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.
The original operation of PCRE was on strings of one-byte characters. However,
there is now also support for UTF-8 character strings. To use this,
PCRE must be built to include UTF-8 support, and you must call
pcre_compile() or pcre_compile2() with the PCRE_UTF8 option. There
is also a special sequence that can be given at the start of a pattern:
(*UTF8)
Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
option. This feature is not Perl-compatible. How setting UTF-8 mode affects
pattern matching is mentioned in several places below. There is also a summary
of UTF-8 features in the
HTML <a href="pcre.html#utf8support">
</a>
section on UTF-8 support
in the main
HREF
pcre
page.
Another special sequence that may appear at the start of a pattern or in combination with (*UTF8) is: (*UCP) This has the same effect as setting the PCRE_UCP option: it causes sequences such as \ed and \ew to use Unicode properties to determine character types, instead of recognizing only characters with codes less than 128 via a lookup table.
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. There are also some more of these special sequences that are concerned with the handling of newlines; they are described below.
The remainder of this document discusses the patterns that are supported by
PCRE when its main matching function, pcre_exec(), is used.
From release 6.0, PCRE offers a second matching function,
pcre_dfa_exec(), which matches using a different algorithm that is not
Perl-compatible. Some of the features discussed below are not available when
pcre_dfa_exec() is used. The advantages and disadvantages of the
alternative function, and how it differs from the normal function, are
discussed in the
HREF
pcrematching
page.
.
.
HTML <a name="newlines"></a>
pcreapi
page has HTML <a href="pcreapi.html#newlines">
</a>
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 pcre_compile() or pcre_compile2(). 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\enb" because LF is no longer a newline. 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.
The newline convention affects the interpretation of the dot metacharacter when
PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
affect what the \eR escape sequence matches. By default, this is any Unicode
newline sequence, for Perl compatibility. However, this can be changed; see the
description of \eR in the section entitled
HTML <a href="#newlineseq">
</a>
"Newline sequences"
below. A change of \eR setting can be combined with a change of newline
convention.
.
.
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:
\e 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:
\e general escape character
^ negate the class, but only if the first character
- indicates character range
JOIN
[ POSIX character class (only if followed by POSIX
syntax)
] terminates the character class
The following sections describe the use of each of the metacharacters.
.
.
For example, if you want to match a * character, you write \e* 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 \e\e.
In UTF-8 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, whitespace in the pattern (other than in a character class) and characters between a # outside a character class and the next newline are ignored. An escaping backslash can be used to include a whitespace 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 \eQ and \eE. This is different from Perl in
that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
Perl, $ and @ cause variable interpolation. Note the following examples:
Pattern PCRE matches Perl matches
JOIN
\eQabc$xyz\eE abc$xyz abc followed by the
contents of $xyz
\eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
\eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
The \eQ...\eE sequence is recognized both inside and outside character classes.
An isolated \eE that is not preceded by \eQ is ignored.
.
.
HTML <a name="digitsafterbackslash"></a>
After \ex, from zero to two hexadecimal digits are read (letters can be in upper or lower case). Any number of hexadecimal digits may appear between \ex{ and }, but the value of the character code must be less than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code point, which is 10FFFF.
If characters other than hexadecimal digits appear between \ex{ and }, or if there is no terminating }, this form of escape is not recognized. Instead, the initial \ex will be interpreted as a basic hexadecimal escape, with no following digits, giving a character whose value is zero.
Characters whose value is less than 256 can be defined by either of the two syntaxes for \ex. There is no difference in the way they are handled. For example, \exdc is exactly the same as \ex{dc}.
After \e0 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 \e0\ex\e07 specifies two binary zeros followed by a BEL character (code value 7). Make sure you supply two digits after the initial zero if the pattern character that follows is itself an octal digit.
The handling of a backslash followed by a digit other than 0 is complicated.
Outside a character class, PCRE reads it and any following digits as a decimal
number. If the number is less than 10, 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
HTML <a href="#backreferences">
</a>
later,
following the discussion of
HTML <a href="#subpattern">
</a>
parenthesized subpatterns.
