add character inside string using on keyup regex javascript

i create function using event on key up to put “/” inside value from input user using regex but still bug, can someone help me ?

$('#validUntil').on('keyup', function(e){

                    $(this).val(function(index, value) {
                        return value
                        // Add "/" separators:
                        .replace(/B(?=(d{2})+(?!d))/g, "/")
                    });
});

for example if i typing 5121 the result must be 51/21