PCREPATTERN(3) Introduction to Library Functions 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 material.

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 pattern. Any number of these items may appear, but they must all
be together right at the start of the pattern string, and the letters
must be in upper case.

UTF support

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 affects pattern
matching is mentioned in several places below. There is also a
summary of features 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 Unicode 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 possessive when what follows
cannot match the repeated item. For example, by default a+b is
treated as a++b. For more details, see the 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 optimizations for quickly
reaching "no match" results. For more details, see the pcreapi
documentation.

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 further 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 behaviour 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() function 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 example 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 characters in the subject. As a
trivial example, the pattern

The quick brown fox

matches a portion of a subject string that is identical to itself.
When caseless matching is specified (the 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 repetitions in the pattern. These are encoded in the
pattern by the use of metacharacters, which do not stand for
themselves but instead are interpreted in some special way.

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

\ general escape character with several uses
^ assert start of string (or line, in multiline mode)
$ assert end of string (or line, in multiline mode)
. match any character except newline (by default)
[ start character class definition
| start of alternative branch
( start 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 character 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 document. The only
characters that are allowed after \c are A-Z, a-z, or one of @, [, \,
], ^, _, or ?. Any other character provokes a compile-time error. The
sequence \c@ encodes character code 0; after \c the letters (in
either case) encode characters 1-26 (hex 01 to hex 1A); [, \, ], ^,
and _ encode characters 27-31 (hex 1B to hex 1F), and \c? becomes
either 255 (hex FF) or 95 (hex 5F).

Thus, apart from \c?, these escapes generate the same character code
values as they do in an ASCII environment, though the meanings of the
values mostly differ. For example, \cG always generates code value 7,
which is BEL in ASCII but DEL in EBCDIC.

The sequence \c? generates DEL (127, hex 7F) in an ASCII environment,
but because 127 is not a control character in EBCDIC, Perl makes it
generate the APC character. Unfortunately, there are several variants
of EBCDIC. In most of them the APC character has the value 255 (hex
FF), but in the one Perl calls POSIX-BC its value is 95 (hex 5F). If
certain other characters have POSIX-BC values, PCRE makes \c?
generate 95; otherwise it generates 255.

After \0 up to two further octal digits are read. If there are fewer
than two digits, just those that are present are used. Thus the
sequence \0\x\015 specifies two binary zeros followed by a CR
character (code value 13). Make sure you supply two digits after the
initial zero if the pattern character that follows is itself an octal
digit.

The escape \o must be followed by a sequence of octal digits,
enclosed in braces. An error occurs if this is not the case. This
escape is a recent addition to Perl; it provides way of specifying
character code points as octal numbers greater than 0777, and it also
allows octal numbers and 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 references. 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 expression, 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 characters "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 example:

\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 introduced by a leading zero, because no more
than three octal digits are ever read.

By default, after \x that is not followed by {, from zero to two
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" character.

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 example, \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 values, 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" codepoints), 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 sequences. 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 enclosed either in angle brackets or single
quotes, is an alternative syntax for referencing a subpattern as a
"subroutine". Details are discussed later. Note that \g{...} (Perl
syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former
is a 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 characters 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 sequences can
appear both inside and outside character classes. They each match one
character of the appropriate type. If the current matching point is
at the end of the subject string, all of them fail, because there is
no character to match.

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 decimal digits, whereas \w matches any Unicode
digit, as well as any Unicode letter, and underscore. 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 sequence. In 8-bit non-UTF-8 mode \R is
equivalent to the following:

(?>\r\n|\n|\x0b|\f|\r|\x85)

