"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [

]>

<refentry id="glib-regex-syntax" revision="11 Jul 2006">

 <refmeta>

  <refentrytitle>Regular expression syntax</refentrytitle>

 </refmeta>

Based on the man page for pcrepattern.

Remember to sync this document with the file docs/pcrepattern.3 in the

pcre package when upgrading to a newer version of pcre.

In sync with PCRE 7.0

-->

<refnamediv>

<refname>Regular expression syntax</refname>

<refpurpose>

syntax and semantics of regular expressions supported by GRegex

</refpurpose>

</refnamediv>

<refsect1>

<title>GRegex regular expression details</title>

<para>

A regular expression is a pattern that is matched against a

string from left to right. Most characters stand for themselves in a

pattern, and match the corresponding characters in the string. As a

trivial example, the pattern

</para>

<programlisting>

The quick brown fox

</programlisting>

<para>

matches a portion of a string that is identical to itself. When

caseless matching is specified (the <varname>G_REGEX_CASELESS</varname> flag), letters are

matched independently of case.

</para>

<para>

The power of regular expressions comes from the ability to include

alternatives and repetitions in the pattern. These are encoded in the

pattern by the use of metacharacters, which do not stand for themselves

but instead are interpreted in some special way.

</para>

<para>

There are two different sets of metacharacters: those that are recognized

anywhere in the pattern except within square brackets, and those

that are recognized in square brackets. Outside square brackets, the

metacharacters are as follows:

</para>

<title>Metacharacters outside square brackets</title>

<tgroup cols="2">

<colspec colnum="1" align="center"/>

  <row>

    <entry>Character</entry>

    <entry>Meaning</entry>

  </row>

  <row>

    <entry>\</entry>

    <entry>general escape character with several uses</entry>

  </row>

  <row>

    <entry>^</entry>

    <entry>assert start of string (or line, in multiline mode)</entry>

  </row>

  <row>

    <entry>$</entry>

    <entry>assert end of string (or line, in multiline mode)</entry>

  </row>

  <row>

    <entry>.</entry>

    <entry>match any character except newline (by default)</entry>

  </row>

  <row>

    <entry>[</entry>

    <entry>start character class definition</entry>

  </row>

  <row>

    <entry>|</entry>

    <entry>start of alternative branch</entry>

  </row>

  <row>

    <entry>(</entry>

    <entry>start subpattern</entry>

  </row>

  <row>

    <entry>)</entry>

    <entry>end subpattern</entry>

  </row>

  <row>

    <entry>?</entry>

    <entry>extends the meaning of (, or 0/1 quantifier, or quantifier minimizer</entry>

  </row>

  <row>

    <entry>*</entry>

    <entry>0 or more quantifier</entry>

  </row>

  <row>

    <entry>+</entry>

    <entry>1 or more quantifier, also "possessive quantifier"</entry>

  </row>

  <row>

    <entry>{</entry>

    <entry>start min/max quantifier</entry>

  </row>

</tgroup>

<para>

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

class". In a character class the only metacharacters are:

</para>

<title>Metacharacters inside square brackets</title>

<tgroup cols="2">

<colspec colnum="1" align="center"/>

  <row>

    <entry>Character</entry>

    <entry>Meaning</entry>

  </row>

  <row>

    <entry>\</entry>

    <entry>general escape character</entry>

  </row>

  <row>

    <entry>^</entry>

    <entry>negate the class, but only if the first character</entry>

  </row>

  <row>

    <entry>-</entry>

    <entry>indicates character range</entry>

  </row>

  <row>

    <entry>[</entry>

    <entry>POSIX character class (only if followed by POSIX syntax)</entry>

  </row>

  <row>

    <entry>]</entry>

    <entry>terminates the character class</entry>

  </row>

</tgroup>

</refsect1>

<refsect1>

<title>Backslash</title>

<para>

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.

