PCRE2PATTERN(3) Library Functions Manual PCRE2PATTERN(3)
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 syn-
tax summary in the pcre2syntax page. PCRE2 tries to match Perl syntax
and semantics as closely as it can. PCRE2 also supports some alterna-
tive 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 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 advan-
tages 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-com-
patible, but are provided to make these options accessible to pattern
writers who are not able to change the program that processes the pat-
tern. 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 the PCRE2_UTF option, or the pattern must start with the
special sequence (*UTF), which is equivalent to setting the relevant
option. 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 deter-
mine character types, instead of recognizing only characters with codes
less than 256 via a lookup table.
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 pat-
tern. 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 opti-
mizations 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 optimiza-
tions 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 pro-
voked by patterns with huge matching trees (a typical example is a pat-
tern 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 set-
ting 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 (line-
feed) character, the two-character sequence CRLF, any of the three pre-
ceding, 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 pat-
tern 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 func-
tion. 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 asser-
tions are true. It also affects the interpretation of the dot metachar-
acter 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_UNI-
CODE) 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 sys-
tem). In the sections below, character code values are ASCII or Uni-
code; 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), letters are
matched independently of case.
The power of regular expressions comes from the ability to include
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 recog-
nized 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 subpattern
) end subpattern
? extends the meaning of (
also 0 or 1 quantifier
also quantifier minimizer
* 0 or more quantifier
+ 1 or more quantifier
also "possessive quantifier"
{ 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 (only if followed by POSIX
syntax)
] terminates the character class
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 number 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 back-
slash, you write \\.
In a UTF mode, only ASCII numbers 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 a pattern is compiled with the PCRE2_EXTENDED option, most white
space in the pattern (other than in a character class), and characters
between a # outside a character class and the next newline, inclusive,
are ignored. An escaping backslash can be used to include a white space
or # character as part of the pattern.
If you want to remove the special meaning from a sequence of charac-
ters, you can do so by putting them between \Q and \E. This is differ-
ent from Perl in that $ and @ are handled as literals in \Q...\E
sequences in PCRE2, whereas in Perl, $ and @ cause variable interpola-
tion. 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 char-
acters 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 than 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)
\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..
\uhhhh character with hex code hhhh (when PCRE2_ALT_BSUX is set)
The \N{U+hhh..} escape sequence is recognized only when the PCRE2_UTF
option is set, that is, when PCRE2 is operating in a Unicode 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.
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 doc-
ument. 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 numeri-
cal character code points, and \g{} to specify backreferences. The fol-
lowing paragraphs describe the old, ambiguous syntax.
The handling of a backslash followed by a digit other than 0 is compli-
cated, and Perl has changed over time, causing PCRE2 also to change.
Outside a character class, PCRE2 reads the digit and any following dig-
its 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 capturing
left parentheses in the expression, the entire sequence is taken as a
backreference. A description of how this works is given later, follow-
ing the discussion of parenthesized subpatterns. 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 char-
acters "8" and "9", and otherwise reads up to three octal digits fol-
lowing the backslash, using them to generate a data character. Any sub-
sequent 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 capturing subpatterns
\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.
By default, after \x that is not followed by {, from zero to two hexa-
decimal digits are read (letters can be in upper or lower case). Any
number of hexadecimal digits may appear between \x{ and }. If a charac-
ter other than a hexadecimal digit appears between \x{ and }, or if
there is no terminating }, an error occurs.
If the PCRE2_ALT_BSUX option is set, the interpretation of \x is as
just described only when it is followed by two hexadecimal digits. Oth-
erwise, it matches 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 matches a literal "u" charac-
ter.
Characters whose value is less than 256 can be defined by either of the
two syntaxes for \x (or by \u in PCRE2_ALT_BSUX mode). There is no dif-
ference in the way they are handled. For example, \xdc is exactly the
same as \x{dc} (or \u00dc in PCRE2_ALT_BSUX mode).
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. However, if the
PCRE2_ALT_BSUX option 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 subpatterns.
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 subpattern as a "subroutine".
Details are discussed later. Note that \g{...} (Perl syntax) and
\g<...> (Oniguruma syntax) are not synonymous. The former is a backref-
erence; 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 charac-
ters" 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 com-
plete 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 con-
trolled 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 effi-
ciency 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 (car-
riage 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 sepa-
rator, 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 abbrevation for "back-
slash 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_UNI-
CODE 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 func-
tion. 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 addi-
tional escape sequences that match characters with specific properties
are available. In 8-bit non-UTF-8 mode, these sequences are of course
limited to testing characters whose code points are less than 256, but
they do work in this mode. 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 Common 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 limited to the Unicode
script names, the 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 "InMu-
sicalSymbols" 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}
Those 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, Bali-
nese, Bamum, Bassa_Vah, Batak, Bengali, Bhaiksuki, Bopomofo, Brahmi,
Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Caucasian_Alba-
nian, Chakma, Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot,
Cyrillic, Deseret, Devanagari, Dogra, Duployan, Egyptian_Hieroglyphs,
Elbasan, 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, Kan-
nada, Katakana, Kayah_Li, Kharoshthi, Khmer, Khojki, Khudawadi, Lao,
Latin, Lepcha, Limbu, Linear_A, Linear_B, Lisu, Lycian, Lydian, Maha-
jani, Makasar, Malayalam, Mandaic, Manichaean, Marchen, Masaram_Gondi,
Medefaidrin, Meetei_Mayek, Mende_Kikakui, Meroitic_Cursive,
Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro, Multani, Myanmar,
Nabataean, New_Tai_Lue, Newa, Nko, Nushu, Ogham, Ol_Chiki, Old_Hungar-
ian, Old_Italic, Old_North_Arabian, Old_Permic, Old_Persian, Old_Sog-
dian, Old_South_Arabian, Old_Turkic, Oriya, Osage, Osmanya,
Pahawh_Hmong, Palmyrene, Pau_Cin_Hau, Phags_Pa, Phoenician,
Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra, Sharada, Sha-
vian, Siddham, SignWriting, Sinhala, Sogdian, Sora_Sompeng, Soyombo,
Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham,
Tai_Viet, Takri, Tamil, Tangut, Telugu, Thaana, Thai, Tibetan, Tifi-
nagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi, Zanabazar_Square.
Each character has exactly one Unicode general category property, spec-
ified by a two-letter abbreviation. For compatibility with Perl, nega-
tion 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 gen-
eral 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 in the range
U+D800 to U+DFFF. Such characters are not valid in Unicode strings and
so cannot be tested by PCRE2, unless UTF validity checking has been
turned off (see the discussion of PCRE2_NO_UTF_CHECK in the pcre2api
page). Perl does not support the Cs property.
