View source for Cookbook/Pattern matching

== Introduction ==
Regular expressions are useful in some situations where the Data.List
library is unwieldy. Posix style regular expressions are available in
the core libraries, and a suite of other regular expression libraries
are [also available], including PCRE and TRE-style regexes.

Bryan O'Sullivan has written [http://www.serpentine.com/blog/2007/02/27/a-haskell-regular-expression-tutorial/ a nice introduction] to using the new regex libraries. This page describes the <tt>(=~)</tt> interface.

== Text.Regex interface ==

Using the Text.Regex module, expressions can be applied with <tt>matchRegex</tt> and <tt>mkRegex</tt> with <tt>Maybe [String]</tt> results. Documentation for this module can be found on the [http://hackage.haskell.org/package/regex-compat Hackage regex-compat page].

<blockquote><pre>$ ghci
Prelude&gt; :m +Text.Regex</pre></blockquote>

For boolean-style (either it succeeded or failed) results:

<blockquote><pre>Prelude Text.Regex&gt; matchRegex (mkRegex "a") "aabc"
Just []

Prelude Text.Regex&gt; matchRegex (mkRegex "z") "aabc"
Nothing</pre></blockquote>

With capturing results in the <tt>[String]</tt>:

<blockquote><pre>Prelude Text.Regex&gt; matchRegex (mkRegex "(a)") "aabc"
Just ["a"]

Prelude Text.Regex&gt; matchRegex (mkRegex "(a+)") "aabc"
Just ["aa"]

Prelude Text.Regex&gt; matchRegex (mkRegex "(a)(b)") "aabc"
Just ["a","b"]

Prelude Text.Regex&gt; matchRegex (mkRegex "(z)") "aabc"
Nothing</pre></blockquote>