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PCREPATTERN(3) Library Functions Manual PCREPATTERN(3)

NAME
PCRE - Perl-compatible regular expressions

PCRE REGULAR EXPRESSION DETAILS

The syntax and semantics of the regular expressions that are supported by PCRE are described
in detail below. There is a quick-reference syntax summary in the pcresyntax page. PCRE tries
to match Perl syntax and semantics as closely as it can. PCRE also supports some alternative
regular expression syntax (which does not conflict with the Perl syntax) in order to provide
some compatibility with regular expressions in Python, .NET, and Oniguruma.

Perl's regular expressions are described in its own documentation, and regular expressions in
general are covered in a number of books, some of which have copious examples. Jeffrey
Friedl's "Mastering Regular Expressions", published by O'Reilly, covers regular expressions
in great detail. This description of PCRE's regular expressions is intended as reference ma‐
terial.

This document discusses the patterns that are supported by PCRE when one its main matching
functions, pcre_exec() (8-bit) or pcre[16|32]_exec() (16- or 32-bit), is used. PCRE also has
alternative matching functions, pcre_dfa_exec() and pcre[16|32_dfa_exec(), which match using
a different algorithm that is not Perl-compatible. Some of the features discussed below are
not available when DFA matching is used. The advantages and disadvantages of the alternative
functions, and how they differ from the normal functions, are discussed in the pcrematching
page.

SPECIAL START-OF-PATTERN ITEMS

A number of options that can be passed to pcre_compile() can also be set by special items at
the start of a pattern. These are not Perl-compatible, but are provided to make these options
accessible to pattern writers who are not able to change the program that processes the 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

The original operation of PCRE was on strings of one-byte characters. However, there is now
also support for UTF-8 strings in the original library, an extra library that supports 16-bit
and UTF-16 character strings, and a third library that supports 32-bit and UTF-32 character
strings. To use these features, PCRE must be built to include appropriate support. When using
UTF strings you must either call the compiling function with the PCRE_UTF8, PCRE_UTF16, or
PCRE_UTF32 option, or the pattern must start with one of these special sequences:

(*UTF8)
(*UTF16)
(*UTF32)
(*UTF)

(*UTF) is a generic sequence that can be used with any of the libraries. Starting a pattern
with such a sequence is equivalent to setting the relevant option. How setting a UTF mode af‐
fects pattern matching is mentioned in several places below. There is also a summary of fea‐
tures in the pcreunicode page.

Some applications that allow their users to supply patterns may wish to restrict them to non-
UTF data for security reasons. If the PCRE_NEVER_UTF option is set at compile time, (*UTF)
etc. are not allowed, and their appearance 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 PCRE_UCP option: it causes sequences such as \d and \w to use Uni‐
code properties to determine character types, instead of recognizing only characters with
codes less than 128 via a lookup table.

Disabling auto-possessification

If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting the
PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE from making quantifiers posses‐
sive when what follows cannot match the repeated item. For example, by default a+b is treated
as a++b. For more details, see the pcreapi documentation.

Disabling start-up optimizations

If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
PCRE_NO_START_OPTIMIZE option either at compile or matching time. This disables several opti‐
mizations for quickly reaching "no match" results. For more details, see the pcreapi documen‐
tation.

Newline conventions

PCRE supports five different conventions for indicating line breaks in strings: a single CR
(carriage return) character, a single LF (linefeed) character, the two-character sequence
CRLF, any of the three preceding, or any Unicode newline sequence. The pcreapi page has fur‐
ther discussion about newlines, and shows how to set the newline convention in the options
arguments for the compiling and matching functions.

It is also possible to specify a newline convention by starting a pattern string with one of
the following five sequences:

(*CR) carriage return
(*LF) linefeed
(*CRLF) carriage return, followed by linefeed
(*ANYCRLF) any of the three above
(*ANY) all Unicode newline sequences

These override the default and the options given to the compiling function. For example, on a
Unix system where LF is the default newline sequence, the pattern

(*CR)a.b

changes the convention to CR. That pattern matches "a\nb" because LF is no longer a newline.
If more than one of these settings is present, the last one is used.

The newline convention affects where the circumflex and dollar assertions are true. It also
affects the interpretation of the dot metacharacter when PCRE_DOTALL is not set, and the be‐
haviour of \N. 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 description of \R in the section entitled "Newline sequences" below. A change of \R
setting can be combined with a change of newline convention.

Setting match and recursion limits

The caller of pcre_exec() can set a limit on the number of times the internal match() func‐
tion is called and on the maximum depth of recursive calls. These facilities are provided to
catch runaway matches that are provoked by patterns with huge matching trees (a typical exam‐
ple is a pattern with nested unlimited repeats) and to avoid running out of system stack by
too much recursion. When one of these limits is reached, pcre_exec() gives an error return.
The limits can also be set by items at the start of the pattern of the form

(*LIMIT_MATCH=d)
(*LIMIT_RECURSION=d)

where d is any number of decimal digits. However, the value of the setting must be less than
the value set (or defaulted) by the caller of pcre_exec() 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.

EBCDIC CHARACTER CODES

PCRE can be compiled to run in an environment that uses EBCDIC as its character code rather
than ASCII or Unicode (typically a mainframe system). In the sections below, character code
values are ASCII or Unicode; in an EBCDIC environment these characters may have different
code values, and there are no code points greater than 255.

CHARACTERS AND METACHARACTERS

A regular expression is a pattern that is matched against a subject string from left to
right. Most characters stand for themselves in a pattern, and match the corresponding charac‐
ters 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 PCRE_CASELESS option), letters are matched independently of case. In a UTF
mode, PCRE always understands the concept of case for characters whose values are less than
128, so caseless matching is always possible. For characters with higher values, the concept
of case is supported if PCRE is compiled with Unicode property support, but not otherwise.
If you want to use caseless matching for characters 128 and above, you must ensure that PCRE
is compiled with Unicode property support as well as with UTF support.

The power of regular expressions comes from the ability to include alternatives and repeti‐
tions in the pattern. These are encoded in the pattern by the use of metacharacters, which do
not stand for themselves but instead are interpreted in some special way.

There are two different sets of metacharacters: those that are recognized anywhere in the
pattern except within square brackets, and those that are recognized within square brackets.
Outside square brackets, the metacharacters are as follows:

\ general escape character with several uses
^ assert start of string (or line, in multiline mode)
$ assert end of string (or line, in multiline mode)
. match any character except newline (by default)
[ start character class definition
| start of alternative branch
( start 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 write \* in the pattern. This escaping
action applies whether or not the following character would otherwise be interpreted as a
metacharacter, so it is always safe to precede a non-alphanumeric with backslash to specify
that it stands for itself. In particular, if you want to match a backslash, you write \\.

In a UTF mode, only ASCII numbers and letters have any special meaning after a backslash. All
other characters (in particular, those whose codepoints are greater than 127) are treated as
literals.

If a pattern is compiled with the PCRE_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 characters, you can do so by
putting them between \Q and \E. This is different from Perl in that $ and @ are handled as
literals in \Q...\E sequences in PCRE, whereas in Perl, $ and @ cause variable interpolation.
Note the following examples:

Pattern PCRE 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

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 char‐
acter class is not terminated.

Non-printing characters

A second use of backslash provides a way of encoding non-printing characters in patterns in a
visible manner. There is no restriction on the appearance of non-printing characters, apart
from the binary zero that terminates 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 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 back reference
\o{ddd..} character with octal code ddd..
\xhh character with hex code hh
\x{hhh..} character with hex code hhh.. (non-JavaScript mode)
\uhhhh character with hex code hhhh (JavaScript mode only)

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 data item (byte or 16-bit value) following \c has a value greater
than 127, a compile-time error occurs. This locks out non-ASCII characters in all modes.

When PCRE is compiled in EBCDIC mode, \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 docu‐
ment. 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 \@ encodes character
code 0; 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 \? becomes either 255 (hex FF) or 95
(hex 5F).

Thus, apart from \?, 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, \G always
generates code value 7, which is BEL in ASCII but DEL in EBCDIC.

The sequence \? 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, PCRE makes \? 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 fol‐
lowed 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 back references 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 character numbers, and \g{} to specify back refer‐
ences. The following paragraphs describe the old, ambiguous syntax.

The handling of a backslash followed by a digit other than 0 is complicated, and Perl has
changed in recent releases, causing PCRE also to change. Outside a character class, PCRE
reads the digit and any following digits as a decimal number. If the number is less than 8,
or if there have been at least that many previous capturing left parentheses in the expres‐
sion, the entire sequence is taken as a back reference. A description of how this works is
given later, following the discussion of parenthesized subpatterns.

