Revision: 978
http://www.exim.org/viewvc/pcre2?view=rev&revision=978
Author: ph10
Date: 2018-08-03 17:56:54 +0100 (Fri, 03 Aug 2018)
Log Message:
-----------
Documentation update.
Modified Paths:
--------------
code/trunk/doc/html/pcre2pattern.html
code/trunk/doc/pcre2.txt
code/trunk/doc/pcre2pattern.3
Modified: code/trunk/doc/html/pcre2pattern.html
===================================================================
--- code/trunk/doc/html/pcre2pattern.html 2018-08-03 09:38:36 UTC (rev 977)
+++ code/trunk/doc/html/pcre2pattern.html 2018-08-03 16:56:54 UTC (rev 978)
@@ -249,7 +249,7 @@
<P>
The newline convention affects where the circumflex and dollar assertions are
true. It also affects the interpretation of the dot metacharacter when
-PCRE2_DOTALL is not set, and the behaviour of \N when not followed by an
+PCRE2_DOTALL is not set, and the behaviour of \N when not followed by an
opening brace. However, it does not affect what the \R escape sequence
matches. By default, this is any Unicode newline sequence, for Perl
compatibility. However, this can be changed; see the next section and the
@@ -357,7 +357,7 @@
If you want to remove the special meaning from a sequence of characters, you
can do so by putting them between \Q and \E. This is different from Perl in
that $ and @ are handled as literals in \Q...\E sequences in PCRE2, whereas
-in Perl, $ and @ cause variable interpolation. Also, Perl does "double-quotish
+in Perl, $ and @ cause variable interpolation. Also, Perl does "double-quotish
backslash interpolation" on any backslashes between \Q and \E which, its
documentation says, "may lead to confusing results". PCRE2 treats a backslash
between \Q and \E just like any other character. Note the following examples:
@@ -400,7 +400,7 @@
\o{ddd..} character with octal code ddd..
\xhh character with hex code hh
\x{hhh..} character with hex code hhh.. (default mode)
- \N{U+hhh..} character with Unicode code point hhh..
+ \N{U+hhh..} character with Unicode code point hhh..
\uhhhh character with hex code hhhh (when PCRE2_ALT_BSUX is set)
</pre>
Note that when \N is not followed by an opening brace (curly bracket) it has
@@ -590,7 +590,7 @@
\D any character that is not a decimal digit
\h any horizontal white space character
\H any character that is not a horizontal white space character
- \N any character that is not a newline
+ \N any character that is not a newline
\s any white space character
\S any character that is not a white space character
\v any vertical white space character
@@ -600,8 +600,8 @@
</pre>
The \N escape sequence has the same meaning as
<a href="#fullstopdot">the "." metacharacter</a>
-when PCRE2_DOTALL is not set, but setting PCRE2_DOTALL does not change the
-meaning of \N. Note that when \N is followed by an opening brace it has a
+when PCRE2_DOTALL is not set, but setting PCRE2_DOTALL does not change the
+meaning of \N. Note that when \N is followed by an opening brace it has a
different meaning. See the section entitled
<a href="#digitsafterbackslash">"Non-printing characters"</a>
above for details. Perl also uses \N{name} to specify characters by Unicode
@@ -1030,8 +1030,8 @@
Unicode supports various kinds of composite character by giving each character
a grapheme breaking property, and having rules that use these properties to
define the boundaries of extended grapheme clusters. The rules are defined in
-Unicode Standard Annex 29, "Unicode Text Segmentation". Unicode 11.0.0
-abandoned the use of some previous properties that had been used for emojis.
+Unicode Standard Annex 29, "Unicode Text Segmentation". Unicode 11.0.0
+abandoned the use of some previous properties that had been used for emojis.
Instead it introduced various emoji-specific properties. PCRE2 uses only the
Extended Pictographic property.
</P>
@@ -1316,7 +1316,7 @@
<P>
The escape sequence \N when not followed by an opening brace behaves like a
dot, except that it is not affected by the PCRE2_DOTALL option. In other words,
-it matches any character except one that signifies the end of a line.
+it matches any character except one that signifies the end of a line.
</P>
<P>
When \N is followed by an opening brace it has a different meaning. See the
@@ -1642,7 +1642,7 @@
xx for PCRE2_EXTENDED_MORE
</pre>
For example, (?im) sets caseless, multiline matching. It is also possible to
-unset these options by preceding the relevant letters with a hyphen, for
+unset these options by preceding the relevant letters with a hyphen, for
example (?-im). The two "extended" options are not independent; unsetting either
one cancels the effects of both of them.
</P>
@@ -1654,9 +1654,9 @@
setting "(?)" is allowed. Needless to say, it has no effect.
</P>
<P>
-If the first character following (? is a circumflex, it causes all of the above
-options to be unset. Thus, (?^) is equivalent to (?-imnsx). Letters may follow
-the circumflex to cause some options to be re-instated, but a hyphen may not
+If the first character following (? is a circumflex, it causes all of the above
+options to be unset. Thus, (?^) is equivalent to (?-imnsx). Letters may follow
+the circumflex to cause some options to be re-instated, but a hyphen may not
appear.
</P>
<P>
@@ -1813,42 +1813,69 @@
<br><a name="SEC16" href="#TOC1">NAMED SUBPATTERNS</a><br>
<P>
Identifying capturing parentheses by number is simple, but it can be very hard
-to keep track of the numbers in complicated regular expressions. Furthermore,
-if an expression is modified, the numbers may change. To help with this
-difficulty, PCRE2 supports the naming of subpatterns. This feature was not
-added to Perl until release 5.10. Python had the feature earlier, and PCRE1
+to keep track of the numbers in complicated patterns. Furthermore, if an
+expression is modified, the numbers may change. To help with this difficulty,
+PCRE2 supports the naming of capturing subpatterns. This feature was not added
+to Perl until release 5.10. Python had the feature earlier, and PCRE1
introduced it at release 4.0, using the Python syntax. PCRE2 supports both the
-Perl and the Python syntax. Perl allows identically numbered subpatterns to
-have different names, but PCRE2 does not.
+Perl and the Python syntax.
</P>
<P>
-In PCRE2, 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
+In PCRE2, a capturing subpattern can be named in one of three ways:
+(?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python. Names
+consist of up to 32 alphanumeric characters and underscores, but must start
+with a non-digit. References to capturing parentheses from other parts of the
+pattern, such as
<a href="#backreferences">backreferences,</a>
<a href="#recursion">recursion,</a>
and
<a href="#conditions">conditions,</a>
-can be made by name as well as by number.
+can all be made by name as well as by number.
</P>
<P>
-Names consist of up to 32 alphanumeric characters and underscores, but must
-start with a non-digit. Named capturing parentheses are still allocated numbers
-as well as names, exactly as if the names were not present. The PCRE2 API
-provides function calls for extracting the name-to-number translation table
-from a compiled pattern. There are also convenience functions for extracting a
-captured substring by name.
+Named capturing parentheses are allocated numbers as well as names, exactly as
+if the names were not present. In both PCRE2 and Perl, capturing subpatterns
+are primarily identified by numbers; any names are just aliases for these
+numbers. The PCRE2 API provides function calls for extracting the complete
+name-to-number translation table from a compiled pattern, as well as
+convenience functions for extracting captured substrings by name.
</P>
<P>
-By default, a name must be unique within a pattern, but it is possible to relax
-this constraint by setting the PCRE2_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.) Duplicate 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:
+<b>Warning:</b> When more than one subpattern has the same number, as described
+in the previous section, a name given to one of them applies to all of them.
+Perl allows identically numbered subpatterns to have different names. Consider
+this pattern, where there are two capturing subpatterns, both numbered 1:
<pre>
+ (?|(?<AA>aa)|(?<BB>bb))
+</pre>
+Perl allows this, with both names AA and BB as aliases of group 1. Thus, after
+a successful match, both names yield the same value (either "aa" or "bb").
+</P>
+<P>
+In an attempt to reduce confusion, PCRE2 does not allow the same group number
+to be associated with more than one name. The example above provokes a
+compile-time error. However, there is still scope for confusion. Consider this
+pattern:
+<pre>
+ (?|(?<AA>aa)|(bb))
+</pre>
+Although the second subpattern number 1 is not explicitly named, the name AA is
+still an alias for subpattern 1. Whether the pattern matches "aa" or "bb", a
+reference by name to group AA yields the matched string.
+</P>
+<P>
+By default, a name must be unique within a pattern, except that duplicate names
+are permitted for subpatterns with the same number, for example:
+<pre>
+ (?|(?<AA>aa)|(?<AA>bb))
+</pre>
+The duplicate name constraint can be disabled by setting the PCRE2_DUPNAMES
+option at compile time, or by the use of (?J) within the pattern. Duplicate
+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:
+<pre>
(?<DN>Mon|Fri|Sun)(?:day)?|
(?<DN>Tue)(?:sday)?|
(?<DN>Wed)(?:nesday)?|
@@ -1856,13 +1883,11 @@
(?<DN>Sat)(?:urday)?
</pre>
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.)
-</P>
-<P>
The convenience functions 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.
+matched. This saves searching to find which numbered subpattern it was. (An
+alternative way of solving this problem is to use a "branch reset" subpattern,
+as described in the previous section.)
</P>
<P>
If you make a backreference to a non-unique named subpattern from elsewhere in
@@ -1878,8 +1903,7 @@
<P>
If you make a subroutine call to a non-unique named subpattern, the one that
corresponds to the first occurrence of the name is used. In the absence of
-duplicate numbers (see the previous section) this is the one with the lowest
-number.
+duplicate numbers this is the one with the lowest number.
</P>
<P>
If you use a named reference in a condition
@@ -1893,14 +1917,6 @@
<a href="pcre2api.html"><b>pcre2api</b></a>
documentation.
</P>
-<P>
-<b>Warning:</b> You cannot use different names to distinguish between two
-subpatterns with the same number because PCRE2 uses only the numbers when
-matching. For this reason, an error is given at compile time if different names
-are given to subpatterns with the same number. However, you can always give the
-same name to subpatterns with the same number, even when PCRE2_DUPNAMES is not
-set.
-</P>
<br><a name="SEC17" href="#TOC1">REPETITION</a><br>
<P>
Repetition is specified by quantifiers, which can follow any of the following
@@ -2327,14 +2343,14 @@
<P>
Assertion subpatterns are not capturing subpatterns. If an assertion contains
capturing subpatterns within it, these are counted for the purposes of
-numbering the capturing subpatterns in the whole pattern. Within each branch of
+numbering the capturing subpatterns in the whole pattern. Within each branch of
an assertion, locally captured substrings may be referenced in the usual way.
-For example, a sequence such as (.)\g{-1} can be used to check that two
+For example, a sequence such as (.)\g{-1} can be used to check that two
adjacent characters are the same.
