/[pcre]/code/trunk/doc/pcrepattern.3
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revision 210 by ph10, Wed Aug 8 14:24:50 2007 UTC revision 211 by ph10, Thu Aug 9 09:52:43 2007 UTC
# Line 168  Thus \ecz becomes hex 1A, but \ec{ becom Line 168  Thus \ecz becomes hex 1A, but \ec{ becom
168  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
169  upper or lower case). Any number of hexadecimal digits may appear between \ex{  upper or lower case). Any number of hexadecimal digits may appear between \ex{
170  and }, but the value of the character code must be less than 256 in non-UTF-8  and }, but the value of the character code must be less than 256 in non-UTF-8
171  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
172  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
173  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
174  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
175  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
176    there is no terminating }, this form of escape is not recognized. Instead, the
177    initial \ex will be interpreted as a basic hexadecimal escape, with no
178    following digits, giving a character whose value is zero.
179  .P  .P
180  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
181  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex. There is no difference in the way they are handled. For
# Line 535  The special property L& is also supporte Line 538  The special property L& is also supporte
538  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
539  a modifier or "other".  a modifier or "other".
540  .P  .P
541    The Cs (Surrogate) property applies only to characters in the range U+D800 to
542    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
543    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
544    (see the discussion of PCRE_NO_UTF8_CHECK in the
545    .\" HREF
546    \fBpcreapi\fP
547    .\"
548    page).
549    .P
550  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The long synonyms for these properties that Perl supports (such as \ep{Letter})
551  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
552  properties with "Is".  properties with "Is".
# Line 1969  documentation. Line 1981  documentation.
1981  .SH "BACTRACKING CONTROL"  .SH "BACTRACKING CONTROL"
1982  .rs  .rs
1983  .sp  .sp
1984  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
1985  are described in the Perl documentation as "experimental and subject to change  are described in the Perl documentation as "experimental and subject to change
1986  or removal in a future version of Perl". It goes on to say: "Their usage in  or removal in a future version of Perl". It goes on to say: "Their usage in
1987  production code should be noted to avoid problems during upgrades." The same  production code should be noted to avoid problems during upgrades." The same
1988  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
1989  .P  .P
1990  Since these verbs are specifically related to backtracking, they can be used  Since these verbs are specifically related to backtracking, they can be used
1991  only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a  only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
1992  backtracking algorithm. They cause an error if encountered by  backtracking algorithm. They cause an error if encountered by
1993  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
1994  .P  .P
1995  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
1996  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. In Perl, they are generally of the form
1997  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB:ARG) but PCRE does not support the use of arguments, so its general
1998  form is just (*VERB). Any number of these verbs may occur in a pattern. There  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 2006  The following verbs act as soon as they
2006     (*ACCEPT)     (*ACCEPT)
2007  .sp  .sp
2008  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2009  pattern. When inside a recursion, only the innermost pattern is ended  pattern. When inside a recursion, only the innermost pattern is ended
2010  immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside  immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2011  capturing parentheses. In Perl, the data so far is captured: in PCRE no data is  capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2012  captured. For example:  captured. For example:
2013  .sp  .sp
2014    A(A|B(*ACCEPT)|C)D    A(A|B(*ACCEPT)|C)D
2015  .sp  .sp
2016  This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is  This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2017  captured.  captured.
2018  .sp  .sp
2019    (*FAIL) or (*F)    (*FAIL) or (*F)
2020  .sp  .sp
2021  This verb causes the match to fail, forcing backtracking to occur. It is  This verb causes the match to fail, forcing backtracking to occur. It is
2022  equivalent to (?!) but easier to read. The Perl documentation notes that it is  equivalent to (?!) but easier to read. The Perl documentation notes that it is
2023  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2024  Perl features that are not present in PCRE. The nearest equivalent is the  Perl features that are not present in PCRE. The nearest equivalent is the
# Line 2014  callout feature, as for example in this Line 2026  callout feature, as for example in this
2026  .sp  .sp
2027    a+(?C)(*FAIL)    a+(?C)(*FAIL)
2028  .sp  .sp
2029  A match with the string "aaaa" always fails, but the callout is taken before  A match with the string "aaaa" always fails, but the callout is taken before
2030  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2031  .  .