Inside a character class, or if the decimal number following \ is greater than 7 and there
have not been that many capturing subpatterns, PCRE handles \8 and \9 as the literal charac‐
ters "8" and "9", and otherwise re-reads up to three octal digits following the backslash,
using them to generate a data character. Any subsequent digits stand for themselves. For ex‐
ample:

\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 back reference
\11 might be a back reference, 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 back reference, otherwise the
character with octal code 113
\377 might be a back reference, otherwise
the value 255 (decimal)
\81 is either a back reference, or the two
characters "8" and "1"

Note that octal values of 100 or greater that are specified using this syntax must not be in‐
troduced 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 hexadecimal digits are read
(letters can be in upper or lower case). Any number of hexadecimal digits may appear between
\x{ and }. If a character other than a hexadecimal digit appears between \x{ and }, or if
there is no terminating }, an error occurs.

If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is as just described
only when it is followed by two hexadecimal digits. Otherwise, it matches a literal "x"
character. In JavaScript 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 JavaScript mode). There is no difference in the way they are handled. For exam‐
ple, \xdc is exactly the same as \x{dc} (or \u00dc in JavaScript mode).

Constraints on character values

Characters that are specified using octal or hexadecimal numbers are limited to certain val‐
ues, as follows:

8-bit non-UTF mode less than 0x100
8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
16-bit non-UTF mode less than 0x10000
16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
32-bit non-UTF mode less than 0x100000000
32-bit UTF-32 mode less than 0x10ffff and a valid codepoint

Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called "surrogate" code‐
points), and 0xffef.

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).

\N is not allowed in a character class. \B, \R, and \X are not special inside a character
class. Like other unrecognized escape sequences, they are treated as the literal characters
"B", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is set. Outside a
character class, these sequences have different meanings.

Unsupported escape sequences

In Perl, the sequences \l, \L, \u, and \U are recognized by its string handler and used to
modify the case of following characters. By default, PCRE does not support these escape se‐
quences. However, if the PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character,
and \u can be used to define a character by code point, as described in the previous section.

Absolute and relative back references

The sequence \g followed by an unsigned or a negative number, optionally enclosed in braces,
is an absolute or relative back reference. A named back reference can be coded as \g{name}.
Back references 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 en‐
closed 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 back reference; 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
\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

There is also the single sequence \N, which matches a non-newline character. This is the
same as the "." metacharacter when PCRE_DOTALL is not set. Perl also uses \N to match charac‐
ters by name; PCRE does not support this.

Each pair of lower and upper case escape sequences partitions the complete set of characters
into two disjoint sets. Any given character matches one, and only one, of each pair. The se‐
quences 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.

For compatibility with Perl, \s did not used to match the VT character (code 11), which made
it different from the the POSIX "space" class. However, Perl added VT at release 5.18, and
PCRE followed suit at release 8.34. The default \s characters are now HT (9), LF (10), VT
(11), FF (12), CR (13), and space (32), which are defined as white space in the "C" locale.
This list may vary if locale-specific matching is taking place. For example, in some locales
the "non-breaking space" character (\xA0) is recognized as white space, and in others the VT
character is not.

A "word" character is an underscore or any character that is a letter or digit. By default,
the definition of letters and digits is controlled by PCRE's low-valued character tables, and
may vary if locale-specific matching is taking place (see "Locale support" in the pcreapi
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 vary for characters in the range 128-255 when
locale-specific matching is happening. These escape sequences retain their original meanings
from before Unicode support was available, mainly for efficiency reasons. If PCRE is compiled
with Unicode property support, and the PCRE_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 deci‐
mal digits, whereas \w matches any Unicode digit, as well as any Unicode letter, and under‐
score. Note also that PCRE_UCP affects \b, and \B because they are defined in terms of \w and
\W. Matching these sequences is noticeably slower when PCRE_UCP is set.

The sequences \h, \H, \v, and \V are features that were added to Perl at release 5.10. In
contrast to the other sequences, which match only ASCII characters by default, these always
match certain high-valued code points, whether or not PCRE_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 codepoints less than 256 are relevant.

Newline sequences

Outside a character class, by default, the escape sequence \R matches any Unicode newline se‐
quence. 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 char‐
acters LF (linefeed, U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (carriage
return, U+000D), or NEL (next line, U+0085). The two-character sequence is treated as a sin‐
gle unit that cannot be split.

In other modes, two additional characters whose codepoints are greater than 255 are added: LS
(line separator, U+2028) and PS (paragraph separator, U+2029). Unicode character property
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 PCRE_BSR_ANYCRLF either at compile time or when
the pattern is matched. (BSR is an abbrevation for "backslash R".) This can be made the de‐
fault when PCRE is built; if this is the case, the other behaviour can be requested via the
PCRE_BSR_UNICODE option. It is also possible to specify these settings by starting a pattern
string with one of the following sequences:

(*BSR_ANYCRLF) CR, LF, or CRLF only
(*BSR_UNICODE) any Unicode newline sequence

These override the default and the options given to the compiling function, but they can
themselves be overridden by options given to a matching function. Note that these special
settings, which are not Perl-compatible, are recognized only at the very start of a pattern,
and that they must be in upper case. If more than one of them is present, the last one is
used. They can be combined with a change of newline convention; for example, a pattern can
start with:

(*ANY)(*BSR_ANYCRLF)

They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or (*UCP) special se‐
quences. Inside a character class, \R is treated as an unrecognized escape sequence, and so
matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.

Unicode character properties

When PCRE is built with Unicode character property support, three additional escape sequences
that match characters with specific properties are available. When in 8-bit non-UTF-8 mode,
these sequences are of course limited to testing characters whose codepoints are less than
256, but they do work in this mode. 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 gen‐
eral category properties, "Any", which matches any character (including newline), and some
special PCRE properties (described in the next section). Other Perl properties such as "In‐
MusicalSymbols" are not currently supported by PCRE. 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:

Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak, Bengali, Bopomofo, Brahmi,
Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Caucasian_Albanian, Chakma, Cham,
Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Duployan, Egyp‐
tian_Hieroglyphs, Elbasan, Ethiopic, Georgian, Glagolitic, Gothic, Grantha, Greek, Gujarati,
Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Imperial_Aramaic, Inherited, Inscrip‐
tional_Pahlavi, Inscriptional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li,
Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Linear_A, Linear_B, Lisu,
Lycian, Lydian, Mahajani, Malayalam, Mandaic, Manichaean, Meetei_Mayek, Mende_Kikakui,
Meroitic_Cursive, Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro, Myanmar, Nabataean,
New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Italic, Old_North_Arabian, Old_Permic, Old_Persian,
Old_South_Arabian, Old_Turkic, Oriya, Osmanya, Pahawh_Hmong, Palmyrene, Pau_Cin_Hau,
Phags_Pa, Phoenician, Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra, Sharada, Sha‐
vian, Siddham, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Tirhuta,
Ugaritic, Vai, Warang_Citi, Yi.

Each character has exactly one Unicode general category property, specified by a two-letter
abbreviation. For compatibility with Perl, negation can be specified by including a circum‐
flex between the opening brace and the property name. For example, \p{^Lu} is the same as
\P{Lu}.

If only one letter is specified with \p or \P, it includes all the general category proper‐
ties that start with that letter. In this case, in the absence of negation, the curly brack‐
ets 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 PCRE, unless UTF va‐
lidity checking has been turned off (see the discussion of PCRE_NO_UTF8_CHECK,
PCRE_NO_UTF16_CHECK and PCRE_NO_UTF32_CHECK in the pcreapi page). Perl does not support the
Cs property.

The long synonyms for property names that Perl supports (such as \p{Letter}) are not sup‐
ported by PCRE, nor is it permitted to prefix any of these properties with "Is".

No character that is in the Unicode table has the Cn (unassigned) property. Instead, this
property is assumed for any code point that is not in the Unicode table.

Specifying caseless matching does not affect these escape sequences. For example, \p{Lu} al‐
ways matches only upper case letters. This is different from the behaviour of current ver‐
sions of Perl.

Matching characters by Unicode property is not fast, because PCRE has to do a multistage ta‐
ble lookup in order to find a character's property. That is why the traditional escape se‐
quences such as \d and \w do not use Unicode properties in PCRE by default, though you can
make them do so by setting the PCRE_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 clus‐
ter", and treats the sequence as an atomic group (see below). Up to and including release
8.31, PCRE matched an earlier, simpler definition that was equivalent to

(?>\PM\pM*)

That is, it matched a character without the "mark" property, followed by zero or more charac‐
ters with the "mark" property. Characters with the "mark" property are typically non-spacing
accents that affect the preceding character.