</P>
<P>
When a branch within an assertion fails to match, any substrings that were
-captured are discarded (as happens with any pattern branch that fails to
+captured are discarded (as happens with any pattern branch that fails to
match). A negative assertion succeeds only when all its branches fail to match;
this means that no captured substrings are ever retained after a successful
negative assertion. When an assertion contains a matching branch, what happens
@@ -2348,7 +2364,7 @@
<a href="#conditions">conditional subpattern</a>
(see below), captured substrings are retained, because matching continues with
the "no" branch of the condition. For other failing negative assertions,
-control passes to the previous backtracking point, thus discarding any captured
+control passes to the previous backtracking point, thus discarding any captured
strings within the assertion.
</P>
<P>
@@ -2957,10 +2973,12 @@
<br><a name="SEC24" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
<P>
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:
+name) is used outside the parentheses to which it refers, it operates a bit
+like a subroutine in a programming language. More accurately, PCRE2 treats the
+referenced subpattern as an independent subpattern which it tries to match at
+the current matching position. The called subpattern may be defined before or
+after the reference. A numbered reference can be absolute or relative, as in
+these examples:
<pre>
(...(absolute)...)...(?2)...
(...(relative)...)...(?-1)...
@@ -2993,6 +3011,13 @@
</pre>
It matches "abcabc". It does not match "abcABC" because the change of
processing option does not affect the called subpattern.
+</P>
+<P>
+The behaviour of
+<a href="#backtrackcontrol">backtracking control verbs</a>
+in subpatterns when called as subroutines is described in the section entitled
+<a href="#btsub">"Backtracking verbs in subroutines"</a>
+below.
<a name="onigurumasubroutines"></a></P>
<br><a name="SEC25" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
<P>
@@ -3111,7 +3136,7 @@
</P>
<P>
A closing parenthesis can be included in a name either as \) or between \Q
-and \E. In addition to backslash processing, if the PCRE2_EXTENDED or
+and \E. In addition to backslash processing, if the PCRE2_EXTENDED or
PCRE2_EXTENDED_MORE option is also set, unescaped whitespace in verb names is
skipped, and #-comments are recognized, exactly as in the rest of the pattern.
PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do not affect verb names unless
@@ -3157,7 +3182,7 @@
documentation.
</P>
<P>
-Experiments with Perl suggest that it too has similar optimizations, and like
+Experiments with Perl suggest that it too has similar optimizations, and like
PCRE2, turning them off can change the result of a match.
</P>
<br><b>
@@ -3185,7 +3210,7 @@
<pre>
(*FAIL) or (*FAIL:NAME)
</pre>
-This verb causes a matching failure, forcing backtracking to occur. It may be
+This verb causes a matching failure, forcing backtracking to occur. It may be
abbreviated to (*F). 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 PCRE2. The
@@ -3197,7 +3222,7 @@
each backtrack happens (in this example, 10 times).
</P>
<P>
-(*ACCEPT:NAME) and (*FAIL:NAME) behave exactly the same as
+(*ACCEPT:NAME) and (*FAIL:NAME) behave exactly the same as
(*MARK:NAME)(*ACCEPT) and (*MARK:NAME)(*FAIL), respectively.
</P>
<br><b>
@@ -3220,7 +3245,7 @@
in the
<a href="pcre2api.html"><b>pcre2api</b></a>
documentation. This applies to all instances of (*MARK), including those inside
-assertions and atomic groups. (There are differences in those cases when
+assertions and atomic groups. (There are differences in those cases when
(*MARK) is used in conjunction with (*SKIP) as described below.)
</P>
<P>
@@ -3300,7 +3325,7 @@
a+(*COMMIT)b
</pre>
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."
+dynamic anchor, or "I've started, so I must finish."
</P>
<P>
The behaviour of (*COMMIT:NAME) is not the same as (*MARK:NAME)(*COMMIT). It is
@@ -3524,7 +3549,7 @@
(*ACCEPT) in a standalone positive assertion causes the assertion to succeed
without any further processing; captured strings and a (*MARK) name (if set)
are retained. In a standalone negative assertion, (*ACCEPT) causes the
-assertion to fail without any further processing; captured substrings and any
+assertion to fail without any further processing; captured substrings and any
(*MARK) name are discarded.
</P>
<P>
@@ -3533,11 +3558,11 @@
retained in both cases.
</P>
<P>
-The remaining verbs act only when a later failure causes a backtrack to
-reach them. This means that their effect is confined to the assertion,
+The remaining verbs act only when a later failure causes a backtrack to
+reach them. This means that their effect is confined to the assertion,
because lookaround assertions are atomic. A backtrack that occurs after an
-assertion is complete does not jump back into the assertion. Note in particular
-that a (*MARK) name that is set in an assertion is not "seen" by an instance of
+assertion is complete does not jump back into the assertion. Note in particular
+that a (*MARK) name that is set in an assertion is not "seen" by an instance of
(*SKIP:NAME) latter in the pattern.
</P>
<P>
Modified: code/trunk/doc/pcre2.txt
===================================================================
--- code/trunk/doc/pcre2.txt 2018-08-03 09:38:36 UTC (rev 977)
+++ code/trunk/doc/pcre2.txt 2018-08-03 16:56:54 UTC (rev 978)
@@ -7393,34 +7393,62 @@
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, PCRE2 supports the naming of sub-
- patterns. This feature was not added to Perl until release 5.10. Python
+ very hard to keep track of the numbers in complicated patterns. Fur-
+ thermore, if an expression is modified, the numbers may change. To help
+ with this difficulty, PCRE2 supports the naming of capturing subpat-
+ terns. This feature was not added to Perl until release 5.10. Python
had the feature earlier, and PCRE1 introduced it at release 4.0, using
- the Python syntax. PCRE2 supports both the Perl and the Python syntax.
- Perl allows identically numbered subpatterns to have different names,
- but PCRE2 does not.
+ the Python syntax. PCRE2 supports both the Perl and the Python syntax.
- In PCRE2, 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.
-
+ In PCRE2, a capturing subpattern can be named in one of three ways:
+ (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python.
Names consist of up to 32 alphanumeric characters and underscores, but
- must start with a non-digit. Named capturing parentheses are still
- allocated numbers as well as names, exactly as if the names were not
- present. The PCRE2 API provides function calls for extracting the name-
- to-number translation table from a compiled pattern. There are also
- convenience functions for extracting a captured substring by name.
+ must start with a non-digit. References to capturing parentheses from
+ other parts of the pattern, such as backreferences, recursion, and con-
+ ditions, can all be made by name as well as by number.
- By default, a name must be unique within a pattern, but it is possible
- to relax this constraint by setting the PCRE2_DUPNAMES option at com-
- pile time. (Duplicate names are also always permitted for subpatterns
- with the same number, set up as described in the previous section.)
+ Named capturing parentheses are allocated numbers as well as names,
+ exactly as if the names were not present. In both PCRE2 and Perl, cap-
+ turing subpatterns are primarily identified by numbers; any names are
+ just aliases for these numbers. The PCRE2 API provides function calls
+ for extracting the complete name-to-number translation table from a
+ compiled pattern, as well as convenience functions for extracting cap-
+ tured substrings by name.
+
+ Warning: When more than one subpattern has the same number, as
+ described in the previous section, a name given to one of them applies
+ to all of them. Perl allows identically numbered subpatterns to have
+ different names. Consider this pattern, where there are two capturing
+ subpatterns, both numbered 1:
+
+ (?|(?<AA>aa)|(?<BB>bb))
+
+ Perl allows this, with both names AA and BB as aliases of group 1.
+ Thus, after a successful match, both names yield the same value (either
+ "aa" or "bb").
+
+ In an attempt to reduce confusion, PCRE2 does not allow the same group
+ number to be associated with more than one name. The example above pro-
+ vokes a compile-time error. However, there is still scope for confu-
+ sion. Consider this pattern:
+
+ (?|(?<AA>aa)|(bb))
+
+ Although the second subpattern number 1 is not explicitly named, the
+ name AA is still an alias for subpattern 1. Whether the pattern matches
+ "aa" or "bb", a reference by name to group AA yields the matched
+ string.
+
+ By default, a name must be unique within a pattern, except that dupli-
+ cate names are permitted for subpatterns with the same number, for
+ example:
+
+ (?|(?<AA>aa)|(?<AA>bb))
+
+ The duplicate name constraint can be disabled by setting the PCRE2_DUP-
+ NAMES option at compile time, or by the use of (?J) within the pattern.
Duplicate names can be useful for patterns where only one instance of
- the named parentheses can match. Suppose you want to match the name 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:
@@ -7432,17 +7460,16 @@
(?<DN>Sat)(?:urday)?
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.)
+ match. The convenience functions 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. (An alternative way of solving this
+ problem is to use a "branch reset" subpattern, as described in the pre-
+ vious section.)
- The convenience functions 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 backreference to a non-unique named subpattern from else-
- where in the pattern, the subpatterns to which the name refers are
- checked in the order in which they appear in the overall pattern. The
+ where in the pattern, the subpatterns to which the name refers are
+ checked in the order in which they appear in the overall pattern. The
first one that is set is used for the reference. For example, this pat-
tern matches both "foofoo" and "barbar" but not "foobar" or "barfoo":
@@ -7450,9 +7477,8 @@
If you make a subroutine call to a non-unique named subpattern, the one
- that corresponds to the first occurrence of the name is used. In the
- absence of duplicate numbers (see the previous section) this is the one
- with the lowest number.
+ that corresponds to the first occurrence of the name is used. In the
+ absence of duplicate numbers 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
@@ -7462,17 +7488,10 @@
details of the interfaces for handling named subpatterns, see the
pcre2api documentation.
- Warning: You cannot use different names to distinguish between two sub-
- patterns with the same number because PCRE2 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 always give the same name to subpatterns with the same number, even
- when PCRE2_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
@@ -7486,17 +7505,17 @@
a parenthesized subpattern (including most 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,}
@@ -7505,26 +7524,26 @@
\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 modes, quantifiers apply to characters rather than to individual
- code units. Thus, for example, \x{100}{2} matches two characters, each
+ code units. Thus, for example, \x{100}{2} matches two characters, each
of which is represented by a two-byte sequence in a UTF-8 string. Simi-
- larly, \X{3} matches three Unicode extended grapheme clusters, each of
- which may be several code units long (and they may be of different
+ larly, \X{3} matches three Unicode extended grapheme clusters, each of
+ which may be several code units 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,}
@@ -7531,24 +7550,24 @@
+ 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 PCRE1 used to give an error at compile
+ Earlier versions of Perl and PCRE1 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-
+ 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
/\*.*\*/
@@ -7557,19 +7576,19 @@
/* 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.
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 pat-
+ and instead matches the minimum number of times possible, so the pat-
tern
/\*.*?\*/
- 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
@@ -7578,45 +7597,45 @@
only way the rest of the pattern matches.
