Luke Wroblewski pointed to Josh Bush’s Masked Input Plugin for JQuery. Luke writes that the plugin “integrate(s) formatting cues directly into an input field to help signal how to correctly answer a question. These formatting instructions are automatically included in an input and cannot be removed by a user. More importantly, a user cannot enter other non-valid characters into the input field. Check out the demo at Masked Input Plugin.”

The video Luke posted to YouTube, and the demo on Bush’s site, show how the input mask works. Great example of an elegantly conceived method for error prevention in forms.