This simple definition was extended in Unicode to include more complicated kinds of composite
character by giving each character a grapheme breaking property, and creating rules that use
these properties to define the boundaries of extended grapheme clusters. In releases of PCRE
later than 8.31, \X matches one of these clusters.

\X always matches at least one character. Then it decides whether to add additional charac‐
ters according to the following rules for ending a cluster:

1. End at the end of the subject string.

2. Do not end between CR and LF; otherwise end after any control character.

3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters are of five
types: L, V, T, LV, and LVT. An L character may be followed by an L, V, LV, or LVT character;
an LV or V character may be followed by a V or T character; an LVT or T character may be
follwed only by a T character.

4. Do not end before extending characters or spacing marks. Characters with the "mark" prop‐
erty always have the "extend" grapheme breaking property.

5. Do not end after prepend characters.

6. Otherwise, end the cluster.

PCRE's additional properties

As well as the standard Unicode properties described above, PCRE supports four more that make
it possible to convert traditional escape sequences such as \w and \s to use Unicode proper‐
ties. PCRE uses these non-standard, non-Perl properties internally when PCRE_UCP is set. How‐
ever, they may also be used explicitly. These properties are:

Xan Any alphanumeric character
Xps Any POSIX space character
Xsp Any Perl space character
Xwd Any Perl "word" character

Xan matches characters that have either the L (letter) or the N (number) property. Xps
matches the characters tab, linefeed, vertical tab, form feed, or carriage return, and any
other character that has the Z (separator) property. Xsp is the same as Xps; it used to ex‐
clude vertical tab, for Perl compatibility, but Perl changed, and so PCRE followed at release
8.34. Xwd matches the same characters as Xan, plus underscore.

There is another non-standard property, Xuc, which matches any character that can be repre‐
sented by a Universal Character Name in C++ and other programming languages. These are the
characters $, @, ` (grave accent), and all characters with Unicode code points greater than
or equal to U+00A0, except for the surrogates U+D800 to U+DFFF. Note that most base (ASCII)
characters are excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH
where H is a hexadecimal digit. Note that the Xuc property does not match these sequences but
the characters that they represent.)

Resetting the match start

The escape sequence \K causes any previously matched characters not to be included in the fi‐
nal matched sequence. For example, the pattern:

foo\Kbar

matches "foobar", but reports that it has matched "bar". This feature is similar to a lookbe‐
hind assertion (described below). However, in this case, the part of the subject before the
real match does not have to be of fixed length, as lookbehind assertions do. The use of \K
does not interfere with the setting of captured substrings. For example, when the pattern

(foo)\Kbar

matches "foobar", the first substring is still set to "foo".

Perl documents that the use of \K within assertions is "not well defined". In PCRE, \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.

Simple assertions

The final use of backslash is for certain simple assertions. An assertion specifies a condi‐
tion 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, by default it matches the corre‐
sponding literal character (for example, \B matches the letter B). However, if the PCRE_EXTRA
option is set, an "invalid escape sequence" error is generated instead.

A word boundary is a position in the subject string where the current character and the pre‐
vious 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 PCRE_UCP option. When this
is done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start of word" or
"end of word" metasequence. However, whatever follows \b normally determines which it is. For
example, the fragment \ba matches "a" at the start of a word.

The \A, \Z, and \z assertions differ from the traditional circumflex and dollar (described in
the next section) in that they only ever match at the very start and end of the subject
string, whatever options are set. Thus, they are independent of multiline mode. These three
assertions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the
behaviour of the circumflex and dollar metacharacters. However, if the startoffset argument
of pcre_exec() 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
match, as specified by the startoffset argument of pcre_exec(). It differs from \A when the
value of startoffset is non-zero. By calling pcre_exec() multiple times with appropriate ar‐
guments, you can mimic Perl's /g option, and it is in this kind of implementation where \G
can be useful.

Note, however, that PCRE's interpretation of \G, as the start of the current match, is subtly
different from Perl's, which defines it as the end of the previous match. In Perl, these can
be different when the previously matched string was empty. Because PCRE 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 start‐
ing match position, and the "anchored" flag is set in the compiled regular expression.

CIRCUMFLEX AND DOLLAR

The circumflex and dollar metacharacters are zero-width assertions. That is, they test for a
particular condition being true without consuming any characters from the subject string.

Outside a character class, in the default matching mode, the circumflex character is an as‐
sertion that is true only if the current matching point is at the start of the subject
string. If the startoffset argument of pcre_exec() is non-zero, circumflex can never match if
the PCRE_MULTILINE option is unset. Inside a character class, circumflex has an entirely dif‐
ferent meaning (see below).

Circumflex need not be the first character of the pattern if a number of alternatives are in‐
volved, but it should be the first thing in each alternative in which it appears if the pat‐
tern is ever to match that branch. If all possible alternatives start with a circumflex, that
is, if the pattern is constrained to match only at the start of the subject, it is said to be
an "anchored" pattern. (There are also other constructs that can cause a pattern to be an‐
chored.)

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). Note, however, that it does not actually match the newline. Dollar need not be the
last character of the pattern if a number of alternatives are involved, but it should be the
last item in any branch in which it appears. Dollar has no special meaning in a character
class.

The meaning of dollar can be changed so that it matches only at the very end of the string,
by setting the PCRE_DOLLAR_ENDONLY option at compile time. This does not affect the \Z asser‐
tion.

The meanings of the circumflex and dollar characters are changed if the PCRE_MULTILINE option
is set. When this is the case, 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. A dollar matches before any newlines in the string, as well as at the very end, when
PCRE_MULTILINE is set. When newline is specified as the two-character sequence CRLF, isolated
CR and LF characters do not indicate newlines.

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 pcre_exec() is non-zero.
The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.

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 PCRE_MULTILINE is set.

FULL STOP (PERIOD, DOT) AND \N

Outside a character class, a dot in the pattern matches any one character in the subject
string except (by default) a character that signifies the end of a line.

When a line ending is defined as a single character, dot never matches that character; when
the two-character sequence CRLF is used, dot does not match CR if it is immediately followed
by LF, but otherwise it matches all characters (including isolated CRs and LFs). When any
Unicode line endings are being recognized, dot does not match CR or LF or any of the other
line ending characters.

The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL option is
set, a dot matches any one character, without exception. If the two-character sequence CRLF
is present in the subject string, it takes two dots to match it.

The handling of dot is entirely independent of the handling of circumflex and dollar, the
only relationship being that they both involve newlines. Dot has no special meaning in a
character class.

The escape sequence \N behaves like a dot, except that it is not affected by the PCRE_DOTALL
option. In other words, it matches any character except one that signifies the end of a line.
Perl also uses \N to match characters by name; PCRE does not support this.

MATCHING A SINGLE DATA UNIT

Outside a character class, the escape sequence \C matches any one data unit, whether or not a
UTF mode is set. In the 8-bit library, one data unit is one byte; in the 16-bit library it is
a 16-bit unit; in the 32-bit library it is a 32-bit unit. Unlike a dot, \C always matches
line-ending characters. The feature is provided in Perl in order to match individual bytes in
UTF-8 mode, but it is unclear how it can usefully be used. Because \C breaks up characters
into individual data units, matching one unit with \C in a UTF mode means that the rest of
the string may start with a malformed UTF character. This has undefined results, because PCRE
assumes that it is dealing with valid UTF strings (and by default it checks this at the start
of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or PCRE_NO_UTF32_CHECK op‐
tion is used).

PCRE does not allow \C to appear in lookbehind assertions (described below) in a UTF mode,
because this would make it impossible to calculate the length of the lookbehind.

In general, the \C escape sequence is best avoided. However, one way of using it that avoids
the problem of malformed UTF characters is to use a lookahead to check the length of the next
character, as in this pattern, which could be used with a UTF-8 string (ignore white space
and line breaks):

(?| (?=[\x00-\x7f])(\C) |
(?=[\x80-\x{7ff}])(\C)(\C) |
(?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
(?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

A group that starts with (?| resets the capturing parentheses numbers in each alternative
(see "Duplicate Subpattern Numbers" below). The assertions at the start of each branch check
the next UTF-8 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively.
The character's individual bytes are then captured by the appropriate number of groups.

SQUARE BRACKETS AND CHARACTER CLASSES

An opening square bracket introduces a character class, terminated by a closing square
bracket. A closing square bracket on its own is not special by default. However, if the
PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square bracket causes a compile-time er‐
ror. 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 back‐
slash.

A character class matches a single character in the subject. In a UTF mode, the character may
be more than one data unit long. 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 no‐
tation for specifying the characters that are in the class by enumerating those that are not.
A class that starts with a circumflex is not an assertion; it still consumes a character from
the subject string, and therefore it fails if the current pointer is at the end of the
string.

