title: Regular expressions: remove non alpha numerics, with an exception
body: <p>To remove all non-alpha numeric chars, the regex would be </p>
  
  <pre><code>x = regexp_replace(somestring, '[^a-zA-Z0-9]+', '', 'g')
  </code></pre>
  
  <p>But what if I want to leave underscores untouched ?</p>