How to use custom masks in MaskedEditExtender?

Question

Hello,

I need to create a textbox which accepts 3 chars and must follow the following format

1. 1st char - only numbers
2. 2nd char - only upper case letters
3. 3rd char - numbers and upper case letters (except zero)

This is what I have so far, but it's not working

<ajaxToolkit:MaskedEditExtender ID="MaskedEditExtender1" runat="server" 
   Mask="9L9" TargetControlID="txtNUIPC3" PromptCharacter="">
</ajaxToolkit:MaskedEditExtender>