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

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-revision 454 by ph10,
Tue Sep 22 09:42:11 2009 UTC
+revision 500 by ph10,
Sat Mar  6 19:00:29 2010 UTC
 Line 21 
 published by O'Reilly, covers regular ex
  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. There is also a special sequence that can be given at the
+ \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
- start of a pattern:
+ is also a special sequence that can be given at the start of a pattern:
  .sp
    (*UTF8)
  .sp
 Line 83 
 string with one of the following five se
    (*ANYCRLF)   any of the three above
    (*ANY)       all Unicode newline sequences
  .sp
- These override the default and the options given to \fBpcre_compile()\fP. For
+ These override the default and the options given to \fBpcre_compile()\fP or
- example, on a Unix system where LF is the default newline sequence, the pattern
+ \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
+ newline sequence, the pattern
  .sp
    (*CR)a.b
  .sp
-Line 206 
 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
 Line 217 
 represents:
    \en        linefeed (hex 0A)
    \er        carriage return (hex 0D)
    \et        tab (hex 09)
-   \eddd      character with octal code ddd, or backreference
+   \eddd      character with octal code ddd, or back reference
    \exhh      character with hex code hh
    \ex{hhh..} character with hex code hhh..
  .sp
 Line 333 
 syntax for referencing a subpattern as 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
 Line 468 
 one of the following sequences:
    (*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, but
+ These override the default and the options given to \fBpcre_compile()\fP or
- they can be overridden by options given to \fBpcre_exec()\fP. Note that these
+ \fBpcre_compile2()\fP, but they can be overridden by options given to
- special settings, which are not Perl-compatible, are recognized only at the
+ \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
- very start of a pattern, and that they must be in upper case. If more than one
+ which are not Perl-compatible, are recognized only at the very start of a
- of them is present, the last one is used. They can be combined with a change of
+ pattern, and that they must be in upper case. If more than one of them is
- newline convention, for example, a pattern can start with:
+ 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
-Line 512 
 Those that are not part of an identified
+Line 513 
 Those that are not part of an identified
  .P
  Arabic,
  Armenian,
+ Avestan,
  Balinese,
+ Bamum,
  Bengali,
  Bopomofo,
  Braille,
  Buginese,
  Buhid,
  Canadian_Aboriginal,
+ Carian,
+ Cham,
  Cherokee,
  Common,
  Coptic,
-Line 527 
 Cypriot,
+Line 532 
 Cypriot,
  Cyrillic,
  Deseret,
  Devanagari,
+ Egyptian_Hieroglyphs,
  Ethiopic,
  Georgian,
  Glagolitic,
-Line 539 
 Hangul,
+Line 545 
 Hangul,
  Hanunoo,
  Hebrew,
  Hiragana,
+ Imperial_Aramaic,
  Inherited,
+ Inscriptional_Pahlavi,
+ Inscriptional_Parthian,
+ Javanese,
+ Kaithi,
  Kannada,
  Katakana,
+ Kayah_Li,
  Kharoshthi,
  Khmer,
  Lao,
  Latin,
+ Lepcha,
  Limbu,
  Linear_B,
+ Lisu,
+ Lycian,
+ Lydian,
  Malayalam,
+ Meetei_Mayek,
  Mongolian,
  Myanmar,
  New_Tai_Lue,
-Line 556 
 Nko,
+Line 573 
 Nko,
  Ogham,
  Old_Italic,
  Old_Persian,
+ Old_South_Arabian,
+ Old_Turkic,
+ Ol_Chiki,
  Oriya,
  Osmanya,
  Phags_Pa,
  Phoenician,
+ Rejang,
  Runic,
+ Samaritan,
+ Saurashtra,
  Shavian,
  Sinhala,
+ Sundanese,
  Syloti_Nagri,
  Syriac,
  Tagalog,
  Tagbanwa,
  Tai_Le,
+ Tai_Tham,
+ Tai_Viet,
  Tamil,
  Telugu,
  Thaana,
-Line 575 
 Thai,
+Line 601 
 Thai,
  Tibetan,
  Tifinagh,
  Ugaritic,
+ Vai,
  Yi.
