3850 lines
162 KiB
HTML
3850 lines
162 KiB
HTML
<html>
|
|
<head>
|
|
<title>pcre2pattern specification</title>
|
|
</head>
|
|
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
|
|
<h1>pcre2pattern man page</h1>
|
|
<p>
|
|
Return to the <a href="index.html">PCRE2 index page</a>.
|
|
</p>
|
|
<p>
|
|
This page is part of the PCRE2 HTML documentation. It was generated
|
|
automatically from the original man page. If there is any nonsense in it,
|
|
please consult the man page, in case the conversion went wrong.
|
|
<br>
|
|
<ul>
|
|
<li><a name="TOC1" href="#SEC1">PCRE2 REGULAR EXPRESSION DETAILS</a>
|
|
<li><a name="TOC2" href="#SEC2">SPECIAL START-OF-PATTERN ITEMS</a>
|
|
<li><a name="TOC3" href="#SEC3">EBCDIC CHARACTER CODES</a>
|
|
<li><a name="TOC4" href="#SEC4">CHARACTERS AND METACHARACTERS</a>
|
|
<li><a name="TOC5" href="#SEC5">BACKSLASH</a>
|
|
<li><a name="TOC6" href="#SEC6">CIRCUMFLEX AND DOLLAR</a>
|
|
<li><a name="TOC7" href="#SEC7">FULL STOP (PERIOD, DOT) AND \N</a>
|
|
<li><a name="TOC8" href="#SEC8">MATCHING A SINGLE CODE UNIT</a>
|
|
<li><a name="TOC9" href="#SEC9">SQUARE BRACKETS AND CHARACTER CLASSES</a>
|
|
<li><a name="TOC10" href="#SEC10">POSIX CHARACTER CLASSES</a>
|
|
<li><a name="TOC11" href="#SEC11">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a>
|
|
<li><a name="TOC12" href="#SEC12">VERTICAL BAR</a>
|
|
<li><a name="TOC13" href="#SEC13">INTERNAL OPTION SETTING</a>
|
|
<li><a name="TOC14" href="#SEC14">GROUPS</a>
|
|
<li><a name="TOC15" href="#SEC15">DUPLICATE GROUP NUMBERS</a>
|
|
<li><a name="TOC16" href="#SEC16">NAMED CAPTURE GROUPS</a>
|
|
<li><a name="TOC17" href="#SEC17">REPETITION</a>
|
|
<li><a name="TOC18" href="#SEC18">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
|
|
<li><a name="TOC19" href="#SEC19">BACKREFERENCES</a>
|
|
<li><a name="TOC20" href="#SEC20">ASSERTIONS</a>
|
|
<li><a name="TOC21" href="#SEC21">NON-ATOMIC ASSERTIONS</a>
|
|
<li><a name="TOC22" href="#SEC22">SCRIPT RUNS</a>
|
|
<li><a name="TOC23" href="#SEC23">CONDITIONAL GROUPS</a>
|
|
<li><a name="TOC24" href="#SEC24">COMMENTS</a>
|
|
<li><a name="TOC25" href="#SEC25">RECURSIVE PATTERNS</a>
|
|
<li><a name="TOC26" href="#SEC26">GROUPS AS SUBROUTINES</a>
|
|
<li><a name="TOC27" href="#SEC27">ONIGURUMA SUBROUTINE SYNTAX</a>
|
|
<li><a name="TOC28" href="#SEC28">CALLOUTS</a>
|
|
<li><a name="TOC29" href="#SEC29">BACKTRACKING CONTROL</a>
|
|
<li><a name="TOC30" href="#SEC30">SEE ALSO</a>
|
|
<li><a name="TOC31" href="#SEC31">AUTHOR</a>
|
|
<li><a name="TOC32" href="#SEC32">REVISION</a>
|
|
</ul>
|
|
<br><a name="SEC1" href="#TOC1">PCRE2 REGULAR EXPRESSION DETAILS</a><br>
|
|
<P>
|
|
The syntax and semantics of the regular expressions that are supported by PCRE2
|
|
are described in detail below. There is a quick-reference syntax summary in the
|
|
<a href="pcre2syntax.html"><b>pcre2syntax</b></a>
|
|
page. PCRE2 tries to match Perl syntax and semantics as closely as it can.
|
|
PCRE2 also supports some alternative regular expression syntax (which does not
|
|
conflict with the Perl syntax) in order to provide some compatibility with
|
|
regular expressions in Python, .NET, and Oniguruma.
|
|
</P>
|
|
<P>
|
|
Perl's regular expressions are described in its own documentation, and regular
|
|
expressions in general are covered in a number of books, some of which have
|
|
copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published
|
|
by O'Reilly, covers regular expressions in great detail. This description of
|
|
PCRE2's regular expressions is intended as reference material.
|
|
</P>
|
|
<P>
|
|
This document discusses the regular expression patterns that are supported by
|
|
PCRE2 when its main matching function, <b>pcre2_match()</b>, is used. PCRE2 also
|
|
has an alternative matching function, <b>pcre2_dfa_match()</b>, which matches
|
|
using a different algorithm that is not Perl-compatible. Some of the features
|
|
discussed below are not available when DFA matching is used. The advantages and
|
|
disadvantages of the alternative function, and how it differs from the normal
|
|
function, are discussed in the
|
|
<a href="pcre2matching.html"><b>pcre2matching</b></a>
|
|
page.
|
|
</P>
|
|
<br><a name="SEC2" href="#TOC1">SPECIAL START-OF-PATTERN ITEMS</a><br>
|
|
<P>
|
|
A number of options that can be passed to <b>pcre2_compile()</b> can also be set
|
|
by special items at the start of a pattern. These are not Perl-compatible, but
|
|
are provided to make these options accessible to pattern writers who are not
|
|
able to change the program that processes the pattern. Any number of these
|
|
items may appear, but they must all be together right at the start of the
|
|
pattern string, and the letters must be in upper case.
|
|
</P>
|
|
<br><b>
|
|
UTF support
|
|
</b><br>
|
|
<P>
|
|
In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either as
|
|
single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32 can be
|
|
specified for the 32-bit library, in which case it constrains the character
|
|
values to valid Unicode code points. To process UTF strings, PCRE2 must be
|
|
built to include Unicode support (which is the default). When using UTF strings
|
|
you must either call the compiling function with one or both of the PCRE2_UTF
|
|
or PCRE2_MATCH_INVALID_UTF options, or the pattern must start with the special
|
|
sequence (*UTF), which is equivalent to setting the relevant PCRE2_UTF. How
|
|
setting a UTF mode affects pattern matching is mentioned in several places
|
|
below. There is also a summary of features in the
|
|
<a href="pcre2unicode.html"><b>pcre2unicode</b></a>
|
|
page.
|
|
</P>
|
|
<P>
|
|
Some applications that allow their users to supply patterns may wish to
|
|
restrict them to non-UTF data for security reasons. If the PCRE2_NEVER_UTF
|
|
option is passed to <b>pcre2_compile()</b>, (*UTF) is not allowed, and its
|
|
appearance in a pattern causes an error.
|
|
</P>
|
|
<br><b>
|
|
Unicode property support
|
|
</b><br>
|
|
<P>
|
|
Another special sequence that may appear at the start of a pattern is (*UCP).
|
|
This has the same effect as setting the PCRE2_UCP option: it causes sequences
|
|
such as \d and \w to use Unicode properties to determine character types,
|
|
instead of recognizing only characters with codes less than 256 via a lookup
|
|
table.
|
|
</P>
|
|
<P>
|
|
Some applications that allow their users to supply patterns may wish to
|
|
restrict them for security reasons. If the PCRE2_NEVER_UCP option is passed to
|
|
<b>pcre2_compile()</b>, (*UCP) is not allowed, and its appearance in a pattern
|
|
causes an error.
|
|
</P>
|
|
<br><b>
|
|
Locking out empty string matching
|
|
</b><br>
|
|
<P>
|
|
Starting a pattern with (*NOTEMPTY) or (*NOTEMPTY_ATSTART) has the same effect
|
|
as passing the PCRE2_NOTEMPTY or PCRE2_NOTEMPTY_ATSTART option to whichever
|
|
matching function is subsequently called to match the pattern. These options
|
|
lock out the matching of empty strings, either entirely, or only at the start
|
|
of the subject.
|
|
</P>
|
|
<br><b>
|
|
Disabling auto-possessification
|
|
</b><br>
|
|
<P>
|
|
If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting
|
|
the PCRE2_NO_AUTO_POSSESS option. This stops PCRE2 from making quantifiers
|
|
possessive when what follows cannot match the repeated item. For example, by
|
|
default a+b is treated as a++b. For more details, see the
|
|
<a href="pcre2api.html"><b>pcre2api</b></a>
|
|
documentation.
|
|
</P>
|
|
<br><b>
|
|
Disabling start-up optimizations
|
|
</b><br>
|
|
<P>
|
|
If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
|
|
PCRE2_NO_START_OPTIMIZE option. This disables several optimizations for quickly
|
|
reaching "no match" results. For more details, see the
|
|
<a href="pcre2api.html"><b>pcre2api</b></a>
|
|
documentation.
|
|
</P>
|
|
<br><b>
|
|
Disabling automatic anchoring
|
|
</b><br>
|
|
<P>
|
|
If a pattern starts with (*NO_DOTSTAR_ANCHOR), it has the same effect as
|
|
setting the PCRE2_NO_DOTSTAR_ANCHOR option. This disables optimizations that
|
|
apply to patterns whose top-level branches all start with .* (match any number
|
|
of arbitrary characters). For more details, see the
|
|
<a href="pcre2api.html"><b>pcre2api</b></a>
|
|
documentation.
|
|
</P>
|
|
<br><b>
|
|
Disabling JIT compilation
|
|
</b><br>
|
|
<P>
|
|
If a pattern that starts with (*NO_JIT) is successfully compiled, an attempt by
|
|
the application to apply the JIT optimization by calling
|
|
<b>pcre2_jit_compile()</b> is ignored.
|
|
</P>
|
|
<br><b>
|
|
Setting match resource limits
|
|
</b><br>
|
|
<P>
|
|
The <b>pcre2_match()</b> function contains a counter that is incremented every
|
|
time it goes round its main loop. The caller of <b>pcre2_match()</b> can set a
|
|
limit on this counter, which therefore limits the amount of computing resource
|
|
used for a match. The maximum depth of nested backtracking can also be limited;
|
|
this indirectly restricts the amount of heap memory that is used, but there is
|
|
also an explicit memory limit that can be set.
|
|
</P>
|
|
<P>
|
|
These facilities are provided to catch runaway matches that are provoked by
|
|
patterns with huge matching trees. A common example is a pattern with nested
|
|
unlimited repeats applied to a long string that does not match. When one of
|
|
these limits is reached, <b>pcre2_match()</b> gives an error return. The limits
|
|
can also be set by items at the start of the pattern of the form
|
|
<pre>
|
|
(*LIMIT_HEAP=d)
|
|
(*LIMIT_MATCH=d)
|
|
(*LIMIT_DEPTH=d)
|
|
</pre>
|
|
where d is any number of decimal digits. However, the value of the setting must
|
|
be less than the value set (or defaulted) by the caller of <b>pcre2_match()</b>
|
|
for it to have any effect. In other words, the pattern writer can lower the
|
|
limits set by the programmer, but not raise them. If there is more than one
|
|
setting of one of these limits, the lower value is used. The heap limit is
|
|
specified in kibibytes (units of 1024 bytes).
|
|
</P>
|
|
<P>
|
|
Prior to release 10.30, LIMIT_DEPTH was called LIMIT_RECURSION. This name is
|
|
still recognized for backwards compatibility.
|
|
</P>
|
|
<P>
|
|
The heap limit applies only when the <b>pcre2_match()</b> or
|
|
<b>pcre2_dfa_match()</b> interpreters are used for matching. It does not apply
|
|
to JIT. The match limit is used (but in a different way) when JIT is being
|
|
used, or when <b>pcre2_dfa_match()</b> is called, to limit computing resource
|
|
usage by those matching functions. The depth limit is ignored by JIT but is
|
|
relevant for DFA matching, which uses function recursion for recursions within
|
|
the pattern and for lookaround assertions and atomic groups. In this case, the
|
|
depth limit controls the depth of such recursion.
|
|
<a name="newlines"></a></P>
|
|
<br><b>
|
|
Newline conventions
|
|
</b><br>
|
|
<P>
|
|
PCRE2 supports six different conventions for indicating line breaks in
|
|
strings: a single CR (carriage return) character, a single LF (linefeed)
|
|
character, the two-character sequence CRLF, any of the three preceding, any
|
|
Unicode newline sequence, or the NUL character (binary zero). The
|
|
<a href="pcre2api.html"><b>pcre2api</b></a>
|
|
page has
|
|
<a href="pcre2api.html#newlines">further discussion</a>
|
|
about newlines, and shows how to set the newline convention when calling
|
|
<b>pcre2_compile()</b>.
|
|
</P>
|
|
<P>
|
|
It is also possible to specify a newline convention by starting a pattern
|
|
string with one of the following sequences:
|
|
<pre>
|
|
(*CR) carriage return
|
|
(*LF) linefeed
|
|
(*CRLF) carriage return, followed by linefeed
|
|
(*ANYCRLF) any of the three above
|
|
(*ANY) all Unicode newline sequences
|
|
(*NUL) the NUL character (binary zero)
|
|
</pre>
|
|
These override the default and the options given to the compiling function. For
|
|
example, on a Unix system where LF is the default newline sequence, the pattern
|
|
<pre>
|
|
(*CR)a.b
|
|
</pre>
|
|
changes the convention to CR. That pattern matches "a\nb" because LF is no
|
|
longer a newline. If more than one of these settings is present, the last one
|
|
is used.
|
|
</P>
|
|
<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
|
|
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
|
|
description of \R in the section entitled
|
|
<a href="#newlineseq">"Newline sequences"</a>
|
|
below. A change of \R setting can be combined with a change of newline
|
|
convention.
|
|
</P>
|
|
<br><b>
|
|
Specifying what \R matches
|
|
</b><br>
|
|
<P>
|
|
It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
|
|
complete set of Unicode line endings) by setting the option PCRE2_BSR_ANYCRLF
|
|
at compile time. This effect can also be achieved by starting a pattern with
|
|
(*BSR_ANYCRLF). For completeness, (*BSR_UNICODE) is also recognized,
|
|
corresponding to PCRE2_BSR_UNICODE.
|
|
</P>
|
|
<br><a name="SEC3" href="#TOC1">EBCDIC CHARACTER CODES</a><br>
|
|
<P>
|
|
PCRE2 can be compiled to run in an environment that uses EBCDIC as its
|
|
character code instead of ASCII or Unicode (typically a mainframe system). In
|
|
the sections below, character code values are ASCII or Unicode; in an EBCDIC
|
|
environment these characters may have different code values, and there are no
|
|
code points greater than 255.
|
|
</P>
|
|
<br><a name="SEC4" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
|
|
<P>
|
|
A regular expression is a pattern that is matched against a subject string from
|
|
left to right. Most characters stand for themselves in a pattern, and match the
|
|
corresponding characters in the subject. As a trivial example, the pattern
|
|
<pre>
|
|
The quick brown fox
|
|
</pre>
|
|
matches a portion of a subject string that is identical to itself. When
|
|
caseless matching is specified (the PCRE2_CASELESS option), letters are matched
|
|
independently of case.
|
|
</P>
|
|
<P>
|
|
The power of regular expressions comes from the ability to include wild cards,
|
|
character classes, alternatives, and repetitions in the pattern. These are
|
|
encoded in the pattern by the use of <i>metacharacters</i>, which do not stand
|
|
for themselves but instead are interpreted in some special way.
|
|
</P>
|
|
<P>
|
|
There are two different sets of metacharacters: those that are recognized
|
|
anywhere in the pattern except within square brackets, and those that are
|
|
recognized within square brackets. Outside square brackets, the metacharacters
|
|
are as follows:
|
|
<pre>
|
|
\ general escape character with several uses
|
|
^ assert start of string (or line, in multiline mode)
|
|
$ assert end of string (or line, in multiline mode)
|
|
. match any character except newline (by default)
|
|
[ start character class definition
|
|
| start of alternative branch
|
|
( start group or control verb
|
|
) end group or control verb
|
|
* 0 or more quantifier
|
|
+ 1 or more quantifier; also "possessive quantifier"
|
|
? 0 or 1 quantifier; also quantifier minimizer
|
|
{ start min/max quantifier
|
|
</pre>
|
|
Part of a pattern that is in square brackets is called a "character class". In
|
|
a character class the only metacharacters are:
|
|
<pre>
|
|
\ general escape character
|
|
^ negate the class, but only if the first character
|
|
- indicates character range
|
|
[ POSIX character class (if followed by POSIX syntax)
|
|
] terminates the character class
|
|
</pre>
|
|
The following sections describe the use of each of the metacharacters.
|
|
</P>
|
|
<br><a name="SEC5" href="#TOC1">BACKSLASH</a><br>
|
|
<P>
|
|
The backslash character has several uses. Firstly, if it is followed by a
|
|
character that is not a digit or a letter, it takes away any special meaning
|
|
that character may have. This use of backslash as an escape character applies
|
|
both inside and outside character classes.
|
|
</P>
|
|
<P>
|
|
For example, if you want to match a * character, you must write \* in the
|
|
pattern. This escaping action applies whether or not the following character
|
|
would otherwise be interpreted as a metacharacter, so it is always safe to
|
|
precede a non-alphanumeric with backslash to specify that it stands for itself.
|
|
In particular, if you want to match a backslash, you write \\.
|
|
</P>
|
|
<P>
|
|
In a UTF mode, only ASCII digits and letters have any special meaning after a
|
|
backslash. All other characters (in particular, those whose code points are
|
|
greater than 127) are treated as literals.
|
|
</P>
|
|
<P>
|
|
If a pattern is compiled with the PCRE2_EXTENDED option, most white space in
|
|
the pattern (other than in a character class), and characters between a #
|
|
outside a character class and the next newline, inclusive, are ignored. An
|
|
escaping backslash can be used to include a white space or # character as part
|
|
of the pattern.
|
|
</P>
|
|
<P>
|
|
If you want to treat all characters in a sequence as literals, 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 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:
|
|
<pre>
|
|
Pattern PCRE2 matches Perl matches
|
|
|
|
\Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
|
|
\Qabc\$xyz\E abc\$xyz abc\$xyz
|
|
\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
|
|
\QA\B\E A\B A\B
|
|
\Q\\E \ \\E
|
|
</pre>
|
|
The \Q...\E sequence is recognized both inside and outside character classes.
