addDashLine = function (val) {
    var comma = '_'
    var num = val.toString().replace(/,/g, '');
    if (num.length != 0 && parseInt(num, 10)) {
        var regex = new RegExp('([0-9]{4})(-?[0-9]+)');
        while (regex.test(num)) {

            num = num.replace(regex, '$1' + comma + '$2');
        }
    }
    return num;
};

alert(addDashLine(123456789))