Regex and callback function when replace

input = input.replace(/\\\//g, '/');
var output = input.replace(/\\u([0-9a-f]{4})/gi, function (m, key, value) {
    //alert(key + '-' + value);
    var hex = m.substring(2);
    var code = parseInt(hex, 16);
    return String.fromCharCode(code);
});