If the PCRE2_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
+ 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 PCRE2_DOTALL option
- (equivalent to Perl's /s) is set, thus allowing the dot to match new-
- lines, 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
+ If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option
+ (equivalent to Perl's /s) is set, thus allowing the dot to match new-
+ lines, 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. PCRE2 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 PCRE2_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 PCRE2_DOTALL in order to obtain this opti-
mization, or alternatively, using ^ to indicate anchoring explicitly.
- However, there are some cases where the optimization cannot be used.
- When .* is inside capturing parentheses that are the subject of a
- backreference elsewhere in the pattern, a match at the start may fail
+ However, there are some cases where the optimization cannot be used.
+ When .* is inside capturing parentheses that are the subject of a
+ backreference 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.
- Another case where implicit anchoring is not applied is when the lead-
- ing .* is inside an atomic group. Once again, a match at the start may
+ Another case where implicit anchoring is not applied is when the lead-
+ ing .* is inside an atomic group. Once again, a match at the start may
fail where a later one succeeds. Consider this pattern:
(?>.*?a)b
- It matches "ab" in the subject "aab". The use of the backtracking con-
- trol verbs (*PRUNE) and (*SKIP) also disable this optimization, and
+ It matches "ab" in the subject "aab". The use of the backtracking con-
+ trol verbs (*PRUNE) and (*SKIP) also disable this optimization, and
there is an option, PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly.
When a capturing subpattern is repeated, the value captured is the sub-
@@ -7625,8 +7644,8 @@
(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))+
@@ -7636,53 +7655,53 @@
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
- exactly the string of characters that an identical standalone pattern
+ An alternative description is that a subpattern of this type matches
+ exactly 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
@@ -7692,46 +7711,46 @@
(abc|xyz){2,3}+
- Possessive quantifiers are always greedy; the setting of the
- PCRE2_UNGREEDY option is ignored. They are a convenient notation for
- the simpler forms of atomic group. However, there is no difference in
+ Possessive quantifiers are always greedy; the setting of the
+ PCRE2_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
+ 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 PCRE1 copied it from there. It ultimately
found its way into Perl at release 5.10.
- PCRE2 has an optimization that automatically "possessifies" certain
- simple 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
+ PCRE2 has an optimization that automatically "possessifies" certain
+ simple 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. This feature can be disabled by the PCRE2_NO_AUTO-
POSSESS option, or starting the pattern with (*NO_AUTO_POSSESS).
- 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 PCRE2 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 PCRE2 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+>)*[!?]
@@ -7742,29 +7761,29 @@
BACKREFERENCES
Outside a character class, a backslash followed by a digit greater than
- 0 (and possibly further digits) is a backreference to a capturing sub-
- pattern earlier (that is, to its left) in the pattern, provided there
+ 0 (and possibly further digits) is a backreference to a capturing sub-
+ 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 8,
- it is always taken as a backreference, 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 8. 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-
+ However, if the decimal number following the backslash is less than 8,
+ it is always taken as a backreference, 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 8. 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 backreference" to a
- subpattern whose number is 8 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 backreference" to a
+ subpattern whose number is 8 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 backreference to any
+ details of the handling of digits following a backslash. There is no
+ such problem when named parentheses are used. A backreference 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 a signed or unsigned number, optionally enclosed in
braces. These examples are all identical:
@@ -7772,9 +7791,9 @@
(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 signed number is a relative reference.
+ digits follow the reference. A signed number is a relative reference.
Consider this example:
(abc(def)ghi)\g{-1}
@@ -7781,37 +7800,37 @@
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.
- The sequence \g{+1} is a reference to the next capturing subpattern.
- This kind of forward reference can be useful it patterns that repeat.
+ The sequence \g{+1} is a reference to the next capturing subpattern.
+ This kind of forward reference can be useful it patterns that repeat.
Perl does not support the use of + in this way.
A backreference matches whatever actually matched the capturing subpat-
- tern in the current subject string, rather than anything matching the
- subpattern itself (see "Subpatterns as subroutines" below for a way of
+ tern 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 backreference, 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 backreference, 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 backreferences 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 backreference syntax, in which \g can be used for both numeric
- and named references, is also supported. We could rewrite the above
+ There are several different ways of writing backreferences 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 backreference syntax, in which \g can be used for both numeric
+ and named references, is also supported. We could rewrite the above
example in any of the following ways:
(?<p1>(?i)rah)\s+\k<p1>
@@ -7819,32 +7838,32 @@
(?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 backreference to the same subpattern. If a
- subpattern has not actually been used in a particular match, any back-
+ There may be more than one backreference 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 PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a backref-
erence 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 backrefer-
- ence number. If the pattern continues with a digit character, some
- delimiter must be used to terminate the backreference. If the
- PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, this can be white
- space. Otherwise, the \g{ syntax or an empty comment (see "Comments"
+ Because there may be many capturing parentheses in a pattern, all dig-
+ its following a backslash are taken as part of a potential backrefer-
+ ence number. If the pattern continues with a digit character, some
+ delimiter must be used to terminate the backreference. If the
+ PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, this can be white
+ space. Otherwise, the \g{ syntax or an empty comment (see "Comments"
below) can be used.
Recursive backreferences
- A backreference 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 backreference 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)+
@@ -7852,74 +7871,74 @@
matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
ation of the subpattern, the backreference 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 backreference. This can be done using alternation, as in the exam-
+ pattern must be such that the first iteration does not need to match
+ the backreference. This can be done using alternation, as in the exam-
ple above, or by a quantifier with a minimum of zero.
- Backreferences 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
+ Backreferences 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
+ An assertion is a test on the characters following or preceding the
current matching point that does not consume any characters. The simple
- assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
+ 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, and in each case an assertion
- may be positive (must succeed for matching to continue) or negative
+ 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, and in each case an assertion
+ may be positive (must succeed for matching to continue) or negative
(must not succeed for matching to continue). An assertion subpattern is
matched in the normal way, except that, when matching continues after a
successful assertion, the matching position in the subject string is as
it was before the assertion was processed.
- Assertion subpatterns are not capturing subpatterns. If an assertion
- contains capturing subpatterns within it, these are counted for the
- purposes of numbering the capturing subpatterns in the whole pattern.
- Within each branch of an assertion, locally captured substrings may be
+ Assertion subpatterns are not capturing subpatterns. If an assertion
+ contains capturing subpatterns within it, these are counted for the
+ purposes of numbering the capturing subpatterns in the whole pattern.
+ Within each branch of an assertion, locally captured substrings may be
referenced in the usual way. For example, a sequence such as (.)\g{-1}
can be used to check that two adjacent characters are the same.
- When a branch within an assertion fails to match, any substrings that
- were captured are discarded (as happens with any pattern branch that
- fails to match). A negative assertion succeeds only when all its
+ When a branch within an assertion fails to match, any substrings that
+ were captured are discarded (as happens with any pattern branch that
+ fails to match). A negative assertion succeeds only when all its
branches fail to match; this means that no captured substrings are ever
- retained after a successful negative assertion. When an assertion con-
+ retained after a successful negative assertion. When an assertion con-
tains a matching branch, what happens depends on the type of assertion.
- For a positive assertion, internally captured substrings in the suc-
- cessful branch are retained, and matching continues with the next pat-
- tern item after the assertion. For a negative assertion, a matching
- branch means that the assertion has failed. If the assertion is being
- used as a condition in a conditional subpattern (see below), captured
- substrings are retained, because matching continues with the "no"
+ For a positive assertion, internally captured substrings in the suc-
+ cessful branch are retained, and matching continues with the next pat-
+ tern item after the assertion. For a negative assertion, a matching
+ branch means that the assertion has failed. If the assertion is being
+ used as a condition in a conditional subpattern (see below), captured
+ substrings are retained, because matching continues with the "no"
branch of the condition. For other failing negative assertions, control
passes to the previous backtracking point, thus discarding any captured
strings within the assertion.
- For compatibility with Perl, most 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. However, an assertion that forms the condition for a condi-
- tional subpattern may not be quantified. In practice, for other asser-
+ For compatibility with Perl, most 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. However, an assertion that forms the condition for a condi-
+ tional subpattern may not be quantified. In practice, for other asser-
tions, 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
@@ -7929,38 +7948,38 @@
\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)
@@ -7969,66 +7988,66 @@
(?<!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
- different lengths, but it is acceptable to PCRE2 if rewritten to use
+ is not permitted, because its single top-level branch can match two
+ different lengths, but it is acceptable to PCRE2 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.
- In UTF-8 and UTF-16 modes, PCRE2 does not allow the \C escape (which
- matches a single code unit even in a UTF mode) to appear in lookbehind
- assertions, because it makes it impossible to calculate the length of
- the lookbehind. The \X and \R escapes, which can match different num-
+ In UTF-8 and UTF-16 modes, PCRE2 does not allow the \C escape (which
+ matches a single code unit even in a UTF mode) to appear in lookbehind
+ assertions, because it makes it impossible to calculate the length of
+ the lookbehind. The \X and \R escapes, which can match different num-
bers of code units, are never permitted in lookbehinds.
- "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
- lookbehinds, as long as the subpattern matches a fixed-length string.
- However, recursion, that is, a "subroutine" call into a group that is
+ "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
+ lookbehinds, as long as the subpattern matches a fixed-length string.
+ However, recursion, that is, a "subroutine" call into a group that is
already active, is not supported.
Perl does not support backreferences in lookbehinds. PCRE2 does support
- them, but only if certain conditions are met. The
- PCRE2_MATCH_UNSET_BACKREF option must not be set, there must be no use
+ them, but only if certain conditions are met. The
+ PCRE2_MATCH_UNSET_BACKREF option must not be set, there must be no use
of (?| in the pattern (it creates duplicate subpattern numbers), and if
- the backreference is by name, the name must be unique. Of course, the
- referenced subpattern must itself be of fixed length. The following
+ the backreference is by name, the name must be unique. Of course, the
+ referenced subpattern must itself be of fixed length. The following
pattern matches words containing at least two characters that begin and
end with the same character:
\b(\w)\w++(?<=\1)
- 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
- proceeds from left to right, PCRE2 will look for each "a" in the sub-
- ject and then see if what follows matches the rest of the pattern. If
+ when applied to a long string that does not match. Because matching
+ proceeds from left to right, PCRE2 will look for each "a" in the sub-
+ ject 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)
@@ -8035,8 +8054,8 @@
there can be no backtracking for the .*+ item because of the possessive
quantifier; it can match only the entire string. The subsequent lookbe-
- hind assertion does a single test on the last four characters. If it
- fails, the match fails immediately. For long strings, this approach
+ hind 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
@@ -8045,18 +8064,18 @@
(?<=\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".
