title: validate email in zk by applying constraint
body: <p>I have applied following constraint for email</p>
  
  <pre><code>&lt;textbox width="400px"
  constraint="/^[A-Za-z0-9_\-\.]+@[A-Za-z0-9_\-\.]{2,}\.[A-Za-z0-9]{2,}(\.[A-Za-z0-9])?/: Please enter an e-mail address in proper format" &gt;
  </code></pre>
  
  <p>In ZK, But this is showing error even if textbox is blank and u click on text box. But i want to validate only when user enters some value in textbox.</p>
  
  <p>Can you please suggest, how can i modify this constraint for my requirement.</p>
  
  <p>Thanks</p>