public static function singular($string)
    {
        $result = $string;
        foreach (self::$_singular as $rule => $replacement) {
            $rule = self::_normalize($rule);
            if (preg_match($rule, $string)) {
                $result = preg_replace($rule, $replacement, $string);
                break;
            }
        }
        return $result;
    }