To set a format validation regex to ensure that users enter a date in the correct format, you can use the regex below. This ensures the format MM/DD/YYYY and also allows for M/D/YYYY
REGEX(((0?[1-9]|1[0-2])\/(0?[1-9]|1[0-9]|2[0-9]|3[01])\/\d{4}))
The regex below allows for the non-US format DD/MM/YYYY and also allows for D/M/YYYY
REGEX(((0?[1-9]|1[0-9]|2[0-9]|3[01])\/(0?[1-9]|1[0-2])\/\d{4}))
Link to this article http://umango.com/KB?article=93