@@ -8064,29 +8083,29 @@
(?<=(?<!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. An absent no-pattern is equivalent to
- an empty string (it always matches). If there are more than two alter-
+ If the condition is satisfied, the yes-pattern is used; otherwise the
+ no-pattern (if present) is used. An absent no-pattern is equivalent to
+ an empty string (it always matches). If there are more than two alter-
natives 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
@@ -8096,57 +8115,57 @@
(?(1) (A|B|C) | (D | (?(2)E|F) | E) )
- There are five kinds of condition: references to subpatterns, refer-
- ences to recursion, two pseudo-conditions called DEFINE and VERSION,
+ There are five kinds of condition: references to subpatterns, refer-
+ ences to recursion, two pseudo-conditions called DEFINE and VERSION,
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
- space to make it more readable (assume the PCRE2_EXTENDED option) and
+ Consider the following pattern, which contains non-significant white
+ space to make it more readable (assume the PCRE2_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
- PCRE1, which had this facility before Perl, the syntax (?(name)...) is
- also recognized. Note, however, that undelimited names consisting of
- the letter R followed by digits are ambiguous (see the following sec-
+ Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
+ used subpattern by name. For compatibility with earlier versions of
+ PCRE1, which had this facility before Perl, the syntax (?(name)...) is
+ also recognized. Note, however, that undelimited names consisting of
+ the letter R followed by digits are ambiguous (see the following sec-
tion).
Rewriting the above example to use a named subpattern gives this:
@@ -8153,31 +8172,31 @@
(?<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
- "Recursion" in this sense refers to any subroutine-like call from one
- part of the pattern to another, whether or not it is actually recur-
- sive. See the sections entitled "Recursive patterns" and "Subpatterns
+ "Recursion" in this sense refers to any subroutine-like call from one
+ part of the pattern to another, whether or not it is actually recur-
+ sive. See the sections entitled "Recursive patterns" and "Subpatterns
as subroutines" below for details of recursion and subpattern calls.
- If a condition is the string (R), and there is no subpattern with the
- name R, the condition is true if matching is currently in a recursion
- or subroutine call to the whole pattern or any subpattern. If digits
- follow the letter R, and there is no subpattern with that name, the
+ If a condition is the string (R), and there is no subpattern with the
+ name R, the condition is true if matching is currently in a recursion
+ or subroutine call to the whole pattern or any subpattern. If digits
+ follow the letter R, and there is no subpattern with that name, the
condition is true if the most recent call is into a subpattern with the
- given number, which must exist somewhere in the overall pattern. This
+ given number, which must exist somewhere in the overall pattern. This
is a contrived example that is equivalent to a+b:
((?(R1)a+|(?1)b))
- However, in both cases, if there is a subpattern with a matching name,
- the condition tests for its being set, as described in the section
- above, instead of testing for recursion. For example, creating a group
- with the name R1 by adding (?<R1>) to the above pattern completely
+ However, in both cases, if there is a subpattern with a matching name,
+ the condition tests for its being set, as described in the section
+ above, instead of testing for recursion. For example, creating a group
+ with the name R1 by adding (?<R1>) to the above pattern completely
changes its meaning.
If a name preceded by ampersand follows the letter R, for example:
@@ -8188,7 +8207,7 @@
of that name (which must exist within the pattern).
This condition does not check the entire recursion stack. It tests only
- the current level. If the name used in a condition of this kind is a
+ the current level. 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.
@@ -8197,10 +8216,10 @@
Defining subpatterns for use by reference only
If the condition is the string (DEFINE), the condition is always false,
- even if there is a group with the name DEFINE. In this case, there may
+ even if there is a group with the name DEFINE. In this case, there may
be only one alternative in the subpattern. It is always skipped if con-
- trol reaches this point in the pattern; the idea of DEFINE is that it
- can be used to define subroutines that can be referenced from else-
+ trol reaches this point in the pattern; the idea of DEFINE is that it
+ can be used to define subroutines that can be referenced from else-
where. (The use of subroutines is described below.) For example, a pat-
tern to match an IPv4 address such as "192.168.23.245" could be written
like this (ignore white space and line breaks):
@@ -8208,53 +8227,53 @@
(?(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.
Checking the PCRE2 version
- Programs that link with a PCRE2 library can check the version by call-
- ing pcre2_config() with appropriate arguments. Users of applications
- that do not have access to the underlying code cannot do this. A spe-
- cial "condition" called VERSION exists to allow such users to discover
+ Programs that link with a PCRE2 library can check the version by call-
+ ing pcre2_config() with appropriate arguments. Users of applications
+ that do not have access to the underlying code cannot do this. A spe-
+ cial "condition" called VERSION exists to allow such users to discover
which version of PCRE2 they are dealing with by using this condition to
- match a string such as "yesno". VERSION must be followed either by "="
+ match a string such as "yesno". VERSION must be followed either by "="
or ">=" and a version number. For example:
(?(VERSION>=10.4)yes|no)
- This pattern matches "yes" if the PCRE2 version is greater or equal to
- 10.4, or "no" otherwise. The fractional part of the version number may
+ This pattern matches "yes" if the PCRE2 version is greater or equal to
+ 10.4, or "no" otherwise. The fractional part of the version number may
not contain more than two digits.
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.
- When an assertion that is a condition contains capturing subpatterns,
- any capturing that occurs in a matching branch is retained afterwards,
+ When an assertion that is a condition contains capturing subpatterns,
+ any capturing that occurs in a matching branch is retained afterwards,
for both positive and negative assertions, because matching always con-
tinues after the assertion, whether it succeeds or fails. (Compare non-
- conditional assertions, when captures are retained only for positive
+ conditional assertions, when captures are retained only for positive
assertions that succeed.)
@@ -8261,44 +8280,44 @@
COMMENTS
There are two ways of including comments in patterns that are processed
- by PCRE2. In both cases, the start of the comment must not be in a
- character class, nor in the middle of any other sequence of related
- characters such as (?: or a subpattern name or number. The characters
+ by PCRE2. In both cases, the start of the comment must not be in a
+ character class, nor in the middle of any other sequence of related
+ characters 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
- PCRE2_EXTENDED or PCRE2_EXTENDED_MORE 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 sequence (?# marks the start of a comment that continues up to the
+ next closing parenthesis. Nested parentheses are not permitted. If the
+ PCRE2_EXTENDED or PCRE2_EXTENDED_MORE 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 characters are interpreted as newlines is controlled
- by an option passed to the compiling function or by a special sequence
+ by an option passed to the compiling function or by a special sequence
at the start of the pattern, as described in the section entitled "New-
line conventions" above. Note that the end of this type of comment is a
- literal newline sequence in the pattern; escape sequences that happen
+ literal newline sequence in the pattern; escape sequences that happen
to represent a newline do not count. For example, consider this pattern
- when PCRE2_EXTENDED is set, and the default newline convention (a sin-
+ when PCRE2_EXTENDED is set, and the default newline convention (a sin-
gle linefeed character) is in force:
abc #comment \n still comment
- On encountering the # character, pcre2_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, pcre2_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:
@@ -8308,109 +8327,109 @@
refers recursively to the pattern in which it appears.
Obviously, PCRE2 cannot support the interpolation of Perl code.
- Instead, it supports special syntax for recursion of the entire pat-
+ Instead, it supports special syntax for recursion of the entire pat-
tern, and also for individual subpattern recursion. After its introduc-
- tion in PCRE1 and Python, this kind of recursion was subsequently
+ tion in PCRE1 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 PCRE2 pattern solves the nested parentheses problem (assume the
+ This PCRE2 pattern solves the nested parentheses problem (assume the
PCRE2_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.
Be aware however, that if duplicate subpattern numbers are in use, rel-
- ative references refer to the earliest subpattern with the appropriate
+ ative references refer to the earliest subpattern with the appropriate
number. Consider, for example:
(?|(a)|(b)) (c) (?-2)
- The first two capturing groups (a) and (b) are both numbered 1, and
- group (c) is number 2. When the reference (?-2) is encountered, the
+ The first two capturing groups (a) and (b) are both numbered 1, and
+ group (c) is number 2. When the reference (?-2) is encountered, the
second most recently opened parentheses has the number 1, but it is the
- first such group (the (a) group) to which the recursion refers. This
- would be the same if an absolute reference (?1) was used. In other
- words, relative references are just a shorthand for computing a group
+ first such group (the (a) group) to which the recursion refers. This
+ would be the same if an absolute reference (?1) was used. In other
+ words, relative references are just a shorthand for computing a group
number.
- 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. The Perl syntax
- for this is (?&name); PCRE1's earlier syntax (?P>name) is also sup-
+ An alternative approach is to use named parentheses. The Perl syntax
+ for this is (?&name); PCRE1's earlier syntax (?P>name) is also sup-
ported. 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.
The example pattern that we have been looking at contains nested unlim-
- ited repeats, and so the use of a possessive quantifier for matching
- strings of non-parentheses is important when applying the pattern to
+ ited repeats, and so the use of a possessive quantifier for matching
+ strings of non-parentheses is important when applying the pattern 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
+ 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 pcre2callout 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.
- 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 PCRE2 and Perl
@@ -8417,65 +8436,67 @@
Some former differences between PCRE2 and Perl no longer exist.
- Before release 10.30, recursion processing in PCRE2 differed from Perl
- in that a recursive subpattern call was always treated as an atomic
- group. That is, once it had matched some of the subject string, it was
- never re-entered, even if it contained untried alternatives and there
- was a subsequent matching failure. (Historical note: PCRE implemented
+ Before release 10.30, recursion processing in PCRE2 differed from Perl
+ in that a recursive subpattern call was always treated as an atomic
+ group. That is, once it had matched some of the subject string, it was
+ never re-entered, even if it contained untried alternatives and there
+ was a subsequent matching failure. (Historical note: PCRE implemented
recursion before Perl did.)
- Starting with release 10.30, recursive subroutine calls are no longer
+ Starting with release 10.30, recursive subroutine calls are no longer
treated as atomic. That is, they can be re-entered to try unused alter-
- natives if there is a matching failure later in the pattern. This is
- now compatible with the way Perl works. If you want a subroutine call
+ natives if there is a matching failure later in the pattern. This is
+ now compatible with the way Perl works. If you want a subroutine call
to be atomic, you must explicitly enclose it in an atomic group.
- Supporting backtracking into recursions simplifies certain types of
+ Supporting backtracking into recursions simplifies certain types of
recursive pattern. For example, this pattern matches palindromic
strings:
^((.)(?1)\2|.?)$
- The second branch in the group matches a single central character in
- the palindrome when there are an odd number of characters, or nothing
- when there are an even number of characters, but in order to work it
- has to be able to try the second case when the rest of the pattern
+ The second branch in the group matches a single central character in
+ the palindrome when there are an odd number of characters, or nothing
+ when there are an even number of characters, but in order to work it
+ has to be able to try the second case when the rest of the pattern
match fails. If you want to match typical palindromic phrases, the pat-
- tern has to ignore all non-word characters, which can be done like
+ tern has to ignore all non-word characters, which can be done like
this:
^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$
- If run with the PCRE2_CASELESS option, this pattern matches phrases
- such as "A man, a plan, a canal: Panama!". Note the use of the posses-
- sive quantifier *+ to avoid backtracking into sequences of non-word
+ If run with the PCRE2_CASELESS option, this pattern matches phrases
+ such as "A man, a plan, a canal: Panama!". Note the use of the posses-
+ sive quantifier *+ to avoid backtracking into sequences of non-word
characters. Without this, PCRE2 takes a great deal longer (ten times or
- more) to match typical phrases, and Perl takes so long that you think
+ more) to match typical phrases, and Perl takes so long that you think
it has gone into a loop.
