RegexdrillLookahead: thousands separators
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 one whole number. Insert a comma between every group of three digits, counting from the right, so 1234567 becomes 1,234,567. Numbers with fewer than four digits stay unchanged.
What this exercise teaches
\B(?=(\d{3})+(?!\d)) matches the empty position where the digits to its right can be split into groups of three all the way to the end. \B keeps the start of the number out, so 123456 does not get a leading comma. This is a classic interview question; in real code Intl.NumberFormat or toLocaleString does the job.
A hint if you get stuck
Match positions, not characters: a spot inside the number, \B, that is followed by one or more groups of exactly three digits and then no further digit. Replace that empty spot with a comma.
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