Te desired by you functionality of RadAutoCompleteBox is currently not provided out of the box. However, you could be able to implement it by your own by creating a custom FilteringBehavior and overriding the FindMatchingItems method. Please, check the following forum post from our community that demonstrates how to achieve a similar to the desired by you functionality - to filter the words with accents as expected: http://www.telerik.com/forums/using-autocompletebox-without-diacritics
Hope this helps.
Regards,
Nasko
Telerik
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