为什么我从正则表达式匹配器中得到这些结果？

Question

I have this code below:

    String content = "title = 123";
    Pattern p = Pattern.compile("(title)");
    Matcher m = p.matcher(content);

    int i = 1;
    while (m.find()) {
        System.out.println("groupCount() = " + m.groupCount());
        System.out.println("i = " + i++ + "     found: " + m.group(0));
        System.out.println("i = " + i++ + "     found: " + m.group(1));
    }

The output is:

groupCount() = 1
i = 1   found: title
i = 2   found: title

Can someone please tell me why I have 1 group count and also if I get correct values in group(0) and group(1) ?

Are there any differences between group(0) and group(1) ?

Answer 1

Seed in the documentation:
groupCount()

Group zero denotes the entire pattern by convention. It is not included in this count.

and
group(int)

Capturing groups are indexed from left to right, starting at one. Group zero denotes the entire pattern, so the expression m.group(0) is equivalent to m.group().

So group 0 is the whole string that will be matched. and you have a capturing group with the whole string . So there is technically 2 group.

0 is the default
1 is your defined

Defined group indexing start from 1 .

Answer 2

Why I have 1 group count?

See Matcher specs :

groupCount() - Returns the number of capturing groups in this matcher's pattern.

You only have 1 capturing group: (title) .

Are there any differences between group(0) and group(1) ?

group(0) is the whole match (ie the string that was matched with the whole pattern), group(1) is only the substring captured with the first numbered capturing group. In your particular case, group(0) and group(1) are equal, because you set a capturing group to the whole pattern. Thus, the whole match (in group(0) ) and the captured substring (in group(1) ) coincide.

Best practice is not use a capturing group around the whole pattern since you always have access to it using group(0) .

Answer 3

group 0 is the entire string which is being used. Since you are capturing everything, group 0 and group 1 (which represents the first captured / matched group) will be same. If you had something like this :

Pattern p = Pattern.compile("(title) = 123"); then you will be able to find the difference between groups 0 and 1.

Note : group() is same as group(0)