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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 documen-

tation, and regular expressions in general are covered in a

number of books, some of which have copious examples. Jef-

frey 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, you must build PCRE to

include UTF-8 support, and then call pcre_compile() with the

PCRE_UTF8 option. How this affects pattern matching is men-

tioned in several places below. There is also a summary of

UTF-8 features in the section on UTF-8 support in the main

pcre page. 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 pcrematching page. 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 new-

lines, and shows how to set the newline convention in the options arguments for the compiling and matching functions. SunOS 5.10 Last change: 1 Introduction to Library Functions PCREPATTERN(3) 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(). 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. 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 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. CHARACTERS AND METACHARACTERS A regular expression is a pattern that is matched against a subject string from left to right. Most characters stand for

themselves in a pattern, and match the corresponding charac-

ters in the subject. As a trivial example, the pattern The quick brown fox matches a portion of a subject string that is identical to itself. When caseless matching is specified (the

PCRE_CASELESS option), letters are matched independently of

case. In UTF-8 mode, PCRE always understands the concept of

case for characters whose values are less than 128, so case-

less 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-8 support.

SunOS 5.10 Last change: 2 Introduction to Library Functions PCREPATTERN(3) 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 inter-

preted 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 brack-

ets. Outside square brackets, the metacharacters are as fol-

lows: \ 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 metacharac-

ters 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 meta-

characters. BACKSLASH The backslash character has several uses. Firstly, if it is

followed by a non-alphanumeric character, 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 SunOS 5.10 Last change: 3 Introduction to Library Functions PCREPATTERN(3) 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 \\.

If a pattern is compiled with the PCRE_EXTENDED option, whi-

tespace 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 pat-

tern. 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 exam-

ples: 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.

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 charac-

ters, apart from the binary zero that terminates a pattern, but when a pattern is being prepared by text editing, it is usually easier to use one of the following escape sequences than the binary character it represents: \a alarm, that is, the BEL character (hex 07)

\cx "control-x", where x is any character

\e escape (hex 1B) \f formfeed (hex 0C) \n linefeed (hex 0A) \r carriage return (hex 0D) \t tab (hex 09) \ddd character with octal code ddd, or backreference \xhh character with hex code hh \x{hhh..} character with hex code hhh.. The precise effect of \cx is as follows: if x is a lower SunOS 5.10 Last change: 4 Introduction to Library Functions PCREPATTERN(3) case letter, it is converted to upper case. Then bit 6 of the character (hex 40) is inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex 7B. After \x, from zero to two hexadecimal digits are read

(letters can be in upper or lower case). Any number of hexa-

decimal digits may appear between \x{ 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 max-

imum 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 \x{ and }, or if there is no terminating }, this form of escape is not recognized. Instead, the initial \x 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 \x. There is no difference in the way they are handled. For example, \xdc is exactly the same as \x{dc}. 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\07 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 later, following the discussion of 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 follow-

ing the backslash, and uses them to generate a data charac-

ter. Any subsequent digits stand for themselves. In non-

UTF-8 mode, the value of a character specified in octal must

be less than \400. In UTF-8 mode, values up to \777 are per-

mitted. For example: \040 is another way of writing a space \40 is the same, provided there are fewer than 40 previous capturing subpatterns \7 is always a back reference SunOS 5.10 Last change: 5 Introduction to Library Functions PCREPATTERN(3) \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 byte consisting entirely of 1 bits \81 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 intro-

duced 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 addi-

tion, inside a character class, the sequence \b is inter-

preted as the backspace character (hex 08), and the sequences \R and \X are interpreted as the characters "R" and "X", respectively. Outside a character class, these sequences have different meanings (see below). 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 dis-

cussed 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. The following are always recognized: \d any decimal digit \D any character that is not a decimal digit \h any horizontal whitespace character \H any character that is not a horizontal whitespace character \s any whitespace character SunOS 5.10 Last change: 6 Introduction to Library Functions PCREPATTERN(3) \S any character that is not a whitespace character \v any vertical whitespace character \V any character that is not a vertical whitespace character \w any "word" character

\W any "non-word" character

Each pair of escape sequences partitions the complete set of characters into two disjoint sets. Any given character matches one, and only one, of each pair. These character type 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, since there is no character to match.

