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

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-revision 227 by ph10,
Tue Aug 21 15:00:15 2007 UTC
+revision 453 by ph10,
Fri Sep 18 19:12:35 2009 UTC
 Line 9 
 are described in detail below. There is
  .\" HREF
  \fBpcresyntax\fP
  .\"
- page. Perl's regular expressions are described in its own documentation, and
+ 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
-Line 18 
 description of PCRE's regular expression
+Line 23 
 description of PCRE's regular expression
  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
  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
- the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
+ the PCRE_UTF8 option. There is also a special sequence that can be given at the
- places below. There is also a summary of UTF-8 features in 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 79 
 example, on a Unix system where LF is th
+Line 91 
 example, on a Unix system where LF is th
  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.
+ 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"
-Line 299 
 parenthesized subpatterns.
+Line 322 
 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 subroutine call.
+ .
+ .
  .SS "Generic character types"
  .rs
  .sp
-Line 334 
 In UTF-8 mode, characters with values gr
+Line 371 
 In UTF-8 mode, characters with values gr
  \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.
+ 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.
-Line 388 
 accented letters, and these are matched
+Line 426 
 accented letters, and these are matched
  is discouraged.
  .
  .
+ .\" HTML <a name="newlineseq"></a>
  .SS "Newline sequences"
  .rs
  .sp
- Outside a character class, the escape sequence \eR matches any Unicode newline
+ Outside a character class, by default, the escape sequence \eR matches any
- sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to
+ Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
- the following:
+ equivalent to the following:
  .sp
    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
  .sp
-Line 413 
 are added: LS (line separator, U+2028) a
+Line 452 
 are added: LS (line separator, U+2028) a
  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, but
+ they can be overridden by options given to \fBpcre_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".
  .
  .
-Line 583 
 cannot be tested by PCRE, unless UTF-8 v
+Line 642 
 cannot be tested by PCRE, unless UTF-8 v
  .\" HREF
  \fBpcreapi\fP
  .\"
- page).
+ page). Perl does not support the Cs property.
  .P
- The long synonyms for these properties that Perl supports (such as \ep{Letter})
+ 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
-Line 960 
 alternative in the subpattern.
+Line 1019 
 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 975 
 PCRE_MULTILINE while unsetting PCRE_DOTA
+Line 1035 
 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 (see below for a description of
  subpatterns) affects only that part of the current pattern that follows it, so
-Line 998 
 branch is abandoned before the option se
+Line 1062 
 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_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
+ \fBNote:\fP There are other PCRE-specific options that can be set by the
- changed in the same way as the Perl-compatible options by using the characters
+ application when the compile or match functions are called. In some cases the
- J, U and X respectively.
+ pattern can contain special leading sequences such as (*CRLF) to override what
+ the application has set or what has been defaulted. Details are given in the
+ 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 1149 
 details of the interfaces for handling n
+Line 1221 
 details of the interfaces for handling n
  \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
+ only the numbers when matching.
  .
  .
  .SH REPETITION
-Line 1197 
 support is available, \eX{3} matches thr
+Line 1273 
 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, the three most common quantifiers have single-character
  abbreviations:
-Line 1839 
 recursively to the pattern in which it a
+Line 1922 
 recursively to the pattern in which it a
  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
  supports special syntax for recursion of the entire pattern, and also for
  individual subpattern recursion. After its introduction in PCRE and Python,
- this kind of recursion was introduced into Perl at release 5.10.
+ this kind of recursion was subsequently introduced into Perl at release 5.10.
  .P
  A special item that consists of (? followed by a number greater than zero and a
  closing parenthesis is a recursive call of the subpattern of the given number,
-Line 1847 
 provided that it occurs inside that subp
+Line 1930 
 provided that it occurs inside that subp
  call, which is described in the next section.) The special item (?R) or (?0) is
  a recursive call of the entire regular expression.
  .P
- 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.
- .P
  This PCRE pattern solves the nested parentheses problem (assume the
  PCRE_EXTENDED option is set so that white space is ignored):
  .sp
-Line 1939 
 different alternatives for the recursive
+Line 2017 
 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*+\4|\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.
+ .
+ .
  .\" HTML <a name="subpatternsassubroutines"></a>
  .SH "SUBPATTERNS AS SUBROUTINES"
  .rs
-Line 1980 
 It matches "abcabc". It does not match "
+Line 2122 
 It matches "abcabc". It does not match "
  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
  .rs
  .sp
-Line 2016 
 description of the interface to the call
+Line 2179 
 description of the interface to the call
  documentation.
  .
  .
- .SH "BACTRACKING CONTROL"
+ .SH "BACKTRACKING CONTROL"
  .rs
  .sp
  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
-Line 2025 
 or removal in a future version of Perl".
+Line 2188 
 or removal in a future version of Perl".
  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
+ Since these verbs are specifically related to backtracking, most of them can be
- only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
+ used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
- backtracking algorithm. They cause an error if encountered by
+ 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 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
  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
-Line 2045 
 The following verbs act as soon as they
+Line 2214 
 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. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
+ immediately. If the (*ACCEPT) is inside capturing parentheses, the data so far
- capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
+ is captured. (This feature was added to PCRE at release 8.00.) For example:
- captured. For example:
  .sp
-   A(A|B(*ACCEPT)|C)D
+   A((?:A|B(*ACCEPT)|C)D)
  .sp
- This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
+ This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
- captured.
+ the outer parentheses.
  .sp
    (*FAIL) or (*F)
  .sp
-Line 2149 
 Cambridge CB2 3QH, England.
+Line 2317 
 Cambridge CB2 3QH, England.
  .rs
  .sp
  .nf
- Last updated: 21 August 2007
+ Last updated: 18 September 2009
- Copyright (c) 1997-2007 University of Cambridge.
+ Copyright (c) 1997-2009 University of Cambridge.
  .fi

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