Check if a String matches specific regular expression

Question

I am not so good with regular expressions and stuff, so I need help. I have to check if a input value matches a specific regular expression format. Here is the format I want to use, 25D8H15M. Here the D means the # of days H means hours and M means minutes. I need the regular expression to check the String. Thanks

Answer 1

Here's the briefest way to code the regex:

if (str.matches("(?!$)(\\d+D)?(\\d\\d?H)?(\\d\\d?M)?"))
    // format is correct

This allows each part to be optional, but the negative look ahead for end-of-input at the start means there must be something there.

Note how with java you don't have to code the start (^) and end ($) of input, because String.matches() must match the whole string, so start and end are implied.

However, this is just a rudimentary regex, because 99D99H99M will pass. The regex for a valid format would be:

if (str.matches("(?!$)(\\d+D)?([0-5]?\\dH)?([0-5]?\\dM)?"))
    // format is correct

This restricts the hours and minutes to 0-59, allowing an optional leading zero for values in the range 0-9.