title: Validate ZipCode in Grails Domain?
body: <p>I have a Grails domain containing a zipCode field: </p>
  
  <pre><code>class SomeClass {
  
   String zipCode
  
  }
  </code></pre>
  
  <p>A valid zip code in my def is that it is either blank or contains only of numbers.</p>
  
  <p><strong>How do I validate that the zipCode field is either blank or a valid zip code containing only numbers?</strong></p>