Revision: 738
http://vcs.pcre.org/viewvc?view=rev&revision=738
Author: ph10
Date: 2011-10-21 10:04:01 +0100 (Fri, 21 Oct 2011)
Log Message:
-----------
Source tidies for 8.20 release.
Modified Paths:
--------------
code/trunk/ChangeLog
code/trunk/NEWS
code/trunk/configure.ac
code/trunk/doc/html/pcrejit.html
code/trunk/doc/html/pcrepattern.html
code/trunk/doc/html/pcreunicode.html
code/trunk/doc/pcre.txt
code/trunk/doc/pcrepattern.3
code/trunk/pcretest.c
Modified: code/trunk/ChangeLog
===================================================================
--- code/trunk/ChangeLog 2011-10-19 17:37:29 UTC (rev 737)
+++ code/trunk/ChangeLog 2011-10-21 09:04:01 UTC (rev 738)
@@ -1,7 +1,7 @@
ChangeLog for PCRE
------------------
-Version 8.20 10-Oct-2011
+Version 8.20 21-Oct-2011
------------------------
1. Change 37 of 8.13 broke patterns like [:a]...[b:] because it thought it had
@@ -68,7 +68,7 @@
12. In some environments, the output of pcretest -C is CRLF terminated. This
broke RunTest's code that checks for the link size. A single white space
- after the value is now allowed for.
+ character after the value is now allowed for.
13. RunTest now checks for the "fr" locale as well as for "fr_FR" and "french".
For "fr", it uses the Windows-specific input and output files.
@@ -103,16 +103,16 @@
19. If the PCRE_NO_START_OPTIMIZE option was set for pcre_compile(), it did not
suppress the check for a minimum subject length at run time. (If it was
given to pcre_exec() or pcre_dfa_exec() it did work.)
-
+
20. Fixed an ASCII-dependent infelicity in pcretest that would have made it
- fail to work when decoding hex characters in data strings in EBCDIC
- environments.
-
-21. It appears that in at least one Mac OS environment, the isxdigit() function
+ fail to work when decoding hex characters in data strings in EBCDIC
+ environments.
+
+21. It appears that in at least one Mac OS environment, the isxdigit() function
is implemented as a macro that evaluates to its argument more than once,
contravening the C 90 Standard (I haven't checked a later standard). There
was an instance in pcretest which caused it to go wrong when processing
- \x{...} escapes in subject strings. The has been rewritten to avoid using
+ \x{...} escapes in subject strings. The has been rewritten to avoid using
things like p++ in the argument of isxdigit().
Modified: code/trunk/NEWS
===================================================================
--- code/trunk/NEWS 2011-10-19 17:37:29 UTC (rev 737)
+++ code/trunk/NEWS 2011-10-21 09:04:01 UTC (rev 738)
@@ -1,14 +1,14 @@
News about PCRE releases
------------------------
-Release 8.20
-------------
+Release 8.20 21-Oct-2011
+------------------------
The main change in this release is the inclusion of Zoltan Herczeg's
just-in-time compiler support, which can be accessed by building PCRE with
--enable-jit. Large performance benefits can be had in many situations. 8.20
also fixes an unfortunate bug that was introduced in 8.13 as well as tidying up
-a couple of infelicities.
+a number of infelicities and differences from Perl.
Release 8.13 16-Aug-2011
Modified: code/trunk/configure.ac
===================================================================
--- code/trunk/configure.ac 2011-10-19 17:37:29 UTC (rev 737)
+++ code/trunk/configure.ac 2011-10-21 09:04:01 UTC (rev 738)
@@ -10,8 +10,8 @@
m4_define(pcre_major, [8])
m4_define(pcre_minor, [20])
-m4_define(pcre_prerelease, [-RC3])
-m4_define(pcre_date, [2011-10-10])
+m4_define(pcre_prerelease, [])
+m4_define(pcre_date, [2011-10-21])
# Libtool shared library interface versions (current:revision:age)
m4_define(libpcre_version, [0:1:0])
Modified: code/trunk/doc/html/pcrejit.html
===================================================================
--- code/trunk/doc/html/pcrejit.html 2011-10-19 17:37:29 UTC (rev 737)
+++ code/trunk/doc/html/pcrejit.html 2011-10-21 09:04:01 UTC (rev 738)
@@ -116,7 +116,7 @@
<P>
The unsupported pattern items are:
<pre>
- \C match a single byte, even in UTF-8 mode
+ \C match a single byte; not supported in UTF-8 mode
(?Cn) callouts
(?(<name>)... conditional test on setting of a named subpattern
(?(R)... conditional test on whole pattern recursion
@@ -275,7 +275,7 @@
</P>
<br><a name="SEC11" href="#TOC1">REVISION</a><br>
<P>
-Last updated: 05 October 2011
+Last updated: 19 October 2011
<br>
Copyright © 1997-2011 University of Cambridge.
<br>
Modified: code/trunk/doc/html/pcrepattern.html
===================================================================
--- code/trunk/doc/html/pcrepattern.html 2011-10-19 17:37:29 UTC (rev 737)
+++ code/trunk/doc/html/pcrepattern.html 2011-10-21 09:04:01 UTC (rev 738)
@@ -968,17 +968,39 @@
<br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
<P>
Outside a character class, the escape sequence \C matches any one byte, both
-in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
+in and out of UTF-8 mode. Unlike a dot, it always matches line-ending
characters. The feature is provided in Perl in order to match individual bytes
-in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
-rest of the string may start with a malformed UTF-8 character. For this reason,
-the \C escape sequence is best avoided.
+in UTF-8 mode, but it is unclear how it can usefully be used. Because \C
+breaks up characters into individual bytes, matching one byte with \C in UTF-8
+mode means that the rest of the string may start with a malformed UTF-8
+character. This has undefined results, because PCRE assumes that it is dealing
+with valid UTF-8 strings (and by default it checks this at the start of
+processing unless the PCRE_NO_UTF8_CHECK option is used).
</P>
<P>
PCRE does not allow \C to appear in lookbehind assertions
<a href="#lookbehind">(described below),</a>
because in UTF-8 mode this would make it impossible to calculate the length of
the lookbehind.
+</P>
+<P>
+In general, the \C escape sequence is best avoided in UTF-8 mode. However, one
+way of using it that avoids the problem of malformed UTF-8 characters is to
+use a lookahead to check the length of the next character, as in this pattern
+(ignore white space and line breaks):
+<pre>
+ (?| (?=[\x00-\x7f])(\C) |
+ (?=[\x80-\x{7ff}])(\C)(\C) |
+ (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
+ (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
+</pre>
+A group that starts with (?| resets the capturing parentheses numbers in each
+alternative (see
+<a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a>
+below). The assertions at the start of each branch check the next UTF-8
+character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
+character's individual bytes are then captured by the appropriate number of
+groups.
<a name="characterclass"></a></P>
<br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
<P>
@@ -2797,7 +2819,7 @@
</P>
<br><a name="SEC28" href="#TOC1">REVISION</a><br>
<P>
-Last updated: 09 October 2011
+Last updated: 19 October 2011
<br>
Copyright © 1997-2011 University of Cambridge.
<br>
Modified: code/trunk/doc/html/pcreunicode.html
===================================================================
--- code/trunk/doc/html/pcreunicode.html 2011-10-19 17:37:29 UTC (rev 737)
+++ code/trunk/doc/html/pcreunicode.html 2011-10-21 09:04:01 UTC (rev 738)
@@ -118,11 +118,14 @@
</P>
<P>
5. The escape sequence \C can be used to match a single byte in UTF-8 mode,
-but its use can lead to some strange effects. This facility is not available in
-the alternative matching function, <b>pcre_dfa_exec()</b>, nor is it supported
-by the JIT optimization of <b>pcre_exec()</b>. If JIT optimization is requested
-for a pattern that contains \C, it will not succeed, and so the matching will
-be carried out by the normal interpretive function.
+but its use can lead to some strange effects because it breaks up multibyte
+characters (see the description of \C in the
+<a href="pcrepattern.html"><b>pcrepattern</b></a>
+documentation). The use of \C is not supported in the alternative matching
+function <b>pcre_dfa_exec()</b>, nor is it supported in UTF-8 mode by the JIT
+optimization of <b>pcre_exec()</b>. If JIT optimization is requested for a UTF-8
+pattern that contains \C, it will not succeed, and so the matching will be
+carried out by the normal interpretive function.
</P>
<P>
6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
@@ -175,7 +178,7 @@
REVISION
</b><br>
<P>
-Last updated: 06 September 2011
+Last updated: 19 October 2011
<br>
Copyright © 1997-2011 University of Cambridge.
<br>
Modified: code/trunk/doc/pcre.txt
===================================================================
--- code/trunk/doc/pcre.txt 2011-10-19 17:37:29 UTC (rev 737)
+++ code/trunk/doc/pcre.txt 2011-10-21 09:04:01 UTC (rev 738)
@@ -4142,127 +4142,147 @@
MATCHING A SINGLE BYTE
Outside a character class, the escape sequence \C matches any one byte,
- both in and out of UTF-8 mode. Unlike a dot, it always matches any
- line-ending characters. The feature is provided in Perl in order to
- match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
- acters into individual bytes, the rest of the string may start with a
- malformed UTF-8 character. For this reason, the \C escape sequence is
- best avoided.
+ both in and out of UTF-8 mode. Unlike a dot, it always matches line-
+ ending characters. The feature is provided in Perl in order to match
+ individual bytes in UTF-8 mode, but it is unclear how it can usefully
+ be used. Because \C breaks up characters into individual bytes, match-
+ ing one byte with \C in UTF-8 mode means that the rest of the string
+ may start with a malformed UTF-8 character. This has undefined results,
+ because PCRE assumes that it is dealing with valid UTF-8 strings (and
+ by default it checks this at the start of processing unless the
+ PCRE_NO_UTF8_CHECK option is used).
- PCRE does not allow \C to appear in lookbehind assertions (described
- below), because in UTF-8 mode this would make it impossible to calcu-
+ PCRE does not allow \C to appear in lookbehind assertions (described
+ below), because in UTF-8 mode this would make it impossible to calcu-
late the length of the lookbehind.
+ In general, the \C escape sequence is best avoided in UTF-8 mode. How-
+ ever, one way of using it that avoids the problem of malformed UTF-8
+ characters is to use a lookahead to check the length of the next char-
+ acter, as in this pattern (ignore white space and line breaks):
+ (?| (?=[\x00-\x7f])(\C) |
+ (?=[\x80-\x{7ff}])(\C)(\C) |
+ (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
+ (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
+
+ A group that starts with (?| resets the capturing parentheses numbers
+ in each alternative (see "Duplicate Subpattern Numbers" below). The
+ assertions at the start of each branch check the next UTF-8 character
+ for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
+ character's individual bytes are then captured by the appropriate num-
+ ber of groups.
+
+
SQUARE BRACKETS AND CHARACTER CLASSES
An opening square bracket introduces a character class, terminated by a
closing square bracket. A closing square bracket on its own is not spe-
cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
a lone closing square bracket causes a compile-time error. If a closing
- square bracket is required as a member of the class, it should be the
- first data character in the class (after an initial circumflex, if
+ square bracket is required as a member of the class, it should be the
+ first data character in the class (after an initial circumflex, if
present) or escaped with a backslash.
- A character class matches a single character in the subject. In UTF-8
+ A character class matches a single character in the subject. In UTF-8
mode, the character may be more than one byte long. A matched character
must be in the set of characters defined by the class, unless the first
- character in the class definition is a circumflex, in which case the
- subject character must not be in the set defined by the class. If a
- circumflex is actually required as a member of the class, ensure it is
+ character in the class definition is a circumflex, in which case the
+ subject character must not be in the set defined by the class. If a
+ circumflex is actually required as a member of the class, ensure it is
not the first character, or escape it with a backslash.
- For example, the character class [aeiou] matches any lower case vowel,
- while [^aeiou] matches any character that is not a lower case vowel.
+ For example, the character class [aeiou] matches any lower case vowel,
+ while [^aeiou] matches any character that is not a lower case vowel.
Note that a circumflex is just a convenient notation for specifying the
- characters that are in the class by enumerating those that are not. A
- class that starts with a circumflex is not an assertion; it still con-
- sumes a character from the subject string, and therefore it fails if
+ characters that are in the class by enumerating those that are not. A
+ class that starts with a circumflex is not an assertion; it still con-
+ sumes a character from the subject string, and therefore it fails if
the current pointer is at the end of the string.
- In UTF-8 mode, characters with values greater than 255 can be included
- in a class as a literal string of bytes, or by using the \x{ escaping
+ In UTF-8 mode, characters with values greater than 255 can be included
+ in a class as a literal string of bytes, or by using the \x{ escaping
mechanism.
- When caseless matching is set, any letters in a class represent both
- their upper case and lower case versions, so for example, a caseless
- [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
- match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
- understands the concept of case for characters whose values are less
- than 128, so caseless matching is always possible. For characters with
- higher values, the concept of case is supported if PCRE is compiled
- with Unicode property support, but not otherwise. If you want to use
- caseless matching in UTF8-mode for characters 128 and above, you must
- ensure that PCRE is compiled with Unicode property support as well as
+ When caseless matching is set, any letters in a class represent both
+ their upper case and lower case versions, so for example, a caseless
+ [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
+ match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
+ understands the concept of case for characters whose values are less
+ than 128, so caseless matching is always possible. For characters with
+ higher values, the concept of case is supported if PCRE is compiled
+ with Unicode property support, but not otherwise. If you want to use
+ caseless matching in UTF8-mode for characters 128 and above, you must
+ ensure that PCRE is compiled with Unicode property support as well as
with UTF-8 support.