  .P
  Each character has exactly one general category property, specified by a
-Line 710 
 For example, when the pattern
+Line 737 
 For example, when the pattern
    (foo)\eKbar
  .sp
  matches "foobar", the first substring is still set to "foo".
+ .P
+ Perl documents that the use of \eK within assertions is "not well defined". In
+ PCRE, \eK is acted upon when it occurs inside positive assertions, but is
+ ignored in negative assertions.
  .
  .
  .\" HTML <a name="smallassertions"></a>
-Line 740 
 different meaning, namely the backspace
+Line 771 
 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 872 
 the lookbehind.
+Line 906 
 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 889 
 For example, the character class [aeiou]
+Line 924 
 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 903 
 caseful version would. In UTF-8 mode, PC
+Line 938 
 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
  Characters that might indicate line breaks are never treated in any special way
  when matching character classes, whatever line-ending sequence is in use, and
-Line 1132 
 is reached, an option setting in one bra
+Line 1167 
 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
-Line 1157 
 stored.
+Line 1193 
 stored.
    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
    # 1            2         2  3        2     3     4
  .sp
- A backreference or a recursive call to a numbered subpattern always refers to
+ A back reference to a numbered subpattern uses the most recent value that is
- the first one in the pattern with the given number.
+ 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.
-Line 1173 
 if an expression is modified, the number
+Line 1226 
 if an expression is modified, the number
  difficulty, PCRE supports the naming of subpatterns. This feature was not
  added to Perl until release 5.10. Python had the feature earlier, and PCRE
  introduced it at release 4.0, using the Python syntax. PCRE now supports both
- the Perl and the Python syntax.
+ the Perl and the Python syntax. Perl allows identically numbered subpatterns to
+ have different names, but PCRE does not.
  .P
  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
  (?'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,
+ back references,
  .\"
  .\" HTML <a href="#recursion">
  .\" </a>
-Line 1200 
 extracting the name-to-number translatio
+Line 1254 
 extracting the name-to-number translatio
  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. This can
+ this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
- be useful for patterns where only one instance of the named parentheses can
+ names are also always permitted for subpatterns with the same number, set up as
- match. Suppose you want to match the name of a weekday, either as a 3-letter
+ described in the previous section.) Duplicate names can be useful for patterns
- abbreviation or as the full name, and in both cases you want to extract the
+ where only one instance of the named parentheses can match. Suppose you want to
- abbreviation. This pattern (ignoring the line breaks) does the job:
+ 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)?|
-Line 1218 
 subpattern, as described in the previous
+Line 1274 
 subpattern, as described in the previous
  .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. If you
+ matched. This saves searching to find which numbered subpattern it was.
- make a reference to a non-unique named subpattern from elsewhere in the
+ .P
- pattern, the one that corresponds to the lowest number is used. For further
+ If you make a back reference to a non-unique named subpattern from elsewhere in
- details of the interfaces for handling named subpatterns, see the
+ 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 (see the previous section) because PCRE uses
+ subpatterns with the same number because PCRE uses only the numbers when
- only the numbers when matching.
+ 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 1247 
 items:
+Line 1318 
 items:
    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 1357 
 worth setting PCRE_DOTALL in order to ob
+Line 1429 
 worth setting PCRE_DOTALL in order to ob
  alternatively using ^ to indicate anchoring explicitly.
  .P
  However, there is one situation where the optimization cannot be used. When .*
- is inside capturing parentheses that are the subject of a backreference
+ is inside capturing parentheses that are the subject of a back reference
  elsewhere in the pattern, a match at the start may fail where a later one
  succeeds. Consider, for example:
  .sp
-Line 1568 
 after the reference.
+Line 1640 
 after the reference.
  .P
  There may be more than one back reference to the same subpattern. If a
  subpattern has not actually been used in a particular match, any back
- references to it always fail. For example, the pattern
+ references to it always fail by default. For example, the pattern
  .sp
    (a|(bc))\e2
  .sp
- always fails if it starts to match "a" rather than "bc". Because there may be
+ always fails if it starts to match "a" rather than "bc". However, if the
- many capturing parentheses in a pattern, all digits following the backslash are
+ PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
- taken as part of a potential back reference number. If the pattern continues
+ unset value matches an empty string.
