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PCRE2PATTERN(3) Library Functions Manual PCRE2PATTERN(3)
PCRE2 - Perl-compatible regular expressions (revised API)
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 pcre2syntax 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.
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.
This document discusses the regular expression patterns that are
supported by PCRE2 when its main matching function,
pcre2_match(), is used. PCRE2 also has an alternative matching
function, pcre2_dfa_match(), 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
pcre2matching page.
A number of options that can be passed to pcre2_compile() 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.
UTF support
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 pcre2unicode page.
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 pcre2_compile(), (*UTF)
is not allowed, and its appearance in a pattern causes an error.
Unicode property support
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. If also causes upper/lower casing operations to use
Unicode properties for characters with code points greater than
127, even when UTF is not set. These behaviours can be changed
within the pattern; see the section entitled "Internal Option
Setting" below.
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 pcre2_compile(), (*UCP) is
not allowed, and its appearance in a pattern causes an error.
Locking out empty string matching
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.
Disabling auto-possessification
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 pcre2api documentation.
Disabling start-up optimizations
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 pcre2api documentation.
Disabling automatic anchoring
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 pcre2api documentation.
Disabling JIT compilation
If a pattern that starts with (*NO_JIT) is successfully compiled,
an attempt by the application to apply the JIT optimization by
calling pcre2_jit_compile() is ignored.
Setting match resource limits
The pcre2_match() function contains a counter that is incremented
every time it goes round its main loop. The caller of
pcre2_match() 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.
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,
pcre2_match() gives an error return. The limits can also be set
by items at the start of the pattern of the form
(*LIMIT_HEAP=d)
(*LIMIT_MATCH=d)
(*LIMIT_DEPTH=d)
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 pcre2_match() 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).
Prior to release 10.30, LIMIT_DEPTH was called LIMIT_RECURSION.
This name is still recognized for backwards compatibility.
The heap limit applies only when the pcre2_match() or
pcre2_dfa_match() 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 pcre2_dfa_match() 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.
Newline conventions
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 pcre2api page has further
discussion about newlines, and shows how to set the newline
convention when calling pcre2_compile().
It is also possible to specify a newline convention by starting a
pattern string with one of the following sequences:
(*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)
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
(*CR)a.b
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.
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 "Newline sequences" below. A change of
\R setting can be combined with a change of newline convention.
Specifying what \R matches
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.
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.
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
The quick brown fox
matches a portion of a subject string that is identical to
itself. When caseless matching is specified (the PCRE2_CASELESS
option or (?i) within the pattern), letters are matched
independently of case. Note that there are two ASCII characters,
K and S, that, in addition to their lower case ASCII equivalents,
are case-equivalent with Unicode U+212A (Kelvin sign) and U+017F
(long S) respectively when either PCRE2_UTF or PCRE2_UCP is set,
unless the PCRE2_EXTRA_CASELESS_RESTRICT option is in force
(either passed to pcre2_compile() or set by (?r) within the
pattern).
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 metacharacters, which do not stand for themselves but
instead are interpreted in some special way.
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:
\ 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
{ potential start of min/max quantifier
Brace characters { and } are also used to enclose data for
constructions such as \g{2} or \k{name}. In almost all uses of
braces, space and/or horizontal tab characters that follow { or
precede } are allowed and are ignored. In the case of
quantifiers, they may also appear before or after the comma. The
exception to this is \u{...} which is an ECMAScript compatibility
feature that is recognized only when the PCRE2_EXTRA_ALT_BSUX
option is set. ECMAScript does not ignore such white space; it
causes the item to be interpreted as literal.
Part of a pattern that is in square brackets is called a
"character class". In a character class the only metacharacters
are:
\ 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
If a pattern is compiled with the PCRE2_EXTENDED option, most
white space in the pattern, other than in a character class,
within a \Q...\E sequence, or 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 a # character
as part of the pattern. If the PCRE2_EXTENDED_MORE option is set,
the same applies, but in addition unescaped space and horizontal
tab characters are ignored inside a character class. Note: only
these two characters are ignored, not the full set of pattern
white space characters that are ignored outside a character
class. Option settings can be changed within a pattern; see the
section entitled "Internal Option Setting" below.
The following sections describe the use of each of the
metacharacters.
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.
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 \\.
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.
If you want to treat all characters in a sequence as literals,
you can do so by putting them between \Q and \E. Note that this
includes white space even when the PCRE2_EXTENDED option is set
so that most other white space is ignored. The behaviour 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:
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
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 then not terminated by a closing square bracket.
Non-printing characters
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:
\a alarm, that is, the BEL character (hex 07)
\cx "control-x", where x is a non-control 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..
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.
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.
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.
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, but unlike other places that also use curly
brackets, spaces are not allowed and would result in the string
being interpreted as a literal. This syntax is from ECMAScript 6.
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.
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.
An error occurs if \c is not followed by a character whose ASCII
code point is in the range 32 to 126. The precise effect of \cx
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 code point less than 32 or
greater than 126, a compile-time error occurs.
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 perlebcdic 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).
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.
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.
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.
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.
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.
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.
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 backreference. A description of how this
works is given later, following the discussion of parenthesized
groups. Otherwise, up to three octal digits are read to form a
character code.
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:
\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
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.
Constraints on character values
Characters that are specified using octal or hexadecimal numbers
are limited to certain values, as follows:
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
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 pcre2_compile() 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.
Escape sequences in character classes
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).
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.
Unsupported escape sequences
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.
Absolute and relative backreferences
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 later, following the discussion of
parenthesized groups.
Absolute and relative subroutine calls
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 later. Note that \g{...}
(Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous.
The former is a backreference; the latter is a subroutine call.
Generic character types
Another use of backslash is for specifying generic character
types:
\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
The \N escape sequence has the same meaning as the "."
metacharacter 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 "Non-printing characters" above for details.
Perl also uses \N{name} to specify characters by Unicode name;
PCRE2 does not support this.
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.
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.
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 "Locale
support" in the pcre2api 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.
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:
\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}, \p{N}, \p{Mn}, or \p{Pc}
The addition of \p{Mn} (non-spacing mark) and the replacement of
an explicit test for underscore with a test for \p{Pc} (connector
punctuation) happened in PCRE2 release 10.43. This brings PCRE2
into line with Perl.
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 other character categories. 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.
The effect of PCRE2_UCP on any one of these escape sequences can
be negated by the options PCRE2_EXTRA_ASCII_BSD,
PCRE2_EXTRA_ASCII_BSS, and PCRE2_EXTRA_ASCII_BSW, respectively.
These options can be set and reset within a pattern by means of
an internal option setting (see below).
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:
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
The vertical space characters are:
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
In 8-bit, non-UTF-8 mode, only the characters with code points
less than 256 are relevant.