In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff) can be in‐
cluded in a class as a literal string of data units, or by using the \x{ escaping mechanism.

When caseless matching is set, any letters in a class represent both their upper case and
lower case versions, so for example, a caseless [aeiou] matches "A" as well as "a", and a
caseless [^aeiou] does not match "A", whereas a caseful version would. In a UTF mode, PCRE
always understands the concept of case for characters whose values are less than 128, so
caseless matching is always possible. For characters with higher values, the concept of case
is supported if PCRE is compiled with Unicode property support, but not otherwise. If you
want to use caseless matching in a UTF mode for characters 128 and above, you must ensure
that PCRE is compiled with Unicode property support as well as with UTF support.

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
PCRE_DOTALL and PCRE_MULTILINE options is used. A class such as [^a] always matches one of
these characters.

The minus (hyphen) character can be used to specify a range of characters in a character
class. For example, [d-m] matches any letter between d and m, inclusive. If a minus character
is required in a class, it must be escaped with a backslash or appear in a position where it
cannot be interpreted as indicating a range, typically as the first or last character in the
class, or immediately after a range. For example, [b-d-z] matches letters in the range b to
d, a hyphen character, or z.

It is not possible to have the literal character "]" as the end character of a range. A pat‐
tern such as [W-]46] is interpreted as a class of two characters ("W" and "-") followed by a
literal string "46]", so it would match "W46]" or "-46]". However, if the "]" is escaped with
a backslash it is interpreted as the end of range, so [W-\]46] is interpreted as a class con‐
taining a range followed by two other characters. The octal or hexadecimal representation of
"]" can also be used to end a range.

An error is generated if a POSIX character class (see below) or an escape sequence other than
one that defines a single character appears at a point where a range ending character is ex‐
pected. For example, [z-\xff] is valid, but [A-\d] and [A-[:digit:]] are not.

Ranges operate in the collating sequence of character values. They can also be used for char‐
acters specified numerically, for example [\000-\037]. Ranges can include any characters that
are valid for the current mode.

If a range that includes letters is used when caseless matching is set, it matches the let‐
ters in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched case‐
lessly, 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. In UTF modes, PCRE supports the con‐
cept of case for characters with values greater than 128 only when it is compiled with Uni‐
code property support.

The character 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 PCRE_UCP option affects the mean‐
ings 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, \N, \R, and \X are not special inside a character class. Like
any other unrecognized escape sequences, they are treated as the literal characters "B", "N",
"R", and "X" by default, but cause an error if the PCRE_EXTRA option is set.

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. PCRE 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 PCRE 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 dif‐
ferent; there may be fewer or more of them. "Space" used to be different to \s, which did not
include VT, for Perl compatibility. However, Perl changed at release 5.18, and PCRE followed
at release 8.34. "Space" and \s now 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. PCRE (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 128 do not match any of the POSIX character
classes. However, if the PCRE_UCP option is passed to pcre_compile(), some of the classes are
changed so that Unicode character properties are used. This is achieved by replacing certain
POSIX classes by other sequences, as follows:

[:alnum:] becomes \p{Xan}
[:alpha:] becomes \p{L}
[:blank:] becomes \h
[: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 han‐
dled specially in UCP mode:

[:graph:] This matches characters that have glyphs that mark the page when printed. In Uni‐
code property terms, it matches all characters with the L, M, N, P, S, or Cf prop‐
erties, except for:

U+061C Arabic Letter Mark
U+180E Mongolian Vowel Separator
U+2066 - U+2069 Various "isolate"s

[:print:] This matches the same characters as [:graph:] plus space characters that are not
controls, that is, characters with the Zs property.

[:punct:] This matches all characters that have the Unicode P (punctuation) property, plus
those characters whose code points are less than 128 that have the S (Symbol) prop‐
erty.

The other POSIX classes are unchanged, and match only characters with code points less than
128.

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". PCRE 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 pat‐
terns. Note that \b matches at the start and the end of a word (see "Simple assertions"
above), and in a Perl-style pattern the preceding or following character normally shows which
is wanted, without the need for the assertions that are used above in order to give exactly
the POSIX behaviour.

VERTICAL BAR

Vertical bar characters are used to separate alternative patterns. For example, the pattern

gilbert|sullivan

matches either "gilbert" or "sullivan". Any number of alternatives may appear, and an empty
alternative is permitted (matching the empty string). The matching process tries each alter‐
native in turn, from left to right, and the first one that succeeds is used. If the alterna‐
tives 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 PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED options
(which are Perl-compatible) can be changed from within the pattern by a sequence of Perl op‐
tion letters enclosed between "(?" and ")". The option letters are

i for PCRE_CASELESS
m for PCRE_MULTILINE
s for PCRE_DOTALL
x for PCRE_EXTENDED

For example, (?im) sets caseless, multiline matching. It is also possible to unset these op‐
tions by preceding the letter with a hyphen, and a combined setting and unsetting such as
(?im-sx), which sets PCRE_CASELESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and
PCRE_EXTENDED, is also permitted. If a letter appears both before and after the hyphen, the
option is unset.

The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be changed in the
same way as the Perl-compatible options by using the characters J, U and X respectively.

When one of these option changes occurs at top level (that is, not inside subpattern paren‐
theses), the change applies to the remainder of the pattern that follows. If the change is
placed right at the start of a pattern, PCRE extracts it into the global options (and it will
therefore show up in data extracted by the pcre_fullinfo() function).

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 PCRE_CASELESS is not used). By this
means, options can be made to have different settings in different parts of the pattern. Any
changes made in one alternative do carry on into subsequent branches within the same subpat‐
tern. 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.

Note: There are other PCRE-specific options that can be set by the application when the com‐
piling or matching functions are called. In some cases the pattern can contain special lead‐
ing sequences such as (*CRLF) to override what the application has set or what has been de‐
faulted. Details are given in the section entitled "Newline sequences" above. There are also
the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading sequences that can be used to set UTF and
Unicode property modes; they are equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32
and the PCRE_UCP options, respectively. The (*UTF) sequence is a generic version that can be
used with any of the libraries. However, the application can set the PCRE_NEVER_UTF option,
which locks out the use of the (*UTF) 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 pat‐
tern matches, that portion of the subject string that matched the subpattern is passed back
to the caller via the ovector argument of the matching function. (This applies only to the
traditional matching functions; the DFA matching functions do 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 numbered 1, 2, and 3, re‐
spectively.

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 cap‐
turing, 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-captur‐
ing subpattern, the option letters may appear between the "?" and the ":". Thus the two pat‐
terns

(?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 set‐
ting 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-captur‐
ing subpattern. For example, consider this pattern:

(?|(Sat)ur|(Sun))day

Because the two alternatives are inside a (?| group, both sets of capturing parentheses are
numbered one. Thus, when the pattern matches, you can look at captured substring number one,
whichever alternative matched. This construct is useful when you want to capture part, but
not all, of one of a number of alternatives. Inside a (?| group, parentheses are numbered as
usual, but the number is reset at the start of each branch. The numbers of any capturing
parentheses that follow the subpattern start after the highest number used in any branch. The
following example is taken from the Perl documentation. The numbers underneath show in which
buffer the captured content will be stored.

# before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4

A back reference 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)/

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 number have matched.

An alternative approach to using this "branch reset" feature is to use duplicate named sub‐
patterns, 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 regular expressions. Furthermore, if an expression is modified,
the numbers may change. To help with this difficulty, PCRE supports the naming of subpat‐
terns. This feature was not added to Perl until release 5.10. Python had the feature earlier,
and PCRE introduced it at release 4.0, using the Python syntax. PCRE now supports both the
Perl and the Python syntax. Perl allows identically numbered subpatterns to have different
names, but PCRE does not.

In PCRE, a subpattern can be named in one of three ways: (?<name>...) or (?'name'...) as in
Perl, or (?P<name>...) as in Python. References to capturing parentheses from other parts of
the pattern, such as back references, recursion, and conditions, can be made by name as well
as by number.

Names consist of up to 32 alphanumeric characters and underscores, but must start with a non-
digit. Named capturing parentheses are still allocated numbers as well as names, exactly as
if the names were not present. The PCRE API provides function calls for extracting the name-
to-number translation table from a compiled pattern. There is also a convenience function for
extracting a captured substring by name.

By default, a name must be unique within a pattern, but it is possible to relax this con‐
straint by setting the PCRE_DUPNAMES option at compile time. (Duplicate names are also always
permitted for subpatterns with the same number, set up as described in the previous section.)
Duplicate names can be useful for patterns where only one instance of the named parentheses
can match. Suppose you want to match the name of a weekday, either as a 3-letter abbreviation
or as the full name, and in both cases you want to extract the abbreviation. This pattern
(ignoring the line breaks) does the job:

There are five capturing substrings, but only one is ever set after a match. (An alternative
way of solving this problem is to use a "branch reset" subpattern, as described in the previ‐
ous section.)

