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<title>POSIX Basic Regular Expression Syntax</title>
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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_regex.syntax.basic_syntax"></a><a class="link" href="basic_syntax.html" title="POSIX Basic Regular Expression Syntax">POSIX Basic Regular
Expression Syntax</a>
</h3></div></div></div>
<h4>
<a name="boost_regex.syntax.basic_syntax.h0"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.synopsis"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.synopsis">Synopsis</a>
</h4>
<p>
The POSIX-Basic regular expression syntax is used by the Unix utility <code class="computeroutput"><span class="identifier">sed</span></code>, and variations are used by <code class="computeroutput"><span class="identifier">grep</span></code> and <code class="computeroutput"><span class="identifier">emacs</span></code>.
You can construct POSIX basic regular expressions in Boost.Regex by passing
the flag <code class="computeroutput"><span class="identifier">basic</span></code> to the regex
constructor (see <a class="link" href="../ref/syntax_option_type.html" title="syntax_option_type"><code class="computeroutput"><span class="identifier">syntax_option_type</span></code></a>), for example:
</p>
<pre class="programlisting"><span class="comment">// e1 is a case sensitive POSIX-Basic expression:</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span> <span class="identifier">e1</span><span class="special">(</span><span class="identifier">my_expression</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">basic</span><span class="special">);</span>
<span class="comment">// e2 a case insensitive POSIX-Basic expression:</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span> <span class="identifier">e2</span><span class="special">(</span><span class="identifier">my_expression</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">basic</span><span class="special">|</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">icase</span><span class="special">);</span>
</pre>
<a name="boost_regex.posix_basic"></a><h4>
<a name="boost_regex.syntax.basic_syntax.h1"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.posix_basic_syntax"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.posix_basic_syntax">POSIX
Basic Syntax</a>
</h4>
<p>
In POSIX-Basic regular expressions, all characters are match themselves except
for the following special characters:
</p>
<pre class="programlisting">.[\*^$</pre>
<h5>
<a name="boost_regex.syntax.basic_syntax.h2"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.wildcard"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.wildcard">Wildcard:</a>
</h5>
<p>
The single character '.' when used outside of a character set will match
any single character except:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
The NULL character when the flag <code class="computeroutput"><span class="identifier">match_no_dot_null</span></code>
is passed to the matching algorithms.
</li>
<li class="listitem">
The newline character when the flag <code class="computeroutput"><span class="identifier">match_not_dot_newline</span></code>
is passed to the matching algorithms.
</li>
</ul></div>
<h5>
<a name="boost_regex.syntax.basic_syntax.h3"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.anchors"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.anchors">Anchors:</a>
</h5>
<p>
A '^' character shall match the start of a line when used as the first character
of an expression, or the first character of a sub-expression.
</p>
<p>
A '$' character shall match the end of a line when used as the last character
of an expression, or the last character of a sub-expression.
</p>
<h5>
<a name="boost_regex.syntax.basic_syntax.h4"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.marked_sub_expressions"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.marked_sub_expressions">Marked sub-expressions:</a>
</h5>
<p>
A section beginning <code class="computeroutput"><span class="special">\(</span></code> and ending
<code class="computeroutput"><span class="special">\)</span></code> acts as a marked sub-expression.
Whatever matched the sub-expression is split out in a separate field by the
matching algorithms. Marked sub-expressions can also repeated, or referred-to
by a back-reference.
</p>
<h5>
<a name="boost_regex.syntax.basic_syntax.h5"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.repeats"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.repeats">Repeats:</a>
</h5>
<p>
Any atom (a single character, a marked sub-expression, or a character class)
can be repeated with the * operator.
