Checking variable length time with JavaScript Regular Expression

My specific requirement is to validate a string ranging from 1 to 4 characters in length (where H stands for hour and M represents minutes):

If the string has 1 character, it should be in the format of H (a digit between 0-9).

A 2-character string should look like HH (the first H can be between 0-2 and the second one between 0-9; however, if the first H is 2, the second one can only be 0-4).

For a 3-character string, the format should be HMM (first H: between 0-9, first M: between 0-5, second M: between 0-9).

A 4-character string should have the format HHMM (first H: between 0-2, second H: between 0-9; but when the first H is 2, the second H can only be 0-4. Additionally, first M: between 0-5, and second M: between 0-9).

The challenge here lies in not knowing the length of the string beforehand and the same number signifying different things depending on its length. Any suggestions on using JavaScript Regular Expressions to handle this validation?

javascriptregex

Answer №1

If you're looking to validate 24-hour time formats using a regular expression, consider the following pattern:

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

This breakdown helps you understand each segment of the regex:

^                   marks the beginning of the string
2[0-3]              matches 20-23 as valid hours
|                   or
[01]?\d             covers hours 00-19
([0-5]\d)?          includes minutes from 00-59, optional
$                   signifies the end of the string

Note: If you prefer to stick with the more common 0-23 hour range, adjust the regex accordingly.

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

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