The convenience function for extracting the data by name returns the substring for the first
(and in this example, the only) subpattern of that name that matched. This saves searching to
find which numbered subpattern it was.

If you make a back reference to a non-unique named subpattern from elsewhere 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 (see the previ‐
ous section) 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 con‐
dition is true. This is the same behaviour as testing by number. For further details of the
interfaces for handling named subpatterns, see the pcreapi documentation.

Warning: You cannot use different names to distinguish between two subpatterns with the same
number because PCRE uses only the numbers when matching. For this reason, an error is given
at compile time if different names are given to subpatterns with the same number. However,
you can always give the same name to subpatterns with the same number, even when PCRE_DUP‐
NAMES is not set.

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 back reference (see next section)
a parenthesized subpattern (including assertions)
a subroutine call to a subpattern (recursive or otherwise)

The general repetition quantifier specifies a minimum and maximum number of permitted
matches, by giving the two numbers in curly brackets (braces), separated by a comma. The num‐
bers must be less than 65536, and the first must be less than or equal to the second. For ex‐
ample:

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, while

\d{8}

matches exactly 8 digits. An opening curly bracket that appears in a position where a quanti‐
fier is not allowed, or one that does not match the syntax of a quantifier, is taken as a
literal character. For example, {,6} is not a quantifier, but a literal string of four char‐
acters.

In UTF modes, quantifiers apply to characters rather than to individual data units. Thus, for
example, \x{100}{2} matches two characters, each of which is represented by a two-byte se‐
quence in a UTF-8 string. Similarly, \X{3} matches three Unicode extended grapheme clusters,
each of which may be several data units long (and they may be of different lengths).

The quantifier {0} is permitted, causing the expression to behave as if the previous item and
the quantifier were not present. This may be useful for subpatterns that are referenced as
subroutines from elsewhere in the pattern (but see also the section entitled "Defining sub‐
patterns 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-character abbreviations:

* is equivalent to {0,}
+ is equivalent to {1,}
? is equivalent to {0,1}

It is possible to construct infinite loops by following a subpattern that can match no char‐
acters with a quantifier that has no upper limit, for example:

(a?)*

Earlier versions of Perl and PCRE 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 broken.

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 ap‐
pear. An attempt to match C comments by applying the pattern

/\*.*\*/

to the string

/* first comment */ not comment /* second comment */

fails, because it matches the entire string owing to the greediness of the .* item.

However, if a quantifier is followed by a question mark, it ceases to be greedy, and instead
matches the minimum number of times possible, so the pattern

/\*.*?\*/

does the right thing with the C comments. The meaning of the various quantifiers is not oth‐
erwise 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 PCRE_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 pro‐
portion to the size of the minimum or maximum.

If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent to Perl's /s) is
set, thus allowing the dot to match newlines, the pattern is implicitly anchored, because
whatever follows will be tried against every character position in the subject string, so
there is no point in retrying the overall match at any position after the first. PCRE nor‐
mally treats such a pattern as though it were preceded by \A.

In cases where it is known that the subject string contains no newlines, it is worth setting
PCRE_DOTALL in order to obtain this optimization, or alternatively using ^ to indicate an‐
choring explicitly.

However, there are some cases where the optimization cannot be used. When .* is inside cap‐
turing parentheses that are the subject of a back reference elsewhere in the pattern, a match
at the start may fail where a later one succeeds. Consider, for example:

(.*)abc\1

If the subject is "xyz123abc123" the match point is the fourth character. For this reason,
such a pattern is not implicitly anchored.

Another case where implicit anchoring is not applied is when the leading .* is inside an
atomic group. Once again, a match at the start may fail where a later one succeeds. Consider
this pattern:

(?>.*?a)b

It matches "ab" in the subject "aab". The use of the backtracking control verbs (*PRUNE) and
(*SKIP) also disable this optimization.

When a capturing subpattern is repeated, the value captured is the substring that matched the
final iteration. For example, after

(tweedle[dume]{3}\s*)+

has matched "tweedledum tweedledee" the value of the captured substring is "tweedledee". How‐
ever, if there are nested capturing subpatterns, the corresponding captured values may have
been set in previous iterations. For example, after

/(a|(b))+/

matches "aba" the value of the second captured substring is "b".

ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") repetition, failure of
what follows normally causes the repeated item to be re-evaluated to see if a different num‐
ber 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 fail‐
ing 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 contains once it has matched,
and a failure further into the pattern is prevented from backtracking into it. Backtracking
past it to previous items, however, works as normal.

An alternative description is that a subpattern of this type matches 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 ex‐
ample can be thought of as a maximizing repeat that must swallow everything it can. So, while
both \d+ and \d+? are prepared to adjust the number of digits they match in order to make the
rest of the pattern match, (?>\d+) can only match an entire sequence of digits.

Atomic groups in general can of course contain arbitrarily complicated 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 PCRE_UNGREEDY option is ignored.
They are a convenient notation for the simpler forms of atomic group. However, there is no
difference in the meaning of a possessive quantifier and the equivalent atomic group, though
there may be a performance difference; possessive quantifiers should be slightly faster.

The possessive quantifier syntax is an extension to the Perl 5.8 syntax. Jeffrey Friedl
originated the idea (and the name) in the first edition of his book. Mike McCloskey liked it,
so implemented it when he built Sun's Java package, and PCRE copied it from there. It ulti‐
mately found its way into Perl at release 5.10.

PCRE 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.

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 fail‐
ing 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 en‐
closed 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 be‐
tween 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 PCRE and Perl have an optimization that allows for fast failure when a single character
is used. They remember the last single character that is required for a match, and fail early
if it is not present in the string.) If the pattern is changed so that it uses an atomic
group, like this:

((?>\D+)|<\d+>)*[!?]

sequences of non-digits cannot be broken, and failure happens quickly.

BACK REFERENCES

Outside a character class, a backslash followed by a digit greater than 0 (and possibly fur‐
ther digits) is a back reference to a capturing subpattern 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 10, it is always taken as
a back reference, and causes an error only if there are not that many capturing left paren‐
theses in the entire pattern. In other words, the parentheses that are referenced need not be
to the left of the reference for numbers less than 10. A "forward back reference" of this
type can make sense when a repetition is involved and the subpattern to the right has partic‐
ipated in an earlier iteration.

It is not possible to have a numerical "forward back reference" to a subpattern whose number
is 10 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 de‐
tails of the handling of digits following a backslash. There is no such problem when named
parentheses are used. A back reference 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 an unsigned number or a nega‐
tive number, optionally enclosed in braces. These examples are all identical:

(ring), \1
(ring), \g1
(ring), \g{1}

An unsigned number specifies an absolute reference without the ambiguity that is present in
the older syntax. It is also useful when literal digits follow the reference. A negative num‐
ber is a relative reference. Consider this example:

(abc(def)ghi)\g{-1}

The sequence \g{-1} is a reference to the most recently started capturing subpattern before
\g, that is, is it equivalent to \2 in this example. Similarly, \g{-2} would be equivalent
to \1. The use of relative references can be helpful in long patterns, and also in patterns
that are created by joining together fragments that contain references within themselves.

A back reference matches whatever actually matched the capturing subpattern in the current
subject string, rather than anything matching the subpattern itself (see "Subpatterns as sub‐
routines" 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 respon‐
sibility". If caseful matching is in force at the time of the back reference, the case of
letters is relevant. For example,

((?i)rah)\s+\1

matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original capturing sub‐
pattern is matched caselessly.

There are several different ways of writing back references to named subpatterns. The .NET
syntax \k{name} and the Perl syntax \k<name> or \k'name' are supported, as is the Python syn‐
tax (?P=name). Perl 5.10's unified back reference 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 refer‐
ence.

There may be more than one back reference to the same subpattern. If a subpattern has not ac‐
tually 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 PCRE_JAVASCRIPT_COM‐
PAT option is set at compile time, a back reference to an unset value matches an empty
string.

Because there may be many capturing parentheses in a pattern, all digits following a back‐
slash are taken as part of a potential back reference number. If the pattern continues with
a digit character, some delimiter must be used to terminate the back reference. If the
PCRE_EXTENDED option is set, this can be white space. Otherwise, the \g{ syntax or an empty
comment (see "Comments" below) can be used.

