JS validation on webtextedit

Question

I have a field that I want to accept 4-6 alphanumeric digits.

Infragistics WebMaskEdit has weird overwriting behavior, prepopulates the box with 6 spaces even though I have promptchar="". I don't want a mask, but I do want the client side validation (filtering and length constraints)

Am I missing an easy way or do I just need to write a custom filter for a WebTextEdit?

<igtxt:WebMaskEdit ID="txtSLINNumber" runat="server" InputMask="AAAAaa" 
DisplayMode="BlankWhenEmpty" PadChar="-" PromptChar="" Width="50px" 
TabIndex="2" EmptyPositionChar="?" />

<%--<igtxt:WebTextEdit ID="txtSLINNumber" Width="50px" TabIndex="2" 
runat="server" />--%>