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) prop-
erty. 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 prop-
erty. 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 vari-
ous emoji-specific properties. PCRE2 uses only the Extended Picto-
graphic 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 char-
acter.
3. Do not break Hangul (a Korean script) syllable sequences. Hangul
characters are of five types: L, V, T, LV, and LVT. An L character may
be followed by an L, V, LV, or LVT character; an LV or V character may
be followed by a V or T character; an LVT or T character may be follwed
only by a T character.
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 charac-
ters.
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 sup-
ports 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 (num-
ber) 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 charac-
ter 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 char-
acters 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 fea-
ture 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 exam-
ple, when the pattern
(foo)\Kbar
matches "foobar", the first substring is still set to "foo".
Perl documents that the use of \K within assertions is "not well
defined". In PCRE2, \K is acted upon when it occurs inside positive
assertions, but is ignored in negative assertions. Note that when a
pattern such as (?=ab\K) matches, the reported start of the match can
be greater than the end of the match. Using \K in a lookbehind asser-
tion at the start of a pattern can also lead to odd effects. For exam-
ple, 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 asser-
tion 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 subpatterns 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. In a
UTF mode, 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" metase-
quence. 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 asser-
tions are not affected by the PCRE2_NOTBOL or PCRE2_NOTEOL options,
which affect only the behaviour of the circumflex and dollar metachar-
acters. 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 startoff-
set 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 con-
suming any characters from the subject string. These two metacharacters
are concerned with matching the starts and ends of lines. If the new-
line 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 argu-
ment of pcre2_match() is non-zero, or if PCRE2_NOTBOL is set, circum-
flex 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 sub-
ject, 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 num-
ber 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 charac-
ter 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) recog-
nizes the two-character sequence CRLF as a newline, this is preferred,
even if the single characters CR and LF are also recognized as new-
lines. 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 charac-
ter in the subject string except (by default) a character that signi-
fies 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 Uni-
code 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 sub-
ject string, it takes two dots to match it.
The handling of dot is entirely independent of the handling of circum-
flex 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 use-
fully 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 charac-
ter 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
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 charac-
ters 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 Subpattern Num-
bers" 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 spe-
cial 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 con-
sumes 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 ver-
sions, 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.
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 fol-
lowed by an opening brace.
The minus (hyphen) character can be used to specify a range of charac-
ters in a character class. For example, [d-m] matches any letter
between d and m, inclusive. If a minus character is required in a
class, it must be escaped with a backslash or appear in a position
where it cannot be interpreted as indicating a range, typically as the
first or last character in the class, or immediately after a range. For
example, [b-d-z] matches letters in the range b to d, a hyphen charac-
ter, 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 charac-
ter of a range. A pattern such as [W-]46] is interpreted as a class of
two characters ("W" and "-") followed by a literal string "46]", so it
would match "W46]" or "-46]". However, if the "]" is escaped with a
backslash it is interpreted as the end of range, so [W-\]46] is inter-
preted as a class containing a range followed by two other characters.
The octal or hexadecimal representation of "]" can also be used to end
a range.
Ranges normally include all code points between the start and end char-
acters, 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). How-
ever, 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 com-
patibility 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 charac-
ters 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 char-
acters 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 (punctua-
tion) 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 asser-
tions" 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 be-
haviour.
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 subpattern (defined below), "succeeds" means matching the
rest of the main pattern as well as the alternative in the subpattern.
INTERNAL OPTION SETTING
The settings of the 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 let-
ters 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 possi-
ble to unset these options by preceding the relevant letters with a
hyphen, for example (?-im). The two "extended" options are not indepen-
dent; 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. Need-
less 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 subpattern parentheses), the change applies to the remainder of
the pattern that follows. An option change within a subpattern (see
below for a description of subpatterns) affects only that part of the
subpattern that follows it, so
(a(?i)b)c
matches abc and aBc and no other strings (assuming PCRE2_CASELESS is
not used). By this means, options can be made to have different set-
tings in different parts of the pattern. Any changes made in one alter-
native do carry on into subsequent branches within the same subpattern.
For example,
(a(?i)b|c)
matches "ab", "aB", "c", and "C", even though when matching "C" the
first branch is abandoned before the option setting. This is because
the effects of option settings happen at compile time. There would be
some very weird behaviour otherwise.
As a convenient shorthand, if any option settings are required at the
start of a non-capturing subpattern (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 that can be set by the
application when the compiling function is called. The pattern can con-
tain 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.
SUBPATTERNS
Subpatterns are delimited by parentheses (round brackets), which can be
nested. Turning part of a pattern into a subpattern 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 sets up the subpattern as a capturing subpattern. This means
that, when the whole pattern matches, the portion of the subject string
that matched the subpattern 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 the capturing subpatterns. 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 num-
bered 1, 2, and 3, respectively.
The fact that plain parentheses fulfil two functions is not always
helpful. There are often times when a grouping subpattern is required
without a capturing requirement. If an opening parenthesis is followed
by a question mark and a colon, the subpattern does not do any captur-
ing, and is not counted when computing the number of any subsequent
capturing subpatterns. For example, if the string "the white queen" is
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 capturing subpatterns is 65535.
As a convenient shorthand, if any option settings are required at the
start of a non-capturing subpattern, the option letters may appear
between the "?" and the ":". Thus the two patterns
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
match exactly the same set of strings. Because alternative branches are
tried from left to right, and options are not reset until the end of
the subpattern is reached, an option setting in one branch does affect
subsequent branches, so the above patterns match "SUNDAY" as well as
"Saturday".
DUPLICATE SUBPATTERN NUMBERS
Perl 5.10 introduced a feature whereby each alternative in a subpattern
uses the same numbers for its capturing parentheses. Such a subpattern
starts with (?| and is itself a non-capturing subpattern. For example,
consider this pattern:
(?|(Sat)ur|(Sun))day
Because the two alternatives are inside a (?| group, both sets of cap-
turing parentheses are numbered one. Thus, when the pattern matches,
you can look at captured substring number one, whichever alternative
matched. This construct is useful when you want to capture part, but
not all, of one of a number of alternatives. Inside a (?| group, paren-
theses are numbered as usual, but the number is reset at the start of
each branch. The numbers of any capturing parentheses that follow the
subpattern start after the highest number used in any branch. The fol-
lowing example is taken from the Perl documentation. The numbers under-
neath 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 numbered subpattern uses the most recent value
that is set for that number by any subpattern. The following pattern
matches "abcabc" or "defdef":
/(?|(abc)|(def))\1/
In contrast, a subroutine call to a numbered subpattern always refers
to the first one in the pattern with the given number. The following
pattern matches "abcabc" or "defabc":
/(?|(abc)|(def))(?1)/
A relative reference such as (?-1) is no different: it is just a conve-
nient way of computing an absolute group number.
