I was building a script to find credit card numbers in files. For PCI compliance reasons.

I wanted to see and or save the credit card numbers in a file but only parts of them, so I still would be compliant.

So I created this small mask_number function to, well... mask found credit card numbers.

To mask a number with * without masking the first 4 and the last 4 numbers in Python using re.sub(), you can use the following regular expression:

import re

def mask_number(number):
    return re.sub(r'(?<=\d{4})\d(?=\d{4})', '*', number)

The regular expression works as follows:

• (?<=\d{4}): Positive look behind to match any digit that has 4 digits behind it

• \d: Match any single digit

• (?=\d{4}): Positive lookahead to match any digit that has 4 digits ahead of it

The re.sub() function will replace all the single digits that match the regular expression with the * character.

Example usage:

>>> mask_number('1234567890123456')
'1234********3456'

>>> mask_number('1234-5678-9012-3456')
'1234-****-****-3456'

In the above example, the function mask_number() takes in a number as a string, and returns a masked string with the first 4 and last 4 digits unmasked and the rest of the digits masked with *.