</p>
<p>
For example <code class="computeroutput"><span class="identifier">a</span><span class="special">*</span></code>
will match any number of letter a's repeated zero or more times (an atom
repeated zero times matches an empty string), so the expression <code class="computeroutput"><span class="identifier">a</span><span class="special">*</span><span class="identifier">b</span></code>
will match any of the following:
</p>
<pre class="programlisting">b
ab
aaaaaaaab
</pre>
<p>
An atom can also be repeated with a bounded repeat:
</p>
<p>
<code class="computeroutput"><span class="identifier">a</span><span class="special">\{</span><span class="identifier">n</span><span class="special">\}</span></code> Matches
'a' repeated exactly n times.
</p>
<p>
<code class="computeroutput"><span class="identifier">a</span><span class="special">\{</span><span class="identifier">n</span><span class="special">,\}</span></code> Matches
'a' repeated n or more times.
</p>
<p>
<code class="computeroutput"><span class="identifier">a</span><span class="special">\{</span><span class="identifier">n</span><span class="special">,</span> <span class="identifier">m</span><span class="special">\}</span></code> Matches 'a' repeated between n and m times
inclusive.
</p>
<p>
For example:
</p>
<pre class="programlisting">^a{2,3}$</pre>
<p>
Will match either of:
</p>
<pre class="programlisting">aa
aaa
</pre>
<p>
But neither of:
</p>
<pre class="programlisting">a
aaaa
</pre>
<p>
It is an error to use a repeat operator, if the preceding construct can not
be repeated, for example:
</p>
<pre class="programlisting">a(*)</pre>
<p>
Will raise an error, as there is nothing for the * operator to be applied
to.
</p>
<h5>
<a name="boost_regex.syntax.basic_syntax.h6"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.back_references"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.back_references">Back
references:</a>
</h5>
<p>
An escape character followed by a digit <span class="emphasis"><em>n</em></span>, where <span class="emphasis"><em>n</em></span>
is in the range 1-9, matches the same string that was matched by sub-expression
<span class="emphasis"><em>n</em></span>. For example the expression:
</p>
<pre class="programlisting">^\(a*\)[^a]*\1$</pre>
<p>
Will match the string:
</p>
<pre class="programlisting">aaabbaaa</pre>
<p>
But not the string:
</p>
<pre class="programlisting">aaabba</pre>
<h5>
<a name="boost_regex.syntax.basic_syntax.h7"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.character_sets"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.character_sets">Character
sets:</a>
</h5>
<p>
A character set is a bracket-expression starting with [ and ending with ],
it defines a set of characters, and matches any single character that is
a member of that set.
</p>
<p>
A bracket expression may contain any combination of the following:
</p>
<h6>
<a name="boost_regex.syntax.basic_syntax.h8"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.single_characters"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.single_characters">Single
characters:</a>
</h6>
<p>
For example <code class="computeroutput"><span class="special">[</span><span class="identifier">abc</span><span class="special">]</span></code>, will match any of the characters 'a', 'b',
or 'c'.
</p>
<h6>
<a name="boost_regex.syntax.basic_syntax.h9"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.character_ranges"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.character_ranges">Character
ranges:</a>
</h6>
<p>
For example <code class="computeroutput"><span class="special">[</span><span class="identifier">a</span><span class="special">-</span><span class="identifier">c</span><span class="special">]</span></code>
will match any single character in the range 'a' to 'c'. By default, for
POSIX-Basic regular expressions, a character <span class="emphasis"><em>x</em></span> is within
the range <span class="emphasis"><em>y</em></span> to <span class="emphasis"><em>z</em></span>, if it collates
within that range; this results in locale specific behavior. This behavior
can be turned off by unsetting the <code class="computeroutput"><span class="identifier">collate</span></code>
option flag when constructing the regular expression - in which case whether
a character appears within a range is determined by comparing the code points
of the characters only.
</p>
<h6>
<a name="boost_regex.syntax.basic_syntax.h10"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.negation"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.negation">Negation:</a>
</h6>
<p>
If the bracket-expression begins with the ^ character, then it matches the
complement of the characters it contains, for example <code class="computeroutput"><span class="special">[^</span><span class="identifier">a</span><span class="special">-</span><span class="identifier">c</span><span class="special">]</span></code> matches any character that is not in the
range a-c.