For compatibility with Perl, \s does not match the VT char-

acter (code 11). This makes it different from the the POSIX "space" class. The \s characters are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is included in a Perl script, \s may match the VT character. In PCRE, it never does.

In UTF-8 mode, characters with values greater than 128 never

match \d, \s, or \w, and always match \D, \S, and \W. This is true even when Unicode character property support is available. These sequences retain their original meanings

from before UTF-8 support was available, mainly for effi-

ciency reasons. The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to the other sequences, these do match certain

high-valued codepoints in UTF-8 mode. 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 SunOS 5.10 Last change: 7 Introduction to Library Functions PCREPATTERN(3)

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 A "word" character is an underscore or any character less than 256 that is a letter or digit. The definition of

letters and digits is controlled by PCRE's low-valued char-

acter 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 128 are used for accented letters, and these are matched by \w. The use of locales with Unicode is discouraged. Newline sequences Outside a character class, by default, the escape sequence \R matches any Unicode newline sequence. This is a Perl 5.10

feature. In non-UTF-8 mode \R is equivalent to the follow-

ing: (?>\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 sin-

gle 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 \R to match only CR, LF, or CRLF (instead of the complete set of Unicode line endings) by

setting the option PCRE_BSR_ANYCRLF either at compile time

or when the pattern is matched. (BSR is an abbrevation for SunOS 5.10 Last change: 8 Introduction to Library Functions PCREPATTERN(3) "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 pos-

sible 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(), but they can be overridden by options given

to pcre_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 exam-

ple, a pattern can start with:

(*ANY)(*BSR_ANYCRLF)

Inside a character class, \R matches the letter "R". 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 not in 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 an extended Unicode sequence The property names represented by xx above are limited to the Unicode script names, the general category properties, and "Any", which matches any character (including newline). Other 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 cer-

tain 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: SunOS 5.10 Last change: 9 Introduction to Library Functions PCREPATTERN(3) Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille,

Buginese, Buhid, Canadian_Aboriginal, Cherokee, Common, Cop-

tic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic, Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin, Limbu,

Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,

Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa,

Phoenician, Runic, Shavian, Sinhala, Syloti_Nagri, Syriac,

Tagalog, Tagbanwa, Tai_Le, Tamil, Telugu, Thaana, Thai,

Tibetan, Tifinagh, Ugaritic, Yi. Each character has exactly one general category property,

specified by a two-letter abbreviation. For compatibility

with Perl, negation can be specified by including a circum-

flex between the opening brace and the property name. For example, \p{^Lu} is the same as \P{Lu}. If only one letter is specified with \p or \P, it includes all the general category 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 SunOS 5.10 Last change: 10 Introduction to Library Functions PCREPATTERN(3) 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 pcreapi

page). The long synonyms for these properties 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 (unas-

signed) 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. The \X escape matches any number of Unicode characters that form an extended Unicode sequence. \X is equivalent to (?>\PM\pM*) That is, it matches a character without the "mark" property,

followed by zero or more characters with the "mark" pro-

perty, and treats the sequence as an atomic group (see SunOS 5.10 Last change: 11 Introduction to Library Functions PCREPATTERN(3) 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 \X

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 \d and \w do not use Unicode pro-

perties in PCRE. Resetting the match start The escape sequence \K, which is a Perl 5.10 feature, 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 inter-

fere with the setting of captured substrings. For example, when the pattern (foo)\Kbar matches "foobar", the first substring is still set to "foo". 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 charac-

ters 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 sub-

ject These assertions may not appear in character classes (but note that \b has a different meaning, namely the backspace SunOS 5.10 Last change: 12 Introduction to Library Functions PCREPATTERN(3) character, inside a character class). 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. 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 imple-

mentation 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 expres-

sion. CIRCUMFLEX AND DOLLAR 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). SunOS 5.10 Last change: 13 Introduction to Library Functions PCREPATTERN(3) 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 alter-

natives 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 con-

structs 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 pat-

tern 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 dol-

lar 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 cir-

cumflex 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) 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. In UTF-8 mode,

the matched character may be more than one byte long. SunOS 5.10 Last change: 14 Introduction to Library Functions PCREPATTERN(3) 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. MATCHING A SINGLE BYTE Outside a character class, the escape sequence \C matches

any one byte, both in and out of UTF-8 mode. Unlike a dot,

it always matches any line-ending characters. The feature is

provided in Perl in order to match individual bytes in UTF-8

mode. Because it breaks up UTF-8 characters into individual

bytes, what remains in the string may be a malformed UTF-8

string. For this reason, the \C escape sequence is best avoided. PCRE does not allow \C to appear in lookbehind assertions

(described below), because in UTF-8 mode this would make it

impossible to calculate the length of the lookbehind. SQUARE BRACKETS AND CHARACTER CLASSES

An opening square bracket introduces a character class, ter-

minated by a closing square bracket. A closing square bracket on its own is not special. 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 cir-

cumflex, if present) or escaped with a backslash. A character class matches a single character in the subject.

