title: regex matching alpha character followed by 4 alphanumerics
body: <p>I need a regex for the following pattern:</p>
  
  <ul>
  <li><p>Total of 5 characters (alpha and numeric, nothing else).</p></li>
  <li><p>first character must be a letter (<code>A</code>, <code>B</code>, or <code>C</code> only)</p></li>
  <li><p>the remaining 4 characters can be number or letter.</p></li>
  </ul>
  
  <p>Clarifcation: the first letter can only be <code>A</code>, <code>B</code>, or <code>C</code>.</p>
  
  <p>Examples:</p>
  
  <ul>
  <li><code>A1234</code> is valid</li>
  <li><code>D1234</code> is invalid</li>
  </ul>