- Another way in which PCRE2 and Perl used to differ in their recursion
- processing is in the handling of captured values. Formerly in Perl,
- when a subpattern was called recursively or as a subpattern (see the
- next section), it had no access to any values that were captured out-
- side the recursion, whereas in PCRE2 these values can be referenced.
+ Another way in which PCRE2 and Perl used to differ in their recursion
+ processing is in the handling of captured values. Formerly in Perl,
+ when a subpattern was called recursively or as a subpattern (see the
+ next section), it had no access to any values that were captured out-
+ side the recursion, whereas in PCRE2 these values can be referenced.
Consider this pattern:
^(.)(\1|a(?2))
- This pattern matches "bab". The first capturing parentheses match "b",
+ This pattern matches "bab". The first capturing parentheses match "b",
then in the second group, when the backreference \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. This match used
+ \1 does now match "b" and so the whole match succeeds. This match used
to fail in Perl, but in later versions (I tried 5.024) it now works.
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
- absolute or relative, as in these examples:
+ 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 a
+ bit like a subroutine in a programming language. More accurately, PCRE2
+ treats the referenced subpattern as an independent subpattern which it
+ tries to match at the current matching position. 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)...
(...(relative)...)...(?-1)...
@@ -8485,48 +8506,52 @@
(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.
- Like recursions, subroutine calls used to be treated as atomic, but
- this changed at PCRE2 release 10.30, so backtracking into subroutine
- calls can now occur. However, any capturing parentheses that are set
+ Like recursions, subroutine calls used to be treated as atomic, but
+ this changed at PCRE2 release 10.30, so backtracking into subroutine
+ calls can now occur. However, 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.
+ The behaviour of backtracking control verbs in subpatterns when called
+ as subroutines is described in the section entitled "Backtracking verbs
+ in subroutines" below.
+
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
- PCRE2 supports an extension to Oniguruma: if a number is preceded by a
+ PCRE2 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 backreference; the latter is a subroutine
+ Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
+ synonymous. The former is a backreference; the latter is a subroutine
call.
@@ -8533,54 +8558,54 @@
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.
- PCRE2 provides a similar feature, but of course it cannot obey arbi-
- trary Perl code. The feature is called "callout". The caller of PCRE2
- provides an external function by putting its entry point in a match
- context using the function pcre2_set_callout(), and then passing that
- context to pcre2_match() or pcre2_dfa_match(). If no match context is
+ PCRE2 provides a similar feature, but of course it cannot obey arbi-
+ trary Perl code. The feature is called "callout". The caller of PCRE2
+ provides an external function by putting its entry point in a match
+ context using the function pcre2_set_callout(), and then passing that
+ context to pcre2_match() or pcre2_dfa_match(). If no match context is
passed, or if the callout entry point is set to NULL, callouts are dis-
abled.
- Within a regular expression, (?C<arg>) indicates a point at which the
- external function is to be called. There are two kinds of callout:
- those with a numerical argument and those with a string argument. (?C)
- on its own with no argument is treated as (?C0). A numerical argument
- allows the application to distinguish between different callouts.
- String arguments were added for release 10.20 to make it possible for
- script languages that use PCRE2 to embed short scripts within patterns
+ Within a regular expression, (?C<arg>) indicates a point at which the
+ external function is to be called. There are two kinds of callout:
+ those with a numerical argument and those with a string argument. (?C)
+ on its own with no argument is treated as (?C0). A numerical argument
+ allows the application to distinguish between different callouts.
+ String arguments were added for release 10.20 to make it possible for
+ script languages that use PCRE2 to embed short scripts within patterns
in a similar way to Perl.
During matching, when PCRE2 reaches a callout point, the external func-
- tion is called. It is provided with the number or string argument of
- the callout, the position in the pattern, and one item of data that is
+ tion is called. It is provided with the number or string argument of
+ the callout, the position in the pattern, and one item of data that is
also set in the match block. The callout function may cause matching to
proceed, to backtrack, or to fail.
- By default, PCRE2 implements a number of optimizations at matching
- time, and one side-effect is that sometimes callouts are skipped. If
- you need all possible callouts to happen, you need to set options that
- disable the relevant optimizations. More details, including a complete
- description of the programming interface to the callout function, are
+ By default, PCRE2 implements a number of optimizations at matching
+ time, and one side-effect is that sometimes callouts are skipped. If
+ you need all possible callouts to happen, you need to set options that
+ disable the relevant optimizations. More details, including a complete
+ description of the programming interface to the callout function, are
given in the pcre2callout documentation.
Callouts with numerical arguments
- If you just want to have a means of identifying different callout
- points, put a number less than 256 after the letter C. For example,
+ If you just want to have a means of identifying different callout
+ points, put a number less than 256 after the letter C. For example,
this pattern has two callout points:
(?C1)abc(?C2)def
- If the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), numerical
- callouts are automatically installed before each item in the pattern.
- They are all numbered 255. If there is a conditional group in the pat-
+ If the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), numerical
+ callouts are automatically installed before each item in the pattern.
+ They are all numbered 255. If there is a conditional group in the pat-
tern whose condition is an assertion, an additional callout is inserted
- just before the condition. An explicit callout may also be set at this
+ just before the condition. An explicit callout may also be set at this
position, as in this example:
(?(?C9)(?=a)abc|def)
@@ -8590,60 +8615,60 @@
Callouts with string arguments
- A delimited string may be used instead of a number as a callout argu-
- ment. The starting delimiter must be one of ` ' " ^ % # $ { and the
+ A delimited string may be used instead of a number as a callout argu-
+ ment. The starting delimiter must be one of ` ' " ^ % # $ { and the
ending delimiter is the same as the start, except for {, where the end-
- ing delimiter is }. If the ending delimiter is needed within the
+ ing delimiter is }. If the ending delimiter is needed within the
string, it must be doubled. For example:
(?C'ab ''c'' d')xyz(?C{any text})pqr
- The doubling is removed before the string is passed to the callout
+ The doubling is removed before the string is passed to the callout
function.
BACKTRACKING CONTROL
- There are a number of special "Backtracking Control Verbs" (to use
- Perl's terminology) that modify the behaviour of backtracking during
- matching. They are generally of the form (*VERB) or (*VERB:NAME). Some
- verbs take either form, possibly behaving differently depending on
+ There are a number of special "Backtracking Control Verbs" (to use
+ Perl's terminology) that modify the behaviour of backtracking during
+ matching. They are generally of the form (*VERB) or (*VERB:NAME). Some
+ verbs take either form, possibly behaving differently depending on
whether or not a name is present.
- By default, for compatibility with Perl, a name is any sequence of
+ By default, for compatibility with Perl, a name is any sequence of
characters that does not include a closing parenthesis. The name is not
- processed in any way, and it is not possible to include a closing
- parenthesis in the name. This can be changed by setting the
- PCRE2_ALT_VERBNAMES option, but the result is no longer Perl-compati-
+ processed in any way, and it is not possible to include a closing
+ parenthesis in the name. This can be changed by setting the
+ PCRE2_ALT_VERBNAMES option, but the result is no longer Perl-compati-
ble.
- When PCRE2_ALT_VERBNAMES is set, backslash processing is applied to
- verb names and only an unescaped closing parenthesis terminates the
- name. However, the only backslash items that are permitted are \Q, \E,
- and sequences such as \x{100} that define character code points. Char-
+ When PCRE2_ALT_VERBNAMES is set, backslash processing is applied to
+ verb names and only an unescaped closing parenthesis terminates the
+ name. However, the only backslash items that are permitted are \Q, \E,
+ and sequences such as \x{100} that define character code points. Char-
acter type escapes such as \d are faulted.
A closing parenthesis can be included in a name either as \) or between
- \Q and \E. In addition to backslash processing, if the PCRE2_EXTENDED
+ \Q and \E. In addition to backslash processing, if the PCRE2_EXTENDED
or PCRE2_EXTENDED_MORE option is also set, unescaped whitespace in verb
names is skipped, and #-comments are recognized, exactly as in the rest
- of the pattern. PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do not affect
+ of the pattern. PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do not affect
verb names unless PCRE2_ALT_VERBNAMES is also set.
- The maximum length of a name is 255 in the 8-bit library and 65535 in
- the 16-bit and 32-bit libraries. If the name is empty, that is, if the
- closing parenthesis immediately follows the colon, the effect is as if
+ The maximum length of a name is 255 in the 8-bit library and 65535 in
+ the 16-bit and 32-bit libraries. 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 pat-
tern.
- Since these verbs are specifically related to backtracking, most of
- them can be used only when the pattern is to be matched using the tra-
+ Since these verbs are specifically related to backtracking, most of
+ them can be used only when the pattern is to be matched using the tra-
ditional matching function, because that uses a backtracking algorithm.
- With the exception of (*FAIL), which behaves like a failing negative
+ With the exception of (*FAIL), which behaves like a failing negative
assertion, the backtracking control verbs cause an error if encountered
by the DFA matching function.
- The behaviour of these verbs in repeated groups, assertions, and in
+ The behaviour of these verbs in repeated groups, assertions, and in
subpatterns called as subroutines (whether or not recursively) is docu-
mented below.
@@ -8651,16 +8676,16 @@
PCRE2 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
+ may know the minimum length of matching subject, or that a particular
character must be present. When one of these optimizations bypasses the
- running of a match, any included backtracking verbs will not, of
+ 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 PCRE2_NO_START_OPTIMIZE option when calling pcre2_com-
- pile(), or by starting the pattern with (*NO_START_OPT). There is more
+ by setting the PCRE2_NO_START_OPTIMIZE option when calling pcre2_com-
+ pile(), or by starting the pattern with (*NO_START_OPT). There is more
discussion of this option in the section entitled "Compiling a pattern"
in the pcre2api documentation.
- Experiments with Perl suggest that it too has similar optimizations,
+ Experiments with Perl suggest that it too has similar optimizations,
and like PCRE2, turning them off can change the result of a match.
Verbs that act immediately
@@ -8669,63 +8694,63 @@
(*ACCEPT) or (*ACCEPT:NAME)
- 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
+ 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) in triggered in a posi-
- tive assertion, the assertion succeeds; in a negative assertion, the
+ tive assertion, the assertion succeeds; in a negative assertion, the
assertion fails.