This is an example of an "atomic group", details of which are given
below. This particular group matches either the two-character
sequence CR followed by LF, or one of the single characters LF
(linefeed, U+000A), VT (vertical tab, U+000B), FF (form feed,
U+000C), CR (carriage return, U+000D), or NEL (next line, U+0085).
The two-character sequence is treated as a single 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 abbreviation for "backslash R".) This can be made
the default 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 sequences. 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 general category properties, "Any", which matches
any character (including newline), and some special PCRE properties
(described in the next section). Other Perl properties such as
"InMusicalSymbols" 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, Egyptian_Hieroglyphs, Elbasan, Ethiopic,
Georgian, Glagolitic, Gothic, Grantha, Greek, Gujarati, Gurmukhi,
Han, Hangul, Hanunoo, Hebrew, Hiragana, Imperial_Aramaic, Inherited,
Inscriptional_Pahlavi, Inscriptional_Parthian, Javanese, Kaithi,
Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Khojki, Khudawadi,
Lao, Latin, Lepcha, Limbu, Linear_A, Linear_B, Lisu, Lycian, Lydian,
Mahajani, 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, Shavian, 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 circumflex between the
opening brace and the property name. For example, \p{^Lu} is the same
as \P{Lu}.

If only one letter is specified with \p or \P, it includes all the
general category properties that start with that letter. In this
case, in the absence of negation, the curly brackets in the escape
sequence are optional; these two examples have the same effect:

\p{L}
\pL

The following general category property codes are supported:

C Other
Cc Control
Cf Format
Cn Unassigned
Co Private use
Cs Surrogate

L Letter
Ll Lower case letter
Lm Modifier letter
Lo Other letter
Lt Title case letter
Lu Upper case letter

M Mark
Mc Spacing mark
Me Enclosing mark
Mn Non-spacing mark

N Number
Nd Decimal number
Nl Letter number
No Other number

P Punctuation
Pc Connector punctuation
Pd Dash punctuation
Pe Close punctuation
Pf Final punctuation
Pi Initial punctuation
Po Other punctuation
Ps Open punctuation

S Symbol
Sc Currency symbol
Sk Modifier symbol
Sm Mathematical symbol
So Other symbol

Z Separator
Zl Line separator
Zp Paragraph separator
Zs Space separator

The special property L& is also supported: it matches a character
that has the Lu, Ll, or Lt property, in other words, a letter that is
not classified as a modifier or "other".

The Cs (Surrogate) property applies only to characters 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 validity 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 supported 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} always matches only upper case letters. This is
different from the behaviour of current versions of Perl.

Matching characters by Unicode property is not fast, because PCRE has
to do a multistage table lookup in order to find a character's
property. That is why the traditional escape sequences such as \d and
\w do not use Unicode properties in 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 cluster", 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 characters 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 characters according to the following rules for ending
a cluster:

1. End at the end of the subject string.

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

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

4. Do not end before extending characters or spacing marks.
Characters with the "mark" property 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 properties. PCRE
uses these non-standard, non-Perl properties internally when PCRE_UCP
is set. However, they may also be used explicitly. These properties
are:

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

Xan matches characters that have either the L (letter) or the N
(number) property. Xps matches the characters tab, linefeed, vertical
tab, form feed, or carriage return, and any other character that has
the Z (separator) property. Xsp is the same as Xps; it used to
exclude 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 represented by a Universal Character Name in
C++ and other programming languages. These are the characters $, @, `
(grave accent), and all characters with Unicode code points greater
than or equal to U+00A0, except for the surrogates U+D800 to U+DFFF.
Note that most base (ASCII) characters are excluded. (Universal
Character Names are of the form \uHHHH or \UHHHHHHHH where H is a
hexadecimal digit. Note that the Xuc property does not match these
sequences but the characters that they represent.)

Resetting the match start

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

foo\Kbar

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

(foo)\Kbar

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

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 condition that has to be met at a particular
point in a match, without consuming any characters from the subject
string. The use of subpatterns for more complicated assertions is
described below. The backslashed assertions are:

\b matches at a word boundary
\B matches when not at a word boundary
\A matches at the start of the subject
\Z matches at the end of the subject
also matches before a newline at the end of the subject
\z matches only at the end of the subject
\G matches at the first matching position in the subject

Inside a character class, \b has a different meaning; it matches the
backspace character. If any other of these assertions appears in a
character class, by default it matches the corresponding 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 previous character do not both match \w or \W (i.e.
one matches \w and the other matches \W), or the start or end of the
string if the first or last character matches \w, respectively. In a
UTF mode, the meanings of \w and \W can be changed by setting the
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 arguments, 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 starting match position, and the "anchored" flag is
set in the compiled regular expression.