|
|
An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
|
|
by \E later in the pattern, the literal interpretation continues to the end of
|
|
the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
|
|
a character class, this causes an error, because the character class is not
|
|
terminated by a closing square bracket.
|
|
<a name="digitsafterbackslash"></a></P>
|
|
<br><b>
|
|
Non-printing characters
|
|
</b><br>
|
|
<P>
|
|
A second use of backslash provides a way of encoding non-printing characters
|
|
in patterns in a visible manner. There is no restriction on the appearance of
|
|
non-printing characters in a pattern, but when a pattern is being prepared by
|
|
text editing, it is often easier to use one of the following escape sequences
|
|
instead of the binary character it represents. In an ASCII or Unicode
|
|
environment, these escapes are as follows:
|
|
<pre>
|
|
\a alarm, that is, the BEL character (hex 07)
|
|
\cx "control-x", where x is any printable ASCII character
|
|
\e escape (hex 1B)
|
|
\f form feed (hex 0C)
|
|
\n linefeed (hex 0A)
|
|
\r carriage return (hex 0D) (but see below)
|
|
\t tab (hex 09)
|
|
\0dd character with octal code 0dd
|
|
\ddd character with octal code ddd, or backreference
|
|
\o{ddd..} character with octal code ddd..
|
|
\xhh character with hex code hh
|
|
\x{hhh..} character with hex code hhh..
|
|
\N{U+hhh..} character with Unicode hex code point hhh..
|
|
</pre>
|
|
By default, after \x that is not followed by {, from zero to two hexadecimal
|
|
digits are read (letters can be in upper or lower case). Any number of
|
|
hexadecimal digits may appear between \x{ and }. If a character other than a
|
|
hexadecimal digit appears between \x{ and }, or if there is no terminating },
|
|
an error occurs.
|
|
</P>
|
|
<P>
|
|
Characters whose code points are less than 256 can be defined by either of the
|
|
two syntaxes for \x or by an octal sequence. There is no difference in the way
|
|
they are handled. For example, \xdc is exactly the same as \x{dc} or \334.
|
|
However, using the braced versions does make such sequences easier to read.
|
|
</P>
|
|
<P>
|
|
Support is available for some ECMAScript (aka JavaScript) escape sequences via
|
|
two compile-time options. If PCRE2_ALT_BSUX is set, the sequence \x followed
|
|
by { is not recognized. Only if \x is followed by two hexadecimal digits is it
|
|
recognized as a character escape. Otherwise it is interpreted as a literal "x"
|
|
character. In this mode, support for code points greater than 256 is provided
|
|
by \u, which must be followed by four hexadecimal digits; otherwise it is
|
|
interpreted as a literal "u" character.
|
|
</P>
|
|
<P>
|
|
PCRE2_EXTRA_ALT_BSUX has the same effect as PCRE2_ALT_BSUX and, in addition,
|
|
\u{hhh..} is recognized as the character specified by hexadecimal code point.
|
|
There may be any number of hexadecimal digits. This syntax is from ECMAScript
|
|
6.
|
|
</P>
|
|
<P>
|
|
The \N{U+hhh..} escape sequence is recognized only when PCRE2 is operating in
|
|
UTF mode. Perl also uses \N{name} to specify characters by Unicode name; PCRE2
|
|
does not support this. Note that when \N is not followed by an opening brace
|
|
(curly bracket) it has an entirely different meaning, matching any character
|
|
that is not a newline.
|
|
</P>
|
|
<P>
|
|
There are some legacy applications where the escape sequence \r is expected to
|
|
match a newline. If the PCRE2_EXTRA_ESCAPED_CR_IS_LF option is set, \r in a
|
|
pattern is converted to \n so that it matches a LF (linefeed) instead of a CR
|
|
(carriage return) character.
|
|
</P>
|
|
<P>
|
|
The precise effect of \cx on ASCII characters is as follows: if x is a lower
|
|
case letter, it is converted to upper case. Then bit 6 of the character (hex
|
|
40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A (A is 41, Z is 5A),
|
|
but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the
|
|
code unit following \c has a value less than 32 or greater than 126, a
|
|
compile-time error occurs.
|
|
</P>
|
|
<P>
|
|
When PCRE2 is compiled in EBCDIC mode, \N{U+hhh..} is not supported. \a, \e,
|
|
\f, \n, \r, and \t generate the appropriate EBCDIC code values. The \c
|
|
escape is processed as specified for Perl in the <b>perlebcdic</b> document. The
|
|
only characters that are allowed after \c are A-Z, a-z, or one of @, [, \, ],
|
|
^, _, or ?. Any other character provokes a compile-time error. The sequence
|
|
\c@ encodes character code 0; after \c the letters (in either case) encode
|
|
characters 1-26 (hex 01 to hex 1A); [, \, ], ^, and _ encode characters 27-31
|
|
(hex 1B to hex 1F), and \c? becomes either 255 (hex FF) or 95 (hex 5F).
|
|
</P>
|
|
<P>
|
|
Thus, apart from \c?, these escapes generate the same character code values as
|
|
they do in an ASCII environment, though the meanings of the values mostly
|
|
differ. For example, \cG always generates code value 7, which is BEL in ASCII
|
|
but DEL in EBCDIC.
|
|
</P>
|
|
<P>
|
|
The sequence \c? generates DEL (127, hex 7F) in an ASCII environment, but
|
|
because 127 is not a control character in EBCDIC, Perl makes it generate the
|
|
APC character. Unfortunately, there are several variants of EBCDIC. In most of
|
|
them the APC character has the value 255 (hex FF), but in the one Perl calls
|
|
POSIX-BC its value is 95 (hex 5F). If certain other characters have POSIX-BC
|
|
values, PCRE2 makes \c? generate 95; otherwise it generates 255.
|
|
</P>
|
|
<P>
|
|
After \0 up to two further octal digits are read. If there are fewer than two
|
|
digits, just those that are present are used. Thus the sequence \0\x\015
|
|
specifies two binary zeros followed by a CR character (code value 13). Make
|
|
sure you supply two digits after the initial zero if the pattern character that
|
|
follows is itself an octal digit.
|
|
</P>
|
|
<P>
|
|
The escape \o must be followed by a sequence of octal digits, enclosed in
|
|
braces. An error occurs if this is not the case. This escape is a recent
|
|
addition to Perl; it provides way of specifying character code points as octal
|
|
numbers greater than 0777, and it also allows octal numbers and backreferences
|
|
to be unambiguously specified.
|
|
</P>
|
|
<P>
|
|
For greater clarity and unambiguity, it is best to avoid following \ by a
|
|
digit greater than zero. Instead, use \o{} or \x{} to specify numerical
|
|
character code points, and \g{} to specify backreferences. The following
|
|
paragraphs describe the old, ambiguous syntax.
|
|
</P>
|
|
<P>
|
|
The handling of a backslash followed by a digit other than 0 is complicated,
|
|
and Perl has changed over time, causing PCRE2 also to change.
|
|
</P>
|
|
<P>
|
|
Outside a character class, PCRE2 reads the digit and any following digits as a
|
|
decimal number. If the number is less than 10, begins with the digit 8 or 9, or
|
|
if there are at least that many previous capture groups in the expression, the
|
|
entire sequence is taken as a <i>backreference</i>. A description of how this
|
|
works is given
|
|
<a href="#backreferences">later,</a>
|
|
following the discussion of
|
|
<a href="#group">parenthesized groups.</a>
|
|
Otherwise, up to three octal digits are read to form a character code.
|
|
</P>
|
|
<P>
|
|
Inside a character class, PCRE2 handles \8 and \9 as the literal characters
|
|
"8" and "9", and otherwise reads up to three octal digits following the
|
|
backslash, using them to generate a data character. Any subsequent digits stand
|
|
for themselves. For example, outside a character class:
|
|
<pre>
|
|
\040 is another way of writing an ASCII space
|
|
\40 is the same, provided there are fewer than 40 previous capture groups
|
|
\7 is always a backreference
|
|
\11 might be a backreference, or another way of writing a tab
|
|
\011 is always a tab
|
|
\0113 is a tab followed by the character "3"
|
|
\113 might be a backreference, otherwise the character with octal code 113
|
|
\377 might be a backreference, otherwise the value 255 (decimal)
|
|
\81 is always a backreference .sp
|
|
</pre>
|
|
Note that octal values of 100 or greater that are specified using this syntax
|
|
must not be introduced by a leading zero, because no more than three octal
|
|
digits are ever read.
|
|
</P>
|
|
<br><b>
|
|
Constraints on character values
|
|
</b><br>
|
|
<P>
|
|
Characters that are specified using octal or hexadecimal numbers are
|
|
limited to certain values, as follows:
|
|
<pre>
|
|
8-bit non-UTF mode no greater than 0xff
|
|
16-bit non-UTF mode no greater than 0xffff
|
|
32-bit non-UTF mode no greater than 0xffffffff
|
|
All UTF modes no greater than 0x10ffff and a valid code point
|
|
</pre>
|
|
Invalid Unicode code points are all those in the range 0xd800 to 0xdfff (the
|
|
so-called "surrogate" code points). The check for these can be disabled by the
|
|
caller of <b>pcre2_compile()</b> by setting the option
|
|
PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES. However, this is possible only in UTF-8
|
|
and UTF-32 modes, because these values are not representable in UTF-16.
|
|
</P>
|
|
<br><b>
|
|
Escape sequences in character classes
|
|
</b><br>
|
|
<P>
|
|
All the sequences that define a single character value can be used both inside
|
|
and outside character classes. In addition, inside a character class, \b is
|
|
interpreted as the backspace character (hex 08).
|
|
</P>
|
|
<P>
|
|
When not followed by an opening brace, \N is not allowed in a character class.
|
|
\B, \R, and \X are not special inside a character class. Like other
|
|
unrecognized alphabetic escape sequences, they cause an error. Outside a
|
|
character class, these sequences have different meanings.
|
|
</P>
|
|
<br><b>
|
|
Unsupported escape sequences
|
|
</b><br>
|
|
<P>
|
|
In Perl, the sequences \F, \l, \L, \u, and \U are recognized by its string
|
|
handler and used to modify the case of following characters. By default, PCRE2
|
|
does not support these escape sequences in patterns. However, if either of the
|
|
PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX options is set, \U matches a "U"
|
|
character, and \u can be used to define a character by code point, as
|
|
described above.
|
|
</P>
|
|
<br><b>
|
|
Absolute and relative backreferences
|
|
</b><br>
|
|
<P>
|
|
The sequence \g followed by a signed or unsigned number, optionally enclosed
|
|
in braces, is an absolute or relative backreference. A named backreference
|
|
can be coded as \g{name}. Backreferences are discussed
|
|
<a href="#backreferences">later,</a>
|
|
following the discussion of
|
|
<a href="#group">parenthesized groups.</a>
|
|
</P>
|
|
<br><b>
|
|
Absolute and relative subroutine calls
|
|
</b><br>
|
|
<P>
|
|
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 capture group as a subroutine. Details are discussed
|
|
<a href="#onigurumasubroutines">later.</a>
|
|
Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
|
|
synonymous. The former is a backreference; the latter is a
|
|
<a href="#groupsassubroutines">subroutine</a>
|
|
call.
|
|
<a name="genericchartypes"></a></P>
|
|
<br><b>
|
|
Generic character types
|
|
</b><br>
|
|
<P>
|
|
Another use of backslash is for specifying generic character types:
|
|
<pre>
|
|
\d any decimal digit
|
|
\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
|
|
\s any white space character
|
|
\S any character that is not a white space character
|
|
\v any vertical white space character
|
|
\V any character that is not a vertical white space character
|
|
\w any "word" character
|
|
\W any "non-word" character
|
|
</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
|
|
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
|
|
name; PCRE2 does not support this.
|
|
</P>
|
|
<P>
|
|
Each pair of lower and upper case escape sequences partitions the complete set
|
|
of characters into two disjoint sets. Any given character matches one, and only
|
|
one, of each pair. The sequences can appear both inside and outside character
|
|
classes. They each match one character of the appropriate type. If the current
|
|
matching point is at the end of the subject string, all of them fail, because
|
|
there is no character to match.
|
|
</P>
|
|
<P>
|
|
The default \s characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
|
|
space (32), which are defined as white space in the "C" locale. This list may
|
|
vary if locale-specific matching is taking place. For example, in some locales
|
|
the "non-breaking space" character (\xA0) is recognized as white space, and in
|
|
others the VT character is not.
|
|
</P>
|
|
<P>
|
|
A "word" character is an underscore or any character that is a letter or digit.
|
|
By default, the definition of letters and digits is controlled by PCRE2's
|
|
low-valued character tables, and may vary if locale-specific matching is taking
|
|
place (see
|
|
<a href="pcre2api.html#localesupport">"Locale support"</a>
|
|
in the
|
|
<a href="pcre2api.html"><b>pcre2api</b></a>
|
|
page). For example, in a French locale such as "fr_FR" in Unix-like systems,
|
|
or "french" in Windows, some character codes greater than 127 are used for
|
|
accented letters, and these are then matched by \w. The use of locales with
|
|
Unicode is discouraged.
|
|
</P>
|
|
<P>
|
|
By default, characters whose code points are greater than 127 never match \d,
|
|
\s, or \w, and always match \D, \S, and \W, although this may be different
|
|
for characters in the range 128-255 when locale-specific matching is happening.
|
|
These escape sequences retain their original meanings from before Unicode
|
|
support was available, mainly for efficiency reasons. If the PCRE2_UCP option
|
|
is set, the behaviour is changed so that Unicode properties are used to
|
|
determine character types, as follows:
|
|
<pre>
|
|
\d any character that matches \p{Nd} (decimal digit)
|
|
\s any character that matches \p{Z} or \h or \v
|
|
\w any character that matches \p{L} or \p{N}, plus underscore
|
|
</pre>
|
|
The upper case escapes match the inverse sets of characters. Note that \d
|
|
matches only decimal digits, whereas \w matches any Unicode digit, as well as
|
|
any Unicode letter, and underscore. Note also that PCRE2_UCP affects \b, and
|
|
\B because they are defined in terms of \w and \W. Matching these sequences
|
|
is noticeably slower when PCRE2_UCP is set.
|
|
</P>
|
|
<P>
|
|
The sequences \h, \H, \v, and \V, in contrast to the other sequences, which
|
|
match only ASCII characters by default, always match a specific list of code
|
|
points, whether or not PCRE2_UCP is set. The horizontal space characters are:
|
|
<pre>
|
|
U+0009 Horizontal tab (HT)
|
|
U+0020 Space
|
|
U+00A0 Non-break space
|
|
U+1680 Ogham space mark
|
|
U+180E Mongolian vowel separator
|
|
U+2000 En quad
|
|
U+2001 Em quad
|
|
U+2002 En space
|
|
U+2003 Em space
|
|
U+2004 Three-per-em space
|
|
U+2005 Four-per-em space
|
|
U+2006 Six-per-em space
|
|
U+2007 Figure space
|
|
U+2008 Punctuation space
|
|
U+2009 Thin space
|
|
U+200A Hair space
|
|
U+202F Narrow no-break space
|
|
U+205F Medium mathematical space
|
|
U+3000 Ideographic space
|
|
</pre>
|
|
The vertical space characters are:
|
|
<pre>
|
|
U+000A Linefeed (LF)
|
|
U+000B Vertical tab (VT)
|
|
U+000C Form feed (FF)
|
|
U+000D Carriage return (CR)
|
|
U+0085 Next line (NEL)
|
|
U+2028 Line separator
|
|
U+2029 Paragraph separator
|
|
</pre>
|
|
In 8-bit, non-UTF-8 mode, only the characters with code points less than 256
|
|
are relevant.
|
|
<a name="newlineseq"></a></P>
|
|
<br><b>
|
|
Newline sequences
|
|
</b><br>
|
|
<P>
|
|
Outside a character class, by default, the escape sequence \R matches any
|
|
Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the
|
|
following:
|
|
<pre>
|
|
(?>\r\n|\n|\x0b|\f|\r|\x85)
|
|
</pre>
|
|
This is an example of an "atomic group", details of which are given
|
|
<a href="#atomicgroup">below.</a>
|
|
This particular group matches either the two-character sequence CR followed by
|
|
LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
|
|
U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
|
|
line, U+0085). Because this is an atomic group, the two-character sequence is
|
|
treated as a single unit that cannot be split.
|
|
</P>
|
|
<P>
|
|
In other modes, two additional characters whose code points are greater than 255
|
|
are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
|
|
Unicode support is not needed for these characters to be recognized.
|
|
</P>
|
|
<P>
|
|
It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
|
|
complete set of Unicode line endings) by setting the option PCRE2_BSR_ANYCRLF
|
|
at compile time. (BSR is an abbrevation for "backslash R".) This can be made
|
|
the default when PCRE2 is built; if this is the case, the other behaviour can
|
|
be requested via the PCRE2_BSR_UNICODE option. It is also possible to specify
|
|
these settings by starting a pattern string with one of the following
|
|
sequences:
|
|
<pre>
|
|
(*BSR_ANYCRLF) CR, LF, or CRLF only
|
|
(*BSR_UNICODE) any Unicode newline sequence
|
|
</pre>
|
|
These override the default and the options given to the compiling function.
|
|
Note that these special settings, which are not Perl-compatible, are recognized
|
|
only at the very start of a pattern, and that they must be in upper case. If
|
|
more than one of them is present, the last one is used. They can be combined
|
|
with a change of newline convention; for example, a pattern can start with:
|
|
<pre>
|
|
(*ANY)(*BSR_ANYCRLF)
|
|
</pre>
|
|
They can also be combined with the (*UTF) or (*UCP) special sequences. Inside a
|
|
character class, \R is treated as an unrecognized escape sequence, and causes
|
|
an error.
