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<title>8. Regular expressions</title>
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<h2 id="CHAPregexp" class="">Chapter 8 - Regular expressions</h2>
<p>
Exim supports the use of regular expressions in many of its options. It
uses the PCRE regular expression library; this provides regular expression
matching that is compatible with Perl 5. The syntax and semantics of
regular expressions is discussed in many Perl reference books, and also in
Jeffrey Friedl’s <span class="docbook_emphasis">Mastering Regular Expressions</span>, which is published by
O’Reilly (see <span class="docbook_emphasis"><a href="http://www.oreilly.com/catalog/regex2/">http://www.oreilly.com/catalog/regex2/</a></span>).
</p>
<p>
The documentation for the syntax and semantics of the regular expressions that
are supported by PCRE is included in the PCRE distribution, and no further
description is included here. The PCRE functions are called from Exim using
the default option settings (that is, with no PCRE options set), except that
the PCRE_CASELESS option is set when the matching is required to be
case-insensitive.
</p>
<p>
In most cases, when a regular expression is required in an Exim configuration,
it has to start with a circumflex, in order to distinguish it from plain text
or an “ends with” wildcard. In this example of a configuration setting, the
second item in the colon-separated list is a regular expression.
</p>
<div class="docbook_literallayout"><pre>
domains = a.b.c : ^\\d{3} : *.y.z : ...
</pre></div>
<p>
The doubling of the backslash is required because of string expansion that
precedes interpretation – see section <a href="ch11.html#SECTlittext" title="11. String expansions">11.1</a> for more discussion
of this issue, and a way of avoiding the need for doubling backslashes. The
regular expression that is eventually used in this example contains just one
backslash. The circumflex is included in the regular expression, and has the
normal effect of “anchoring” it to the start of the string that is being
matched.
</p>
<p>
There are, however, two cases where a circumflex is not required for the
recognition of a regular expression: these are the <span class="docbook_option">match</span> condition in a
string expansion, and the <span class="docbook_option">matches</span> condition in an Exim filter file. In
these cases, the relevant string is always treated as a regular expression; if
it does not start with a circumflex, the expression is not anchored, and can
match anywhere in the subject string.
</p>
<p>
In all cases, if you want a regular expression to match at the end of a string,
you must code the $ metacharacter to indicate this. For example:
</p>
<div class="docbook_literallayout"><pre>
domains = ^\\d{3}\\.example
</pre></div>
<p>
matches the domain <span class="docbook_emphasis">123.example</span>, but it also matches <span class="docbook_emphasis">123.example.com</span>.
You need to use:
</p>
<div class="docbook_literallayout"><pre>
domains = ^\\d{3}\\.example\$
</pre></div>
<p>
if you want <span class="docbook_emphasis">example</span> to be the top-level domain. The backslash before the
$ is needed because string expansion also interprets dollar characters.
</p>
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