Newline sequences
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:
(?>\r\n|\n|\x0b|\f|\r|\x85)
This is an example of an "atomic group", details of which are
given below. 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.
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.
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
abbreviation 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:
(*BSR_ANYCRLF) CR, LF, or CRLF only
(*BSR_UNICODE) any Unicode newline sequence
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:
(*ANY)(*BSR_ANYCRLF)
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.
Unicode character properties
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.
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).
The extra escape sequences that provide property support are:
\p{xx} a character with the xx property
\P{xx} a character without the xx property
\X a Unicode extended grapheme cluster
The property names represented by xx above are not case-
sensitive, and in accordance with Unicode's "loose matching"
rules, spaces, hyphens, and underscores are ignored. There is
support for Unicode script names, Unicode general category
properties, "Any", which matches any character (including
newline), Bidi_Class, a number of binary (yes/no) properties, and
some special PCRE2 properties (described below). Certain 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.
Script properties for \p and \P
There are three different syntax forms for matching a script.
Each Unicode character has a basic script and, optionally, a list
of other scripts ("Script Extensions") with which it is commonly
used. Using the Adlam script as an example, \p{sc:Adlam} matches
characters whose basic script is Adlam, whereas \p{scx:Adlam}
matches, in addition, characters that have Adlam in their
extensions list. The full names "script" and "script extensions"
for the property types are recognized, and a equals sign is an
alternative to the colon. If a script name is given without a
property type, for example, \p{Adlam}, it is treated as
\p{scx:Adlam}. Perl changed to this interpretation at release
5.26 and PCRE2 changed at release 10.40.
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
recognized script names and their 4-character abbreviations can
be obtained by running this command:
pcre2test -LS
The general category property for \p and \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}.
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:
\p{L}
\pL
The following general category property codes are supported:
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
The special property LC, which has the synonym 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".
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 pcre2api page).
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".
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.
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.
Binary (yes/no) properties for \p and \P
Unicode defines a number of binary properties, that is,
properties whose only values are true or false. You can obtain a
list of those that are recognized by \p and \P, along with their
abbreviations, by running this command:
pcre2test -LP
The Bidi_Class property for \p and \P
\p{Bidi_Class:<class>} matches a character with the given
class
\p{BC:<class>} matches a character with the given
class
The recognized classes are:
AL Arabic letter
AN Arabic number
B paragraph separator
BN boundary neutral
CS common separator
EN European number
ES European separator
ET European terminator
FSI first strong isolate
L left-to-right
LRE left-to-right embedding
LRI left-to-right isolate
LRO left-to-right override
NSM non-spacing mark
ON other neutral
PDF pop directional format
PDI pop directional isolate
R right-to-left
RLE right-to-left embedding
RLI right-to-left isolate
RLO right-to-left override
S segment separator
WS which space
An equals sign may be used instead of a colon. The class names
are case-insensitive; only the short names listed above are
recognized.
Extended grapheme clusters
The \X escape matches any number of Unicode characters that form
an "extended grapheme cluster", and treats the sequence as an
atomic group (see below). 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.
\X always matches at least one character. Then it decides whether
to add additional characters according to the following rules for
ending a cluster:
1. End at the end of the subject string.
2. Do not end between CR and LF; otherwise end after any control
character.
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 followed only by a T character.
4. Do not end before extending characters or spacing marks or the
zero-width joiner (ZWJ) character. Characters with the "mark"
property always have the "extend" grapheme breaking property.
5. Do not end after prepend characters.
6. Do not end within emoji modifier sequences or emoji ZWJ (zero-
width joiner) sequences. An emoji ZWJ sequence consists of a
character with the Extended_Pictographic property, optionally
followed by one or more characters with the Extend property,
followed by the ZWJ character, followed by another
Extended_Pictographic character.
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.
8. Otherwise, end the cluster.
PCRE2's additional properties
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:
Xan Any alphanumeric character
Xps Any POSIX space character
Xsp Any Perl space character
Xwd Any Perl "word" character
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 those that match Mn (non-spacing mark) or Pc
(connector punctuation, which includes underscore).
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.)
Resetting the match start
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:
foo\Kbar
matches "foobar", but reports that it has matched "bar". \K does
not interact with anchoring in any way. The pattern:
^foo\Kbar
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 (described
below), but the part of the pattern that precedes \K is not
constrained to match a limited number of characters, as is
required for a lookbehind assertion. The use of \K does not
interfere with the setting of captured substrings. For example,
when the pattern
(foo)\Kbar
matches "foobar", the first substring is still set to "foo".
From version 5.32.0 Perl forbids the use of \K in lookaround
assertions. From release 10.38 PCRE2 also forbids this by
default. However, the PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK option can
be used when calling pcre2_compile() to re-enable the previous
behaviour. When this option is set, \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:
(?<=\Kfoo)bar
If the subject is "foobar", a call to pcre2_match() 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.
Simple assertions
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 below. The backslashed assertions are:
\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
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.
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.
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 startoffset argument of pcre2_match() 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.
The \G assertion is true only when the current matching position
is at the start point of the matching process, as specified by
the startoffset argument of pcre2_match(). It differs from \A
when the value of startoffset is non-zero. By calling
pcre2_match() 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.
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.
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.
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.
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 startoffset argument of pcre2_match() 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 (see below).
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.)
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.
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.
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.
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
startoffset argument of pcre2_match() is non-zero. The
PCRE2_DOLLAR_ENDONLY option is ignored if PCRE2_MULTILINE is set.
When the newline convention (see "Newline conventions" 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.)
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.
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. One or more characters may be
specified as line terminators (see "Newline conventions" above).
Dot never matches a single line-ending character. When the two-
character sequence CRLF is the only line ending, dot does not
match CR if it is immediately followed by LF, but otherwise it
matches all characters (including isolated CRs and LFs). When
ANYCRLF is selected for line endings, no occurrences of CR of LF
match dot. When all Unicode line endings are being recognized,
dot does not match CR or LF or any of the other line ending
characters.
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.
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.
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.
When \N is followed by an opening brace it has a different
meaning. See the section entitled "Non-printing characters" above
for details. Perl also uses \N{name} to specify characters by
Unicode name; PCRE2 does not support this.
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.
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).
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.
PCRE2 does not allow \C to appear in lookbehind assertions
(described below) 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 pcre2_dfa_match() 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.
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.
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):
(?| (?=[\x00-\x7f])(\C) |
(?=[\x80-\x{7ff}])(\C)(\C) |
(?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
(?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
In this example, a group that starts with (?| resets the
capturing parentheses numbers in each alternative (see "Duplicate
Group Numbers" below). The assertions at the start of each branch
check the next UTF-8 character for values whose encoding uses 1,
2, 3, or 4 bytes, respectively. The character's individual bytes
are then captured by the appropriate number of \C groups.