If a condition test for a subpattern's having matched refers to a non-
unique number, the test is true if any of the subpatterns of that num-
ber have matched.
An alternative approach to using this "branch reset" feature is to use
duplicate named subpatterns, as described in the next section.
NAMED SUBPATTERNS
Identifying capturing parentheses by number is simple, but it can be
very hard to keep track of the numbers in complicated patterns. Fur-
thermore, if an expression is modified, the numbers may change. To help
with this difficulty, PCRE2 supports the naming of capturing subpat-
terns. This feature was not added to Perl until release 5.10. Python
had the feature earlier, and PCRE1 introduced it at release 4.0, using
the Python syntax. PCRE2 supports both the Perl and the Python syntax.
In PCRE2, a capturing subpattern can be named in one of three ways:
(?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python.
Names consist of up to 32 alphanumeric characters and underscores, but
must start with a non-digit. References to capturing parentheses from
other parts of the pattern, such as backreferences, recursion, and con-
ditions, can all be made by name as well as by number.
Named capturing parentheses are allocated numbers as well as names,
exactly as if the names were not present. In both PCRE2 and Perl, cap-
turing subpatterns are primarily identified by numbers; any names are
just aliases for these numbers. The PCRE2 API provides function calls
for extracting the complete name-to-number translation table from a
compiled pattern, as well as convenience functions for extracting cap-
tured substrings by name.
Warning: When more than one subpattern has the same number, as
described in the previous section, a name given to one of them applies
to all of them. Perl allows identically numbered subpatterns to have
different names. Consider this pattern, where there are two capturing
subpatterns, both numbered 1:
(?|(?<AA>aa)|(?<BB>bb))
Perl allows this, with both names AA and BB as aliases of group 1.
Thus, after a successful match, both names yield the same value (either
"aa" or "bb").
In an attempt to reduce confusion, PCRE2 does not allow the same group
number to be associated with more than one name. The example above pro-
vokes a compile-time error. However, there is still scope for confu-
sion. Consider this pattern:
(?|(?<AA>aa)|(bb))
Although the second subpattern number 1 is not explicitly named, the
name AA is still an alias for subpattern 1. Whether the pattern matches
"aa" or "bb", a reference by name to group AA yields the matched
string.
By default, a name must be unique within a pattern, except that dupli-
cate names are permitted for subpatterns with the same number, for
example:
(?|(?<AA>aa)|(?<AA>bb))
The duplicate name constraint can be disabled by setting the PCRE2_DUP-
NAMES option at compile time, or by the use of (?J) within the pattern.
Duplicate names can be useful for patterns where only one instance of
the named parentheses can match. Suppose you want to match the name of
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:
(?<DN>Mon|Fri|Sun)(?:day)?|
(?<DN>Tue)(?:sday)?|
(?<DN>Wed)(?:nesday)?|
(?<DN>Thu)(?:rsday)?|
(?<DN>Sat)(?:urday)?
There are five capturing substrings, 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)
subpattern of that name that matched. This saves searching to find
which numbered subpattern it was. (An alternative way of solving this
problem is to use a "branch reset" subpattern, as described in the pre-
vious section.)
If you make a backreference to a non-unique named subpattern from else-
where in the pattern, the subpatterns to which the name refers are
checked in the order in which they appear in the overall pattern. The
first one that is set is used for the reference. For example, this pat-
tern matches both "foofoo" and "barbar" but not "foobar" or "barfoo":
(?:(?<n>foo)|(?<n>bar))\k<n>
If you make a subroutine call to a non-unique named subpattern, the one
that corresponds to the first occurrence of the name is used. In the
absence of duplicate numbers this is the one with the lowest number.
If you use a named reference in a condition test (see the section about
conditions below), either to check whether a subpattern has matched, or
to check for recursion, all subpatterns with the same name are tested.
If the condition is true for any one of them, the overall condition is
true. This is the same behaviour as testing by number. For further
details of the interfaces for handling named subpatterns, see the
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 \X escape sequence
the \R escape sequence
an escape such as \d or \pL that matches a single character
a character class
a backreference
a parenthesized subpattern (including most assertions)
a subroutine call to a subpattern (recursive or otherwise)
The general repetition quantifier specifies a minimum and maximum num-
ber of permitted matches, by giving the two numbers in curly brackets
(braces), separated by a comma. The numbers must be less than 65536,
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 exam-
ple, {,6} is not a quantifier, but a literal string of four characters.
In UTF modes, quantifiers apply to characters rather than to individual
code units. Thus, for example, \x{100}{2} matches two characters, each
of which is represented by a two-byte sequence in a UTF-8 string. Simi-
larly, \X{3} matches three Unicode extended grapheme clusters, each of
which may be several code units long (and they may be of different
lengths).
The quantifier {0} is permitted, causing the expression to behave as if
the previous item and the quantifier were not present. This may be use-
ful for subpatterns that are referenced as subroutines from elsewhere
in the pattern (but see also the section entitled "Defining subpatterns
for use by reference only" below). Items other than subpatterns that
have a {0} quantifier are omitted from the compiled pattern.
For convenience, the three most common quantifiers have single-charac-
ter abbreviations:
* is equivalent to {0,}
+ is equivalent to {1,}
? is equivalent to {0,1}
It is possible to construct infinite loops by following a subpattern
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 if any repetition of the
subpattern does in fact match no characters, the loop is forcibly bro-
ken.
By default, the quantifiers are "greedy", that is, they match as much
as possible (up to the maximum number of permitted times), without
causing the rest of the pattern to fail. The classic example of where
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.
If a quantifier is followed by a question mark, it ceases to be greedy,
and instead matches the minimum number of times possible, so the pat-
tern
/\*.*?\*/
does the right thing with the C comments. The meaning of the various
quantifiers is not otherwise changed, just the preferred number of
matches. Do not confuse this use of question mark with its use as a
quantifier in its own right. Because it has two uses, it can sometimes
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 subpattern is quantified with a minimum repeat
count that is greater than 1 or with a limited maximum, more memory is
required for the compiled pattern, in proportion to the size of the
minimum or maximum.