CIRCUMFLEX AND DOLLAR

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

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

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

The dollar character is an assertion that is true only if the current
matching point is at the end of the subject string, or immediately
before a newline at the end of the string (by default). 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 assertion.

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 option 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 error. If a
closing square bracket is required as a member of the class, it
should be the first data character in the class (after an initial
circumflex, if present) or escaped with a backslash.

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 notation for
specifying the characters that are in the class by enumerating those
that are not. A class that starts with a circumflex is not an
assertion; it still consumes a character from the subject string, and
therefore it fails if the current pointer is at the end of the
string.

In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than
255 (0xffff) can be included 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 pattern such as [W-]46] is interpreted as a
class of two characters ("W" and "-") followed by a literal string
"46]", so it would match "W46]" or "-46]". However, if the "]" is
escaped with a backslash it is interpreted as the end of range, so
[W-\]46] is interpreted as a class containing a range followed by two
other characters. The octal or hexadecimal representation of "]" can
also be used to end a range.

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 expected. 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 characters 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 letters in either case. For example, [W-c] is
equivalent to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF
mode, if character tables for a French locale are in use, [\xc8-\xcb]
matches accented E characters in both cases. In UTF modes, PCRE
supports the concept of case for characters with values greater than
128 only when it is compiled with Unicode 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 meanings of \d, \s, \w and their upper case partners,
just as it does when they appear outside a character class, as
described in the section entitled "Generic character types" above.
The escape sequence \b has a different meaning inside a character
class; it matches the backspace character. The sequences \B, \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 different; 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 handled specially in UCP mode:

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

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


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

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

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

VERTICAL BAR

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

gilbert|sullivan

matches either "gilbert" or "sullivan". Any number of alternatives
may appear, and an empty alternative is permitted (matching the empty
string). The matching process tries each alternative in turn, from
left to right, and the first one that succeeds is used. If the
alternatives are within a 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 option 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 options 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 parentheses), the change applies to the remainder
of the pattern that follows. An option change within a subpattern
(see below for a description of subpatterns) affects only that part
of the subpattern that follows it, so

(a(?i)b)c

matches abc and aBc and no other strings (assuming 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
subpattern. For example,

(a(?i)b|c)

matches "ab", "aB", "c", and "C", even though when matching "C" the
first branch is abandoned before the option setting. This is because
the effects of option settings happen at compile time. There would be
some very weird behaviour otherwise.

Note: There are other PCRE-specific options that can be set by the
application when the compiling or matching functions are called. In
some cases the pattern can contain special leading sequences such as
(*CRLF) to override what the application has set or what has been
defaulted. Details are given in the section entitled "Newline
sequences" above. There are also the (*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 pattern 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, respectively.

The fact that plain parentheses fulfil two functions is not always
helpful. There are often times when a grouping subpattern is
required without a capturing requirement. If an opening parenthesis
is followed by a question mark and a colon, the subpattern does not
do any capturing, and is not counted when computing the number of any
subsequent capturing subpatterns. For example, if the string "the
white queen" is matched against the pattern

the ((?:red|white) (king|queen))

the captured substrings are "white queen" and "queen", and are
numbered 1 and 2. The maximum number of capturing subpatterns is
65535.

As a convenient shorthand, if any option settings are required at the
start of a non-capturing subpattern, the option letters may appear
between the "?" and the ":". Thus the two patterns

(?i:saturday|sunday)
(?:(?i)saturday|sunday)

match exactly the same set of strings. Because alternative branches
are tried from left to right, and options are not reset until the end
of the subpattern is reached, an option setting in one branch does
affect subsequent branches, so the above patterns match "SUNDAY" as
well as "Saturday".

DUPLICATE SUBPATTERN NUMBERS

Perl 5.10 introduced a feature whereby each alternative in a
subpattern uses the same numbers for its capturing parentheses. Such
a subpattern starts with (?| and is itself a non-capturing
subpattern. For example, consider this pattern:

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

Because the two alternatives are inside a (?| group, both sets of
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 subpatterns, as described in the next section.

