title: How to validate phone numbers using regex
body: <p>I'm trying to put together a comprehensive regex to validate phone numbers. Ideally it would handle international formats, but it must handle US formats, including the following:</p>
  
  <ul>
  <li><code>1-234-567-8901</code></li>
  <li><code>1-234-567-8901 x1234</code></li>
  <li><code>1-234-567-8901 ext1234</code></li>
  <li><code>1 (234) 567-8901</code></li>
  <li><code>1.234.567.8901</code></li>
  <li><code>1/234/567/8901</code></li>
  <li><code>12345678901</code></li>
  </ul>
  
  <p>I'll answer with my current attempt, but I'm hoping somebody has something better and/or more elegant.</p>