Reproduce code:

$username = new Zend_Form_Element_Text('username');
$username->addValidator('stringLength', true, array(5, 20))
         ->addValidator('regex', true, array('/^[a-zA-Z0-9_]*$/'))
         ->addFilter('StringToLower')
         ->setRequired(true);
$form = new Zend_Form();
$form->addElement($username);

$form->isValid(array('username' => '#'));
echo var_export( $form->getMessages(), 1), "\n\n";

Expected results:

array (
  'username' =>
  array (
    'stringLengthTooShort' => '\'#\' is less than 5 characters long',
  ),
)

Actual result:

array (
  'username' => 
  array (
    'stringLengthTooShort' => '\'#\' is less than 5 characters long',
    'regexNotMatch' => '\'#\' does not match against pattern \'/^[a-zA-Z0-9_]*$/\'',
  ),
)