- Characters that might indicate line breaks are never treated in any
- special way when matching character classes, whatever line-ending
- sequence is in use, and whatever setting of the PCRE_DOTALL and
+ Characters that might indicate line breaks are never treated in any
+ special way when matching character classes, whatever line-ending
+ sequence is in use, and whatever setting of the PCRE_DOTALL and
PCRE_MULTILINE options is used. A class such as [^a] always matches one
of these characters.
- The minus (hyphen) character can be used to specify a range of charac-
- ters in a character class. For example, [d-m] matches any letter
- between d and m, inclusive. If a minus character is required in a
- class, it must be escaped with a backslash or appear in a position
- where it cannot be interpreted as indicating a range, typically as the
+ The minus (hyphen) character can be used to specify a range of charac-
+ ters in a character class. For example, [d-m] matches any letter
+ between d and m, inclusive. If a minus character is required in a
+ class, it must be escaped with a backslash or appear in a position
+ where it cannot be interpreted as indicating a range, typically as the
first or last character in the class.
It is not possible to have the literal character "]" as the end charac-
- ter of a range. A pattern such as [W-]46] is interpreted as a class of
- two characters ("W" and "-") followed by a literal string "46]", so it
- would match "W46]" or "-46]". However, if the "]" is escaped with a
- backslash it is interpreted as the end of range, so [W-\]46] is inter-
- preted as a class containing a range followed by two other characters.
- The octal or hexadecimal representation of "]" can also be used to end
+ ter of a range. A pattern such as [W-]46] is interpreted as a class of
+ two characters ("W" and "-") followed by a literal string "46]", so it
+ would match "W46]" or "-46]". However, if the "]" is escaped with a
+ backslash it is interpreted as the end of range, so [W-\]46] is inter-
+ preted as a class containing a range followed by two other characters.
+ The octal or hexadecimal representation of "]" can also be used to end
a range.
- Ranges operate in the collating sequence of character values. They can
- also be used for characters specified numerically, for example
- [\000-\037]. In UTF-8 mode, ranges can include characters whose values
+ Ranges operate in the collating sequence of character values. They can
+ also be used for characters specified numerically, for example
+ [\000-\037]. In UTF-8 mode, ranges can include characters whose values
are greater than 255, for example [\x{100}-\x{2ff}].
If a range that includes letters is used when caseless matching is set,
it matches the letters in either case. For example, [W-c] is equivalent
- to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
- character tables for a French locale are in use, [\xc8-\xcb] matches
- accented E characters in both cases. In UTF-8 mode, PCRE supports the
- concept of case for characters with values greater than 128 only when
+ to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
+ character tables for a French locale are in use, [\xc8-\xcb] matches
+ accented E characters in both cases. In UTF-8 mode, PCRE supports the
+ concept of case for characters with values greater than 128 only when
it is compiled with Unicode property support.
- The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
+ The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
\w, and \W may appear in a character class, and add the characters that
- they match to the class. For example, [\dABCDEF] matches any hexadeci-
- mal digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of
- \d, \s, \w and their upper case partners, just as it does when they
- appear outside a character class, as described in the section entitled
+ they match to the class. For example, [\dABCDEF] matches any hexadeci-
+ mal digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of
+ \d, \s, \w and their upper case partners, just as it does when they
+ appear outside a character class, as described in the section entitled
"Generic character types" above. The escape sequence \b has a different
- meaning inside a character class; it matches the backspace character.
- The sequences \B, \N, \R, and \X are not special inside a character
- class. Like any other unrecognized escape sequences, they are treated
- as the literal characters "B", "N", "R", and "X" by default, but cause
+ meaning inside a character class; it matches the backspace character.
+ The sequences \B, \N, \R, and \X are not special inside a character
+ class. Like any other unrecognized escape sequences, they are treated
+ as the literal characters "B", "N", "R", and "X" by default, but cause
an error if the PCRE_EXTRA option is set.
- A circumflex can conveniently be used with the upper case character
- types to specify a more restricted set of characters than the matching
- lower case type. For example, the class [^\W_] matches any letter or
+ A circumflex can conveniently be used with the upper case character
+ types to specify a more restricted set of characters than the matching
+ lower case type. For example, the class [^\W_] matches any letter or
digit, but not underscore, whereas [\w] includes underscore. A positive
character class should be read as "something OR something OR ..." and a
negative class as "NOT something AND NOT something AND NOT ...".
- The only metacharacters that are recognized in character classes are
- backslash, hyphen (only where it can be interpreted as specifying a
- range), circumflex (only at the start), opening square bracket (only
- when it can be interpreted as introducing a POSIX class name - see the
- next section), and the terminating closing square bracket. However,
+ The only metacharacters that are recognized in character classes are
+ backslash, hyphen (only where it can be interpreted as specifying a
+ range), circumflex (only at the start), opening square bracket (only
+ when it can be interpreted as introducing a POSIX class name - see the
+ next section), and the terminating closing square bracket. However,
escaping other non-alphanumeric characters does no harm.
POSIX CHARACTER CLASSES
Perl supports the POSIX notation for character classes. This uses names
- enclosed by [: and :] within the enclosing square brackets. PCRE also
+ enclosed by [: and :] within the enclosing square brackets. PCRE also
supports this notation. For example,
[01[:alpha:]%]
@@ -4285,24 +4305,24 @@
word "word" characters (same as \w)
xdigit hexadecimal digits
- The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
- and space (32). Notice that this list includes the VT character (code
+ The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
+ and space (32). Notice that this list includes the VT character (code
11). This makes "space" different to \s, which does not include VT (for
Perl compatibility).
- The name "word" is a Perl extension, and "blank" is a GNU extension
- from Perl 5.8. Another Perl extension is negation, which is indicated
+ The name "word" is a Perl extension, and "blank" is a GNU extension
+ from Perl 5.8. Another Perl extension is negation, which is indicated
by a ^ character after the colon. For example,
[12[:^digit:]]
- matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
+ matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
these are not supported, and an error is given if they are encountered.
- By default, in UTF-8 mode, characters with values greater than 128 do
- not match any of the POSIX character classes. However, if the PCRE_UCP
- option is passed to pcre_compile(), some of the classes are changed so
+ By default, in UTF-8 mode, characters with values greater than 128 do
+ not match any of the POSIX character classes. However, if the PCRE_UCP
+ option is passed to pcre_compile(), some of the classes are changed so
that Unicode character properties are used. This is achieved by replac-
ing the POSIX classes by other sequences, as follows:
@@ -4315,31 +4335,31 @@
[:upper:] becomes \p{Lu}
[:word:] becomes \p{Xwd}
- Negated versions, such as [:^alpha:] use \P instead of \p. The other
+ Negated versions, such as [:^alpha:] use \P instead of \p. The other
POSIX classes are unchanged, and match only characters with code points
less than 128.
VERTICAL BAR
- Vertical bar characters are used to separate alternative patterns. For
+ Vertical bar characters are used to separate alternative patterns. For
example, the pattern
gilbert|sullivan
- matches either "gilbert" or "sullivan". Any number of alternatives may
- appear, and an empty alternative is permitted (matching the empty
+ matches either "gilbert" or "sullivan". Any number of alternatives may
+ appear, and an empty alternative is permitted (matching the empty
string). The matching process tries each alternative in turn, from left
- to right, and the first one that succeeds is used. If the alternatives
- are within a subpattern (defined below), "succeeds" means matching the
+ to right, and the first one that succeeds is used. If the alternatives
+ are within a subpattern (defined below), "succeeds" means matching the
rest of the main pattern as well as the alternative in the subpattern.
INTERNAL OPTION SETTING
- The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
- PCRE_EXTENDED options (which are Perl-compatible) can be changed from
- within the pattern by a sequence of Perl option letters enclosed
+ The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
+ PCRE_EXTENDED options (which are Perl-compatible) can be changed from
+ within the pattern by a sequence of Perl option letters enclosed
between "(?" and ")". The option letters are
i for PCRE_CASELESS
@@ -4349,47 +4369,47 @@
For example, (?im) sets caseless, multiline matching. It is also possi-
ble to unset these options by preceding the letter with a hyphen, and a
- combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
- LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
- is also permitted. If a letter appears both before and after the
+ combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
+ LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
+ is also permitted. If a letter appears both before and after the
hyphen, the option is unset.
- 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 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.
- When one of these option changes occurs at top level (that is, not
- inside subpattern parentheses), the change applies to the remainder of
+ 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 there-
fore show up in data extracted by the pcre_fullinfo() function).
- An option change within a subpattern (see below for a description of
- subpatterns) affects only that part of the subpattern that follows it,
+ An option change within a subpattern (see below for a description of
+ subpatterns) affects only that part of the subpattern that follows it,
so
(a(?i)b)c
matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
- used). By this means, options can be made to have different settings
- in different parts of the pattern. Any changes made in one alternative
- do carry on into subsequent branches within the same subpattern. For
+ used). By this means, options can be made to have different settings
+ in different parts of the pattern. Any changes made in one alternative
+ do carry on into subsequent branches within the same subpattern. For
example,
(a(?i)b|c)
- matches "ab", "aB", "c", and "C", even though when matching "C" the
- first branch is abandoned before the option setting. This is because
- the effects of option settings happen at compile time. There would be
+ matches "ab", "aB", "c", and "C", even though when matching "C" the
+ first 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.
- Note: There are other PCRE-specific options that can be set by the
- application when the compile or match functions are called. In some
+ Note: There are other PCRE-specific options that can be set by the
+ application when the compile or match functions are called. In some
cases the 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 "Newline sequences" above.
- There are also the (*UTF8) and (*UCP) leading sequences that can be
- used to set UTF-8 and Unicode property modes; they are equivalent to
+ to override what the application has set or what has been defaulted.
+ Details are given in the section entitled "Newline sequences" above.
+ There are also the (*UTF8) and (*UCP) leading sequences that can be
+ used to set UTF-8 and Unicode property modes; they are equivalent to
setting the PCRE_UTF8 and the PCRE_UCP options, respectively.
@@ -4402,15 +4422,15 @@
cat(aract|erpillar|)
- matches "cataract", "caterpillar", or "cat". Without the parentheses,
+ matches "cataract", "caterpillar", or "cat". Without the parentheses,
it would match "cataract", "erpillar" or an empty string.
- 2. It sets up the subpattern as a capturing subpattern. This means
- that, when the whole pattern matches, that portion of the subject
+ 2. It sets up the subpattern as a capturing subpattern. This means
+ that, when the whole pattern matches, that portion of the subject
string that matched the subpattern is passed back to the caller via the
- ovector argument of pcre_exec(). Opening parentheses are counted from
- left to right (starting from 1) to obtain numbers for the capturing
- subpatterns. For example, if the string "the red king" is matched
+ ovector argument of pcre_exec(). Opening parentheses are counted from
+ left to right (starting from 1) to obtain numbers for the capturing
+ subpatterns. For example, if the string "the red king" is matched
against the pattern
the ((red|white) (king|queen))
@@ -4418,12 +4438,12 @@
the captured substrings are "red king", "red", and "king", and are num-
bered 1, 2, and 3, respectively.
- The fact that plain parentheses fulfil two functions is not always
- helpful. There are often times when a grouping subpattern is required
- without a capturing requirement. If an opening parenthesis is followed
- by a question mark and a colon, the subpattern does not do any captur-
- ing, and is not counted when computing the number of any subsequent
- capturing subpatterns. For example, if the string "the white queen" is
+ The fact that plain parentheses fulfil two functions is not always
+ helpful. There are often times when a grouping subpattern is required
+ without a capturing requirement. If an opening parenthesis is followed
+ by a question mark and a colon, the subpattern does not do any captur-
+ ing, and is not counted when computing the number of any subsequent
+ capturing subpatterns. For example, if the string "the white queen" is
matched against the pattern
the ((?:red|white) (king|queen))
@@ -4431,37 +4451,37 @@
the captured substrings are "white queen" and "queen", and are numbered
1 and 2. The maximum number of capturing subpatterns is 65535.
- As a convenient shorthand, if any option settings are required at the
- start of a non-capturing subpattern, the option letters may appear
+ As a convenient shorthand, if any option settings are required at the
+ start of a non-capturing subpattern, the option letters may appear
between the "?" and the ":". Thus the two patterns
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
match exactly the same set of strings. Because alternative branches are
- tried from left to right, and options are not reset until the end of
- the subpattern is reached, an option setting in one branch does affect
- subsequent branches, so the above patterns match "SUNDAY" as well as
+ tried from left to right, and options are not reset until the end of
+ the subpattern is reached, an option setting in one branch does affect
+ subsequent branches, so the above patterns match "SUNDAY" as well as
"Saturday".
DUPLICATE SUBPATTERN NUMBERS
Perl 5.10 introduced a feature whereby each alternative in a subpattern
- uses the same numbers for its capturing parentheses. Such a subpattern
- starts with (?| and is itself a non-capturing subpattern. For example,
+ uses the same numbers for its capturing parentheses. Such a subpattern
+ starts with (?| and is itself a non-capturing subpattern. For example,
consider this pattern:
(?|(Sat)ur|(Sun))day
- Because the two alternatives are inside a (?| group, both sets of cap-
- turing parentheses are numbered one. Thus, when the pattern matches,
- you can look at captured substring number one, whichever alternative
- matched. This construct is useful when you want to capture part, but
+ Because the two alternatives are inside a (?| group, both sets of cap-
+ turing parentheses are numbered one. Thus, when the pattern matches,
+ you can look at captured substring number one, whichever alternative
+ matched. This construct is useful when you want to capture part, but
not all, of one of a number of alternatives. Inside a (?| group, paren-
- theses are numbered as usual, but the number is reset at the start of
- each branch. The numbers of any capturing parentheses that follow the
- subpattern start after the highest number used in any branch. The fol-
+ theses are numbered as usual, but the number is reset at the start of
+ each branch. The numbers of any capturing parentheses that follow the
+ subpattern start after the highest number used in any branch. The fol-
lowing example is taken from the Perl documentation. The numbers under-
neath show in which buffer the captured content will be stored.