Inside a character class, or if the decimal number is greater than 9 and there
have not been that many capturing subpatterns, PCRE re-reads up to three octal
digits following the backslash, and uses them to generate a data character. Any
subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
character specified in octal must be less than \e400. In UTF-8 mode, values up
to \e777 are permitted. For example:
\e040 is another way of writing a space
JOIN
\e40 is the same, provided there are fewer than 40
previous capturing subpatterns
\e7 is always a back reference
JOIN
\e11 might be a back reference, or another way of
writing a tab
\e011 is always a tab
\e0113 is a tab followed by the character "3"
JOIN
\e113 might be a back reference, otherwise the
character with octal code 113
JOIN
\e377 might be a back reference, otherwise
the byte consisting entirely of 1 bits
JOIN
\e81 is either a back reference, or a binary zero
followed by the two characters "8" and "1"
Note that octal values of 100 or greater must not be introduced by a leading
zero, because no more than three octal digits are ever read.
All the sequences that define a single character value can be used both inside and outside character classes. In addition, inside a character class, the sequence \eb is interpreted as the backspace character (hex 08). The sequences \eB, \eN, \eR, and \eX 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. Outside a character class, these sequences have different meanings. . .
</a>
later,
following the discussion of HTML <a href="#subpattern">
</a>
parenthesized subpatterns.
. .
</a>
later.
Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are not synonymous. The former is a back reference; the latter is a HTML <a href="#subpatternsassubroutines">
</a>
subroutine
call. . . HTML <a name="genericchartypes"></a>
</a>
the "." metacharacter
when PCRE_DOTALL is not set.
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, \es does not match the VT character (code 11). This makes it different from the the POSIX "space" class. The \es characters are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is included in a Perl script, \es may match the VT character. In PCRE, it never does.
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
HTML <a href="pcreapi.html#localesupport">
</a>
"Locale support"
in the
HREF
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 128 are used for
accented letters, and these are then matched by \ew. The use of locales with
Unicode is discouraged.
By default, in UTF-8 mode, characters with values greater than 128 never match \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain their original meanings from before UTF-8 support was available, mainly for efficiency reasons. However, 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: \ed any character that \ep{Nd} matches (decimal digit) \es any character that \ep{Z} matches, plus HT, LF, FF, CR \ew any character that \ep{L} or \ep{N} matches, plus underscore The upper case escapes match the inverse sets of characters. Note that \ed matches only decimal digits, whereas \ew matches any Unicode digit, as well as any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and \eB because they are defined in terms of \ew and \eW. Matching these sequences is noticeably slower when PCRE_UCP is set.
The sequences \eh, \eH, \ev, and \eV 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 codepoints in
UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
are:
U+0009 Horizontal tab
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
U+000B Vertical tab
U+000C Formfeed
U+000D Carriage return
U+0085 Next line
U+2028 Line separator
U+2029 Paragraph separator
.
.
HTML <a name="newlineseq"></a>
</a>
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 (formfeed, 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 UTF-8 mode, 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 \eR to match only CR, LF, or CRLF (instead of the
complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
either at compile time or when the pattern is matched. (BSR is an abbrevation
for "backslash R".) This can be made the 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 pcre_compile() or
pcre_compile2(), but they can be overridden by options given to
pcre_exec() or pcre_dfa_exec(). 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) or (*UCP) special sequences. Inside
a character class, \eR is treated as an unrecognized escape sequence, and so
matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
.
.
HTML <a name="uniextseq"></a>
</a>
next section).
Other Perl properties such as "InMusicalSymbols" are not currently supported by PCRE. Note that \eP{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: \ep{Greek} \eP{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, Bengali, Bopomofo, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyptian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam, Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic, Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian, Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai, 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, \ep{^Lu} is the same as \eP{Lu}.
If only one letter is specified with \ep or \eP, 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: \ep{L} \epL 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 UTF-8 strings (see RFC 3629) and so
cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
(see the discussion of PCRE_NO_UTF8_CHECK in the
HREF
pcreapi
page). Perl does not support the Cs property.
