public function setFields() : void
    {
        preg_match_all('#' . self::EXP_TOKEN . '(.*?)' . self::EXP_TOKEN . '#', $this->getExpression(), $matches);
        if (empty($matches[1])) {
            throw new InvalidArgumentException("No tokens found in expression");
        }
        $this->fields = $matches[1];
    }