title: adding phone number extension to phonenumber regex
body: <p>Here is my current regex:</p>
  
  <p>/\b\d{3}[-.]?\d{3}[-.]?\d{4}?\b/gm</p>
  
  <p>It matches:</p>
  
  <ul>
  <li>444-555-1234</li>
  <li>246.555.8888</li>
  <li>1235554567</li>
  </ul>
  
  <p>What I need now is the <strong>possibility, if available</strong> for:</p>
  
  <ul>
  <li>1234567890x1234</li>
  <li>444-555-1234x1234</li>
  <li>246.555.8888x1234</li>
  </ul>
  
  <p>Any help would be appreciated.</p>
  
  <p>Also, can someone explain the purposes of the <code>\b</code> and how the <code>[-.]</code> becomes optional if exists in string</p>