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.
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.
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.
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. Note that there are two ASCII
characters, K and S, that, in addition to their lower case ASCII
equivalents, are case-equivalent with Unicode U+212A (Kelvin
sign) and U+017F (long S) respectively when either PCRE2_UTF or
PCRE2_UCP is set.
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.
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 "Generic
character types" 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.
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.
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 \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.
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.
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.
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.
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.
A circumflex can conveniently be used with the upper case
character types to specify a more restricted set of characters
than the matching lower case type. For example, the class [^\W_]
matches any letter or digit, but not underscore, whereas [\w]
includes underscore. A positive character class should be read as
"something OR something OR ..." and a negative class as "NOT
something AND NOT something AND NOT ...".
The only metacharacters that are recognized in character classes
are backslash, hyphen (only where it can be interpreted as
specifying a range), circumflex (only at the start), opening
square bracket (only when it can be interpreted as introducing a
POSIX class name, 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.
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,
[01[:alpha:]%]
matches "0", "1", any alphabetic character, or "%". The supported
class names are:
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
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, as do "word" and \w.
The name "word" is a Perl extension, and "blank" is a GNU
extension from Perl 5.8. Another Perl extension is negation,
which is indicated by a ^ character after the colon. For example,
[12[:^digit:]]
matches "1", "2", or any non-digit. 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.
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, in UCP mode, unless
certain options are set (see below), some of the classes are
changed so that Unicode character properties are used. This is
achieved by replacing POSIX classes with other sequences, as
follows:
[: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}
Negated versions, such as [:^alpha:] use \P instead of \p. Four
other POSIX classes are handled specially in UCP mode:
[: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:
U+061C Arabic Letter Mark
U+180E Mongolian Vowel Separator
U+2066 - U+2069 Various "isolate"s
[:print:]
This matches the same characters as [:graph:] plus space
characters that are not controls, that is, characters with
the Zs property.
[: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.
[:xdigit:]
In addition to the ASCII hexadecimal digits, this also
matches the "fullwidth" versions of those characters,
whose Unicode code points start at U+FF10. This is a
change that was made in PCRE release 10.43 for Perl
compatibility.
The other POSIX classes are unchanged by PCRE2_UCP, and match
only characters with code points less than 256.
There are two options that can be used to restrict the POSIX
classes to ASCII characters when PCRE2_UCP is set. The option
PCRE2_EXTRA_ASCII_DIGIT affects just [:digit:] and [:xdigit:].
Within a pattern, this can be set and unset by (?aT) and (?-aT).
The PCRE2_EXTRA_ASCII_POSIX option disables UCP processing for
all POSIX classes, including [:digit:] and [:xdigit:]. Within a
pattern, (?aP) and (?-aP) set and unset both these options for
consistency.
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:
[[:<:]] is converted to \b(?=\w)
[[:>:]] is converted to \b(?<=\w)
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 "Simple assertions" 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. Note also
that the PCRE2_UCP option changes the meaning of \w (and
therefore \b) by default, so it also affects these POSIX
sequences.
Vertical bar characters are used to separate alternative
patterns. For example, the pattern
gilbert|sullivan
matches either "gilbert" or "sullivan". Any number of
alternatives may appear, and an empty alternative is permitted
(matching the empty 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 (defined
below), "succeeds" means matching the rest of the main pattern as
well as the alternative in the group.
The settings of several options can be changed within a pattern
by a sequence of letters enclosed between "(?" and ")". The
following are Perl-compatible, and are described in detail in the
pcre2api documentation. The option letters are:
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
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.
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.
If the first character following (? is a circumflex, it causes
all of the above options to be unset. Letters may follow the
circumflex to cause some options to be re-instated, but a hyphen
may not appear.
Some PCRE2-specific options can be changed by the same mechanism
using these pairs or individual letters:
aD for PCRE2_EXTRA_ASCII_BSD
aS for PCRE2_EXTRA_ASCII_BSS
aW for PCRE2_EXTRA_ASCII_BSW
aP for PCRE2_EXTRA_ASCII_POSIX and PCRE2_EXTRA_ASCII_DIGIT
aT for PCRE2_EXTRA_ASCII_DIGIT
r for PCRE2_EXTRA_CASELESS_RESTRICT
J for PCRE2_DUPNAMES
U for PCRE2_UNGREEDY
However, except for 'r', these are not unset by (?^), which is
equivalent to (?-imnrsx). If 'a' is not followed by any of the
upper case letters shown above, it sets (or unsets) all the ASCII
options.
PCRE2_EXTRA_ASCII_DIGIT has no additional effect when
PCRE2_EXTRA_ASCII_POSIX is set, but including it in (?aP) means
that (?-aP) suppresses all ASCII restrictions for POSIX classes.
When one of these option changes occurs at top level (that is,
not inside group parentheses), the change applies until a
subsequent change, or the end of the pattern. An option change
within a group (see below for a description of groups) affects
only that part of the group that follows it. At the end of the
group these options are reset to the state they were before the
group. For example,
(a(?i)b)c
matches abc and aBc and no other strings (assuming PCRE2_CASELESS
is not set externally). Any changes made in one alternative do
carry on into subsequent branches within the same group. For
example,
(a(?i)b|c)
matches "ab", "aB", "c", and "C", even though when matching "C"
the first branch is abandoned before the option setting. This is
because the effects of option settings happen at compile time.
There would be some very weird behaviour otherwise.
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
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
match exactly the same set of strings.
Note: 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 "Newline sequences" 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 or PCRE2_NEVER_UCP options, which lock out the
use of the (*UTF) and (*UCP) sequences.
Groups are delimited by parentheses (round brackets), which can
be nested. Turning part of a pattern into a group does two
things:
1. It localizes a set of alternatives. For example, the pattern
cat(aract|erpillar|)
matches "cataract", "caterpillar", or "cat". Without the
parentheses, it would match "cataract", "erpillar" or an empty
string.
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.)
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
the ((red|white) (king|queen))
the captured substrings are "red king", "red", and "king", and
are numbered 1, 2, and 3, respectively.
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
the ((?:red|white) (king|queen))
the captured substrings are "white queen" and "queen", and are
numbered 1 and 2. The maximum number of capture groups is 65535.
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
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
match exactly the same set of strings. Because alternative
branches are tried from left to right, and options are not reset
until the end of the group is reached, an option setting in one
branch does affect subsequent branches, so the above patterns
match "SUNDAY" as well as "Saturday".
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:
(?|(Sat)ur|(Sun))day
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.