In UTF-8 mode, the character may occupy more than one byte.

A matched character must be in the set of characters defined

by the class, unless the first character in the class defin-

ition 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. SunOS 5.10 Last change: 15 Introduction to Library Functions PCREPATTERN(3) 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 con-

venient notation for specifying the characters that are in the class by enumerating those that are not. A class that

starts with a circumflex is not an assertion: it still con-

sumes a character from the subject string, and therefore it fails if the current pointer is at the end of the string.

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 \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 case-

ful 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 char-

acters 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 charac-

ters 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 inter-

preted 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 "-") fol-

lowed by a literal string "46]", so it would match "W46]" or

"-46]". However, if the "]" is escaped with a backslash it

is interpreted as the end of range, so [W-\]46] is inter-

preted as a class containing a range followed by two other characters. The octal or hexadecimal representation of "]" can also be used to end a range. Ranges operate in the collating sequence of character values. They can also be used for characters specified SunOS 5.10 Last change: 16 Introduction to Library Functions PCREPATTERN(3)

numerically, for example [\000-\037]. In UTF-8 mode, ranges

can include characters whose values are greater than 255,

for example [\x{100}-\x{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 [][\\^_`wxyzabc], matched

caselessly, and in non-UTF-8 mode, if character tables for a

French locale are in use, [\xc8-\xcb] 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 types \d, \D, \p, \P, \s, \S, \w, and \W may also appear in a character class, and add the characters that they match to the class. For example, [\dABCDEF] matches any hexadecimal digit. 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. The only metacharacters that are recognized in character

classes are backslash, hyphen (only where it can be inter-

preted as specifying a range), circumflex (only at the

start), opening square bracket (only when it can be inter-

preted as introducing a POSIX class name - see the next sec-

tion), 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 sup-

ported 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 space white space (not quite the same as \s) SunOS 5.10 Last change: 17 Introduction to Library Functions PCREPATTERN(3) upper upper case letters word "word" characters (same as \w) xdigit hexadecimal digits The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and space (32). Notice that this list

includes the VT character (code 11). This makes "space" dif-

ferent to \s, which does not include VT (for Perl compati-

bility). 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.

In UTF-8 mode, characters with values greater than 128 do

not match any of the POSIX character classes. 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 alter-

natives 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 sub-

pattern (defined below), "succeeds" means matching the rest

of the main pattern as well as the alternative in the sub-

pattern. 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

SunOS 5.10 Last change: 18 Introduction to Library Functions PCREPATTERN(3)

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 an option change 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 current pattern 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 oth-

erwise.

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 lead-

ing sequences to override what the application has set or what has been defaulted. Details are given in the section entitled "Newline sequences" above. SUBPATTERNS SunOS 5.10 Last change: 19 Introduction to Library Functions PCREPATTERN(3) Subpatterns are delimited by parentheses (round brackets),

which can be nested. Turning part of a pattern into a sub-

pattern does two things:

1. It localizes a set of alternatives. For example, the pat-

tern cat(aract|erpillar|)

matches one of the words "cat", "cataract", or "caterpil-

lar". 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 pcre_exec().

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 sub-

pattern 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 captur-

ing 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 sub-

patterns 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 SunOS 5.10 Last change: 20 Introduction to Library Functions PCREPATTERN(3) 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 pat-

tern: (?|(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 buffers 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 backreference or a recursive call to a numbered subpattern always refers to the first one in the pattern with the given number. 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 compli-

cated regular expressions. Furthermore, if an expression is

modified, the numbers may change. To help with this diffi-

culty, 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. SunOS 5.10 Last change: 21 Introduction to Library Functions PCREPATTERN(3) In PCRE, a subpattern can be named in one of three ways: (?...) or (?'name'...) as in Perl, or (?P...) as in Python. References to capturing parentheses from other parts of the pattern, such as backreferences, recursion, and conditions, can be made by name as well as by number.