|
|
<a name="uniextseq"></a></P>
|
|
<br><b>
|
|
Unicode character properties
|
|
</b><br>
|
|
<P>
|
|
When PCRE2 is built with Unicode support (the default), three additional escape
|
|
sequences that match characters with specific properties are available. They
|
|
can be used in any mode, though in 8-bit and 16-bit non-UTF modes these
|
|
sequences are of course limited to testing characters whose code points are
|
|
less than U+0100 and U+10000, respectively. In 32-bit non-UTF mode, code points
|
|
greater than 0x10ffff (the Unicode limit) may be encountered. These are all
|
|
treated as being in the Unknown script and with an unassigned type. The extra
|
|
escape sequences are:
|
|
<pre>
|
|
\p{<i>xx</i>} a character with the <i>xx</i> property
|
|
\P{<i>xx</i>} a character without the <i>xx</i> property
|
|
\X a Unicode extended grapheme cluster
|
|
</pre>
|
|
The property names represented by <i>xx</i> above are case-sensitive. There is
|
|
support for Unicode script names, Unicode general category properties, "Any",
|
|
which matches any character (including newline), and some special PCRE2
|
|
properties (described in the
|
|
<a href="#extraprops">next section).</a>
|
|
Other Perl properties such as "InMusicalSymbols" are not supported by PCRE2.
|
|
Note that \P{Any} does not match any characters, so always causes a match
|
|
failure.
|
|
</P>
|
|
<P>
|
|
Sets of Unicode characters are defined as belonging to certain scripts. A
|
|
character from one of these sets can be matched using a script name. For
|
|
example:
|
|
<pre>
|
|
\p{Greek}
|
|
\P{Han}
|
|
</pre>
|
|
Unassigned characters (and in non-UTF 32-bit mode, characters with code points
|
|
greater than 0x10FFFF) are assigned the "Unknown" script. Others that are not
|
|
part of an identified script are lumped together as "Common". The current list
|
|
of scripts is:
|
|
</P>
|
|
<P>
|
|
Adlam,
|
|
Ahom,
|
|
Anatolian_Hieroglyphs,
|
|
Arabic,
|
|
Armenian,
|
|
Avestan,
|
|
Balinese,
|
|
Bamum,
|
|
Bassa_Vah,
|
|
Batak,
|
|
Bengali,
|
|
Bhaiksuki,
|
|
Bopomofo,
|
|
Brahmi,
|
|
Braille,
|
|
Buginese,
|
|
Buhid,
|
|
Canadian_Aboriginal,
|
|
Carian,
|
|
Caucasian_Albanian,
|
|
Chakma,
|
|
Cham,
|
|
Cherokee,
|
|
Common,
|
|
Coptic,
|
|
Cuneiform,
|
|
Cypriot,
|
|
Cyrillic,
|
|
Deseret,
|
|
Devanagari,
|
|
Dogra,
|
|
Duployan,
|
|
Egyptian_Hieroglyphs,
|
|
Elbasan,
|
|
Elymaic,
|
|
Ethiopic,
|
|
Georgian,
|
|
Glagolitic,
|
|
Gothic,
|
|
Grantha,
|
|
Greek,
|
|
Gujarati,
|
|
Gunjala_Gondi,
|
|
Gurmukhi,
|
|
Han,
|
|
Hangul,
|
|
Hanifi_Rohingya,
|
|
Hanunoo,
|
|
Hatran,
|
|
Hebrew,
|
|
Hiragana,
|
|
Imperial_Aramaic,
|
|
Inherited,
|
|
Inscriptional_Pahlavi,
|
|
Inscriptional_Parthian,
|
|
Javanese,
|
|
Kaithi,
|
|
Kannada,
|
|
Katakana,
|
|
Kayah_Li,
|
|
Kharoshthi,
|
|
Khmer,
|
|
Khojki,
|
|
Khudawadi,
|
|
Lao,
|
|
Latin,
|
|
Lepcha,
|
|
Limbu,
|
|
Linear_A,
|
|
Linear_B,
|
|
Lisu,
|
|
Lycian,
|
|
Lydian,
|
|
Mahajani,
|
|
Makasar,
|
|
Malayalam,
|
|
Mandaic,
|
|
Manichaean,
|
|
Marchen,
|
|
Masaram_Gondi,
|
|
Medefaidrin,
|
|
Meetei_Mayek,
|
|
Mende_Kikakui,
|
|
Meroitic_Cursive,
|
|
Meroitic_Hieroglyphs,
|
|
Miao,
|
|
Modi,
|
|
Mongolian,
|
|
Mro,
|
|
Multani,
|
|
Myanmar,
|
|
Nabataean,
|
|
Nandinagari,
|
|
New_Tai_Lue,
|
|
Newa,
|
|
Nko,
|
|
Nushu,
|
|
Nyakeng_Puachue_Hmong,
|
|
Ogham,
|
|
Ol_Chiki,
|
|
Old_Hungarian,
|
|
Old_Italic,
|
|
Old_North_Arabian,
|
|
Old_Permic,
|
|
Old_Persian,
|
|
Old_Sogdian,
|
|
Old_South_Arabian,
|
|
Old_Turkic,
|
|
Oriya,
|
|
Osage,
|
|
Osmanya,
|
|
Pahawh_Hmong,
|
|
Palmyrene,
|
|
Pau_Cin_Hau,
|
|
Phags_Pa,
|
|
Phoenician,
|
|
Psalter_Pahlavi,
|
|
Rejang,
|
|
Runic,
|
|
Samaritan,
|
|
Saurashtra,
|
|
Sharada,
|
|
Shavian,
|
|
Siddham,
|
|
SignWriting,
|
|
Sinhala,
|
|
Sogdian,
|
|
Sora_Sompeng,
|
|
Soyombo,
|
|
Sundanese,
|
|
Syloti_Nagri,
|
|
Syriac,
|
|
Tagalog,
|
|
Tagbanwa,
|
|
Tai_Le,
|
|
Tai_Tham,
|
|
Tai_Viet,
|
|
Takri,
|
|
Tamil,
|
|
Tangut,
|
|
Telugu,
|
|
Thaana,
|
|
Thai,
|
|
Tibetan,
|
|
Tifinagh,
|
|
Tirhuta,
|
|
Ugaritic,
|
|
Unknown,
|
|
Vai,
|
|
Wancho,
|
|
Warang_Citi,
|
|
Yi,
|
|
Zanabazar_Square.
|
|
</P>
|
|
<P>
|
|
Each character has exactly one Unicode general category property, specified by
|
|
a two-letter abbreviation. For compatibility with Perl, negation can be
|
|
specified by including a circumflex between the opening brace and the property
|
|
name. For example, \p{^Lu} is the same as \P{Lu}.
|
|
</P>
|
|
<P>
|
|
If only one letter is specified with \p or \P, it includes all the general
|
|
category properties that start with that letter. In this case, in the absence
|
|
of negation, the curly brackets in the escape sequence are optional; these two
|
|
examples have the same effect:
|
|
<pre>
|
|
\p{L}
|
|
\pL
|
|
</pre>
|
|
The following general category property codes are supported:
|
|
<pre>
|
|
C Other
|
|
Cc Control
|
|
Cf Format
|
|
Cn Unassigned
|
|
Co Private use
|
|
Cs Surrogate
|
|
|
|
L Letter
|
|
Ll Lower case letter
|
|
Lm Modifier letter
|
|
Lo Other letter
|
|
Lt Title case letter
|
|
Lu Upper case letter
|
|
|
|
M Mark
|
|
Mc Spacing mark
|
|
Me Enclosing mark
|
|
Mn Non-spacing mark
|
|
|
|
N Number
|
|
Nd Decimal number
|
|
Nl Letter number
|
|
No Other number
|
|
|
|
P Punctuation
|
|
Pc Connector punctuation
|
|
Pd Dash punctuation
|
|
Pe Close punctuation
|
|
Pf Final punctuation
|
|
Pi Initial punctuation
|
|
Po Other punctuation
|
|
Ps Open punctuation
|
|
|
|
S Symbol
|
|
Sc Currency symbol
|
|
Sk Modifier symbol
|
|
Sm Mathematical symbol
|
|
So Other symbol
|
|
|
|
Z Separator
|
|
Zl Line separator
|
|
Zp Paragraph separator
|
|
Zs Space separator
|
|
</pre>
|
|
The special property L& is also supported: it matches a character that has
|
|
the Lu, Ll, or Lt property, in other words, a letter that is not classified as
|
|
a modifier or "other".
|
|
</P>
|
|
<P>
|
|
The Cs (Surrogate) property applies only to characters whose code points are in
|
|
the range U+D800 to U+DFFF. These characters are no different to any other
|
|
character when PCRE2 is not in UTF mode (using the 16-bit or 32-bit library).
|
|
However, they are not valid in Unicode strings and so cannot be tested by PCRE2
|
|
in UTF mode, unless UTF validity checking has been turned off (see the
|
|
discussion of PCRE2_NO_UTF_CHECK in the
|
|
<a href="pcre2api.html"><b>pcre2api</b></a>
|
|
page).
|
|
</P>
|
|
<P>
|
|
The long synonyms for property names that Perl supports (such as \p{Letter})
|
|
are not supported by PCRE2, nor is it permitted to prefix any of these
|
|
properties with "Is".
|
|
</P>
|
|
<P>
|
|
No character that is in the Unicode table has the Cn (unassigned) property.
|
|
Instead, this property is assumed for any code point that is not in the
|
|
Unicode table.
|
|
</P>
|
|
<P>
|
|
Specifying caseless matching does not affect these escape sequences. For
|
|
example, \p{Lu} always matches only upper case letters. This is different from
|
|
the behaviour of current versions of Perl.
|
|
</P>
|
|
<P>
|
|
Matching characters by Unicode property is not fast, because PCRE2 has to do a
|
|
multistage table lookup in order to find a character's property. That is why
|
|
the traditional escape sequences such as \d and \w do not use Unicode
|
|
properties in PCRE2 by default, though you can make them do so by setting the
|
|
PCRE2_UCP option or by starting the pattern with (*UCP).
|
|
</P>
|
|
<br><b>
|
|
Extended grapheme clusters
|
|
</b><br>
|
|
<P>
|
|
The \X escape matches any number of Unicode characters that form an "extended
|
|
grapheme cluster", and treats the sequence as an atomic group
|
|
<a href="#atomicgroup">(see below).</a>
|
|
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.
|
|
Instead it introduced various emoji-specific properties. PCRE2 uses only the
|
|
Extended Pictographic property.
|
|
</P>
|
|
<P>
|
|
\X always matches at least one character. Then it decides whether to add
|
|
additional characters according to the following rules for ending a cluster:
|
|
</P>
|
|
<P>
|
|
1. End at the end of the subject string.
|
|
</P>
|
|
<P>
|
|
2. Do not end between CR and LF; otherwise end after any control character.
|
|
</P>
|
|
<P>
|
|
3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
|
|
are of five types: L, V, T, LV, and LVT. An L character may be followed by an
|
|
L, V, LV, or LVT character; an LV or V character may be followed by a V or T
|
|
character; an LVT or T character may be follwed only by a T character.
|
|
</P>
|
|
<P>
|
|
4. Do not end before extending characters or spacing marks or the "zero-width
|
|
joiner" character. Characters with the "mark" property always have the
|
|
"extend" grapheme breaking property.
|
|
</P>
|
|
<P>
|
|
5. Do not end after prepend characters.
|
|
</P>
|
|
<P>
|
|
6. Do not break within emoji modifier sequences or emoji zwj sequences. That
|
|
is, do not break between characters with the Extended_Pictographic property.
|
|
Extend and ZWJ characters are allowed between the characters.
|
|
</P>
|
|
<P>
|
|
7. Do not break within emoji flag sequences. That is, do not break between
|
|
regional indicator (RI) characters if there are an odd number of RI characters
|
|
before the break point.
|
|
</P>
|
|
<P>
|
|
8. Otherwise, end the cluster.
|
|
<a name="extraprops"></a></P>
|
|
<br><b>
|
|
PCRE2's additional properties
|
|
</b><br>
|
|
<P>
|
|
As well as the standard Unicode properties described above, PCRE2 supports four
|
|
more that make it possible to convert traditional escape sequences such as \w
|
|
and \s to use Unicode properties. PCRE2 uses these non-standard, non-Perl
|
|
properties internally when PCRE2_UCP is set. However, they may also be used
|
|
explicitly. These properties are:
|
|
<pre>
|
|
Xan Any alphanumeric character
|
|
Xps Any POSIX space character
|
|
Xsp Any Perl space character
|
|
Xwd Any Perl "word" character
|
|
</pre>
|
|
Xan matches characters that have either the L (letter) or the N (number)
|
|
property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
|
|
carriage return, and any other character that has the Z (separator) property.
|
|
Xsp is the same as Xps; in PCRE1 it used to exclude vertical tab, for Perl
|
|
compatibility, but Perl changed. Xwd matches the same characters as Xan, plus
|
|
underscore.
|
|
</P>
|
|
<P>
|
|
There is another non-standard property, Xuc, which matches any character that
|
|
can be represented by a Universal Character Name in C++ and other programming
|
|
languages. These are the characters $, @, ` (grave accent), and all characters
|
|
with Unicode code points greater than or equal to U+00A0, except for the
|
|
surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
|
|
excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH
|
|
where H is a hexadecimal digit. Note that the Xuc property does not match these
|
|
sequences but the characters that they represent.)
|
|
<a name="resetmatchstart"></a></P>
|
|
<br><b>
|
|
Resetting the match start
|
|
</b><br>
|
|
<P>
|
|
In normal use, the escape sequence \K causes any previously matched characters
|
|
not to be included in the final matched sequence that is returned. For example,
|
|
the pattern:
|
|
<pre>
|
|
foo\Kbar
|
|
</pre>
|
|
matches "foobar", but reports that it has matched "bar". \K does not interact
|
|
with anchoring in any way. The pattern:
|
|
<pre>
|
|
^foo\Kbar
|
|
</pre>
|
|
matches only when the subject begins with "foobar" (in single line mode),
|
|
though it again reports the matched string as "bar". This feature is similar to
|
|
a lookbehind assertion
|
|
<a href="#lookbehind">(described below).</a>
|
|
However, in this case, the part of the subject before the real match does not
|
|
have to be of fixed length, as lookbehind assertions do. The use of \K does
|
|
not interfere with the setting of
|
|
<a href="#group">captured substrings.</a>
|
|
For example, when the pattern
|
|
<pre>
|
|
(foo)\Kbar
|
|
</pre>
|
|
matches "foobar", the first substring is still set to "foo".
|
|
</P>
|
|
<P>
|
|
Perl documents that the use of \K within assertions is "not well defined". In
|
|
PCRE2, \K is acted upon when it occurs inside positive assertions, but is
|
|
ignored in negative assertions. Note that when a pattern such as (?=ab\K)
|
|
matches, the reported start of the match can be greater than the end of the
|
|
match. Using \K in a lookbehind assertion at the start of a pattern can also
|
|
lead to odd effects. For example, consider this pattern:
|
|
<pre>
|
|
(?<=\Kfoo)bar
|
|
</pre>
|
|
If the subject is "foobar", a call to <b>pcre2_match()</b> with a starting
|
|
offset of 3 succeeds and reports the matching string as "foobar", that is, the
|
|
start of the reported match is earlier than where the match started.
|
|
<a name="smallassertions"></a></P>
|
|
<br><b>
|
|
Simple assertions
|
|
</b><br>
|
|
<P>
|
|
The final use of backslash is for certain simple assertions. An assertion
|
|
specifies a condition that has to be met at a particular point in a match,
|
|
without consuming any characters from the subject string. The use of
|
|
groups for more complicated assertions is described
|
|
<a href="#bigassertions">below.</a>
|
|
The backslashed assertions are:
|
|
<pre>
|
|
\b matches at a word boundary
|
|
\B matches when not at a word boundary
|
|
\A matches at the start of the subject
|
|
\Z matches at the end of the subject
|
|
also matches before a newline at the end of the subject
|
|
\z matches only at the end of the subject
|
|
\G matches at the first matching position in the subject
|
|
</pre>
|
|
Inside a character class, \b has a different meaning; it matches the backspace
|
|
character. If any other of these assertions appears in a character class, an
|
|
"invalid escape sequence" error is generated.
|
|
</P>
|
|
<P>
|
|
A word boundary is a position in the subject string where the current character
|
|
and the previous character do not both match \w or \W (i.e. one matches
|
|
\w and the other matches \W), or the start or end of the string if the
|
|
first or last character matches \w, respectively. When PCRE2 is built with
|
|
Unicode support, the meanings of \w and \W can be changed by setting the
|
|
PCRE2_UCP option. When this is done, it also affects \b and \B. Neither PCRE2
|
|
nor Perl has a separate "start of word" or "end of word" metasequence. However,
|
|
whatever follows \b normally determines which it is. For example, the fragment
|
|
\ba matches "a" at the start of a word.
|
|
</P>
|
|
<P>
|
|
The \A, \Z, and \z assertions differ from the traditional circumflex and
|
|
dollar (described in the next section) in that they only ever match at the very
|
|
start and end of the subject string, whatever options are set. Thus, they are
|
|
independent of multiline mode. These three assertions are not affected by the
|
|
PCRE2_NOTBOL or PCRE2_NOTEOL options, which affect only the behaviour of the
|
|
circumflex and dollar metacharacters. However, if the <i>startoffset</i>
|
|
argument of <b>pcre2_match()</b> is non-zero, indicating that matching is to
|
|
start at a point other than the beginning of the subject, \A can never match.
|
|
The difference between \Z and \z is that \Z matches before a newline at the
|
|
end of the string as well as at the very end, whereas \z matches only at the
|
|
end.
|
|
</P>
|
|
<P>
|
|
The \G assertion is true only when the current matching position is at the
|
|
start point of the matching process, as specified by the <i>startoffset</i>
|
|
argument of <b>pcre2_match()</b>. It differs from \A when the value of
|
|
<i>startoffset</i> is non-zero. By calling <b>pcre2_match()</b> multiple times
|
|
with appropriate arguments, you can mimic Perl's /g option, and it is in this
|
|
kind of implementation where \G can be useful.
|
|
</P>
|
|
<P>
|
|
Note, however, that PCRE2's implementation of \G, being true at the starting
|
|
character of the matching process, is subtly different from Perl's, which
|
|
defines it as true at the end of the previous match. In Perl, these can be
|
|
different when the previously matched string was empty. Because PCRE2 does just
|
|
one match at a time, it cannot reproduce this behaviour.
|
|
</P>
|
|
<P>
|
|
If all the alternatives of a pattern begin with \G, the expression is anchored
|
|
to the starting match position, and the "anchored" flag is set in the compiled
|
|
regular expression.
|
|
</P>
|
|
<br><a name="SEC6" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
|
|
<P>
|
|
The circumflex and dollar metacharacters are zero-width assertions. That is,
|
|
they test for a particular condition being true without consuming any
|
|
characters from the subject string. These two metacharacters are concerned with
|
|
matching the starts and ends of lines. If the newline convention is set so that
|
|
only the two-character sequence CRLF is recognized as a newline, isolated CR
|
|
and LF characters are treated as ordinary data characters, and are not
|
|
recognized as newlines.