The long synonyms for property names that Perl supports (such as \ep{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, \ep{Lu} always matches only upper case letters.
The \eX escape matches any number of Unicode characters that form an extended
Unicode sequence. \eX is equivalent to
(?>\ePM\epM*)
That is, it matches a character without the "mark" property, followed by zero
or more characters with the "mark" property, and treats the sequence as an
atomic group
HTML <a href="#atomicgroup">
</a>
(see below).
Characters with the "mark" property are typically accents that affect the
preceding character. None of them have codepoints less than 256, so in
non-UTF-8 mode \eX matches any one character.
Matching characters by Unicode property is not fast, because PCRE has to search
a structure that contains data for over fifteen thousand characters. That is
why the traditional escape sequences such as \ed and \ew do not use Unicode
properties in PCRE by default, though you can make them do so by setting the
PCRE_UCP option for pcre_compile() or by starting the pattern with
(*UCP).
.
.
HTML <a name="extraprops"></a>
</a>
(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 \eK does not interfere with the setting of HTML <a href="#subpattern">
</a>
captured substrings.
For example, when the pattern (foo)\eKbar matches "foobar", the first substring is still set to "foo".
Perl documents that the use of \eK within assertions is "not well defined". In
PCRE, \eK is acted upon when it occurs inside positive assertions, but is
ignored in negative assertions.
.
.
HTML <a name="smallassertions"></a>
</a>
below.
The backslashed assertions are: \eb matches at a word boundary \eB matches when not at a word boundary \eA matches at the start of the subject \eZ matches at the end of the subject also matches before a newline at the end of the subject \ez matches only at the end of the subject \eG matches at the first matching position in the subject Inside a character class, \eb 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, \eB 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 \ew or \eW (i.e. one matches \ew and the other matches \eW), or the start or end of the string if the first or last character matches \ew, respectively. In UTF-8 mode, the meanings of \ew and \eW can be changed by setting the PCRE_UCP option. When this is done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start of word" or "end of word" metasequence. However, whatever follows \eb normally determines which it is. For example, the fragment \eba matches "a" at the start of a word.
The \eA, \eZ, and \ez 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, \eA can never match. The difference between \eZ and \ez is that \eZ matches before a newline at the end of the string as well as at the very end, whereas \ez matches only at the end.
The \eG 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 \eA 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 \eG can be useful.
Note, however, that PCRE's interpretation of \eG, 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 \eG, the expression is anchored to the starting match position, and the "anchored" flag is set in the compiled regular expression. . .
</a>
(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.)
A 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). 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 \eZ 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\enabc" (where \en 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 \eA, \eZ, and \ez can be used to match the start and
end of the subject in both modes, and if all branches of a pattern start with
\eA it is always anchored, whether or not PCRE_MULTILINE is set.
.
.
HTML <a name="fullstopdot"></a>
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 \eN 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. . .
PCRE does not allow \eC to appear in lookbehind assertions
HTML <a href="#lookbehind">
</a>
(described below),
because in UTF-8 mode this would make it impossible to calculate the length of
the lookbehind.
.
.
HTML <a name="characterclass"></a>
A character class matches a single character in the subject. In UTF-8 mode, the character may be more than one byte 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 mode, characters with values greater than 255 can be included in a class as a literal string of bytes, or by using the \ex{ 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 UTF-8 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 UTF8-mode for characters 128 and above, you must ensure that PCRE is compiled with Unicode property support as well as with UTF-8 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.
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-\e]46] is interpreted as a class containing a range followed by two other characters. The octal or hexadecimal representation of "]" can also be used to end a range.
Ranges operate in the collating sequence of character values. They can also be used for characters specified numerically, for example [\e000-\e037]. In UTF-8 mode, ranges can include characters whose values are greater than 255, for example [\ex{100}-\ex{2ff}].