</para>

<para>

For example, if you want to match a * character, you write \* in the

pattern. This escaping action applies whether or not the following

character would otherwise be interpreted as a metacharacter, so it is

always safe to precede a non-alphanumeric with backslash to specify

that it stands for itself. In particular, if you want to match a

backslash, you write \\.

</para>

<para>

If a pattern is compiled with the <varname>G_REGEX_EXTENDED</varname>

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.

</para>

<para>

Note that the C compiler interprets backslash in strings itself, therefore

you need to duplicate all \ characters when you put a regular expression

in a C string, like "\\d{3}".

</para>

<para>

If you want to remove the special meaning from a sequence of characters,

you can do so by putting them between \Q and \E.

The \Q...\E sequence is recognized both inside and outside character

classes.

</para>

<refsect2>

<title>Non-printing characters</title>

<para>

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:

</para>

<title>Non-printing characters</title>

<tgroup cols="2">

<colspec colnum="1" align="center"/>

  <row>

    <entry>Escape</entry>

    <entry>Meaning</entry>

  </row>

  <row>

    <entry>\a</entry>

    <entry>alarm, that is, the BEL character (hex 07)</entry>

  </row>

  <row>

    <entry>\cx</entry>

    <entry>"control-x", where x is any character</entry>

  </row>

  <row>

    <entry>\e</entry>

    <entry>escape (hex 1B)</entry>

  </row>

  <row>

    <entry>\f</entry>

    <entry>formfeed (hex 0C)</entry>

  </row>

  <row>

    <entry>\n</entry>

    <entry>newline (hex 0A)</entry>

  </row>

  <row>

    <entry>\r</entry>

    <entry>carriage return (hex 0D)</entry>

  </row>

  <row>

    <entry>\t</entry>

    <entry>tab (hex 09)</entry>

  </row>

  <row>

    <entry>\ddd</entry>

    <entry>character with octal code ddd, or backreference</entry>

  </row>

  <row>

    <entry>\xhh</entry>

    <entry>character with hex code hh</entry>

  </row>

  <row>

    <entry>\x{hhh..}</entry>

    <entry>character with hex code hhh..</entry>

  </row>

</tgroup>

<para>

The precise effect of \cx 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 \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;

becomes hex 7B.

</para>

<para>

After \x, from zero to two hexadecimal digits are read (letters can be

in upper or lower case). Any number of hexadecimal digits may appear

between \x{ and }, but the value of the character code

must be less than 2**31 (that is, the maximum hexadecimal value is

7FFFFFFF). 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.

</para>

<para>

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}.

</para>

<para>

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.

</para>

<para>

The handling of a backslash followed by a digit other than 0 is complicated.

Outside a character class, GRegex 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.

</para>

<para>

Inside a character class, or if the decimal number is greater than 9

and there have not been that many capturing subpatterns, GRegex re-reads

up to three octal digits following the backslash, and uses them to generate

a data character. Any subsequent digits stand for themselves. For example:

</para>

<title>Non-printing characters</title>

<tgroup cols="2">

<colspec colnum="1" align="center"/>

  <row>

    <entry>Escape</entry>

    <entry>Meaning</entry>

  </row>

  <row>

    <entry>\040</entry>

    <entry>is another way of writing a space</entry>

  </row>

  <row>

    <entry>\40</entry>

    <entry>is the same, provided there are fewer than 40 previous capturing subpatterns</entry>

  </row>

  <row>

    <entry>\7</entry>

    <entry>is always a back reference</entry>

  </row>

  <row>

    <entry>\11</entry>

    <entry>might be a back reference, or another way of writing a tab</entry>

  </row>

  <row>

    <entry>\011</entry>

    <entry>is always a tab</entry>

  </row>

  <row>

    <entry>\0113</entry>

    <entry>is a tab followed by the character "3"</entry>

  </row>

  <row>

    <entry>\113</entry>

    <entry>might be a back reference, otherwise the character with octal code 113</entry>

  </row>

  <row>

    <entry>\377</entry>

    <entry>might be a back reference, otherwise the byte consisting entirely of 1 bits</entry>

  </row>

  <row>

    <entry>\81</entry>

    <entry>is either a back reference, or a binary zero followed by the two characters "8" and "1"</entry>

  </row>

</tgroup>

<para>

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.

