Is there a correct way to mask a 15 digit credit card number?
Solution 1:
There is no standard really. The vast majority of sites display the last four with four X's in front them whether the credit card number is 15 or 16 digits so that would probably be the most recognizable to your users. But if you really want to format it so you show the actual structure of the card number then do it as xxxx-xxxxxx-x0000 despite the goofiness. Users may find showing an extra character (xxxx-xxxxxx-00000) is "unsafe" and I'm sure you don't want to give anyone an opportunity to complain or do a chargeback.