Sorting files in a list
<p>Let's say I have a list of files</p>

<pre><code>files = ['s1.txt', 'ai1.txt', 's2.txt', 'ai3.txt']
</code></pre>

<p>and I need to sort them into sub-lists based off of their number so that</p>

<pre><code>files = [['s1.txt', 'ai1.txt'], ['s2.txt'], ['ai3.txt']]
</code></pre>

<p>I could write a bunch of loops, however I am wondering if there is a better way to do this?</p>
