java.util.regex.Matcher confused group

Question

I'm having trouble getting the right group of a regex match. My code boils down to following:

Pattern fileNamePattern = Pattern.compile("\\w+_\\w+_\\w+_(\\w+)_(\\d*_\\d*)\\.xml");
Matcher fileNameMatcher = fileNamePattern.matcher("test_test_test_test_20110101_0000.xml");

System.out.println(fileNameMatcher.groupCount());

if (fileNameMatcher.matches()) {
    for (int i = 0; i < fileNameMatcher.groupCount(); ++i) {
        System.out.println(fileNameMatcher.group(i));
    }
}

I expect the output to be:

2
test
20110101_0000

However its:

2
test_test_test_test_20110101_0000.xml
test

Does anyone have an explanation?

Frank Schmitt · Accepted Answer · 2011-04-19 13:04:05Z

6

Group(0) is the whole match, and group(1), group(2), ... are the sub-groups matched by the regular expression.
Why do you expect "test" to be contained in your groups? You didn't define a group to match test (your regex contains only the group \d*_\d*).

answered Apr 19, 2011 at 13:04

Frank Schmitt

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2 Comments

Will Over a year ago

Thank you very much! Was not aware of that default behavior.

Will Over a year ago

And yes I missed a pair of parenthesis in my code, just edited it for correctness.

axtavt · Accepted Answer · 2011-04-19 13:01:29Z

2

Group 0 is the whole match. Real groups start with 1, i.e. you need this:

System.out.println(fileNameMatcher.group(i + 1));

answered Apr 19, 2011 at 13:01

axtavt

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1 Comment

Will Over a year ago

Thank you very much! Was not aware of that default behavior.

NPE · Accepted Answer · 2011-04-19 13:02:31Z

2

group(0) should be the entire match ("test_test_test_test_20110101_0000.xml");
group(1) should be the sole capture group in your regex ("20110101_0000").

This is what I am getting. I am puzzled as to why you'd be getting a different value for group(1).

answered Apr 19, 2011 at 13:02

NPE

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1 Comment

Will Over a year ago

Thanks I got it now :) Sorry for the confusion I forgot some parenthesis in my code, see my edit.

RobbySherwood · Accepted Answer · 2014-02-24 20:23:33Z

2

actually your for loop should INCLUDE groupCount() using "<=" :

for (int i = 0; i <= fileNameMatcher.groupCount(); ++i) {
    System.out.println(fileNameMatcher.group(i));
}

thus your output then will be:

2
test_test_test_test_20110101_0000.xml
test
20110101_0000

the groupCount() will not count group 0 matching the whole string.

first group will be "test" as matched by (\w+) and

second group will be "20110101_0000" as matched by (\d*_\d*)

answered Feb 24, 2014 at 20:23

RobbySherwood

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