path: root/docs/reference/glib/regex-syntax.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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]>
<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>

<table frame="all" colsep="1" rowsep="1">
<title>Metacharacters outside square brackets</title>
<tgroup cols="2">
<colspec colnum="1" align="center"/>
<thead>
  <row>
    <entry>Character</entry>
    <entry>Meaning</entry>
  </row>
</thead>
<tbody>
  <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>
</tbody>
</tgroup>
</table>

<para>
Part of a pattern that is in square brackets is called a "character
class". In a character class the only metacharacters are:
</para>

<table frame="all" colsep="1" rowsep="1">
<title>Metacharacters inside square brackets</title>
<tgroup cols="2">
<colspec colnum="1" align="center"/>
<thead>
  <row>
    <entry>Character</entry>
    <entry>Meaning</entry>
  </row>
</thead>
<tbody>
  <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>
</tbody>
</tgroup>
</table>
</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>

<table frame="all" colsep="1" rowsep="1">
<title>Non-printing characters</title>
<tgroup cols="2">
<colspec colnum="1" align="center"/>
<thead>
  <row>
    <entry>Escape</entry>
    <entry>Meaning</entry>
  </row>
</thead>
<tbody>
  <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>
</tbody>
</tgroup>
</table>

<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>

<table frame="all" colsep="1" rowsep="1">
<title>Non-printing characters</title>
<tgroup cols="2">
<colspec colnum="1" align="center"/>
<thead>
  <row>
    <entry>Escape</entry>
    <entry>Meaning</entry>
  </row>
</thead>
<tbody>
  <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>
</tbody>
</tgroup>
</table>

<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>

<table frame="all" colsep="1" rowsep="1">
<title>Generic characters</title>
<tgroup cols="2">
<colspec colnum="1" align="center"/>
<thead>
  <row>
    <entry>Escape</entry>
    <entry>Meaning</entry>
  </row>
</thead>
<tbody>
  <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>
</tbody>
</tgroup>
</table>

<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>

<table frame="all" colsep="1" rowsep="1">
<title>Generic character types</title>
<tgroup cols="2">
<colspec colnum="1" align="center"/>
<thead>
  <row>
    <entry>Escape</entry>
    <entry>Meaning</entry>
  </row>
</thead>
<tbody>
  <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>
</tbody>
</tgroup>
</table>

<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>

<table frame="all" colsep="1" rowsep="1">
<title>Property codes</title>
<tgroup cols="2">
<colspec colnum="1" align="center"/>
<thead>
  <row>
    <entry>Code</entry>
    <entry>Meaning</entry>
  </row>
</thead>
<tbody>
  <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>
</tbody>
</tgroup>
</table>

<para>
The special property L&amp; 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>
(?&gt;\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>

<table frame="all" colsep="1" rowsep="1">
<title>Simple assertions</title>
<tgroup cols="2">
<colspec colnum="1" align="center"/>
<thead>
  <row>
    <entry>Escape</entry>
    <entry>Meaning</entry>
  </row>
</thead>
<tbody>
  <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>
</tbody>
</tgroup>
</table>

<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
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
circumflex, if present) or escaped with a backslash.
</para>

<para>
A character class matches a single character in the string.  A matched character
must be in the set of characters defined by the class, unless the first
character in the class definition is a circumflex, in which case the
string 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.
</para>

<para>
For example, the character class [aeiou] matches any lower case vowel,
while [^aeiou] matches any character that is not a lower case vowel.
Note that a circumflex is just a convenient notation for specifying the
characters that are in the class by enumerating those that are not. A
class that starts with a circumflex is not an assertion: it still consumes
a character from the string, and therefore it fails if the current pointer
is at the end of the string.
</para>

<para>
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.
</para>

<para>
When caseless matching is set, any letters in a class represent both
their upper case and lower case versions, so for example, a caseless
[aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
match "A", whereas a caseful version would.
</para>

<para>
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 <varname>G_REGEX_DOTALL</varname>
and <varname>G_REGEX_MULTILINE</varname> options is used. A class such as [^a]
always matches one of these characters.
</para>

<para>
The minus (hyphen) character can be used to specify a range of characters in
a character class. For example, [d-m] matches any letter
between d and m, inclusive. If a minus character is required in a
class, it must be escaped with a backslash or appear in a position
where it cannot be interpreted as indicating a range, typically as the
first or last character in the class.
</para>