NAMED SUBPATTERNS

Identifying capturing parentheses by number is simple, but it can be
very hard to keep track of the numbers in complicated regular
expressions. Furthermore, if an expression is modified, the numbers
may change. To help with this difficulty, PCRE supports the naming of
subpatterns. 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 constraint 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:

(?<DN>Mon|Fri|Sun)(?:day)?|
(?<DN>Tue)(?:sday)?|
(?<DN>Wed)(?:nesday)?|
(?<DN>Thu)(?:rsday)?|
(?<DN>Sat)(?:urday)?

There are five capturing substrings, but only one is ever set after a
match. (An alternative way of solving this problem is to use a
"branch reset" subpattern, as described in the previous 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 pattern 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 previous 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 condition 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_DUPNAMES 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 numbers must be less
than 65536, and the first must be less than or equal to the second.
For example:

z{2,4}

matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
special character. If the second number is omitted, but the comma is
present, there is no upper limit; if the second number and the comma
are both omitted, the quantifier specifies an exact number of
required matches. Thus

[aeiou]{3,}

matches at least 3 successive vowels, but may match many more, while

\d{8}

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

In UTF modes, quantifiers apply to characters rather than to
individual data units. Thus, for example, \x{100}{2} matches two
characters, each of which is represented by a two-byte sequence in a
UTF-8 string. Similarly, \X{3} matches three Unicode extended
grapheme clusters, each of which may be several 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
subpatterns for use by reference only" below). Items other than
subpatterns that have a {0} quantifier are omitted from the compiled
pattern.

For convenience, the three most common quantifiers have single-
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 characters 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 appear. An attempt to match C comments by
applying the pattern

/\*.*\*/

to the string

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

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

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

/\*.*?\*/

does the right thing with the C comments. The meaning of the various
quantifiers is not otherwise changed, just the preferred number of
matches. Do not confuse this use of question mark with its use as a
quantifier in its own right. Because it has two uses, it can
sometimes appear doubled, as in

\d??\d

which matches one digit by preference, but can match two if that is
the only way the rest of the pattern matches.

If the 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 proportion 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 normally treats such a pattern as
though it were preceded by \A.

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

However, there are some cases where the optimization cannot be used.
When .* is inside capturing parentheses that are the subject of a
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". However, 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 number of repeats allows the
rest of the pattern to match. Sometimes it is useful to prevent this,
either to change the nature of the match, or to cause it fail earlier
than it otherwise might, when the author of the pattern knows there
is no point in carrying on.

Consider, for example, the pattern \d+foo when applied to the subject
line

123456bar

After matching all 6 digits and then failing to match "foo", the
normal action of the matcher is to try again with only 5 digits
matching the \d+ item, and then with 4, and so on, before ultimately
failing. "Atomic grouping" (a term taken from Jeffrey Friedl's book)
provides the means for specifying that once a subpattern has matched,
it is not to be re-evaluated in this way.

If we use atomic grouping for the previous example, the matcher gives
up immediately on failing to match "foo" the first time. The notation
is a kind of special parenthesis, starting with (?> as in this
example:

(?>\d+)foo

This kind of parenthesis "locks up" the part of the pattern it
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 example can be thought of as a maximizing
repeat that must swallow everything it can. So, while both \d+ and
\d+? are prepared to adjust the number of digits they match in order
to make the rest of the pattern match, (?>\d+) can only match an
entire sequence of digits.

Atomic groups in general can of course contain arbitrarily
complicated 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
ultimately 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 failing matches taking a
very long time indeed. The pattern

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

matches an unlimited number of substrings that either consist of non-
digits, or digits enclosed in <>, followed by either ! or ?. When it
matches, it runs quickly. However, if it is applied to

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

it takes a long time before reporting failure. This is because the
string can be divided between the internal \D+ repeat and the
external * repeat in a large number of ways, and all have to be
tried. (The example uses [!?] rather than a single character at the
end, because both 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 further 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 parentheses 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 participated 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 details 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 negative 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 number is a relative
reference. Consider this example:

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

The sequence \g{-1} is a reference to the most recently started
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 subroutines"
below for a way of doing that). So the pattern

(sens|respons)e and \1ibility

matches "sense and sensibility" and "response and responsibility",
but not "sense and responsibility". If caseful matching is in force
at the time of the 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 subpattern 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 syntax (?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 reference.

