protected function _getRule($rule)
    {
        if ($rule instanceof Zend_Filter_Interface) {
            return $rule;
        }

        $rule = (string) $rule;

        $className  = $this->getPluginLoader()->load($rule);
        $ruleObject = new $className();
        if (!$ruleObject instanceof Zend_Filter_Interface) {

            throw new Zend_Filter_Exception('No class named ' . $rule . ' implementing Zend_Filter_Interface could be found');
        }

        return $ruleObject;
    }