If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option
(equivalent to Perl's /s) is set, thus allowing the dot to match new-
lines, the pattern is implicitly anchored, because whatever follows
will be tried against every character position in the subject string,
so there is no point in retrying the overall match at any position
after the first. PCRE2 normally treats such a pattern as though it were
preceded by \A.
In cases where it is known that the subject string contains no new-
lines, it is worth setting PCRE2_DOTALL in order to obtain this opti-
mization, or alternatively, using ^ to indicate anchoring explicitly.
However, there are some cases where the optimization cannot be used.
When .* is inside capturing parentheses that are the subject of a
backreference elsewhere in the pattern, a match at the start may fail
where a later one succeeds. Consider, for example:
(.*)abc\1
If the subject is "xyz123abc123" the match point is the fourth charac-
ter. For this reason, such a pattern is not implicitly anchored.
Another case where implicit anchoring is not applied is when the lead-
ing .* is inside an atomic group. Once again, a match at the start may
fail where a later one succeeds. Consider this pattern:
(?>.*?a)b
It matches "ab" in the subject "aab". The use of the backtracking con-
trol verbs (*PRUNE) and (*SKIP) also disable this optimization, and
there is an option, PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly.
When a capturing subpattern is repeated, the value captured is the sub-
string 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 capturing subpatterns,
the corresponding captured values may have been set in previous itera-
tions. 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 subpattern has matched, it is not
to be re-evaluated in this way.
If we use atomic grouping for the previous example, the matcher gives
up immediately on failing to match "foo" the first time. The notation
is a kind of special parenthesis, starting with (?> as in this example:
(?>\d+)foo
This kind of parenthesis "locks up" the part of the pattern it con-
tains once it has matched, and a failure further into the pattern is
prevented from backtracking into it. Backtracking past it to previous
items, however, works as normal.
An alternative description is that a subpattern of this type matches
exactly the string of characters that an identical standalone pattern
would match, if anchored at the current point in the subject string.
Atomic grouping subpatterns are not capturing subpatterns. Simple cases
such as the above example can be thought of as a maximizing repeat that
must swallow everything it can. So, while both \d+ and \d+? are pre-
pared to adjust the number of digits they match in order to make the
rest of the pattern match, (?>\d+) can only match an entire sequence of
digits.
Atomic groups in general can of course contain arbitrarily complicated
subpatterns, and can be nested. However, when the subpattern for an
atomic 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 syn-
tax. Jeffrey Friedl originated the idea (and the name) in the first
edition of his book. Mike McCloskey liked it, so implemented it when he
built Sun's Java package, and PCRE1 copied it from there. It ultimately
found its way into Perl at release 5.10.
PCRE2 has an optimization that automatically "possessifies" certain
simple pattern constructs. For example, the sequence A+B is treated as
A++B because there is no point in backtracking into a sequence of A's
when B must follow. This feature can be disabled by the PCRE2_NO_AUTO-
POSSESS option, or starting the pattern with (*NO_AUTO_POSSESS).
When a pattern contains an unlimited repeat inside a subpattern that
can itself be repeated an unlimited number of times, the use of an
atomic group is the only way to avoid some failing matches taking a
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 charac-
ter that is required for a match, and fail early if it is not present
in the string.) If the pattern is changed so that it uses an atomic
group, like this:
((?>\D+)|<\d+>)*[!?]
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 capturing sub-
pattern earlier (that is, to its left) in the pattern, provided there
have been that many previous capturing left parentheses.
However, if the decimal number following the backslash is less than 8,
it is always taken as a backreference, and causes an error only if
there are not that many capturing left parentheses in the entire pat-
tern. In other words, the parentheses that are referenced need not be
to the left of the reference for numbers less than 8. A "forward back-
reference" of this type can make sense when a repetition is involved
and the subpattern to the right has participated in an earlier itera-
tion.
It is not possible to have a numerical "forward backreference" to a
subpattern whose number is 8 or more using this syntax because a
sequence such as \50 is interpreted as a character defined in octal.
See the subsection entitled "Non-printing characters" above for further
details of the handling of digits following a backslash. There is no
such problem when named parentheses are used. A backreference to any
subpattern is possible using named parentheses (see below).
Another way of avoiding the ambiguity inherent in the use of digits
following a backslash is to use the \g escape sequence. This escape
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 ambigu-
ity that is present in the older syntax. It is also useful when literal
digits follow the reference. A signed number is a relative reference.
Consider this example:
(abc(def)ghi)\g{-1}
The sequence \g{-1} is a reference to the most recently started captur-
ing subpattern before \g, that is, is it equivalent to \2 in this exam-
ple. Similarly, \g{-2} would be equivalent to \1. The use of relative
references can be helpful in long patterns, and also in patterns that
are created by joining together fragments that contain references
within themselves.
The sequence \g{+1} is a reference to the next capturing subpattern.
This kind of forward reference can be useful it patterns that repeat.
Perl does not support the use of + in this way.
A backreference matches whatever actually matched the capturing subpat-
tern in the current subject string, rather than anything matching the
subpattern itself (see "Subpatterns as subroutines" below for a way of
doing that). So the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsibility", but
not "sense and responsibility". If caseful matching is in force at the
time of the backreference, the case of letters is relevant. For exam-
ple,
((?i)rah)\s+\1
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
original capturing subpattern is matched caselessly.
There are several different ways of writing backreferences to named
subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
\k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
unified backreference syntax, in which \g can be used for both numeric
and named references, is also supported. We could rewrite the above
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 subpattern that is referenced by name may appear in the pattern
before or after the reference.
There may be more than one backreference to the same subpattern. If a
subpattern has not actually been used in a particular match, any back-
references to it always fail by default. For example, the pattern
(a|(bc))\2
always fails if it starts to match "a" rather than "bc". However, if
the PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a backref-
erence to an unset value matches an empty string.
Because there may be many capturing parentheses in a pattern, all dig-
its following a backslash are taken as part of a potential backrefer-
ence number. If the pattern continues with a digit character, some
delimiter must be used to terminate the backreference. If the
PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, this can be white
space. Otherwise, the \g{ syntax or an empty comment (see "Comments"
below) can be used.
Recursive backreferences
A backreference that occurs inside the parentheses to which it refers
fails when the subpattern is first used, so, for example, (a\1) never
matches. However, such references can be useful inside repeated sub-
patterns. For example, the pattern
(a|b\1)+
matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
ation of the subpattern, the backreference matches the character string
corresponding to the previous iteration. In order for this to work, the
pattern must be such that the first iteration does not need to match
the backreference. This can be done using alternation, as in the exam-
ple above, or by a quantifier with a minimum of zero.
