public function getTrackNumber()
    {
        if ($this->trackNumber === null) {
            $match = [];
            preg_match(self::$REGEX_TRACK_NUMBER, $this->output, $match);
            $this->trackNumber = (int)(array_key_exists(2, $match) ? $match[2] : 0);
        }

        return $this->trackNumber;
    }