<para>
It is not possible to have the literal character "]" as the end character
of a range. A pattern such as [W-]46] is interpreted as a class of
two characters ("W" and "-") followed by a literal string "46]", so it
would match "W46]" or "-46]". However, if the "]" is escaped with a
backslash it is interpreted as the end of range, so [W-\]46] is interpreted
as a class containing a range followed by two other characters.
The octal or hexadecimal representation of "]" can also be used to end
a range.
</para>

<para>
Ranges operate in the collating sequence of character values. They can
also be used for characters specified 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}].
</para>

<para>
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.
</para>

<para>
The only metacharacters that are recognized in character classes are
backslash, hyphen (only where it can be interpreted as specifying a
range), circumflex (only at the start), opening square bracket (only
when it can be interpreted as introducing a POSIX class name - see the
next section), and the terminating closing square bracket. However,
escaping other non-alphanumeric characters does no harm.
</para>
</refsect1>

<refsect1>
<title>Posix character classes</title>
<para>
GRegex supports the POSIX notation for character classes. This uses names
enclosed by [: and :] within the enclosing square brackets. For example,
</para>

<programlisting>
[01[:alpha:]%]
</programlisting>

<para>
matches "0", "1", any alphabetic character, or "%". The supported class
names are
</para>

<table frame="all" colsep="1" rowsep="1">
<title>Posix classes</title>
<tgroup cols="2">
<colspec colnum="1" align="center"/>
<thead>
  <row>
    <entry>Name</entry>
    <entry>Meaning</entry>
  </row>
</thead>
<tbody>
  <row>
    <entry>alnum</entry>
    <entry>letters and digits</entry>
  </row>
  <row>
    <entry>alpha</entry>
    <entry>letters</entry>
  </row>
  <row>
    <entry>ascii</entry>
    <entry>character codes 0 - 127</entry>
  </row>
  <row>
    <entry>blank</entry>
    <entry>space or tab only</entry>
  </row>
  <row>
    <entry>cntrl</entry>
    <entry>control characters</entry>
  </row>
  <row>
    <entry>digit</entry>
    <entry>decimal digits (same as \d)</entry>
  </row>
  <row>
    <entry>graph</entry>
    <entry>printing characters, excluding space</entry>
  </row>
  <row>
    <entry>lower</entry>
    <entry>lower case letters</entry>
  </row>
  <row>
    <entry>print</entry>
    <entry>printing characters, including space</entry>
  </row>
  <row>
    <entry>punct</entry>
    <entry>printing characters, excluding letters and digits</entry>
  </row>
  <row>
    <entry>space</entry>
    <entry>white space (not quite the same as \s)</entry>
  </row>
  <row>
    <entry>upper</entry>
    <entry>upper case letters</entry>
  </row>
  <row>
    <entry>word</entry>
    <entry>"word" characters (same as \w)</entry>
  </row>
  <row>
    <entry>xdigit</entry>
    <entry>hexadecimal digits</entry>
  </row>
</tbody>
</tgroup>
</table>

<para>
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" different to \s, which does not include VT (for
Perl compatibility).
</para>

<para>
The name "word" is a Perl extension, and "blank" is a GNU extension.
Another Perl extension is negation, which is indicated by a ^ character
after the colon. For example,
</para>

<programlisting>
[12[:^digit:]]
</programlisting>

<para>
matches "1", "2", or any non-digit. GRegex 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.
</para>

<para>
In UTF-8 mode, characters with values greater than 128 do not match any
of the POSIX character classes.
</para>
</refsect1>

<refsect1>
<title>Vertical bar</title>
<para>
Vertical bar characters are used to separate alternative patterns. For
example, the pattern
</para>

<programlisting>
 gilbert|sullivan
</programlisting>

<para>
matches either "gilbert" or "sullivan". Any number of alternatives may
appear, and an empty alternative is permitted (matching the empty
string). The matching process tries each alternative in turn, from
left to right, and the first one that succeeds is used. If the alternatives are within a subpattern (defined below), "succeeds" means matching the rest of the main pattern as well as the alternative in the subpattern.
</para>
</refsect1>

<refsect1>
<title>Internal option setting</title>
<para>
The settings of the <varname>G_REGEX_CASELESS</varname>, <varname>G_REGEX_MULTILINE</varname>, <varname>G_REGEX_MULTILINE</varname>,
and <varname>G_REGEX_EXTENDED</varname> options can be changed from within the pattern by a
sequence of Perl-style option letters enclosed between "(?" and ")". The
option letters are
</para>