Backreferences of this type cause the group that they reference to be
treated as an atomic group. Once the whole group has been matched, a
subsequent matching failure cannot cause backtracking into the middle
of the group.
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 subpatterns. There are two
kinds: those that look ahead of the current position in the subject
string, and those that look behind it, and in each case an assertion
may be positive (must succeed for matching to continue) or negative
(must not succeed for matching to continue). An assertion subpattern is
matched in the normal way, except that, when matching continues after a
successful assertion, the matching position in the subject string is as
it was before the assertion was processed.
Assertion subpatterns are not capturing subpatterns. If an assertion
contains capturing subpatterns within it, these are counted for the
purposes of numbering the capturing subpatterns in the whole pattern.
Within each branch of an assertion, locally captured substrings may be
referenced in the usual way. For example, a sequence such as (.)\g{-1}
can be used to check that two adjacent characters are the same.
When a branch within an assertion fails to match, any substrings that
were captured are discarded (as happens with any pattern branch that
fails to match). A negative assertion succeeds only when all its
branches fail to match; this means that no captured substrings are ever
retained after a successful negative assertion. When an assertion con-
tains a matching branch, what happens depends on the type of assertion.
For a positive assertion, internally captured substrings in the suc-
cessful branch are retained, and matching continues with the next pat-
tern item after the assertion. For a negative assertion, a matching
branch means that the assertion has failed. If the assertion is being
used as a condition in a conditional subpattern (see below), captured
substrings are retained, because matching continues with the "no"
branch of the condition. For other failing negative assertions, control
passes to the previous backtracking point, thus discarding any captured
strings within the assertion.
For compatibility with Perl, most assertion subpatterns may be
repeated; though it makes no sense to assert the same thing several
times, the side effect of capturing parentheses may occasionally be
useful. However, an assertion that forms the condition for a condi-
tional subpattern may not be quantified. In practice, for other asser-
tions, there only three cases:
(1) If the quantifier is {0}, the assertion is never obeyed during
matching. However, it may contain internal capturing parenthesized
groups that are called from elsewhere via the subroutine mechanism.
(2) If quantifier is {0,n} where n is greater than zero, it is treated
as if it were {0,1}. At run time, the rest of the pattern match is
tried with and without the assertion, the order depending on the greed-
iness of the quantifier.
(3) If the minimum repetition is greater than zero, the quantifier is
ignored. The assertion is obeyed just once when encountered during
matching.
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 semi-
colon 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 sev-
eral 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 cur-
rent position, the assertion fails.
In UTF-8 and UTF-16 modes, PCRE2 does not allow the \C escape (which
matches a single code unit even in a UTF mode) to appear in lookbehind
assertions, because it makes it impossible to calculate the length of
the lookbehind. The \X and \R escapes, which can match different num-
bers of code units, are never permitted in lookbehinds.
"Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
lookbehinds, as long as the subpattern matches a fixed-length string.
However, recursion, that is, a "subroutine" call into a group that is
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 subpattern numbers), and if
the backreference is by name, the name must be unique. Of course, the
referenced subpattern must itself be of fixed length. The following
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 sub-
ject and then see if what follows matches the rest of the pattern. If
the pattern is specified as
^.*abcd$
the initial .* matches the entire string at first, but when this fails
(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 lookbe-
hind assertion does a single test on the last four characters. If it
fails, the match fails immediately. For long strings, this approach
makes a significant difference to the processing time.
Using multiple assertions
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" pre-
ceded by six characters, the first of which are digits and the last
three of which are not "999". For example, it doesn't match "123abc-
foo". 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".
CONDITIONAL SUBPATTERNS
It is possible to cause the matching process to obey a subpattern con-
ditionally or to choose between two alternative subpatterns, depending
on the result of an assertion, or whether a specific capturing subpat-
tern has already been matched. The two possible forms of conditional
subpattern are:
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
If the condition is satisfied, the yes-pattern is used; otherwise the
no-pattern (if present) is used. An absent no-pattern is equivalent to
an empty string (it always matches). If there are more than two alter-
natives in the subpattern, a compile-time error occurs. Each of the two
alternatives may itself contain nested subpatterns of any form, includ-
ing conditional subpatterns; the restriction to two alternatives
applies only at the level of the condition. 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 subpatterns, refer-
ences to recursion, two pseudo-conditions called DEFINE and VERSION,
and assertions.
Checking for a used subpattern by number
If the text between the parentheses consists of a sequence of digits,
the condition is true if a capturing subpattern of that number has pre-
viously matched. If there is more than one capturing subpattern with
the same number (see the earlier section about duplicate subpattern
numbers), the condition is true if any of them have matched. An alter-
native notation is to precede the digits with a plus or minus sign. In
this case, the subpattern number is relative rather than absolute. The
most recently opened parentheses can be referenced by (?(-1), the next
most recent by (?(-2), and so on. Inside loops it can also make sense
to refer to subsequent groups. The next parentheses to be opened can be
referenced as (?(+1), and so on. (The value zero in any of these forms
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 sec-
ond part matches one or more characters that are not parentheses. The
third part is a conditional subpattern that tests whether or not the
first set of parentheses matched. If they did, that is, if subject
started with an opening parenthesis, the condition is true, and so the
yes-pattern is executed and a closing parenthesis is required. Other-
wise, since no-pattern is not present, the subpattern matches nothing.
In other words, this pattern matches a sequence of non-parentheses,
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 subpattern by name
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
used subpattern by name. For compatibility with earlier versions of
PCRE1, which had this facility before Perl, the syntax (?(name)...) is
also recognized. Note, however, that undelimited names consisting of
the letter R followed by digits are ambiguous (see the following sec-
tion).
Rewriting the above example to use a named subpattern gives this:
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
If the name used in a condition of this kind is a duplicate, the test
is applied to all subpatterns of the same name, and is true if any one
of them has matched.
Checking for pattern recursion
"Recursion" in this sense refers to any subroutine-like call from one
part of the pattern to another, whether or not it is actually recur-
sive. See the sections entitled "Recursive patterns" and "Subpatterns
as subroutines" below for details of recursion and subpattern calls.
If a condition is the string (R), and there is no subpattern with the
name R, the condition is true if matching is currently in a recursion
or subroutine call to the whole pattern or any subpattern. If digits
follow the letter R, and there is no subpattern with that name, the
condition is true if the most recent call is into a subpattern with the
given number, which must exist somewhere in the overall pattern. This
is a contrived example that is equivalent to a+b:
((?(R1)a+|(?1)b))
However, in both cases, if there is a subpattern with a matching name,
the condition tests for its being set, as described in the section
above, instead of testing for recursion. For example, creating a group
with the name R1 by adding (?<R1>) to the above pattern completely
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 subpattern
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 subpatterns of the same name, and
is true if any one of them is the most recent recursion.