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 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character tables for a French locale are in use, [\exc8-\excb] matches accented E characters in both cases. In UTF-8 mode, 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 \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
\eV, \ew, and \eW may appear in a character class, and add the characters that
they match to the class. For example, [\edABCDEF] matches any hexadecimal
digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
and their upper case partners, just as it does when they appear outside a
character class, as described in the section entitled
HTML <a href="#genericchartypes">
</a>
"Generic character types"
above. The escape sequence \eb has a different meaning inside a character
class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
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 [^\eW_] matches any letter or digit, but not underscore, whereas [\ew] 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 - see the next section), and the terminating closing square bracket. However, escaping other non-alphanumeric characters does no harm. . .
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, in UTF-8 mode, 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 the POSIX classes by other sequences, as follows: [:alnum:] becomes \ep{Xan} [:alpha:] becomes \ep{L} [:blank:] becomes \eh [:digit:] becomes \ep{Nd} [:lower:] becomes \ep{Ll} [:space:] becomes \ep{Xps} [:upper:] becomes \ep{Lu} [:word:] becomes \ep{Xwd} Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX classes are unchanged, and match only characters with code points less than 128. . .
</a>
(defined below),
"succeeds" means matching the rest of the main pattern as well as the alternative in the subpattern. . .
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. If the change is placed right at the start of a pattern, PCRE extracts it into the global options (and it will therefore show up in data extracted by the pcre_fullinfo() function).
An option change within a subpattern (see below for a description of subpatterns) affects only that part of the subpattern that follows it, so (a(?i)b)c matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). By this means, options can be made to have different settings in different parts of the pattern. Any changes made in one alternative do carry on into subsequent branches within the same 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 compile or match 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
HTML <a href="#newlineseq">
</a>
"Newline sequences"
above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
to set UTF-8 and Unicode property modes; they are equivalent to setting the
PCRE_UTF8 and the PCRE_UCP options, respectively.
.
.
HTML <a name="subpattern"></a>
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".
.
.
HTML <a name="dupsubpatternnumber"></a>
</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. . .
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
HTML <a href="#backreferences">
</a>
back references,
HTML <a href="#recursion">
</a>
recursion,
and
HTML <a href="#conditions">
</a>
conditions,
can be made by name as well as by number.
Names consist of up to 32 alphanumeric characters and underscores. 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 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
HTML <a href="#conditions">
</a>
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
HREF
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 give the same name to subpatterns with the same number, even when PCRE_DUPNAMES is not set. . .
In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of which is represented by a two-byte sequence. Similarly, when Unicode property support is available, \eX{3} matches three Unicode extended sequences, each of which may be several bytes 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
HTML <a href="#subpatternsassubroutines">
</a>
subroutines
from elsewhere in the pattern (but see also the section entitled
HTML <a href="#subdefine">
</a>
"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 /\e*.*\e*/ 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 /\e*.*?\e*/ 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 \ed??\ed 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 \eA.
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 is one situation 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\e1 If the subject is "xyz123abc123" the match point is the fourth character. For this reason, such a pattern is not implicitly anchored.
When a capturing subpattern is repeated, the value captured is the substring
that matched the final iteration. For example, after
(tweedle[dume]{3}\es*)+
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".
.
.
HTML <a name="atomicgroup"></a>
Consider, for example, the pattern \ed+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 \ed+ 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: (?>\ed+)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 \ed+ and \ed+? are prepared to adjust the number of digits they match in order to make the rest of the pattern match, (?>\ed+) 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 \ed++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
(\eD+|<\ed+>)*[!?]
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 \eD+ 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:
((?>\eD+)|<\ed+>)*[!?]
sequences of non-digits cannot be broken, and failure happens quickly.
.
.
HTML <a name="backreferences"></a>
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 \e50 is
interpreted as a character defined in octal. See the subsection entitled
"Non-printing characters"
HTML <a href="#digitsafterbackslash">
</a>
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 \eg 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), \e1 (ring), \eg1 (ring), \eg{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)\eg{-1} The sequence \eg{-1} is a reference to the most recently started capturing subpattern before \eg, that is, is it equivalent to \e2 in this example. Similarly, \eg{-2} would be equivalent to \e1. 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
HTML <a href="#subpatternsassubroutines">
</a>
"Subpatterns as subroutines"
below for a way of doing that). So the pattern
(sens|respons)e and \e1ibility
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)\es+\e1
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 \ek{name} and the Perl syntax \ek<name> or \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified back reference syntax, in which \eg 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)\es+\ek<p1> (?'p1'(?i)rah)\es+\ek{p1} (?P<p1>(?i)rah)\es+(?P=p1) (?<p1>(?i)rah)\es+\eg{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))\e2 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
whitespace. Otherwise, the \eg{ syntax or an empty comment (see
HTML <a href="#comments">
</a>
"Comments"
below) can be used.
