public function parse($key)
    {
        $key = trim($key);

        if ($this->parser->string_helper->startsWith('#', $key)) {
            return false;
        }

        if (!ctype_alnum(str_replace('_', '', $key)) || $this->parser->string_helper->startsWithNumber($key)) {
            throw new ParseException(
                sprintf('Key can only contain alphanumeric and underscores and can not start with a number: %s', $key),
                $key,
                $this->parser->line_num
            );
        }

        return $key;
    }