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

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-revision 208 by ph10,
Mon Aug  6 15:23:29 2007 UTC
+revision 451 by ph10,
Wed Sep 16 11:05:51 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 44 
 discussed in the
+Line 56 
 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. 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
-Line 153 
 represents:
+Line 214 
 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 168 
 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
-Line 258 
 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 293 
 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 347 
 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 372 
 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 535 
 The special property L& is also supporte
+Line 635 
 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
+ 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
-Line 910 
 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 925 
 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 948 
 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 1099 
 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 1147 
 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 1930 
 It matches "abcabc". It does not match "
+Line 2063 
 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 1966 
 description of the interface to the call
+Line 2120 
 description of the interface to the call
  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 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
+ 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 the (*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 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 1986 
 Cambridge CB2 3QH, England.
+Line 2258 
 Cambridge CB2 3QH, England.
  .rs
  .sp
  .nf
- Last updated: 06 August 2007
+ Last updated: 16 September 2009
- Copyright (c) 1997-2007 University of Cambridge.
+ Copyright (c) 1997-2009 University of Cambridge.
  .fi

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