Filter a listbox with textbox

Hi.
So i currently filter the listbox with some javascript i have found.
What it does is going to the item i type on the textbox.
It will not filter the listbox so it will contain the specific matches.
I was reading for autocomplete but i don't seem to find this 2 way usage.
I was also reading this article:
http://stackoverflow.com/questions/1688651/jquery-listbox-textbox-filter
But i'm completely lost on how i can implement it.
Any ideas?
Thanks.
...