title: Regex for fixed length string, starting with multiple words
body: <p>I'm trying to make a regex (JS flavor) that matches a string that is exactly 17 alphanumeric characters in length and must start with either "AB, "DE" or "GH". After these 3 possibilities, any alphanumeric character is accepted.</p>
  
  <p>Match:</p>
  
  <pre><code>AB163829F13246915
  DET639601BA167860
  GHF1973771A002957
  </code></pre>
  
  <p>Don't match</p>
  
  <pre><code>XYZ63829F13246915
  AAA639601BA167860
  BBC1973771A002957
  </code></pre>
  
  <p>So far I have this regex which I'm testing on <a href="http://regexpal.com/">http://regexpal.com/</a></p>
  
  <pre><code>^(AB|)[a-zA-Z0-9]{17}$
  </code></pre>
  
  <p>Not sure why the pipe character is required for it to match my first example, or why it fails when I add "DE" after the pipe.</p>
  
  <p>Anyone?</p>