<table frame="all" colsep="1" rowsep="1">
<title>Option settings</title>
<tgroup cols="2">
<colspec colnum="1" align="center"/>
<thead>
  <row>
    <entry>Option</entry>
    <entry>Flag</entry>
  </row>
</thead>
<tbody>
  <row>
    <entry>i</entry>
    <entry><varname>G_REGEX_CASELESS</varname></entry>
  </row>
  <row>
    <entry>m</entry>
    <entry><varname>G_REGEX_MULTILINE</varname></entry>
  </row>
  <row>
    <entry>s</entry>
    <entry><varname>G_REGEX_DOTALL</varname></entry>
  </row>
  <row>
    <entry>x</entry>
    <entry><varname>G_REGEX_EXTENDED</varname></entry>
  </row>
</tbody>
</tgroup>
</table>

<para>
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 <varname>G_REGEX_CASELESS</varname>
and <varname>G_REGEX_MULTILINE</varname> while unsetting <varname>G_REGEX_DOTALL</varname> and <varname>G_REGEX_EXTENDED</varname>,
is also permitted. If a letter appears both before and after the
hyphen, the option is unset.
</para>

<para>
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.
</para>

<para>
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
</para>

<programlisting>
(a(?i)b)c
</programlisting>

<para>
matches abc and aBc and no other strings (assuming <varname>G_REGEX_CASELESS</varname> 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,
</para>

<programlisting>
(a(?i)b|c)
</programlisting>

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

<para>
The options <varname>G_REGEX_UNGREEDY</varname> and
<varname>G_REGEX_EXTRA</varname> and <varname>G_REGEX_DUPNAMES</varname>
can be changed in the same way as the Perl-compatible options by using
the characters U, X and J respectively.
</para>
</refsect1>

<refsect1>
<title>Subpatterns</title>
<para>
Subpatterns are delimited by parentheses (round brackets), which can be
nested. Turning part of a pattern into a subpattern does two things:
</para>

<itemizedlist>
<listitem><para>
It localizes a set of alternatives. For example, the pattern
cat(aract|erpillar|) matches one of the words "cat", "cataract", or
"caterpillar". Without the parentheses, it would match "cataract",
"erpillar" or an empty string.
</para></listitem>
<listitem><para>
It sets up the subpattern as a capturing subpattern. This means
that, when the whole pattern matches, that portion of the
string that matched the subpattern can be obtained using <function>g_match_info_fetch()</function>.
Opening parentheses are counted from left to right (starting from 1, as
subpattern 0 is the whole matched string) to obtain numbers for the
capturing subpatterns.
</para></listitem>
</itemizedlist>

<para>
For example, if the string "the red king" is matched against the pattern
</para>

<programlisting>
the ((red|white) (king|queen))
</programlisting>

<para>
the captured substrings are "red king", "red", and "king", and are numbered 1, 2, and 3, respectively.
</para>

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

<programlisting>
the ((?:red|white) (king|queen))
</programlisting>

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

<para>
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
</para>

<programlisting>
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
</programlisting>

<para>
match exactly the same set of strings. Because alternative branches are
tried from left to right, and options are not reset until the end of
the subpattern is reached, an option setting in one branch does affect
subsequent branches, so the above patterns match "SUNDAY" as well as
"Saturday".
</para>
</refsect1>

<refsect1>
<title>Named subpatterns</title>
<para>
Identifying capturing parentheses by number is simple, but it can be
very hard to keep track of the numbers in complicated regular expressions.
Furthermore, if an expression is modified, the numbers may
change. To help with this difficulty, GRegex supports the naming of
subpatterns.  A subpattern can be named in one of three ways: (?&lt;name&gt;...) or
(?'name'...) as in Perl, or (?P&lt;name&gt;...) 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.
</para>

<para>
Names consist of up to 32 alphanumeric characters and underscores. Named
capturing parentheses are still allocated numbers as well as names, exactly as
if the names were not present.
By default, a name must be unique within a pattern, but it is possible to relax
this constraint by setting the <varname>G_REGEX_DUPNAMES</varname> 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:
</para>

<programlisting>
(?&lt;DN&gt;Mon|Fri|Sun)(?:day)?|
(?&lt;DN&gt;Tue)(?:sday)?|
(?&lt;DN&gt;Wed)(?:nesday)?|
(?&lt;DN&gt;Thu)(?:rsday)?|
(?&lt;DN&gt;Sat)(?:urday)?
</programlisting>

