A string expression, defined by a developer, that governs what a user is allowed to enter in as input in a text box.
An input mask is in general a set of rules, that entered data must conform to, mainly used for the purposes of data integrity by preventing transcription errors. The syntax of this string expression differs from implementation to implementation, but the fundamental input types are all supported.
Some frequent uses of input masks include entry of telephone numbers, ZIP or postal codes, times and dates.