Java正则表达式捕获数字组

Question

I'm having a heck of a time getting this regular expression to output the groups I expect:

Pattern p = Pattern.compile("(point) (.*)");
Matcher m = p.matcher("point 0 0 255 0 0");

if (m.find()) {
    for (int i = 0; i < m.groupCount(); i++) {
        System.out.println("-- " + m.group(i));
    }
}

I am hoping to see this output:

point 0 0 255 0 0   //  this is group 0
point
0 0 255 0 0

Instead, I get group 0 and 1 - nothing from the last group.

I don't know what I'm doing wrong.

In the long run, I'm trying to match (point|rect|line) and a variable number of integers.

Thanks...

Answer 1

You are not printing all the groups. Try for (int i = 0; i <= m.groupCount(); i++)

Answer 2

Group 0 always refers to the originally matched string - in your example, point 0 0 255 0 0 . Therefore, the numbered capture groups in your pattern are referred to by a 1-based index - In your example group 1 , group 2 .

This is a common mannerism with other programming languages - JavaScript and Perl also denote the index-zero-match (also referred to as \\0 in "replace" functions) as the whole matched string.

Therefore, to output your expected capture groups, you will need to loop from 0 to 2 or from 1 to 2:

for (int i = 0; i <= m.groupCount(); i++) {
    //             ^--  changed "<" to "<="
    System.out.println("-- " + m.group(i));
}

Output:
point 0 0 255 0 0
point
0 0 255 0 0

Answer 3

Have a look at the MatchResult reference on https://docs.oracle.com/javase/7/docs/api/java/util/regex/MatchResult.html and check 'groupCount' section. It is said that group zero denotes the entire pattern by convention and not included in this count.