Infopath 2013 edit for date probably needs a regular expression

In an InfoPath 2013 form that uses a custom list, I want to be able to create an edit for a date. This would occur when the user does not use the date picker but uses enters a value. If the user enters a value that is not a valid date, InfoPath will show there is a problem.

However I need to generate a custom error message saying the date is an invalid date format. I know that I can use some kind of a regular expression, but I do not know what regular expression to use. I know the internal storage for the date is different than what is displayed by InfoPath.

Thus can you show me what custom edit pattern I can use to edit a date field?