There may be more than one back reference to the same subpattern. If
a subpattern has not actually been used in a particular match, any
back references to it always fail by default. For example, the
pattern

(a|(bc))\2

always fails if it starts to match "a" rather than "bc". However, if
the PCRE_JAVASCRIPT_COMPAT 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 backslash 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 subpattern 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 subpattern, the back reference matches the character
string corresponding to the previous iteration. In order for this to
work, the pattern must be such that the first iteration does not need
to match the 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 assertion 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 subpatterns in the whole
pattern. However, substring capturing is carried out only for
positive assertions. (Perl sometimes, but not always, does do
capturing in negative assertions.)

WARNING: If a positive assertion containing one or more capturing
subpatterns succeeds, but failure to match later in the pattern
causes backtracking over this assertion, the captures within the
assertion are reset only if no higher numbered captures are already
set. This is, unfortunately, a fundamental limitation of the current
implementation, and as PCRE1 is now in maintenance-only status, it is
unlikely ever to change.

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 occasionally 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 subroutine mechanism.

(2) If quantifier is {0,n} where n is greater than zero, it is
treated as if it were {0,1}. At run time, the rest of the pattern
match is tried with and without the assertion, the order depending on
the 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 similar pattern

(?!foo)bar

does not find an occurrence of "bar" that is preceded by something
other than "foo"; it finds any occurrence of "bar" whatsoever,
because the assertion (?!foo) is always true when the next three
characters are "bar". A lookbehind assertion is needed to achieve the
other effect.

If you want to force a matching failure at some point in a pattern,
the most convenient way to do it is with (?!) because an empty string
always matches, so an assertion that requires there not to be an
empty string must always fail. The backtracking control verb (*FAIL)
or (*F) is a synonym for (?!).

Lookbehind assertions

Lookbehind assertions start with (?<= for positive assertions and
(?<! for negative assertions. For example,

(?<!foo)bar

does find an occurrence of "bar" that is not preceded by "foo". The
contents of a lookbehind assertion are restricted such that all the
strings it matches must have a fixed length. However, if there are
several top-level alternatives, they do not all have to have the same
fixed length. Thus

(?<=bullock|donkey)

is permitted, but

(?<!dogs?|cats?)

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

(?<=ab(c|de))

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

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

In 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 efficient matching of fixed-length strings at
the end of subject strings. Consider a simple pattern such as

abcd$

when applied to a long string that does not match. Because matching
proceeds from left to right, 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 subsequent lookbehind assertion does a single test
on the last four characters. If it fails, the match fails
immediately. For long strings, this approach makes a significant
difference to the processing time.

Using multiple assertions

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

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

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

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

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

Assertions can be nested in any combination. For example,

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

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

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

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

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 occurs. Each of
the two alternatives may itself contain nested subpatterns of any
form, including 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 subsequent groups. The next
parentheses to be opened can be referenced as (?(+1), and so on. (The
value zero in any of these forms is not used; it provokes a compile-
time error.)

Consider the following pattern, which contains non-significant white
space to make it more readable (assume the PCRE_EXTENDED option) and
to divide it into three parts for ease of discussion:

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

The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The
second part matches one or more characters that are not parentheses.
The third part is a conditional subpattern that tests whether or not
the first set of parentheses matched. If they did, that is, if
subject started with an opening parenthesis, the condition is true,
and so the yes-pattern is executed and a closing parenthesis is
required. Otherwise, since no-pattern is not present, the subpattern
matches nothing. In other words, this pattern matches a sequence of
non-parentheses, optionally enclosed in parentheses.

If you were embedding this pattern in a larger one, you could use a
relative reference:

...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...

This makes the fragment independent of the parentheses in the larger
pattern.

Checking for a used subpattern by name

Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
used subpattern by name. For compatibility with earlier versions of
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 subpatterns 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 patterns 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 subroutines is described
below.) For example, a pattern to match an IPv4 address such as
"192.168.23.245" could be written like this (ignore white space and
line breaks):

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

The first part of the pattern is a DEFINE group inside which 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 containing non-
significant white space, and with the two alternatives on the second
line:

(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )

The condition is a positive lookahead assertion that matches an
optional sequence of non-letters followed by a letter. In other
words, it tests for the presence of at least one letter in the
subject. If a letter is found, the subject is matched against the
first alternative; otherwise it is matched against the second. This
pattern matches strings in one of the two forms dd-aaa-dd or dd-dd-
dd, where aaa are letters and dd are digits.

