RegexdrillLookahead: mask all but the last four digits
A regex exercise from Regexdrill. Solve it with a pattern box with live highlighting in your browser, checked against hidden test strings, and it is checked the moment you submit.
The task
Each line is an account number made only of digits. Replace every digit with * except the last four. Numbers with four digits or fewer stay unchanged.
What this exercise teaches
\d(?=\d{4}) matches a digit only if at least four digits come after it, so the last four never qualify. Because the lookahead takes up no characters, the next match can start at the very next digit.
A hint if you get stuck
Match one digit at a time, but only when four more digits follow it: \d(?=\d{4}). The lookahead checks them without using them up.
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