protected function placeholder(&$placeholder)
    {
        if ($this->match(
            $this->utf8
                ? '([\pL\w\-_]+|#[{][$][\pL\w\-_]+[}])'
                : '([\w\-_]+|#[{][$][\w\-_]+[}])',
            $m
        )) {
            $placeholder = $m[1];

            return true;
        }

        return false;
    }