.TH PCREPATTERN 3

.SH NAME

PCRE - Perl-compatible regular expressions

.SH "PCRE REGULAR EXPRESSION DETAILS"

.rs

.sp

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

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\fBpcresyntax\fP

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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.

.P

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.

.P

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 \fBpcre_compile()\fP with

the PCRE_UTF8 option. How this affects pattern matching is mentioned in several

places below. There is also a summary of UTF-8 features in the

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section on UTF-8 support

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in the main

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\fBpcre\fP

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page.

.P

The remainder of this document discusses the patterns that are supported by

PCRE when its main matching function, \fBpcre_exec()\fP, is used.

From release 6.0, PCRE offers a second matching function,

\fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not

Perl-compatible. Some of the features discussed below are not available when

\fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the

alternative function, and how it differs from the normal function, are

discussed in the

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\fBpcrematching\fP

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page.

.

.

.SH "NEWLINE CONVENTIONS"

.rs

.sp

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

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\fBpcreapi\fP

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page has

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further discussion

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about newlines, and shows how to set the newline convention in the

\fIoptions\fP arguments for the compiling and matching functions.

.P

It is also possible to specify a newline convention by starting a pattern

string with one of the following five sequences:

.sp

  (*CR)        carriage return

  (*LF)        linefeed

  (*CRLF)      carriage return, followed by linefeed

  (*ANYCRLF)   any of the three above

  (*ANY)       all Unicode newline sequences

.sp

These override the default and the options given to \fBpcre_compile()\fP. For

example, on a Unix system where LF is the default newline sequence, the pattern

.sp

  (*CR)a.b

.sp

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.

.P

The newline convention 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

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"Newline sequences"

.\"

below. A change of \eR setting can be combined with a change of newline

convention.

.

.

.SH "CHARACTERS AND METACHARACTERS"

.rs

.sp

A regular expression is a pattern that is matched against a subject string from

left to right. Most characters stand for themselves in a pattern, and match the

corresponding characters in the subject. As a trivial example, the pattern

.sp

  The quick brown fox

.sp

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 caseless matching is

always possible. For characters with higher values, the concept of case is

supported if PCRE is compiled with Unicode property support, but not otherwise.

If you want to use caseless matching for characters 128 and above, you must

ensure that PCRE is compiled with Unicode property support as well as with

UTF-8 support.

.P

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

\fImetacharacters\fP, which do not stand for themselves but instead are

interpreted in some special way.

.P

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:

.sp

  \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

.sp

Part of a pattern that is in square brackets is called a "character class". In

a character class the only metacharacters are:

.sp

  \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

.sp

The following sections describe the use of each of the metacharacters.

.

.

.SH BACKSLASH

.rs

.sp

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.

.P

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.

.P

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.

.P

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:

.sp

  Pattern            PCRE matches   Perl matches

.sp

.\" 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

.sp

The \eQ...\eE sequence is recognized both inside and outside character classes.

.

.

.\" HTML <a name="digitsafterbackslash"></a>

.SS "Non-printing characters"

.rs

.sp

A second use of backslash provides a way of encoding non-printing characters

in patterns in a visible manner. There is no restriction on the appearance of

non-printing characters, apart from the binary zero that terminates a pattern,

but when a pattern is being prepared by text editing, it is usually easier to

use one of the following escape sequences than the binary character it

represents:

.sp

  \ea        alarm, that is, the BEL character (hex 07)

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

  \ee        escape (hex 1B)

  \ef        formfeed (hex 0C)

  \en        linefeed (hex 0A)

  \er        carriage return (hex 0D)

  \et        tab (hex 09)

  \eddd      character with octal code ddd, or backreference

  \exhh      character with hex code hh

  \ex{hhh..} character with hex code hhh..

.sp

The precise effect of \ecx is as follows: if x is a lower case letter, it

is converted to upper case. Then bit 6 of the character (hex 40) is inverted.

Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex

7B.

.P

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.

.P

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.

.P

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}.

