Question

This is my requirement for validating a string that could be between 1 and 4 characters long (H stands for hour and M stand for minutes):

1 character = H (digit between 0-9)

2 characters = HH (first H is digit between 0-2, second is digit between 0-9, but if the first one is 2 the second one can only be 0-4)

3 characters = HMM (first H is digit between 0-9, first M is digit between 0-5, second M is digit between 0-9)

4 characters = HHMM (first H is digit between 0-2, second is digit between 0-9, but if the first one is 2 the second one can only be 0-4, first M is digit between 0-5, second M is digit between 0-9)

What makes this challenging is that I don’t know the length of the string in advance and the same number could mean different things depending on how long the string is. Any ideas about a JavaScript RegularExpression to validate this?

Answer 1

You can use a regular expression like this:

^(2[0-4]|[01]?\d)([0-5]\d)?$

Breakdown:

^                   beginning of string
2[0-4]              digit 2 followed by digit 0-4
|                   or
[01]?\d             digit 0-1, optional, followed by any digit
([0-5]\d)?          digit 0-5 followed by any digit, as whole optional
$                   end of string

Note: Perhaps you mean to have hours 0-23 instead of 0-24:

^(2[0-3]|[01]?\d)([0-5]\d)?$