</para>

<para>

All the sequences that define a single character can be used both inside

and outside character classes. In addition, inside a character class, the

sequence \b is interpreted 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).

</para>

</refsect2>

<refsect2>

<title>Absolute and relative back references</title>

<para>

The sequence \g followed by a positive or negative number, optionally enclosed

in braces, is an absolute or relative back reference. Back references are

discussed later, following the discussion of parenthesized subpatterns.

</para>

</refsect2>

<refsect2>

<title>Generic character types</title>

<para>

Another use of backslash is for specifying generic character types.

The following are always recognized:

</para>

<title>Generic characters</title>

<tgroup cols="2">

<colspec colnum="1" align="center"/>

  <row>

    <entry>Escape</entry>

    <entry>Meaning</entry>

  </row>

  <row>

    <entry>\d</entry>

    <entry>any decimal digit</entry>

  </row>

  <row>

    <entry>\D</entry>

    <entry>any character that is not a decimal digit</entry>

  </row>

  <row>

    <entry>\s</entry>

    <entry>any whitespace character</entry>

  </row>

  <row>

    <entry>\S</entry>

    <entry>any character that is not a whitespace character</entry>

  </row>

  <row>

    <entry>\w</entry>

    <entry>any "word" character</entry>

  </row>

  <row>

    <entry>\W</entry>

    <entry>any "non-word" character</entry>

  </row>

</tgroup>

<para>

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.

</para>

<para>

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 passed string, all

of them fail, since there is no character to match.

</para>

<para>

For compatibility with Perl, \s does not match the VT character (code

11). This makes it different from the POSIX "space" class. The \s

characters are HT (9), LF (10), FF (12), CR (13), and space (32).

</para>

<para>

A "word" character is an underscore or any character less than 256 that

is a letter or digit.</para>

<para>

Characters with values greater than 128 never match \d,

\s, or \w, and always match \D, \S, and \W.

</para>

</refsect2>

<refsect2>

<title>Newline sequences</title>

<para>Outside a character class, the escape sequence \R matches any Unicode

newline sequence.

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), NEL (next

line, U+0085), LS (line separator, U+2028), or PS (paragraph separator, U+2029).

The two-character sequence is treated as a single unit that

cannot be split. Inside a character class, \R matches the letter "R".</para>

</refsect2>

<refsect2>

<title>Unicode character properties</title>

<para>

To support generic character types there are three additional escape

sequences, they are:

</para>

<title>Generic character types</title>

<tgroup cols="2">

<colspec colnum="1" align="center"/>

  <row>

    <entry>Escape</entry>

    <entry>Meaning</entry>

  </row>

  <row>

    <entry>\p{xx}</entry>

    <entry>a character with the xx property</entry>

  </row>

  <row>

    <entry>\P{xx}</entry>

    <entry>a character without the xx property</entry>

  </row>

  <row>

    <entry>\X</entry>

    <entry>an extended Unicode sequence</entry>

  </row>

</tgroup>

<para>

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. Note that \P{Any} does not match any characters,

so always causes a match failure.

</para>

<para>

Sets of Unicode characters are defined as belonging to certain scripts. A

character from one of these sets can be matched using a script name. For

example, \p{Greek} or \P{Han}.

</para>

<para>

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

"Common". The current list of scripts can be found in the documentation for

the #GUnicodeScript enumeration. Script names for use with \p{} can be

found by replacing all spaces with underscores, e.g. for Linear B use

\p{Linear_B}.

</para>

<para>

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 circumflex between the opening brace and the property name. For

example, \p{^Lu} is the same as \P{Lu}.

