You maybe could make use of the OnChange event and apply a filter to the form. As the user types, the filter is modified for each keystroke. If they type a invalid character combination, the records disappear. As long as the underlying record source is not huge or you're not going over a network as you filter, performance should be acceptable.
Maybe that's not conducive to what you're trying to do anyway. Just thought I'd throw it in here.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.