<para>
There are five capturing substrings, but only one is ever set after a match.
The 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 elsewhere in the
pattern, the one that corresponds to the lowest number is used.
</para>
</refsect1>

<refsect1>
<title>Repetition</title>
<para>
Repetition is specified by quantifiers, which can follow any of the
following items:
</para>

<itemizedlist>
<listitem><para>a literal data character</para></listitem>
<listitem><para>the dot metacharacter</para></listitem>
<listitem><para>the \C escape sequence</para></listitem>
<listitem><para>the \X escape sequence (in UTF-8 mode)</para></listitem>
<listitem><para>the \R escape sequence</para></listitem>
<listitem><para>an escape such as \d that matches a single character</para></listitem>
<listitem><para>a character class</para></listitem>
<listitem><para>a back reference (see next section)</para></listitem>
<listitem><para>a parenthesized subpattern (unless it is an assertion)</para></listitem>
</itemizedlist>

<para>
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:
</para>

<programlisting>
z{2,4}
</programlisting>

<para>
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
</para>

<programlisting>
[aeiou]{3,}
</programlisting>

<para>
matches at least 3 successive vowels, but may match many more, while
</para>

<programlisting>
\d{8}
</programlisting>

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

<para>
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,
\X{3} matches three Unicode extended sequences, each of which may be
several bytes long (and they may be of different lengths).
</para>

<para>
The quantifier {0} is permitted, causing the expression to behave as if
the previous item and the quantifier were not present.
</para>

<para>
For convenience, the three most common quantifiers have single-character
abbreviations:
</para>

<table frame="all" colsep="1" rowsep="1">
<title>Abbreviations for quantifiers</title>
<tgroup cols="2">
<colspec colnum="1" align="center"/>
<thead>
  <row>
    <entry>Abbreviation</entry>
    <entry>Meaning</entry>
  </row>
</thead>
<tbody>
  <row>
    <entry>*</entry>
    <entry>is equivalent to {0,}</entry>
  </row>
  <row>
    <entry>+</entry>
    <entry>is equivalent to {1,}</entry>
  </row>
  <row>
    <entry>?</entry>
    <entry>is equivalent to {0,1}</entry>
  </row>
</tbody>
</tgroup>
</table>

<para>
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:
</para>

<programlisting>
(a?)*
</programlisting>

<para>
Because there are cases where this can be useful, such patterns are
accepted, but if any repetition of the subpattern does in fact match
no characters, the loop is forcibly broken.
</para>

<para>
By default, the quantifiers are "greedy", that is, they match as much
as possible (up to the maximum number of permitted times), without
causing the rest of the pattern to fail. The classic example of where
this gives problems is in trying to match comments in C programs. These
appear between /* and */ and within the comment, individual * and /
characters may appear. An attempt to match C comments by applying the
pattern
</para>

<programlisting>
/\*.*\*/
</programlisting>

<para>
to the string
</para>

<programlisting>
/* first comment */  not comment  /* second comment */
</programlisting>

<para>
fails, because it matches the entire string owing to the greediness of
the .* item.
</para>

<para>
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
</para>

<programlisting>
/\*.*?\*/
</programlisting>

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

<programlisting>
\d??\d
</programlisting>

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

<para>
If the <varname>G_REGEX_UNGREEDY</varname> flag is set, 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.
</para>

<para>
When a parenthesized subpattern is quantified with a minimum repeat
count that is greater than 1 or with a limited maximum, more memory is
required for the compiled pattern, in proportion to the size of the
minimum or maximum.
</para>

<para>
If a pattern starts with .* or .{0,} and the <varname>G_REGEX_DOTALL</varname> flag
is set, thus allowing the dot to match newlines, the
pattern is implicitly anchored, because whatever follows will be tried
against every character position in the string, so there is no
point in retrying the overall match at any position after the first.
GRegex normally treats such a pattern as though it were preceded by \A.
</para>

<para>
In cases where it is known that the string contains no newlines, it
is worth setting <varname>G_REGEX_DOTALL</varname> in order to obtain this optimization,
or alternatively using ^ to indicate anchoring explicitly.
</para>

<para>
However, there is one situation where the optimization cannot 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:
</para>

<programlisting>
(.*)abc\1
</programlisting>

<para>
If the string is "xyz123abc123" the match point is the fourth character.
For this reason, such a pattern is not implicitly anchored.
</para>

