Question

I have a regex to validate a string containing ‘HHMM’ or ‘HHMM,HHMM’
This is what I am using:

^[0-2][0-9][0-5][0-9],[0-2][0-9][0-5][0-9]$

This sort of works in that 1247 is ok 3247 is not.
Similarly 1247,2200 is ok but 1247,2290 is not.

However, to allow for 24 hour clock values such as 2500 cannot be allowed.
Is there a way to put a check in regex that if first part of hour is 2 that second part can only be 0-4?

Answer 1

Yes, use | to separate the 0x and 1x cases from 2y:

^(?:[01][0-9]|2[0-4])[0-5][0-9],(?:[01][0-9]|2[0-4])[0-5][0-9]$

The (?:...) creates a non-capturing group, limiting the scope of the | operator. If you did want to capture the HH value (e.g. to use it in a replacement later or something), use (...) instead.

Update: Allowing 2400 but avoiding all other invalid times like 2411 can be done with another |:

(?:(?:[01][0-9]|2[0-3])[0-5][0-9]|2400)

However I would argue that it’s better to just disallow any 24MM (including 2400) at all since it really should be written 00MM. In this case the regex just becomes:

^(?:[01][0-9]|2[0-3])[0-5][0-9],(?:[01][0-9]|2[0-3])[0-5][0-9]$