Recursive back references

A back reference that occurs inside the parentheses to which it refers fails when the subpat‐
tern is first used, so, for example, (a\1) never matches. However, such references can be
useful inside repeated subpatterns. For example, the pattern

(a|b\1)+

matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of the subpat‐
tern, the back reference matches the character string corresponding to the previous itera‐
tion. In order for this to work, the pattern must be such that the first iteration does not
need to match the back reference. This can be done using alternation, as in the example
above, or by a quantifier with a minimum of zero.

Back references 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 actually 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. An asser‐
tion subpattern is matched in the normal way, except that it does not cause the current
matching position to be changed.

Assertion subpatterns are not capturing subpatterns. If such an assertion contains capturing
subpatterns within it, these are counted for the purposes of numbering the capturing subpat‐
terns in the whole pattern. However, substring capturing is carried out only for positive as‐
sertions. (Perl sometimes, but not always, does do capturing in negative assertions.)

For compatibility with Perl, assertion subpatterns may be repeated; though it makes no sense
to assert the same thing several times, the side effect of capturing parentheses may occa‐
sionally be useful. In practice, 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 sub‐
routine 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 greediness 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 semicolon in the match, and

foo(?!bar)

matches any occurrence of "foo" that is not followed by "bar". Note that the apparently simi‐
lar 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 ef‐
fect.

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 asser‐
tions. 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. How‐
ever, if there are several top-level alternatives, they do not all have to have the same
fixed length. Thus

(?<=bullock|donkey)

is permitted, but

(?<!dogs?|cats?)

causes an error at compile time. Branches that match different length strings are permitted
only at the top level of a lookbehind assertion. This is an extension compared with Perl,
which requires all branches to match the same length of string. An assertion such as

(?<=ab(c|de))

is not permitted, because its single top-level branch can match two different lengths, but it
is acceptable to PCRE 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 asser‐
tion to get round the fixed-length restriction.

The implementation of lookbehind assertions is, for each alternative, to temporarily move the
current position back by the fixed length and then try to match. If there are insufficient
characters before the current position, the assertion fails.

In a UTF mode, PCRE does not allow the \C escape (which matches a single data unit even in a
UTF mode) to appear in lookbehind assertions, because it makes it impossible to calculate the
length of the lookbehind. The \X and \R escapes, which can match different numbers of data
units, are also not permitted.

"Subroutine" calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long as
the subpattern matches a fixed-length string. Recursion, however, is not supported.

Possessive quantifiers can be used in conjunction with lookbehind assertions to specify effi‐
cient matching of fixed-length strings at the end of subject strings. Consider a simple pat‐
tern such as

abcd$

when applied to a long string that does not match. Because matching proceeds from left to
right, PCRE will look for each "a" in the subject and then see if what follows matches the
rest of the pattern. If the pattern is specified as

^.*abcd$

the initial .* matches the entire string at first, but when this fails (because there is no
following "a"), it backtracks to match all but the last character, then all but the last two
characters, and so on. Once again the search for "a" covers the entire string, from right to
left, so we are no better off. However, if the pattern is written as

^.*+(?<=abcd)

there can be no backtracking for the .*+ item; it can match only the entire string. The sub‐
sequent lookbehind assertion does a single test on the last four characters. If it fails, the
match fails immediately. For long strings, this approach makes a significant difference to
the processing time.

Using multiple assertions

Several assertions (of any sort) may occur in succession. For example,

(?<=\d{3})(?<!999)foo

matches "foo" preceded by three digits that are not "999". Notice that each of the assertions
is applied independently at the same point in the subject string. First there is a check that
the previous three characters are all digits, and then there is a check that the same three
characters are not "999". This pattern does not match "foo" preceded by six characters, the
first of which are digits and the last three of which are not "999". For example, it doesn't
match "123abcfoo". A pattern to do that is

(?<=\d{3}...)(?<!999)foo

This time the first assertion looks at the preceding six characters, checking that the first
three are digits, and then the second assertion checks that the preceding three characters
are not "999".

Assertions can be nested in any combination. For example,

(?<=(?<!foo)bar)baz

matches an occurrence of "baz" that is preceded by "bar" which in turn is not preceded by
"foo", while

(?<=\d{3}(?!999)...)foo

is another pattern that matches "foo" preceded by three digits and any three characters that
are not "999".

CONDITIONAL SUBPATTERNS

It is possible to cause the matching process to obey a subpattern conditionally or to choose
between two alternative subpatterns, depending on the result of an assertion, or whether a
specific capturing subpattern 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. If there are more than two alternatives in the subpattern, a compile-time error oc‐
curs. 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 four kinds of condition: references to subpatterns, references to recursion, a
pseudo-condition called DEFINE, 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 previously 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 alternative 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 subse‐
quent 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 PCRE_EXTENDED option) and to divide it into three parts for ease of dis‐
cussion:

( $ )? [^()]+ (?(1) $ )

The first part matches an optional opening parenthesis, and if that character is present,
sets it as the first captured substring. The second part matches one or more characters that
are not parentheses. The third part is a conditional 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 paren‐
thesis is required. Otherwise, since no-pattern is not present, the subpattern matches noth‐
ing. 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 PCRE, which had this facility before Perl, the
syntax (?(name)...) is also recognized.

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 sub‐
patterns of the same name, and is true if any one of them has matched.

Checking for pattern recursion

If the condition is the string (R), and there is no subpattern with the name R, the condition
is true if a recursive call to the whole pattern or any subpattern has been made. If digits
or a name preceded by ampersand follow the letter R, for example:

(?(R3)...) or (?(R&name)...)

the condition is true if the most recent recursion is into a subpattern whose number or name
is given. This condition does not check the entire recursion stack. 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. The syntax for recursive pat‐
terns is described below.

Defining subpatterns for use by reference only

If the condition is the string (DEFINE), and there is no subpattern with the name DEFINE, the
condition is always false. In this case, there may be only one alternative in the subpattern.
It is always skipped if control reaches this point in the pattern; the idea of DEFINE is that
it can be used to define subroutines that can be referenced from elsewhere. (The use of sub‐
routines is described below.) For example, a pattern to match an IPv4 address such as
"192.168.23.245" could be written like this (ignore white space and line breaks):

(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
\b (?&byte) (\.(?&byte)){3} \b

The first part of the pattern is a DEFINE group inside which a another group named "byte" is
defined. This matches an individual component of an IPv4 address (a number less than 256).
When matching takes place, this part of the pattern is skipped because DEFINE acts like a
false condition. The rest of the pattern uses references to the named group to match the four
dot-separated components of an IPv4 address, insisting on a word boundary at each end.

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 contain‐
ing 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-let‐
ters 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.

COMMENTS

There are two ways of including comments in patterns that are processed by PCRE. 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 charac‐
ters 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 parenthe‐
sis. Nested parentheses are not permitted. If the PCRE_EXTENDED 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 the options passed to a compiling function or by a special se‐
quence at the start of the pattern, as described in the section entitled "Newline conven‐
tions" 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, con‐
sider this pattern when PCRE_EXTENDED is set, and the default newline convention is in force:

abc #comment \n still comment

On encountering the # character, pcre_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 com‐
ment. Only an actual character with the code value 0x0a (the default newline) does so.

RECURSIVE PATTERNS

Consider the problem of matching a string in parentheses, allowing for unlimited nested
parentheses. Without the use of recursion, the best that can be done is to use a pattern that
matches up to some fixed depth of nesting. It is not possible to handle an arbitrary nesting
depth.

For some time, Perl has provided a facility that allows regular expressions to recurse
(amongst other things). It does this by interpolating Perl code in the expression at run
time, and the code can refer to the expression itself. A Perl pattern using code interpola‐
tion 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, PCRE cannot support the interpolation of Perl code. Instead, it supports special
syntax for recursion of the entire pattern, and also for individual subpattern recursion. Af‐
ter its introduction in PCRE 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 PCRE pattern solves the nested parentheses problem (assume the PCRE_EXTENDED option is
set so that white space is ignored):

$ ( [^()]++ | (?R) )* $

First it matches an opening parenthesis. Then it matches any number of substrings which can
either be a sequence of non-parentheses, or a recursive match of the pattern itself (that is,
a correctly parenthesized substring). Finally there is a closing parenthesis. Note the use
of a possessive quantifier to avoid backtracking into sequences of non-parentheses.

If this were part of a larger pattern, you would not want to recurse the entire pattern, so
instead you could use this:

( $ ( [^()]++ | (?1) )* $ )

We have put the pattern into parentheses, and caused the recursion to refer to them instead
of the whole pattern.

In a larger pattern, keeping track of parenthesis numbers can be tricky. This is made easier
by the use of relative references. Instead of (?1) in the pattern above you can write (?-2)
to refer to the second most recently opened parentheses preceding the recursion. In other
words, a negative number counts capturing parentheses leftwards from the point at which it is
encountered.