</para>

<para>

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:

</para>

<programlisting>

\p{L}

\pL

</programlisting>

<para>

In addition to the two-letter category codes listed in the

documentation for the #GUnicodeType enumeration, the following

general category property codes are supported:

</para>

<title>Property codes</title>

<tgroup cols="2">

<colspec colnum="1" align="center"/>

  <row>

    <entry>Code</entry>

    <entry>Meaning</entry>

  </row>

  <row>

    <entry>C</entry>

    <entry>Other</entry>

  </row>

  <row>

    <entry>L</entry>

    <entry>Letter</entry>

  </row>

  <row>

    <entry>M</entry>

    <entry>Mark</entry>

  </row>

  <row>

    <entry>N</entry>

    <entry>Number</entry>

  </row>

  <row>

    <entry>P</entry>

    <entry>Punctuation</entry>

  </row>

  <row>

    <entry>S</entry>

    <entry>Symbol</entry>

  </row>

  <row>

    <entry>Z</entry>

    <entry>Separator</entry>

  </row>

</tgroup>

<para>

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".

</para>

<para>

The long synonyms for these properties that Perl supports (such as \ep{Letter})

are not supported by GRegex, nor is it permitted to prefix any of these

properties with "Is".

</para>

<para>

No character that is in the Unicode table has the Cn (unassigned) property.

Instead, this property is assumed for any code point that is not in the

Unicode table.

</para>

<para>

Specifying caseless matching does not affect these escape sequences.

For example, \p{Lu} always matches only upper case letters.

</para>

<para>

The \X escape matches any number of Unicode characters that form an

extended Unicode sequence. \X is equivalent to

</para>

<programlisting>

(?>\PM\pM*)

</programlisting>

<para>

That is, it matches a character without the "mark" property, followed

by zero or more characters with the "mark" property, and treats the

sequence as an atomic group (see below). Characters with the "mark"

property are typically accents that affect the preceding character.

</para>

<para>

Matching characters by Unicode property is not fast, because GRegex 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 properties.

</para>

</refsect2>

<refsect2>

<title>Simple assertions</title>

<para>

The final use of backslash is for certain simple assertions. An

assertion specifies a condition that has to be met at a particular point in

a match, without consuming any characters from the string. The

use of subpatterns for more complicated assertions is described below.

The backslashed assertions are:

</para>

<title>Simple assertions</title>

<tgroup cols="2">

<colspec colnum="1" align="center"/>

  <row>

    <entry>Escape</entry>

    <entry>Meaning</entry>

  </row>

  <row>

    <entry>\b</entry>

    <entry>matches at a word boundary</entry>

  </row>

  <row>

    <entry>\B</entry>

    <entry>matches when not at a word boundary</entry>

  </row>

  <row>

    <entry>\A</entry>

    <entry>matches at the start of the string</entry>

  </row>

  <row>

    <entry>\Z</entry>

    <entry>matches at the end of the string or before a newline at the end of the string</entry>

  </row>

  <row>

    <entry>\z</entry>

    <entry>matches only at the end of the string</entry>

  </row>

  <row>

    <entry>\G</entry>

    <entry>matches at first matching position in the string</entry>

  </row>

</tgroup>

<para>

These assertions may not appear in character classes (but note that \b

has a different meaning, namely the backspace character, inside a

character class).

</para>

<para>

A word boundary is a position in the 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.

</para>

<para>

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 string, whatever options are

set. Thus, they are independent of multiline mode. These three assertions

are not affected by the <varname>G_REGEX_MATCH_NOTBOL</varname> or <varname>G_REGEX_MATCH_NOTEOL</varname> options,

which affect only the behaviour of the circumflex and dollar metacharacters.

However, if the start_position argument of a matching function is non-zero,

indicating that matching is to start at a point other than the beginning of

the string, \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 at the

very end, whereas \z matches only at the end.

