title: RegEx: HTML whitelist
body: <p>Being weak on regular expressions, I've been working with them to improve.  One concept I've been trying to do is to remove all HTML elements except for a list of allowed ones.</p>
  
  <p>I've managed to do the reverse -- remove a specified list of elements:</p>
  
  <pre><code>&lt;\/?(strong|em|a)[^&gt;]*&gt;
  </code></pre>
  
  <p>However I want the opposite, and remove every element but.</p>