title: Regular expression to match letters, numbers, and certain symbols
body: <p>I need to validate a username in PHP. It can be:</p>
  
  <ul>
  <li>Letters (upper and lower case)</li>
  <li>Numbers</li>
  <li>Any of these symbols :.,?!@</li>
  <li>Up to 15 characters OR 16 if the last character is one of the following #$^ (it can also be 15 or less with one of these 3 characters at the end only)</li>
  </ul>
  
  <p>How do I do this?</p>