@@ -4469,58 +4489,58 @@
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4
- A back reference to a numbered subpattern uses the most recent value
- that is set for that number by any subpattern. The following pattern
+ A back reference to a numbered subpattern uses the most recent value
+ that is set for that number by any subpattern. The following pattern
matches "abcabc" or "defdef":
/(?|(abc)|(def))\1/
- In contrast, a subroutine call to a numbered subpattern always refers
- to the first one in the pattern with the given number. The following
+ In contrast, a subroutine call to a numbered subpattern always refers
+ to the first one in the pattern with the given number. The following
pattern matches "abcabc" or "defabc":
/(?|(abc)|(def))(?1)/
- If a condition test for a subpattern's having matched refers to a non-
- unique number, the test is true if any of the subpatterns of that num-
+ If a condition test for a subpattern's having matched refers to a non-
+ unique number, the test is true if any of the subpatterns of that num-
ber have matched.
- An alternative approach to using this "branch reset" feature is to use
+ An alternative approach to using this "branch reset" feature is to use
duplicate named subpatterns, as described in the next section.
NAMED SUBPATTERNS
- Identifying capturing parentheses by number is simple, but it can be
- very hard to keep track of the numbers in complicated regular expres-
- sions. Furthermore, if an expression is modified, the numbers may
- change. To help with this difficulty, PCRE supports the naming of sub-
+ Identifying capturing parentheses by number is simple, but it can be
+ very hard to keep track of the numbers in complicated regular expres-
+ sions. Furthermore, if an expression is modified, the numbers may
+ change. To help with this difficulty, PCRE supports the naming of sub-
patterns. This feature was not added to Perl until release 5.10. Python
- had the feature earlier, and PCRE introduced it at release 4.0, using
- the Python syntax. PCRE now supports both the Perl and the Python syn-
- tax. Perl allows identically numbered subpatterns to have different
+ had the feature earlier, and PCRE introduced it at release 4.0, using
+ the Python syntax. PCRE now supports both the Perl and the Python syn-
+ tax. Perl allows identically numbered subpatterns to have different
names, but PCRE does not.
- In PCRE, a subpattern can be named in one of three ways: (?<name>...)
- or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
- to capturing parentheses from other parts of the pattern, such as back
- references, recursion, and conditions, can be made by name as well as
+ In PCRE, a subpattern can be named in one of three ways: (?<name>...)
+ or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
+ to capturing parentheses from other parts of the pattern, such as back
+ references, recursion, and conditions, can be made by name as well as
by number.
- Names consist of up to 32 alphanumeric characters and underscores.
- Named capturing parentheses are still allocated numbers as well as
- names, exactly as if the names were not present. The PCRE API provides
+ Names consist of up to 32 alphanumeric characters and underscores.
+ Named capturing parentheses are still allocated numbers as well as
+ names, exactly as if the names were not present. The PCRE API provides
function calls for extracting the name-to-number translation table from
a compiled pattern. There is also a convenience function for extracting
a captured substring by name.
- By default, a name must be unique within a pattern, but it is possible
+ By default, a name must be unique within a pattern, but it is possible
to relax this constraint by setting the PCRE_DUPNAMES option at compile
- time. (Duplicate names are also always permitted for subpatterns with
- the same number, set up as described in the previous section.) Dupli-
- cate names can be useful for patterns where only one instance of the
- named parentheses can match. Suppose you want to match the name of a
- weekday, either as a 3-letter abbreviation or as the full name, and in
+ time. (Duplicate names are also always permitted for subpatterns with
+ the same number, set up as described in the previous section.) Dupli-
+ cate names can be useful for patterns where only one instance of the
+ named parentheses can match. Suppose you want to match the name of a
+ weekday, either as a 3-letter abbreviation or as the full name, and in
both cases you want to extract the abbreviation. This pattern (ignoring
the line breaks) does the job:
@@ -4530,38 +4550,38 @@
(?<DN>Thu)(?:rsday)?|
(?<DN>Sat)(?:urday)?
- There are five capturing substrings, but only one is ever set after a
+ There are five capturing substrings, but only one is ever set after a
match. (An alternative way of solving this problem is to use a "branch
reset" subpattern, as described in the previous section.)
- The convenience function for extracting the data by name returns the
- substring for the first (and in this example, the only) subpattern of
- that name that matched. This saves searching to find which numbered
+ The convenience function for extracting the data by name returns the
+ substring for the first (and in this example, the only) subpattern of
+ that name that matched. This saves searching to find which numbered
subpattern it was.
- If you make a back reference to a non-unique named subpattern from
- elsewhere in the pattern, the one that corresponds to the first occur-
+ If you make a back reference to a non-unique named subpattern from
+ elsewhere in the pattern, the one that corresponds to the first occur-
rence of the name is used. In the absence of duplicate numbers (see the
- previous section) this is the one with the lowest number. If you use a
- named reference in a condition test (see the section about conditions
- below), either to check whether a subpattern has matched, or to check
- for recursion, all subpatterns with the same name are tested. If the
- condition is true for any one of them, the overall condition is true.
+ previous section) this is the one with the lowest number. If you use a
+ named reference in a condition test (see the section about conditions
+ below), either to check whether a subpattern has matched, or to check
+ for recursion, all subpatterns with the same name are tested. If the
+ condition is true for any one of them, the overall condition is true.
This is the same behaviour as testing by number. For further details of
the interfaces for handling named subpatterns, see the pcreapi documen-
tation.
Warning: You cannot use different names to distinguish between two sub-
- patterns with the same number because PCRE uses only the numbers when
+ patterns with the same number because PCRE uses only the numbers when
matching. For this reason, an error is given at compile time if differ-
- ent names are given to subpatterns with the same number. However, you
- can give the same name to subpatterns with the same number, even when
+ ent names are given to subpatterns with the same number. However, you
+ can give the same name to subpatterns with the same number, even when
PCRE_DUPNAMES is not set.
REPETITION
- Repetition is specified by quantifiers, which can follow any of the
+ Repetition is specified by quantifiers, which can follow any of the
following items:
a literal data character
@@ -4575,17 +4595,17 @@
a parenthesized subpattern (including assertions)
a subroutine call to a subpattern (recursive or otherwise)
- The general repetition quantifier specifies a minimum and maximum num-
- ber of permitted matches, by giving the two numbers in curly brackets
- (braces), separated by a comma. The numbers must be less than 65536,
+ The general repetition quantifier specifies a minimum and maximum num-
+ ber of permitted matches, by giving the two numbers in curly brackets
+ (braces), separated by a comma. The numbers must be less than 65536,
and the first must be less than or equal to the second. For example:
z{2,4}
- matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
- special character. If the second number is omitted, but the comma is
- present, there is no upper limit; if the second number and the comma
- are both omitted, the quantifier specifies an exact number of required
+ matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
+ special character. If the second number is omitted, but the comma is
+ present, there is no upper limit; if the second number and the comma
+ are both omitted, the quantifier specifies an exact number of required
matches. Thus
[aeiou]{3,}
@@ -4594,50 +4614,50 @@
\d{8}
- matches exactly 8 digits. An opening curly bracket that appears in a
- position where a quantifier is not allowed, or one that does not match
- the syntax of a quantifier, is taken as a literal character. For exam-
+ matches exactly 8 digits. An opening curly bracket that appears in a
+ position where a quantifier is not allowed, or one that does not match
+ the syntax of a quantifier, is taken as a literal character. For exam-
ple, {,6} is not a quantifier, but a literal string of four characters.
- In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
+ In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
acters, each of which is represented by a two-byte sequence. Similarly,
when Unicode property support is available, \X{3} matches three Unicode
- extended sequences, each of which may be several bytes long (and they
+ extended sequences, each of which may be several bytes long (and they
may be of different lengths).
The quantifier {0} is permitted, causing the expression to behave as if
the previous item and the quantifier were not present. This may be use-
- ful for subpatterns that are referenced as subroutines from elsewhere
+ ful for subpatterns that are referenced as subroutines from elsewhere
in the pattern (but see also the section entitled "Defining subpatterns
- for use by reference only" below). Items other than subpatterns that
+ for use by reference only" below). Items other than subpatterns that
have a {0} quantifier are omitted from the compiled pattern.
- For convenience, the three most common quantifiers have single-charac-
+ For convenience, the three most common quantifiers have single-charac-
ter abbreviations:
* is equivalent to {0,}
+ is equivalent to {1,}
? is equivalent to {0,1}
- It is possible to construct infinite loops by following a subpattern
+ It is possible to construct infinite loops by following a subpattern
that can match no characters with a quantifier that has no upper limit,
for example:
(a?)*
Earlier versions of Perl and PCRE used to give an error at compile time
- for such patterns. However, because there are cases where this can be
- useful, such patterns are now accepted, but if any repetition of the
- subpattern does in fact match no characters, the loop is forcibly bro-
+ for such patterns. However, because there are cases where this can be
+ useful, such patterns are now accepted, but if any repetition of the
+ subpattern does in fact match no characters, the loop is forcibly bro-
ken.
- By default, the quantifiers are "greedy", that is, they match as much
- as possible (up to the maximum number of permitted times), without
- causing the rest of the pattern to fail. The classic example of where
+ By default, the quantifiers are "greedy", that is, they match as much
+ as possible (up to the maximum number of permitted times), without
+ causing the rest of the pattern to fail. The classic example of where
this gives problems is in trying to match comments in C programs. These
- appear between /* and */ and within the comment, individual * and /
- characters may appear. An attempt to match C comments by applying the
+ appear between /* and */ and within the comment, individual * and /
+ characters may appear. An attempt to match C comments by applying the
pattern
/\*.*\*/
@@ -4646,19 +4666,19 @@
/* first comment */ not comment /* second comment */
- fails, because it matches the entire string owing to the greediness of
+ fails, because it matches the entire string owing to the greediness of
the .* item.
- However, if a quantifier is followed by a question mark, it ceases to
+ However, if a quantifier is followed by a question mark, it ceases to
be greedy, and instead matches the minimum number of times possible, so
the pattern
/\*.*?\*/
- does the right thing with the C comments. The meaning of the various
- quantifiers is not otherwise changed, just the preferred number of
- matches. Do not confuse this use of question mark with its use as a
- quantifier in its own right. Because it has two uses, it can sometimes
+ does the right thing with the C comments. The meaning of the various
+ quantifiers is not otherwise changed, just the preferred number of
+ matches. Do not confuse this use of question mark with its use as a
+ quantifier in its own right. Because it has two uses, it can sometimes
appear doubled, as in
\d??\d
@@ -4666,36 +4686,36 @@
which matches one digit by preference, but can match two if that is the
only way the rest of the pattern matches.
- If the PCRE_UNGREEDY option is set (an option that is not available in
- Perl), the quantifiers are not greedy by default, but individual ones
- can be made greedy by following them with a question mark. In other
+ If the PCRE_UNGREEDY option is set (an option that is not available in
+ Perl), the quantifiers are not greedy by default, but individual ones
+ can be made greedy by following them with a question mark. In other
words, it inverts the default behaviour.
- When a parenthesized subpattern is quantified with a minimum repeat
- count that is greater than 1 or with a limited maximum, more memory is
- required for the compiled pattern, in proportion to the size of the
+ When a parenthesized subpattern is quantified with a minimum repeat
+ count that is greater than 1 or with a limited maximum, more memory is
+ required for the compiled pattern, in proportion to the size of the
minimum or maximum.
If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
- alent to Perl's /s) is set, thus allowing the dot to match newlines,
- the pattern is implicitly anchored, because whatever follows will be
- tried against every character position in the subject string, so there
- is no point in retrying the overall match at any position after the
- first. PCRE normally treats such a pattern as though it were preceded
+ alent to Perl's /s) is set, thus allowing the dot to match newlines,
+ the pattern is implicitly anchored, because whatever follows will be
+ tried against every character position in the subject string, so there
+ is no point in retrying the overall match at any position after the
+ first. PCRE normally treats such a pattern as though it were preceded
by \A.
- In cases where it is known that the subject string contains no new-
- lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
+ In cases where it is known that the subject string contains no new-
+ lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
mization, or alternatively using ^ to indicate anchoring explicitly.
- However, there is one situation where the optimization cannot be used.
+ However, there is one situation where the optimization cannot be used.
When .* is inside capturing parentheses that are the subject of a back
reference elsewhere in the pattern, a match at the start may fail where
a later one succeeds. Consider, for example:
(.*)abc\1
- If the subject is "xyz123abc123" the match point is the fourth charac-
+ If the subject is "xyz123abc123" the match point is the fourth charac-
ter. For this reason, such a pattern is not implicitly anchored.
When a capturing subpattern is repeated, the value captured is the sub-
@@ -4704,8 +4724,8 @@
(tweedle[dume]{3}\s*)+
has matched "tweedledum tweedledee" the value of the captured substring
- is "tweedledee". However, if there are nested capturing subpatterns,
- the corresponding captured values may have been set in previous itera-
+ is "tweedledee". However, if there are nested capturing subpatterns,
+ the corresponding captured values may have been set in previous itera-
tions. For example, after
/(a|(b))+/
@@ -4715,53 +4735,53 @@
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
- With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
- repetition, failure of what follows normally causes the repeated item
- to be re-evaluated to see if a different number of repeats allows the
- rest of the pattern to match. Sometimes it is useful to prevent this,
- either to change the nature of the match, or to cause it fail earlier
- than it otherwise might, when the author of the pattern knows there is
+ With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
+ repetition, failure of what follows normally causes the repeated item
+ to be re-evaluated to see if a different number of repeats allows the
+ rest of the pattern to match. Sometimes it is useful to prevent this,
+ either to change the nature of the match, or to cause it fail earlier
+ than it otherwise might, when the author of the pattern knows there is
no point in carrying on.
