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

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-revision 210 by ph10,
Wed Aug  8 14:24:50 2007 UTC
+revision 247 by ph10,
Mon Sep 17 09:38:32 2007 UTC
 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. 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 202 
 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 217 
 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 347 
 accented letters, and these are matched
+Line 399 
 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 425 
 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 608 
 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 910 
 alternative in the subpattern.
+Line 992 
 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 1008 
 PCRE_MULTILINE while unsetting PCRE_DOTA
  permitted. If a letter appears both before and after the hyphen, the option is
  unset.
  .P
+ The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
+ changed in the same way as the Perl-compatible options by using the characters
+ J, U and X respectively.
+ .P
  When an option change 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
-Line 947 
 matches "ab", "aB", "c", and "C", even t
+Line 1034 
 matches "ab", "aB", "c", and "C", even t
  branch is abandoned before the option setting. This is because the effects of
  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
- changed in the same way as the Perl-compatible options by using the characters
- J, U and X respectively.
  .
  .
  .\" HTML <a name="subpattern"></a>
-Line 1966 
 description of the interface to the call
+Line 2049 
 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
  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
-Line 1994 
 The following verbs act as soon as they
+Line 2077 
 The following verbs act as soon as they
     (*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
-Line 2014 
 callout feature, as for example in this
+Line 2097 
 callout feature, as for example in this
  .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
-Line 2073 
 for a pattern-based if-then-else block:
+Line 2156 
 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).
  .
  .
-Line 2099 
 Cambridge CB2 3QH, England.
+Line 2182 
 Cambridge CB2 3QH, England.
  .rs
  .sp
  .nf
- Last updated: 08 August 2007
+ Last updated: 14 September 2007
  Copyright (c) 1997-2007 University of Cambridge.
  .fi

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