Regex Editor & Tester

Matches zero-width positions inside digit strings where a thousands separator should be inserted. The \B avoids the start of a number, while the lookahead (?=(\d{3})+(?!\d)) asserts that all remaining digits form complete groups of three with no partial group after them. Used with a replace (e.g., inserting a comma), it formats numbers like 1234567 as 1,234,567. Commonly used in text editors, code cleanup, and data formatting tasks with global multiline flags.