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
characters that make up a comment play no part in the pattern
matching.

The sequence (?# marks the start of a comment that continues up to
the next closing parenthesis. Nested parentheses are not permitted.
If the 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
sequence at the start of the pattern, as described in the section
entitled "Newline conventions" above. Note that the end of this type
of comment is a literal newline sequence in the pattern; escape
sequences that happen to represent a newline do not count. For
example, consider this pattern when 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 comment. Only an actual
character with the code value 0x0a (the default newline) does so.

RECURSIVE PATTERNS

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

For some time, Perl has provided a facility that allows regular
expressions to recurse (amongst other things). It does this by
interpolating Perl code in the expression at run time, and the code
can refer to the expression itself. A Perl pattern using code
interpolation to solve the parentheses problem can be created like
this:

$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

The (?p{...}) item interpolates Perl code at run time, and in this
case refers recursively to the pattern in which it appears.

Obviously, 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. After 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 parentheses that are
referenced. They are always non-recursive subroutine calls, as
described in the next section.

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

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

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

This particular example pattern that we have been looking at contains
nested unlimited repeats, and so the use of a possessive quantifier
for matching strings of non-parentheses is important when applying
the pattern to strings that do not match. For example, when this
pattern is applied to

(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

it yields "no match" quickly. However, if a possessive quantifier is
not used, the match runs for a very long time indeed because there
are so many different ways the + and * repeats can carve up the
subject, and all have to be tested before failure can be reported.

At the end of a match, the values of capturing parentheses are those
from the outermost level. If you want to obtain intermediate values,
a callout function can be used (see below and the 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 final 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_ERROR_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 digits are allowed in
nested brackets (that is, when recursing), whereas any characters are
permitted at the outer level.

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

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

Differences in recursion processing between 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 number of
characters (for example, "a", "aba", "abcba", "abcdcba"):

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

The idea is that it either matches a single character, or two
identical characters surrounding 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 alternative 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
alternatives 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 quantifier *+ 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
succeeds. 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 responsibility". If instead the pattern

(sens|respons)e and (?1)ibility

is used, it does match "sense and responsibility" as well as the
other two strings. Another example is given in the discussion of
DEFINE above.

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 afterwards.

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 enclosed either in angle brackets or single
quotes, is an alternative syntax for referencing a subpattern as a
subroutine, possibly recursively. Here are two of the examples used
above, 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 feature is called "callout". The caller of
PCRE provides an external function by putting its entry 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 conditional group in the
pattern whose condition is an assertion, an additional callout is
inserted just before the condition. An explicit callout may also be
set at this position, as in this example:

(?(?C9)(?=a)abc|def)

Note that this applies only to assertion conditions, not to other
types of condition.

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 described in the Perl documentation as
"experimental and subject to change or removal in a future 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
failing 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 subject, or that
a particular character must be present. When one of these
optimizations bypasses the running of a match, any included
backtracking verbs will not, of course, be processed. You can
suppress the start-of-match optimizations by setting the
PCRE_NO_START_OPTIMIZE 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. However, when it is inside a subpattern
that is called as a subroutine, only that subpattern is ended
successfully. Matching then continues at the outer level. If
(*ACCEPT) in triggered in a positive assertion, the assertion
succeeds; in a negative assertion, the assertion fails.

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

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

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

(*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 combined 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) below).

(*MARK:NAME) or (*:NAME)

A name is always required with this verb. There may be as many
instances of (*MARK) as you like in a pattern, and their names do not
have to be unique.

When a match succeeds, the name of the last-encountered (*MARK:NAME),
(*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 assertions or failing
positive assertions.

After a partial match or a failed match, the last encountered name in
the entire match process is returned. For example:

re> /X(*MARK:A)Y|X(*MARK:B)Z/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 follows, 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 assertion. (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 assertion.
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 (*COMMIT) may be triggered first, so merely passing
(*COMMIT) during a match does not always guarantee that a match must
be at this starting point.