Names consist of up to 32 alphanumeric characters and under-

scores. 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 pat-

tern. 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. This 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: (?Mon|Fri|Sun)(?:day)?| (?Tue)(?:sday)?| (?Wed)(?:nesday)?| (?Thu)(?:rsday)?| (?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 reference to a non-unique named subpattern from else-

where in the pattern, the one that corresponds to the lowest number is used. For further details of the interfaces for handling named subpatterns, see the pcreapi documentation. REPETITION Repetition is specified by quantifiers, which can follow any of the following items: a literal data character the dot metacharacter the \C escape sequence

the \X escape sequence (in UTF-8 mode with Unicode

SunOS 5.10 Last change: 22 Introduction to Library Functions PCREPATTERN(3) properties) the \R escape sequence an escape such as \d that matches a single character a character class a back reference (see next section) a parenthesized subpattern (unless it is an assertion) 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 quantif-

ier, but a literal string of four characters.

In UTF-8 mode, quantifiers apply to UTF-8 characters rather

than to individual bytes. Thus, for example, \x{100}{2}

matches two UTF-8 characters, each of which is represented

by a two-byte sequence. Similarly, when Unicode property

support is available, \X{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 refer-

enced as subroutines from elsewhere in the pattern. Items

other than subpatterns that have a {0} quantifier are omit-

ted from the compiled pattern. For convenience, the three most common quantifiers have

single-character abbreviations:

SunOS 5.10 Last change: 23 Introduction to Library Functions PCREPATTERN(3) * 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 per-

mitted 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 / charac-

ters 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 pre-

ferred number of matches. Do not confuse this use of ques-

tion 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. SunOS 5.10 Last change: 24 Introduction to Library Functions PCREPATTERN(3)

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 max-

imum, 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 charac-

ter 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 pre-

ceded 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 indi-

cate anchoring explicitly.

However, there is one situation where the optimization can-

not be used. When .* is inside capturing parentheses that are the subject of a backreference elsewhere in the pattern, a match at the start may fail where a later one succeeds. Consider, for example: (.*)abc\1 If the subject is "xyz123abc123" the match point is the fourth character. For this reason, such a pattern is not implicitly anchored. When a capturing subpattern is repeated, the value captured

is the substring that matched the final iteration. For exam-

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

has matched "tweedledum tweedledee" the value of the cap-

tured substring is "tweedledee". However, if there are nested capturing subpatterns, the corresponding captured

values may have been set in previous iterations. For exam-

ple, after /(a|(b))+/ matches "aba" the value of the second captured substring is SunOS 5.10 Last change: 25 Introduction to Library Functions PCREPATTERN(3) "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 dif-

ferent 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 specify-

ing 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. Back-

tracking past it to previous items, however, works as nor-

mal. An alternative description is that a subpattern of this type

matches the string of characters that an identical stan-

dalone 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. SunOS 5.10 Last change: 26 Introduction to Library Functions PCREPATTERN(3) 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 nota-

tion 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 per-

formance 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 back-

tracking into a sequence of A's when B must follow.

When a pattern contains an unlimited repeat inside a subpat-

tern 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 con-

sist 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+ SunOS 5.10 Last change: 27 Introduction to Library Functions PCREPATTERN(3) 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 hap-

pens 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 refer-

ence" 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, which is a feature introduced in Perl 5.10. 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} SunOS 5.10 Last change: 28 Introduction to Library Functions PCREPATTERN(3) 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 exam-

ple:

(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. 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 them-

selves.

A back reference matches whatever actually matched the cap-

turing 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 responsi-

bility", 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 case-

lessly. There are several different ways of writing back references to named subpatterns. The .NET syntax \k{name} and the Perl syntax \k 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 refer-

ences, is also supported. We could rewrite the above example in any of the following ways: (?(?i)rah)\s+\k (?'p1'(?i)rah)\s+\k{p1} (?P(?i)rah)\s+(?P=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 sub-

pattern. If a subpattern has not actually been used in a SunOS 5.10 Last change: 29 Introduction to Library Functions PCREPATTERN(3) particular match, any back references to it always fail. For example, the pattern (a|(bc))\2 always fails if it starts to match "a" rather than "bc".

