title: symfony2 entity validation regexp a-z A-Z 0-9
body: <p>Is there a built-in way in symfony2 to validate a string (in my case the username and one other property) against the classic a-z, A-Z and 0-9 rule?</p>
  
  <p>Would I have to write that in regexp myself as a custom validator? (if so, hint where to look is appreciated)</p>