Regex Tester & Validator

This regex matches zero-width positions within a sequence of digits where a thousands separator (comma) should be inserted. It uses a lookbehind to ensure the position follows a digit, and a lookahead to verify that the upcoming digits form one or more groups of exactly three digits followed by a non-digit boundary. It is commonly used for formatting large numbers with commas (e.g., 1234567 -> 1,234,567). Note that some older JavaScript engines have limited lookbehind support, which may cause issues on certain devices.