title: Match only the nth occurrence using a regular expression
body: <p>I have a string with 3 dates in it like this:</p>
  
  <pre><code>XXXXX_20160207_20180208_XXXXXXX_20190408T160742_xxxxx
  </code></pre>
  
  <p>I want to select the 2nd date in the string, the <code>20180208</code> one.</p>
  
  <p>Is there away to do this purely in the <code>regex</code>, with have to resort to pulling out the 2 match in code. I'm using <code>C#</code> if that matters.</p>
  
  <p>Thanks for any help.</p>