I’m using the REGEX below to effectively check if a string is a YYYY-MM-DD

Question

Asked: May 26, 20262026-05-26T10:48:57+00:00 2026-05-26T10:48:57+00:00

I’m using the REGEX below to effectively check if a string is a YYYY-MM-DD date.

[0-9]{4}-[0-9]{2}-[0-9]{2}

How do I do the reverse and use a similar REGEX to check a string is NOT a date in this format.

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Editorial Team · Answer 1 · 2026-05-26T10:48:58+00:00

You could use a negative look-ahead to solve this:

 (?![0-9]{4}-[0-9]{2}-[0-9]{2})

Edit: After watching the post by Lucasus I’ve made a new regex to have a more strict validation.

New Regex:

(?!([0-9]{4})-([1-9]|0[1-9]|1[012])-([1-9]|0[1-9]|[12][0-9]|3[01]))

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