At "top level", all these recursion test conditions are false.
Defining subpatterns for use by reference only
If the condition is the string (DEFINE), the condition is always false,
even if there is a group with the name DEFINE. In this case, there may
be only one alternative in the subpattern. It is always skipped if con-
trol reaches this point in the pattern; the idea of DEFINE is that it
can be used to define subroutines that can be referenced from else-
where. (The use of subroutines is described below.) For example, a pat-
tern to match an IPv4 address such as "192.168.23.245" could be written
like this (ignore white space and line breaks):
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
\b (?&byte) (\.(?&byte)){3} \b
The first part of the pattern is a DEFINE group inside which a another
group named "byte" is defined. This matches an individual component of
an IPv4 address (a number less than 256). When matching takes place,
this part of the pattern is skipped because DEFINE acts like a false
condition. The rest of the pattern uses references to the named group
to match the four dot-separated components of an IPv4 address, insist-
ing on a word boundary at each end.
Checking the PCRE2 version
Programs that link with a PCRE2 library can check the version by call-
ing pcre2_config() with appropriate arguments. Users of applications
that do not have access to the underlying code cannot do this. A spe-
cial "condition" called VERSION exists to allow such users to discover
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 an
assertion. This may be a positive or negative lookahead or lookbehind
assertion. Consider this pattern, again containing non-significant
white space, and with the two alternatives on the second line:
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
The condition is a positive lookahead assertion that matches an
optional sequence of non-letters followed by a letter. In other words,
it tests for the presence of at least one letter in the subject. If a
letter is found, the subject is matched against the first alternative;
otherwise it is matched against the second. This pattern matches
strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
letters and dd are digits.
When an assertion that is a condition contains capturing subpatterns,
any capturing that occurs in a matching branch is retained afterwards,
for both positive and negative assertions, because matching always con-
tinues after the assertion, whether it succeeds or fails. (Compare non-
conditional assertions, when captures are retained only for positive
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 subpattern name or number. The characters
that make up a comment play no part in the pattern matching.
The sequence (?# marks the start of a comment that continues up to the
next closing parenthesis. Nested parentheses are not permitted. If the
PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, an unescaped #
character also introduces a comment, which in this case continues to
immediately after the next newline character or character sequence in
the 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 "New-
line conventions" above. Note that the end of this type of comment is a
literal newline sequence in the pattern; escape sequences that happen
to represent a newline do not count. For example, consider this pattern
when PCRE2_EXTENDED is set, and the default newline convention (a sin-
gle linefeed character) is in force:
abc #comment \n still comment
On encountering the # character, pcre2_compile() skips along, looking
for a newline in the pattern. The sequence \n is still literal at this
stage, so it does not terminate the comment. Only an actual character
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 expres-
sions to recurse (amongst other things). It does this by interpolating
Perl code in the expression at run time, and the code can refer to the
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 pat-
tern, and also for individual subpattern recursion. After its introduc-
tion in PCRE1 and Python, this kind of recursion was subsequently
introduced into Perl at release 5.10.
A special item that consists of (? followed by a number greater than
zero and a closing parenthesis is a recursive subroutine call of the
subpattern of the given number, provided that it occurs inside that
subpattern. (If not, it is a non-recursive subroutine call, which is
described in the next section.) The special item (?R) or (?0) is a
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 parenthe-
sized substring). Finally there is a closing parenthesis. Note the use
of a possessive quantifier to avoid backtracking into sequences of non-
parentheses.
If this were part of a larger pattern, you would not want to recurse
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 subpattern numbers are in use, rel-
ative references refer to the earliest subpattern with the appropriate
number. Consider, for example:
(?|(a)|(b)) (c) (?-2)
The first two capturing groups (a) and (b) are both numbered 1, and
group (c) is number 2. When the reference (?-2) is encountered, the
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 subsequently opened parentheses, by
writing references such as (?+2). However, these cannot be recursive
because the reference is not inside the parentheses that are refer-
enced. They are always non-recursive subroutine calls, as described in
the next section.
An alternative approach is to use named parentheses. The Perl syntax
for this is (?&name); PCRE1's earlier syntax (?P>name) is also sup-
ported. We could rewrite the above example as follows:
(?<pn> \( ( [^()]++ | (?&pn) )* \) )
If there is more than one subpattern with the same name, the earliest
one is used.
The example pattern that we have been looking at contains nested unlim-
ited repeats, and so the use of a possessive quantifier for matching
strings of non-parentheses is important when applying the pattern to
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 documenta-
tion). If the pattern above is matched against
(ab(cd)ef)
the value for the inner capturing parentheses (numbered 2) is "ef",
which is the last value taken on at the top level. If a capturing sub-
pattern is not matched at the top level, its final captured value is
unset, even if it was (temporarily) set at a deeper level during the
matching process.
Do not confuse the (?R) item with the condition (R), which tests for
recursion. Consider this pattern, which matches text in angle brack-
ets, allowing for arbitrary nesting. Only digits are allowed in nested
brackets (that is, when recursing), whereas any characters are permit-
ted at the outer level.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
In this pattern, (?(R) is the start of a conditional subpattern, with
two different alternatives for the recursive and non-recursive cases.
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 subpattern call was always treated as an atomic
group. That is, once it had matched some of the subject string, it was
never re-entered, even if it contained untried alternatives and there
was a subsequent matching failure. (Historical note: PCRE implemented
recursion before Perl did.)
Starting with release 10.30, recursive subroutine calls are no longer
treated as atomic. That is, they can be re-entered to try unused alter-
natives if there is a matching failure later in the pattern. This is
now compatible with the way Perl works. If you want a subroutine call
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 pat-
tern has to ignore all non-word characters, which can be done like
this:
^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$
If run with the PCRE2_CASELESS option, this pattern matches phrases
such as "A man, a plan, a canal: Panama!". Note the use of the posses-
sive quantifier *+ to avoid backtracking into sequences of non-word
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 subpattern was called recursively or as a subpattern (see the
next section), it had no access to any values that were captured out-
side the recursion, whereas in PCRE2 these values can be referenced.
Consider this pattern:
^(.)(\1|a(?2))
This pattern matches "bab". The first capturing parentheses match "b",
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.
