Error: Error: Operation "create" on target "cs513" of type "null" failed:
No Handler for type rwt.clientscripting.Listener
Properties: 
code = var handleEvent = function( event ) {

  var regexp = /^[0-9]*$/;
  var text = event.widget.getText();
  if( text.match( regexp ) === null ) {
    event.widget.setBackground( [ 255, 255, 128 ] );
    event.widget.setToolTip( "Only digits allowed!" );
  } else {
    event.widget.setBackground( null );
    event.widget.setToolTip( null );
  }

};