# before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4
A backreference to a capture group uses the most recent value
that is set for the group. The following pattern matches "abcabc"
or "defdef":
/(?|(abc)|(def))\1/
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":
/(?|(abc)|(def))(?1)/
A relative reference such as (?-1) is no different: it is just a
convenient way of computing an absolute group number.
If a condition test for a group's having matched refers to a non-
unique number, the test is true if any group with that number has
matched.
An alternative approach to using this "branch reset" feature is
to use duplicate named groups, as described in the next section.
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.
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 128 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:
^[_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
References to capture groups from other parts of the pattern,
such as backreferences, recursion, and conditions, can all be
made by name as well as by number.
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.
Warning: 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:
(?|(?<AA>aa)|(?<BB>bb))
Perl allows this, with both names AA and BB as aliases of group
1. Thus, after a successful match, both names yield the same
value (either "aa" or "bb").
In an attempt to reduce confusion, PCRE2 does not allow the same
group number to be associated with more than one name. The
example above provokes a compile-time error. However, there is
still scope for confusion. Consider this pattern:
(?|(?<AA>aa)|(bb))
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.
By default, a name must be unique within a pattern, except that
duplicate names are permitted for groups with the same number,
for example:
(?|(?<AA>aa)|(?<AA>bb))
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
"Internal Option Setting" above.
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:
(?J)
(?<DN>Mon|Fri|Sun)(?:day)?|
(?<DN>Tue)(?:sday)?|
(?<DN>Wed)(?:nesday)?|
(?<DN>Thu)(?:rsday)?|
(?<DN>Sat)(?:urday)?
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.)
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":
(?J)(?:(?<n>foo)|(?<n>bar))\k<n>
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.
If you use a named reference in a condition test (see the section
about conditions 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 pcre2api documentation.
Repetition is specified by quantifiers, which may follow any one
of these items:
a literal data character
the dot metacharacter
the \C escape sequence
the \R escape sequence
the \X escape sequence
any escape sequence that matches a single character
a character class
a backreference
a parenthesized group (including lookaround assertions)
a subroutine call (recursive or otherwise)
If a quantifier does not follow a repeatable item, an error
occurs. The general repetition quantifier specifies a minimum and
maximum number of permitted matches by giving two numbers in
curly brackets (braces), separated by a comma. The numbers must
be less than 65536, and the first must be less than or equal to
the second. For example,
z{2,4}
matches "zz", "zzz", or "zzzz". A closing brace on its own is not
a special character. If the second number is omitted, but the
comma is present, there is no upper limit; if the second number
and the comma are both omitted, the quantifier specifies an exact
number of required matches. Thus
[aeiou]{3,}
matches at least 3 successive vowels, but may match many more,
whereas
\d{8}
matches exactly 8 digits. If the first number is omitted, the
lower limit is taken as zero; in this case the upper limit must
be present.
X{,4} is interpreted as X{0,4}
This is a change in behaviour that happened in Perl 5.34.0 and
PCRE2 10.43. In earlier versions such a sequence was not
interpreted as a quantifier. Other regular expression engines may
behave either way.
If the characters that follow an opening brace do not match the
syntax of a quantifier, the brace is taken as a literal
character. In particular, this means that {,} is a literal string
of three characters.
Note that not every opening brace is potentially the start of a
quantifier because braces are used in other items such as
\N{U+345} or \k{name}.
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).
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
subroutines from elsewhere in the pattern (but see also the
section entitled "Defining capture groups for use by reference
only" below). Except for parenthesized groups, items that have a
{0} quantifier are omitted from the compiled pattern.
For convenience, the three most common quantifiers have single-
character abbreviations:
* is equivalent to {0,}
+ is equivalent to {1,}
? is equivalent to {0,1}
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:
(a?)*
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.
By default, quantifiers are "greedy", that is, they match as much
as possible (up to the maximum number of permitted repetitions),
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
/\*.*\*/
to the string
/* first comment */ not comment /* second comment */
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
/\*.*?\*/
does the right thing with C comments. The meaning of the various
quantifiers is not otherwise changed, just the preferred number
of matches. Do not confuse this use of question mark with its use
as a quantifier in its own right. Because it has two uses, it
can sometimes appear doubled, as in
\d??\d
which matches one digit by preference, but can match two if that
is the only way the rest of the pattern matches.
If the PCRE2_UNGREEDY option is set (an option that is not
available in Perl), the quantifiers are not greedy by default,
but individual ones can be made greedy by following them with a
question mark. In other words, it inverts the default behaviour.
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.
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.
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.
However, there are some cases where the optimization cannot be
used. When .* is inside capturing parentheses that are the
subject of a backreference elsewhere in the pattern, a match at
the start may fail where a later one succeeds. Consider, for
example:
(.*)abc\1
If the subject is "xyz123abc123" the match point is the fourth
character. For this reason, such a pattern is not implicitly
anchored.
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:
(?>.*?a)b
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.
When a capture group is repeated, the value captured is the
substring that matched the final iteration. For example, after
(tweedle[dume]{3}\s*)+
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
(a|(b))+
matches "aba" the value of the second captured substring is "b".
With both maximizing ("greedy") and minimizing ("ungreedy" or
"lazy") repetition, failure of what follows normally causes the
repeated item to be re-evaluated to see if a different number of
repeats allows the rest of the pattern to match. Sometimes it is
useful to prevent this, either to change the nature of the match,
or to cause it fail earlier than it otherwise might, when the
author of the pattern knows there is no point in carrying on.
Consider, for example, the pattern \d+foo when applied to the
subject line
123456bar
After matching all 6 digits and then failing to match "foo", the
normal action of the matcher is to try again with only 5 digits
matching the \d+ item, and then with 4, and so on, before
ultimately failing. "Atomic grouping" (a term taken from Jeffrey
Friedl's book) provides the means for specifying that once a
group has matched, it is not to be re-evaluated in this way.
If we use atomic grouping for the previous example, the matcher
gives up immediately on failing to match "foo" the first time.
The notation is a kind of special parenthesis, starting with (?>
as in this example:
(?>\d+)foo
Perl 5.28 introduced an experimental alphabetic form starting
with (* which may be easier to remember:
(*atomic:\d+)foo
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.
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.
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.
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
\d++foo
Note that a possessive quantifier can be used with an entire
group, for example:
(abc|xyz){2,3}+
Possessive quantifiers are always greedy; the setting of the
PCRE2_UNGREEDY option is ignored. They are a convenient notation
for the simpler forms of atomic group. However, there is no
difference in the meaning of a possessive quantifier and the
equivalent atomic group, though there may be a performance
difference; possessive quantifiers should be slightly faster.
The possessive quantifier syntax is an extension to the Perl 5.8
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.
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).
