Regular Expressions Cookbook, 2nd Edition

4.6. Validate Traditional Time Formats

Problem

You want to validate times in various traditional time formats, such as hh:mm and hh:mm:ss in both 12-hour and 24-hour formats.

Solution

Hours and minutes, 12-hour clock:

^(1[0-2]|0?[1-9]):([0-5]?[0-9])(●?[AP]M)?$

Regex options: None

Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Hours and minutes, 24-hour clock:

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

Regex options: None

Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Hours, minutes, and seconds, 12-hour clock:

^(1[0-2]|0?[1-9]):([0-5]?[0-9]):([0-5]?[0-9])(●?[AP]M)?$

Regex options: None

Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Hours, minutes, and seconds, 24-hour clock:

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

Regex options: None

Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

The question marks in all of the preceding regular expressions make leading zeros optional. Remove the question marks to make leading zeros mandatory.

Discussion

Validating times is considerably easier than validating dates. Every hour has 60 minutes, and every minute has 60 seconds. This means we don’t need any complicated alternations in the regex. For the minutes and seconds, we don’t use alternation at all. ‹[0-5]?[0-9]› matches a digit between 0 and 5, followed by a digit between 0 and 9. This correctly matches any number between 0 and 59. The question mark after the first character class makes it optional. This way, a single digit between 0 and 9 is ...