- Consider, for example, the pattern \d+foo when applied to the subject
+ Consider, for example, the pattern \d+foo when applied to the subject
line
123456bar
After matching all 6 digits and then failing to match "foo", the normal
- action of the matcher is to try again with only 5 digits matching the
- \d+ item, and then with 4, and so on, before ultimately failing.
- "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
- the means for specifying that once a subpattern has matched, it is not
+ action of the matcher is to try again with only 5 digits matching the
+ \d+ item, and then with 4, and so on, before ultimately failing.
+ "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
+ the means for specifying that once a subpattern has matched, it is not
to be re-evaluated in this way.
- If we use atomic grouping for the previous example, the matcher gives
- up immediately on failing to match "foo" the first time. The notation
+ If we use atomic grouping for the previous example, the matcher gives
+ up immediately on failing to match "foo" the first time. The notation
is a kind of special parenthesis, starting with (?> as in this example:
(?>\d+)foo
- This kind of parenthesis "locks up" the part of the pattern it con-
- tains once it has matched, and a failure further into the pattern is
- prevented from backtracking into it. Backtracking past it to previous
+ This kind of parenthesis "locks up" the part of the pattern it con-
+ tains once it has matched, and a failure further into the pattern is
+ prevented from backtracking into it. Backtracking past it to previous
items, however, works as normal.
- An alternative description is that a subpattern of this type matches
- the string of characters that an identical standalone pattern would
+ An alternative description is that a subpattern of this type matches
+ the string of characters that an identical standalone pattern would
match, if anchored at the current point in the subject string.
Atomic grouping subpatterns are not capturing subpatterns. Simple cases
such as the above example can be thought of as a maximizing repeat that
- must swallow everything it can. So, while both \d+ and \d+? are pre-
- pared to adjust the number of digits they match in order to make the
+ must swallow everything it can. So, while both \d+ and \d+? are pre-
+ pared to adjust the number of digits they match in order to make the
rest of the pattern match, (?>\d+) can only match an entire sequence of
digits.
- Atomic groups in general can of course contain arbitrarily complicated
- subpatterns, and can be nested. However, when the subpattern for an
+ Atomic groups in general can of course contain arbitrarily complicated
+ subpatterns, and can be nested. However, when the subpattern for an
atomic group is just a single repeated item, as in the example above, a
- simpler notation, called a "possessive quantifier" can be used. This
- consists of an additional + character following a quantifier. Using
+ simpler notation, called a "possessive quantifier" can be used. This
+ consists of an additional + character following a quantifier. Using
this notation, the previous example can be rewritten as
\d++foo
@@ -4771,45 +4791,45 @@
(abc|xyz){2,3}+
- Possessive quantifiers are always greedy; the setting of the
+ 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 quantifier and the equivalent atomic group,
- though there may be a performance difference; possessive quantifiers
+ simpler forms of atomic group. However, there is no difference in the
+ meaning of a possessive quantifier and the equivalent atomic group,
+ though there may be a performance difference; possessive quantifiers
should be slightly faster.
- The possessive quantifier syntax is an extension to the Perl 5.8 syn-
- tax. Jeffrey Friedl originated the idea (and the name) in the first
+ The possessive quantifier syntax is an extension to the Perl 5.8 syn-
+ tax. Jeffrey Friedl originated the idea (and the name) in the first
edition of his book. Mike McCloskey liked it, so implemented it when he
- built Sun's Java package, and PCRE copied it from there. It ultimately
+ built Sun's Java package, and PCRE copied it from there. It ultimately
found its way into Perl at release 5.10.
PCRE has an optimization that automatically "possessifies" certain sim-
- ple pattern constructs. For example, the sequence A+B is treated as
- A++B because there is no point in backtracking into a sequence of A's
+ ple pattern constructs. For example, the sequence A+B is treated as
+ A++B because there is no point in backtracking into a sequence of A's
when B must follow.
- When a pattern contains an unlimited repeat inside a subpattern that
- can itself be repeated an unlimited number of times, the use of an
- atomic group is the only way to avoid some failing matches taking a
+ When a pattern contains an unlimited repeat inside a subpattern that
+ can itself be repeated an unlimited number of times, the use of an
+ atomic group is the only way to avoid some failing matches taking a
very long time indeed. The pattern
(\D+|<\d+>)*[!?]
- matches an unlimited number of substrings that either consist of non-
- digits, or digits enclosed in <>, followed by either ! or ?. When it
+ matches an unlimited number of substrings that either consist of non-
+ digits, or digits enclosed in <>, followed by either ! or ?. When it
matches, it runs quickly. However, if it is applied to
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
- it takes a long time before reporting failure. This is because the
- string can be divided between the internal \D+ repeat and the external
- * repeat in a large number of ways, and all have to be tried. (The
- example uses [!?] rather than a single character at the end, because
- both PCRE and Perl have an optimization that allows for fast failure
- when a single character is used. They remember the last single charac-
- ter that is required for a match, and fail early if it is not present
- in the string.) If the pattern is changed so that it uses an atomic
+ it takes a long time before reporting failure. This is because the
+ string can be divided between the internal \D+ repeat and the external
+ * repeat in a large number of ways, and all have to be tried. (The
+ example uses [!?] rather than a single character at the end, because
+ both PCRE and Perl have an optimization that allows for fast failure
+ when a single character is used. They remember the last single charac-
+ ter that is required for a match, and fail early if it is not present
+ in the string.) If the pattern is changed so that it uses an atomic
group, like this:
((?>\D+)|<\d+>)*[!?]
@@ -4821,28 +4841,28 @@
Outside a character class, a backslash followed by a digit greater than
0 (and possibly further digits) is a back reference to a capturing sub-
- pattern earlier (that is, to its left) in the pattern, provided there
+ pattern earlier (that is, to its left) in the pattern, provided there
have been that many previous capturing left parentheses.
However, if the decimal number following the backslash is less than 10,
- it is always taken as a back reference, and causes an error only if
- there are not that many capturing left parentheses in the entire pat-
- tern. In other words, the parentheses that are referenced need not be
- to the left of the reference for numbers less than 10. A "forward back
- reference" of this type can make sense when a repetition is involved
- and the subpattern to the right has participated in an earlier itera-
+ it is always taken as a back reference, and causes an error only if
+ there are not that many capturing left parentheses in the entire pat-
+ tern. In other words, the parentheses that are referenced need not be
+ to the left of the reference for numbers less than 10. A "forward back
+ reference" of this type can make sense when a repetition is involved
+ and the subpattern to the right has participated in an earlier itera-
tion.
- It is not possible to have a numerical "forward back reference" to a
- subpattern whose number is 10 or more using this syntax because a
- sequence such as \50 is interpreted as a character defined in octal.
+ It is not possible to have a numerical "forward back reference" to a
+ subpattern whose number is 10 or more using this syntax because a
+ sequence such as \50 is interpreted as a character defined in octal.
See the subsection entitled "Non-printing characters" above for further
- details of the handling of digits following a backslash. There is no
- such problem when named parentheses are used. A back reference to any
+ details of the handling of digits following a backslash. There is no
+ such problem when named parentheses are used. A back reference to any
subpattern is possible using named parentheses (see below).
- Another way of avoiding the ambiguity inherent in the use of digits
- following a backslash is to use the \g escape sequence. This escape
+ Another way of avoiding the ambiguity inherent in the use of digits
+ following a backslash is to use the \g escape sequence. This escape
must be followed by an unsigned number or a negative number, optionally
enclosed in braces. These examples are all identical:
@@ -4850,7 +4870,7 @@
(ring), \g1
(ring), \g{1}
- An unsigned number specifies an absolute reference without the ambigu-
+ An unsigned number specifies an absolute reference without the ambigu-
ity that is present in the older syntax. It is also useful when literal
digits follow the reference. A negative number is a relative reference.
Consider this example:
@@ -4859,33 +4879,33 @@
The sequence \g{-1} is a reference to the most recently started captur-
ing subpattern before \g, that is, is it equivalent to \2 in this exam-
- ple. Similarly, \g{-2} would be equivalent to \1. The use of relative
- references can be helpful in long patterns, and also in patterns that
- are created by joining together fragments that contain references
+ ple. Similarly, \g{-2} would be equivalent to \1. The use of relative
+ references can be helpful in long patterns, and also in patterns that
+ are created by joining together fragments that contain references
within themselves.
- A back reference matches whatever actually matched the capturing sub-
- pattern in the current subject string, rather than anything matching
+ A back reference matches whatever actually matched the capturing sub-
+ pattern in the current subject string, rather than anything matching
the subpattern itself (see "Subpatterns as subroutines" below for a way
of doing that). So the pattern
(sens|respons)e and \1ibility
- matches "sense and sensibility" and "response and responsibility", but
- not "sense and responsibility". If caseful matching is in force at the
- time of the back reference, the case of letters is relevant. For exam-
+ matches "sense and sensibility" and "response and responsibility", but
+ not "sense and responsibility". If caseful matching is in force at the
+ time of the back reference, the case of letters is relevant. For exam-
ple,
((?i)rah)\s+\1
- matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
+ matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
original capturing subpattern is matched caselessly.
- There are several different ways of writing back references to named
- subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
- \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
+ There are several different ways of writing back references to named
+ subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
+ \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
unified back reference syntax, in which \g can be used for both numeric
- and named references, is also supported. We could rewrite the above
+ and named references, is also supported. We could rewrite the above
example in any of the following ways:
(?<p1>(?i)rah)\s+\k<p1>
@@ -4893,83 +4913,83 @@
(?P<p1>(?i)rah)\s+(?P=p1)
(?<p1>(?i)rah)\s+\g{p1}
- A subpattern that is referenced by name may appear in the pattern
+ A subpattern that is referenced by name may appear in the pattern
before or after the reference.
- There may be more than one back reference to the same subpattern. If a
- subpattern has not actually been used in a particular match, any back
+ There may be more than one back reference to the same subpattern. If a
+ subpattern has not actually been used in a particular match, any back
references to it always fail by default. For example, the pattern
(a|(bc))\2
- always fails if it starts to match "a" rather than "bc". However, if
+ always fails if it starts to match "a" rather than "bc". However, if
the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer-
ence to an unset value matches an empty string.
- Because there may be many capturing parentheses in a pattern, all dig-
- its following a backslash are taken as part of a potential back refer-
- ence number. If the pattern continues with a digit character, some
- delimiter must be used to terminate the back reference. If the
+ Because there may be many capturing parentheses in a pattern, all dig-
+ its following a backslash are taken as part of a potential back refer-
+ ence number. If the pattern continues with a digit character, some
+ delimiter must be used to terminate the back reference. If the
PCRE_EXTENDED option is set, this can be whitespace. Otherwise, the \g{
syntax or an empty comment (see "Comments" below) can be used.
Recursive back references
- A back reference that occurs inside the parentheses to which it refers
- fails when the subpattern is first used, so, for example, (a\1) never
- matches. However, such references can be useful inside repeated sub-
+ A back reference that occurs inside the parentheses to which it refers
+ fails when the subpattern is first used, so, for example, (a\1) never
+ matches. However, such references can be useful inside repeated sub-
patterns. For example, the pattern
(a|b\1)+
matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
- ation of the subpattern, the back reference matches the character
- string corresponding to the previous iteration. In order for this to
- work, the pattern must be such that the first iteration does not need
- to match the back reference. This can be done using alternation, as in
+ ation of the subpattern, the back reference matches the character
+ string corresponding to the previous iteration. In order for this to
+ work, the pattern must be such that the first iteration does not need
+ to match the back reference. This can be done using alternation, as in
the example above, or by a quantifier with a minimum of zero.
- Back references of this type cause the group that they reference to be
- treated as an atomic group. Once the whole group has been matched, a
- subsequent matching failure cannot cause backtracking into the middle
+ Back references of this type cause the group that they reference to be
+ treated as an atomic group. Once the whole group has been matched, a
+ subsequent matching failure cannot cause backtracking into the middle
of the group.
ASSERTIONS
- An assertion is a test on the characters following or preceding the
- current matching point that does not actually consume any characters.
- The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
+ An assertion is a test on the characters following or preceding the
+ current matching point that does not actually consume any characters.
+ The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
described above.
- More complicated assertions are coded as subpatterns. There are two
- kinds: those that look ahead of the current position in the subject
- string, and those that look behind it. An assertion subpattern is
- matched in the normal way, except that it does not cause the current
+ More complicated assertions are coded as subpatterns. There are two
+ kinds: those that look ahead of the current position in the subject
+ string, and those that look behind it. An assertion subpattern is
+ matched in the normal way, except that it does not cause the current
matching position to be changed.
- Assertion subpatterns are not capturing subpatterns. If such an asser-
- tion contains capturing subpatterns within it, these are counted for
- the purposes of numbering the capturing subpatterns in the whole pat-
- tern. However, substring capturing is carried out only for positive
+ Assertion subpatterns are not capturing subpatterns. If such an asser-
+ tion contains capturing subpatterns within it, these are counted for
+ the purposes of numbering the capturing subpatterns in the whole pat-
+ tern. However, substring capturing is carried out only for positive
assertions, because it does not make sense for negative assertions.
- For compatibility with Perl, assertion subpatterns may be repeated;
- though it makes no sense to assert the same thing several times, the
- side effect of capturing parentheses may occasionally be useful. In
+ For compatibility with Perl, assertion subpatterns may be repeated;
+ though it makes no sense to assert the same thing several times, the
+ side effect of capturing parentheses may occasionally be useful. In
practice, there only three cases:
- (1) If the quantifier is {0}, the assertion is never obeyed during
- matching. However, it may contain internal capturing parenthesized
+ (1) If the quantifier is {0}, the assertion is never obeyed during
+ matching. However, it may contain internal capturing parenthesized
groups that are called from elsewhere via the subroutine mechanism.