</p>
<h6>
<a name="boost_regex.syntax.basic_syntax.h11"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.character_classes"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.character_classes">Character
classes:</a>
</h6>
<p>
An expression of the form <code class="computeroutput"><span class="special">[[:</span><span class="identifier">name</span><span class="special">:]]</span></code>
matches the named character class "name", for example <code class="computeroutput"><span class="special">[[:</span><span class="identifier">lower</span><span class="special">:]]</span></code> matches any lower case character. See
<a class="link" href="character_classes.html" title="Character Class Names">character class names</a>.
</p>
<h6>
<a name="boost_regex.syntax.basic_syntax.h12"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.collating_elements"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.collating_elements">Collating
Elements:</a>
</h6>
<p>
An expression of the form <code class="computeroutput"><span class="special">[[.</span><span class="identifier">col</span><span class="special">.]</span></code> matches
the collating element <span class="emphasis"><em>col</em></span>. A collating element is any
single character, or any sequence of characters that collates as a single
unit. Collating elements may also be used as the end point of a range, for
example: <code class="computeroutput"><span class="special">[[.</span><span class="identifier">ae</span><span class="special">.]-</span><span class="identifier">c</span><span class="special">]</span></code>
matches the character sequence "ae", plus any single character
in the range "ae"-c, assuming that "ae" is treated as
a single collating element in the current locale.
</p>
<p>
Collating elements may be used in place of escapes (which are not normally
allowed inside character sets), for example <code class="computeroutput"><span class="special">[[.^.]</span><span class="identifier">abc</span><span class="special">]</span></code> would
match either one of the characters 'abc^'.
</p>
<p>
As an extension, a collating element may also be specified via its symbolic
name, for example:
</p>
<pre class="programlisting">[[.NUL.]]</pre>
<p>
matches a 'NUL' character. See <a class="link" href="collating_names.html" title="Collating Names">collating
element names</a>.
</p>
<h6>
<a name="boost_regex.syntax.basic_syntax.h13"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.equivalence_classes"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.equivalence_classes">Equivalence
classes:</a>
</h6>
<p>
An expression of the form <code class="computeroutput"><span class="special">[[=</span><span class="identifier">col</span><span class="special">=]]</span></code>,
matches any character or collating element whose primary sort key is the
same as that for collating element <span class="emphasis"><em>col</em></span>, as with collating
elements the name <span class="emphasis"><em>col</em></span> may be a <a class="link" href="collating_names.html" title="Collating Names">collating
symbolic name</a>. A primary sort key is one that ignores case, accentation,
or locale-specific tailorings; so for example <code class="computeroutput"><span class="special">[[=</span><span class="identifier">a</span><span class="special">=]]</span></code> matches
any of the characters: a, &#192;, &#193;, &#194;, &#195;, &#196;, &#197;, A, &#224;, &#225;, &#226;, &#227;, &#228; and &#229;. Unfortunately implementation
of this is reliant on the platform's collation and localisation support;
this feature can not be relied upon to work portably across all platforms,
or even all locales on one platform.
</p>
<h6>
<a name="boost_regex.syntax.basic_syntax.h14"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.combinations"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.combinations">Combinations:</a>
</h6>
<p>
All of the above can be combined in one character set declaration, for example:
<code class="computeroutput"><span class="special">[[:</span><span class="identifier">digit</span><span class="special">:]</span><span class="identifier">a</span><span class="special">-</span><span class="identifier">c</span><span class="special">[.</span><span class="identifier">NUL</span><span class="special">.]].</span></code>
</p>
<h5>
<a name="boost_regex.syntax.basic_syntax.h15"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.escapes"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.escapes">Escapes</a>
</h5>
<p>
With the exception of the escape sequences \{, \}, \(, and \), which are
documented above, an escape followed by any character matches that character.
