How do you match military time? [duplicate]

Question

I'm trying to create a valid Java regex for matching strings representing standard "military time":

String militaryTimeRegex = "^([01]\d|2[0-3]):?([0-5]\d)$";

This gives me a compiler error:

Invalid escape sequence (valid ones are \b \t \n \f \r \" \' \ )

Where am I going wrong?!?

Answer 1

Make sure you use double backslashes for escaping characters:

String militaryTimeRegex = "^([01]\\d|2[0-3]):?([0-5]\\d)$";

Single backslashes indicate the beginning of an escape sequence. You need to use \\ to get the character as it appears in the String.

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To answer your comment, you are currently only matching 19:00. You need to account for the additional :00 at the end of the String in your pattern:

String militaryTimeRegex = "^([01]\\d|2[0-3]):?([0-5]\\d):?([0-5]\\d)$";

Answer 2

In Java, you need to double-escape all the \ characters:

String militaryTimeRegex = "^([01]\\d|2[0-3]):([0-5]\\d):([0-5]\\d)$";

Why? because \ is the escape character for strings, and if you need a literal \ to appear somewhere inside a string, then you have to escape it, too: \\.