- with a digit character, some delimiter must be used to terminate the back
+ .P
- reference. If the PCRE_EXTENDED option is set, this can be whitespace.
+ Because there may be many capturing parentheses in a pattern, all digits
- Otherwise an empty comment (see
+ following a backslash are taken as part of a potential back reference number.
+ If the pattern continues with a digit character, some delimiter must be used to
+ terminate the back reference. If the PCRE_EXTENDED option is set, this can be
+ whitespace. Otherwise, the \eg{ syntax or an empty comment (see
  .\" HTML <a href="#comments">
  .\" </a>
  "Comments"
  .\"
  below) can be used.
- .P
+ .
+ .SS "Recursive back references"
+ .rs
+ .sp
  A back reference that occurs inside the parentheses to which it refers fails
  when the subpattern is first used, so, for example, (a\e1) never matches.
  However, such references can be useful inside repeated subpatterns. For
-Line 1597 
 to the previous iteration. In order for
+Line 1675 
 to the previous iteration. In order for
  that the first iteration does not need to match the back reference. This can be
  done using alternation, as in the example above, or by a quantifier with a
  minimum of zero.
+ .P
+ Back references of this type cause the group that they reference to be treated
+ as an
+ .\" HTML <a href="#atomicgroup">
+ .\" </a>
+ atomic group.
+ .\"
+ Once the whole group has been matched, a subsequent matching failure cannot
+ cause backtracking into the middle of the group.
  .
  .
  .\" HTML <a name="bigassertions"></a>
-Line 1650 
 lookbehind assertion is needed to achiev
+Line 1737 
 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 1690 
 In some cases, the Perl 5.10 escape sequ
+Line 1779 
 In some cases, the Perl 5.10 escape sequ
  .\" </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
-Line 1708 
 different numbers of bytes, are also not
+Line 1797 
 different numbers of bytes, are also not
  "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,
-Line 1716 
 Recursion,
+Line 1805 
 Recursion,
  however, is not supported.
  .P
  Possessive quantifiers can be used in conjunction with lookbehind assertions to
- specify efficient matching at the end of the subject string. Consider a simple
+ specify efficient matching of fixed-length strings at the end of subject
- pattern such as
+ strings. Consider a simple pattern such as
  .sp
    abcd$
  .sp
-Line 1781 
 characters that are not "999".
+Line 1870 
 characters that are not "999".
  .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 1798 
 recursion, a pseudo-condition called DEF
+Line 1887 
 recursion, a pseudo-condition called DEF
  .rs
  .sp
  If the text between the parentheses consists of a sequence of digits, the
- condition is true if the capturing subpattern of that number has previously
+ condition is true if a capturing subpattern of that number has previously
- matched. An alternative notation is to precede the digits with a plus or minus
+ matched. If there is more than one capturing subpattern with the same number
- sign. In this case, the subpattern number is relative rather than absolute.
+ (see the earlier
- 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
+ .\" HTML <a href="#recursion">
- refer to subsequent groups with constructs such as (?(+2).
+ .\" </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
-Line 1844 
 Rewriting the above example to use a nam
+Line 1941 
 Rewriting the above example to use a nam
  .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
-Line 1855 
 letter R, for example:
+Line 1955 
 letter R, for example:
  .sp
    (?(R3)...) or (?(R&name)...)
  .sp
- the condition is true if the most recent recursion is into the subpattern whose
+ 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.
+ 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>
- Recursive patterns
+ The syntax for recursive patterns
  .\"
- are described below.
+ is described below.
  .
  .SS "Defining subpatterns for use by reference only"
  .rs
-Line 1873 
 If the condition is the string (DEFINE),
+Line 1975 
 If the condition is the string (DEFINE),
  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"
-Line 1887 
 written like this (ignore whitespace and
+Line 1989 
 written like this (ignore whitespace and
  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.
+ pattern is skipped because DEFINE acts like a false condition. The rest of the
- .P
+ pattern uses references to the named group to match the four dot-separated
- The rest of the pattern uses references to the named group to match the four
+ components of an IPv4 address, insisting on a word boundary at each end.