- (2) If quantifier is {0,n} where n is greater than zero, it is treated
- as if it were {0,1}. At run time, the rest of the pattern match is
+ (2) If quantifier is {0,n} where n is greater than zero, it is treated
+ as if it were {0,1}. At run time, the rest of the pattern match is
tried with and without the assertion, the order depending on the greed-
iness of the quantifier.
- (3) If the minimum repetition is greater than zero, the quantifier is
- ignored. The assertion is obeyed just once when encountered during
+ (3) If the minimum repetition is greater than zero, the quantifier is
+ ignored. The assertion is obeyed just once when encountered during
matching.
Lookahead assertions
@@ -4979,38 +4999,38 @@
\w+(?=;)
- matches a word followed by a semicolon, but does not include the semi-
+ matches a word followed by a semicolon, but does not include the semi-
colon in the match, and
foo(?!bar)
- matches any occurrence of "foo" that is not followed by "bar". Note
+ matches any occurrence of "foo" that is not followed by "bar". Note
that the apparently similar pattern
(?!foo)bar
- does not find an occurrence of "bar" that is preceded by something
- other than "foo"; it finds any occurrence of "bar" whatsoever, because
+ does not find an occurrence of "bar" that is preceded by something
+ other than "foo"; it finds any occurrence of "bar" whatsoever, because
the assertion (?!foo) is always true when the next three characters are
"bar". A lookbehind assertion is needed to achieve the other effect.
If you want to force a matching failure at some point in a pattern, the
- most convenient way to do it is with (?!) because an empty string
- always matches, so an assertion that requires there not to be an empty
+ most convenient way to do it is with (?!) because an empty string
+ always matches, so an assertion that requires there not to be an empty
string must always fail. The backtracking control verb (*FAIL) or (*F)
is a synonym for (?!).
Lookbehind assertions
- Lookbehind assertions start with (?<= for positive assertions and (?<!
+ Lookbehind assertions start with (?<= for positive assertions and (?<!
for negative assertions. For example,
(?<!foo)bar
- does find an occurrence of "bar" that is not preceded by "foo". The
- contents of a lookbehind assertion are restricted such that all the
+ does find an occurrence of "bar" that is not preceded by "foo". The
+ contents of a lookbehind assertion are restricted such that all the
strings it matches must have a fixed length. However, if there are sev-
- eral top-level alternatives, they do not all have to have the same
+ eral top-level alternatives, they do not all have to have the same
fixed length. Thus
(?<=bullock|donkey)
@@ -5019,61 +5039,61 @@
(?<!dogs?|cats?)
- causes an error at compile time. Branches that match different length
- strings are permitted only at the top level of a lookbehind assertion.
+ causes an error at compile time. Branches that match different length
+ strings are permitted only at the top level of a lookbehind assertion.
This is an extension compared with Perl, which requires all branches to
match the same length of string. An assertion such as
(?<=ab(c|de))
- is not permitted, because its single top-level branch can match two
+ is not permitted, because its single top-level branch can match two
different lengths, but it is acceptable to PCRE if rewritten to use two
top-level branches:
(?<=abc|abde)
- In some cases, the escape sequence \K (see above) can be used instead
+ In some cases, the escape sequence \K (see above) can be used instead
of a lookbehind assertion to get round the fixed-length restriction.
- The implementation of lookbehind assertions is, for each alternative,
- to temporarily move the current position back by the fixed length and
+ The implementation of lookbehind assertions is, for each alternative,
+ to temporarily move the current position back by the fixed length and
then try to match. If there are insufficient characters before the cur-
rent position, the assertion fails.
PCRE does not allow the \C escape (which matches a single byte in UTF-8
- mode) to appear in lookbehind assertions, because it makes it impossi-
- ble to calculate the length of the lookbehind. The \X and \R escapes,
+ mode) to appear in lookbehind assertions, because it makes it impossi-
+ ble to calculate the length of the lookbehind. The \X and \R escapes,
which can match different numbers of bytes, are also not permitted.
- "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
- lookbehinds, as long as the subpattern matches a fixed-length string.
+ "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
+ lookbehinds, as long as the subpattern matches a fixed-length string.
Recursion, however, is not supported.
- Possessive quantifiers can be used in conjunction with lookbehind
+ Possessive quantifiers can be used in conjunction with lookbehind
assertions to specify efficient matching of fixed-length strings at the
end of subject strings. Consider a simple pattern such as
abcd$
- when applied to a long string that does not match. Because matching
+ when applied to a long string that does not match. Because matching
proceeds from left to right, PCRE will look for each "a" in the subject
- and then see if what follows matches the rest of the pattern. If the
+ and then see if what follows matches the rest of the pattern. If the
pattern is specified as
^.*abcd$
- the initial .* matches the entire string at first, but when this fails
+ the initial .* matches the entire string at first, but when this fails
(because there is no following "a"), it backtracks to match all but the
- last character, then all but the last two characters, and so on. Once
- again the search for "a" covers the entire string, from right to left,
+ last character, then all but the last two characters, and so on. Once
+ again the search for "a" covers the entire string, from right to left,
so we are no better off. However, if the pattern is written as
^.*+(?<=abcd)
- there can be no backtracking for the .*+ item; it can match only the
- entire string. The subsequent lookbehind assertion does a single test
- on the last four characters. If it fails, the match fails immediately.
- For long strings, this approach makes a significant difference to the
+ there can be no backtracking for the .*+ item; it can match only the
+ entire string. The subsequent lookbehind assertion does a single test
+ on the last four characters. If it fails, the match fails immediately.
+ For long strings, this approach makes a significant difference to the
processing time.
Using multiple assertions
@@ -5082,18 +5102,18 @@
(?<=\d{3})(?<!999)foo
- matches "foo" preceded by three digits that are not "999". Notice that
- each of the assertions is applied independently at the same point in
- the subject string. First there is a check that the previous three
- characters are all digits, and then there is a check that the same
+ matches "foo" preceded by three digits that are not "999". Notice that
+ each of the assertions is applied independently at the same point in
+ the subject string. First there is a check that the previous three
+ characters are all digits, and then there is a check that the same
three characters are not "999". This pattern does not match "foo" pre-
- ceded by six characters, the first of which are digits and the last
- three of which are not "999". For example, it doesn't match "123abc-
+ ceded by six characters, the first of which are digits and the last
+ three of which are not "999". For example, it doesn't match "123abc-
foo". A pattern to do that is
(?<=\d{3}...)(?<!999)foo
- This time the first assertion looks at the preceding six characters,
+ This time the first assertion looks at the preceding six characters,
checking that the first three are digits, and then the second assertion
checks that the preceding three characters are not "999".
@@ -5101,29 +5121,29 @@
(?<=(?<!foo)bar)baz
- matches an occurrence of "baz" that is preceded by "bar" which in turn
+ matches an occurrence of "baz" that is preceded by "bar" which in turn
is not preceded by "foo", while
(?<=\d{3}(?!999)...)foo
- is another pattern that matches "foo" preceded by three digits and any
+ is another pattern that matches "foo" preceded by three digits and any
three characters that are not "999".
CONDITIONAL SUBPATTERNS
- It is possible to cause the matching process to obey a subpattern con-
- ditionally or to choose between two alternative subpatterns, depending
- on the result of an assertion, or whether a specific capturing subpat-
- tern has already been matched. The two possible forms of conditional
+ It is possible to cause the matching process to obey a subpattern con-
+ ditionally or to choose between two alternative subpatterns, depending
+ on the result of an assertion, or whether a specific capturing subpat-
+ tern has already been matched. The two possible forms of conditional
subpattern are:
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
- If the condition is satisfied, the yes-pattern is used; otherwise the
- no-pattern (if present) is used. If there are more than two alterna-
- tives in the subpattern, a compile-time error occurs. Each of the two
+ If the condition is satisfied, the yes-pattern is used; otherwise the
+ no-pattern (if present) is used. If there are more than two alterna-
+ tives in the subpattern, a compile-time error occurs. Each of the two
alternatives may itself contain nested subpatterns of any form, includ-
ing conditional subpatterns; the restriction to two alternatives
applies only at the level of the condition. This pattern fragment is an
@@ -5132,73 +5152,73 @@
(?(1) (A|B|C) | (D | (?(2)E|F) | E) )
- There are four kinds of condition: references to subpatterns, refer-
+ There are four kinds of condition: references to subpatterns, refer-
ences to recursion, a pseudo-condition called DEFINE, and assertions.
Checking for a used subpattern by number
- If the text between the parentheses consists of a sequence of digits,
+ If the text between the parentheses consists of a sequence of digits,
the condition is true if a capturing subpattern of that number has pre-
- viously matched. If there is more than one capturing subpattern with
- the same number (see the earlier section about duplicate subpattern
- numbers), the condition is true if any of them have matched. An alter-
- native notation is to precede the digits with a plus or minus sign. In
- this case, the subpattern number is relative rather than absolute. The
- most recently opened parentheses can be referenced by (?(-1), the next
- most recent by (?(-2), and so on. Inside loops it can also make sense
+ viously matched. If there is more than one capturing subpattern with
+ the same number (see the earlier section about duplicate subpattern
+ numbers), the condition is true if any of them have matched. An alter-
+ native notation is to precede the digits with a plus or minus sign. In
+ this case, the subpattern number is relative rather than absolute. The
+ most recently opened parentheses can be referenced by (?(-1), the next
+ most recent by (?(-2), and so on. Inside loops it can also make sense
to refer to subsequent groups. The next parentheses to be opened can be
- referenced as (?(+1), and so on. (The value zero in any of these forms
+ referenced as (?(+1), and so on. (The value zero in any of these forms
is not used; it provokes a compile-time error.)
- Consider the following pattern, which contains non-significant white
+ Consider the following pattern, which contains non-significant white
space to make it more readable (assume the PCRE_EXTENDED option) and to
divide it into three parts for ease of discussion:
( \( )? [^()]+ (?(1) \) )
- The first part matches an optional opening parenthesis, and if that
+ The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The sec-
- ond part matches one or more characters that are not parentheses. The
- third part is a conditional subpattern that tests whether or not the
- first set of parentheses matched. If they did, that is, if subject
- started with an opening parenthesis, the condition is true, and so the
- yes-pattern is executed and a closing parenthesis is required. Other-
- wise, since no-pattern is not present, the subpattern matches nothing.
- In other words, this pattern matches a sequence of non-parentheses,
+ ond part matches one or more characters that are not parentheses. The
+ third part is a conditional subpattern that tests whether or not the
+ first set of parentheses matched. If they did, that is, if subject
+ started with an opening parenthesis, the condition is true, and so the
+ yes-pattern is executed and a closing parenthesis is required. Other-
+ wise, since no-pattern is not present, the subpattern matches nothing.
+ In other words, this pattern matches a sequence of non-parentheses,
optionally enclosed in parentheses.
- If you were embedding this pattern in a larger one, you could use a
+ If you were embedding this pattern in a larger one, you could use a
relative reference:
...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
- This makes the fragment independent of the parentheses in the larger
+ This makes the fragment independent of the parentheses in the larger
pattern.
Checking for a used subpattern by name
- Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
- used subpattern by name. For compatibility with earlier versions of
- PCRE, which had this facility before Perl, the syntax (?(name)...) is
- also recognized. However, there is a possible ambiguity with this syn-
- tax, because subpattern names may consist entirely of digits. PCRE
- looks first for a named subpattern; if it cannot find one and the name
- consists entirely of digits, PCRE looks for a subpattern of that num-
- ber, which must be greater than zero. Using subpattern names that con-
+ Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
+ used subpattern by name. For compatibility with earlier versions of
+ PCRE, which had this facility before Perl, the syntax (?(name)...) is
+ also recognized. However, there is a possible ambiguity with this syn-
+ tax, because subpattern names may consist entirely of digits. PCRE
+ looks first for a named subpattern; if it cannot find one and the name
+ consists entirely of digits, PCRE looks for a subpattern of that num-
+ ber, which must be greater than zero. Using subpattern names that con-
sist entirely of digits is not recommended.
Rewriting the above example to use a named subpattern gives this:
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
- If the name used in a condition of this kind is a duplicate, the test
- is applied to all subpatterns of the same name, and is true if any one
+ If the name used in a condition of this kind is a duplicate, the test
+ is applied to all subpatterns of the same name, and is true if any one
of them has matched.
Checking for pattern recursion
If the condition is the string (R), and there is no subpattern with the
- name R, the condition is true if a recursive call to the whole pattern
+ name R, the condition is true if a recursive call to the whole pattern
or any subpattern has been made. If digits or a name preceded by amper-
sand follow the letter R, for example:
@@ -5206,51 +5226,51 @@
the condition is true if the most recent recursion is into a subpattern
whose number or name is given. This condition does not check the entire
- recursion stack. If the name used in a condition of this kind is a
+ recursion stack. If the name used in a condition of this kind is a
duplicate, the test is applied to all subpatterns of the same name, and
is true if any one of them is the most recent recursion.
- At "top level", all these recursion test conditions are false. The
+ At "top level", all these recursion test conditions are false. The
syntax for recursive patterns is described below.