.
Back references of this type cause the group that they reference to be treated
as an
HTML <a href="#atomicgroup">
</a>
atomic group.
Once the whole group has been matched, a subsequent matching failure cannot
cause backtracking into the middle of the group.
.
.
HTML <a name="bigassertions"></a>
</a>
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, and may not be repeated, because it makes no sense to assert the same thing several times. If any kind of 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, because it does not make sense for negative assertions. . .
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 (?!).
.
.
HTML <a name="lookbehind"></a>
</a>
(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.
PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode) to appear in lookbehind assertions, because it makes it impossible to calculate the length of the lookbehind. The \eX and \eR escapes, which can match different numbers of bytes, are also not permitted.
HTML <a href="#subpatternsassubroutines">
</a>
"Subroutine"
calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
as the subpattern matches a fixed-length string.
HTML <a href="#recursion">
</a>
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. . .
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
(?<=\ed{3}(?!999)...)foo
is another pattern that matches "foo" preceded by three digits and any three
characters that are not "999".
.
.
HTML <a name="conditions"></a>
There are four kinds of condition: references to subpatterns, references to recursion, a pseudo-condition called DEFINE, and assertions. .
HTML <a href="#recursion">
</a>
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: ( \e( )? [^()]+ (?(1) \e) ) 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... ( \e( )? [^()]+ (?(-1) \e) ) ... This makes the fragment independent of the parentheses in the larger pattern. .
Rewriting the above example to use a named subpattern gives this: (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) ) 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. .
At "top level", all these recursion test conditions are false.
HTML <a href="#recursion">
</a>
The syntax for recursive patterns
is described below.
.
HTML <a name="subdefine"></a>
</a>
"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 whitespace and line breaks): (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) ) \eb (?&byte) (\e.(?&byte)){3} \eb 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. .
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 pcre_compile() or by a special
sequence at the start of the pattern, as described in the section entitled
HTML <a href="#newlines">
</a>
"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 \en still comment
On encountering the # character, pcre_compile() skips along, looking for
a newline in the pattern. The sequence \en 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.
.
.
HTML <a name="recursion"></a>
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: $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}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 call of the subpattern of the given number,
provided that it occurs inside that subpattern. (If not, it is a
HTML <a href="#subpatternsassubroutines">
</a>
"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): \e( ( [^()]++ | (?R) )* \e) 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: ( \e( ( [^()]++ | (?1) )* \e) ) 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
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"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> \e( ( [^()]++ | (?&pn) )* \e) ) 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
HREF
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 value is unset, even if it is (temporarily)
set at a deeper level.
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) \ed++ | [^<>]*+) | (?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.
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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)\e2|.)$ 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)\e2|.?)$ 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)\e2|)|((.)(?3)\e4|.)) If you want to match typical palindromic phrases, the pattern has to ignore all non-word characters, which can be done like this: ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$ 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.
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Like recursive subpatterns, a subroutine 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. Any capturing parentheses that are set during the subroutine call revert to their previous values afterwards.
When a subpattern is used as a subroutine, processing options such as
case-independence are fixed when the subpattern is defined. They 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.
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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. 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 pcre_compile(), callouts are automatically installed before each item in the pattern. They are all numbered 255.
During matching, when PCRE reaches a callout point (and pcre_callout is
set), 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 pcre_exec(). The callout function
may cause matching to proceed, to backtrack, or to fail altogether. A complete
description of the interface to the callout function is given in the
HREF
pcrecallout
documentation.
