Question about autocomplete and xml-source

Question about autocomplete and xml-source

I use the example from the ui-page, which reads data out of an xml-source and then provides it to the autocomplete-feature. Now my problem is, that, if I enter my search term "berl", autocomplete returns every entry, containing "berl" in it. Instead of this, I want only to get the entries out of the xml-data, beginning with "berl" - Does anyone how I can realize this?