Defining subpatterns for use by reference only
- If the condition is the string (DEFINE), and there is no subpattern
- with the name DEFINE, the condition is always false. In this case,
- there may be only one alternative in the subpattern. It is always
- skipped if control reaches this point in the pattern; the idea of
- DEFINE is that it can be used to define subroutines that can be refer-
- enced from elsewhere. (The use of subroutines is described below.) For
- example, a pattern to match an IPv4 address such as "192.168.23.245"
+ If the condition is the string (DEFINE), and there is no subpattern
+ with the name DEFINE, the condition is always false. In this case,
+ there may be only one alternative in the subpattern. It is always
+ skipped if control reaches this point in the pattern; the idea of
+ DEFINE is that it can be used to define subroutines that can be refer-
+ enced from elsewhere. (The use of subroutines is described below.) For
+ example, a pattern to match an IPv4 address such as "192.168.23.245"
could be written like this (ignore whitespace and line breaks):
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
\b (?&byte) (\.(?&byte)){3} \b
- The first part of the pattern is a DEFINE group inside which a another
- group named "byte" is defined. This matches an individual component of
- an IPv4 address (a number less than 256). When matching takes place,
- this part of the pattern is skipped because DEFINE acts like a false
- condition. The rest of the pattern uses references to the named group
- to match the four dot-separated components of an IPv4 address, insist-
+ The first part of the pattern is a DEFINE group inside which a another
+ group named "byte" is defined. This matches an individual component of
+ an IPv4 address (a number less than 256). When matching takes place,
+ this part of the pattern is skipped because DEFINE acts like a false
+ condition. The rest of the pattern uses references to the named group
+ to match the four dot-separated components of an IPv4 address, insist-
ing on a word boundary at each end.
Assertion conditions
- If the condition is not in any of the above formats, it must be an
- assertion. This may be a positive or negative lookahead or lookbehind
- assertion. Consider this pattern, again containing non-significant
+ If the condition is not in any of the above formats, it must be an
+ assertion. This may be a positive or negative lookahead or lookbehind
+ assertion. Consider this pattern, again containing non-significant
white space, and with the two alternatives on the second line:
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
- The condition is a positive lookahead assertion that matches an
- optional sequence of non-letters followed by a letter. In other words,
- it tests for the presence of at least one letter in the subject. If a
- letter is found, the subject is matched against the first alternative;
- otherwise it is matched against the second. This pattern matches
- strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
+ The condition is a positive lookahead assertion that matches an
+ optional sequence of non-letters followed by a letter. In other words,
+ it tests for the presence of at least one letter in the subject. If a
+ letter is found, the subject is matched against the first alternative;
+ otherwise it is matched against the second. This pattern matches
+ strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
letters and dd are digits.
@@ -5259,41 +5279,41 @@
There are two ways of including comments in patterns that are processed
by PCRE. In both cases, the start of the comment must not be in a char-
acter class, nor in the middle of any other sequence of related charac-
- ters such as (?: or a subpattern name or number. The characters that
+ ters such as (?: or a subpattern name or number. The characters that
make up a comment play no part in the pattern matching.
- The sequence (?# marks the start of a comment that continues up to the
- next closing parenthesis. Nested parentheses are not permitted. If the
+ The sequence (?# marks the start of a comment that continues up to the
+ next closing parenthesis. Nested parentheses are not permitted. If the
PCRE_EXTENDED option is set, an unescaped # character also introduces a
- comment, which in this case continues to immediately after the next
- newline character or character sequence in the pattern. Which charac-
+ comment, which in this case continues to immediately after the next
+ newline character or character sequence in the pattern. Which charac-
ters are interpreted as newlines is controlled by the options passed to
pcre_compile() or by a special sequence at the start of the pattern, as
- described in the section entitled "Newline conventions" above. Note
- that the end of this type of comment is a literal newline sequence in
+ described in the section entitled "Newline conventions" above. Note
+ that the end of this type of comment is a literal newline sequence in
the pattern; escape sequences that happen to represent a newline do not
- count. For example, consider this pattern when PCRE_EXTENDED is set,
+ count. For example, consider this pattern when PCRE_EXTENDED is set,
and the default newline convention is in force:
abc #comment \n still comment
- On encountering the # character, pcre_compile() skips along, looking
- for a newline in the pattern. The sequence \n is still literal at this
- stage, so it does not terminate the comment. Only an actual character
+ On encountering the # character, pcre_compile() skips along, looking
+ for a newline in the pattern. The sequence \n is still literal at this
+ stage, so it does not terminate the comment. Only an actual character
with the code value 0x0a (the default newline) does so.
RECURSIVE PATTERNS
- Consider the problem of matching a string in parentheses, allowing for
- unlimited nested parentheses. Without the use of recursion, the best
- that can be done is to use a pattern that matches up to some fixed
- depth of nesting. It is not possible to handle an arbitrary nesting
+ Consider the problem of matching a string in parentheses, allowing for
+ unlimited nested parentheses. Without the use of recursion, the best
+ that can be done is to use a pattern that matches up to some fixed
+ depth of nesting. It is not possible to handle an arbitrary nesting
depth.
For some time, Perl has provided a facility that allows regular expres-
- sions to recurse (amongst other things). It does this by interpolating
- Perl code in the expression at run time, and the code can refer to the
+ sions to recurse (amongst other things). It does this by interpolating
+ Perl code in the expression at run time, and the code can refer to the
expression itself. A Perl pattern using code interpolation to solve the
parentheses problem can be created like this:
@@ -5303,201 +5323,201 @@
refers recursively to the pattern in which it appears.
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 subsequently introduced
+ 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 subsequently introduced
into Perl at release 5.10.
- A special item that consists of (? followed by a number greater than
- zero and a closing parenthesis is a recursive subroutine call of the
- subpattern of the given number, provided that it occurs inside that
- subpattern. (If not, it is a non-recursive subroutine call, which is
- described in the next section.) The special item (?R) or (?0) is a
+ A special item that consists of (? followed by a number greater than
+ zero and a closing parenthesis is a recursive subroutine call of the
+ subpattern of the given number, provided that it occurs inside that
+ subpattern. (If not, it is a non-recursive subroutine call, which is
+ described in the next section.) The special item (?R) or (?0) is a
recursive call of the entire regular expression.
- This PCRE pattern solves the nested parentheses problem (assume the
+ This PCRE pattern solves the nested parentheses problem (assume the
PCRE_EXTENDED option is set so that white space is ignored):
\( ( [^()]++ | (?R) )* \)
- First it matches an opening parenthesis. Then it matches any number of
- substrings which can either be a sequence of non-parentheses, or a
- recursive match of the pattern itself (that is, a correctly parenthe-
+ First it matches an opening parenthesis. Then it matches any number of
+ substrings which can either be a sequence of non-parentheses, or a
+ recursive match of the pattern itself (that is, a correctly parenthe-
sized substring). Finally there is a closing parenthesis. Note the use
of a possessive quantifier to avoid backtracking into sequences of non-
parentheses.
- If this were part of a larger pattern, you would not want to recurse
+ If this were part of a larger pattern, you would not want to recurse
the entire pattern, so instead you could use this:
( \( ( [^()]++ | (?1) )* \) )
- We have put the pattern into parentheses, and caused the recursion to
+ We have put the pattern into parentheses, and caused the recursion to
refer to them instead of the whole pattern.
- In a larger pattern, keeping track of parenthesis numbers can be
- tricky. This is made easier by the use of relative references. Instead
+ In a larger pattern, keeping track of parenthesis numbers can be
+ tricky. This is made easier by the use of relative references. Instead
of (?1) in the pattern above you can write (?-2) to refer to the second
- most recently opened parentheses preceding the recursion. In other
- words, a negative number counts capturing parentheses leftwards from
+ most recently opened parentheses preceding the recursion. In other
+ words, a negative number counts capturing parentheses leftwards from
the point at which it is encountered.
- It is also possible to refer to subsequently opened parentheses, by
- writing references such as (?+2). However, these cannot be recursive
- because the reference is not inside the parentheses that are refer-
- enced. They are always non-recursive subroutine calls, as described in
+ It is also possible to refer to subsequently opened parentheses, by
+ writing references such as (?+2). However, these cannot be recursive
+ because the reference is not inside the parentheses that are refer-
+ enced. They are always non-recursive subroutine calls, as described in
the next section.
- An alternative approach is to use named parentheses instead. The Perl
- syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
+ An alternative approach is to use named parentheses instead. The Perl
+ syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
supported. We could rewrite the above example as follows:
(?<pn> \( ( [^()]++ | (?&pn) )* \) )
- If there is more than one subpattern with the same name, the earliest
+ If there is more than one subpattern with the same name, the earliest
one is used.
- This particular example pattern that we have been looking at contains
+ This particular example pattern that we have been looking at contains
nested unlimited repeats, and so the use of a possessive quantifier for
matching strings of non-parentheses is important when applying the pat-
- tern to strings that do not match. For example, when this pattern is
+ tern to strings that do not match. For example, when this pattern is
applied to
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
- it yields "no match" quickly. However, if a possessive quantifier is
- not used, the match runs for a very long time indeed because there are
- so many different ways the + and * repeats can carve up the subject,
+ it yields "no match" quickly. However, if a possessive quantifier is
+ not used, the match runs for a very long time indeed because there are
+ so many different ways the + and * repeats can carve up the subject,
and all have to be tested before failure can be reported.
- At the end of a match, the values of capturing parentheses are those
- from the outermost level. If you want to obtain intermediate values, a
- callout function can be used (see below and the pcrecallout documenta-
+ At the end of a match, the values of capturing parentheses are those
+ from the outermost level. If you want to obtain intermediate values, a
+ callout function can be used (see below and the pcrecallout documenta-
tion). If the pattern above is matched against
(ab(cd)ef)
- the value for the inner capturing parentheses (numbered 2) is "ef",
- which is the last value taken on at the top level. If a capturing sub-
- pattern is not matched at the top level, its final captured value is
- unset, even if it was (temporarily) set at a deeper level during the
+ the value for the inner capturing parentheses (numbered 2) is "ef",
+ which is the last value taken on at the top level. If a capturing sub-
+ pattern is not matched at the top level, its final captured value is
+ unset, even if it was (temporarily) set at a deeper level during the
matching process.
- If there are more than 15 capturing parentheses in a pattern, PCRE has
- to obtain extra memory to store data during a recursion, which it does
+ If there are more than 15 capturing parentheses in a pattern, PCRE has
+ to obtain extra memory to store data during a recursion, which it does
by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
- Do not confuse the (?R) item with the condition (R), which tests for
- recursion. Consider this pattern, which matches text in angle brack-
- ets, allowing for arbitrary nesting. Only digits are allowed in nested
- brackets (that is, when recursing), whereas any characters are permit-
+ Do not confuse the (?R) item with the condition (R), which tests for
+ recursion. Consider this pattern, which matches text in angle brack-
+ ets, allowing for arbitrary nesting. Only digits are allowed in nested
+ brackets (that is, when recursing), whereas any characters are permit-
ted at the outer level.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
- In this pattern, (?(R) is the start of a conditional subpattern, with
- two different alternatives for the recursive and non-recursive cases.
+ In this pattern, (?(R) is the start of a conditional subpattern, with
+ two different alternatives for the recursive and non-recursive cases.
The (?R) item is the actual recursive call.
Differences in recursion processing between PCRE and Perl
- Recursion processing in PCRE differs from Perl in two important ways.
- In PCRE (like Python, but unlike Perl), a recursive subpattern call is
+ Recursion processing in PCRE differs from Perl in two important ways.
+ 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 palin-
- dromic string that contains an odd number of characters (for example,
+ alternatives and there is a subsequent matching failure. This can be
+ illustrated by the following pattern, which purports to match a palin-
+ dromic string that contains an odd number of characters (for example,
"a", "aba", "abcba", "abcdcba"):
^(.|(.)(?1)\2)$
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.
+ 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":
- At the top level, the first character is matched, but as it is not at
+ 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 alterna-
tive is taken and the recursion kicks in. The recursive call to subpat-
- tern 1 successfully matches the next character ("b"). (Note that the
+ tern 1 successfully matches the next character ("b"). (Note that the
beginning and end of line tests are not part of the recursion).
- 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
+ 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:
^((.)(?1)\2|.)$
- 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
+ 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.
- To change the pattern so that it matches all palindromic strings, not
- just those with an odd number of characters, it is tempting to change
+ To change the pattern so that it matches all palindromic strings, not
+ just those with an odd number of characters, it is tempting to change
the pattern to this:
^((.)(?1)\2|.?)$
- 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 alter-
+ 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 alter-
natives at the higher level:
^(?:((.)(?1)\2|)|((.)(?3)\4|.))
- If you want to match typical palindromic phrases, the pattern has to
+ If you want to match typical palindromic phrases, the pattern has to
ignore all non-word characters, which can be done like this:
^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
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 backtrack-
- ing into sequences of non-word characters. Without this, PCRE takes a
- great deal longer (ten times or more) to match typical phrases, and
+ Perl. Note the use of the possessive quantifier *+ to avoid backtrack-
+ ing 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.
- WARNING: The palindrome-matching patterns above work only if the sub-
- ject string does not start with a palindrome that is shorter than the
- entire string. For example, although "abcba" is correctly matched, if
- the subject is "ababa", PCRE finds the palindrome "aba" at the start,
- then fails at top level because the end of the string does not follow.
- Once again, it cannot jump back into the recursion to try other alter-
+ WARNING: The palindrome-matching patterns above work only if the sub-
+ ject string does not start with a palindrome that is shorter than the
+ entire string. For example, although "abcba" is correctly matched, if
+ the subject is "ababa", PCRE finds the palindrome "aba" at the start,
+ then fails at top level because the end of the string does not follow.
+ Once again, it cannot jump back into the recursion to try other alter-
natives, so the entire match fails.
- The second way in which PCRE and Perl differ in their recursion pro-
- cessing is in the handling of captured values. In Perl, when a subpat-
- tern is called recursively or as a subpattern (see the next section),
- it has no access to any values that were captured outside the recur-
- sion, whereas in PCRE these values can be referenced. Consider this
+ The second way in which PCRE and Perl differ in their recursion pro-
+ cessing is in the handling of captured values. In Perl, when a subpat-
+ tern is called recursively or as a subpattern (see the next section),
+ it has no access to any values that were captured outside the recur-
+ sion, whereas in PCRE these values can be referenced. Consider this
pattern:
^(.)(\1|a(?2))
- In PCRE, this pattern matches "bab". The first capturing parentheses
- match "b", then in the second group, when the back reference \1 fails
- to match "b", the second alternative matches "a" and then recurses. In
- the recursion, \1 does now match "b" and so the whole match succeeds.