|
|
</P>
|
|
<P>
|
|
Outside a character class, in the default matching mode, the circumflex
|
|
character is an assertion that is true only if the current matching point is at
|
|
the start of the subject string. If the <i>startoffset</i> argument of
|
|
<b>pcre2_match()</b> is non-zero, or if PCRE2_NOTBOL is set, circumflex can
|
|
never match if the PCRE2_MULTILINE option is unset. Inside a character class,
|
|
circumflex has an entirely different meaning
|
|
<a href="#characterclass">(see below).</a>
|
|
</P>
|
|
<P>
|
|
Circumflex need not be the first character of the pattern if a number of
|
|
alternatives are involved, but it should be the first thing in each alternative
|
|
in which it appears if the pattern is ever to match that branch. If all
|
|
possible alternatives start with a circumflex, that is, if the pattern is
|
|
constrained to match only at the start of the subject, it is said to be an
|
|
"anchored" pattern. (There are also other constructs that can cause a pattern
|
|
to be anchored.)
|
|
</P>
|
|
<P>
|
|
The dollar character is an assertion that is true only if the current matching
|
|
point is at the end of the subject string, or immediately before a newline at
|
|
the end of the string (by default), unless PCRE2_NOTEOL is set. Note, however,
|
|
that it does not actually match the newline. Dollar need not be the last
|
|
character of the pattern if a number of alternatives are involved, but it
|
|
should be the last item in any branch in which it appears. Dollar has no
|
|
special meaning in a character class.
|
|
</P>
|
|
<P>
|
|
The meaning of dollar can be changed so that it matches only at the very end of
|
|
the string, by setting the PCRE2_DOLLAR_ENDONLY option at compile time. This
|
|
does not affect the \Z assertion.
|
|
</P>
|
|
<P>
|
|
The meanings of the circumflex and dollar metacharacters are changed if the
|
|
PCRE2_MULTILINE option is set. When this is the case, a dollar character
|
|
matches before any newlines in the string, as well as at the very end, and a
|
|
circumflex matches immediately after internal newlines as well as at the start
|
|
of the subject string. It does not match after a newline that ends the string,
|
|
for compatibility with Perl. However, this can be changed by setting the
|
|
PCRE2_ALT_CIRCUMFLEX option.
|
|
</P>
|
|
<P>
|
|
For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
|
|
\n represents a newline) in multiline mode, but not otherwise. Consequently,
|
|
patterns that are anchored in single line mode because all branches start with
|
|
^ are not anchored in multiline mode, and a match for circumflex is possible
|
|
when the <i>startoffset</i> argument of <b>pcre2_match()</b> is non-zero. The
|
|
PCRE2_DOLLAR_ENDONLY option is ignored if PCRE2_MULTILINE is set.
|
|
</P>
|
|
<P>
|
|
When the newline convention (see
|
|
<a href="#newlines">"Newline conventions"</a>
|
|
below) recognizes the two-character sequence CRLF as a newline, this is
|
|
preferred, even if the single characters CR and LF are also recognized as
|
|
newlines. For example, if the newline convention is "any", a multiline mode
|
|
circumflex matches before "xyz" in the string "abc\r\nxyz" rather than after
|
|
CR, even though CR on its own is a valid newline. (It also matches at the very
|
|
start of the string, of course.)
|
|
</P>
|
|
<P>
|
|
Note that the sequences \A, \Z, and \z can be used to match the start and
|
|
end of the subject in both modes, and if all branches of a pattern start with
|
|
\A it is always anchored, whether or not PCRE2_MULTILINE is set.
|
|
<a name="fullstopdot"></a></P>
|
|
<br><a name="SEC7" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
|
|
<P>
|
|
Outside a character class, a dot in the pattern matches any one character in
|
|
the subject string except (by default) a character that signifies the end of a
|
|
line.
|
|
</P>
|
|
<P>
|
|
When a line ending is defined as a single character, dot never matches that
|
|
character; when the two-character sequence CRLF is used, dot does not match CR
|
|
if it is immediately followed by LF, but otherwise it matches all characters
|
|
(including isolated CRs and LFs). When any Unicode line endings are being
|
|
recognized, dot does not match CR or LF or any of the other line ending
|
|
characters.
|
|
</P>
|
|
<P>
|
|
The behaviour of dot with regard to newlines can be changed. If the
|
|
PCRE2_DOTALL option is set, a dot matches any one character, without exception.
|
|
If the two-character sequence CRLF is present in the subject string, it takes
|
|
two dots to match it.
|
|
</P>
|
|
<P>
|
|
The handling of dot is entirely independent of the handling of circumflex and
|
|
dollar, the only relationship being that they both involve newlines. Dot has no
|
|
special meaning in a character class.
|
|
</P>
|
|
<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.
|
|
</P>
|
|
<P>
|
|
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
|
|
name; PCRE2 does not support this.
|
|
</P>
|
|
<br><a name="SEC8" href="#TOC1">MATCHING A SINGLE CODE UNIT</a><br>
|
|
<P>
|
|
Outside a character class, the escape sequence \C matches any one code unit,
|
|
whether or not a UTF mode is set. In the 8-bit library, one code unit is one
|
|
byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is a
|
|
32-bit unit. Unlike a dot, \C always matches line-ending characters. The
|
|
feature is provided in Perl in order to match individual bytes in UTF-8 mode,
|
|
but it is unclear how it can usefully be used.
|
|
</P>
|
|
<P>
|
|
Because \C breaks up characters into individual code units, matching one unit
|
|
with \C in UTF-8 or UTF-16 mode means that the rest of the string may start
|
|
with a malformed UTF character. This has undefined results, because PCRE2
|
|
assumes that it is matching character by character in a valid UTF string (by
|
|
default it checks the subject string's validity at the start of processing
|
|
unless the PCRE2_NO_UTF_CHECK or PCRE2_MATCH_INVALID_UTF option is used).
|
|
</P>
|
|
<P>
|
|
An application can lock out the use of \C by setting the
|
|
PCRE2_NEVER_BACKSLASH_C option when compiling a pattern. It is also possible to
|
|
build PCRE2 with the use of \C permanently disabled.
|
|
</P>
|
|
<P>
|
|
PCRE2 does not allow \C to appear in lookbehind assertions
|
|
<a href="#lookbehind">(described below)</a>
|
|
in UTF-8 or UTF-16 modes, because this would make it impossible to calculate
|
|
the length of the lookbehind. Neither the alternative matching function
|
|
<b>pcre2_dfa_match()</b> nor the JIT optimizer support \C in these UTF modes.
|
|
The former gives a match-time error; the latter fails to optimize and so the
|
|
match is always run using the interpreter.
|
|
</P>
|
|
<P>
|
|
In the 32-bit library, however, \C is always supported (when not explicitly
|
|
locked out) because it always matches a single code unit, whether or not UTF-32
|
|
is specified.
|
|
</P>
|
|
<P>
|
|
In general, the \C escape sequence is best avoided. However, one way of using
|
|
it that avoids the problem of malformed UTF-8 or UTF-16 characters is to use a
|
|
lookahead to check the length of the next character, as in this pattern, which
|
|
could be used with a UTF-8 string (ignore white space and line breaks):
|
|
<pre>
|
|
(?| (?=[\x00-\x7f])(\C) |
|
|
(?=[\x80-\x{7ff}])(\C)(\C) |
|
|
(?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
|
|
(?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
|
|
</pre>
|
|
In this example, a group that starts with (?| resets the capturing parentheses
|
|
numbers in each alternative (see
|
|
<a href="#dupgroupnumber">"Duplicate Group Numbers"</a>
|
|
below). The assertions at the start of each branch check the next UTF-8
|
|
character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
|
|
character's individual bytes are then captured by the appropriate number of
|
|
\C groups.
|
|
<a name="characterclass"></a></P>
|
|
<br><a name="SEC9" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
|
|
<P>
|
|
An opening square bracket introduces a character class, terminated by a closing
|
|
square bracket. A closing square bracket on its own is not special by default.
|
|
If a closing square bracket is required as a member of the class, it should be
|
|
the first data character in the class (after an initial circumflex, if present)
|
|
or escaped with a backslash. This means that, by default, an empty class cannot
|
|
be defined. However, if the PCRE2_ALLOW_EMPTY_CLASS option is set, a closing
|
|
square bracket at the start does end the (empty) class.
|
|
</P>
|
|
<P>
|
|
A character class matches a single character in the subject. A matched
|
|
character must be in the set of characters defined by the class, unless the
|
|
first character in the class definition is a circumflex, in which case the
|
|
subject character must not be in the set defined by the class. If a circumflex
|
|
is actually required as a member of the class, ensure it is not the first
|
|
character, or escape it with a backslash.
|
|
</P>
|
|
<P>
|
|
For example, the character class [aeiou] matches any lower case vowel, while
|
|
[^aeiou] matches any character that is not a lower case vowel. Note that a
|
|
circumflex is just a convenient notation for specifying the characters that
|
|
are in the class by enumerating those that are not. A class that starts with a
|
|
circumflex is not an assertion; it still consumes a character from the subject
|
|
string, and therefore it fails if the current pointer is at the end of the
|
|
string.
|
|
</P>
|
|
<P>
|
|
Characters in a class may be specified by their code points using \o, \x, or
|
|
\N{U+hh..} in the usual way. When caseless matching is set, any letters in a
|
|
class represent both their upper case and lower case versions, so for example,
|
|
a caseless [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
|
|
match "A", whereas a caseful version would.
|
|
</P>
|
|
<P>
|
|
Characters that might indicate line breaks are never treated in any special way
|
|
when matching character classes, whatever line-ending sequence is in use, and
|
|
whatever setting of the PCRE2_DOTALL and PCRE2_MULTILINE options is used. A
|
|
class such as [^a] always matches one of these characters.
|
|
</P>
|
|
<P>
|
|
The generic character type escape sequences \d, \D, \h, \H, \p, \P, \s,
|
|
\S, \v, \V, \w, and \W may appear in a character class, and add the
|
|
characters that they match to the class. For example, [\dABCDEF] matches any
|
|
hexadecimal digit. In UTF modes, the PCRE2_UCP option affects the meanings of
|
|
\d, \s, \w and their upper case partners, just as it does when they appear
|
|
outside a character class, as described in the section entitled
|
|
<a href="#genericchartypes">"Generic character types"</a>
|
|
above. The escape sequence \b has a different meaning inside a character
|
|
class; it matches the backspace character. The sequences \B, \R, and \X are
|
|
not special inside a character class. Like any other unrecognized escape
|
|
sequences, they cause an error. The same is true for \N when not followed by
|
|
an opening brace.
|
|
</P>
|
|
<P>
|
|
The minus (hyphen) character can be used to specify a range of characters in a
|
|
character class. For example, [d-m] matches any letter between d and m,
|
|
inclusive. If a minus character is required in a class, it must be escaped with
|
|
a backslash or appear in a position where it cannot be interpreted as
|
|
indicating a range, typically as the first or last character in the class,
|
|
or immediately after a range. For example, [b-d-z] matches letters in the range
|
|
b to d, a hyphen character, or z.
|
|
</P>
|
|
<P>
|
|
Perl treats a hyphen as a literal if it appears before or after a POSIX class
|
|
(see below) or before or after a character type escape such as as \d or \H.
|
|
However, unless the hyphen is the last character in the class, Perl outputs a
|
|
warning in its warning mode, as this is most likely a user error. As PCRE2 has
|
|
no facility for warning, an error is given in these cases.
|
|
</P>
|
|
<P>
|
|
It is not possible to have the literal character "]" as the end character of a
|
|
range. A pattern such as [W-]46] is interpreted as a class of two characters
|
|
("W" and "-") followed by a literal string "46]", so it would match "W46]" or
|
|
"-46]". However, if the "]" is escaped with a backslash it is interpreted as
|
|
the end of range, so [W-\]46] is interpreted as a class containing a range
|
|
followed by two other characters. The octal or hexadecimal representation of
|
|
"]" can also be used to end a range.
|
|
</P>
|
|
<P>
|
|
Ranges normally include all code points between the start and end characters,
|
|
inclusive. They can also be used for code points specified numerically, for
|
|
example [\000-\037]. Ranges can include any characters that are valid for the
|
|
current mode. In any UTF mode, the so-called "surrogate" characters (those
|
|
whose code points lie between 0xd800 and 0xdfff inclusive) may not be specified
|
|
explicitly by default (the PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES option disables
|
|
this check). However, ranges such as [\x{d7ff}-\x{e000}], which include the
|
|
surrogates, are always permitted.
|
|
</P>
|
|
<P>
|
|
There is a special case in EBCDIC environments for ranges whose end points are
|
|
both specified as literal letters in the same case. For compatibility with
|
|
Perl, EBCDIC code points within the range that are not letters are omitted. For
|
|
example, [h-k] matches only four characters, even though the codes for h and k
|
|
are 0x88 and 0x92, a range of 11 code points. However, if the range is
|
|
specified numerically, for example, [\x88-\x92] or [h-\x92], all code points
|
|
are included.
|
|
</P>
|
|
<P>
|
|
If a range that includes letters is used when caseless matching is set, it
|
|
matches the letters in either case. For example, [W-c] is equivalent to
|
|
[][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
|
|
tables for a French locale are in use, [\xc8-\xcb] matches accented E
|
|
characters in both cases.
|
|
</P>
|
|
<P>
|
|
A circumflex can conveniently be used with the upper case character types to
|
|
specify a more restricted set of characters than the matching lower case type.
|
|
For example, the class [^\W_] matches any letter or digit, but not underscore,
|
|
whereas [\w] includes underscore. A positive character class should be read as
|
|
"something OR something OR ..." and a negative class as "NOT something AND NOT
|
|
something AND NOT ...".
|
|
</P>
|
|
<P>
|
|
The only metacharacters that are recognized in character classes are backslash,
|
|
hyphen (only where it can be interpreted as specifying a range), circumflex
|
|
(only at the start), opening square bracket (only when it can be interpreted as
|
|
introducing a POSIX class name, or for a special compatibility feature - see
|
|
the next two sections), and the terminating closing square bracket. However,
|
|
escaping other non-alphanumeric characters does no harm.
|
|
</P>
|
|
<br><a name="SEC10" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
|
|
<P>
|
|
Perl supports the POSIX notation for character classes. This uses names
|
|
enclosed by [: and :] within the enclosing square brackets. PCRE2 also supports
|
|
this notation. For example,
|
|
<pre>
|
|
[01[:alpha:]%]
|
|
</pre>
|
|
matches "0", "1", any alphabetic character, or "%". The supported class names
|
|
are:
|
|
<pre>
|
|
alnum letters and digits
|
|
alpha letters
|
|
ascii character codes 0 - 127
|
|
blank space or tab only
|
|
cntrl control characters
|
|
digit decimal digits (same as \d)
|
|
graph printing characters, excluding space
|
|
lower lower case letters
|
|
print printing characters, including space
|
|
punct printing characters, excluding letters and digits and space
|
|
space white space (the same as \s from PCRE2 8.34)
|
|
upper upper case letters
|
|
word "word" characters (same as \w)
|
|
xdigit hexadecimal digits
|
|
</pre>
|
|
The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
|
|
and space (32). If locale-specific matching is taking place, the list of space
|
|
characters may be different; there may be fewer or more of them. "Space" and
|
|
\s match the same set of characters.
|
|
</P>
|
|
<P>
|
|
The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
|
|
5.8. Another Perl extension is negation, which is indicated by a ^ character
|
|
after the colon. For example,
|
|
<pre>
|
|
[12[:^digit:]]
|
|
</pre>
|
|
matches "1", "2", or any non-digit. PCRE2 (and Perl) also recognize the POSIX
|
|
syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
|
|
supported, and an error is given if they are encountered.
|
|
</P>
|
|
<P>
|
|
By default, characters with values greater than 127 do not match any of the
|
|
POSIX character classes, although this may be different for characters in the
|
|
range 128-255 when locale-specific matching is happening. However, if the
|
|
PCRE2_UCP option is passed to <b>pcre2_compile()</b>, some of the classes are
|
|
changed so that Unicode character properties are used. This is achieved by
|
|
replacing certain POSIX classes with other sequences, as follows:
|
|
<pre>
|
|
[:alnum:] becomes \p{Xan}
|
|
[:alpha:] becomes \p{L}
|
|
[:blank:] becomes \h
|
|
[:cntrl:] becomes \p{Cc}
|
|
[:digit:] becomes \p{Nd}
|
|
[:lower:] becomes \p{Ll}
|
|
[:space:] becomes \p{Xps}
|
|
[:upper:] becomes \p{Lu}
|
|
[:word:] becomes \p{Xwd}
|
|
</pre>
|
|
Negated versions, such as [:^alpha:] use \P instead of \p. Three other POSIX
|
|
classes are handled specially in UCP mode:
|
|
</P>
|
|
<P>
|
|
[:graph:]
|
|
This matches characters that have glyphs that mark the page when printed. In
|
|
Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
|
|
properties, except for:
|
|
<pre>
|
|
U+061C Arabic Letter Mark
|
|
U+180E Mongolian Vowel Separator
|
|
U+2066 - U+2069 Various "isolate"s
|
|
|
|
</PRE>
|
|
</P>
|
|
<P>
|
|
[:print:]
|
|
This matches the same characters as [:graph:] plus space characters that are
|
|
not controls, that is, characters with the Zs property.
|
|
</P>
|
|
<P>
|
|
[:punct:]
|
|
This matches all characters that have the Unicode P (punctuation) property,
|
|
plus those characters with code points less than 256 that have the S (Symbol)
|
|
property.
|
|
</P>
|
|
<P>
|
|
The other POSIX classes are unchanged, and match only characters with code
|
|
points less than 256.
|
|
</P>
|
|
<br><a name="SEC11" href="#TOC1">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a><br>
|
|
<P>
|
|
In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly
|
|
syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of
|
|
word". PCRE2 treats these items as follows:
|
|
<pre>
|
|
[[:<:]] is converted to \b(?=\w)
|
|
[[:>:]] is converted to \b(?<=\w)
|
|
</pre>
|
|
Only these exact character sequences are recognized. A sequence such as
|
|
[a[:<:]b] provokes error for an unrecognized POSIX class name. This support is
|
|
not compatible with Perl. It is provided to help migrations from other
|
|
environments, and is best not used in any new patterns. Note that \b matches
|
|
at the start and the end of a word (see
|
|
<a href="#smallassertions">"Simple assertions"</a>
|
|
above), and in a Perl-style pattern the preceding or following character
|
|
normally shows which is wanted, without the need for the assertions that are
|
|
used above in order to give exactly the POSIX behaviour.