<para>
When a capturing subpattern is repeated, the value captured is the
substring that matched the final iteration. For example, after
</para>

<programlisting>
(tweedle[dume]{3}\s*)+
</programlisting>

<para>
has matched "tweedledum tweedledee" the value of the captured substring
is "tweedledee". However, if there are nested capturing subpatterns,
the corresponding captured values may have been set in previous iterations.
For example, after
</para>

<programlisting>
/(a|(b))+/
</programlisting>

<para>
matches "aba" the value of the second captured substring is "b".
</para>
</refsect1>

<refsect1>
<title>Atomic grouping and possessive quantifiers</title>
<para>
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
repetition, failure of what follows normally causes the repeated
item to be re-evaluated to see if a different number
of repeats allows the rest of the pattern to match. Sometimes it
is useful to prevent this, either to change the nature of the
match, or to cause it fail earlier than it otherwise might, when the
author of the pattern knows there is no point in carrying on.
</para>

<para>
Consider, for example, the pattern \d+foo when applied to the string
</para>

<programlisting>
123456bar
</programlisting>

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

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

<programlisting>
(?>\d+)foo
</programlisting>

<para>
This kind of parenthesis "locks up" the part of the pattern it contains
once it has matched, and a failure further into the pattern is
prevented from backtracking into it. Backtracking past it to previous
items, however, works as normal.
</para>

<para>
An alternative description is that a subpattern of this type matches
the string of characters that an identical standalone pattern would
match, if anchored at the current point in the string.
</para>

<para>
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.
</para>

<para>
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
</para>

<programlisting>
\d++foo
</programlisting>

<para>
Possessive quantifiers are always greedy; the setting of the
<varname>G_REGEX_UNGREEDY</varname> option is ignored. They are a convenient notation for the
simpler forms of atomic group. However, there is no difference in the
meaning of a possessive quantifier and the equivalent
atomic group, though there may be a performance difference;
possessive quantifiers should be slightly faster.
</para>

<para>
The possessive quantifier syntax is an extension to the Perl syntax.
It was invented by Jeffrey Friedl in the first edition of his book and
then implemented by Mike McCloskey in Sun's Java package.
It ultimately found its way into Perl at release 5.10.
</para>

<para>
GRegex has an optimization that automatically "possessifies" certain simple
pattern constructs. For example, the sequence A+B is treated as A++B because
there is no point in backtracking into a sequence of A's when B must follow.
</para>

<para>
When a pattern contains an unlimited repeat inside a subpattern that
can itself be repeated an unlimited number of times, the use of an
atomic group is the only way to avoid some failing matches taking a
very long time indeed. The pattern
</para>
 
<programlisting>
(\D+|&lt;\d+&gt;)*[!?]
</programlisting>

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

<programlisting>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
</programlisting>

<para>
it takes a long time before reporting failure. This is because the
string can be divided between the internal \D+ repeat and the external
* repeat in a large number of ways, and all have to be tried. (The
example uses [!?] rather than a single character at the end, because
GRegex has an optimization that allows for fast failure
when a single character is used. It 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:
</para>

<programlisting>
((?>\D+)|&lt;\d+&gt;)*[!?]
</programlisting>

<para>
sequences of non-digits cannot be broken, and failure happens quickly.
</para>
</refsect1>

<refsect1>
<title>Back references</title>
<para>
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.
</para>

<para>
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.
</para>

<para>
It is not possible to have a numerical "forward back reference" to subpattern
whose number is 10 or more using this syntax because a sequence such as \e50 is
interpreted as a character defined in octal. See the subsection entitled
"Non-printing characters" 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).
</para>

<para>
Another way of avoiding the ambiguity inherent in the use of digits following a
backslash is to use the \g escape sequence (introduced in Perl 5.10.)
This escape must be followed by a positive or a negative number,
optionally enclosed in braces.
</para>

<para>
A positive 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 "(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 themselves.
</para>

<para>
A back reference matches whatever actually matched the capturing subpattern
in the current string, rather than anything matching
the subpattern itself (see "Subpatterns as subroutines" below for a way
of doing that). So the pattern
</para>

<programlisting>
(sens|respons)e and \1ibility
</programlisting>

<para>
matches "sense and sensibility" and "response and responsibility", but
not "sense and responsibility". If caseful matching is in force at the
time of the back reference, the case of letters is relevant. For example,
</para>

