title: adding a Company tax id filter using gform_field_validation on gravity forms
body: <p>I want to add a filter to make sure people signing up add a valid Company Tax id number in Europe. All EU tax numbers go e.g. DE12345678, so that the first 2 characters are letters and the rest are numbers. 
  Q: How do i add an "if" statement for each character?
  <code>add_filter('gform_field_validation_1_44', 'tax-id-check', 10 ,4);
  function tax-id-check( $result, $value, $form, $field ) { if ....
  </code></p>