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 paren‐
theses that are referenced. They are always non-recursive subroutine calls, as described in
the next section.

An alternative approach is to use named parentheses instead. The Perl syntax for this is
(?&name); PCRE's earlier syntax (?P>name) is also supported. We could rewrite the above exam‐
ple as follows:

(?<pn> $ ( [^()]++ | (?&pn) )* $ )

If there is more than one subpattern with the same name, the earliest one is used.

This particular example pattern that we have been looking at contains nested unlimited re‐
peats, 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 pat‐
tern 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 pcrecallout documentation). If the pattern above is matched against

(ab(cd)ef)

the value for the inner capturing parentheses (numbered 2) is "ef", which is the last value
taken on at the top level. If a capturing subpattern is not matched at the top level, its fi‐
nal captured value is unset, even if it was (temporarily) set at a deeper level during the
matching process.

If there are more than 15 capturing parentheses in a pattern, PCRE has to obtain extra memory
to store data during a recursion, which it does by using pcre_malloc, freeing it via
pcre_free afterwards. If no memory can be obtained, the match fails with the PCRE_ER‐
ROR_NOMEMORY error.

Do not confuse the (?R) item with the condition (R), which tests for recursion. Consider
this pattern, which matches text in angle brackets, allowing for arbitrary nesting. Only dig‐
its are allowed in nested brackets (that is, when recursing), whereas any characters are per‐
mitted at the outer level.

< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >

In this pattern, (?(R) is the start of a conditional subpattern, with two different alterna‐
tives for the recursive and non-recursive cases. The (?R) item is the actual recursive call.

Differences in recursion processing between PCRE and Perl

Recursion processing in PCRE differs from Perl in two important ways. In PCRE (like Python,
but unlike Perl), a recursive subpattern call is always treated as an atomic group. That is,
once it has matched some of the subject string, it is never re-entered, even if it contains
untried alternatives and there is a subsequent matching failure. This can be illustrated by
the following pattern, which purports to match a palindromic string that contains an odd num‐
ber of characters (for example, "a", "aba", "abcba", "abcdcba"):

^(.|(.)(?1)\2)$

The idea is that it either matches a single character, or two identical characters surround‐
ing a sub-palindrome. In Perl, this pattern works; in PCRE it does not if the pattern is
longer than three characters. Consider the subject string "abcba":

At the top level, the first character is matched, but as it is not at the end of the string,
the first alternative fails; the second alternative is taken and the recursion kicks in. The
recursive call to subpattern 1 successfully matches the next character ("b"). (Note that the
beginning and end of line tests are not part of the recursion).

Back at the top level, the next character ("c") is compared with what subpattern 2 matched,
which was "a". This fails. Because the recursion is treated as an atomic group, there are now
no backtracking points, and so the entire match fails. (Perl is able, at this point, to re-
enter the recursion and try the second alternative.) However, if the pattern is written with
the alternatives in the other order, things are different:

^((.)(?1)\2|.)$

This time, the recursing alternative is tried first, and continues to recurse until it runs
out of characters, at which point the recursion fails. But this time we do have another al‐
ternative to try at the higher level. That is the big difference: in the previous case the
remaining alternative is at a deeper recursion level, which PCRE cannot use.

To change the pattern so that it matches all palindromic strings, not just those with an odd
number of characters, it is tempting to change the pattern to this:

^((.)(?1)\2|.?)$

Again, this works in Perl, but not in PCRE, and for the same reason. When a deeper recursion
has matched a single character, it cannot be entered again in order to match an empty string.
The solution is to separate the two cases, and write out the odd and even cases as alterna‐
tives at the higher level:

^(?:((.)(?1)\2|)|((.)(?3)\4|.))

If you want to match typical palindromic phrases, the pattern has to ignore all non-word
characters, which can be done like this:

^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$

If run with the PCRE_CASELESS option, this pattern matches phrases such as "A man, a plan, a
canal: Panama!" and it works well in both PCRE and Perl. Note the use of the possessive quan‐
tifier *+ to avoid backtracking into sequences of non-word characters. Without this, PCRE
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.

WARNING: The palindrome-matching patterns above work only if the subject string does not
start with a palindrome that is shorter than the entire string. For example, although
"abcba" is correctly matched, if the subject is "ababa", PCRE finds the palindrome "aba" at
the start, then fails at top level because the end of the string does not follow. Once again,
it cannot jump back into the recursion to try other alternatives, so the entire match fails.

The second way in which PCRE and Perl differ in their recursion processing is in the handling
of captured values. In Perl, when a subpattern is called recursively or as a subpattern (see
the next section), it has no access to any values that were captured outside the recursion,
whereas in PCRE these values can be referenced. Consider this pattern:

^(.)(\1|a(?2))

In PCRE, this pattern matches "bab". The first capturing parentheses match "b", then in the
second group, when the back reference \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 suc‐
ceeds. In Perl, the pattern fails to match because inside the recursive call \1 cannot access
the externally set value.

SUBPATTERNS AS SUBROUTINES

If the syntax for a recursive subpattern call (either by number or by name) is used outside
the parentheses to which it refers, it operates like a subroutine in a programming language.
The called subpattern may be defined before or after the reference. A numbered reference can
be absolute or relative, as in these examples:

(...(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 respon‐
sibility". 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.

All subroutine calls, whether recursive or not, are always treated as atomic groups. That is,
once a subroutine has matched some of the subject string, it is never re-entered, even if it
contains untried alternatives and there is a subsequent matching failure. Any capturing
parentheses that are set during the subroutine call revert to their previous values after‐
wards.

Processing options such as case-independence are fixed when a subpattern 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.

ONIGURUMA SUBROUTINE SYNTAX

For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number en‐
closed 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,
rewritten using this syntax:

(?<pn> $ ( (?>[^()]+) | \g<pn> )* $ )
(sens|respons)e and \g'1'ibility

PCRE 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 back reference; the latter is a subroutine call.

CALLOUTS

Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl code to be
obeyed in the middle of matching a regular expression. This makes it possible, amongst other
things, to extract different substrings that match the same pair of parentheses when there is
a repetition.

PCRE provides a similar feature, but of course it cannot obey arbitrary Perl code. The fea‐
ture is called "callout". The caller of PCRE provides an external function by putting its en‐
try point in the global variable pcre_callout (8-bit library) or pcre[16|32]_callout (16-bit
or 32-bit library). By default, this variable contains NULL, which disables all calling out.

Within a regular expression, (?C) indicates the points at which the external function is to
be called. If you want to identify different callout points, you can put a number less than
256 after the letter C. The default value is zero. For example, this pattern has two callout
points:

(?C1)abc(?C2)def

If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are automatically
installed before each item in the pattern. They are all numbered 255. If there is a condi‐
tional group in the pattern whose condition is an assertion, an additional callout is in‐
serted 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.

During matching, when PCRE reaches a callout point, the external function is called. It is
provided with the number of the callout, the position in the pattern, and, optionally, one
item of data originally supplied by the caller of the matching function. The callout function
may cause matching to proceed, to backtrack, or to fail altogether.

By default, PCRE implements a number of optimizations at compile time and 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, and a
complete description of the interface to the callout function, are given in the pcrecallout
documentation.

BACKTRACKING CONTROL

Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which are still de‐
scribed in the Perl documentation as "experimental and subject to change or removal in a fu‐
ture version of Perl". It goes on to say: "Their usage in production code should be noted to
avoid problems during upgrades." The same remarks apply to the PCRE features described in
this section.

The new verbs make use of what was previously invalid syntax: an opening parenthesis followed
by an asterisk. They are generally of the form (*VERB) or (*VERB:NAME). Some may take either
form, possibly behaving differently depending on whether or not a name is present. A name is
any sequence of characters that does not include a closing parenthesis. The maximum length of
name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit libraries. If the name is
empty, that is, if the closing parenthesis immediately follows the colon, the effect is as if
the colon were not there. Any number of these verbs may occur in a pattern.

Since these verbs are specifically related to backtracking, most of them can be used only
when the pattern is to be matched using one of the traditional matching functions, because
these use a backtracking algorithm. With the exception of (*FAIL), which behaves like a fail‐
ing negative assertion, the backtracking control verbs cause an error if encountered by a DFA
matching function.

The behaviour of these verbs in repeated groups, assertions, and in subpatterns called as
subroutines (whether or not recursively) is documented below.

Optimizations that affect backtracking verbs

PCRE 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 sub‐
ject, or that a particular character must be present. When one of these optimizations by‐
passes the running of a match, any included backtracking verbs will not, of course, be pro‐
cessed. You can suppress the start-of-match optimizations by setting the PCRE_NO_START_OPTI‐
MIZE option when calling pcre_compile() or pcre_exec(), or by starting the pattern with
(*NO_START_OPT). There is more discussion of this option in the section entitled "Option bits
for pcre_exec()" in the pcreapi documentation.

