Regex to Match ISO 8601 Duration Format

This regex validates ISO 8601 duration format strings. Here is the token-by-token breakdown:

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

P — Matches the literal P (period) character that begins every ISO 8601 duration. This is mandatory and stands for period.

(?!$) — A negative lookahead ensuring the string does not end immediately after P. A bare P is not a valid duration.

(\d+Y)? — Optional capturing group 1 matches the years component: one or more digits followed by Y.

(\d+M)? — Optional capturing group 2 matches the months component: digits followed by M.

(\d+W)? — Optional capturing group 3 matches the weeks component: digits followed by W.

(\d+D)? — Optional capturing group 4 matches the days component: digits followed by D.

(?:T(?!$) — An optional non-capturing group for the time portion, starting with the literal T time designator. The negative lookahead (?!$) ensures T is not the last character.

(\d+H)? — Optional capturing group 5 matches hours: digits followed by H.

(\d+M)? — Optional capturing group 6 matches minutes in the time section: digits followed by M.

(\d+(?:.\d+)?S)? — Optional capturing group 7 matches seconds: digits with an optional decimal fraction followed by S.

)? — Makes the entire time section optional.

$ — Anchors at the end of the string.

ISO 8601 durations are used in APIs, scheduling systems, video metadata, and configuration files. Examples include P1Y (one year), PT1H30M (1 hour 30 minutes), and P2DT12H (2 days 12 hours).