const markerRegexp = /\((\d+)x(\d+)\)/;

const decompress = (input, deep) => {
    let len = 0;
    let marker = null;
    do {
        marker = markerRegexp.exec(input);
        if (marker === null) {
            len += input.length;
            break;
        }

        let pos = marker.index;
        if (pos > 0) {
            len += pos;
        }

        let chars = parseInt(marker[1], 10),
            times = parseInt(marker[2], 10);
        if (!deep) {
            len += chars * times;
        } else {
            let str = input.substr(pos + marker[0].length, chars);
            len += decompress(str, true) * times;
        }
        input = input.substring(pos + marker[0].length + chars);
        marker = null;
    } while(true);
    return len;
}

const input = require('fs').readFileSync('d9.txt', 'utf8'); // no \n at EOF
//const input = `(25x3)(3x3)ABC(2x3)XY(5x2)PQRSTX(18x9)(3x2)TWO(5x7)SEVEN`;
console.log(`result: \n${decompress(input, true)}`);