Regex to Match Numbers with Thousands Separators

This regex validates numbers formatted with comma-based thousands separators, a common convention in English-speaking countries. Here is the token-by-token breakdown:

^ — Anchors the match at the start of the string.

-? — Optionally matches a minus sign for negative numbers.

\d{1,3} — Matches one to three digits at the beginning of the number. This is the leading group before the first comma. Numbers can start with 1 digit (like 1,000), 2 digits (like 12,000), or 3 digits (like 123,000).

(,\d{3})* — Capturing group that matches zero or more comma-separated groups of exactly three digits. Each group consists of a comma followed by exactly three digits. This enforces correct placement of thousands separators: every three digits from right to left.

(.\d+)? — An optional capturing group for the decimal portion. It matches a literal dot followed by one or more digits. The decimal portion does not use thousands separators.

$ — Anchors the match at the end of the string.

No flags are used since this validates a single number string.

Thousands separators make large numbers more readable. For example, 1000000 becomes 1,000,000 and 1234567.89 becomes 1,234,567.89. This format is standard in the United States, United Kingdom, and many other countries.

This pattern is useful for validating user input in financial forms, currency fields, data import tools, and spreadsheet applications. For European-style formatting where dots are separators and commas are decimal marks, the characters would need to be swapped.