Why can't I validate input on a masked variable type?

I am having an issue where I can't validate data in a masked field and am wondering if anyone knows why, and/or has a solution?

What I'm trying to do is validate a TIN by using an onChange client script. I've validated the script works if the field is a single line text, but when I try it on a masked variable, it immediately removes any input and shows the field message.

Has anyone accomplished this?

function onChange(control, oldValue, newValue, isLoading) {
    if (isLoading || newValue == '') {
        return;
    }

   //Allow only numbers or XX-XXXXXXX on the TIN field   
	var regexp = /^\d{2}-?\d{7}$/;
    if (!regexp.test(newValue)) {        
        g_form.setValue('tin_masked', '');
        var myElement = g_form.getControl('tin_masked');
        g_form.flash("label_" + myElement.id, "#FFFACD", 0);
        g_form.showFieldMsg('tin_masked', "Please enter TIN in the format ##-#######", 'error');
		return false;
    }
}