Experiments with Perl suggest that it too has similar optimizations, sometimes leading to
anomalous results.

Verbs that act immediately

The following verbs act as soon as they are encountered. They may not be followed by a name.

(*ACCEPT)

This verb causes the match to end successfully, skipping the remainder of the pattern. How‐
ever, 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 positive assertion, the assertion succeeds; in a negative assertion, the assertion fails.

If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For example:

A((?:A|B(*ACCEPT)|C)D)

This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by the outer paren‐
theses.

(*FAIL) or (*F)

This verb causes a matching failure, forcing backtracking to occur. It is equivalent to (?!)
but easier to read. The Perl documentation notes that it is probably useful only when com‐
bined with (?{}) or (??{}). Those are, of course, Perl features that are not present in PCRE.
The nearest equivalent is the callout feature, as for example in this pattern:

a+(?C)(*FAIL)

A match with the string "aaaa" always fails, but the callout is taken before each backtrack
happens (in this example, 10 times).

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) be‐
low).

(*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), (*PRUNE:NAME), or
(*THEN:NAME) on the matching path is passed back to the caller as described in the section
entitled "Extra data for pcre_exec()" in the pcreapi documentation. Here is an example of
pcretest output, where the /K modifier requests the retrieval and outputting of (*MARK) data:

re> /X(*MARK:A)Y|X(*MARK:B)Z/K
data> XY
0: XY
MK: A
XZ
0: XZ
MK: B

The (*MARK) name is tagged with "MK:" in this output, and in this example it indicates which
of the two alternatives matched. This is a more efficient way of obtaining this information
than putting each alternative in its own capturing parentheses.

If a verb with a name is encountered in a positive assertion that is true, the name is
recorded and passed back if it is the last-encountered. This does not happen for negative as‐
sertions 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/K
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
PCRE_NO_START_OPTIMIZE option (see above) to ensure that the match is always attempted.

Verbs that act after backtracking

The following verbs do nothing when they are encountered. Matching continues with what fol‐
lows, but if there is no subsequent match, causing a backtrack to the verb, a failure is
forced. That is, backtracking cannot pass to the left of the verb. However, when one of these
verbs appears inside an atomic group or an 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.
In this situation, backtracking can "jump back" to the left of the entire atomic group or as‐
sertion. (Remember also, as stated above, that this localization also applies in subroutine
calls.)

These verbs differ in exactly what kind of failure occurs when backtracking reaches them. The
behaviour described below is what happens when the verb is not in a subroutine or an asser‐
tion. Subsequent sections cover these special cases.

(*COMMIT)

This verb, which may not be followed by a name, causes the whole match to fail outright if
there is a later matching failure that causes backtracking to reach it. Even if the pattern
is unanchored, no further attempts to find a match by advancing the starting point take
place. If (*COMMIT) is the only backtracking verb that is encountered, once it has been
passed pcre_exec() is committed to finding a match at the current starting point, or not at
all. For example:

a+(*COMMIT)b

This matches "xxaab" but not "aacaab". It can be thought of as a kind of dynamic anchor, or
"I've started, so I must finish." The name of the most recently passed (*MARK) in the path is
passed back when (*COMMIT) forces a match failure.

If there is more than one backtracking verb in a pattern, a different one that follows (*COM‐
MIT) may be triggered first, so merely passing (*COMMIT) during a match does not always guar‐
antee 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 PCRE's
start-of-match optimizations are turned off, as shown in this output from pcretest:

re> /(*COMMIT)abc/
data> xyzabc
0: abc
data> xyzabc\Y
No match

For this pattern, PCRE 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 at‐
tempt then succeeds. In the second set of data, the escape sequence \Y is interpreted by the
pcretest program. It causes the PCRE_NO_START_OPTIMIZE option to be set when pcre_exec() is
called. This disables the optimization that skips along to the first character. The pattern
is now applied starting at "x", and so the (*COMMIT) causes the match to fail without trying
any other starting points.

(*PRUNE) or (*PRUNE:NAME)

This verb causes the match to fail at the current starting position in the subject if there
is a later matching failure that causes backtracking to reach it. If the pattern is unan‐
chored, the normal "bumpalong" advance to the next starting character then happens. Back‐
tracking can occur as usual to the left of (*PRUNE), before it is reached, or when matching
to the right of (*PRUNE), but if there is no match to the right, backtracking cannot cross
(*PRUNE). In simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
possessive quantifier, but there are some uses of (*PRUNE) that cannot be expressed in any
other way. In an anchored pattern (*PRUNE) has the same effect as (*COMMIT).

The behaviour of (*PRUNE:NAME) is the 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).

(*SKIP)

This verb, when given without a name, is like (*PRUNE), except that if the pattern is unan‐
chored, the "bumpalong" advance is not to the next character, but to the position in the sub‐
ject where (*SKIP) was encountered. (*SKIP) signifies that whatever text was matched leading
up to it cannot be part of a successful match. 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 quantifer does not have the same effect as this example; although it would
suppress backtracking during the first match attempt, the second attempt would start at the
second character instead of skipping on to "c".

(*SKIP:NAME)

When (*SKIP) has an associated name, its behaviour is modified. When it is triggered, the
previous path through the pattern is searched for the most recent (*MARK) that has the same
name. If one is found, the "bumpalong" advance is to the subject position that corresponds to
that (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a matching name
is found, the (*SKIP) is ignored.

Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores names that are
set by (*PRUNE:NAME) or (*THEN:NAME).

(*THEN) or (*THEN:NAME)

This verb causes a skip to the next innermost alternative when backtracking reaches it. That
is, it cancels any further backtracking within the current alternative. Its name comes from
the observation that it can be used for a pattern-based if-then-else block:

( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

If the COND1 pattern matches, FOO is tried (and possibly further items after the end of the
group if FOO succeeds); on failure, the matcher skips to the second alternative and tries
COND2, without backtracking into COND1. If that succeeds and BAR fails, COND3 is tried. If
subsequently BAZ fails, there are no more alternatives, so there is a backtrack to whatever
came before the entire group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).

The behaviour of (*THEN:NAME) is the 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).

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 be‐
yond 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; in‐
stead 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 subpattern to fail because there are no more alter‐
natives 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 alter‐
native 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 match‐
ing fails. (*THEN) is the weakest, carrying on the match at the next alternative. (*PRUNE)
comes next, failing the match at the current starting position, but allowing an advance to
the next character (for an unanchored pattern). (*SKIP) is similar, except that the advance
may be more than one character. (*COMMIT) is the strongest, causing the entire match to fail.

More than one backtracking verb

If more than one backtracking verb is present in a pattern, the one that is backtracked onto
first acts. For example, consider this pattern, where A, B, etc. are complex pattern frag‐
ments:

(A(*COMMIT)B(*THEN)C|ABD)

If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to fail. How‐
ever, 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 trig‐
gered, and its action is taken. There can never be a backtrack onto (*COMMIT).

Backtracking verbs in repeated groups

PCRE 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, but PCRE fails because the (*COMMIT) in the second
repeat of the group acts.

Backtracking verbs in assertions

(*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.

(*ACCEPT) in a positive assertion causes the assertion to succeed without any further pro‐
cessing. In a negative assertion, (*ACCEPT) causes the assertion to fail without any further
processing.

The other backtracking verbs are not treated specially if they appear in a positive asser‐
tion. In particular, (*THEN) skips to the next alternative in the innermost enclosing group
that has alternations, whether or not this is within the assertion.

Negative assertions are, however, different, in order to ensure that changing a positive as‐
sertion into a negative assertion changes its result. Backtracking into (*COMMIT), (*SKIP),
or (*PRUNE) causes a negative assertion to be true, without considering any further alterna‐
tive branches in the assertion. Backtracking into (*THEN) causes it to skip to the next en‐
closing alternative within the assertion (the normal behaviour), but if the assertion does
not have such an alternative, (*THEN) behaves like (*PRUNE).

Backtracking verbs in subroutines

These behaviours occur whether or not the subpattern is called recursively. Perl's treatment
of subroutines is different in some cases.

(*FAIL) in a subpattern called as a subroutine has its normal effect: it forces an immediate
backtrack.

(*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to succeed with‐
out any further processing. Matching then continues after the subroutine call.

(*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause the subroutine
match to fail.

(*THEN) skips to the next alternative in the innermost enclosing group within the subpattern
that has alternatives. If there is no such group within the subpattern, (*THEN) causes the
subroutine match to fail.