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
(\D+|<\d+>)*[!?]
matches an unlimited number of substrings that either consist of
non-digits, or digits enclosed in <>, followed by either ! or ?.
When it matches, it runs quickly. However, if it is applied to
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
it takes a long time before reporting failure. This is because
the string can be divided between the internal \D+ repeat and the
external * repeat in a large number of ways, and all have to be
tried. (The example uses [!?] rather than a single character at
the end, because both PCRE2 and Perl have an optimization that
allows for fast failure when a single character is used. They
remember the last single 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:
((?>\D+)|<\d+>)*[!?]
sequences of non-digits cannot be broken, and failure happens
quickly.
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.
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.
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"
above 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).
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:
(ring), \1
(ring), \g1
(ring), \g{1}
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:
(abc(def)ghi)\g{-1}
The sequence \g{-1} is a reference to the capture group whose
number is one less than the number of the next group to be
started, so in this example (where the next group would be
numbered 3) is it equivalent to \2, and \g{-2} would be
equivalent to \1. Note that if this construct is inside a capture
group, that group is included in the count, so in this example
\g{-2} also refers to group 1:
(A)(\g{-2}B)
The use of relative references can be helpful in long patterns,
and also in patterns that are created by joining together
fragments that contain references within themselves.
The sequence \g{+1} is a reference to the next capture group that
is started after this item, and \g{+2} refers to the one after
that, and so on. This kind of forward reference can be useful in
patterns that repeat. Perl does not support the use of + in this
way.
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 "Groups as
subroutines" below for a way of doing that). So the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and
responsibility", but not "sense and responsibility". If caseful
matching is in force at the time of the backreference, the case
of letters is relevant. For example,
((?i)rah)\s+\1
matches "rah rah" and "RAH RAH", but not "RAH rah", even though
the original capture group is matched caselessly.
There are several different ways of writing backreferences to
named capture groups. The .NET syntax is \k{name}, the Python
syntax is (?=name), and the original Perl syntax is \k<name> or
\k'name'. All of these are now supported by both Perl and PCRE2.
Perl 5.10's unified backreference syntax, in which \g can be used
for both numeric and named references, is also supported by
PCRE2. We could rewrite the above example in any of the
following ways:
(?<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}
A capture group that is referenced by name may appear in the
pattern before or after the reference.
There may be more than one backreference to the same group. If a
group has not actually been used in a particular match,
backreferences to it always fail by default. For example, the
pattern
(a|(bc))\2
always fails if it starts to match "a" rather than "bc". However,
if the PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a
backreference to an unset value matches an empty string.
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 "Comments" below) can be used.
Recursive backreferences
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
(a|b\1)+
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.
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 atomic group. This restriction no longer applies, and
backtracking into such groups can occur as normal.
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 above.
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.
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 "Non-
atomic assertions" below, but they are not Perl-compatible.
A lookaround assertion may appear as the condition in a
conditional group (see below). In this case, the result of
matching the assertion determines which branch of the condition
is followed.
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.
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.
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 conditional group
(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.
Most assertion groups may be repeated; though it makes no sense
to assert the same thing several times, the side effect of
capturing in positive assertions may occasionally be useful.
However, an assertion that forms the condition for a conditional
group may not be quantified. PCRE2 used to restrict the
repetition of assertions, but from release 10.35 the only
restriction is that an unlimited maximum repetition is changed to
be one more than the minimum. For example, {3,} is treated as
{3,4}.
Alphabetic assertion names
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:
(*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 (?<!
For example, (*pla:foo) is the same assertion as (?=foo). In the
following sections, the various assertions are described using
the original symbolic forms.
Lookahead assertions
Lookahead assertions start with (?= for positive assertions and
(?! for negative assertions. For example,
\w+(?=;)
matches a word followed by a semicolon, but does not include the
semicolon in the match, and
foo(?!bar)
matches any occurrence of "foo" that is not followed by "bar".
Note that the apparently similar pattern
(?!foo)bar
does not find an occurrence of "bar" that is preceded by
something other than "foo"; it finds any occurrence of "bar"
whatsoever, because the assertion (?!foo) is always true when the
next three characters are "bar". A lookbehind assertion is needed
to achieve the other effect.
If you want to force a matching failure at some point in a
pattern, the most convenient way to do it is with (?!) because an
empty string always matches, so an assertion that requires there
not to be an empty string must always fail. The backtracking
control verb (*FAIL) or (*F) is a synonym for (?!).
Lookbehind assertions
Lookbehind assertions start with (?<= for positive assertions and
(?<! for negative assertions. For example,
(?<!foo)bar
does find an occurrence of "bar" that is not preceded by "foo".
The contents of a lookbehind assertion are restricted such that
there must be a known maximum to the lengths of all the strings
it matches. There are two cases:
If every top-level alternative matches a fixed length, for
example
(?<=colour|color)
there is a limit of 65535 characters to the lengths, which do not
have to be the same, as this example demonstrates. This is the
only kind of lookbehind supported by PCRE2 versions earlier than
10.43 and by the alternative matching function pcre2_dfa_match().
In PCRE2 10.43 and later, pcre2_match() supports lookbehind
assertions in which one or more top-level alternatives can match
more than one string length, for example
(?<=colou?r)
The maximum matching length for any branch of the lookbehind is
limited to a value set by the calling program (default 255
characters). Unlimited repetition (for example \d*) is not
supported. In some cases, the escape sequence \K (see above) can
be used instead of a lookbehind assertion at the start of a
pattern to get round the length limit restriction.
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.
"Subroutine" calls (see below) such as (?2) or (?&X) are
permitted in lookbehinds, as long as the called capture group
matches a limited-length string. However, recursion, that is, a
"subroutine" call into a group that is already active, is not
supported.
PCRE2 supports backreferences in lookbehinds, 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 limited length substring. The following pattern
matches words containing at least two characters that begin and
end with the same character:
\b(\w)\w++(?<=\1)
Possessive quantifiers can be used in conjunction with lookbehind
assertions to specify efficient matching at the end of subject
strings. Consider a simple pattern such as
abcd$
when applied to a long string that does not match. Because
matching proceeds from left to right, PCRE2 will look for each
"a" in the subject and then see if what follows matches the rest
of the pattern. If the pattern is specified as
^.*abcd$
the initial .* matches the entire string at first, but when this
fails (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
^.*+(?<=abcd)
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.
Using multiple assertions
Several assertions (of any sort) may occur in succession. For
example,
(?<=\d{3})(?<!999)foo
matches "foo" preceded by three digits that are not "999". Notice
that each of the assertions is applied independently at the same
point in the subject string. First there is a check that the
previous three characters are all digits, and then there is a
check that the same three characters are not "999". This pattern
does not 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
(?<=\d{3}...)(?<!999)foo
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".
