Question

I am using regex to figure out what format the input date is. This is one of the patterns i am using

    ^((18[5-9]|19[0-9]|20[0-9])\\d)(0?[1-9]|1[012])(0?[1-9]|[12][0-9]|3[01])$

so the constraint is to have the year between 1850 and 2099. If I pass for instance this string as date 20011212 when I am extracting the year, month and day from it, this is what I get: year: 2001, month: 200, day :12. Any Idea why?

    pattern = Pattern.compile(PATTERN);
    matcher = pattern.matcher(dateString);
    if (matcher.matches()){
       matcher.reset();
       if (matcher.find()){
          Integer.parseInt(matcher.group(1));
          Integer.parseInt(matcher.group(2));
          Integer.parseInt(matcher.group(3));
       }
    }

The code is simplified, but even on this simplified version, it returns erroneous results. Thank you for any suggestions/solutions.

Answer 1

In a regex everything you put inside (...) is a capturing group. You have two groups in the year, those are both capturing groups:

group(1) = ((18[5-9]|19[0-9]|20[0-9])\\d)
group(2) = (18[5-9]|19[0-9]|20[0-9])
group(3) = (0?[1-9]|1[012])
group(4) = (0?[1-9]|[12][0-9]|3[01])

You can also use non capturing blocks like this: (?:...)

So your pattern should be:

^((?:18[5-9]|19[0-9]|20[0-9])\\d)(0?[1-9]|1[012])(0?[1-9]|[12][0-9]|3[01])$