Regex to Match ISO Week Number Dates

This regex validates ISO 8601 week date strings, which represent dates by year, week number, and optionally day of the week. Here is the token-by-token breakdown:

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

\d{4} — Matches exactly four digits for the ISO year. Note that the ISO week-numbering year may differ from the calendar year for dates near the year boundary.

-W — Matches the literal -W separator that identifies this as a week date format. The W prefix distinguishes week dates from ordinal dates and calendar dates.

(?:0[1-9]|[1-4][0-9]|5[0-3]) — A non-capturing group matching the week number from 01 to 53. 0[1-9] matches weeks 01-09, [1-4][0-9] matches weeks 10-49, and 5[0-3] matches weeks 50-53. Most years have 52 weeks, but some ISO years have 53 weeks.

(?:-[1-7])? — An optional non-capturing group for the day of the week. When present, it consists of a hyphen followed by a digit from 1 (Monday) to 7 (Sunday). ISO 8601 defines Monday as the first day of the week.

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

No flags are used since this validates a single week date string.

ISO week dates are used in business and industrial planning, particularly in European countries. They are common in manufacturing schedules, fiscal reporting, and project management. Examples include: 2024-W01 (first week of 2024), 2024-W52-5 (Friday of week 52), and 2023-W53 (the rare 53rd week).

This pattern is useful for date validation in scheduling systems, timesheet applications, and data import tools that handle ISO week dates.