Assertions can be nested in any combination. For example,
(?<=(?<!foo)bar)baz
matches an occurrence of "baz" that is preceded by "bar" which in
turn is not preceded by "foo", while
(?<=\d{3}(?!999)...)foo
is another pattern that matches "foo" preceded by three digits
and any three characters that are not "999".
Traditional 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:
(*non_atomic_positive_lookahead: or (*napla: or (?*
(*non_atomic_positive_lookbehind: or (*naplb: or (?<*
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:
^(?x)(*napla: .* \b(\w++)) (?> .*? \b\1\b ){2}
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.
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 lookahead (?= 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.
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.
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.
There is one exception to backtracking into a non-atomic
assertion. If an (*ACCEPT) control verb is triggered, the
assertion succeeds atomically. That is, a subsequent match
failure cannot backtrack into the assertion.
Non-atomic assertions are not supported by the alternative
matching function pcre2_dfa_match(). They are supported by JIT,
but only if they do not contain any control verbs such as
(*ACCEPT). (This may change in future). Note that assertions that
appear as conditions for conditional groups (see below) must be
atomic.
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 "Script Runs" in the
pcre2unicode documentation.
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:
\s+(*sr:\S+)
To be sure that they are all from the Latin script (for example),
a lookahead can be used:
\s+(?=\p{Latin})(*sr:\S+)
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:
\s+(?=[0-9_.]*\p{Latin})(*sr:\S+)
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:
(*asr:...) is the same as (*sr:(?>...))
Note that the atomic group is inside the script run. Putting it
outside would not prevent backtracking into the script run
pattern.
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, pcre2_dfa_match()
because they use the same mechanism as capturing parentheses.
Warning: The (*ACCEPT) control verb (see below) should not be
used within a script run group, because it causes an immediate
exit from the group, bypassing the script run checking.
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:
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
If the condition is satisfied, the yes-pattern is used; otherwise
the no-pattern (if present) is used. An absent no-pattern is
equivalent to an empty string (it always matches). If there are
more than two 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:
(?(1) (A|B|C) | (D | (?(2)E|F) | E) )
There are five kinds of condition: references to capture groups,
references to recursion, two pseudo-conditions called DEFINE and
VERSION, and assertions.
Checking for a used capture group by number
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 section about duplicate
group numbers), the condition is true if any of them have
matched. An alternative notation, which is a PCRE2 extension, not
supported by Perl, 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 (which could be
enclosing this condition) 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 to
be opened 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.
Consider the following pattern, which contains non-significant
white space to make it more readable (assume the PCRE2_EXTENDED
option) and to divide it into three parts for ease of discussion:
( \( )? [^()]+ (?(1) \) )
The first part matches an optional opening parenthesis, and if
that 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.
If you were embedding this pattern in a larger one, you could use
a relative reference:
...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
This makes the fragment independent of the parentheses in the
larger pattern.
Checking for a used capture group by name
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:
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
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.
Checking for pattern recursion
"Recursion" in this sense refers to any subroutine-like call from
one part of the pattern to another, whether or not it is actually
recursive. See the sections entitled "Recursive patterns" and
"Groups as subroutines" below for details of recursion and
subroutine calls.
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:
((?(R1)a+|(?1)b))
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.
If a name preceded by ampersand follows the letter R, for
example:
(?(R&name)...)
the condition is true if the most recent recursion is into a
group of that name (which must exist within the pattern).
This condition does not check the entire recursion stack. It
tests only the current level. If the name used in a condition of
this kind is a 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.
At "top level", all these recursion test conditions are false.
Defining capture groups for use by reference only
If the condition is the string (DEFINE), the condition is always
false, even if there is a group with the name DEFINE. In this
case, there may 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 subroutines is described below.) For example, a pattern to
match an IPv4 address such as "192.168.23.245" could be written
like this (ignore white space and line breaks):
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
\b (?&byte) (\.(?&byte)){3} \b
The first part of the pattern is a DEFINE group inside which
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.
Checking the PCRE2 version
Programs that link with a PCRE2 library can check the version by
calling pcre2_config() 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:
(?(VERSION>=10.4)yes|no)
This pattern matches "yes" if the PCRE2 version is greater or
equal to 10.4, or "no" otherwise. The fractional part of the
version number may not contain more than two digits.
Assertion conditions
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 non-atomic
assertions.
Consider this pattern, again containing non-significant white
space, and with the two alternatives on the second line:
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
The condition is a positive lookahead assertion that matches an
optional sequence of non-letters followed by a letter. In other
words, it tests for the presence of at least one letter in the
subject. If a letter is found, the subject is matched against the
first alternative; otherwise it is matched against the second.
This pattern matches strings in one of the two forms dd-aaa-dd or
dd-dd-dd, where aaa are letters and dd are digits.
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.)
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.
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 "Newline
conventions" above. Note that the end of this type of comment is
a literal newline sequence in the pattern; escape sequences that
happen to represent a newline do not count. For example, consider
this pattern when PCRE2_EXTENDED is set, and the default newline
convention (a single linefeed character) is in force:
abc #comment \n still comment
On encountering the # character, pcre2_compile() skips along,
looking for a newline in the pattern. The sequence \n is still
literal at this stage, so it does not terminate the comment. Only
an actual character with the code value 0x0a (the default
newline) does so.
Consider the problem of matching a string in parentheses,
allowing for unlimited nested parentheses. Without the use of
recursion, the best that can be done is to use a pattern that
matches up to some fixed depth of nesting. It is not possible to
handle an arbitrary nesting depth.
For some time, Perl has provided a facility that allows regular
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:
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
The (?p{...}) item interpolates Perl code at run time, and in
this case refers recursively to the pattern in which it appears.
Obviously, PCRE2 cannot support the interpolation of Perl code.
Instead, it supports special syntax for recursion of the entire
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.
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 non-recursive
subroutine call, which is described in the next section.) The
special item (?R) or (?0) is a recursive call of the entire
regular expression.
This PCRE2 pattern solves the nested parentheses problem (assume
the PCRE2_EXTENDED option is set so that white space is ignored):
\( ( [^()]++ | (?R) )* \)
First it matches an opening parenthesis. Then it matches any
number of substrings which can either be a sequence of non-
parentheses, or a recursive match of the pattern itself (that is,
a correctly parenthesized substring). Finally there is a closing
parenthesis. Note the use of a possessive quantifier to avoid
backtracking into sequences of non-parentheses.
If this were part of a larger pattern, you would not want to
recurse the entire pattern, so instead you could use this:
( \( ( [^()]++ | (?1) )* \) )
We have put the pattern into parentheses, and caused the
recursion to refer to them instead of the whole pattern.