<programlisting>
((?i)rah)\s+\1
</programlisting>

<para>
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
original capturing subpattern is matched caselessly.
</para>

<para>
Back references to named subpatterns use the Perl syntax \k&lt;name&gt; or \k'name'
or the Python syntax (?P=name). We could rewrite the above example in either of
the following ways:
</para>

<programlisting>
(?&lt;p1&gt;(?i)rah)\s+\k&lt;p1&gt;
(?P&lt;p1&gt;(?i)rah)\s+(?P=p1)
</programlisting>

<para>
A subpattern that is referenced by name may appear in the pattern before or
after the reference.
</para>

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

<programlisting>
(a|(bc))\2
</programlisting>

<para>
always fails if it starts to match "a" rather than "bc". Because there
may be many capturing parentheses in a pattern, all digits following
the backslash are taken as part of a potential back reference number.
If the pattern continues with a digit character, some delimiter must be
used to terminate the back reference. If the <varname>G_REGEX_EXTENDED</varname> flag is
set, this can be whitespace. Otherwise an empty comment (see "Comments" below) can be used.
</para>

<para>
A back reference that occurs inside the parentheses to which it refers
fails when the subpattern is first used, so, for example, (a\1) never
matches. However, such references can be useful inside repeated subpatterns.
For example, the pattern
</para>

<programlisting>
(a|b\1)+
</programlisting>

<para>
matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration
of the subpattern, the back reference matches the character
string corresponding to the previous iteration. In order for this to
work, the pattern must be such that the first iteration does not need
to match the back reference. This can be done using alternation, as in
the example above, or by a quantifier with a minimum of zero.
</para>
</refsect1>

<refsect1>
<title>Assertions</title>
<para>
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.
</para>

<para>
More complicated assertions are coded as subpatterns. There are two
kinds: those that look ahead of the current position in the
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.
</para>

<para>
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.
</para>

<refsect2>
<title>Lookahead assertions</title>
<para>
Lookahead assertions start with (?= for positive assertions and (?! for
negative assertions. For example,
</para>

<programlisting>
\w+(?=;)
</programlisting>

<para>
matches a word followed by a semicolon, but does not include the semicolon
in the match, and
</para>

<programlisting>
foo(?!bar)
</programlisting>

<para>
matches any occurrence of "foo" that is not followed by "bar". Note
that the apparently similar pattern
</para>

<programlisting>
(?!foo)bar
</programlisting>

<para>
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.
</para>

<para>
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.
</para>
</refsect2>

<refsect2>
<title>Lookbehind assertions</title>
<para>
Lookbehind assertions start with (?&lt;= for positive assertions and (?&lt;!
for negative assertions. For example,
</para>

<programlisting>
(?&lt;!foo)bar
</programlisting>

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

<programlisting>
(?&lt;=bullock|donkey)
</programlisting>

<para>
is permitted, but
</para>

<programlisting>
(?&lt;!dogs?|cats?)
</programlisting>

<para>
causes an error at compile time. Branches that match different length
strings are permitted only at the top level of a lookbehind assertion.
An assertion such as
</para>

<programlisting>
(?&lt;=ab(c|de))
</programlisting>

<para>
is not permitted, because its single top-level branch can match two
different lengths, but it is acceptable if rewritten to use two top-
level branches:
</para>

<programlisting>
(?&lt;=abc|abde)
</programlisting>

<para>
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.
</para>

<para>
GRegex 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 different numbers of bytes, are also not permitted.
</para>

<para>
Possessive quantifiers can be used in conjunction with lookbehind assertions to
specify efficient matching at the end of the subject string. Consider a simple
pattern such as
</para>

<programlisting>
abcd$
</programlisting>

<para>
when applied to a long string that does not match. Because matching
proceeds from left to right, GRegex will look for each "a" in the string
and then see if what follows matches the rest of the pattern. If the
pattern is specified as
</para>

<programlisting>
^.*abcd$
</programlisting>

<para>
the initial .* matches the entire string at first, but when this fails
(because there is no following "a"), it backtracks to match all but the
last character, then all but the last two characters, and so on. Once
again the search for "a" covers the entire string, from right to left,
so we are no better off. However, if the pattern is written as
</para>

<programlisting>
^.*+(?&lt;=abcd)
</programlisting>

<para>
there can be no backtracking for the .*+ item; it can match only the
entire string. The subsequent lookbehind assertion does a single test
on the last four characters. If it fails, the match fails immediately.
For long strings, this approach makes a significant difference to the
processing time.
</para>
</refsect2>

