Basic masking
Input
mask_all_but_last4('1234567890')
Output
'******7890'
Input length 10 -> first 6 characters masked, final 4 kept.
Full lesson preview
Mask all but the last four characters
Replace all characters in a string with a mask character except for the final four characters.
Python practice15 minString PatternsIntermediateLast updated March 18, 2026
Problem statement
Many systems display only the last few characters of sensitive identifiers (e.g., credit cards, account numbers) and mask the rest. Implement a function mask_all_but_last4(s, mask_char='*') that converts the input to a string and replaces every character except the final four with mask_char. If the string has length 4 or less, return it unchanged. The mask_char must be a single-character string.
Task
Write a function that masks all characters of an input (converted to string) except the last four, using a single-character mask.
Examples
Input format
A value s (any type) and optionally a single-character mask_char.
Output format
A string with all but the last four characters replaced by mask_char.
Constraints
Do not use external libraries. Convert non-string inputs to string. mask_char must be a single-character string.
Samples
Sample 1
Input
mask_all_but_last4('abcd')
Output
'abcd'
Strings of length 4 are returned unchanged.