|
|
</P>
|
|
<br><a name="SEC12" href="#TOC1">VERTICAL BAR</a><br>
|
|
<P>
|
|
Vertical bar characters are used to separate alternative patterns. For example,
|
|
the pattern
|
|
<pre>
|
|
gilbert|sullivan
|
|
</pre>
|
|
matches either "gilbert" or "sullivan". Any number of alternatives may appear,
|
|
and an empty alternative is permitted (matching the empty string). The matching
|
|
process tries each alternative in turn, from left to right, and the first one
|
|
that succeeds is used. If the alternatives are within a group
|
|
<a href="#group">(defined below),</a>
|
|
"succeeds" means matching the rest of the main pattern as well as the
|
|
alternative in the group.
|
|
<a name="internaloptions"></a></P>
|
|
<br><a name="SEC13" href="#TOC1">INTERNAL OPTION SETTING</a><br>
|
|
<P>
|
|
The settings of the PCRE2_CASELESS, PCRE2_MULTILINE, PCRE2_DOTALL,
|
|
PCRE2_EXTENDED, PCRE2_EXTENDED_MORE, and PCRE2_NO_AUTO_CAPTURE options can be
|
|
changed from within the pattern by a sequence of letters enclosed between "(?"
|
|
and ")". These options are Perl-compatible, and are described in detail in the
|
|
<a href="pcre2api.html"><b>pcre2api</b></a>
|
|
documentation. The option letters are:
|
|
<pre>
|
|
i for PCRE2_CASELESS
|
|
m for PCRE2_MULTILINE
|
|
n for PCRE2_NO_AUTO_CAPTURE
|
|
s for PCRE2_DOTALL
|
|
x for PCRE2_EXTENDED
|
|
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
|
|
example (?-im). The two "extended" options are not independent; unsetting either
|
|
one cancels the effects of both of them.
|
|
</P>
|
|
<P>
|
|
A combined setting and unsetting such as (?im-sx), which sets PCRE2_CASELESS
|
|
and PCRE2_MULTILINE while unsetting PCRE2_DOTALL and PCRE2_EXTENDED, is also
|
|
permitted. Only one hyphen may appear in the options string. If a letter
|
|
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>
|
|
<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
|
|
appear.
|
|
</P>
|
|
<P>
|
|
The PCRE2-specific options PCRE2_DUPNAMES and PCRE2_UNGREEDY can be changed in
|
|
the same way as the Perl-compatible options by using the characters J and U
|
|
respectively. However, these are not unset by (?^).
|
|
</P>
|
|
<P>
|
|
When one of these option changes occurs at top level (that is, not inside
|
|
group parentheses), the change applies to the remainder of the pattern
|
|
that follows. An option change within a group (see below for a description
|
|
of groups) affects only that part of the group that follows it, so
|
|
<pre>
|
|
(a(?i)b)c
|
|
</pre>
|
|
matches abc and aBc and no other strings (assuming PCRE2_CASELESS is not used).
|
|
By this means, options can be made to have different settings in different
|
|
parts of the pattern. Any changes made in one alternative do carry on
|
|
into subsequent branches within the same group. For example,
|
|
<pre>
|
|
(a(?i)b|c)
|
|
</pre>
|
|
matches "ab", "aB", "c", and "C", even though when matching "C" the first
|
|
branch is abandoned before the option setting. This is because the effects of
|
|
option settings happen at compile time. There would be some very weird
|
|
behaviour otherwise.
|
|
</P>
|
|
<P>
|
|
As a convenient shorthand, if any option settings are required at the start of
|
|
a non-capturing group (see the next section), the option letters may
|
|
appear between the "?" and the ":". Thus the two patterns
|
|
<pre>
|
|
(?i:saturday|sunday)
|
|
(?:(?i)saturday|sunday)
|
|
</pre>
|
|
match exactly the same set of strings.
|
|
</P>
|
|
<P>
|
|
<b>Note:</b> There are other PCRE2-specific options, applying to the whole
|
|
pattern, which can be set by the application when the compiling function is
|
|
called. In addition, the pattern can contain special leading sequences such as
|
|
(*CRLF) to override what the application has set or what has been defaulted.
|
|
Details are given in the section entitled
|
|
<a href="#newlineseq">"Newline sequences"</a>
|
|
above. There are also the (*UTF) and (*UCP) leading sequences that can be used
|
|
to set UTF and Unicode property modes; they are equivalent to setting the
|
|
PCRE2_UTF and PCRE2_UCP options, respectively. However, the application can set
|
|
the PCRE2_NEVER_UTF and PCRE2_NEVER_UCP options, which lock out the use of the
|
|
(*UTF) and (*UCP) sequences.
|
|
<a name="group"></a></P>
|
|
<br><a name="SEC14" href="#TOC1">GROUPS</a><br>
|
|
<P>
|
|
Groups are delimited by parentheses (round brackets), which can be nested.
|
|
Turning part of a pattern into a group does two things:
|
|
<br>
|
|
<br>
|
|
1. It localizes a set of alternatives. For example, the pattern
|
|
<pre>
|
|
cat(aract|erpillar|)
|
|
</pre>
|
|
matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
|
|
match "cataract", "erpillar" or an empty string.
|
|
<br>
|
|
<br>
|
|
2. It creates a "capture group". This means that, when the whole pattern
|
|
matches, the portion of the subject string that matched the group is passed
|
|
back to the caller, separately from the portion that matched the whole pattern.
|
|
(This applies only to the traditional matching function; the DFA matching
|
|
function does not support capturing.)
|
|
</P>
|
|
<P>
|
|
Opening parentheses are counted from left to right (starting from 1) to obtain
|
|
numbers for capture groups. For example, if the string "the red king" is
|
|
matched against the pattern
|
|
<pre>
|
|
the ((red|white) (king|queen))
|
|
</pre>
|
|
the captured substrings are "red king", "red", and "king", and are numbered 1,
|
|
2, and 3, respectively.
|
|
</P>
|
|
<P>
|
|
The fact that plain parentheses fulfil two functions is not always helpful.
|
|
There are often times when grouping is required without capturing. If an
|
|
opening parenthesis is followed by a question mark and a colon, the group
|
|
does not do any capturing, and is not counted when computing the number of any
|
|
subsequent capture groups. For example, if the string "the white queen"
|
|
is matched against the pattern
|
|
<pre>
|
|
the ((?:red|white) (king|queen))
|
|
</pre>
|
|
the captured substrings are "white queen" and "queen", and are numbered 1 and
|
|
2. The maximum number of capture groups is 65535.
|
|
</P>
|
|
<P>
|
|
As a convenient shorthand, if any option settings are required at the start of
|
|
a non-capturing group, the option letters may appear between the "?" and the
|
|
":". Thus the two patterns
|
|
<pre>
|
|
(?i:saturday|sunday)
|
|
(?:(?i)saturday|sunday)
|
|
</pre>
|
|
match exactly the same set of strings. Because alternative branches are tried
|
|
from left to right, and options are not reset until the end of the group is
|
|
reached, an option setting in one branch does affect subsequent branches, so
|
|
the above patterns match "SUNDAY" as well as "Saturday".
|
|
<a name="dupgroupnumber"></a></P>
|
|
<br><a name="SEC15" href="#TOC1">DUPLICATE GROUP NUMBERS</a><br>
|
|
<P>
|
|
Perl 5.10 introduced a feature whereby each alternative in a group uses the
|
|
same numbers for its capturing parentheses. Such a group starts with (?| and is
|
|
itself a non-capturing group. For example, consider this pattern:
|
|
<pre>
|
|
(?|(Sat)ur|(Sun))day
|
|
</pre>
|
|
Because the two alternatives are inside a (?| group, both sets of capturing
|
|
parentheses are numbered one. Thus, when the pattern matches, you can look
|
|
at captured substring number one, whichever alternative matched. This construct
|
|
is useful when you want to capture part, but not all, of one of a number of
|
|
alternatives. Inside a (?| group, parentheses are numbered as usual, but the
|
|
number is reset at the start of each branch. The numbers of any capturing
|
|
parentheses that follow the whole group start after the highest number used in
|
|
any branch. The following example is taken from the Perl documentation. The
|
|
numbers underneath show in which buffer the captured content will be stored.
|
|
<pre>
|
|
# before ---------------branch-reset----------- after
|
|
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
|
|
# 1 2 2 3 2 3 4
|
|
</pre>
|
|
A backreference to a capture group uses the most recent value that is set for
|
|
the group. The following pattern matches "abcabc" or "defdef":
|
|
<pre>
|
|
/(?|(abc)|(def))\1/
|
|
</pre>
|
|
In contrast, a subroutine call to a capture group always refers to the
|
|
first one in the pattern with the given number. The following pattern matches
|
|
"abcabc" or "defabc":
|
|
<pre>
|
|
/(?|(abc)|(def))(?1)/
|
|
</pre>
|
|
A relative reference such as (?-1) is no different: it is just a convenient way
|
|
of computing an absolute group number.
|
|
</P>
|
|
<P>
|
|
If a
|
|
<a href="#conditions">condition test</a>
|
|
for a group's having matched refers to a non-unique number, the test is
|
|
true if any group with that number has matched.
|
|
</P>
|
|
<P>
|
|
An alternative approach to using this "branch reset" feature is to use
|
|
duplicate named groups, as described in the next section.
|
|
</P>
|
|
<br><a name="SEC16" href="#TOC1">NAMED CAPTURE GROUPS</a><br>
|
|
<P>
|
|
Identifying capture groups by number is simple, but it can be very hard 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 capture groups. 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.
|
|
</P>
|
|
<P>
|
|
In PCRE2, a capture group can be named in one of three ways: (?<name>...) or
|
|
(?'name'...) as in Perl, or (?P<name>...) as in Python. Names may be up to 32
|
|
code units long. When PCRE2_UTF is not set, they may contain only ASCII
|
|
alphanumeric characters and underscores, but must start with a non-digit. When
|
|
PCRE2_UTF is set, the syntax of group names is extended to allow any Unicode
|
|
letter or Unicode decimal digit. In other words, group names must match one of
|
|
these patterns:
|
|
<pre>
|
|
^[_A-Za-z][_A-Za-z0-9]*\z when PCRE2_UTF is not set
|
|
^[_\p{L}][_\p{L}\p{Nd}]*\z when PCRE2_UTF is set
|
|
</pre>
|
|
References to capture groups 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 all be made by name as well as by number.
|
|
</P>
|
|
<P>
|
|
Named capture groups are allocated numbers as well as names, exactly as
|
|
if the names were not present. In both PCRE2 and Perl, capture groups
|
|
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>
|
|
<b>Warning:</b> When more than one capture group 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 groups to have different names.
|
|
Consider this pattern, where there are two capture groups, 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 group number 1 is not explicitly named, the name AA is
|
|
still an alias for any group 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 groups 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, as described
|
|
in the section entitled
|
|
<a href="#internaloptions">"Internal Option Setting"</a>
|
|
above.
|
|
</P>
|
|
<P>
|
|
Duplicate names can be useful for patterns where only one instance of the named
|
|
capture group 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>
|
|
(?J)
|
|
(?<DN>Mon|Fri|Sun)(?:day)?|
|
|
(?<DN>Tue)(?:sday)?|
|
|
(?<DN>Wed)(?:nesday)?|
|
|
(?<DN>Thu)(?:rsday)?|
|
|
(?<DN>Sat)(?:urday)?
|
|
</pre>
|
|
There are five capture groups, but only one is ever set after a match. The
|
|
convenience functions for extracting the data by name returns the substring for
|
|
the first (and in this example, the only) group of that name that matched. This
|
|
saves searching to find which numbered group it was. (An alternative way of
|
|
solving this problem is to use a "branch reset" group, as described in the
|
|
previous section.)
|
|
</P>
|
|
<P>
|
|
If you make a backreference to a non-unique named group from elsewhere in the
|
|
pattern, the groups 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 pattern matches both "foofoo" and "barbar" but not
|
|
"foobar" or "barfoo":
|
|
<pre>
|
|
(?J)(?:(?<n>foo)|(?<n>bar))\k<n>
|
|
|
|
</PRE>
|
|
</P>
|
|
<P>
|
|
If you make a subroutine call to a non-unique named group, the one 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.
|
|
</P>
|
|
<P>
|
|
If you use a named reference in a condition
|
|
test (see the
|
|
<a href="#conditions">section about conditions</a>
|
|
below), either to check whether a capture group has matched, or to check for
|
|
recursion, all groups with the same name are tested. If the condition is true
|
|
for any one of them, the overall condition is true. This is the same behaviour
|
|
as testing by number. For further details of the interfaces for handling named
|
|
capture groups, see the
|
|
<a href="pcre2api.html"><b>pcre2api</b></a>
|
|
documentation.
|
|
</P>
|
|
<br><a name="SEC17" href="#TOC1">REPETITION</a><br>
|
|
<P>
|
|
Repetition is specified by quantifiers, which can follow any of the following
|
|
items:
|
|
<pre>
|
|
a literal data character
|
|
the dot metacharacter
|
|
the \C escape sequence
|
|
the \R escape sequence
|
|
the \X escape sequence
|
|
an escape such as \d or \pL that matches a single character
|
|
a character class
|
|
a backreference
|
|
a parenthesized group (including most assertions)
|
|
a subroutine call (recursive or otherwise)
|
|
</pre>
|
|
The general repetition quantifier specifies a minimum and maximum number 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,
|
|
<pre>
|
|
z{2,4}
|
|
</pre>
|
|
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
|
|
<pre>
|
|
[aeiou]{3,}
|
|
</pre>
|
|
matches at least 3 successive vowels, but may match many more, whereas
|
|
<pre>
|
|
\d{8}
|
|
</pre>
|
|
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 example, {,6} is not a
|
|
quantifier, but a literal string of four characters.
|
|
</P>
|
|
<P>
|
|
In UTF modes, quantifiers apply to characters rather than to individual 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. Similarly,
|
|
\X{3} matches three Unicode extended grapheme clusters, each of which may be
|
|
several code units long (and they may be of different lengths).
|
|
</P>
|
|
<P>
|
|
The quantifier {0} is permitted, causing the expression to behave as if the
|
|
previous item and the quantifier were not present. This may be useful for
|
|
capture groups that are referenced as
|
|
<a href="#groupsassubroutines">subroutines</a>
|
|
from elsewhere in the pattern (but see also the section entitled
|
|
<a href="#subdefine">"Defining capture groups for use by reference only"</a>
|
|
below). Except for parenthesized groups, items that have a {0} quantifier are
|
|
omitted from the compiled pattern.
|
|
</P>
|
|
<P>
|
|
For convenience, the three most common quantifiers have single-character
|
|
abbreviations:
|
|
<pre>
|
|
* is equivalent to {0,}
|
|
+ is equivalent to {1,}
|
|
? is equivalent to {0,1}
|
|
</pre>
|
|
It is possible to construct infinite loops by following a group that can match
|
|
no characters with a quantifier that has no upper limit, for example:
|
|
<pre>
|
|
(a?)*
|
|
</pre>
|
|
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 whenever an iteration of such a group matches no
|
|
characters, matching moves on to the next item in the pattern instead of
|
|
repeatedly matching an empty string. This does not prevent backtracking into
|
|
any of the iterations if a subsequent item fails to match.
|
|
</P>
|
|
<P>
|
|
By default, 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 pattern
|
|
<pre>
|
|
/\*.*\*/
|
|
</pre>
|
|
to the string
|
|
<pre>
|
|
/* first comment */ not comment /* second comment */
|
|
</pre>
|
|
fails, because it matches the entire string owing to the greediness of the .*
|
|
item. However, if a quantifier is followed by a question mark, it ceases to be
|
|
greedy, and instead matches the minimum number of times possible, so the
|
|
pattern
|
|
<pre>
|
|
/\*.*?\*/
|
|
</pre>
|
|
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
|
|
<pre>
|
|
\d??\d
|
|
</pre>
|
|
which matches one digit by preference, but can match two if that is the only
|
|
way the rest of the pattern matches.
|
|
</P>
|
|
<P>
|
|
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 words, it inverts the
|
|
default behaviour.
|
|
</P>
|
|
<P>
|
|
When a parenthesized group 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.
|
|
</P>
|
|
<P>
|
|
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 newlines, the pattern is
|
|
implicitly anchored, because whatever follows will be tried against every
|
|
character position in the subject string, so there is no point in retrying the
|
|
overall match at any position after the first. PCRE2 normally treats such a
|
|
pattern as though it were preceded by \A.
|
|
</P>
|
|
<P>
|
|
In cases where it is known that the subject string contains no newlines, it is
|
|
worth setting PCRE2_DOTALL in order to obtain this optimization, or
|
|
alternatively, using ^ to indicate anchoring explicitly.
|
|
</P>
|
|
<P>
|
|
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:
|
|
<pre>
|
|
(.*)abc\1
|
|
</pre>
|
|
If the subject is "xyz123abc123" the match point is the fourth character. For
|
|
this reason, such a pattern is not implicitly anchored.
|
|
</P>
|
|
<P>
|
|
Another case where implicit anchoring is not applied is when the leading .* is
|
|
inside an atomic group. Once again, a match at the start may fail where a later
|
|
one succeeds. Consider this pattern:
|
|
<pre>
|
|
(?>.*?a)b
|
|
</pre>
|
|
It matches "ab" in the subject "aab". The use of the backtracking control verbs
|
|
(*PRUNE) and (*SKIP) also disable this optimization, and there is an option,
|
|
PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly.
|
|
</P>
|
|
<P>
|
|
When a capture group is repeated, the value captured is the substring that
|
|
matched the final iteration. For example, after
|
|
<pre>
|
|
(tweedle[dume]{3}\s*)+
|
|
</pre>
|
|
has matched "tweedledum tweedledee" the value of the captured substring is
|
|
"tweedledee". However, if there are nested capture groups, the corresponding
|
|
captured values may have been set in previous iterations. For example, after
|
|
<pre>
|
|
(a|(b))+
|
|
</pre>
|
|
matches "aba" the value of the second captured substring is "b".
|
|
<a name="atomicgroup"></a></P>
|
|
<br><a name="SEC18" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
Consider, for example, the pattern \d+foo when applied to the subject line
|
|
<pre>
|
|
123456bar
|
|
</pre>
|
|
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 group has matched, it is not to be re-evaluated in this way.