<refsect2>
<title>Using multiple assertions</title>
<para>
Several assertions (of any sort) may occur in succession. For example,
</para>

<programlisting>
(?&lt;=\d{3})(?&lt;!999)foo
</programlisting>

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

<programlisting>
(?&lt;=\d{3}...)(?&lt;!999)foo
</programlisting>

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

<para>
Assertions can be nested in any combination. For example,
</para>

<programlisting>
(?&lt;=(?&lt;!foo)bar)baz
</programlisting>

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

<programlisting>
(?&lt;=\d{3}(?!999)...)foo
</programlisting>

<para>
is another pattern that matches "foo" preceded by three digits and any
three characters that are not "999".
</para>
</refsect2>
</refsect1>

<refsect1>
<title>Conditional subpatterns</title>
<para>
It is possible to cause the matching process to obey a subpattern
conditionally or to choose between two alternative subpatterns, depending
on the result of an assertion, or whether a previous capturing subpattern
matched or not. The two possible forms of conditional subpattern are
</para>

<programlisting>
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
</programlisting>

<para>
If the condition is satisfied, the yes-pattern is used; otherwise the
no-pattern (if present) is used. If there are more than two alternatives
in the subpattern, a compile-time error occurs.
</para>

<para>
There are four kinds of condition: references to subpatterns, references to
recursion, a pseudo-condition called DEFINE, and assertions.
</para>

<refsect2>
<title>Checking for a used subpattern by number</title>
<para>
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.
</para>

<para>
Consider the following pattern, which contains non-significant white space
to make it more readable (assume the <varname>G_REGEX_EXTENDED</varname>)
and to divide it into three parts for ease of discussion:
</para>

<programlisting>
( \( )?    [^()]+    (?(1) \) )
</programlisting>

<para>
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 string 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.
</para>
</refsect2>

<refsect2>
<title>Checking for a used subpattern by name</title>
<para>
Perl uses the syntax (?(&lt;name&gt;)...) or (?('name')...) to test for a used
subpattern by name, the Python syntax (?(name)...) is also recognized. However,
there is a possible ambiguity with this syntax, because subpattern names may
consist entirely of digits. GRegex looks first for a named subpattern; if it
cannot find one and the name consists entirely of digits, GRegex looks for a
subpattern of that number, which must be greater than zero. Using subpattern
names that consist entirely of digits is not recommended.
</para>

<para>
Rewriting the above example to use a named subpattern gives this:
</para>

<programlisting>
(?&lt;OPEN&gt; \( )?    [^()]+    (?(&lt;OPEN&gt;) \) )
</programlisting>
</refsect2>

<refsect2>
<title>Checking for pattern recursion</title>
<para>
If the condition is the string (R), and there is no subpattern with the name R,
the condition is true if a recursive call to the whole pattern or any
subpattern has been made. If digits or a name preceded by ampersand follow the
letter R, for example:
</para>

<programlisting>
(?(R3)...)
(?(R&amp;name)...)
</programlisting>

<para>
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.
</para>

<para>
At "top level", all these recursion test conditions are false. Recursive
patterns are described below.
</para>
</refsect2>

<refsect2>
<title>Defining subpatterns for use by reference only</title>
<para>
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):
</para>

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

<para>
The first part of the pattern is a DEFINE group inside which 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.
</para>

<para>
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.
</para>
</refsect2>

<refsect2>
<title>Assertion conditions</title>
<para>
If the condition is not in any of the above formats, it must be an
assertion. This may be a positive or negative lookahead or lookbehind
assertion. Consider this pattern, again containing non-significant
white space, and with the two alternatives on the second line:
</para>

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

<para>
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 string. If a
letter is found, the string 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.
</para>
</refsect2>
</refsect1>

<refsect1>
<title>Comments</title>
<para>
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.
</para>

<para>
If the <varname>G_REGEX_EXTENDED</varname> option is set, an unescaped #
character outside a character class introduces a comment that continues to
immediately after the next newline in the pattern.
</para>
</refsect1>

<refsect1>
<title>Recursive patterns</title>
<para>
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.
</para>

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

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

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

<para>
Obviously, GRegex cannot support the interpolation of Perl code. Instead, it
supports special syntax for recursion of the entire pattern, and also for
individual subpattern recursion. This kind of recursion was introduced into
Perl at release 5.10.
</para>

