parentheses, it would match "cataract", "erpillar" or the empty string.
+ parentheses, it would 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
-Line 841 
 the string "the white queen" is matched
+Line 1122 
 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 857 
 is reached, an option setting in one bra
+Line 1137 
 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
+ buffers 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 backreference 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 recursive or "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>
+ backreferences,
+ .\"
+ .\" 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 backreference 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 884 
 Repetition is specified by quantifiers,
+Line 1280 
 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 \eR escape sequence
    an escape such as \ed that matches a single character
    a character class
    a back reference (see next section)
    a parenthesized subpattern (unless it is an assertion)
+   a recursive or "subroutine" call to a subpattern
  .sp
  The general repetition quantifier specifies a minimum and maximum number of
  permitted matches, by giving the two numbers in curly brackets (braces),
-Line 922 
 support is available, \eX{3} matches thr
+Line 1320 
 support is available, \eX{3} matches thr
  which may be several bytes 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. 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 973 
 own right. Because it has two uses, it c
+Line 1378 
 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 983 
 is greater than 1 or with a limited maxi
+Line 1388 
 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 995 
 alternatively using ^ to indicate anchor
+Line 1400 
 alternatively using ^ to indicate anchor
  .P
  However, there is one situation where the optimization cannot be used. When .*
  is inside capturing parentheses that are the subject of a backreference
- 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 1022 
 matches "aba" the value of the second ca
+Line 1427 
 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 1039 
 item, and then with 4, and so on, before
+Line 1444 
 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 1069 
 previous example can be rewritten as
+Line 1474 
 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 1117 
 However, if the decimal number following
+Line 1535 
 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, which is a feature introduced in
+ Perl 5.10. 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. 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 1144 
 back reference, the case of letters is r
+Line 1593 
 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"
-Line 1210 
 because it does not make sense for negat
+Line 1672 
 because it does not make sense for negat
  .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 1233 
 lookbehind assertion is needed to achiev
+Line 1695 
 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 Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
+ synonym for (?!).
  .
  .
  .\" HTML <a name="lookbehind"></a>
-Line 1246 
 negative assertions. For example,
+Line 1710 
 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 1257 
 is permitted, but
+Line 1721 
 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 (5.8 and 5.10), which requires all branches to
  match the same 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 Perl 5.10 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
  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
  to appear in lookbehind assertions, because it makes it impossible to calculate
- the length of the lookbehind. The \eX escape, which can match different numbers
+ the length of the lookbehind. The \eX and \eR escapes, which can match
- of bytes, is also not permitted.
+ different numbers of bytes, 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
- Atomic groups can be used in conjunction with lookbehind assertions to specify
+ Possessive quantifiers can be used in conjunction with lookbehind assertions to
- efficient matching at the end of the subject string. Consider a simple pattern
+ specify efficient matching of fixed-length strings at the end of subject
- such as
+ strings. Consider a simple pattern such as
  .sp
    abcd$
  .sp
-Line 1295 
 then all but the last two characters, an
+Line 1780 
 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 1341 
 is another pattern that matches "foo" pr
+Line 1822 
 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)
-Line 1356 
 If the condition is satisfied, the yes-p
+Line 1838 
 If the condition is satisfied, the yes-p
  no-pattern (if present) is used. If there are more than two alternatives in the
  subpattern, a compile-time error occurs.
  .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 been set. 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. In
+ looping constructs it can also make sense to refer to subsequent groups with
+ constructs such as (?(+2).
+ .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
-Line 1375 
 parenthesis is required. Otherwise, sinc
+Line 1876 
 parenthesis is required. Otherwise, sinc
  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.
+ .
+ .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 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 1404 
 closing parenthesis. Nested parentheses
+Line 1979 
 closing parenthesis. Nested parentheses
  that make up a comment play no part in the pattern matching at all.
  .P
  If the PCRE_EXTENDED option is set, an unescaped # character outside a
- character class introduces a comment that continues up to the next newline
+ character class introduces a comment that continues to immediately after the
- character in the pattern.
+ next newline in the pattern.
  .
  .
+ .\" 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
+ .P
- recursion of the entire pattern, and also for individual subpattern recursion.
+ Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
- .P
+ supports special syntax for recursion of the entire pattern, and also for
- The special item that consists of (? followed by a number greater than zero and
+ individual subpattern recursion. After its introduction in PCRE and Python,
- a closing parenthesis is a recursive call of the subpattern of the given
+ this kind of recursion was subsequently introduced into Perl at release 5.10.
- number, provided that it occurs inside that subpattern. (If not, it is a
+ .P
- "subroutine" call, which is described in the next section.) The special item
+ A special item that consists of (? followed by a number greater than zero and a
- (?R) is a recursive call of the entire regular expression.
+ closing parenthesis is a recursive call of the subpattern of the given number,
- .P
+ provided that it occurs inside that subpattern. (If not, it is a
- A recursive subpattern call is always treated as an atomic group. That is, once
+ .\" HTML <a href="#subpatternsassubroutines">
- it has matched some of the subject string, it is never re-entered, even if
+ .\" </a>
- it contains untried alternatives and there is a subsequent matching failure.
