Mask Credit Card

Instructions

Create a function maskify to mask digits of a credit card number with #.

Requirements:

• Do not mask the first digit and the last four digits
• Do not mask non-digit chars
• Do not mask if the input is less than 6
• Return '' when input is empty

Examples:

• 1234-5678-9012 converts to 1###-####-9012
• 123456789012 converts to 1#######9012