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Since these verbs are specifically related to backtracking, most of them can be used only when the pattern is to be matched using pcre_exec(), which uses a backtracking algorithm. With the exception of (*FAIL), which behaves like a failing negative assertion, they cause an error if encountered by pcre_dfa_exec().
If any of these verbs are used in an assertion or subroutine subpattern (including recursive subpatterns), their effect is confined to that subpattern; it does not extend to the surrounding pattern. Note that such subpatterns are processed as anchored at the point where they are tested.
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, with differing behaviour, depending on whether or not an argument is present. An name is a sequence of letters, digits, and underscores. 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.
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 suppresses 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). . .
When a match succeeds, the name of the last-encountered (*MARK) is passed back
to the caller via the pcre_extra data structure, as described in the
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section on pcre_extra
in the
HREF
pcreapi
documentation. No data is returned for a partial match. Here is an example of
pcretest output, where the /K modifier requests the retrieval and
outputting of (*MARK) data:
/X(*MARK:A)Y|X(*MARK:B)Z/K
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.
A name may also be returned after a failed match if the final path through the pattern involves (*MARK). However, unless (*MARK) used in conjunction with (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the starting point for matching is advanced, the final check is often with an empty string, causing a failure before (*MARK) is reached. For example: /X(*MARK:A)Y|X(*MARK:B)Z/K XP No match There are three potential starting points for this match (starting with X, starting with P, and with an empty string). If the pattern is anchored, the result is different: /^X(*MARK:A)Y|^X(*MARK:B)Z/K XP No match, mark = B PCRE's start-of-match optimizations can also interfere with this. For example, if, as a result of a call to pcre_study(), it knows the minimum subject length for a match, a shorter subject will not be scanned at all.
Note that similar anomalies (though different in detail) exist in Perl, no doubt for the same reasons. The use of (*MARK) data after a failed match of an unanchored pattern is not recommended, unless (*COMMIT) is involved. . .
These verbs differ in exactly what kind of failure occurs when backtracking reaches them. (*COMMIT) This verb, which may not be followed by a name, causes the whole match to fail outright if the rest of the pattern does not match. Even if the pattern is unanchored, no further attempts to find a match by advancing the starting point take place. Once (*COMMIT) 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.
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 pcretest example: /(*COMMIT)abc/ xyzabc 0: abc xyzabc\eY No match PCRE knows that any match must start with "a", so the optimization skips along the subject to "a" before running the first match attempt, which succeeds. When the optimization is disabled by the \eY escape in the second subject, the match starts at "x" and so the (*COMMIT) causes it 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 the rest of the pattern does not match. 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. The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the match fails completely; the name is passed back if this is the final attempt. (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored pattern (*PRUNE) has the same effect as (*COMMIT). (*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 quantifer does not have the same effect as this example; although it would suppress backtracking during the first match attempt, the second attempt would start at the second character instead of skipping on to "c". (*SKIP:NAME) When (*SKIP) has an associated name, its behaviour is modified. If the following pattern fails to match, 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, normal "bumpalong" of one character happens (the (*SKIP) is ignored). (*THEN) or (*THEN:NAME) This verb causes a skip to the next alternation in the innermost enclosing group if the rest of the pattern does not match. That is, it cancels pending backtracking, but only within the current alternation. 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. The behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the overall match fails. If (*THEN) is not directly inside an alternation, it acts like (*PRUNE). .
The above verbs provide four different "strengths" of control when subsequent matching fails. (*THEN) is the weakest, carrying on the match at the next alternation. (*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.
If more than one is present in a pattern, the "stongest" one wins. For example, consider this pattern, where A, B, etc. are complex pattern fragments: (A(*COMMIT)B(*THEN)C|D) Once A has matched, PCRE is committed to this match, at the current starting position. If subsequently B matches, but C does not, the normal (*THEN) action of trying the next alternation (that is, D) does not happen because (*COMMIT) overrides. . .
Philip Hazel University Computing Service Cambridge CB2 3QH, England.. .
Last updated: 21 November 2010 Copyright (c) 1997-2010 University of Cambridge.