In a larger pattern, keeping track of parenthesis numbers can be
tricky. This is made easier by the use of relative references.
Instead 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.
Be aware however, that if duplicate capture group numbers are in
use, relative references refer to the earliest group with the
appropriate number. Consider, for example:
(?|(a)|(b)) (c) (?-2)
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.
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 non-recursive subroutine
calls, as described in the next section.
An alternative approach is to use named parentheses. The Perl
syntax for this is (?&name); PCRE1's earlier syntax (?P>name) is
also supported. We could rewrite the above example as follows:
(?<pn> \( ( [^()]++ | (?&pn) )* \) )
If there is more than one group with the same name, the earliest
one is used.
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
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
it yields "no match" quickly. However, if a possessive quantifier
is not used, the match runs for a very long time indeed because
there are so many different ways the + and * repeats can carve up
the subject, and all have to be tested before failure can be
reported.
At the end of a match, the values of capturing parentheses are
those from the outermost level. If you want to obtain
intermediate values, a callout function can be used (see below
and the pcre2callout documentation). If the pattern above is
matched against
(ab(cd)ef)
the value for the inner capturing parentheses (numbered 2) is
"ef", which is the last value taken on at the top level. If a
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.
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.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
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.
Differences in recursion processing between PCRE2 and Perl
Some former differences between PCRE2 and Perl no longer exist.
Before release 10.30, recursion processing in PCRE2 differed from
Perl in that a recursive 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.)
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.
Supporting backtracking into recursions simplifies certain types
of recursive pattern. For example, this pattern matches
palindromic strings:
^((.)(?1)\2|.?)$
The second branch in the group matches a single central character
in the palindrome when there are an odd number of characters, or
nothing when there are an even number of characters, but in order
to work it has to be able to try the second case when the rest of
the pattern 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:
^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$
If run with the PCRE2_CASELESS option, this pattern matches
phrases such as "A man, a plan, a canal: Panama!". Note the use
of the 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.
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:
^(.)(\1|a(?2))
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.
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:
(...(absolute)...)...(?2)...
(...(relative)...)...(?-1)...
(...(?+1)...(relative)...
An earlier example pointed out that the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and
responsibility", but not "sense and responsibility". If instead
the pattern
(sens|respons)e and (?1)ibility
is used, it does match "sense and responsibility" as well as the
other two strings. Another example is given in the discussion of
DEFINE above.
Like recursions, subroutine calls used to be treated as atomic,
but this changed at PCRE2 release 10.30, so backtracking into
subroutine calls can now occur. However, any capturing
parentheses that are set during the subroutine call revert to
their previous values afterwards.
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:
(abc)(?i:(?-1))
It matches "abcabc". It does not match "abcABC" because the
change of processing option does not affect the called group.
The behaviour of backtracking control verbs in groups when called
as subroutines is described in the section entitled "Backtracking
verbs in subroutines" below.
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:
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
(sens|respons)e and \g'1'ibility
PCRE2 supports an extension to Oniguruma: if a number is preceded
by a plus or a minus sign it is taken as a relative reference.
For example:
(abc)(?i:\g<-1>)
Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax)
are not synonymous. The former is a backreference; the latter is
a subroutine call.
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.
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 pcre2_set_callout(), and
then passing that context to pcre2_match() or pcre2_dfa_match().
If no match context is passed, or if the callout entry point is
set to NULL, callouts are disabled.
Within a regular expression, (?C<arg>) indicates a point at which
the external function is to be called. There are two kinds of
callout: those with a numerical argument and those with a string
argument. (?C) on its own with no argument is treated as (?C0). A
numerical argument allows the application to distinguish between
different callouts. String arguments were added for release 10.20
to make it possible for script languages that use PCRE2 to embed
short scripts within patterns in a similar way to Perl.
During matching, when PCRE2 reaches a callout point, the external
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.
By default, PCRE2 implements a number of optimizations at
matching time, and one side-effect is that sometimes callouts are
skipped. If you need all possible callouts to happen, you need to
set options that disable the relevant optimizations. More
details, including a complete description of the programming
interface to the callout function, are given in the pcre2callout
documentation.
Callouts with numerical arguments
If you just want to have a means of identifying different callout
points, put a number less than 256 after the letter C. For
example, this pattern has two callout points:
(?C1)abc(?C2)def
If the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(),
numerical callouts are automatically installed before each item
in the pattern. They are all numbered 255. If there is a
conditional group in the 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:
(?(?C9)(?=a)abc|def)
Note that this applies only to assertion conditions, not to other
types of condition.
Callouts with string arguments
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:
(?C'ab ''c'' d')xyz(?C{any text})pqr
The doubling is removed before the string is passed to the
callout function.
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.
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.
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.
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.
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.
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.
The behaviour of these verbs in repeated groups, assertions, and
in capture groups called as subroutines (whether or not
recursively) is documented below.
Optimizations that affect backtracking verbs
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 pcre2_compile(), or
by starting the pattern with (*NO_START_OPT). There is more
discussion of this option in the section entitled "Compiling a
pattern" in the pcre2api documentation.
Experiments with Perl suggest that it too has similar
optimizations, and like PCRE2, turning them off can change the
result of a match.
Verbs that act immediately
The following verbs act as soon as they are encountered.
(*ACCEPT) or (*ACCEPT:NAME)
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.
If (*ACCEPT) is inside capturing parentheses, the data so far is
captured. For example:
A((?:A|B(*ACCEPT)|C)D)
This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is
captured by the outer parentheses.
(*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,
(A(*ACCEPT)??B)C
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".
Warning: (*ACCEPT) should not be used within a script run group,
because it causes an immediate exit from the group, bypassing the
script run checking.
(*FAIL) or (*FAIL:NAME)
This verb causes a matching failure, forcing backtracking to
occur. It may be abbreviated to (*F). It is equivalent to (?!)
but easier to 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:
a+(?C)(*FAIL)
A match with the string "aaaa" always fails, but the callout is
taken before each backtrack happens (in this example, 10 times).
(*ACCEPT:NAME) and (*FAIL:NAME) behave the same as
(*MARK:NAME)(*ACCEPT) and (*MARK:NAME)(*FAIL), respectively, that
is, a (*MARK) is recorded just before the verb acts.
Recording which path was taken
There is one verb whose main purpose is to track how a match was
arrived at, though it also has a secondary use in conjunction
with advancing the match starting point (see (*SKIP) below).
(*MARK:NAME) or (*:NAME)
A name is always required with this verb. For all the other
backtracking control verbs, a NAME argument is optional.
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 "Other information about the match" in the
pcre2api 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.