- If (*ACCEPT) is inside capturing parentheses, the data so far is cap-
+ If (*ACCEPT) is inside capturing parentheses, the data so far is cap-
tured. 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 (*FAIL:NAME)
- This verb causes a matching failure, forcing backtracking to occur. It
- may be abbreviated to (*F). It is equivalent to (?!) but easier to
+ This verb causes a matching failure, forcing backtracking to occur. It
+ may be abbreviated to (*F). 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 PCRE2. The nearest equivalent is the callout fea-
+ are not present in PCRE2. The nearest equivalent is the callout fea-
ture, as for example in this pattern:
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).
- (*ACCEPT:NAME) and (*FAIL:NAME) behave exactly the same as
+ (*ACCEPT:NAME) and (*FAIL:NAME) behave exactly the same as
(*MARK:NAME)(*ACCEPT) and (*MARK:NAME)(*FAIL), respectively.
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:NAME) on
the matching path is passed back to the caller as described in the sec-
tion entitled "Other information about the match" in the pcre2api docu-
- mentation. This applies to all instances of (*MARK), including those
- inside assertions and atomic groups. (There are differences in those
- cases when (*MARK) is used in conjunction with (*SKIP) as described
+ mentation. This applies to all instances of (*MARK), including those
+ inside assertions and atomic groups. (There are differences in those
+ cases when (*MARK) is used in conjunction with (*SKIP) as described
below.)
- As well as (*MARK), the (*COMMIT), (*PRUNE) and (*THEN) verbs may have
- associated NAME arguments. Whichever is last on the matching path is
+ As well as (*MARK), the (*COMMIT), (*PRUNE) and (*THEN) verbs may have
+ associated NAME arguments. Whichever is last on the matching path is
passed back. See below for more details of these other verbs.
- Here is an example of pcre2test output, where the "mark" modifier
+ Here is an example of pcre2test output, where the "mark" modifier
requests the retrieval and outputting of (*MARK) data:
re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
@@ -8737,16 +8762,16 @@
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 a verb with a name is encountered in a positive assertion that is
- true, the name is recorded and passed back if it is the last-encoun-
+ If a verb with a name is encountered in a positive assertion that is
+ true, the name is recorded and passed back if it is the last-encoun-
tered. This does not happen for negative assertions or failing positive
assertions.
- After a partial match or a failed match, the last encountered name in
+ After a partial match or a failed match, the last encountered name in
the entire match process is returned. For example:
re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
@@ -8753,38 +8778,38 @@
data> XP
No match, mark = B
- Note that in this unanchored example the mark is retained from the
+ Note that in this unanchored example the mark is retained from the
match attempt that started at the letter "X" in the subject. Subsequent
match attempts starting at "P" and then with an empty string do not get
as far as the (*MARK) item, but nevertheless do not reset it.
- If you are interested in (*MARK) values after failed matches, you
- should probably set the PCRE2_NO_START_OPTIMIZE option (see above) to
+ If you are interested in (*MARK) values after failed matches, you
+ should probably set the PCRE2_NO_START_OPTIMIZE option (see above) to
ensure that the match is always attempted.
Verbs that act after backtracking
The following verbs do nothing when they are encountered. Matching con-
- tinues with what follows, but if there is a subsequent match failure,
- causing a backtrack to the verb, a failure is forced. That is, back-
- tracking cannot pass to the left of the verb. However, when one of
+ tinues with what follows, but if there is a subsequent match failure,
+ causing a backtrack to the verb, a failure is forced. That is, back-
+ tracking cannot pass to the left of the verb. However, when one of
these verbs appears inside an atomic group or in a lookaround assertion
- that is true, its effect is confined to that group, because once the
- group has been matched, there is never any backtracking into it. Back-
+ that is true, its effect is confined to that group, because once the
+ group has been matched, there is never any backtracking into it. Back-
tracking from beyond an assertion or an atomic group ignores the entire
group, and seeks a preceeding backtracking point.
- These verbs differ in exactly what kind of failure occurs when back-
- tracking reaches them. The behaviour described below is what happens
- when the verb is not in a subroutine or an assertion. Subsequent sec-
+ These verbs differ in exactly what kind of failure occurs when back-
+ tracking reaches them. The behaviour described below is what happens
+ when the verb is not in a subroutine or an assertion. Subsequent sec-
tions cover these special cases.
(*COMMIT) or (*COMMIT:NAME)
- This verb causes the whole match to fail outright if there is a later
+ This verb causes the whole match to fail outright if there is a later
matching failure that causes backtracking to reach it. Even if the pat-
- tern is unanchored, no further attempts to find a match by advancing
- the starting point take place. If (*COMMIT) is the only backtracking
+ tern is unanchored, no further attempts to find a match by advancing
+ the starting point take place. If (*COMMIT) is the only backtracking
verb that is encountered, once it has been passed pcre2_match() is com-
mitted to finding a match at the current starting point, or not at all.
For example:
@@ -8791,22 +8816,22 @@
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 behaviour of (*COMMIT:NAME) is not the same as (*MARK:NAME)(*COM-
- MIT). It is like (*MARK:NAME) in that the name is remembered for pass-
- ing back to the caller. However, (*SKIP:NAME) searches only for names
- set with (*MARK), ignoring those set by (*COMMIT), (*PRUNE) and
+ The behaviour of (*COMMIT:NAME) is not the same as (*MARK:NAME)(*COM-
+ MIT). It is like (*MARK:NAME) in that the name is remembered for pass-
+ ing back to the caller. However, (*SKIP:NAME) searches only for names
+ set with (*MARK), ignoring those set by (*COMMIT), (*PRUNE) and
(*THEN).
- If there is more than one backtracking verb in a pattern, a different
- one that follows (*COMMIT) may be triggered first, so merely passing
+ If there is more than one backtracking verb in a pattern, a different
+ one that follows (*COMMIT) may be triggered first, so merely passing
(*COMMIT) during a match does not always guarantee that a match must be
at this starting point.
- Note that (*COMMIT) at the start of a pattern is not the same as an
- anchor, unless PCRE2'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 PCRE2's start-of-match optimizations are turned off, as
shown in this output from pcre2test:
re> /(*COMMIT)abc/
@@ -8817,63 +8842,63 @@
data> xyzabc
No match
- For the first pattern, PCRE2 knows that any match must start with "a",
- so the optimization skips along the subject to "a" before applying the
- pattern to the first set of data. The match attempt then succeeds. The
- second pattern disables the optimization that skips along to the first
- character. The pattern is now applied starting at "x", and so the
- (*COMMIT) causes the match to fail without trying any other starting
+ For the first pattern, PCRE2 knows that any match must start with "a",
+ so the optimization skips along the subject to "a" before applying the
+ pattern to the first set of data. The match attempt then succeeds. The
+ second pattern disables the optimization that skips along to the first
+ character. The pattern is now applied starting at "x", and so the
+ (*COMMIT) causes the match 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
+ This verb causes the match to fail at the current starting position in
the subject if there is a later matching failure that causes backtrack-
- ing to reach it. 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 alternative to an atomic group or possessive quan-
+ ing to reach it. 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 alternative to an atomic group or possessive quan-
tifier, but there are some uses of (*PRUNE) that cannot be expressed in
- any other way. In an anchored pattern (*PRUNE) has the same effect as
+ any other way. In an anchored pattern (*PRUNE) has the same effect as
(*COMMIT).
The behaviour of (*PRUNE:NAME) is not the same as (*MARK:NAME)(*PRUNE).
It is like (*MARK:NAME) in that the name is remembered for passing back
- to the caller. However, (*SKIP:NAME) searches only for names set with
+ to the caller. However, (*SKIP:NAME) searches only for names set with
(*MARK), ignoring those set by (*COMMIT), (*PRUNE) or (*THEN).
(*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
- it cannot be part of a successful match if there is a later mismatch.
+ tered. (*SKIP) signifies that whatever text was matched leading up to
+ it cannot be part of a successful match if there is a later mismatch.
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. When
- such a (*SKIP) is triggered, 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 corre-
- sponds to that (*MARK) instead of to where (*SKIP) was encountered. If
+ When (*SKIP) has an associated name, its behaviour is modified. When
+ such a (*SKIP) is triggered, 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 corre-
+ sponds to that (*MARK) instead of to where (*SKIP) was encountered. If
no (*MARK) with a matching name is found, the (*SKIP) is ignored.
- The search for a (*MARK) name uses the normal backtracking mechanism,
- which means that it does not see (*MARK) settings that are inside
+ The search for a (*MARK) name uses the normal backtracking mechanism,
+ which means that it does not see (*MARK) settings that are inside
atomic groups or assertions, because they are never re-entered by back-
tracking. Compare the following pcre2test examples:
@@ -8887,106 +8912,106 @@
0: b
1: b
- In the first example, the (*MARK) setting is in an atomic group, so it
+ In the first example, the (*MARK) setting is in an atomic group, so it
is not seen when (*SKIP:X) triggers, causing the (*SKIP) to be ignored.
- This allows the second branch of the pattern to be tried at the first
- character position. In the second example, the (*MARK) setting is not
- in an atomic group. This allows (*SKIP:X) to find the (*MARK) when it
+ This allows the second branch of the pattern to be tried at the first
+ character position. In the second example, the (*MARK) setting is not
+ in an atomic group. This allows (*SKIP:X) to find the (*MARK) when it
backtracks, and this causes a new matching attempt to start at the sec-
- ond character. This time, the (*MARK) is never seen because "a" does
+ ond character. This time, the (*MARK) is never seen because "a" does
not match "b", so the matcher immediately jumps to the second branch of
the pattern.
- Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It
- ignores names that are set by (*COMMIT:NAME), (*PRUNE:NAME) or
+ Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It
+ ignores names that are set by (*COMMIT:NAME), (*PRUNE:NAME) or
(*THEN:NAME).
(*THEN) or (*THEN:NAME)
- This verb causes a skip to the next innermost alternative when back-
- tracking reaches it. That is, it cancels any further backtracking
- within the current alternative. Its name comes from the observation
+ This verb causes a skip to the next innermost alternative when back-
+ tracking reaches it. That is, it cancels any further backtracking
+ 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. If that succeeds and BAR fails, COND3 is tried. If subse-
- quently BAZ fails, there are no more alternatives, so there is a back-
- track to whatever came before the entire group. 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. If that succeeds and BAR fails, COND3 is tried. If subse-
+ quently BAZ fails, there are no more alternatives, so there is a back-
+ track to whatever came before the entire group. If (*THEN) is not
inside an alternation, it acts like (*PRUNE).
- The behaviour of (*THEN:NAME) is not the same as (*MARK:NAME)(*THEN).
+ The behaviour of (*THEN:NAME) is not the same as (*MARK:NAME)(*THEN).
It is like (*MARK:NAME) in that the name is remembered for passing back
- to the caller. However, (*SKIP:NAME) searches only for names set with
+ to the caller. However, (*SKIP:NAME) searches only for names set with
(*MARK), ignoring those set by (*COMMIT), (*PRUNE) and (*THEN).