.P

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.

.P

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 \fIback reference\fP. A description of how this works is given

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later,

.\"

following the discussion of

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.\" </a>

parenthesized subpatterns.

.\"

.P

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:

.sp

  \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"

.sp

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.

.P

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), and the

sequences \eR and \eX are interpreted as the characters "R" and "X",

respectively. Outside a character class, these sequences have different

meanings

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.\" </a>

(see below).

.\"

.

.

.SS "Absolute and relative back references"

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.sp

The sequence \eg 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 \eg{name}. Back references are discussed

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later,

.\"

following the discussion of

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.\" </a>

parenthesized subpatterns.

.\"

.

.

.SS "Absolute and relative subroutine calls"

.rs

.sp

For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or

a number enclosed either in angle brackets or single quotes, is an alternative

syntax for referencing a subpattern as a "subroutine". Details are discussed

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.\" </a>

later.

.\"

Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP

synonymous. The former is a back reference; the latter is a subroutine call.

.

.

.SS "Generic character types"

.rs

.sp

Another use of backslash is for specifying generic character types. The

following are always recognized:

.sp

  \ed     any decimal digit

  \eD     any character that is not a decimal digit

  \eh     any horizontal whitespace character

  \eH     any character that is not a horizontal whitespace character

  \es     any whitespace character

  \eS     any character that is not a whitespace character

  \ev     any vertical whitespace character

  \eV     any character that is not a vertical whitespace character

  \ew     any "word" character

  \eW     any "non-word" character

.sp

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.

.P

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.

.P

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.

.P

In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or

\ew, and always match \eD, \eS, and \eW. 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 efficiency

reasons.

.P

The sequences \eh, \eH, \ev, and \eV 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:

.sp

  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

.sp

The vertical space characters are:

.sp

  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

.P

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 character tables, and may vary if locale-specific matching is taking

place (see

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.\" </a>

"Locale support"

.\"

in the

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\fBpcreapi\fP

.\"

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 \ew. The use of locales with Unicode

is discouraged.

.

.

.\" HTML <a name="newlineseq"></a>

.SS "Newline sequences"

.rs

.sp

Outside a character class, by default, the escape sequence \eR matches any

Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is

equivalent to the following:

.sp

  (?>\er\en|\en|\ex0b|\ef|\er|\ex85)

.sp

This is an example of an "atomic group", details of which are given

.\" HTML <a href="#atomicgroup">

.\" </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.

.P

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.

.P

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:

.sp

  (*BSR_ANYCRLF)   CR, LF, or CRLF only

  (*BSR_UNICODE)   any Unicode newline sequence

.sp

These override the default and the options given to \fBpcre_compile()\fP, but

they can be overridden by options given to \fBpcre_exec()\fP. 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:

.sp

  (*ANY)(*BSR_ANYCRLF)

.sp

Inside a character class, \eR matches the letter "R".

.

.

.\" HTML <a name="uniextseq"></a>

.SS Unicode character properties

.rs

.sp

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:

.sp

  \ep{\fIxx\fP}   a character with the \fIxx\fP property

  \eP{\fIxx\fP}   a character without the \fIxx\fP property

  \eX       an extended Unicode sequence

.sp

The property names represented by \fIxx\fP 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 \eP{Any} does not match any

characters, so always causes a match failure.

.P

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:

.sp

  \ep{Greek}

  \eP{Han}

.sp

Those that are not part of an identified script are lumped together as

"Common". The current list of scripts is:

.P

Arabic,

Armenian,

Balinese,

Bengali,

Bopomofo,

Braille,

Buginese,

Buhid,

Canadian_Aboriginal,

Cherokee,

Common,

Coptic,

Cuneiform,

Cypriot,

Cyrillic,

Deseret,

Devanagari,

Ethiopic,

Georgian,

Glagolitic,

Gothic,

Greek,

Gujarati,

Gurmukhi,

Han,

Hangul,

Hanunoo,

Hebrew,

Hiragana,