title: Replace Placeholder HTML comment with HTML element
body: <p>I'm developing an iOS app that needs to display HTML content inside a HTML powered textview (DTCoreText).
  For whatever reaso nthe client has decided to provide videos inside special HTML comments that I'm supposed to turn into a  tag.</p>
  
  <p>The comment format is as such</p>
  
  <p><code>&lt;!-- placeholder_video:url_of_video.mp4 --&gt;</code></p>
  
  <p>I was hoping I could write a regex to match the entire comment, extract the content and replace it with a  element pointing to the correct URL through NSRegularExpression's stringByReplacingMatchesInString:options:range:withTemplate: but I can't for the life of me figure out Regular Expressions.</p>
  
  <p>The best I could come up with is</p>
  
  <pre><code>(?&lt;=&lt;!-- placeholder_video:)(.*)(?=--&gt;)
  </code></pre>
  
  <p>Which matches the comment's content (the mp4 URL), but I need it to match the entire comment instead and extract the content as a sub pattern (that I would later access through \1 if I understand correctly) so I can use a replace pattern to quickly replace the comment with the proper <code>&lt;video src="url_of_video.mp4"&gt;</code> string </p>
  
  <p>Can it be done? Or am I better off trying to do it in two passes instead? (match the entire comment then run another regex on that comment to extract the URL and replace the former?</p>