- In Perl, the pattern fails to match because inside the recursive call
+ In PCRE, this pattern matches "bab". The first capturing parentheses
+ match "b", then in the second group, when the back reference \1 fails
+ to match "b", the second alternative matches "a" and then recurses. In
+ the recursion, \1 does now match "b" and so the whole match succeeds.
+ In Perl, the pattern fails to match because inside the recursive call
\1 cannot access the externally set value.
SUBPATTERNS AS SUBROUTINES
- If the syntax for a recursive subpattern call (either by number or by
- name) is used outside the parentheses to which it refers, it operates
- like a subroutine in a programming language. The called subpattern may
- be defined before or after the reference. A numbered reference can be
+ If the syntax for a recursive subpattern call (either by number or by
+ name) is used outside the parentheses to which it refers, it operates
+ like a subroutine in a programming language. The called subpattern may
+ be defined before or after the reference. A numbered reference can be
absolute or relative, as in these examples:
(...(absolute)...)...(?2)...
@@ -5508,179 +5528,179 @@
(sens|respons)e and \1ibility
- matches "sense and sensibility" and "response and responsibility", but
+ matches "sense and sensibility" and "response and responsibility", but
not "sense and responsibility". If instead the pattern
(sens|respons)e and (?1)ibility
- is used, it does match "sense and responsibility" as well as the other
- two strings. Another example is given in the discussion of DEFINE
+ is used, it does match "sense and responsibility" as well as the other
+ two strings. Another example is given in the discussion of DEFINE
above.
- All subroutine calls, whether recursive or not, are always treated as
- atomic groups. That is, once a subroutine has matched some of the sub-
+ All subroutine calls, whether recursive or not, are always treated as
+ atomic groups. That is, once a subroutine has matched some of the sub-
ject string, it is never re-entered, even if it contains untried alter-
- natives and there is a subsequent matching failure. Any capturing
- parentheses that are set during the subroutine call revert to their
+ natives and there is a subsequent matching failure. Any capturing
+ parentheses that are set during the subroutine call revert to their
previous values afterwards.
- Processing options such as case-independence are fixed when a subpat-
- tern is defined, so if it is used as a subroutine, such options cannot
+ Processing options such as case-independence are fixed when a subpat-
+ tern is defined, so if it is used as a subroutine, such options cannot
be changed for different calls. For example, consider this pattern:
(abc)(?i:(?-1))
- It matches "abcabc". It does not match "abcABC" because the change of
+ It matches "abcabc". It does not match "abcABC" because the change of
processing option does not affect the called subpattern.
ONIGURUMA SUBROUTINE SYNTAX
- For compatibility with Oniguruma, the non-Perl syntax \g followed by a
+ For compatibility with Oniguruma, the non-Perl syntax \g 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, rewrit-
+ an alternative syntax for referencing a subpattern as a subroutine,
+ possibly recursively. Here are two of the examples used above, rewrit-
ten using this syntax:
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
(sens|respons)e and \g'1'ibility
- PCRE supports an extension to Oniguruma: if a number is preceded by a
+ 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:
(abc)(?i:\g<-1>)
- Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
- synonymous. The former is a back reference; the latter is a subroutine
+ Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
+ synonymous. The former is a back reference; the latter is a subroutine
call.
CALLOUTS
Perl has a feature whereby using the sequence (?{...}) causes arbitrary
- Perl code to be obeyed in the middle of matching a regular expression.
+ Perl code to be obeyed in the middle of matching a regular expression.
This makes it possible, amongst other things, to extract different sub-
strings that match the same pair of parentheses when there is a repeti-
tion.
PCRE provides a similar feature, but of course it cannot obey arbitrary
Perl code. The feature is called "callout". The caller of PCRE provides
- an external function by putting its entry point in the global variable
- pcre_callout. By default, this variable contains NULL, which disables
+ an external function by putting its entry point in the global variable
+ pcre_callout. By default, this variable contains NULL, which disables
all calling out.
- Within a regular expression, (?C) indicates the points at which the
- external function is to be called. If you want to identify different
- callout points, you can put a number less than 256 after the letter C.
- The default value is zero. For example, this pattern has two callout
+ Within a regular expression, (?C) indicates the points at which the
+ external function is to be called. If you want to identify different
+ callout points, you can put a number less than 256 after the letter C.
+ The default value is zero. For example, this pattern has two callout
points:
(?C1)abc(?C2)def
If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
- automatically installed before each item in the pattern. They are all
+ automatically installed before each item in the pattern. They are all
numbered 255.
During matching, when PCRE reaches a callout point (and pcre_callout is
- set), the external function is called. It is provided with the number
- of the callout, the position in the pattern, and, optionally, one item
- of data originally supplied by the caller of pcre_exec(). The callout
- function may cause matching to proceed, to backtrack, or to fail alto-
+ set), the external function is called. It is provided with the number
+ of the callout, the position in the pattern, and, optionally, one item
+ of data originally supplied by the caller of pcre_exec(). The callout
+ function may cause matching to proceed, to backtrack, or to fail alto-
gether. A complete description of the interface to the callout function
is given in the pcrecallout documentation.
BACKTRACKING CONTROL
- Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
+ Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
which are described in the Perl documentation as "experimental and sub-
- ject 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
+ ject 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.
- Since these verbs are specifically related to backtracking, most of
- them can be used only when the pattern is to be matched using
+ Since these verbs are specifically related to backtracking, most of
+ them can be used only when the pattern is to be matched using
pcre_exec(), which uses a backtracking algorithm. With the exception of
(*FAIL), which behaves like a failing negative assertion, they cause an
error if encountered by pcre_dfa_exec().
- If any of these verbs are used in an assertion or in a subpattern that
+ If any of these verbs are used in an assertion or in a subpattern that
is called as a subroutine (whether or not recursively), their effect is
confined to that subpattern; it does not extend to the surrounding pat-
- tern, with one exception: a *MARK that is encountered in a positive
+ tern, with one exception: a *MARK that is encountered in a positive
assertion is passed back (compare capturing parentheses in assertions).
Note that such subpatterns are processed as anchored at the point where
they are tested. Note also that Perl's treatment of subroutines is dif-
ferent in some cases.
- The new verbs make use of what was previously invalid syntax: an open-
+ The new verbs make use of what was previously invalid syntax: an open-
ing parenthesis followed by an asterisk. They are generally of the form
- (*VERB) or (*VERB:NAME). Some may take either form, with differing be-
- haviour, depending on whether or not an argument is present. A name is
+ (*VERB) or (*VERB:NAME). Some may take either form, with differing be-
+ haviour, depending on whether or not an argument is present. A name is
any sequence of characters that does not include a closing parenthesis.
- If the name is empty, that is, if the closing parenthesis immediately
- follows the colon, the effect is as if the colon were not there. Any
+ If the name is empty, that is, if the closing parenthesis immediately
+ follows the colon, the effect is as if the colon were not there. Any
number of these verbs may occur in a pattern.
- PCRE contains some optimizations that are used to speed up matching by
+ PCRE contains some optimizations that are used to speed up matching by
running some checks at the start of each match attempt. For example, it
- may know the minimum length of matching subject, or that a particular
- character must be present. When one of these optimizations suppresses
- the running of a match, any included backtracking verbs will not, of
+ may know the minimum length of matching subject, or that a particular
+ character must be present. When one of these optimizations suppresses
+ the running of a match, any included backtracking verbs will not, of
course, be processed. You can suppress the start-of-match optimizations
- by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com-
+ by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com-
pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT).
Verbs that act immediately
- The following verbs act as soon as they are encountered. They may not
+ The following verbs act as soon as they are encountered. They may not
be followed by a name.
(*ACCEPT)
- This verb causes the match to end successfully, skipping the remainder
- of the pattern. However, when it is inside a subpattern that is called
- as a subroutine, only that subpattern is ended successfully. Matching
- then continues at the outer level. If (*ACCEPT) is inside capturing
+ This verb causes the match to end successfully, skipping the remainder
+ of the pattern. However, when it is inside a subpattern that is called
+ as a subroutine, only that subpattern is ended successfully. Matching
+ then continues at the outer level. If (*ACCEPT) is inside capturing
parentheses, the data so far is captured. For example:
A((?:A|B(*ACCEPT)|C)D)
- This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap-
+ This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap-
tured by the outer parentheses.
(*FAIL) or (*F)
- This verb causes a matching failure, 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 pat-
+ This verb causes a matching failure, 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 pat-
tern:
a+(?C)(*FAIL)
- A match with the string "aaaa" always fails, but the callout is taken
+ A match with the string "aaaa" always fails, but the callout is taken
before each backtrack happens (in this example, 10 times).
Recording which path was taken
- There is one verb whose main purpose is to track how a match was
- arrived at, though it also has a secondary use in conjunction with
+ There is one verb whose main purpose is to track how a match was
+ arrived at, though it also has a secondary use in conjunction with
advancing the match starting point (see (*SKIP) below).
(*MARK:NAME) or (*:NAME)
- A name is always required with this verb. There may be as many
- instances of (*MARK) as you like in a pattern, and their names do not
+ A name is always required with this verb. There may be as many
+ instances of (*MARK) as you like in a pattern, and their names do not
have to be unique.
- When a match succeeds, the name of the last-encountered (*MARK) is
- passed back to the caller via the pcre_extra data structure, as
+ When a match succeeds, the name of the last-encountered (*MARK) is
+ passed back to the caller via the pcre_extra data structure, as
described in the section on pcre_extra in the pcreapi documentation. No
- data is returned for a partial match. Here is an example of pcretest
- output, where the /K modifier requests the retrieval and outputting of
+ data is returned for a partial match. Here is an example of pcretest
+ output, where the /K modifier requests the retrieval and outputting of
(*MARK) data:
/X(*MARK:A)Y|X(*MARK:B)Z/K
@@ -5692,17 +5712,17 @@
MK: B
The (*MARK) name is tagged with "MK:" in this output, and in this exam-
- ple it indicates which of the two alternatives matched. This is a more
- efficient way of obtaining this information than putting each alterna-
+ ple it indicates which of the two alternatives matched. This is a more
+ efficient way of obtaining this information than putting each alterna-
tive in its own capturing parentheses.
If (*MARK) is encountered in a positive assertion, its name is recorded
and passed back if it is the last-encountered. This does not happen for
negative assertions.
- A name may also be returned after a failed match if the final path
- through the pattern involves (*MARK). However, unless (*MARK) used in
- conjunction with (*COMMIT), this is unlikely to happen for an unan-
+ A name may also be returned after a failed match if the final path
+ through the pattern involves (*MARK). However, unless (*MARK) used in
+ conjunction with (*COMMIT), this is unlikely to happen for an unan-
chored pattern because, as the starting point for matching is advanced,
the final check is often with an empty string, causing a failure before
(*MARK) is reached. For example:
@@ -5712,56 +5732,56 @@
No match
There are three potential starting points for this match (starting with
- X, starting with P, and with an empty string). If the pattern is
+ X, starting with P, and with an empty string). If the pattern is
anchored, the result is different:
/^X(*MARK:A)Y|^X(*MARK:B)Z/K
XP
No match, mark = B
- PCRE's start-of-match optimizations can also interfere with this. For
- example, if, as a result of a call to pcre_study(), it knows the mini-
- mum subject length for a match, a shorter subject will not be scanned
+ PCRE's start-of-match optimizations can also interfere with this. For
+ example, if, as a result of a call to pcre_study(), it knows the mini-
+ mum subject length for a match, a shorter subject will not be scanned
at all.
Note that similar anomalies (though different in detail) exist in Perl,
- no doubt for the same reasons. The use of (*MARK) data after a failed
- match of an unanchored pattern is not recommended, unless (*COMMIT) is
+ no doubt for the same reasons. The use of (*MARK) data after a failed
+ match of an unanchored pattern is not recommended, unless (*COMMIT) is
involved.
Verbs that act after backtracking
The following verbs do nothing when they are encountered. Matching con-
- tinues with what follows, but if there is no subsequent match, causing
- a backtrack to the verb, a failure is forced. That is, backtracking
- cannot pass to the left of the verb. However, when one of these verbs
- appears inside an atomic group, its effect is confined to that group,
- because once the group has been matched, there is never any backtrack-
- ing into it. In this situation, backtracking can "jump back" to the
- left of the entire atomic group. (Remember also, as stated above, that
+ tinues with what follows, but if there is no subsequent match, causing
+ a backtrack to the verb, a failure is forced. That is, backtracking
+ cannot pass to the left of the verb. However, when one of these verbs
+ appears inside an atomic group, its effect is confined to that group,
+ because once the group has been matched, there is never any backtrack-
+ ing into it. In this situation, backtracking can "jump back" to the
+ left of the entire atomic group. (Remember also, as stated above, that
this localization also applies in subroutine calls and assertions.)
- These verbs differ in exactly what kind of failure occurs when back-
+ These verbs differ in exactly what kind of failure occurs when back-
tracking reaches them.
(*COMMIT)
- This verb, which may not be followed by a name, causes the whole match
+ This verb, which may not be followed by a name, 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 starting point take place. Once (*COMMIT) has been passed,
- pcre_exec() is committed to finding a match at the current starting
+ pcre_exec() is committed to finding a match at the current starting
point, or not at all. For example:
a+(*COMMIT)b
- This matches "xxaab" but not "aacaab". It can be thought of as a kind
+ 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." The name of the
- most recently passed (*MARK) in the path is passed back when (*COMMIT)
+ most recently passed (*MARK) in the path is passed back when (*COMMIT)
forces a match failure.
