Regex conditional always returning else

Question

I'm trying to write a regex for military time (0000-2359), but it lets through any hour up to 29. Why doesn't the expression throw the error for 24XX+?

    while(true)
    {
        try
        {       
            sInput = input.nextLine();

            // If the input is a properly formatted time break the loop
            // otherwise throw invalidTimeFormatException
            if(Pattern.matches("[0-2](?:(?=2)[0-3]|[0-9])[0-5][0-9]", sInput))
            {
                // This will only happen if the time is properly formatted
                // thanks to the regular expression above.
                break;
            }

            throw invalidTimeFormatException;
        }
        catch(Exception e)
        {
            System.out.println(e.getMessage());
        }           
    }

ach · Accepted Answer · 2013-09-23 14:56:58Z

7

I know that this was already answered and accepted, but I'd suggest using a more simple and explicit regex instead of using lookbehinds:

([0-1][0-9]|2[0-3])[0-5][0-9]

It's shorter, supported in every regex engine, and (to me at least) much clearer.

edited Sep 23, 2013 at 14:56

answered Sep 23, 2013 at 14:51

ach

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Reinstate Monica -- notmaynard · Accepted Answer · 2013-09-23 14:57:07Z

2

You want a look-behind (?<=2) rather than look-ahead (?=2).

As it is, it's matching "first character 0, 1, or 2; next character, if it's a 2 then 0-3, otherwise 0-9; etc."

Edit: Actually, you'll need a negative look-behind (?<!2) to ensure the previous character is not 2 to match [0-9], and it needs to not be a non-capturing group:

[0-2]((?<=2)[0-3]|(?<!2)[0-9])[0-5][0-9]
                  \____/
                     |
add negative look-behind here

edited Sep 23, 2013 at 14:57

answered Sep 23, 2013 at 14:31

Reinstate Monica -- notmaynard

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4 Comments

user768417 Over a year ago

Doing this still gives the same problem.

Reinstate Monica -- notmaynard Over a year ago

Can't type this morning. Try it now.

user768417 Over a year ago

Works! Thank you. Just one question, why ?<=2 and not ?<2?

Reinstate Monica -- notmaynard Over a year ago

I don't know. That's just the way they did it, I guess. It may have been for a very good reason, or for none at all. In some flavors (e.g. Ruby, and apparently Java 7) ?< by itself symbolizes the start of a named group (and so needs to be differentiated), but that was probably created afterwards.

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