public function rule($str_to_change, $str_new)
    {
        if (!is_scalar($str_to_change) || !is_scalar($str_new)) {
            throw new \InvalidArgumentException('New rule to replace character must be two valid string or scalar value.');
        }

        if (strlen($str_new) > 0) {
            if (preg_match('/[^a-z0-9-]/', (string) $str_new)) {
                throw new \InvalidArgumentException('Replacement string contains not allowed characters!');
            }
        }

        $this->arr_rules[(string) $str_to_change] = (string) $str_new;

        return $this;
    }