SUBPATTERNS AS SUBROUTINES
If the syntax for a recursive subpattern call (either by number or by
name) is used outside the parentheses to which it refers, it operates a
bit like a subroutine in a programming language. More accurately, PCRE2
treats the referenced subpattern as an independent subpattern which it
tries to match at the current matching position. The called subpattern
may be defined before or after the reference. A numbered reference can
be absolute or relative, as in these examples:
(...(absolute)...)...(?2)...
(...(relative)...)...(?-1)...
(...(?+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 subpat-
tern is defined, so if it is used as a subroutine, such options cannot
be changed for different calls. For example, consider this pattern:
(abc)(?i:(?-1))
It matches "abcabc". It does not match "abcABC" because the change of
processing option does not affect the called subpattern.
The behaviour of backtracking control verbs in subpatterns when called
as subroutines is described in the section entitled "Backtracking verbs
in subroutines" below.
ONIGURUMA SUBROUTINE SYNTAX
For compatibility with Oniguruma, the non-Perl syntax \g followed by a
name or a number enclosed either in angle brackets or single quotes, is
an alternative syntax for referencing a subpattern as a subroutine,
possibly recursively. Here are two of the examples used above, rewrit-
ten using this syntax:
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
(sens|respons)e and \g'1'ibility
PCRE2 supports an extension to Oniguruma: if a number is preceded by a
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 sub-
strings that match the same pair of parentheses when there is a repeti-
tion.
PCRE2 provides a similar feature, but of course it cannot obey arbi-
trary Perl code. The feature is called "callout". The caller of PCRE2
provides an external function by putting its entry point in a match
context using the function pcre2_set_callout(), and then passing that
context to pcre2_match() or pcre2_dfa_match(). If no match context is
passed, or if the callout entry point is set to NULL, callouts are dis-
abled.
Within a regular expression, (?C<arg>) indicates a point at which the
external function is to be called. There are two kinds of callout:
those with a numerical argument and those with a string argument. (?C)
on its own with no argument is treated as (?C0). A numerical argument
allows the application to distinguish between different callouts.
String arguments were added for release 10.20 to make it possible for
script languages that use PCRE2 to embed short scripts within patterns
in a similar way to Perl.
During matching, when PCRE2 reaches a callout point, the external func-
tion is called. It is provided with the number or string argument of
the callout, the position in the pattern, and one item of data that is
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 pat-
tern whose condition is an assertion, an additional callout is inserted
just before the condition. An explicit callout may also be set at this
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 argu-
ment. The starting delimiter must be one of ` ' " ^ % # $ { and the
ending delimiter is the same as the start, except for {, where the end-
ing delimiter is }. If the ending delimiter is needed within the
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, possibly behaving differently depending on
whether or not a name is present.
By default, for compatibility with Perl, a name is any sequence of
characters that does not include a closing parenthesis. The name is not
processed in any way, and it is not possible to include a closing
parenthesis in the name. This can be changed by setting the
PCRE2_ALT_VERBNAMES option, but the result is no longer Perl-compati-
ble.
When PCRE2_ALT_VERBNAMES is set, backslash processing is applied to
verb names and only an unescaped closing parenthesis terminates the
name. However, the only backslash items that are permitted are \Q, \E,
and sequences such as \x{100} that define character code points. Char-
acter type escapes such as \d are faulted.
A closing parenthesis can be included in a name either as \) or between
\Q and \E. In addition to backslash processing, if the PCRE2_EXTENDED
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 pat-
tern.
Since these verbs are specifically related to backtracking, most of
them can be used only when the pattern is to be matched using the tra-
ditional 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
subpatterns called as subroutines (whether or not recursively) is docu-
mented 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_com-
pile(), or by starting the pattern with (*NO_START_OPT). There is more
discussion of this option in the section entitled "Compiling a pattern"
in the pcre2api documentation.
Experiments with Perl suggest that it too has similar optimizations,
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 subpattern that is called
as a subroutine, only that subpattern is ended successfully. Matching
then continues at the outer level. If (*ACCEPT) in triggered in a posi-
tive assertion, the assertion succeeds; in a negative assertion, the
assertion fails.
If (*ACCEPT) is inside capturing parentheses, the data so far is cap-
tured. For example:
A((?:A|B(*ACCEPT)|C)D)
This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap-
tured by the outer parentheses.
(*FAIL) or (*FAIL:NAME)
This verb causes a matching failure, forcing backtracking to occur. It
may be abbreviated to (*F). It is equivalent to (?!) but easier to
read. The Perl documentation notes that it is probably useful only when
combined with (?{}) or (??{}). Those are, of course, Perl features that
are not present in PCRE2. The nearest equivalent is the callout fea-
ture, 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 exactly the same as
(*MARK:NAME)(*ACCEPT) and (*MARK:NAME)(*FAIL), respectively.
Recording which path was taken
There is one verb whose main purpose is to track how a match was
arrived at, though it also has a secondary use in conjunction with
advancing the match starting point (see (*SKIP) below).
(*MARK:NAME) or (*:NAME)
A name is always required with this verb. There may be as many
instances of (*MARK) as you like in a pattern, and their names do not
have to be unique.
When a match succeeds, the name of the last-encountered (*MARK:NAME) on
the matching path is passed back to the caller as described in the sec-
tion entitled "Other information about the match" in the pcre2api docu-
mentation. This applies to all instances of (*MARK), including those
inside assertions and atomic groups. (There are differences in those
cases when (*MARK) is used in conjunction with (*SKIP) as described
below.)
As well as (*MARK), the (*COMMIT), (*PRUNE) and (*THEN) verbs may have
associated NAME arguments. Whichever is last on the matching path is
passed back. See below for more details of these other verbs.
Here is an example of pcre2test output, where the "mark" modifier
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 exam-
ple it indicates which of the two alternatives matched. This is a more
efficient way of obtaining this information than putting each alterna-
tive in its own capturing parentheses.
If a verb with a name is encountered in a positive assertion that is
true, the name is recorded and passed back if it is the last-encoun-
tered. This does not happen for negative assertions or failing positive
assertions.
After a partial match or a failed match, the last encountered name in
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 con-
tinues with what follows, but if there is a subsequent match failure,
causing a backtrack to the verb, a failure is forced. That is, back-
tracking cannot pass to the left of the verb. However, when one of
these verbs appears inside an atomic group or in a lookaround assertion
that is true, its effect is confined to that group, because once the
group has been matched, there is never any backtracking into it. Back-
tracking from beyond an assertion or an atomic group ignores the entire
group, and seeks a preceeding backtracking point.
