"Input masks integrate 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 (video below).
When implementing input masks, it’s a good idea to surface the formatting right up front and not gradually reveal it as people enter information into an input field. Contrast the example above with the one at Dynamic Field Masking (video below)." (Continued via Functioning Form, LukeW) [Usability Resources]