2032  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2033  .rs  .rs
2034  .sp  .sp
2035  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2036  with what follows, but if there is no subsequent match, a failure is forced.  with what follows, but if there is no subsequent match, a failure is forced.
2037  The verbs differ in exactly what kind of failure occurs.  The verbs differ in exactly what kind of failure occurs.
2038  .sp  .sp
2039    (*COMMIT)    (*COMMIT)
2040  .sp  .sp
2041  This verb causes the whole match to fail outright if the rest of the pattern  This verb causes the whole match to fail outright if the rest of the pattern
2042  does not match. Even if the pattern is unanchored, no further attempts to find  does not match. Even if the pattern is unanchored, no further attempts to find
2043  a match by advancing the start point take place. Once (*COMMIT) has been  a match by advancing the start point take place. Once (*COMMIT) has been
2044  passed, \fBpcre_exec()\fP is committed to finding a match at the current  passed, \fBpcre_exec()\fP is committed to finding a match at the current
2045  starting point, or not at all. For example:  starting point, or not at all. For example:
2046  .sp  .sp
2047    a+(*COMMIT)b    a+(*COMMIT)b
2048  .sp  .sp
2049  This matches "xxaab" but not "aacaab". It can be thought of as a kind of  This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2050  dynamic anchor, or "I've started, so I must finish."  dynamic anchor, or "I've started, so I must finish."
2051  .sp  .sp
2052    (*PRUNE)    (*PRUNE)
2053  .sp  .sp
2054  This verb causes the match to fail at the current position if the rest of the  This verb causes the match to fail at the current position if the rest of the
2055  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2056  advance to the next starting character then happens. Backtracking can occur as  advance to the next starting character then happens. Backtracking can occur as
2057  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2058  if there is no match to the right, backtracking cannot cross (*PRUNE).  if there is no match to the right, backtracking cannot cross (*PRUNE).
2059  In simple cases, the use of (*PRUNE) is just an alternative to an atomic  In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2060  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2061  be expressed in any other way.  be expressed in any other way.
2062  .sp  .sp
2063    (*SKIP)    (*SKIP)
2064  .sp  .sp
2065  This verb is like (*PRUNE), except that if the pattern is unanchored, the  This verb is like (*PRUNE), except that if the pattern is unanchored, the
2066  "bumpalong" advance is not to the next character, but to the position in the  "bumpalong" advance is not to the next character, but to the position in the
2067  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2068  was matched leading up to it cannot be part of a successful match. Consider:  was matched leading up to it cannot be part of a successful match. Consider:
2069  .sp  .sp
2070    a+(*SKIP)b    a+(*SKIP)b
2071  .sp  .sp
2072  If the subject is "aaaac...", after the first match attempt fails (starting at  If the subject is "aaaac...", after the first match attempt fails (starting at
2073  the first character in the string), the starting point skips on to start the  the first character in the string), the starting point skips on to start the
2074  next attempt at "c". Note that a possessive quantifer does not have the same  next attempt at "c". Note that a possessive quantifer does not have the same
2075  effect in this example; although it would suppress backtracking during the  effect in this example; although it would suppress backtracking during the
2076  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2077  instead of skipping on to "c".  instead of skipping on to "c".
2078  .sp  .sp
2079    (*THEN)    (*THEN)
2080  .sp  .sp
2081  This verb causes a skip to the next alternation if the rest of the pattern does  This verb causes a skip to the next alternation if the rest of the pattern does
2082  not match. That is, it cancels pending backtracking, but only within the  not match. That is, it cancels pending backtracking, but only within the
2083  current alternation. Its name comes from the observation that it can be used  current alternation. Its name comes from the observation that it can be used
# Line 2073  for a pattern-based if-then-else block: Line 2085  for a pattern-based if-then-else block:
2085  .sp  .sp
2086    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2087  .sp  .sp
2088  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
2089  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure the matcher skips to the
2090  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2091  is used outside of any alternation, it acts exactly like (*PRUNE).  is used outside of any alternation, it acts exactly like (*PRUNE).
2092  .  .
2093  .  .
# Line 2099  Cambridge CB2 3QH, England. Line 2111  Cambridge CB2 3QH, England.
2111  .rs  .rs
2112  .sp  .sp
2113  .nf  .nf
2114  Last updated: 08 August 2007  Last updated: 09 August 2007
2115  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2007 University of Cambridge.
2116  .fi  .fi

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