These verbs differ in exactly what kind of failure occurs when back-
tracking reaches them. The behaviour described below is what happens
when the verb is not in a subroutine or an assertion. Subsequent sec-
tions 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 pat-
tern is unanchored, no further attempts to find a match by advancing
the starting point take place. If (*COMMIT) is the only backtracking
verb that is encountered, once it has been passed pcre2_match() is com-
mitted to finding a match at the current starting point, or not at all.
For example:
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)(*COM-
MIT). It is like (*MARK:NAME) in that the name is remembered for pass-
ing back to the caller. However, (*SKIP:NAME) searches only for names
set with (*MARK), ignoring those set by (*COMMIT), (*PRUNE) and
(*THEN).
If there is more than one backtracking verb in a pattern, a different
one that follows (*COMMIT) may be triggered first, so merely passing
(*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 backtrack-
ing to reach it. If the pattern is unanchored, the normal "bumpalong"
advance to the next starting character then happens. Backtracking can
occur as usual to the left of (*PRUNE), before it is reached, or when
matching to the right of (*PRUNE), but if there is no match to the
right, backtracking cannot cross (*PRUNE). In simple cases, the use of
(*PRUNE) is just an alternative to an atomic group or possessive quan-
tifier, but there are some uses of (*PRUNE) that cannot be expressed in
any other way. In an anchored pattern (*PRUNE) has the same effect as
(*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 (*COMMIT), (*PRUNE) or (*THEN).
(*SKIP)
This verb, when given without a name, is like (*PRUNE), except that if
the pattern is unanchored, the "bumpalong" advance is not to the next
character, but to the position in the subject where (*SKIP) was encoun-
tered. (*SKIP) signifies that whatever text was matched leading up to
it cannot be part of a successful match if there is a later mismatch.
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 quan-
tifer does not have the same effect as this example; although it would
suppress backtracking during the first match attempt, the second
attempt would start at the second character instead of skipping on to
"c".
(*SKIP:NAME)
When (*SKIP) has an associated name, its behaviour is modified. When
such a (*SKIP) is triggered, the previous path through the pattern is
searched for the most recent (*MARK) that has the same name. If one is
found, the "bumpalong" advance is to the subject position that corre-
sponds to that (*MARK) instead of to where (*SKIP) was encountered. If
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 back-
tracking. 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 sec-
ond character. This time, the (*MARK) is never seen because "a" does
not match "b", so the matcher immediately jumps to the second branch of
the pattern.
Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It
ignores names that are set by (*COMMIT:NAME), (*PRUNE:NAME) or
(*THEN:NAME).
(*THEN) or (*THEN:NAME)
This verb causes a skip to the next innermost alternative when back-
tracking reaches it. That is, it cancels any further backtracking
within the current alternative. Its name comes from the observation
that it can be used for a pattern-based if-then-else block:
( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
If the COND1 pattern matches, FOO is tried (and possibly further items
after the end of the group if FOO succeeds); on failure, the matcher
skips to the second alternative and tries COND2, without backtracking
into COND1. If that succeeds and BAR fails, COND3 is tried. If subse-
quently BAZ fails, there are no more alternatives, so there is a back-
track to whatever came before the entire group. If (*THEN) is not
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 (*COMMIT), (*PRUNE) and (*THEN).
A subpattern that does not contain a | character is just a part of the
enclosing alternative; it is not a nested alternation with only one
alternative. The effect of (*THEN) extends beyond such a subpattern to
the enclosing alternative. Consider this pattern, where A, B, etc. are
complex pattern fragments that do not contain any | characters at this
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 subpattern containing (*THEN) is given an alternative,
it behaves differently:
A (B(*THEN)C | (*FAIL)) | D
The effect of (*THEN) is now confined to the inner subpattern. After a
failure in C, matching moves to (*FAIL), which causes the whole subpat-
tern to fail because there are no more alternatives to try. In this
case, matching does now backtrack into A.
Note that a conditional subpattern is not considered as having two
alternatives, because only one is ever used. In other words, the |
character in a conditional subpattern has a different meaning. Ignoring
white space, consider:
^.*? (?(?=a) a | b(*THEN)c )
If the subject is "ba", this pattern does not match. Because .*? is
ungreedy, it initially matches zero characters. The condition (?=a)
then fails, the character "b" is matched, but "c" is not. At this
point, matching does not backtrack to .*? as might perhaps be expected
from the presence of the | character. The conditional subpattern is
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 pat-
tern, where A, B, etc. are complex pattern fragments:
(A(*COMMIT)B(*THEN)C|ABD)
If A matches but B fails, the backtrack to (*COMMIT) causes the entire
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 subpattern.
(*ACCEPT) in a standalone positive assertion causes the assertion to
succeed without any further processing; captured strings and a (*MARK)
name (if set) are retained. In a standalone negative assertion,
(*ACCEPT) causes the assertion to fail without any further processing;
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 their effect is confined to the assertion,
because lookaround assertions are atomic. A backtrack that occurs after
an assertion is complete does not jump back into the assertion. Note in
particular that a (*MARK) name that is set in an assertion is not
"seen" by an instance of (*SKIP:NAME) latter in the pattern.
The effect of (*THEN) is not allowed to escape beyond an assertion. If
there are no more branches to try, (*THEN) causes a positive assertion
to be false, and a negative assertion to be true.
The other backtracking verbs are not treated specially if they appear
in a standalone positive assertion. In a conditional positive asser-
tion, backtracking (from within the assertion) into (*COMMIT), (*SKIP),
or (*PRUNE) causes the condition to be false. However, for both stand-
alone and conditional negative assertions, backtracking into (*COMMIT),
(*SKIP), or (*PRUNE) causes the assertion to be true, without consider-
ing any further alternative branches.
Backtracking verbs in subroutines
These behaviours occur whether or not the subpattern is called recur-
sively.
(*ACCEPT) in a subpattern called as a subroutine causes the subroutine
match to succeed without any further processing. Matching then contin-
ues after the subroutine call. Perl documents this behaviour. Perl's
treatment of the other verbs in subroutines is different in some cases.
(*FAIL) in a subpattern 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 subpattern called as a sub-
routine. There is then a backtrack at the outer level.
(*THEN), when triggered, skips to the next alternative in the innermost
enclosing group within the subpattern that has alternatives (its normal
behaviour). However, if there is no such group within the subroutine
subpattern, 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
University Computing Service
Cambridge, England.
REVISION
Last updated: 04 September 2018
Copyright (c) 1997-2018 University of Cambridge.
PCRE2 10.32 04 September 2018 PCRE2PATTERN(3)
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