Because there may be many capturing parentheses in a pat-

tern, all digits following the backslash are taken as part

of a potential back reference number. If the pattern contin-

ues 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 an empty comment (see "Comments" below) can be used. 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 pat-

tern (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 itera-

tion. In order for this to work, the pattern must be such that the first iteration does not need to match the back reference. This can be done using alternation, as in the example above, or by a quantifier with a minimum of zero. 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 posi-

tion 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. SunOS 5.10 Last change: 30 Introduction to Library Functions PCREPATTERN(3) 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. Lookbehind assertions

Lookbehind assertions start with (?<= for positive asser-

tions and (?

length. However, if there are several top-level alterna-

tives, they do not all have to have the same fixed length. Thus (?<=bullock|donkey) is permitted, but (?

causes an error at compile time. Branches that match dif-

ferent length strings are permitted only at the top level of a lookbehind assertion. This is an extension compared with Perl (at least for 5.8), which requires all branches to SunOS 5.10 Last change: 31 Introduction to Library Functions PCREPATTERN(3) 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 if rewrit-

ten to use two top-level branches:

(?<=abc|abde) In some cases, the Perl 5.10 escape sequence \K (see above) can be used instead of a lookbehind assertion; this is not

restricted to a fixed-length.

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 \C 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 \X and \R escapes, which can match dif-

ferent numbers of bytes, are also not permitted.

Possessive quantifiers can be used in conjunction with look-

behind assertions to specify efficient matching at the end of the subject string. 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 back-

tracks 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 SunOS 5.10 Last change: 32 Introduction to Library Functions PCREPATTERN(3) makes a significant difference to the processing time. Using multiple assertions Several assertions (of any sort) may occur in succession. For example, (?<=\d{3})(?

It is possible to cause the matching process to obey a sub-

pattern conditionally or to choose between two alternative subpatterns, depending on the result of an assertion, or whether a previous capturing subpattern matched or not. 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; oth-

erwise the no-pattern (if present) is used. If there are

more than two alternatives in the subpattern, a compile-time

SunOS 5.10 Last change: 33 Introduction to Library Functions PCREPATTERN(3) error occurs.

There are four kinds of condition: references to subpat-

terns, 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 the capturing subpattern

of that number has previously matched. An alternative nota-

tion 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. In looping constructs it can also make sense to refer to subsequent groups with constructs such as (?(+2).

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 the first set of parentheses matched or not. 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 (?()...) 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. However, SunOS 5.10 Last change: 34 Introduction to Library Functions PCREPATTERN(3)

there is a possible ambiguity with this syntax, because sub-

pattern names may consist entirely of digits. PCRE looks first for a named subpattern; if it cannot find one and the

name consists entirely of digits, PCRE looks for a subpat-

tern of that number, which must be greater than zero. Using subpattern names that consist entirely of digits is not recommended. Rewriting the above example to use a named subpattern gives this: (? $ )? [^()]+ (?() $ ) Checking for pattern recursion

If the condition is the string (R), and there is no subpat-

tern 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 the subpattern whose number or name is given. This condition does not check the entire recursion stack. At "top level", all these recursion test conditions are false. Recursive patterns are 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 could be written like this (ignore whitespace and line breaks):

(?(DEFINE) (? 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. SunOS 5.10 Last change: 35 Introduction to Library Functions PCREPATTERN(3) 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 looka-

head 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

The sequence (?# marks the start of a comment that continues

up to the next closing parenthesis. Nested parentheses are not permitted. The characters that make up a comment play no part in the pattern matching at all.

If the PCRE_EXTENDED option is set, an unescaped # character

outside a character class introduces a comment that contin-

ues to immediately after the next newline in the pattern. 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 reg-

ular 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;

SunOS 5.10 Last change: 36 Introduction to Library Functions PCREPATTERN(3) 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 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 "subroutine" call, which is described in the next section.) The special item (?R) or (?0) is a recursive call of the entire regular expression.