This can be used to make the special characters
</p>
<pre class="programlisting">.[\*^$</pre>
<p>
"ordinary". Note that the escape character loses its special meaning
inside a character set, so <code class="computeroutput"><span class="special">[\^]</span></code>
will match either a literal '\' or a '^'.
</p>
<h4>
<a name="boost_regex.syntax.basic_syntax.h16"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.what_gets_matched"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.what_gets_matched">What
Gets Matched</a>
</h4>
<p>
When there is more that one way to match a regular expression, the "best"
possible match is obtained using the <a class="link" href="leftmost_longest_rule.html" title="The Leftmost Longest Rule">leftmost-longest
rule</a>.
</p>
<h4>
<a name="boost_regex.syntax.basic_syntax.h17"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.variations"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.variations">Variations</a>
</h4>
<a name="boost_regex.grep_syntax"></a><h5>
<a name="boost_regex.syntax.basic_syntax.h18"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.grep"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.grep">Grep</a>
</h5>
<p>
When an expression is compiled with the flag <code class="computeroutput"><span class="identifier">grep</span></code>
set, then the expression is treated as a newline separated list of <a class="link" href="basic_syntax.html#boost_regex.posix_basic">POSIX-Basic expressions</a>, a match
is found if any of the expressions in the list match, for example:
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span> <span class="identifier">e</span><span class="special">(</span><span class="string">"abc\ndef"</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">grep</span><span class="special">);</span>
</pre>
<p>
will match either of the <a class="link" href="basic_syntax.html#boost_regex.posix_basic">POSIX-Basic
expressions</a> "abc" or "def".
</p>
<p>
As its name suggests, this behavior is consistent with the Unix utility grep.
</p>
<h5>
<a name="boost_regex.syntax.basic_syntax.h19"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.emacs"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.emacs">emacs</a>
</h5>
<p>
In addition to the <a class="link" href="basic_syntax.html#boost_regex.posix_basic">POSIX-Basic features</a>
the following characters are also special:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Character
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
+
</p>
</td>
<td>
<p>
repeats the preceding atom one or more times.
</p>
</td>
</tr>
<tr>
<td>
<p>
?
</p>
</td>
<td>
<p>
repeats the preceding atom zero or one times.
</p>
</td>
</tr>
<tr>
<td>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
?
</li></ul></div>
</td>
<td>
<p>
A non-greedy version of *.
</p>
</td>
</tr>
<tr>
<td>
<p>
+?
</p>
</td>
<td>
<p>
A non-greedy version of +.
</p>
</td>
</tr>
<tr>
<td>
<p>
??
</p>
</td>
<td>
<p>
A non-greedy version of ?.
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
And the following escape sequences are also recognised:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Escape
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
\|
</p>
</td>
<td>
<p>
specifies an alternative.
</p>
</td>
</tr>
<tr>
<td>
<p>
\(?: ... )
</p>
</td>
<td>
<p>
is a non-marking grouping construct - allows you to lexically group
something without spitting out an extra sub-expression.
</p>
</td>
</tr>
<tr>
<td>
<p>
\w
</p>
</td>
<td>
<p>
matches any word character.
</p>
</td>
</tr>
<tr>
<td>
<p>
\W
</p>
</td>
<td>
<p>
matches any non-word character.
</p>
</td>
</tr>
<tr>
<td>
<p>
\sx
</p>
</td>
<td>
<p>
matches any character in the syntax group x, the following emacs
groupings are supported: 's', ' ', '_', 'w', '.', ')', '(', '"',
'\'', '&gt;' and '&lt;'. Refer to the emacs docs for details.
</p>
</td>
</tr>
<tr>
<td>
<p>
\Sx
</p>
</td>
<td>
<p>
matches any character not in the syntax grouping x.
