[Alexandroppolus]

var str = 'AAAABBBCCXYZDDDDEEEFFFAAAAAABBBBBBBBBBBBBBBBBBBBBBBBBBBBbbb';

function rle(s) {
  return s.replace(/(.)\1*/g, function (m, c) { return c + (m.length > 1 ? m.length : ''); });
}

alert(rle(str))