Filtering in ajax ComboBox based on StartsWith

Hi..

I am looking for a particular feature using ComboBox and may be jquery/javascript.

The feature is as follows :

When a user types something in ComboBox, the dropdown list should be filtered. 

For example, suppose I have a Country ComboBox with values US,UK,India,Indonesia. Now when a user types 'U' in the combobox, the dropdown displayed should contain only US and UK. The filtering should be based on "StartsWith" and not "contains".

I googled this alot but still I didn't get any useful script to implement this.

Help me out !!!