Provided by: libpcre2-dev_10.39-3ubuntu0.1_amd64 
NAME
PCRE2 - Perl-compatible regular expressions (revised API)
PCRE2 REGULAR EXPRESSION DETAILS
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.
SPECIAL START-OF-PATTERN ITEMS
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.
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.
EBCDIC CHARACTER CODES
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.
CHARACTERS AND METACHARACTERS
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.
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
{ start min/max quantifier
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, and characters between a # outside a character class and the next newline, inclusive,
are ignored. An escaping backslash can be used to include a white space or 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.
BACKSLASH
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. This is different from Perl in that $ and @ are handled as literals in \Q...\E sequences in
PCRE2, whereas in Perl, $ and @ cause variable interpolation. Also, Perl does "double-quotish backslash
interpolation" on any backslashes between \Q and \E which, its documentation says, "may lead to confusing
results". PCRE2 treats a backslash between \Q and \E just like any other character. Note the following
examples:
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 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 any printable ASCII character
\e escape (hex 1B)
\f form feed (hex 0C)
\n linefeed (hex 0A)
\r carriage return (hex 0D) (but see below)
\t tab (hex 09)
\0dd character with octal code 0dd
\ddd character with octal code ddd, or backreference
\o{ddd..} character with octal code ddd..
\xhh character with hex code hh
\x{hhh..} character with hex code hhh..
\N{U+hhh..} character with Unicode hex code point hhh..
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. 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.
The precise effect of \cx on ASCII characters is as follows: if x is a lower case letter, it is converted
to upper case. Then bit 6 of the character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A
(A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the code unit
following \c has a value less than 32 or greater than 126, a compile-time error occurs.
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} or \p{N}, plus underscore
The upper case escapes match the inverse sets of characters. Note that \d matches only decimal digits,
whereas \w matches any Unicode digit, as well as any Unicode letter, and underscore. Note also that
PCRE2_UCP affects \b, and \B because they are defined in terms of \w and \W. Matching these sequences is
noticeably slower when PCRE2_UCP is set.
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. The extra escape sequences 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 case-sensitive. There is support for Unicode script names,
Unicode general category properties, "Any", which matches any character (including newline), and some
special PCRE2 properties (described in the next section). 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.
Sets of Unicode characters are defined as belonging to certain scripts. A character from one of these
sets can be matched using a script name. For example:
\p{Greek}
\P{Han}
Unassigned characters (and in non-UTF 32-bit mode, characters with code points greater than 0x10FFFF) are
assigned the "Unknown" script. Others that are not part of an identified script are lumped together as
"Common". The current list of scripts is:
Adlam, Ahom, Anatolian_Hieroglyphs, Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak,
Bengali, Bhaiksuki, Bopomofo, Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian,
Caucasian_Albanian, Chakma, Cham, Cherokee, Chorasmian, Common, Coptic, Cuneiform, Cypriot, Cypro_Minoan,
Cyrillic, Deseret, Devanagari, Dives_Akuru, Dogra, Duployan, Egyptian_Hieroglyphs, Elbasan, Elymaic,
Ethiopic, Georgian, Glagolitic, Gothic, Grantha, Greek, Gujarati, Gunjala_Gondi, Gurmukhi, Han, Hangul,
Hanifi_Rohingya, Hanunoo, Hatran, Hebrew, Hiragana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi,
Inscriptional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khitan_Small_Script,
Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Linear_A, Linear_B, Lisu, Lycian, Lydian, Mahajani,
Makasar, Malayalam, Mandaic, Manichaean, Marchen, Masaram_Gondi, Medefaidrin, Meetei_Mayek,
Mende_Kikakui, Meroitic_Cursive, Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro, Multani, Myanmar,
Nabataean, Nandinagari, New_Tai_Lue, Newa, Nko, Nushu, Nyakeng_Puachue_Hmong, Ogham, Ol_Chiki,
Old_Hungarian, Old_Italic, Old_North_Arabian, Old_Permic, Old_Persian, Old_Sogdian, Old_South_Arabian,
Old_Turkic, Old_Uyghur, Oriya, Osage, Osmanya, Pahawh_Hmong, Palmyrene, Pau_Cin_Hau, Phags_Pa,
Phoenician, Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra, Sharada, Shavian, Siddham,
SignWriting, Sinhala, Sogdian, Sora_Sompeng, Soyombo, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Tangsa, Tangut, Telugu, Thaana, Thai, Tibetan, Tifinagh,
Tirhuta, Toto, Ugaritic, Unknown, Vai, Vithkuqi, Wancho, Warang_Citi, Yezidi, Yi, Zanabazar_Square.
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 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.
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).
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" character.
Characters with the "mark" property always have the "extend" grapheme breaking property.
