[рони]

gsdev99,

var s = 'AAAABBBCCXYZDDDDEEEFFFAAAAAABBBBBBBBBBBBBBBBBBBBBBBBBBBBbbb';
// A4B3C2XYZD4E3F3A6B28b3

function rle(s) {

    return s.replace(/(.)\1+/g, function(a, c) {
    return c + a.length
});
}

alert(rle(s))