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 pcre2test output, where the "mark"
modifier requests the retrieval and outputting of (*MARK) data:
re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
data> XY
0: XY
MK: A
XZ
0: XZ
MK: B
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.
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.
After a partial match or a failed match, the last encountered
name in the entire match process is returned. For example:
re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
data> XP
No match, mark = B
Note that in this unanchored example the mark is retained from
the match attempt that started at the letter "X" in the subject.
Subsequent match attempts starting at "P" and then with an empty
string do not get as far as the (*MARK) item, but nevertheless do
not reset it.
If you are interested in (*MARK) values after failed matches, you
should probably set the PCRE2_NO_START_OPTIMIZE option (see
above) to ensure that the match is always attempted.
Verbs that act after backtracking
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.
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.
(*COMMIT) or (*COMMIT:NAME)
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 pcre2_match() is committed to finding a match at the
current starting point, or not at all. For example:
a+(*COMMIT)b
This matches "xxaab" but not "aacaab". It can be thought of as a
kind of dynamic anchor, or "I've started, so I must finish."
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.
If there is more than one backtracking verb in a pattern, a
different one that follows (*COMMIT) may be triggered first, so
merely passing (*COMMIT) during a match does not always guarantee
that a match must be at this starting point.
Note that (*COMMIT) at the start of a pattern is not the same as
an anchor, unless PCRE2's start-of-match optimizations are turned
off, as shown in this output from pcre2test:
re> /(*COMMIT)abc/
data> xyzabc
0: abc
data>
re> /(*COMMIT)abc/no_start_optimize
data> xyzabc
No match
For the first pattern, PCRE2 knows that any match must start with
"a", so the optimization skips along the subject to "a" before
applying the pattern to the first set of data. The match attempt
then succeeds. The second pattern disables the optimization that
skips along to the first character. The pattern is now applied
starting at "x", and so the (*COMMIT) causes the match to fail
without trying any other starting points.
(*PRUNE) or (*PRUNE:NAME)
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).
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.
(*SKIP)
This verb, when given without a name, is like (*PRUNE), except
that if the pattern is unanchored, the "bumpalong" advance is not
to the next 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:
a+(*SKIP)b
If the subject is "aaaac...", after the first match attempt fails
(starting at the first character in the string), the starting
point skips on to start the next attempt at "c". Note that a
possessive quantifier 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".
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.
(*SKIP:NAME)
When (*SKIP) has an associated name, its behaviour is modified.
When such a (*SKIP) is triggered, the previous path through the
pattern is searched for the most recent (*MARK) that has the same
name. If one is found, the "bumpalong" advance is to the subject
position that corresponds to that (*MARK) instead of to where
(*SKIP) was encountered. If no (*MARK) with a matching name is
found, the (*SKIP) is ignored.
The search for a (*MARK) name uses the normal backtracking
mechanism, which means that it does not see (*MARK) settings that
are inside atomic groups or assertions, because they are never
re-entered by backtracking. Compare the following pcre2test
examples:
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
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.
Note that (*SKIP:NAME) searches only for names set by
(*MARK:NAME). It ignores names that are set by other backtracking
verbs.
(*THEN) or (*THEN:NAME)
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:
( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )
...
If the COND1 pattern matches, FOO is tried (and possibly further
items after the end of the group if FOO succeeds); on failure,
the matcher skips to the second alternative and tries COND2,
without backtracking into COND1. If that succeeds and BAR fails,
COND3 is tried. If 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).
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.
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:
A (B(*THEN)C) | D
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:
A (B(*THEN)C | (*FAIL)) | D
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.
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:
^.*? (?(?=a) a | b(*THEN)c )
If the subject is "ba", this pattern does not match. Because .*?
is ungreedy, it initially matches zero characters. The condition
(?=a) then fails, the character "b" is matched, but "c" is not.
At this point, matching does not backtrack to .*? as might
perhaps be expected from the presence of the | character. The
conditional 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.)
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.
More than one backtracking verb
If more than one backtracking verb is present in a pattern, the
one that is backtracked onto first acts. For example, consider
this pattern, where A, B, etc. are complex pattern fragments:
(A(*COMMIT)B(*THEN)C|ABD)
If A matches but B fails, the backtrack to (*COMMIT) causes the
entire 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 but the last of them has no effect. Consider
this example:
...(*COMMIT)(*PRUNE)...
If there is a matching failure to the right, backtracking onto
(*PRUNE) causes it to be triggered, and its action is taken.
There can never be a backtrack onto (*COMMIT).
Backtracking verbs in repeated groups
PCRE2 sometimes differs from Perl in its handling of backtracking
verbs in repeated groups. For example, consider:
/(a(*COMMIT)b)+ac/
If the subject is "abac", Perl matches unless its optimizations
are disabled, but PCRE2 always fails because the (*COMMIT) in the
second repeat of the group acts.
Backtracking verbs in assertions
(*FAIL) in any assertion has its normal effect: it forces an
immediate backtrack. The behaviour of the other backtracking
verbs depends on whether or not the assertion is standalone or
acting as the condition in a conditional group.
(*ACCEPT) in a standalone positive assertion causes the assertion
to succeed without any further processing; captured strings and a
mark name (if set) are retained. In a standalone negative
assertion, (*ACCEPT) causes the assertion to fail without any
further processing; captured substrings and any mark name are
discarded.
If the assertion is a condition, (*ACCEPT) causes the condition
to be true for a positive assertion and false for a negative one;
captured substrings are retained in both cases.
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.
PCRE2 now supports non-atomic positive assertions, as described
in the section entitled "Non-atomic assertions" 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.
The effect of (*THEN) is not allowed to escape beyond an
assertion. If there are no more branches to try, (*THEN) causes a
positive assertion to be false, and a negative assertion to be
true.
The other backtracking verbs are not treated specially if they
appear in a standalone positive assertion. In a conditional
positive 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.
Backtracking verbs in subroutines
These behaviours occur whether or not the group is called
recursively.
(*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.
(*FAIL) in a group called as a subroutine has its normal effect:
it forces an immediate backtrack.
(*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.
(*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.
pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2syntax(3),
pcre2(3).
Philip Hazel
Retired from University Computing Service
Cambridge, England.
Last updated: 04 June 2024
Copyright (c) 1997-2024 University of Cambridge.
This page is part of the PCRE (Perl Compatible Regular
Expressions) project. Information about the project can be found
at ⟨http://www.pcre.org/⟩. If you have a bug report for this
manual page, see
⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩. This page was
obtained from the tarball fetched from
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discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
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manual page), send a mail to [email protected]
PCRE2 10.44 04 June 2024 PCRE2PATTERN(3)
Pages that refer to this page: grep(1), journalctl(1), pcre2grep(1), pcre2test(1), pcre2syntax(3)
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