</para>

<para>

The \G assertion is true only when the current matching position is at

the start point of the match, as specified by the start_position argument

to the matching functions. It differs from \A when the value of startoffset is

non-zero.

</para>

<para>

Note, however, that the 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.

</para>

<para>

If all the alternatives of a pattern begin with \G, the expression is

anchored to the starting match position, and the "anchored" flag is set

in the compiled regular expression.

</para>

</refsect2>

</refsect1>

<refsect1>

<title>Circumflex and dollar</title>

<para>

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 string. If the start_position argument to

the matching functions is non-zero, circumflex can never match if the

<varname>G_REGEX_MULTILINE</varname> option is unset. Inside a character class, circumflex

has an entirely different meaning (see below).

</para>

<para>

Circumflex need not be the first character of the pattern if a number

of alternatives are involved, but it should be the first thing in each

alternative in which it appears if the pattern is ever to match that

branch. If all possible alternatives start with a circumflex, that is,

if the pattern is constrained to match only at the start of the string,

it is said to be an "anchored" pattern. (There are also other

constructs that can cause a pattern to be anchored.)

</para>

<para>

A dollar character is an assertion that is true only if the current

matching point is at the end of the string, or immediately

before a newline at the end of the string (by default). Dollar need not

be the last character of the pattern if a number of alternatives are

involved, but it should be the last item in any branch in which it

appears. Dollar has no special meaning in a character class.

</para>

<para>

The meaning of dollar can be changed so that it matches only at the

very end of the string, by setting the <varname>G_REGEX_DOLLAR_ENDONLY</varname> option at

compile time. This does not affect the \Z assertion.

</para>

<para>

The meanings of the circumflex and dollar characters are changed if the

<varname>G_REGEX_MULTILINE</varname> option is set. When this is the case,

a circumflex matches immediately after internal newlines as well as at the

start of the string. It does not match after a newline that ends the string.

A dollar matches before any newlines in the string, as well as at the very

end, when <varname>G_REGEX_MULTILINE</varname> is set. When newline is

specified as the two-character sequence CRLF, isolated CR and LF characters

do not indicate newlines.

</para>

<para>

For example, the pattern /^abc$/ matches the string "def\nabc" (where

\n represents a newline) in multiline mode, but not otherwise. Consequently,

patterns that are anchored in single line mode because all branches start with

^ are not anchored in multiline mode, and a match for circumflex is possible

when the <varname>start_position</varname> argument of a matching function

is non-zero. The <varname>G_REGEX_DOLLAR_ENDONLY</varname> option is ignored

if <varname>G_REGEX_MULTILINE</varname> is set.

</para>

<para>

Note that the sequences \A, \Z, and \z can be used to match the start and

end of the string in both modes, and if all branches of a pattern start with

\A it is always anchored, whether or not <varname>G_REGEX_MULTILINE</varname>

is set.

</para>

</refsect1>

<refsect1>

<title>Full stop (period, dot)</title>

<para>

Outside a character class, a dot in the pattern matches any one character

in the string, including a non-printing character, but not (by

default) newline. In UTF-8 a character might be more than one byte long.

</para>

<para>

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.

</para>

<para>

If the <varname>G_REGEX_DOTALL</varname> flag is set, dots match newlines

as well. The handling of dot is entirely independent of the handling of circumflex

and dollar, the only relationship being that they both involve newline

characters. Dot has no special meaning in a character class.

</para>

<para>

The behaviour of dot with regard to newlines can be changed. If the

<varname>G_REGEX_DOTALL</varname> option is set, a dot matches any one

character, without exception. If newline is defined as the two-character

sequence CRLF, it takes two dots to match it.

</para>

<para>

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.

</para>

</refsect1>

<refsect1>

<title>Matching a single byte</title>

<para>

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.

</para>

<para>

GRegex 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.

</para>

</refsect1>

<refsect1>

<title>Square brackets and character classes</title>

<para>

An opening square bracket introduces a character class, terminated by a