title: Regular expression to allow range of numbers, or null
body: <p>I have the following Regular Expression, how can I modify it to also allow null?</p>
  
  <p><code>[0-9]{5}|[0-9]{10}</code></p>
  
  <p>I would like it to allow a 5 digit number, a 10 digit number, or null</p>
  
  <p>Thanks</p>