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+revision 461 by ph10,
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 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,
  PCRE must be built to include UTF-8 support, and you must call
  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
  is also a special sequence that can be given at the start of a pattern:
 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 or
  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
  newline sequence, the pattern
  .sp
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 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
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 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 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
 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. 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
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 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 by default.
  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
  bracket causes a compile-time error. If a closing square bracket is required as
  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.
 Line 1163 
 stored.
    / ( 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))\1/
+   /(?|(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)/
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 is also a convenience function for extra
  .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
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 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.
  .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
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 test (see the
  .\" </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
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 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),
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 references to it always fail by default.
  .sp
    (a|(bc))\e2
  .sp
  always fails if it starts to match "a" rather than "bc". However, if the
  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
  unset value matches an empty string.
  .P
  Because there may be many capturing parentheses in a pattern, all digits
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 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
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 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,
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 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 specific capturing subpattern has
  already been matched. The two possible forms of conditional subpattern are:
  .sp
    (?(condition)yes-pattern)
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 recursion, a pseudo-condition called DEF
  .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>
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 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"
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 letter R, for example:
  .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
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 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"
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 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 2026 
 PCRE_EXTENDED option is set so that whit
  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. 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
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 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 2143 
 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.
 Line 2294 
 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
  confined to that subpattern; it does not extend to the surrounding pattern.
  Note that assertion subpatterns are processed as anchored at the point where
  they are tested.
  .P
  The new verbs make use of what was previously invalid syntax: an opening
 Line 2319 
 captured. (This feature was added to PCR
  .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 2400 
 is used outside of any alternation, it a
  .SH "SEE ALSO"
  .rs
  .sp
  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
  \fBpcresyntax\fP(3), \fBpcre\fP(3).
  .
  .

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