|
|
</P>
|
|
<P>
|
|
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:
|
|
<pre>
|
|
(?>\d+)foo
|
|
</pre>
|
|
Perl 5.28 introduced an experimental alphabetic form starting with (* which may
|
|
be easier to remember:
|
|
<pre>
|
|
(*atomic:\d+)foo
|
|
</pre>
|
|
This kind of parenthesized group "locks up" the part of the pattern it
|
|
contains 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.
|
|
</P>
|
|
<P>
|
|
An alternative description is that a group 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.
|
|
</P>
|
|
<P>
|
|
Atomic groups are not capture groups. 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 prepared 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.
|
|
</P>
|
|
<P>
|
|
Atomic groups in general can of course contain arbitrarily complicated
|
|
expressions, and can be nested. However, when the contents of 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 this notation, the
|
|
previous example can be rewritten as
|
|
<pre>
|
|
\d++foo
|
|
</pre>
|
|
Note that a possessive quantifier can be used with an entire group, for
|
|
example:
|
|
<pre>
|
|
(abc|xyz){2,3}+
|
|
</pre>
|
|
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 should be slightly faster.
|
|
</P>
|
|
<P>
|
|
The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
|
|
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 found its way into Perl at release
|
|
5.10.
|
|
</P>
|
|
<P>
|
|
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_AUTOPOSSESS option, or starting
|
|
the pattern with (*NO_AUTO_POSSESS).
|
|
</P>
|
|
<P>
|
|
When a pattern contains an unlimited repeat inside a group 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
|
|
<pre>
|
|
(\D+|<\d+>)*[!?]
|
|
</pre>
|
|
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
|
|
<pre>
|
|
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
|
|
</pre>
|
|
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 character 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:
|
|
<pre>
|
|
((?>\D+)|<\d+>)*[!?]
|
|
</pre>
|
|
sequences of non-digits cannot be broken, and failure happens quickly.
|
|
<a name="backreferences"></a></P>
|
|
<br><a name="SEC19" href="#TOC1">BACKREFERENCES</a><br>
|
|
<P>
|
|
Outside a character class, a backslash followed by a digit greater than 0 (and
|
|
possibly further digits) is a backreference to a capture group earlier (that
|
|
is, to its left) in the pattern, provided there have been that many previous
|
|
capture groups.
|
|
</P>
|
|
<P>
|
|
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 capture groups in the entire pattern. In other words, the group that is
|
|
referenced need not be to the left of the reference for numbers less than 8. A
|
|
"forward backreference" of this type can make sense when a repetition is
|
|
involved and the group to the right has participated in an earlier iteration.
|
|
</P>
|
|
<P>
|
|
It is not possible to have a numerical "forward backreference" to a group 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"
|
|
<a href="#digitsafterbackslash">above</a>
|
|
for further details of the handling of digits following a backslash. Other
|
|
forms of backreferencing do not suffer from this restriction. In particular,
|
|
there is no problem when named capture groups are used (see below).
|
|
</P>
|
|
<P>
|
|
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:
|
|
<pre>
|
|
(ring), \1
|
|
(ring), \g1
|
|
(ring), \g{1}
|
|
</pre>
|
|
An unsigned number specifies an absolute reference without the ambiguity that
|
|
is present in the older syntax. It is also useful when literal digits follow
|
|
the reference. A signed number is a relative reference. Consider this example:
|
|
<pre>
|
|
(abc(def)ghi)\g{-1}
|
|
</pre>
|
|
The sequence \g{-1} is a reference to the most recently started capture group
|
|
before \g, that is, is it equivalent to \2 in this example. 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.
|
|
</P>
|
|
<P>
|
|
The sequence \g{+1} is a reference to the next capture group. This kind of
|
|
forward reference can be useful in patterns that repeat. Perl does not support
|
|
the use of + in this way.
|
|
</P>
|
|
<P>
|
|
A backreference matches whatever actually most recently matched the capture
|
|
group in the current subject string, rather than anything at all that matches
|
|
the group (see
|
|
<a href="#groupsassubroutines">"Groups as subroutines"</a>
|
|
below for a way of doing that). So the pattern
|
|
<pre>
|
|
(sens|respons)e and \1ibility
|
|
</pre>
|
|
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 example,
|
|
<pre>
|
|
((?i)rah)\s+\1
|
|
</pre>
|
|
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
|
|
capture group is matched caselessly.
|
|
</P>
|
|
<P>
|
|
There are several different ways of writing backreferences to named capture
|
|
groups. 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:
|
|
<pre>
|
|
(?<p1>(?i)rah)\s+\k<p1>
|
|
(?'p1'(?i)rah)\s+\k{p1}
|
|
(?P<p1>(?i)rah)\s+(?P=p1)
|
|
(?<p1>(?i)rah)\s+\g{p1}
|
|
</pre>
|
|
A capture group that is referenced by name may appear in the pattern before or
|
|
after the reference.
|
|
</P>
|
|
<P>
|
|
There may be more than one backreference to the same group. If a group has not
|
|
actually been used in a particular match, backreferences to it always fail by
|
|
default. For example, the pattern
|
|
<pre>
|
|
(a|(bc))\2
|
|
</pre>
|
|
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 backreference to an
|
|
unset value matches an empty string.
|
|
</P>
|
|
<P>
|
|
Because there may be many capture groups in a pattern, all digits following a
|
|
backslash are taken as part of a potential backreference 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
|
|
<a href="#comments">"Comments"</a>
|
|
below) can be used.
|
|
</P>
|
|
<br><b>
|
|
Recursive backreferences
|
|
</b><br>
|
|
<P>
|
|
A backreference that occurs inside the group to which it refers fails when the
|
|
group is first used, so, for example, (a\1) never matches. However, such
|
|
references can be useful inside repeated groups. For example, the pattern
|
|
<pre>
|
|
(a|b\1)+
|
|
</pre>
|
|
matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
|
|
the group, 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 example above, or by a quantifier with a minimum
|
|
of zero.
|
|
</P>
|
|
<P>
|
|
For versions of PCRE2 less than 10.25, backreferences of this type used to
|
|
cause the group that they reference to be treated as an
|
|
<a href="#atomicgroup">atomic group.</a>
|
|
This restriction no longer applies, and backtracking into such groups can occur
|
|
as normal.
|
|
<a name="bigassertions"></a></P>
|
|
<br><a name="SEC20" href="#TOC1">ASSERTIONS</a><br>
|
|
<P>
|
|
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
|
|
<a href="#smallassertions">above.</a>
|
|
</P>
|
|
<P>
|
|
More complicated assertions are coded as parenthesized groups. 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
|
|
match for the assertion to be true) or negative (must not match for the
|
|
assertion to be true). An assertion group is matched in the normal way,
|
|
and if it is true, matching continues after it, but with the matching position
|
|
in the subject string reset to what it was before the assertion was processed.
|
|
</P>
|
|
<P>
|
|
The Perl-compatible lookaround assertions are atomic. If an assertion is true,
|
|
but there is a subsequent matching failure, there is no backtracking into the
|
|
assertion. However, there are some cases where non-atomic assertions can be
|
|
useful. PCRE2 has some support for these, described in the section entitled
|
|
<a href="#nonatomicassertions">"Non-atomic assertions"</a>
|
|
below, but they are not Perl-compatible.
|
|
</P>
|
|
<P>
|
|
A lookaround assertion may appear as the condition in a
|
|
<a href="#conditions">conditional group</a>
|
|
(see below). In this case, the result of matching the assertion determines
|
|
which branch of the condition is followed.
|
|
</P>
|
|
<P>
|
|
Assertion groups are not capture groups. If an assertion contains capture
|
|
groups within it, these are counted for the purposes of numbering the capture
|
|
groups 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.
|
|
</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
|
|
match). A negative assertion is true 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
|
|
depends on the type of assertion.
|
|
</P>
|
|
<P>
|
|
For a positive assertion, internally captured substrings in the successful
|
|
branch are retained, and matching continues with the next pattern item after
|
|
the assertion. For a negative assertion, a matching branch means that the
|
|
assertion is not true. If such an assertion is being used as a condition in a
|
|
<a href="#conditions">conditional group</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
|
|
strings within the assertion.
|
|
</P>
|
|
<P>
|
|
For compatibility with Perl, most assertion groups may be repeated; though it
|
|
makes no sense to assert the same thing several times, the side effect of
|
|
capturing may occasionally be useful. However, an assertion that forms the
|
|
condition for a conditional group may not be quantified. In practice, for
|
|
other assertions, there only three cases:
|
|
<br>
|
|
<br>
|
|
(1) If the quantifier is {0}, the assertion is never obeyed during matching.
|
|
However, it may contain internal capture groups that are called from elsewhere
|
|
via the
|
|
<a href="#groupsassubroutines">subroutine mechanism.</a>
|
|
<br>
|
|
<br>
|
|
(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 greediness of the quantifier.
|
|
<br>
|
|
<br>
|
|
(3) If the minimum repetition is greater than zero, the quantifier is ignored.
|
|
The assertion is obeyed just once when encountered during matching.
|
|
</P>
|
|
<br><b>
|
|
Alphabetic assertion names
|
|
</b><br>
|
|
<P>
|
|
Traditionally, symbolic sequences such as (?= and (?<= have been used to
|
|
specify lookaround assertions. Perl 5.28 introduced some experimental
|
|
alphabetic alternatives which might be easier to remember. They all start with
|
|
(* instead of (? and must be written using lower case letters. PCRE2 supports
|
|
the following synonyms:
|
|
<pre>
|
|
(*positive_lookahead: or (*pla: is the same as (?=
|
|
(*negative_lookahead: or (*nla: is the same as (?!
|
|
(*positive_lookbehind: or (*plb: is the same as (?<=
|
|
(*negative_lookbehind: or (*nlb: is the same as (?<!
|
|
</pre>
|
|
For example, (*pla:foo) is the same assertion as (?=foo). In the following
|
|
sections, the various assertions are described using the original symbolic
|
|
forms.
|
|
</P>
|
|
<br><b>
|
|
Lookahead assertions
|
|
</b><br>
|
|
<P>
|
|
Lookahead assertions start with (?= for positive assertions and (?! for
|
|
negative assertions. For example,
|
|
<pre>
|
|
\w+(?=;)
|
|
</pre>
|
|
matches a word followed by a semicolon, but does not include the semicolon in
|
|
the match, and
|
|
<pre>
|
|
foo(?!bar)
|
|
</pre>
|
|
matches any occurrence of "foo" that is not followed by "bar". Note that the
|
|
apparently similar pattern
|
|
<pre>
|
|
(?!foo)bar
|
|
</pre>
|
|
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.
|
|
</P>
|
|
<P>
|
|
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 string must always fail.
|
|
The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
|
|
<a name="lookbehind"></a></P>
|
|
<br><b>
|
|
Lookbehind assertions
|
|
</b><br>
|
|
<P>
|
|
Lookbehind assertions start with (?<= for positive assertions and (?<! for
|
|
negative assertions. For example,
|
|
<pre>
|
|
(?<!foo)bar
|
|
</pre>
|
|
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 several top-level alternatives, they
|
|
do not all have to have the same fixed length. Thus
|
|
<pre>
|
|
(?<=bullock|donkey)
|
|
</pre>
|
|
is permitted, but
|
|
<pre>
|
|
(?<!dogs?|cats?)
|
|
</pre>
|
|
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
|
|
<pre>
|
|
(?<=ab(c|de))
|
|
</pre>
|
|
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:
|
|
<pre>
|
|
(?<=abc|abde)
|
|
</pre>
|
|
In some cases, the escape sequence \K
|
|
<a href="#resetmatchstart">(see above)</a>
|
|
can be used instead of a lookbehind assertion to get round the fixed-length
|
|
restriction.
|
|
</P>
|
|
<P>
|
|
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 current position, the
|
|
assertion fails.
|
|
</P>
|
|
<P>
|
|
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 numbers of code units, are never
|
|
permitted in lookbehinds.
|
|
</P>
|
|
<P>
|
|
<a href="#groupsassubroutines">"Subroutine"</a>
|
|
calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
|
|
as the called capture group matches a fixed-length string. However,
|
|
<a href="#recursion">recursion,</a>
|
|
that is, a "subroutine" call into a group that is already active,
|
|
is not supported.
|
|
</P>
|
|
<P>
|
|
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 of (?| in the pattern (it creates
|
|
duplicate group numbers), and if the backreference is by name, the name
|
|
must be unique. Of course, the referenced group must itself match a fixed
|
|
length substring. The following pattern matches words containing at least two
|
|
characters that begin and end with the same character:
|
|
<pre>
|
|
\b(\w)\w++(?<=\1)
|
|
</PRE>
|
|
</P>
|
|
<P>
|
|
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
|
|
<pre>
|
|
abcd$
|
|
</pre>
|
|
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 subject and then see if
|
|
what follows matches the rest of the pattern. If the pattern is specified as
|
|
<pre>
|
|
^.*abcd$
|
|
</pre>
|
|
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, so we are no better off. However,
|
|
if the pattern is written as
|
|
<pre>
|
|
^.*+(?<=abcd)
|
|
</pre>
|
|
there can be no backtracking for the .*+ item because of the possessive
|
|
quantifier; it can match only the entire string. The subsequent lookbehind
|
|
assertion does a single test on the last four characters. If it fails, the
|
|
match fails immediately. For long strings, this approach makes a significant
|
|
difference to the processing time.
|
|
</P>
|
|
<br><b>
|
|
Using multiple assertions
|
|
</b><br>
|
|
<P>
|
|
Several assertions (of any sort) may occur in succession. For example,
|
|
<pre>
|
|
(?<=\d{3})(?<!999)foo
|
|
</pre>
|
|
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 <i>not</i> match "foo" preceded by six characters, the first
|
|
of which are digits and the last three of which are not "999". For example, it
|
|
doesn't match "123abcfoo". A pattern to do that is
|
|
<pre>
|
|
(?<=\d{3}...)(?<!999)foo
|
|
</pre>
|
|
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".
|
|
</P>
|
|
<P>
|
|
Assertions can be nested in any combination. For example,
|
|
<pre>
|
|
(?<=(?<!foo)bar)baz
|
|
</pre>
|
|
matches an occurrence of "baz" that is preceded by "bar" which in turn is not
|
|
preceded by "foo", while
|
|
<pre>
|
|
(?<=\d{3}(?!999)...)foo
|
|
</pre>
|
|
is another pattern that matches "foo" preceded by three digits and any three
|
|
characters that are not "999".
|
|
<a name="nonatomicassertions"></a></P>
|
|
<br><a name="SEC21" href="#TOC1">NON-ATOMIC ASSERTIONS</a><br>
|
|
<P>
|
|
The traditional Perl-compatible lookaround assertions are atomic. That is, if
|
|
an assertion is true, but there is a subsequent matching failure, there is no
|
|
backtracking into the assertion. However, there are some cases where non-atomic
|
|
positive assertions can be useful. PCRE2 provides these using the following
|
|
syntax:
|
|
<pre>
|
|
(*non_atomic_positive_lookahead: or (*napla: or (?*
|
|
(*non_atomic_positive_lookbehind: or (*naplb: or (?<*
|
|
</pre>
|
|
Consider the problem of finding the right-most word in a string that also
|
|
appears earlier in the string, that is, it must appear at least twice in total.
|
|
This pattern returns the required result as captured substring 1:
|
|
<pre>
|
|
^(?x)(*napla: .* \b(\w++)) (?> .*? \b\1\b ){2}
|
|
</pre>
|
|
For a subject such as "word1 word2 word3 word2 word3 word4" the result is
|
|
"word3". How does it work? At the start, ^(?x) anchors the pattern and sets the
|
|
"x" option, which causes white space (introduced for readability) to be
|
|
ignored. Inside the assertion, the greedy .* at first consumes the entire
|
|
string, but then has to backtrack until the rest of the assertion can match a
|
|
word, which is captured by group 1. In other words, when the assertion first
|
|
succeeds, it captures the right-most word in the string.
|
|
</P>
|
|
<P>
|
|
The current matching point is then reset to the start of the subject, and the
|
|
rest of the pattern match checks for two occurrences of the captured word,
|
|
using an ungreedy .*? to scan from the left. If this succeeds, we are done, but
|
|
if the last word in the string does not occur twice, this part of the pattern
|
|
fails. If a traditional atomic lookhead (?= or (*pla: had been used, the
|
|
assertion could not be re-entered, and the whole match would fail. The pattern
|
|
would succeed only if the very last word in the subject was found twice.
|
|
</P>
|
|
<P>
|
|
Using a non-atomic lookahead, however, means that when the last word does not
|
|
occur twice in the string, the lookahead can backtrack and find the second-last
|
|
word, and so on, until either the match succeeds, or all words have been
|
|
tested.
|
|
</P>
|
|
<P>
|
|
Two conditions must be met for a non-atomic assertion to be useful: the
|
|
contents of one or more capturing groups must change after a backtrack into the
|
|
assertion, and there must be a backreference to a changed group later in the
|
|
pattern. If this is not the case, the rest of the pattern match fails exactly
|
|
as before because nothing has changed, so using a non-atomic assertion just
|
|
wastes resources.
|
|
</P>
|
|
<P>
|
|
Non-atomic assertions are not supported by the alternative matching function
|
|
<b>pcre2_dfa_match()</b>. They are also not supported by JIT (but may be in
|
|
future). Note that assertions that appear as conditions for
|
|
<a href="#conditions">conditional groups</a>
|
|
(see below) must be atomic.
|
|
</P>
|
|
<br><a name="SEC22" href="#TOC1">SCRIPT RUNS</a><br>
|
|
<P>
|
|
In concept, a script run is a sequence of characters that are all from the same
|
|
Unicode script such as Latin or Greek. However, because some scripts are
|
|
commonly used together, and because some diacritical and other marks are used
|
|
with multiple scripts, it is not that simple. There is a full description of
|
|
the rules that PCRE2 uses in the section entitled
|
|
<a href="pcre2unicode.html#scriptruns">"Script Runs"</a>
|
|
in the
|
|
<a href="pcre2unicode.html"><b>pcre2unicode</b></a>
|
|
documentation.