- dot-separated components of an IPv4 address, insisting on a word boundary at
- each end.
  .
  .SS "Assertion conditions"
  .rs
-Line 1952 
 this kind of recursion was subsequently
+Line 2052 
 this kind of recursion was subsequently
  .P
  A special item that consists of (? followed by a number greater than zero and a
  closing parenthesis is a recursive call of the subpattern of the given number,
  provided that it occurs inside that subpattern. (If not, it is a
  .\" HTML <a href="#subpatternsassubroutines">
  .\" </a>
  "subroutine"
-Line 1963 
 a recursive call of the entire regular e
+Line 2063 
 a recursive call of the entire regular e
  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.
  .P
  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
- is made easier by the use of relative references. (A Perl 5.10 feature.)
+ is made easier by the use of relative references (a Perl 5.10 feature).
  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
  most recently opened parentheses preceding the recursion. In other words, a
  negative number counts capturing parentheses leftwards from the point at which
-Line 1998 
 An alternative approach is to use named
+Line 2099 
 An alternative approach is to use named
  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) )
+   (?<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 atomic grouping for matching strings of
+ unlimited repeats, and so the use of a possessive quantifier for matching
- non-parentheses is important when applying the pattern to strings that do not
+ strings of non-parentheses is important when applying the pattern to strings
- match. For example, when this pattern is applied to
+ 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
- below and the
  .\" HREF
  \fBpcrecallout\fP
  .\"
-Line 2026 
 documentation). If the pattern above is
+Line 2126 
 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 2058 
 is the actual recursive call.
+Line 2155 
 is the actual recursive call.
  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
-Line 2084 
 alternatives in the other order, things
+Line 2181 
 alternatives in the other order, things
  .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*+\4|\eW*+.\eW*+))\eW*+$
+   ^\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>
-Line 2141 
 matches "sense and sensibility" and "res
+Line 2245 
 matches "sense and sensibility" and "res
  is used, it does match "sense and responsibility" as well as the other two
  strings. Another example is given in the discussion of DEFINE above.
  .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
-Line 2228 
 a backtracking algorithm. With the excep
+Line 2333 
 a backtracking algorithm. With the excep
  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 subpattern, their effect is
+ If any of these verbs are used in an assertion or subroutine subpattern
- confined to that subpattern; it does not extend to the surrounding pattern.
+ (including recursive subpatterns), their effect is confined to that subpattern;
- Note that assertion subpatterns are processed as anchored at the point where
+ it does not extend to the surrounding pattern. Note that such subpatterns are
- they are tested.
+ processed as anchored at the point where they are tested.
  .P
  The new verbs make use of what was previously invalid syntax: an opening
  parenthesis followed by an asterisk. In Perl, they are generally of the form
-Line 2248 
 The following verbs act as soon as they
+Line 2353 
 The following verbs act as soon as they
  .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 the (*ACCEPT) is inside capturing parentheses, the data so far
+ immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
- is captured. (This feature was added to PCRE at release 8.00.) For example:
+ 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)
-Line 2280 
 The verbs differ in exactly what kind of
+Line 2385 
 The verbs differ in exactly what kind of
  .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 start point take place. Once (*COMMIT) has been
+ 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
-Line 2312 
 was matched leading up to it cannot be p
+Line 2417 
 was matched leading up to it cannot be p
  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 in this example; although it would suppress backtracking during the
+ 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
-Line 2334 
 is used outside of any alternation, it a
+Line 2439 
 is used outside of any alternation, it a
  .SH "SEE ALSO"
  .rs
  .sp
- \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
+ \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
+ \fBpcresyntax\fP(3), \fBpcre\fP(3).
  .
  .
  .SH AUTHOR
-Line 2351 
 Cambridge CB2 3QH, England.
+Line 2457 
 Cambridge CB2 3QH, England.
  .rs
  .sp
  .nf
- Last updated: 22 September 2009
+ Last updated: 06 March 2010
- Copyright (c) 1997-2009 University of Cambridge.
+ Copyright (c) 1997-2010 University of Cambridge.
  .fi

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