5. Do not end after prepend characters.
6. Do not break within emoji modifier sequences or emoji zwj sequences. That is, do not break between
characters with the Extended_Pictographic property. Extend and ZWJ characters are allowed between the
characters.
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 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). However,
in this case, the part of the subject before the real match does not have to be of fixed length, as
lookbehind assertions do. The use of \K does not interfere with the setting of 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.
CIRCUMFLEX AND DOLLAR
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.
FULL STOP (PERIOD, DOT) AND \N
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.
When a line ending is defined as a single character, dot never matches that character; when the two-
character sequence CRLF is used, dot does not match CR if it is immediately followed by LF, but otherwise
it matches all characters (including isolated CRs and LFs). When any Unicode line endings are being
recognized, dot does not match CR or LF or any of the other line ending characters.
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.
MATCHING A SINGLE CODE UNIT
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.
SQUARE BRACKETS AND CHARACTER CLASSES
An opening square bracket introduces a character class, terminated by a closing square bracket. A closing
square bracket on its own is not 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 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.
POSIX CHARACTER CLASSES
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.
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, if the PCRE2_UCP option is passed to pcre2_compile(), some of the classes are changed
so that Unicode character properties are used. This is achieved by replacing certain POSIX classes with
other sequences, as follows:
[: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. Three 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.
The other POSIX classes are unchanged, and match only characters with code points less than 256.
COMPATIBILITY FEATURE FOR WORD BOUNDARIES
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.
VERTICAL BAR
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.
INTERNAL OPTION SETTING
The settings of the PCRE2_CASELESS, PCRE2_MULTILINE, PCRE2_DOTALL, PCRE2_EXTENDED, PCRE2_EXTENDED_MORE,
and PCRE2_NO_AUTO_CAPTURE options can be changed from within the pattern by a sequence of letters
enclosed between "(?" and ")". These options are Perl-compatible, and are described in detail in the
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.
Thus, (?^) is equivalent to (?-imnsx). Letters may follow the circumflex to cause some options to be re-
instated, but a hyphen may not appear.
The PCRE2-specific options PCRE2_DUPNAMES and PCRE2_UNGREEDY can be changed in the same way as the Perl-
compatible options by using the characters J and U respectively. However, these are not unset by (?^).
When one of these option changes occurs at top level (that is, not inside group parentheses), the change
applies to the remainder of the pattern that follows. An option change within a group (see below for a
description of groups) affects only that part of the group that follows it, so
(a(?i)b)c
matches abc and aBc and no other strings (assuming PCRE2_CASELESS is not used). By this means, options
can be made to have different settings in different parts of the pattern. Any changes made in one
alternative do carry on into subsequent branches within the same group. For example,
(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 and
PCRE2_NEVER_UCP options, which lock out the use of the (*UTF) and (*UCP) sequences.
GROUPS
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".
DUPLICATE GROUP NUMBERS
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.
NAMED CAPTURE GROUPS
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 32 code units long. When PCRE2_UTF is not set, they may
contain only ASCII alphanumeric characters and underscores, but must start with a non-digit. When
PCRE2_UTF is set, the syntax of group names is extended to allow any Unicode letter or Unicode decimal
digit. In other words, group names must match one of these patterns:
^[_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
Repetition is specified by quantifiers, which can follow any of the following items:
a literal data character
the dot metacharacter
the \C escape sequence
the \R escape sequence
the \X escape sequence
an escape such as \d or \pL that matches a single character
a character class
a backreference
a parenthesized group (including lookaround assertions)
a subroutine call (recursive or otherwise)
The general repetition quantifier specifies a minimum and maximum number of permitted matches, by giving
the two numbers in curly brackets (braces), separated by a comma. The numbers must be less than 65536,
and the first must be less than or equal to the second. For example,
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. An opening curly bracket that appears in a position where a quantifier is not
allowed, or one that does not match the syntax of a quantifier, is taken as a literal character. For
example, {,6} is not a quantifier, but a literal string of four characters.
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 times), without causing the rest of the pattern to fail. The classic example of where this
gives problems is in trying to match comments in C programs. These appear between /* and */ and within
the comment, individual * and / characters may appear. An attempt to match C comments by applying the
pattern
/\*.*\*/
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 the C comments. The meaning of the various quantifiers is not otherwise
changed, just the preferred number of matches. Do not confuse this use of question mark with its use as
a quantifier in its own right. Because it has two uses, it can sometimes appear doubled, as in
\d??\d
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".
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") repetition, failure of what follows
normally causes the repeated item to be re-evaluated to see if a different number of repeats allows the
rest of the pattern to match. Sometimes it is useful to prevent this, either to change the nature of the
match, or to cause it fail earlier than it otherwise might, when the author of the pattern knows there is
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.
BACKREFERENCES
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 most recently started capture group before \g, that is, is it
equivalent to \2 in this example. Similarly, \g{-2} would be equivalent to \1. The use of relative
references can be helpful in long patterns, and also in patterns that are created by joining together
fragments that contain references within themselves.