In PCRE (like Python, but unlike Perl), a recursive subpat-

tern 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 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. 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. (A Perl 5.10 feature.) 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. SunOS 5.10 Last change: 37 Introduction to Library Functions PCREPATTERN(3) 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 "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) )* $ ) 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 atomic

grouping for matching strings of non-parentheses is impor-

tant 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 atomic grouping 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 set for any capturing sub-

patterns are those from the outermost level of the recursion at which the subpattern value is set. 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 capturing parentheses is "ef", which is the last value taken on at the top level. If additional parentheses are added, giving $ ( ( (?>[^()]+) | (?R) )* ) $ ^ ^ ^ ^ the string they capture is "ab(cd)ef", the contents of the top level parentheses. 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

SunOS 5.10 Last change: 38 Introduction to Library Functions PCREPATTERN(3) 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 recurs-

ing), whereas any characters are permitted at the outer level. < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >

In this pattern, (?(R) is the start of a conditional subpat-

tern, with two different alternatives for the recursive and

non-recursive cases. The (?R) item is the actual recursive

call. SUBPATTERNS AS SUBROUTINES If the syntax for a recursive subpattern reference (either by number or by name) is used outside the parentheses to

which it refers, it operates like a subroutine in a program-

ming 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 responsi-

bility", 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 dis-

cussion of DEFINE above. 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. When a subpattern is used as a subroutine, processing

options such as case-independence are fixed when the

SunOS 5.10 Last change: 39 Introduction to Library Functions PCREPATTERN(3) 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 sub-

pattern. 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 recur-

sively. Here are two of the examples used above, rewritten using this syntax: (? $ ( (?>[^()]+) | \g )* $ ) (sens|respons)e and \g'1'ibility

PCRE supports an extension to Oniguruma: if a number is pre-

ceded 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 syn-

tax) 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 repeti-

tion. 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: SunOS 5.10 Last change: 40 Introduction to Library Functions PCREPATTERN(3) (?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 sup-

plied by the caller of pcre_exec(). The callout function may

cause matching to proceed, to backtrack, or to fail alto-

gether. A complete description of the interface to the cal-

lout function is given in the pcrecallout documentation. BACKTRACKING CONTROL

Perl 5.10 introduced a number of "Special Backtracking Con-

trol Verbs", which are 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. 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 algo-

rithm. With the exception of (*FAIL), which behaves like a

failing negative assertion, they cause an error if encoun-

tered by pcre_dfa_exec().

The new verbs make use of what was previously invalid syn-

tax: an opening parenthesis followed by an asterisk. In Perl, they are generally of the form (*VERB:ARG) but PCRE does not support the use of arguments, so its general form is just (*VERB). Any number of these verbs may occur in a pattern. There are two kinds: Verbs that act immediately The following verbs act as soon as they are encountered: (*ACCEPT) This verb causes the match to end successfully, skipping the remainder of the pattern. When inside a recursion, only the innermost pattern is ended immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside capturing parentheses. In Perl, the data so far is captured: in PCRE no data is captured. For example: SunOS 5.10 Last change: 41 Introduction to Library Functions PCREPATTERN(3) A(A|B(*ACCEPT)|C)D This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is captured. (*FAIL) or (*F) This verb causes the match to fail, 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). 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, a failure is forced. The verbs differ in exactly what kind of failure occurs. (*COMMIT) This verb 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 start 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." (*PRUNE) This verb causes the match to fail at the current position 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), or when matching to the right

of (*PRUNE), but if there is no match to the right, back-

tracking cannot cross (*PRUNE). In simple cases, the use of SunOS 5.10 Last change: 42 Introduction to Library Functions PCREPATTERN(3)

(*PRUNE) is just an alternative to an atomic group or pos-

sessive quantifier, but there are some uses of (*PRUNE) that cannot be expressed in any other way. (*SKIP) This verb is like (*PRUNE), except that if the pattern is

unanchored, the "bumpalong" advance is not to the next char-

acter, 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 in this example; although it would suppress back-

tracking during the first match attempt, the second attempt would start at the second character instead of skipping on to "c". (*THEN) This verb causes a skip to the next alternation 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. If (*THEN) is used outside of any alternation, it acts exactly like (*PRUNE).

ATTRIBUTES

See attributes(5) for descriptions of the following attri-

butes:

_______________________________________

| ATTRIBUTE TYPE | ATTRIBUTE VALUE|

|____________________|__________________|_

| Availability | library/pcre |

|____________________|__________________|_

| Interface Stability| Uncommitted |

|____________________|_________________|

NOTES Source for PCRE is available on http://opensolaris.org. SunOS 5.10 Last change: 44