Hi,
I tired latest verion Radcontrol Q2 2012 and found out the RadMaskedTextBox doesn't work with RegularExpressionValidator on IE 8 when you put wrong format input and
tab out the RadMaskedTextBox. TheRegularExpressionValidator doesn't show the error message even if the input doesn't match with Validation Expression. But works on the Firefox 12.
Note: I mean tab out not click button.
Following is my testing code:
I also tested on your demo site and same behavior:
http://demos.telerik.com/aspnet-ajax/input/examples/common/validation/defaultcs.aspx
Alan