|
|
</P>
|
|
<P>
|
|
If part of a pattern is enclosed between (*script_run: or (*sr: and a closing
|
|
parenthesis, it fails if the sequence of characters that it matches are not a
|
|
script run. After a failure, normal backtracking occurs. Script runs can be
|
|
used to detect spoofing attacks using characters that look the same, but are
|
|
from different scripts. The string "paypal.com" is an infamous example, where
|
|
the letters could be a mixture of Latin and Cyrillic. This pattern ensures that
|
|
the matched characters in a sequence of non-spaces that follow white space are
|
|
a script run:
|
|
<pre>
|
|
\s+(*sr:\S+)
|
|
</pre>
|
|
To be sure that they are all from the Latin script (for example), a lookahead
|
|
can be used:
|
|
<pre>
|
|
\s+(?=\p{Latin})(*sr:\S+)
|
|
</pre>
|
|
This works as long as the first character is expected to be a character in that
|
|
script, and not (for example) punctuation, which is allowed with any script. If
|
|
this is not the case, a more creative lookahead is needed. For example, if
|
|
digits, underscore, and dots are permitted at the start:
|
|
<pre>
|
|
\s+(?=[0-9_.]*\p{Latin})(*sr:\S+)
|
|
|
|
</PRE>
|
|
</P>
|
|
<P>
|
|
In many cases, backtracking into a script run pattern fragment is not
|
|
desirable. The script run can employ an atomic group to prevent this. Because
|
|
this is a common requirement, a shorthand notation is provided by
|
|
(*atomic_script_run: or (*asr:
|
|
<pre>
|
|
(*asr:...) is the same as (*sr:(?>...))
|
|
</pre>
|
|
Note that the atomic group is inside the script run. Putting it outside would
|
|
not prevent backtracking into the script run pattern.
|
|
</P>
|
|
<P>
|
|
Support for script runs is not available if PCRE2 is compiled without Unicode
|
|
support. A compile-time error is given if any of the above constructs is
|
|
encountered. Script runs are not supported by the alternate matching function,
|
|
<b>pcre2_dfa_match()</b> because they use the same mechanism as capturing
|
|
parentheses.
|
|
</P>
|
|
<P>
|
|
<b>Warning:</b> The (*ACCEPT) control verb
|
|
<a href="#acceptverb">(see below)</a>
|
|
should not be used within a script run group, because it causes an immediate
|
|
exit from the group, bypassing the script run checking.
|
|
<a name="conditions"></a></P>
|
|
<br><a name="SEC23" href="#TOC1">CONDITIONAL GROUPS</a><br>
|
|
<P>
|
|
It is possible to cause the matching process to obey a pattern fragment
|
|
conditionally or to choose between two alternative fragments, depending on
|
|
the result of an assertion, or whether a specific capture group has
|
|
already been matched. The two possible forms of conditional group are:
|
|
<pre>
|
|
(?(condition)yes-pattern)
|
|
(?(condition)yes-pattern|no-pattern)
|
|
</pre>
|
|
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 alternatives in the
|
|
group, a compile-time error occurs. Each of the two alternatives may itself
|
|
contain nested groups of any form, including conditional groups; the
|
|
restriction to two alternatives applies only at the level of the condition
|
|
itself. This pattern fragment is an example where the alternatives are complex:
|
|
<pre>
|
|
(?(1) (A|B|C) | (D | (?(2)E|F) | E) )
|
|
|
|
</PRE>
|
|
</P>
|
|
<P>
|
|
There are five kinds of condition: references to capture groups, references to
|
|
recursion, two pseudo-conditions called DEFINE and VERSION, and assertions.
|
|
</P>
|
|
<br><b>
|
|
Checking for a used capture group by number
|
|
</b><br>
|
|
<P>
|
|
If the text between the parentheses consists of a sequence of digits, the
|
|
condition is true if a capture group of that number has previously matched. If
|
|
there is more than one capture group with the same number (see the earlier
|
|
<a href="#recursion">section about duplicate group numbers),</a>
|
|
the condition is true if any of them have matched. An alternative notation is
|
|
to precede the digits with a plus or minus sign. In this case, the group number
|
|
is relative rather than absolute. The most recently opened capture group 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 capture group
|
|
can be referenced as (?(+1), and so on. (The value zero in any of these forms
|
|
is not used; it provokes a compile-time error.)
|
|
</P>
|
|
<P>
|
|
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:
|
|
<pre>
|
|
( \( )? [^()]+ (?(1) \) )
|
|
</pre>
|
|
The first part matches an optional opening parenthesis, and if that
|
|
character is present, sets it as the first captured substring. The second part
|
|
matches one or more characters that are not parentheses. The third part is a
|
|
conditional group that tests whether or not the first capture group
|
|
matched. If it 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. Otherwise, since no-pattern is not present, the
|
|
conditional group matches nothing. In other words, this pattern matches a
|
|
sequence of non-parentheses, optionally enclosed in parentheses.
|
|
</P>
|
|
<P>
|
|
If you were embedding this pattern in a larger one, you could use a relative
|
|
reference:
|
|
<pre>
|
|
...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
|
|
</pre>
|
|
This makes the fragment independent of the parentheses in the larger pattern.
|
|
</P>
|
|
<br><b>
|
|
Checking for a used capture group by name
|
|
</b><br>
|
|
<P>
|
|
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
|
|
capture group 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 section). Rewriting the above example
|
|
to use a named group gives this:
|
|
<pre>
|
|
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
|
|
</pre>
|
|
If the name used in a condition of this kind is a duplicate, the test is
|
|
applied to all groups of the same name, and is true if any one of them has
|
|
matched.
|
|
</P>
|
|
<br><b>
|
|
Checking for pattern recursion
|
|
</b><br>
|
|
<P>
|
|
"Recursion" in this sense refers to any subroutine-like call from one part of
|
|
the pattern to another, whether or not it is actually recursive. See the
|
|
sections entitled
|
|
<a href="#recursion">"Recursive patterns"</a>
|
|
and
|
|
<a href="#groupsassubroutines">"Groups as subroutines"</a>
|
|
below for details of recursion and subroutine calls.
|
|
</P>
|
|
<P>
|
|
If a condition is the string (R), and there is no capture group with the name
|
|
R, the condition is true if matching is currently in a recursion or subroutine
|
|
call to the whole pattern or any capture group. If digits follow the letter R,
|
|
and there is no group with that name, the condition is true if the most recent
|
|
call is into a group with the given number, which must exist somewhere in the
|
|
overall pattern. This is a contrived example that is equivalent to a+b:
|
|
<pre>
|
|
((?(R1)a+|(?1)b))
|
|
</pre>
|
|
However, in both cases, if there is a capture group 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.
|
|
</P>
|
|
<P>
|
|
If a name preceded by ampersand follows the letter R, for example:
|
|
<pre>
|
|
(?(R&name)...)
|
|
</pre>
|
|
the condition is true if the most recent recursion is into a group of that name
|
|
(which must exist within the pattern).
|
|
</P>
|
|
<P>
|
|
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 duplicate, the
|
|
test is applied to all groups of the same name, and is true if any one of
|
|
them is the most recent recursion.
|
|
</P>
|
|
<P>
|
|
At "top level", all these recursion test conditions are false.
|
|
<a name="subdefine"></a></P>
|
|
<br><b>
|
|
Defining capture groups for use by reference only
|
|
</b><br>
|
|
<P>
|
|
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 be only one
|
|
alternative in the rest of the conditional group. It is always skipped if
|
|
control reaches this point in the pattern; the idea of DEFINE is that it can be
|
|
used to define subroutines that can be referenced from elsewhere. (The use of
|
|
<a href="#groupsassubroutines">subroutines</a>
|
|
is described below.) For example, a pattern to match an IPv4 address such as
|
|
"192.168.23.245" could be written like this (ignore white space and line
|
|
breaks):
|
|
<pre>
|
|
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
|
|
\b (?&byte) (\.(?&byte)){3} \b
|
|
</pre>
|
|
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, insisting on a word boundary at each end.
|
|
</P>
|
|
<br><b>
|
|
Checking the PCRE2 version
|
|
</b><br>
|
|
<P>
|
|
Programs that link with a PCRE2 library can check the version by calling
|
|
<b>pcre2_config()</b> with appropriate arguments. Users of applications that do
|
|
not have access to the underlying code cannot do this. A special "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 "=" or ">=" and a version number.
|
|
For example:
|
|
<pre>
|
|
(?(VERSION>=10.4)yes|no)
|
|
</pre>
|
|
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.
|
|
</P>
|
|
<br><b>
|
|
Assertion conditions
|
|
</b><br>
|
|
<P>
|
|
If the condition is not in any of the above formats, it must be a parenthesized
|
|
assertion. This may be a positive or negative lookahead or lookbehind
|
|
assertion. However, it must be a traditional atomic assertion, not one of the
|
|
PCRE2-specific
|
|
<a href="#nonatomicassertions">non-atomic assertions.</a>
|
|
</P>
|
|
<P>
|
|
Consider this pattern, again containing non-significant white space, and with
|
|
the two alternatives on the second line:
|
|
<pre>
|
|
(?(?=[^a-z]*[a-z])
|
|
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
|
|
</pre>
|
|
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.
|
|
</P>
|
|
<P>
|
|
When an assertion that is a condition contains capture groups, any
|
|
capturing that occurs in a matching branch is retained afterwards, for both
|
|
positive and negative assertions, because matching always continues after the
|
|
assertion, whether it succeeds or fails. (Compare non-conditional assertions,
|
|
for which captures are retained only for positive assertions that succeed.)
|
|
<a name="comments"></a></P>
|
|
<br><a name="SEC24" href="#TOC1">COMMENTS</a><br>
|
|
<P>
|
|
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 group name or number. The characters that make up a comment play
|
|
no part in the pattern matching.
|
|
</P>
|
|
<P>
|
|
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 at the start of the pattern, as
|
|
described in the section entitled
|
|
<a href="#newlines">"Newline conventions"</a>
|
|
above. Note that the end of this type of comment is a literal newline sequence
|
|
in the pattern; escape sequences that happen to represent a newline do not
|
|
count. For example, consider this pattern when PCRE2_EXTENDED is set, and the
|
|
default newline convention (a single linefeed character) is in force:
|
|
<pre>
|
|
abc #comment \n still comment
|
|
</pre>
|
|
On encountering the # character, <b>pcre2_compile()</b> 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.
|
|
<a name="recursion"></a></P>
|
|
<br><a name="SEC25" href="#TOC1">RECURSIVE PATTERNS</a><br>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
For some time, Perl has provided a facility that allows regular expressions 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:
|
|
<pre>
|
|
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
|
|
</pre>
|
|
The (?p{...}) item interpolates Perl code at run time, and in this case refers
|
|
recursively to the pattern in which it appears.
|
|
</P>
|
|
<P>
|
|
Obviously, PCRE2 cannot support the interpolation of Perl code. Instead, it
|
|
supports special syntax for recursion of the entire pattern, and also for
|
|
individual capture group recursion. After its introduction in PCRE1 and Python,
|
|
this kind of recursion was subsequently introduced into Perl at release 5.10.
|
|
</P>
|
|
<P>
|
|
A special item that consists of (? followed by a number greater than zero and a
|
|
closing parenthesis is a recursive subroutine call of the capture group of the
|
|
given number, provided that it occurs inside that group. (If not, it is a
|
|
<a href="#groupsassubroutines">non-recursive subroutine</a>
|
|
call, which is described in the next section.) The special item (?R) or (?0) is
|
|
a recursive call of the entire regular expression.
|
|
</P>
|
|
<P>
|
|
This PCRE2 pattern solves the nested parentheses problem (assume the
|
|
PCRE2_EXTENDED option is set so that white space is ignored):
|
|
<pre>
|
|
\( ( [^()]++ | (?R) )* \)
|
|
</pre>
|
|
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 parenthesized substring).
|
|
Finally there is a closing parenthesis. Note the use of a possessive quantifier
|
|
to avoid backtracking into sequences of non-parentheses.
|
|
</P>
|
|
<P>
|
|
If this were part of a larger pattern, you would not want to recurse the entire
|
|
pattern, so instead you could use this:
|
|
<pre>
|
|
( \( ( [^()]++ | (?1) )* \) )
|
|
</pre>
|
|
We have put the pattern into parentheses, and caused the recursion to refer to
|
|
them instead of the whole pattern.
|
|
</P>
|
|
<P>
|
|
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 the point at which it is encountered.
|
|
</P>
|
|
<P>
|
|
Be aware however, that if
|
|
<a href="#dupgroupnumber">duplicate capture group numbers</a>
|
|
are in use, relative references refer to the earliest group with the
|
|
appropriate number. Consider, for example:
|
|
<pre>
|
|
(?|(a)|(b)) (c) (?-2)
|
|
</pre>
|
|
The first two capture 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 number.
|
|
</P>
|
|
<P>
|
|
It is also possible to refer to subsequent capture groups, by writing
|
|
references such as (?+2). However, these cannot be recursive because the
|
|
reference is not inside the parentheses that are referenced. They are always
|
|
<a href="#groupsassubroutines">non-recursive subroutine</a>
|
|
calls, as described in the next section.
|
|
</P>
|
|
<P>
|
|
An alternative approach is to use named parentheses. The Perl syntax for this
|
|
is (?&name); PCRE1's earlier syntax (?P>name) is also supported. We could
|
|
rewrite the above example as follows:
|
|
<pre>
|
|
(?<pn> \( ( [^()]++ | (?&pn) )* \) )
|
|
</pre>
|
|
If there is more than one group with the same name, the earliest one is
|
|
used.
|
|
</P>
|
|
<P>
|
|
The example pattern that we have been looking at contains nested unlimited
|
|
repeats, and so the use of a possessive quantifier for matching strings of
|
|
non-parentheses is important when applying the pattern to strings that do not
|
|
match. For example, when this pattern is applied to
|
|
<pre>
|
|
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
|
|
</pre>
|
|
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.
|
|
</P>
|
|
<P>
|
|
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
|
|
<a href="pcre2callout.html"><b>pcre2callout</b></a>
|
|
documentation). If the pattern above is matched against
|
|
<pre>
|
|
(ab(cd)ef)
|
|
</pre>
|
|
the value for the inner capturing parentheses (numbered 2) is "ef", which is
|
|
the last value taken on at the top level. If a capture group 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.
|
|
</P>
|
|
<P>
|
|
Do not confuse the (?R) item with the condition (R), which tests for recursion.
|
|
Consider this pattern, which matches text in angle brackets, allowing for
|
|
arbitrary nesting. Only digits are allowed in nested brackets (that is, when
|
|
recursing), whereas any characters are permitted at the outer level.
|
|
<pre>
|
|
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
|
|
</pre>
|
|
In this pattern, (?(R) is the start of a conditional group, with two different
|
|
alternatives for the recursive and non-recursive cases. The (?R) item is the
|
|
actual recursive call.
|
|
<a name="recursiondifference"></a></P>
|
|
<br><b>
|
|
Differences in recursion processing between PCRE2 and Perl
|
|
</b><br>
|
|
<P>
|
|
Some former differences between PCRE2 and Perl no longer exist.
|
|
</P>
|
|
<P>
|
|
Before release 10.30, recursion processing in PCRE2 differed from Perl in that
|
|
a recursive subroutine 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.)
|
|
</P>
|
|
<P>
|
|
Starting with release 10.30, recursive subroutine calls are no longer treated
|
|
as atomic. That is, they can be re-entered to try unused alternatives 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.
|
|
</P>
|
|
<P>
|
|
Supporting backtracking into recursions simplifies certain types of recursive
|
|
pattern. For example, this pattern matches palindromic strings:
|
|
<pre>
|
|
^((.)(?1)\2|.?)$
|
|
</pre>
|
|
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 pattern has to ignore all non-word characters,
|
|
which can be done like this:
|
|
<pre>
|
|
^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$
|
|
</pre>
|
|
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 possessive 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 it has gone into a loop.
|
|
</P>
|
|
<P>
|
|
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
|
|
group was called recursively or as a subroutine (see the next section), it
|
|
had no access to any values that were captured outside the recursion, whereas
|
|
in PCRE2 these values can be referenced. Consider this pattern:
|
|
<pre>
|
|
^(.)(\1|a(?2))
|
|
</pre>
|
|
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 to fail in Perl, but in
|
|
later versions (I tried 5.024) it now works.
|
|
<a name="groupsassubroutines"></a></P>
|
|
<br><a name="SEC26" href="#TOC1">GROUPS AS SUBROUTINES</a><br>
|
|
<P>
|
|
If the syntax for a recursive group 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 group
|
|
as an independent subpattern which it tries to match at the current matching
|
|
position. The called group 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)...
|
|
(...(?+1)...(relative)...
|
|
</pre>
|
|
An earlier example pointed out that the pattern
|
|
<pre>
|
|
(sens|respons)e and \1ibility
|
|
</pre>
|
|
matches "sense and sensibility" and "response and responsibility", but not
|
|
"sense and responsibility". If instead the pattern
|
|
<pre>
|
|
(sens|respons)e and (?1)ibility
|
|
</pre>
|
|
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.
|
|
</P>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
Processing options such as case-independence are fixed when a group is
|
|
defined, so if it is used as a subroutine, such options cannot be changed for
|
|
different calls. For example, consider this pattern:
|
|
<pre>
|
|
(abc)(?i:(?-1))
|
|
</pre>
|
|
It matches "abcabc". It does not match "abcABC" because the change of
|
|
processing option does not affect the called group.
|
|
</P>
|
|
<P>
|
|
The behaviour of
|
|
<a href="#backtrackcontrol">backtracking control verbs</a>
|
|
in groups 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="SEC27" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
|
|
<P>
|
|
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 calling a group as a subroutine, possibly recursively. Here are two
|
|
of the examples used above, rewritten using this syntax:
|
|
<pre>
|
|
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
|
|
(sens|respons)e and \g'1'ibility
|
|
</pre>
|
|
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:
|
|
<pre>
|
|
(abc)(?i:\g<-1>)
|
|
</pre>
|
|
Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
|
|
synonymous. The former is a backreference; the latter is a subroutine call.
|
|
</P>
|
|
<br><a name="SEC28" href="#TOC1">CALLOUTS</a><br>
|
|
<P>
|
|
Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
|
|
code to be obeyed in the middle of matching a regular expression. This makes it
|
|
possible, amongst other things, to extract different substrings that match the
|
|
same pair of parentheses when there is a repetition.
|
|
</P>
|
|
<P>
|
|
PCRE2 provides a similar feature, but of course it cannot obey arbitrary 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
|
|
<b>pcre2_set_callout()</b>, and then passing that context to <b>pcre2_match()</b>
|
|
or <b>pcre2_dfa_match()</b>. If no match context is passed, or if the callout
|
|
entry point is set to NULL, callouts are disabled.