+ "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).
- 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 (a Perl 5.10 feature).
- named parentheses). We could rewrite the above example as follows:
+ Instead of (?1) in the pattern above you can write (?-2) to refer to the second
- .sp
+ most recently opened parentheses preceding the recursion. In other words, a
-   (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )
+ negative number counts capturing parentheses leftwards from the point at which
- .sp
+ it is encountered.
- This particular example pattern contains nested unlimited repeats, and so the
+ .P
- use of atomic grouping for matching strings of non-parentheses is important
+ It is also possible to refer to subsequently opened parentheses, by writing
- when applying the pattern to strings that do not match. For example, when this
+ references such as (?+2). However, these cannot be recursive because the
- pattern is applied to
+ reference is not inside the parentheses that are referenced. They are always
+ .\" HTML <a href="#subpatternsassubroutines">
+ .\" </a>
+ "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 1484 
 documentation). If the pattern above is
+Line 2084 
 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 value is unset, even if it is (temporarily)
-   \e( ( ( (?>[^()]+) | (?R) )* ) \e)
+ set at a deeper level.
-      ^                        ^
+ .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 1509 
 different alternatives for the recursive
+Line 2106 
 different alternatives for the recursive
  is the actual recursive call.
  .
  .
+ .\" HTML <a name="recursiondifference"></a>
+ .SS "Recursion difference from Perl"
+ .rs
+ .sp
+ 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 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.
+ .
+ .
  .\" HTML <a name="subpatternsassubroutines"></a>
  .SH "SUBPATTERNS AS SUBROUTINES"
  .rs
  .sp
  If the syntax for a recursive subpattern reference (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 1526 
 matches "sense and sensibility" and "res
+Line 2201 
 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
- Like recursive subpatterns, a "subroutine" call is always treated as an atomic
+ Like recursive subpatterns, a subroutine 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.
+ matching failure. Any capturing parentheses that are set during the subroutine
+ call revert to their previous values afterwards.
+ .P
+ When a subpattern is used as a subroutine, processing options such as
+ case-independence are fixed when the subpattern is defined. They 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 1553 
 function is to be called. If you want to
+Line 2258 
 function is to be called. If you want to
  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
  automatically installed before each item in the pattern. They are all numbered
-Line 1569 
 description of the interface to the call
+Line 2274 
 description of the interface to the call
  \fBpcrecallout\fP
  .\"
  documentation.
+ .
+ .
+ .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 \fBpcre_exec()\fP, which uses
+ a backtracking algorithm. With the exception of (*FAIL), which behaves like a
+ failing negative assertion, they cause an error if encountered by
+ \fBpcre_dfa_exec()\fP.
+ .P
+ If any of these verbs are used in an assertion or subroutine subpattern
+ (including recursive subpatterns), their effect is confined to that subpattern;
+ it does not extend to the surrounding pattern. Note that such subpatterns are
+ processed as anchored at the point where they are tested.
  .P
- .in 0
+ The new verbs make use of what was previously invalid syntax: an opening
- Last updated: 24 January 2006
+ parenthesis followed by an asterisk. In Perl, they are generally of the form
- .br
+ (*VERB:ARG) but PCRE does not support the use of arguments, so its general
- Copyright (c) 1997-2006 University of Cambridge.
+ form is just (*VERB). Any number of these verbs may occur in a pattern. There
+ are two kinds:
+ .
+ .SS "Verbs that act immediately"
+ .rs
+ .sp
+ The following verbs act as soon as they are encountered:
+ .sp
+    (*ACCEPT)
+ .sp
+ This verb causes the match to end successfully, skipping the remainder of the
+ pattern. When inside a recursion, only the innermost pattern is ended
+ immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
+ captured. (This feature was added to PCRE at release 8.00.) 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 the match to fail, 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 "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, a failure is forced.
+ The verbs differ in exactly what kind of failure occurs.
+ .sp
+   (*COMMIT)
+ .sp
+ This verb 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."
+ .sp
+   (*PRUNE)
+ .sp
+ This verb causes the match to fail at the current position 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), 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.
+ .sp
+   (*SKIP)
+ .sp
+ This verb 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
+   (*THEN)
+ .sp
+ This verb causes a skip to the next alternation if the rest of the pattern does
+ not match. That is, it cancels pending backtracking, but only within the
+ current alternation. 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. If (*THEN)
+ is used outside of any alternation, it acts exactly like (*PRUNE).
+ .
+ .
+ .SH "SEE ALSO"
+ .rs
+ .sp
+ \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
+ \fBpcresyntax\fP(3), \fBpcre\fP(3).
+ .
+ .
+ .SH AUTHOR
+ .rs
+ .sp
+ .nf
+ Philip Hazel
+ University Computing Service
+ Cambridge CB2 3QH, England.
+ .fi
+ .
+ .
+ .SH REVISION
+ .rs
+ .sp
+ .nf
+ Last updated: 18 October 2009
+ Copyright (c) 1997-2009 University of Cambridge.
+ .fi

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