Note that (*COMMIT) at the start of a pattern is not the same as an
anchor, unless 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 attempt 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 unanchored, the normal
"bumpalong" advance to the next starting character then happens.
Backtracking can occur as usual to the left of (*PRUNE), before it is
reached, or when matching to the right of (*PRUNE), but if there is
no match to the right, backtracking cannot cross (*PRUNE). In simple
cases, the use of (*PRUNE) is just an alternative to an atomic group
or possessive quantifier, but there are some uses of (*PRUNE) that
cannot be expressed in any other way. In an anchored pattern (*PRUNE)
has the same effect as (*COMMIT).

The behaviour of (*PRUNE:NAME) is 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 unanchored, the "bumpalong" advance is not to the
next character, but to the position in the subject where (*SKIP) was
encountered. (*SKIP) signifies that whatever text was matched leading
up to it cannot be part of a successful match. Consider:

a+(*SKIP)b

If the subject is "aaaac...", after the first match attempt fails
(starting at the first character in the string), the starting point
skips on to start the next attempt at "c". Note that a possessive
quantifier does not have the same effect as this example; although it
would suppress backtracking during the first match attempt, the
second attempt would start at the second character instead of
skipping on to "c".

(*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 beyond such a
subpattern to the enclosing alternative. Consider this pattern, where
A, B, etc. are complex pattern fragments that do not contain any |
characters at this level:

A (B(*THEN)C) | D

If A and B are matched, but there is a failure in C, matching does
not backtrack into A; instead it moves to the next alternative, that
is, D. However, if the subpattern containing (*THEN) is given an
alternative, it behaves differently:

A (B(*THEN)C | (*FAIL)) | D

The effect of (*THEN) is now confined to the inner subpattern. After
a failure in C, matching moves to (*FAIL), which causes the whole
subpattern to fail because there are no more alternatives to try. In
this case, matching does now backtrack into A.

Note that a conditional subpattern is not considered as having two
alternatives, because only one is ever used. In other words, the |
character in a conditional subpattern has a different meaning.
Ignoring white space, consider:

^.*? (?(?=a) a | b(*THEN)c )

If the subject is "ba", this pattern does not match. Because .*? is
ungreedy, it initially matches zero characters. The condition (?=a)
then fails, the character "b" is matched, but "c" is not. At this
point, matching does not backtrack to .*? as might perhaps be
expected from the presence of the | character. The conditional
subpattern is part of the single alternative that comprises the whole
pattern, and so the match fails. (If there was a backtrack into .*?,
allowing it to match "b", the match would succeed.)

The verbs just described provide four different "strengths" of
control when subsequent matching fails. (*THEN) is the weakest,
carrying on the match at the next alternative. (*PRUNE) comes next,
failing the match at the current starting position, but allowing an
advance to the next character (for an unanchored pattern). (*SKIP) is
similar, except that the advance may be more than one character.
(*COMMIT) is the strongest, causing the entire match to fail.

More than one backtracking verb

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

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

If A matches but B fails, the backtrack to (*COMMIT) causes the
entire match to fail. However, if A and B match, but C fails, the
backtrack to (*THEN) causes the next alternative (ABD) to be tried.
This behaviour is consistent, but is not always the same as Perl's.
It means that if two or more backtracking verbs appear in succession,
all the the last of them has no effect. Consider this example:

...(*COMMIT)(*PRUNE)...

If there is a matching failure to the right, backtracking onto
(*PRUNE) causes it to be triggered, and its action is taken. There
can never be a backtrack onto (*COMMIT).

Backtracking verbs in repeated groups

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 processing. 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 assertion. 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 assertion into a negative assertion changes its
result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a
negative assertion to be true, without considering any further
alternative branches in the assertion. Backtracking into (*THEN)
causes it to skip to the next enclosing 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 without 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.

SEE ALSO

pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3),
pcre16(3), pcre32(3).

AUTHOR

Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.

REVISION

Last updated: 23 October 2016
Copyright (c) 1997-2016 University of Cambridge.

PCRE 8.40 23 October 2016 PCREPATTERN(3)