Usually when I enter and save my credit card number in online shop or mobile app, only last four numbers are shown as a reminder. Example (not actual digits):
Your card: **** **** **** 1234
Recently I used an app, where card number was shown as below (not actual digits):
Your card: 1234 56** **** 1234
Are there any good practices or something enforced by Visa/MC or law regulating how to mask card number properly?