</p>
</td>
</tr>
<tr>
<td>
<p>
\c and \C
</p>
</td>
<td>
<p>
These are not supported.
</p>
</td>
</tr>
<tr>
<td>
<p>
\`
</p>
</td>
<td>
<p>
matches zero characters only at the start of a buffer (or string
being matched).
</p>
</td>
</tr>
<tr>
<td>
<p>
\'
</p>
</td>
<td>
<p>
matches zero characters only at the end of a buffer (or string
being matched).
</p>
</td>
</tr>
<tr>
<td>
<p>
\b
</p>
</td>
<td>
<p>
matches zero characters at a word boundary.
</p>
</td>
</tr>
<tr>
<td>
<p>
\B
</p>
</td>
<td>
<p>
matches zero characters, not at a word boundary.
</p>
</td>
</tr>
<tr>
<td>
<p>
\&lt;
</p>
</td>
<td>
<p>
matches zero characters only at the start of a word.
</p>
</td>
</tr>
<tr>
<td>
<p>
\&gt;
</p>
</td>
<td>
<p>
matches zero characters only at the end of a word.
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
Finally, you should note that emacs style regular expressions are matched
according to the <a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.what_gets_matched">Perl
"depth first search" rules</a>. Emacs expressions are matched
this way because they contain Perl-like extensions, that do not interact
well with the <a class="link" href="leftmost_longest_rule.html" title="The Leftmost Longest Rule">POSIX-style
leftmost-longest rule</a>.
</p>
<h4>
<a name="boost_regex.syntax.basic_syntax.h20"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.options"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.options">Options</a>
</h4>
<p>
There are a <a class="link" href="../ref/syntax_option_type/syntax_option_type_basic.html" title="Options for POSIX Basic Regular Expressions">variety
of flags</a> that may be combined with the <code class="computeroutput"><span class="identifier">basic</span></code>
and <code class="computeroutput"><span class="identifier">grep</span></code> options when constructing
the regular expression, in particular note that the <a class="link" href="../ref/syntax_option_type/syntax_option_type_basic.html" title="Options for POSIX Basic Regular Expressions"><code class="computeroutput"><span class="identifier">newline_alt</span></code>, <code class="computeroutput"><span class="identifier">no_char_classes</span></code>,
<code class="computeroutput"><span class="identifier">no</span><span class="special">-</span><span class="identifier">intervals</span></code>, <code class="computeroutput"><span class="identifier">bk_plus_qm</span></code>
and <code class="computeroutput"><span class="identifier">bk_plus_vbar</span></code></a> options
all alter the syntax, while the <a class="link" href="../ref/syntax_option_type/syntax_option_type_basic.html" title="Options for POSIX Basic Regular Expressions"><code class="computeroutput"><span class="identifier">collate</span></code> and <code class="computeroutput"><span class="identifier">icase</span></code>
options</a> modify how the case and locale sensitivity are to be applied.
</p>
<h4>
<a name="boost_regex.syntax.basic_syntax.h21"></a>
<span class="phrase"><a name="boost_regex.syntax.basic_syntax.references"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.references">References</a>
</h4>
<p>
<a href="http://www.opengroup.org/onlinepubs/000095399/basedefs/xbd_chap09.html" target="_top">IEEE
Std 1003.1-2001, Portable Operating System Interface (POSIX ), Base Definitions
and Headers, Section 9, Regular Expressions (FWD.1).</a>
</p>
<p>
<a href="http://www.opengroup.org/onlinepubs/000095399/utilities/grep.html" target="_top">IEEE
Std 1003.1-2001, Portable Operating System Interface (POSIX ), Shells and
Utilities, Section 4, Utilities, grep (FWD.1).</a>
</p>
<p>
<a href="http://www.gnu.org/software/emacs/" target="_top">Emacs Version 21.3.</a>
</p>
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 1998-2013 John Maddock<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
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