title: How to use angular ui-mask to mask a phone number with optional extension?
body: <p>We have the demo <a href="http://angular-ui.github.io/ui-utils/#/mask" rel="noreferrer">here</a> on the angular-ui page.</p>
  
  <p>Wondering how we can write a mask with optional extension number.</p>
  
  <p><code>(999) 999-9999 ext. 999</code> will make the extension required.</p>