- Note that (*COMMIT) at the start of a pattern is not the same as an
- anchor, unless PCRE's start-of-match optimizations are turned off, as
+ Note that (*COMMIT) at the start of a pattern is not the same as an
+ anchor, unless PCRE's start-of-match optimizations are turned off, as
shown in this pcretest example:
/(*COMMIT)abc/
@@ -5770,115 +5790,115 @@
xyzabc\Y
No match
- PCRE knows that any match must start with "a", so the optimization
- skips along the subject to "a" before running the first match attempt,
- which succeeds. When the optimization is disabled by the \Y escape in
+ PCRE knows that any match must start with "a", so the optimization
+ skips along the subject to "a" before running the first match attempt,
+ which succeeds. When the optimization is disabled by the \Y escape in
the second subject, the match starts at "x" and so the (*COMMIT) causes
it to fail without trying any other starting points.
(*PRUNE) or (*PRUNE:NAME)
- This verb causes the match to fail at the current starting position in
- the subject 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), before it is reached, 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 alter-
- native to an atomic group or possessive quantifier, but there are some
+ This verb causes the match to fail at the current starting position in
+ the subject 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), before it is reached, 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 alter-
+ native to an atomic group or possessive quantifier, but there are some
uses of (*PRUNE) that cannot be expressed in any other way. The behav-
- iour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
- match fails completely; the name is passed back if this is the final
- attempt. (*PRUNE:NAME) does not pass back a name if the match suc-
- ceeds. In an anchored pattern (*PRUNE) has the same effect as (*COM-
+ iour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
+ match fails completely; the name is passed back if this is the final
+ attempt. (*PRUNE:NAME) does not pass back a name if the match suc-
+ ceeds. In an anchored pattern (*PRUNE) has the same effect as (*COM-
MIT).
(*SKIP)
- This verb, when given without a name, is like (*PRUNE), except that if
- the pattern is unanchored, the "bumpalong" advance is not to the next
+ This verb, when given without a name, 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 encoun-
- tered. (*SKIP) signifies that whatever text was matched leading up to
+ tered. (*SKIP) signifies that whatever text was matched leading up to
it cannot be part of a successful match. Consider:
a+(*SKIP)b
- If the subject is "aaaac...", after the first match attempt fails
- (starting at the first character in the string), the starting point
+ 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 quan-
- tifer does not have the same effect as 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
+ tifer does not have the same effect as 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".
(*SKIP:NAME)
- When (*SKIP) has an associated name, its behaviour is modified. If the
+ When (*SKIP) has an associated name, its behaviour is modified. If the
following pattern fails to match, the previous path through the pattern
- is searched for the most recent (*MARK) that has the same name. If one
- is found, the "bumpalong" advance is to the subject position that cor-
- responds to that (*MARK) instead of to where (*SKIP) was encountered.
- If no (*MARK) with a matching name is found, normal "bumpalong" of one
+ is searched for the most recent (*MARK) that has the same name. If one
+ is found, the "bumpalong" advance is to the subject position that cor-
+ responds to that (*MARK) instead of to where (*SKIP) was encountered.
+ If no (*MARK) with a matching name is found, normal "bumpalong" of one
character happens (that is, the (*SKIP) is ignored).
(*THEN) or (*THEN:NAME)
- This verb causes a skip to the next innermost alternative if the rest
- of the pattern does not match. That is, it cancels pending backtrack-
- ing, but only within the current alternative. Its name comes from the
+ This verb causes a skip to the next innermost alternative if the rest
+ of the pattern does not match. That is, it cancels pending backtrack-
+ ing, but only within the current alternative. Its name comes from the
observation that it can be used for a pattern-based if-then-else block:
( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
- 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. The behaviour of (*THEN:NAME) is exactly the same as
- (*MARK:NAME)(*THEN) if the overall match fails. If (*THEN) is not
+ 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. The behaviour of (*THEN:NAME) is exactly the same as
+ (*MARK:NAME)(*THEN) if the overall match fails. If (*THEN) is not
inside an alternation, it acts like (*PRUNE).
- Note that a subpattern that does not contain a | character is just a
- part of the enclosing alternative; it is not a nested alternation with
- only one alternative. The effect of (*THEN) extends beyond such a sub-
- pattern to the enclosing alternative. Consider this pattern, where A,
+ Note that a subpattern that does not contain a | character is just a
+ part of the enclosing alternative; it is not a nested alternation with
+ only one alternative. The effect of (*THEN) extends beyond such a sub-
+ pattern to the enclosing alternative. Consider this pattern, where A,
B, etc. are complex pattern fragments that do not contain any | charac-
ters at this level:
A (B(*THEN)C) | D
- If A and B are matched, but there is a failure in C, matching does not
+ If A and B are matched, but there is a failure in C, matching does not
backtrack into A; instead it moves to the next alternative, that is, D.
- However, if the subpattern containing (*THEN) is given an alternative,
+ However, if the subpattern containing (*THEN) is given an alternative,
it behaves differently:
A (B(*THEN)C | (*FAIL)) | D
- The effect of (*THEN) is now confined to the inner subpattern. After a
+ The effect of (*THEN) is now confined to the inner subpattern. After a
failure in C, matching moves to (*FAIL), which causes the whole subpat-
- tern to fail because there are no more alternatives to try. In this
+ tern to fail because there are no more alternatives to try. In this
case, matching does now backtrack into A.
Note also that a conditional subpattern is not considered as having two
- alternatives, because only one is ever used. In other words, the |
+ alternatives, because only one is ever used. In other words, the |
character in a conditional subpattern has a different meaning. Ignoring
white space, consider:
^.*? (?(?=a) a | b(*THEN)c )
- If the subject is "ba", this pattern does not match. Because .*? is
- ungreedy, it initially matches zero characters. The condition (?=a)
- then fails, the character "b" is matched, but "c" is not. At this
- point, matching does not backtrack to .*? as might perhaps be expected
- from the presence of the | character. The conditional subpattern is
+ If the subject is "ba", this pattern does not match. Because .*? is
+ ungreedy, it initially matches zero characters. The condition (?=a)
+ then fails, the character "b" is matched, but "c" is not. At this
+ point, matching does not backtrack to .*? as might perhaps be expected
+ from the presence of the | character. The conditional subpattern is
part of the single alternative that comprises the whole pattern, and so
- the match fails. (If there was a backtrack into .*?, allowing it to
+ the match fails. (If there was a backtrack into .*?, allowing it to
match "b", the match would succeed.)
- The verbs just described provide four different "strengths" of control
+ The verbs just described provide four different "strengths" of control
when subsequent matching fails. (*THEN) is the weakest, carrying on the
- match at the next alternative. (*PRUNE) comes next, failing the match
- at the current starting position, but allowing an advance to the next
- character (for an unanchored pattern). (*SKIP) is similar, except that
+ match at the next alternative. (*PRUNE) comes next, failing the match
+ at the current starting position, but allowing an advance to the next
+ character (for an unanchored pattern). (*SKIP) is similar, except that
the advance may be more than one character. (*COMMIT) is the strongest,
causing the entire match to fail.
@@ -5888,8 +5908,8 @@
(A(*COMMIT)B(*THEN)C|D)
- Once A has matched, PCRE is committed to this match, at the current
- starting position. If subsequently B matches, but C does not, the nor-
+ Once A has matched, PCRE is committed to this match, at the current
+ starting position. If subsequently B matches, but C does not, the nor-
mal (*THEN) action of trying the next alternative (that is, D) does not
happen because (*COMMIT) overrides.
@@ -5908,7 +5928,7 @@
REVISION
- Last updated: 09 October 2011
+ Last updated: 19 October 2011
Copyright (c) 1997-2011 University of Cambridge.
------------------------------------------------------------------------------
@@ -6383,42 +6403,43 @@
gle byte.
5. The escape sequence \C can be used to match a single byte in UTF-8
- mode, but its use can lead to some strange effects. This facility is
- not available in the alternative matching function, pcre_dfa_exec(),
- nor is it supported by the JIT optimization of pcre_exec(). If JIT
- optimization is requested for a pattern that contains \C, it will not
- succeed, and so the matching will be carried out by the normal inter-
- pretive function.
+ mode, but its use can lead to some strange effects because it breaks up
+ multibyte characters (see the description of \C in the pcrepattern doc-
+ umentation). The use of \C is not supported in the alternative matching
+ function pcre_dfa_exec(), nor is it supported in UTF-8 mode by the JIT
+ optimization of pcre_exec(). If JIT optimization is requested for a
+ UTF-8 pattern that contains \C, it will not succeed, and so the match-
+ ing will be carried out by the normal interpretive function.
- 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
+ 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
test characters of any code value, but, by default, the characters that
- PCRE recognizes as digits, spaces, or word characters remain the same
- set as before, all with values less than 256. This remains true even
- when PCRE is built to include Unicode property support, because to do
+ PCRE recognizes as digits, spaces, or word characters remain the same
+ set as before, all with values less than 256. This remains true even
+ when PCRE is built to include Unicode property support, because to do
otherwise would slow down PCRE in many common cases. Note in particular
that this applies to \b and \B, because they are defined in terms of \w
- and \W. If you really want to test for a wider sense of, say, "digit",
- you can use explicit Unicode property tests such as \p{Nd}. Alterna-
- tively, if you set the PCRE_UCP option, the way that the character
- escapes work is changed so that Unicode properties are used to deter-
- mine which characters match. There are more details in the section on
+ and \W. If you really want to test for a wider sense of, say, "digit",
+ you can use explicit Unicode property tests such as \p{Nd}. Alterna-
+ tively, if you set the PCRE_UCP option, the way that the character
+ escapes work is changed so that Unicode properties are used to deter-
+ mine which characters match. There are more details in the section on
generic character types in the pcrepattern documentation.
- 7. Similarly, characters that match the POSIX named character classes
+ 7. Similarly, characters that match the POSIX named character classes
are all low-valued characters, unless the PCRE_UCP option is set.
- 8. However, the horizontal and vertical whitespace matching escapes
- (\h, \H, \v, and \V) do match all the appropriate Unicode characters,
+ 8. However, the horizontal and vertical whitespace matching escapes
+ (\h, \H, \v, and \V) do match all the appropriate Unicode characters,
whether or not PCRE_UCP is set.
- 9. Case-insensitive matching applies only to characters whose values
- are less than 128, unless PCRE is built with Unicode property support.
- Even when Unicode property support is available, PCRE still uses its
- own character tables when checking the case of low-valued characters,
- so as not to degrade performance. The Unicode property information is
+ 9. Case-insensitive matching applies only to characters whose values
+ are less than 128, unless PCRE is built with Unicode property support.
+ Even when Unicode property support is available, PCRE still uses its
+ own character tables when checking the case of low-valued characters,
+ so as not to degrade performance. The Unicode property information is
used only for characters with higher values. Furthermore, PCRE supports
- case-insensitive matching only when there is a one-to-one mapping
- between a letter's cases. There are a small number of many-to-one map-
+ case-insensitive matching only when there is a one-to-one mapping
+ between a letter's cases. There are a small number of many-to-one map-
pings in Unicode; these are not supported by PCRE.
@@ -6431,7 +6452,7 @@
REVISION
- Last updated: 06 September 2011
+ Last updated: 19 October 2011
Copyright (c) 1997-2011 University of Cambridge.
------------------------------------------------------------------------------
@@ -6534,7 +6555,7 @@
The unsupported pattern items are:
- \C match a single byte, even in UTF-8 mode
+ \C match a single byte; not supported in UTF-8 mode
(?Cn) callouts
(?(<name>)... conditional test on setting of a named subpattern
(?(R)... conditional test on whole pattern recursion
@@ -6691,7 +6712,7 @@
REVISION
- Last updated: 05 October 2011
+ Last updated: 19 October 2011
Copyright (c) 1997-2011 University of Cambridge.
------------------------------------------------------------------------------
Modified: code/trunk/doc/pcrepattern.3
===================================================================
--- code/trunk/doc/pcrepattern.3 2011-10-19 17:37:29 UTC (rev 737)
+++ code/trunk/doc/pcrepattern.3 2011-10-21 09:04:01 UTC (rev 738)
@@ -989,23 +989,23 @@
the lookbehind.
.P
In general, the \eC escape sequence is best avoided in UTF-8 mode. However, one
-way of using it that avoids the problem of malformed UTF-8 characters is to
-use a lookahead to check the length of the next character, as in this pattern
+way of using it that avoids the problem of malformed UTF-8 characters is to
+use a lookahead to check the length of the next character, as in this pattern
(ignore white space and line breaks):
.sp
(?| (?=[\ex00-\ex7f])(\eC) |
- (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
- (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
+ (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
+ (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
(?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
.sp
-A group that starts with (?| resets the capturing parentheses numbers in each
-alternative (see
+A group that starts with (?| resets the capturing parentheses numbers in each
+alternative (see
.\" HTML <a href="#dupsubpatternnumber">
.\" </a>
"Duplicate Subpattern Numbers"
.\"
-below). The assertions at the start of each branch check the next UTF-8
-character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
+below). The assertions at the start of each branch check the next UTF-8
+character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
character's individual bytes are then captured by the appropriate number of
groups.
.
Modified: code/trunk/pcretest.c
===================================================================
--- code/trunk/pcretest.c 2011-10-19 17:37:29 UTC (rev 737)
+++ code/trunk/pcretest.c 2011-10-21 09:04:01 UTC (rev 738)
@@ -2346,12 +2346,12 @@
{
unsigned char *pt = p;
c = 0;
-
+
/* We used to have "while (isxdigit(*(++pt)))" here, but it fails
when isxdigit() is a macro that refers to its argument more than
once. This is banned by the C Standard, but apparently happens in at
least one MacOS environment. */
-
+
for (pt++; isxdigit(*pt); pt++)
c = c * 16 + tolower(*pt) - ((isdigit(*pt))? '0' : 'a' - 10);
if (*pt == '}')
