Selection api why you so verbose? :(

gistfile1.js

    function f_rel(el, start, end) {
        // end may be < 0 and is relative to start, start must be >=0
        f_abs(el, start, start + end);
    }
    function f_abs(el, start, end){
        // end may be 0 < end < start, start must be >=0

        var s = getSelection();
        s.removeAllRanges();

        var r = new Range();
        r.setStart(el, 0);
        s.addRange(r);

        if (typeof start === 'number' && start >= 0) {
            if (typeof end === 'number') {
                var d = end > start ? 'forward' : 'backward';
                end = Math.abs(Math.abs(end) - start); // -5 - 10 = -15, 5 - 10 = -5
            } else {
                end = 0;
            }
        } else {
            start = 0;
        }

        // modify api is quite tedious...
        while (start--) {
            s.modify('move', 'forward', 'character');
        }
        while (end--) {
            s.modify('extend', d, 'character');
        }
    }