- 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 subpattern to
- the enclosing alternative. Consider this pattern, where A, B, etc. are
- complex pattern fragments that do not contain any | characters at this
+ 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 subpattern to
+ the enclosing alternative. Consider this pattern, where A, B, etc. are
+ complex pattern fragments that do not contain any | characters 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 that a conditional subpattern is not considered as having two
- alternatives, because only one is ever used. In other words, the |
+ Note that a conditional subpattern is not considered as having two
+ 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.
More than one backtracking verb
- If more than one backtracking verb is present in a pattern, the one
- that is backtracked onto first acts. For example, consider this pat-
+ If more than one backtracking verb is present in a pattern, the one
+ that is backtracked onto first acts. For example, consider this pat-
tern, where A, B, etc. are complex pattern fragments:
(A(*COMMIT)B(*THEN)C|ABD)
- If A matches but B fails, the backtrack to (*COMMIT) causes the entire
+ If A matches but B fails, the backtrack to (*COMMIT) causes the entire
match to fail. However, if A and B match, but C fails, the backtrack to
- (*THEN) causes the next alternative (ABD) to be tried. This behaviour
- is consistent, but is not always the same as Perl's. It means that if
- two or more backtracking verbs appear in succession, all the the last
+ (*THEN) causes the next alternative (ABD) to be tried. This behaviour
+ is consistent, but is not always the same as Perl's. It means that if
+ two or more backtracking verbs appear in succession, all the the last
of them has no effect. Consider this example:
...(*COMMIT)(*PRUNE)...
If there is a matching failure to the right, backtracking onto (*PRUNE)
- causes it to be triggered, and its action is taken. There can never be
+ causes it to be triggered, and its action is taken. There can never be
a backtrack onto (*COMMIT).
Backtracking verbs in repeated groups
@@ -8996,42 +9021,42 @@
/(a(*COMMIT)b)+ac/
- If the subject is "abac", Perl matches unless its optimizations are
- disabled, but PCRE2 always fails because the (*COMMIT) in the second
+ If the subject is "abac", Perl matches unless its optimizations are
+ disabled, but PCRE2 always fails because the (*COMMIT) in the second
repeat of the group acts.
Backtracking verbs in assertions
- (*FAIL) in any assertion has its normal effect: it forces an immediate
- backtrack. The behaviour of the other backtracking verbs depends on
- whether or not the assertion is standalone or acting as the condition
+ (*FAIL) in any assertion has its normal effect: it forces an immediate
+ backtrack. The behaviour of the other backtracking verbs depends on
+ whether or not the assertion is standalone or acting as the condition
in a conditional subpattern.
- (*ACCEPT) in a standalone positive assertion causes the assertion to
- succeed without any further processing; captured strings and a (*MARK)
- name (if set) are retained. In a standalone negative assertion,
- (*ACCEPT) causes the assertion to fail without any further processing;
+ (*ACCEPT) in a standalone positive assertion causes the assertion to
+ succeed without any further processing; captured strings and a (*MARK)
+ name (if set) are retained. In a standalone negative assertion,
+ (*ACCEPT) causes the assertion to fail without any further processing;
captured substrings and any (*MARK) name are discarded.
- If the assertion is a condition, (*ACCEPT) causes the condition to be
- true for a positive assertion and false for a negative one; captured
+ If the assertion is a condition, (*ACCEPT) causes the condition to be
+ true for a positive assertion and false for a negative one; captured
substrings are retained in both cases.
The remaining verbs act only when a later failure causes a backtrack to
- reach them. This means that their effect is confined to the assertion,
+ reach them. This means that their effect is confined to the assertion,
because lookaround assertions are atomic. A backtrack that occurs after
an assertion is complete does not jump back into the assertion. Note in
- particular that a (*MARK) name that is set in an assertion is not
+ particular that a (*MARK) name that is set in an assertion is not
"seen" by an instance of (*SKIP:NAME) latter in the pattern.
- The effect of (*THEN) is not allowed to escape beyond an assertion. If
- there are no more branches to try, (*THEN) causes a positive assertion
+ The effect of (*THEN) is not allowed to escape beyond an assertion. If
+ there are no more branches to try, (*THEN) causes a positive assertion
to be false, and a negative assertion to be true.
- The other backtracking verbs are not treated specially if they appear
- in a standalone positive assertion. In a conditional positive asser-
+ The other backtracking verbs are not treated specially if they appear
+ in a standalone positive assertion. In a conditional positive asser-
tion, backtracking (from within the assertion) into (*COMMIT), (*SKIP),
- or (*PRUNE) causes the condition to be false. However, for both stand-
+ or (*PRUNE) causes the condition to be false. However, for both stand-
alone and conditional negative assertions, backtracking into (*COMMIT),
(*SKIP), or (*PRUNE) causes the assertion to be true, without consider-
ing any further alternative branches.
@@ -9038,28 +9063,28 @@
Backtracking verbs in subroutines
- These behaviours occur whether or not the subpattern is called recur-
+ These behaviours occur whether or not the subpattern is called recur-
sively.
- (*ACCEPT) in a subpattern called as a subroutine causes the subroutine
- match to succeed without any further processing. Matching then contin-
- ues after the subroutine call. Perl documents this behaviour. Perl's
+ (*ACCEPT) in a subpattern called as a subroutine causes the subroutine
+ match to succeed without any further processing. Matching then contin-
+ ues after the subroutine call. Perl documents this behaviour. Perl's
treatment of the other verbs in subroutines is different in some cases.
- (*FAIL) in a subpattern called as a subroutine has its normal effect:
+ (*FAIL) in a subpattern called as a subroutine has its normal effect:
it forces an immediate backtrack.
(*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine
cause the subroutine match to fail.
- (*THEN) skips to the next alternative in the innermost enclosing group
- within the subpattern that has alternatives. If there is no such group
+ (*THEN) skips to the next alternative in the innermost enclosing group
+ within the subpattern that has alternatives. If there is no such group
within the subpattern, (*THEN) causes the subroutine match to fail.
SEE ALSO
- pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2syntax(3),
+ pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2syntax(3),
pcre2(3).
Modified: code/trunk/doc/pcre2pattern.3
===================================================================
--- code/trunk/doc/pcre2pattern.3 2018-08-03 09:38:36 UTC (rev 977)
+++ code/trunk/doc/pcre2pattern.3 2018-08-03 16:56:54 UTC (rev 978)
@@ -218,7 +218,7 @@
.P
The newline convention affects where the circumflex and dollar assertions are
true. It also affects the interpretation of the dot metacharacter when
-PCRE2_DOTALL is not set, and the behaviour of \eN when not followed by an
+PCRE2_DOTALL is not set, and the behaviour of \eN when not followed by an
opening brace. However, it does not affect what the \eR escape sequence
matches. By default, this is any Unicode newline sequence, for Perl
compatibility. However, this can be changed; see the next section and the
@@ -331,7 +331,7 @@
If you want to remove the special meaning from a sequence of characters, you
can do so by putting them between \eQ and \eE. This is different from Perl in
that $ and @ are handled as literals in \eQ...\eE sequences in PCRE2, whereas
-in Perl, $ and @ cause variable interpolation. Also, Perl does "double-quotish
+in Perl, $ and @ cause variable interpolation. Also, Perl does "double-quotish
backslash interpolation" on any backslashes between \eQ and \eE which, its
documentation says, "may lead to confusing results". PCRE2 treats a backslash
between \eQ and \eE just like any other character. Note the following examples:
@@ -377,7 +377,7 @@
\eo{ddd..} character with octal code ddd..
\exhh character with hex code hh
\ex{hhh..} character with hex code hhh.. (default mode)
- \eN{U+hhh..} character with Unicode code point hhh..
+ \eN{U+hhh..} character with Unicode code point hhh..
\euhhhh character with hex code hhhh (when PCRE2_ALT_BSUX is set)
.sp
Note that when \eN is not followed by an opening brace (curly bracket) it has
@@ -581,7 +581,7 @@
\eD any character that is not a decimal digit
\eh any horizontal white space character
\eH any character that is not a horizontal white space character
- \eN any character that is not a newline
+ \eN any character that is not a newline
\es any white space character
\eS any character that is not a white space character
\ev any vertical white space character
@@ -594,8 +594,8 @@
.\" </a>
the "." metacharacter
.\"
-when PCRE2_DOTALL is not set, but setting PCRE2_DOTALL does not change the
-meaning of \eN. Note that when \eN is followed by an opening brace it has a
+when PCRE2_DOTALL is not set, but setting PCRE2_DOTALL does not change the
+meaning of \eN. Note that when \eN is followed by an opening brace it has a
different meaning. See the section entitled
.\" HTML <a href="#digitsafterbackslash">
.\" </a>
@@ -1029,8 +1029,8 @@
Unicode supports various kinds of composite character by giving each character
a grapheme breaking property, and having rules that use these properties to
define the boundaries of extended grapheme clusters. The rules are defined in
-Unicode Standard Annex 29, "Unicode Text Segmentation". Unicode 11.0.0
-abandoned the use of some previous properties that had been used for emojis.
+Unicode Standard Annex 29, "Unicode Text Segmentation". Unicode 11.0.0
+abandoned the use of some previous properties that had been used for emojis.
Instead it introduced various emoji-specific properties. PCRE2 uses only the
Extended Pictographic property.
.P
@@ -1310,7 +1310,7 @@
.P
The escape sequence \eN when not followed by an opening brace behaves like a
dot, except that it is not affected by the PCRE2_DOTALL option. In other words,
-it matches any character except one that signifies the end of a line.
+it matches any character except one that signifies the end of a line.
.P
When \eN is followed by an opening brace it has a different meaning. See the
section entitled
@@ -1643,7 +1643,7 @@
xx for PCRE2_EXTENDED_MORE
.sp
For example, (?im) sets caseless, multiline matching. It is also possible to
-unset these options by preceding the relevant letters with a hyphen, for
+unset these options by preceding the relevant letters with a hyphen, for
example (?-im). The two "extended" options are not independent; unsetting either
one cancels the effects of both of them.
.P
@@ -1653,9 +1653,9 @@
appears both before and after the hyphen, the option is unset. An empty options
setting "(?)" is allowed. Needless to say, it has no effect.
.P
-If the first character following (? is a circumflex, it causes all of the above
-options to be unset. Thus, (?^) is equivalent to (?-imnsx). Letters may follow
-the circumflex to cause some options to be re-instated, but a hyphen may not
+If the first character following (? is a circumflex, it causes all of the above
+options to be unset. Thus, (?^) is equivalent to (?-imnsx). Letters may follow
+the circumflex to cause some options to be re-instated, but a hyphen may not
appear.