|
|
</P>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
During matching, when PCRE2 reaches a callout point, the external function 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.
|
|
</P>
|
|
<P>
|
|
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
|
|
<a href="pcre2callout.html"><b>pcre2callout</b></a>
|
|
documentation.
|
|
</P>
|
|
<br><b>
|
|
Callouts with numerical arguments
|
|
</b><br>
|
|
<P>
|
|
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:
|
|
<pre>
|
|
(?C1)abc(?C2)def
|
|
</pre>
|
|
If the PCRE2_AUTO_CALLOUT flag is passed to <b>pcre2_compile()</b>, numerical
|
|
callouts are automatically installed before each item in the pattern. They are
|
|
all numbered 255. If there is a conditional group in the pattern whose
|
|
condition is an assertion, an additional callout is inserted just before the
|
|
condition. An explicit callout may also be set at this position, as in this
|
|
example:
|
|
<pre>
|
|
(?(?C9)(?=a)abc|def)
|
|
</pre>
|
|
Note that this applies only to assertion conditions, not to other types of
|
|
condition.
|
|
</P>
|
|
<br><b>
|
|
Callouts with string arguments
|
|
</b><br>
|
|
<P>
|
|
A delimited string may be used instead of a number as a callout argument. The
|
|
starting delimiter must be one of ` ' " ^ % # $ { and the ending delimiter is
|
|
the same as the start, except for {, where the ending delimiter is }. If the
|
|
ending delimiter is needed within the string, it must be doubled. For
|
|
example:
|
|
<pre>
|
|
(?C'ab ''c'' d')xyz(?C{any text})pqr
|
|
</pre>
|
|
The doubling is removed before the string is passed to the callout function.
|
|
<a name="backtrackcontrol"></a></P>
|
|
<br><a name="SEC29" href="#TOC1">BACKTRACKING CONTROL</a><br>
|
|
<P>
|
|
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,
|
|
and may behave differently depending on whether or not a name argument is
|
|
present. The names are not required to be unique within the pattern.
|
|
</P>
|
|
<P>
|
|
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-compatible.
|
|
</P>
|
|
<P>
|
|
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. Character type escapes such as \d
|
|
are faulted.
|
|
</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
|
|
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
|
|
PCRE2_ALT_VERBNAMES is also set.
|
|
</P>
|
|
<P>
|
|
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 pattern. Except for
|
|
(*ACCEPT), they may not be quantified.
|
|
</P>
|
|
<P>
|
|
Since these verbs are specifically related to backtracking, most of them can be
|
|
used only when the pattern is to be matched using the traditional matching
|
|
function, because that uses a backtracking algorithm. 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.
|
|
</P>
|
|
<P>
|
|
The behaviour of these verbs in
|
|
<a href="#btrepeat">repeated groups,</a>
|
|
<a href="#btassert">assertions,</a>
|
|
and in
|
|
<a href="#btsub">capture groups called as subroutines</a>
|
|
(whether or not recursively) is documented below.
|
|
<a name="nooptimize"></a></P>
|
|
<br><b>
|
|
Optimizations that affect backtracking verbs
|
|
</b><br>
|
|
<P>
|
|
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 character must be
|
|
present. When one of these optimizations bypasses the running of a match, any
|
|
included backtracking verbs will not, of course, be processed. You can suppress
|
|
the start-of-match optimizations by setting the PCRE2_NO_START_OPTIMIZE option
|
|
when calling <b>pcre2_compile()</b>, or by starting the pattern with
|
|
(*NO_START_OPT). There is more discussion of this option in the section
|
|
entitled
|
|
<a href="pcre2api.html#compiling">"Compiling a pattern"</a>
|
|
in the
|
|
<a href="pcre2api.html"><b>pcre2api</b></a>
|
|
documentation.
|
|
</P>
|
|
<P>
|
|
Experiments with Perl suggest that it too has similar optimizations, and like
|
|
PCRE2, turning them off can change the result of a match.
|
|
<a name="acceptverb"></a></P>
|
|
<br><b>
|
|
Verbs that act immediately
|
|
</b><br>
|
|
<P>
|
|
The following verbs act as soon as they are encountered.
|
|
<pre>
|
|
(*ACCEPT) or (*ACCEPT:NAME)
|
|
</pre>
|
|
This verb causes the match to end successfully, skipping the remainder of the
|
|
pattern. However, when it is inside a capture group that is called as a
|
|
subroutine, only that group is ended successfully. Matching then continues
|
|
at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
|
|
assertion succeeds; in a negative assertion, the assertion fails.
|
|
</P>
|
|
<P>
|
|
If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
|
|
example:
|
|
<pre>
|
|
A((?:A|B(*ACCEPT)|C)D)
|
|
</pre>
|
|
This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
|
|
the outer parentheses.
|
|
</P>
|
|
<P>
|
|
(*ACCEPT) is the only backtracking verb that is allowed to be quantified
|
|
because an ungreedy quantification with a minimum of zero acts only when a
|
|
backtrack happens. Consider, for example,
|
|
<pre>
|
|
(A(*ACCEPT)??B)C
|
|
</pre>
|
|
where A, B, and C may be complex expressions. After matching "A", the matcher
|
|
processes "BC"; if that fails, causing a backtrack, (*ACCEPT) is triggered and
|
|
the match succeeds. In both cases, all but C is captured. Whereas (*COMMIT)
|
|
(see below) means "fail on backtrack", a repeated (*ACCEPT) of this type means
|
|
"succeed on backtrack".
|
|
</P>
|
|
<P>
|
|
<b>Warning:</b> (*ACCEPT) should not be used within a script run group, because
|
|
it causes an immediate exit from the group, bypassing the script run checking.
|
|
<pre>
|
|
(*FAIL) or (*FAIL:NAME)
|
|
</pre>
|
|
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 feature, as for example in this pattern:
|
|
<pre>
|
|
a+(?C)(*FAIL)
|
|
</pre>
|
|
A match with the string "aaaa" always fails, but the callout is taken before
|
|
each backtrack happens (in this example, 10 times).
|
|
</P>
|
|
<P>
|
|
(*ACCEPT:NAME) and (*FAIL:NAME) behave the same as (*MARK:NAME)(*ACCEPT) and
|
|
(*MARK:NAME)(*FAIL), respectively, that is, a (*MARK) is recorded just before
|
|
the verb acts.
|
|
</P>
|
|
<br><b>
|
|
Recording which path was taken
|
|
</b><br>
|
|
<P>
|
|
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).
|
|
<pre>
|
|
(*MARK:NAME) or (*:NAME)
|
|
</pre>
|
|
A name is always required with this verb. For all the other backtracking
|
|
control verbs, a NAME argument is optional.
|
|
</P>
|
|
<P>
|
|
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 section entitled
|
|
<a href="pcre2api.html#matchotherdata">"Other information about the match"</a>
|
|
in the
|
|
<a href="pcre2api.html"><b>pcre2api</b></a>
|
|
documentation. This applies to all instances of (*MARK) and other verbs,
|
|
including those inside assertions and atomic groups. However, there are
|
|
differences in those cases when (*MARK) is used in conjunction with (*SKIP) as
|
|
described below.
|
|
</P>
|
|
<P>
|
|
The mark name that was last encountered on the matching path is passed back. A
|
|
verb without a NAME argument is ignored for this purpose. Here is an example of
|
|
<b>pcre2test</b> output, where the "mark" modifier requests the retrieval and
|
|
outputting of (*MARK) data:
|
|
<pre>
|
|
re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
|
|
data> XY
|
|
0: XY
|
|
MK: A
|
|
XZ
|
|
0: XZ
|
|
MK: B
|
|
</pre>
|
|
The (*MARK) name is tagged with "MK:" in this output, and in this example it
|
|
indicates which of the two alternatives matched. This is a more efficient way
|
|
of obtaining this information than putting each alternative in its own
|
|
capturing parentheses.
|
|
</P>
|
|
<P>
|
|
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-encountered. This does not
|
|
happen for negative assertions or failing positive assertions.
|
|
</P>
|
|
<P>
|
|
After a partial match or a failed match, the last encountered name in the
|
|
entire match process is returned. For example:
|
|
<pre>
|
|
re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
|
|
data> XP
|
|
No match, mark = B
|
|
</pre>
|
|
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.
|
|
</P>
|
|
<P>
|
|
If you are interested in (*MARK) values after failed matches, you should
|
|
probably set the PCRE2_NO_START_OPTIMIZE option
|
|
<a href="#nooptimize">(see above)</a>
|
|
to ensure that the match is always attempted.
|
|
</P>
|
|
<br><b>
|
|
Verbs that act after backtracking
|
|
</b><br>
|
|
<P>
|
|
The following verbs do nothing when they are encountered. Matching continues
|
|
with what follows, but if there is a subsequent match failure, causing a
|
|
backtrack to the verb, a failure is forced. That is, backtracking cannot pass
|
|
to the left of the verb. However, when one of these verbs appears inside an
|
|
atomic group 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. Backtracking from beyond an assertion or an atomic group
|
|
ignores the entire group, and seeks a preceding backtracking point.
|
|
</P>
|
|
<P>
|
|
These verbs differ in exactly what kind of failure occurs when backtracking
|
|
reaches them. The behaviour described below is what happens when the verb is
|
|
not in a subroutine or an assertion. Subsequent sections cover these special
|
|
cases.
|
|
<pre>
|
|
(*COMMIT) or (*COMMIT:NAME)
|
|
</pre>
|
|
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 pattern 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 <b>pcre2_match()</b> is committed to finding a match at
|
|
the current starting point, or not at all. For example:
|
|
<pre>
|
|
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."
|
|
</P>
|
|
<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
|
|
caller. However, (*SKIP:NAME) searches only for names that are set with
|
|
(*MARK), ignoring those set by any of the other backtracking verbs.
|
|
</P>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
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 <b>pcre2test</b>:
|
|
<pre>
|
|
re> /(*COMMIT)abc/
|
|
data> xyzabc
|
|
0: abc
|
|
data>
|
|
re> /(*COMMIT)abc/no_start_optimize
|
|
data> xyzabc
|
|
No match
|
|
</pre>
|
|
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.
|
|
<pre>
|
|
(*PRUNE) or (*PRUNE:NAME)
|
|
</pre>
|
|
This verb causes the match to fail at the current starting position in the
|
|
subject if there is a later matching failure that causes backtracking 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 quantifier, 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 (*COMMIT).
|
|
</P>
|
|
<P>
|
|
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 (*MARK),
|
|
ignoring those set by other backtracking verbs.
|
|
<pre>
|
|
(*SKIP)
|
|
</pre>
|
|
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 encountered. (*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:
|
|
<pre>
|
|
a+(*SKIP)b
|
|
</pre>
|
|
If the subject is "aaaac...", after the first match attempt fails (starting at
|
|
the first character in the string), the starting point skips on to start the
|
|
next attempt at "c". Note that a possessive quantifer does not have the same
|
|
effect 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".
|
|
</P>
|
|
<P>
|
|
If (*SKIP) is used to specify a new starting position that is the same as the
|
|
starting position of the current match, or (by being inside a lookbehind)
|
|
earlier, the position specified by (*SKIP) is ignored, and instead the normal
|
|
"bumpalong" occurs.
|
|
<pre>
|
|
(*SKIP:NAME)
|
|
</pre>
|
|
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 corresponds to that (*MARK) instead of
|
|
to where (*SKIP) was encountered. If no (*MARK) with a matching name is found,
|
|
the (*SKIP) is ignored.
|
|
</P>
|
|
<P>
|
|
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 backtracking. Compare the
|
|
following <b>pcre2test</b> examples:
|
|
<pre>
|
|
re> /a(?>(*MARK:X))(*SKIP:X)(*F)|(.)/
|
|
data: abc
|
|
0: a
|
|
1: a
|
|
data:
|
|
re> /a(?:(*MARK:X))(*SKIP:X)(*F)|(.)/
|
|
data: abc
|
|
0: b
|
|
1: b
|
|
</pre>
|
|
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 backtracks, and this causes a new
|
|
matching attempt to start at the second 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.
|
|
</P>
|
|
<P>
|
|
Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
|
|
names that are set by other backtracking verbs.
|
|
<pre>
|
|
(*THEN) or (*THEN:NAME)
|
|
</pre>
|
|
This verb causes a skip to the next innermost alternative when backtracking
|
|
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:
|
|
<pre>
|
|
( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
|
|
</pre>
|
|
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 subsequently BAZ fails, there are no
|
|
more alternatives, so there is a backtrack to whatever came before the entire
|
|
group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
|
|
</P>
|
|
<P>
|
|
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 (*MARK),
|
|
ignoring those set by other backtracking verbs.
|
|
</P>
|
|
<P>
|
|
A group 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 group to the enclosing alternative.
|
|
Consider this pattern, where A, B, etc. are complex pattern fragments that do
|
|
not contain any | characters at this level:
|
|
<pre>
|
|
A (B(*THEN)C) | D
|
|
</pre>
|
|
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 group containing (*THEN) is given an alternative, it
|
|
behaves differently:
|
|
<pre>
|
|
A (B(*THEN)C | (*FAIL)) | D
|
|
</pre>
|
|
The effect of (*THEN) is now confined to the inner group. After a failure in C,
|
|
matching moves to (*FAIL), which causes the whole group to fail because there
|
|
are no more alternatives to try. In this case, matching does backtrack into A.
|
|
</P>
|
|
<P>
|
|
Note that a conditional group is not considered as having two alternatives,
|
|
because only one is ever used. In other words, the | character in a conditional
|
|
group has a different meaning. Ignoring white space, consider:
|
|
<pre>
|
|
^.*? (?(?=a) a | b(*THEN)c )
|
|
</pre>
|
|
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 group 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 match "b", the match would succeed.)
|
|
</P>
|
|
<P>
|
|
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 the advance may be more
|
|
than one character. (*COMMIT) is the strongest, causing the entire match to
|
|
fail.
|
|
</P>
|
|
<br><b>
|
|
More than one backtracking verb
|
|
</b><br>
|
|
<P>
|
|
If more than one backtracking verb is present in a pattern, the one that is
|
|
backtracked onto first acts. For example, consider this pattern, where A, B,
|
|
etc. are complex pattern fragments:
|
|
<pre>
|
|
(A(*COMMIT)B(*THEN)C|ABD)
|
|
</pre>
|
|
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 of them has no effect. Consider this
|
|
example:
|
|
<pre>
|
|
...(*COMMIT)(*PRUNE)...
|
|
</pre>
|
|
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 a backtrack
|
|
onto (*COMMIT).
|
|
<a name="btrepeat"></a></P>
|
|
<br><b>
|
|
Backtracking verbs in repeated groups
|
|
</b><br>
|
|
<P>
|
|
PCRE2 sometimes differs from Perl in its handling of backtracking verbs in
|
|
repeated groups. For example, consider:
|
|
<pre>
|
|
/(a(*COMMIT)b)+ac/
|
|
</pre>
|
|
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.
|
|
<a name="btassert"></a></P>
|
|
<br><b>
|
|
Backtracking verbs in assertions
|
|
</b><br>
|
|
<P>
|
|
(*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
|
|
group.
|
|
</P>
|
|
<P>
|
|
(*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.
|
|
</P>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
The remaining verbs act only when a later failure causes a backtrack to
|
|
reach them. This means that, for the Perl-compatible assertions, their effect
|
|
is confined to the assertion, because Perl lookaround assertions are atomic. A
|
|
backtrack that occurs after such 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 (*SKIP:NAME) later in the pattern.
|
|
</P>
|
|
<P>
|
|
PCRE2 now supports non-atomic positive assertions, as described in the section
|
|
entitled
|
|
<a href="#nonatomicassertions">"Non-atomic assertions"</a>
|
|
above. These assertions must be standalone (not used as conditions). They are
|
|
not Perl-compatible. For these assertions, a later backtrack does jump back
|
|
into the assertion, and therefore verbs such as (*COMMIT) can be triggered by
|
|
backtracks from later in the pattern.
|
|
</P>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
The other backtracking verbs are not treated specially if they appear in a
|
|
standalone positive assertion. In a conditional positive assertion,
|
|
backtracking (from within the assertion) into (*COMMIT), (*SKIP), or (*PRUNE)
|
|
causes the condition to be false. However, for both standalone and conditional
|
|
negative assertions, backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes
|
|
the assertion to be true, without considering any further alternative branches.
|
|
<a name="btsub"></a></P>
|
|
<br><b>
|
|
Backtracking verbs in subroutines
|
|
</b><br>
|
|
<P>
|
|
These behaviours occur whether or not the group is called recursively.
|
|
</P>
|
|
<P>
|
|
(*ACCEPT) in a group called as a subroutine causes the subroutine match to
|
|
succeed without any further processing. Matching then continues after the
|
|
subroutine call. Perl documents this behaviour. Perl's treatment of the other
|
|
verbs in subroutines is different in some cases.
|
|
</P>
|
|
<P>
|
|
(*FAIL) in a group called as a subroutine has its normal effect: it forces
|
|
an immediate backtrack.
|
|
</P>
|
|
<P>
|
|
(*COMMIT), (*SKIP), and (*PRUNE) cause the subroutine match to fail when
|
|
triggered by being backtracked to in a group called as a subroutine. There is
|
|
then a backtrack at the outer level.
|
|
</P>
|
|
<P>
|
|
(*THEN), when triggered, skips to the next alternative in the innermost
|
|
enclosing group that has alternatives (its normal behaviour). However, if there
|
|
is no such group within the subroutine's group, the subroutine match fails and
|
|
there is a backtrack at the outer level.
|
|
</P>
|
|
<br><a name="SEC30" href="#TOC1">SEE ALSO</a><br>
|
|
<P>
|
|
<b>pcre2api</b>(3), <b>pcre2callout</b>(3), <b>pcre2matching</b>(3),
|
|
<b>pcre2syntax</b>(3), <b>pcre2</b>(3).
|
|
</P>
|
|
<br><a name="SEC31" href="#TOC1">AUTHOR</a><br>
|
|
<P>
|
|
Philip Hazel
|
|
<br>
|
|
University Computing Service
|
|
<br>
|
|
Cambridge, England.
|
|
<br>
|
|
</P>
|
|
<br><a name="SEC32" href="#TOC1">REVISION</a><br>
|
|
<P>
|
|
Last updated: 29 December 2019
|
|
<br>
|
|
Copyright © 1997-2019 University of Cambridge.
|
|
<br>
|
|
<p>
|
|
Return to the <a href="index.html">PCRE2 index page</a>.
|
|
</p>
|