Only accepting upper-case and integers in a MaskedEditExtender

Question

I can't figure out why this isn't working:

<td><asp:TextBox ID="TextBox_Tracking_BenOpt" runat="server" AutoPostBack="True" 
         Height="16px" Width="100px"></asp:TextBox>
<AJAXControls:MaskedEditExtender ID="MeeBenOpt" runat="server" Mask="AAAAA" 
              MaskType="None" TargetControlID="TextBox_Tracking_BenOpt" 
              PromptCharacter="_" InputDirection="LeftToRight" 
              Filtered="ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890" />
<asp:RegularExpressionValidator Display="Dynamic" 
              ControlToValidate="TextBox_Tracking_BenOpt" ID="RevBenOpt" 
              ValidationExpression="^[0-9A-Z]{5,}$" runat="server" ErrorMessage="X" 
              ForeColor="Red" Font-Size="Large"></asp:RegularExpressionValidator></td> 

It works in that it only allows 5 characters and they're alphanumeric, but I only want to allow numbers and upper-case letters. How do I get rid of the lower-case letters?