title: Why does this backreference not work inside a lookbehind?
body: <p>Matching a repeated character in regex is simple with a backreference:</p>
  
  <pre><code>(.)\1
  </code></pre>
  
  <p><a href="http://regexstorm.net/tester?p=(.)%5c1&amp;i=aabccdefghhij" rel="noreferrer">Test it here.</a></p>
  
  <p>However, I would like to match the character <em>after</em> the pair of characters, so I thought I could simply put this in a lookbehind:</p>
  
  <pre><code>(?&lt;=(.)\1).
  </code></pre>
  
  <p><a href="http://regexstorm.net/tester?p=(%3f%3c%3d(.)%5c1).&amp;i=aabccdefghhij" rel="noreferrer">Unfortunately, this doesn't match anything.</a></p>
  
  <p>Why is that? In other flavours I wouldn't be surprised because there are strong restrictions on lookbehinds, but .NET usually supports arbitrarily complicated patterns inside lookbehinds.</p>