.P
The PCRE2-specific options PCRE2_DUPNAMES and PCRE2_UNGREEDY can be changed in
@@ -1815,17 +1815,18 @@
.rs
.sp
Identifying capturing parentheses by number is simple, but it can be very hard
-to keep track of the numbers in complicated regular expressions. Furthermore,
-if an expression is modified, the numbers may change. To help with this
-difficulty, PCRE2 supports the naming of subpatterns. This feature was not
-added to Perl until release 5.10. Python had the feature earlier, and PCRE1
+to keep track of the numbers in complicated patterns. Furthermore, if an
+expression is modified, the numbers may change. To help with this difficulty,
+PCRE2 supports the naming of capturing subpatterns. This feature was not added
+to Perl until release 5.10. Python had the feature earlier, and PCRE1
introduced it at release 4.0, using the Python syntax. PCRE2 supports both the
-Perl and the Python syntax. Perl allows identically numbered subpatterns to
-have different names, but PCRE2 does not.
+Perl and the Python syntax.
.P
-In PCRE2, 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
+In PCRE2, a capturing subpattern can be named in one of three ways:
+(?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python. Names
+consist of up to 32 alphanumeric characters and underscores, but must start
+with a non-digit. References to capturing parentheses from other parts of the
+pattern, such as
.\" HTML <a href="#backreferences">
.\" </a>
backreferences,
@@ -1839,24 +1840,48 @@
.\" </a>
conditions,
.\"
-can be made by name as well as by number.
+can all be made by name as well as by number.
.P
-Names consist of up to 32 alphanumeric characters and underscores, but must
-start with a non-digit. Named capturing parentheses are still allocated numbers
-as well as names, exactly as if the names were not present. The PCRE2 API
-provides function calls for extracting the name-to-number translation table
-from a compiled pattern. There are also convenience functions for extracting a
-captured substring by name.
+Named capturing parentheses are allocated numbers as well as names, exactly as
+if the names were not present. In both PCRE2 and Perl, capturing subpatterns
+are primarily identified by numbers; any names are just aliases for these
+numbers. The PCRE2 API provides function calls for extracting the complete
+name-to-number translation table from a compiled pattern, as well as
+convenience functions for extracting captured substrings by name.
.P
-By default, a name must be unique within a pattern, but it is possible to relax
-this constraint by setting the PCRE2_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.) Duplicate 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:
+\fBWarning:\fP When more than one subpattern has the same number, as described
+in the previous section, a name given to one of them applies to all of them.
+Perl allows identically numbered subpatterns to have different names. Consider
+this pattern, where there are two capturing subpatterns, both numbered 1:
.sp
+ (?|(?<AA>aa)|(?<BB>bb))
+.sp
+Perl allows this, with both names AA and BB as aliases of group 1. Thus, after
+a successful match, both names yield the same value (either "aa" or "bb").
+.P
+In an attempt to reduce confusion, PCRE2 does not allow the same group number
+to be associated with more than one name. The example above provokes a
+compile-time error. However, there is still scope for confusion. Consider this
+pattern:
+.sp
+ (?|(?<AA>aa)|(bb))
+.sp
+Although the second subpattern number 1 is not explicitly named, the name AA is
+still an alias for subpattern 1. Whether the pattern matches "aa" or "bb", a
+reference by name to group AA yields the matched string.
+.P
+By default, a name must be unique within a pattern, except that duplicate names
+are permitted for subpatterns with the same number, for example:
+.sp
+ (?|(?<AA>aa)|(?<AA>bb))
+.sp
+The duplicate name constraint can be disabled by setting the PCRE2_DUPNAMES
+option at compile time, or by the use of (?J) within the pattern. Duplicate
+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:
+.sp
(?<DN>Mon|Fri|Sun)(?:day)?|
(?<DN>Tue)(?:sday)?|
(?<DN>Wed)(?:nesday)?|
@@ -1864,12 +1889,11 @@
(?<DN>Sat)(?:urday)?
.sp
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.)
-.P
The convenience functions 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.
+matched. This saves searching to find which numbered subpattern it was. (An
+alternative way of solving this problem is to use a "branch reset" subpattern,
+as described in the previous section.)
.P
If you make a backreference to a non-unique named subpattern from elsewhere in
the pattern, the subpatterns to which the name refers are checked in the order
@@ -1882,8 +1906,7 @@
.P
If you make a subroutine call to a non-unique named subpattern, the one that
corresponds to the first occurrence of the name is used. In the absence of
-duplicate numbers (see the previous section) this is the one with the lowest
-number.
+duplicate numbers this is the one with the lowest number.
.P
If you use a named reference in a condition
test (see the
@@ -1901,13 +1924,6 @@
\fBpcre2api\fP
.\"
documentation.
-.P
-\fBWarning:\fP You cannot use different names to distinguish between two
-subpatterns with the same number because PCRE2 uses only the numbers when
-matching. For this reason, an error is given at compile time if different names
-are given to subpatterns with the same number. However, you can always give the
-same name to subpatterns with the same number, even when PCRE2_DUPNAMES is not
-set.
.
.
.SH REPETITION
@@ -2336,13 +2352,13 @@
.P
Assertion subpatterns are not capturing subpatterns. If an assertion contains
capturing subpatterns within it, these are counted for the purposes of
-numbering the capturing subpatterns in the whole pattern. Within each branch of
+numbering the capturing subpatterns in the whole pattern. Within each branch of
an assertion, locally captured substrings may be referenced in the usual way.
-For example, a sequence such as (.)\eg{-1} can be used to check that two
+For example, a sequence such as (.)\eg{-1} can be used to check that two
adjacent characters are the same.
.P
When a branch within an assertion fails to match, any substrings that were
-captured are discarded (as happens with any pattern branch that fails to
+captured are discarded (as happens with any pattern branch that fails to
match). A negative assertion succeeds only when all its branches fail to match;
this means that no captured substrings are ever retained after a successful
negative assertion. When an assertion contains a matching branch, what happens
@@ -2358,7 +2374,7 @@
.\"
(see below), captured substrings are retained, because matching continues with
the "no" branch of the condition. For other failing negative assertions,
-control passes to the previous backtracking point, thus discarding any captured
+control passes to the previous backtracking point, thus discarding any captured
strings within the assertion.
.P
For compatibility with Perl, most assertion subpatterns may be repeated; though
@@ -2982,10 +2998,12 @@
.rs
.sp
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:
+name) is used outside the parentheses to which it refers, it operates a bit
+like a subroutine in a programming language. More accurately, PCRE2 treats the
+referenced subpattern as an independent subpattern which it tries to match at
+the current matching position. The called subpattern may be defined before or
+after the reference. A numbered reference can be absolute or relative, as in
+these examples:
.sp
(...(absolute)...)...(?2)...
(...(relative)...)...(?-1)...
@@ -3016,6 +3034,18 @@
.sp
It matches "abcabc". It does not match "abcABC" because the change of
processing option does not affect the called subpattern.
+.P
+The behaviour of
+.\" HTML <a href="#backtrackcontrol">
+.\" </a>
+backtracking control verbs
+.\"
+in subpatterns when called as subroutines is described in the section entitled
+.\" HTML <a href="#btsub">
+.\" </a>
+"Backtracking verbs in subroutines"
+.\"
+below.
.
.
.\" HTML <a name="onigurumasubroutines"></a>
@@ -3137,7 +3167,7 @@
are faulted.
.P
A closing parenthesis can be included in a name either as \e) or between \eQ
-and \eE. In addition to backslash processing, if the PCRE2_EXTENDED or
+and \eE. In addition to backslash processing, if the PCRE2_EXTENDED or
PCRE2_EXTENDED_MORE option is also set, unescaped whitespace in verb names is
skipped, and #-comments are recognized, exactly as in the rest of the pattern.
PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do not affect verb names unless
@@ -3194,7 +3224,7 @@
.\"
documentation.
.P
-Experiments with Perl suggest that it too has similar optimizations, and like
+Experiments with Perl suggest that it too has similar optimizations, and like
PCRE2, turning them off can change the result of a match.
.
.
@@ -3221,7 +3251,7 @@
.sp
(*FAIL) or (*FAIL:NAME)
.sp
-This verb causes a matching failure, forcing backtracking to occur. It may be
+This verb causes a matching failure, forcing backtracking to occur. It may be
abbreviated to (*F). 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 PCRE2. The
@@ -3232,7 +3262,7 @@
A match with the string "aaaa" always fails, but the callout is taken before
each backtrack happens (in this example, 10 times).
.P
-(*ACCEPT:NAME) and (*FAIL:NAME) behave exactly the same as
+(*ACCEPT:NAME) and (*FAIL:NAME) behave exactly the same as
(*MARK:NAME)(*ACCEPT) and (*MARK:NAME)(*FAIL), respectively.
.
.
@@ -3259,7 +3289,7 @@
\fBpcre2api\fP
.\"
documentation. This applies to all instances of (*MARK), including those inside
-assertions and atomic groups. (There are differences in those cases when
+assertions and atomic groups. (There are differences in those cases when
(*MARK) is used in conjunction with (*SKIP) as described below.)
.P
As well as (*MARK), the (*COMMIT), (*PRUNE) and (*THEN) verbs may have
@@ -3336,7 +3366,7 @@
a+(*COMMIT)b
.sp
This matches "xxaab" but not "aacaab". It can be thought of as a kind of
-dynamic anchor, or "I've started, so I must finish."
+dynamic anchor, or "I've started, so I must finish."
.P
The behaviour of (*COMMIT:NAME) is not the same as (*MARK:NAME)(*COMMIT). It is
like (*MARK:NAME) in that the name is remembered for passing back to the
@@ -3424,7 +3454,7 @@
data: abc
0: b
1: b
-.sp
+.sp
In the first example, the (*MARK) setting is in an atomic group, so it is not
seen when (*SKIP:X) triggers, causing the (*SKIP) to be ignored. This allows
the second branch of the pattern to be tried at the first character position.
@@ -3551,7 +3581,7 @@
(*ACCEPT) in a standalone positive assertion causes the assertion to succeed
without any further processing; captured strings and a (*MARK) name (if set)
are retained. In a standalone negative assertion, (*ACCEPT) causes the
-assertion to fail without any further processing; captured substrings and any
+assertion to fail without any further processing; captured substrings and any
(*MARK) name are discarded.
.P
If the assertion is a condition, (*ACCEPT) causes the condition to be true for
@@ -3558,11 +3588,11 @@
a positive assertion and false for a negative one; captured substrings are
retained in both cases.
.P
-The remaining verbs act only when a later failure causes a backtrack to
-reach them. This means that their effect is confined to the assertion,
+The remaining verbs act only when a later failure causes a backtrack to
+reach them. This means that their effect is confined to the assertion,
because lookaround assertions are atomic. A backtrack that occurs after an
-assertion is complete does not jump back into the assertion. Note in particular
-that a (*MARK) name that is set in an assertion is not "seen" by an instance of
+assertion is complete does not jump back into the assertion. Note in particular
+that a (*MARK) name that is set in an assertion is not "seen" by an instance of
(*SKIP:NAME) latter in the pattern.
.P
The effect of (*THEN) is not allowed to escape beyond an assertion. If there
