An additional suggestion for this it to set it up not just so that the data conforms to a certain format, but also to make sure that the value entered into the field meets certain other conditions.
For example, you may be running a site that requires people belong to a certain email domain to join, like .org or .gov. You could set up a mask field on so that not only do you verify that an email is entered, but that the email ends in either .org or .gov.