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+revision 227 by ph10,
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 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. 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
-Line 40 
 discussed in the
+Line 44 
 discussed in the
  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. 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.
+ .
+ .
  .SH "CHARACTERS AND METACHARACTERS"
  .rs
  .sp
-Line 149 
 represents:
+Line 191 
 represents:
    \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 164 
 Thus \ecz becomes hex 1A, but \ec{ becom
+Line 206 
 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
-Line 240 
 meanings
+Line 285 
 meanings
  .SS "Absolute and relative back references"
  .rs
  .sp
- The sequence \eg followed by a positive or negative number, optionally enclosed
+ The sequence \eg followed by an unsigned or a negative number, optionally
- in braces, is an absolute or relative back reference. A named back reference
+ enclosed in braces, is an absolute or relative back reference. A named back
- can be coded as \eg{name}. Back references are discussed
+ reference can be coded as \eg{name}. Back references are discussed
  .\" HTML <a href="#backreferences">
  .\" </a>
  later,
-Line 531 
 The special property L& is also supporte
+Line 576 
 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 Cs (Surrogate) property applies only to characters in the range U+D800 to
+ 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).
+ .P
  The long synonyms for these properties that Perl supports (such as \ep{Letter})
  are not supported by PCRE, nor is it permitted to prefix any of these
  properties with "Is".
-Line 1290 
 previous example can be rewritten as
+Line 1344 
 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 of a possessive
-Line 1364 
 subpattern is possible using named paren
+Line 1423 
 subpattern is possible using named paren
  .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 a positive or a negative number,
+ Perl 5.10. This escape must be followed by an unsigned number or a negative
- optionally enclosed in braces. These examples are all identical:
+ number, optionally enclosed in braces. These examples are all identical:
  .sp
    (ring), \e1
    (ring), \eg1
    (ring), \eg{1}
  .sp
- A positive number specifies an absolute reference without the ambiguity that is
+ An unsigned number specifies an absolute reference without the ambiguity that
- present in the older syntax. It is also useful when literal digits follow the
+ is present in the older syntax. It is also useful when literal digits follow
- reference. A negative number is a relative reference. Consider this example:
+ the reference. A negative number is a relative reference. Consider this
+ example:
  .sp
    (abc(def)ghi)\eg{-1}
  .sp
-Line 1956 
 description of the interface to the call
+Line 2016 
 description of the interface to the call
  documentation.
  .
  .
+ .SH "BACTRACKING 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, they can be used
+ only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
+ backtracking algorithm. They cause an error if encountered by
+ \fBpcre_dfa_exec()\fP.
+ .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
+ (*VERB:ARG) but PCRE does not support the use of arguments, so its general
+ 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. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
+ capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
+ captured. For example:
+ .sp
+   A(A|B(*ACCEPT)|C)D
+ .sp
+ This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
+ captured.
+ .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 start 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 in 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
-Line 1976 
 Cambridge CB2 3QH, England.
+Line 2149 
 Cambridge CB2 3QH, England.
  .rs
  .sp
  .nf
- Last updated: 19 June 2007
+ Last updated: 21 August 2007
  Copyright (c) 1997-2007 University of Cambridge.
  .fi

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