<p>what's the quickest way to extract a 5 digit number from a string in c#. </p>

<p>I've got </p>

<pre><code>string.Join(null, System.Text.RegularExpressions.Regex.Split(expression, "[^\\d]"));
</code></pre>

<p>Any others?</p>

Do you mean convert a string to a number?  Or find the first 5 digit string and then make it a number?  Either way, you'll probably be using
decimal.Parse
or int.Parse.
I'm of the opinion that Regular Expressions are the wrong approach.  A more efficient approach would simply to walk through the string looking for a digit, and then advancing 4 characters and seeing if they are all digits.  If they are, you've got your substring.  It's not as robust, no, but it doesn't have the overhead either.