The sequence \g{+1} is a reference to the next capture group. This kind of forward reference can be
useful in patterns that repeat. Perl does not support the use of + in this way.
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
\k{name} and the Perl syntax \k<name> or \k'name' are supported, as is the Python syntax (?P=name). Perl
5.10's unified backreference syntax, in which \g can be used for both numeric and named references, is
also supported. We could rewrite the above example in any of the following ways:
(?<p1>(?i)rah)\s+\k<p1>
(?'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.
ASSERTIONS
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 all the strings it matches must have a fixed length. However, if there are
several top-level alternatives, they do not all have to have the same fixed length. Thus
(?<=bullock|donkey)
is permitted, but
(?<!dogs?|cats?)
causes an error at compile time. Branches that match different length strings are permitted only at the
top level of a lookbehind assertion. This is an extension compared with Perl, which requires all branches
to match the same length of string. An assertion such as
(?<=ab(c|de))
is not permitted, because its single top-level branch can match two different lengths, but it is
acceptable to PCRE2 if rewritten to use two top-level branches:
(?<=abc|abde)
In some cases, the escape sequence \K (see above) can be used instead of a lookbehind assertion to get
round the fixed-length restriction.
The implementation of lookbehind assertions is, for each alternative, to temporarily move the current
position back by the fixed length and then try to match. If there are insufficient characters before the
current position, the assertion fails.
In UTF-8 and UTF-16 modes, PCRE2 does not allow the \C escape (which matches a single code unit even in a
UTF mode) to appear in lookbehind assertions, because it makes it impossible to calculate the length of
the lookbehind. The \X and \R escapes, which can match different 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 fixed-length string. However, recursion, that is, a "subroutine" call into a
group that is already active, is not supported.
Perl does not support backreferences in lookbehinds. PCRE2 does support them, but only if certain
conditions are met. The PCRE2_MATCH_UNSET_BACKREF option must not be set, there must be no use of (?| in
the pattern (it creates duplicate group numbers), and if the backreference is by name, the name must be
unique. Of course, the referenced group must itself match a fixed length substring. The following pattern
matches words containing at least two characters that begin and end with the same character:
\b(\w)\w++(?<=\1)
Possessive quantifiers can be used in conjunction with lookbehind assertions to specify efficient
matching of fixed-length strings at the end of subject strings. Consider a simple pattern such as
abcd$
when applied to a long string that does not match. Because matching proceeds from left to right, PCRE2
will look for each "a" in the 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".
NON-ATOMIC ASSERTIONS
The traditional Perl-compatible lookaround assertions are atomic. That is, if an assertion is true, but
there is a subsequent matching failure, there is no backtracking into the assertion. However, there are
some cases where non-atomic positive assertions can be useful. PCRE2 provides these using the following
syntax:
(*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 lookhead (?= or (*pla: had been used, the assertion could not be re-
entered, and the whole match would fail. The pattern would succeed only if the very last word in the
subject was found twice.
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.
SCRIPT RUNS
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.
CONDITIONAL GROUPS
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 is to precede the digits with a plus or minus sign. In this case, the
group number is relative rather than absolute. The most recently opened capture group can be referenced
by (?(-1), the next most recent by (?(-2), and so on. Inside loops it can also make sense to refer to
subsequent groups. The next capture group can be referenced as (?(+1), and so on. (The value zero in any
of these forms is not used; it provokes a compile-time error.)
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 PCRE2-specific 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.)
COMMENTS
There are two ways of including comments in patterns that are processed by PCRE2. In both cases, the
start of the comment must not be in a character class, nor in the middle of any other sequence of related
characters such as (?: or a 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.
RECURSIVE PATTERNS
Consider the problem of matching a string in parentheses, allowing for unlimited nested parentheses.
Without the use of recursion, the best that can be done is to use a pattern that matches up to some fixed
depth of nesting. It is not possible to handle an arbitrary nesting 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.
GROUPS AS SUBROUTINES
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.
ONIGURUMA SUBROUTINE SYNTAX
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.
CALLOUTS
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.
BACKTRACKING CONTROL
There are a number of special "Backtracking Control Verbs" (to use Perl's terminology) that modify the
behaviour of backtracking during matching. They are generally of the form (*VERB) or (*VERB:NAME). Some
verbs take either form, 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 the the last of them has no effect. Consider this example:
...(*COMMIT)(*PRUNE)...
If there is a matching failure to the right, backtracking onto (*PRUNE) causes it to be triggered, and
its action is taken. There can never be 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.
SEE ALSO
pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2syntax(3), pcre2(3).
AUTHOR
Philip Hazel
Retired from University Computing Service
Cambridge, England.
REVISION
Last updated: 30 August 2021
Copyright (c) 1997-2021 University of Cambridge.