<para>
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.
</para>

<para>
In GRegex (like Python, but unlike Perl), a recursive subpattern call is always
treated as an atomic group. That is, once it has matched some of the subject
string, it is never re-entered, even if it contains untried alternatives and
there is a subsequent matching failure.
</para>

<para>
This pattern solves the nested parentheses problem (assume the
<varname>G_REGEX_EXTENDED</varname> option is set so that white space is
ignored):
</para>

<programlisting>
\( ( (?&gt;[^()]+) | (?R) )* \)
</programlisting>

<para>
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.
</para>

<para>
If this were part of a larger pattern, you would not want to recurse
the entire pattern, so instead you could use this:
</para>

<programlisting>
( \( ( (?&gt;[^()]+) | (?1) )* \) )
</programlisting>

<para>
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. It may be more convenient to
use named parentheses instead.
The Perl syntax for this is (?&amp;name); GRegex also supports the(?P>name)
syntax. We could rewrite the above example as follows:
</para>

<programlisting>
(?&lt;pn&gt; \( ( (?&gt;[^()]+) | (?&amp;pn) )* \) )
</programlisting>

<para>
If there is more than one subpattern with the same name, the earliest one is
used. This particular example pattern contains nested unlimited repeats, and so
the use of atomic grouping for matching strings of non-parentheses is important
when applying the pattern to strings that do not match.
For example, when this pattern is applied to
</para>

<programlisting>
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
</programlisting>

<para>
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 string, and all
have to be tested before failure can be reported.
</para>

<para>
At the end of a match, the values set for any capturing subpatterns are
those from the outermost level of the recursion at which the subpattern
value is set.

<!-- Callouts are not supported by GRegex
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
</para>

<programlisting>
(ab(cd)ef)
</programlisting>

<para>
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
</para>

<programlisting>
\( ( ( (?&gt;[^()]+) | (?R) )* ) \)
   ^                        ^
   ^                        ^
</programlisting>

<para>
the string they capture is "ab(cd)ef", the contents of the top level
parentheses.
</para>

<para>
Do not confuse the (?R) item with the condition (R), which tests for
recursion. Consider this pattern, which matches text in angle brackets,
allowing for arbitrary nesting. Only digits are allowed in nested
brackets (that is, when recursing), whereas any characters are permitted
at the outer level.
</para>

<programlisting>
&lt; (?: (?(R) \d++ | [^&lt;&gt;]*+) | (?R)) * &gt;
</programlisting>

<para>
In this pattern, (?(R) is the start of a conditional subpattern, with
two different alternatives for the recursive and non-recursive cases.
The (?R) item is the actual recursive call.
</para>
</refsect1>

<refsect1>
<title>Subpatterns as subroutines</title>
<para>
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 programming language. The "called" subpattern may
be defined before or after the reference. An earlier example pointed out
that the pattern
</para>

<programlisting>
(sens|respons)e and \1ibility
</programlisting>

<para>
matches "sense and sensibility" and "response and responsibility", but
not "sense and responsibility". If instead the pattern
</para>

<programlisting>
(sens|respons)e and (?1)ibility
</programlisting>

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

<para>
Like recursive subpatterns, a "subroutine" call is always treated as an atomic
group. That is, once it has matched some of the string, it is never
re-entered, even if it contains untried alternatives and there is a subsequent
matching failure.
</para>

<para>
When a subpattern is used as a subroutine, processing options such as
case-independence are fixed when the subpattern is defined. They cannot be
changed for different calls. For example, consider this pattern:
</para>

<programlisting>
(abc)(?i:(?1))
</programlisting>

<para>
It matches "abcabc". It does not match "abcABC" because the change of
processing option does not affect the called subpattern.
</para>
</refsect1>

<refsect1>
<title>Copyright</title>
<para>
This document was copied and adapted from the PCRE documentation,
specifically from the man page for pcrepattern.
The original copyright note is:
</para>

<programlisting>
Copyright (c) 1997-2006 University of Cambridge.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:

    * Redistributions of source code must retain the above copyright notice,
      this list of conditions and the following disclaimer.

    * Redistributions in binary form must reproduce the above copyright
      notice, this list of conditions and the following disclaimer in the
      documentation and/or other materials provided with the distribution.

    * Neither the name of the University of Cambridge nor the name of Google
      Inc. nor the names of their contributors may be used to endorse or
      promote products derived from this software without specific prior
      written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE.
</programlisting>
</refsect1>

</refentry>