Validation on #rules too eager

Assume the following element:

$form['field'] = array(
    '#type' => 'textfield',
    '#title' => 'Foobar',
    '#rules' => array('alpha'), // or 'digit'
  );

The validation itself is working fine, but when you delete the value after the validation mechanism has been activated, you'll get a validation error if you leave the field empty.
If you deactivate clientside_validation an empty field is accepted by fapi_validation.

I assume this is caused by wrong regexp.