java - How can I count the number of matches for a regex?

Question

Let's say I have a string which contains this:

HelloxxxHelloxxxHello

I compile a pattern to look for 'Hello'

Pattern pattern = Pattern.compile("Hello");
Matcher matcher = pattern.matcher("HelloxxxHelloxxxHello");

It should find three matches. How can I get a count of how many matches there were?

I've tried various loops and using the matcher.groupCount() but it didn't work.

Answer 1

matcher.find() does not find all matches, only the next match.

Solution for Java 9+

long matches = matcher.results().count();

Solution for Java 8 and older

You'll have to do the following. (Starting from Java 9, there is a nicer solution)

int count = 0;
while (matcher.find())
    count++;

Btw, matcher.groupCount() is something completely different.

Complete example:

import java.util.regex.*;

class Test {
    public static void main(String[] args) {
        String hello = "HelloxxxHelloxxxHello";
        Pattern pattern = Pattern.compile("Hello");
        Matcher matcher = pattern.matcher(hello);

        int count = 0;
        while (matcher.find())
            count++;

        System.out.println(count);    // prints 3
    }
}

Handling overlapping matches

When counting matches of aa in aaaa the above snippet will give you 2.

aaaa
aa
  aa

To get 3 matches, i.e. this behavior:

aaaa
aa
 aa
  aa

You have to search for a match at index <start of last match> + 1 as follows:

String hello = "aaaa";
Pattern pattern = Pattern.compile("aa");
Matcher matcher = pattern.matcher(hello);

int count = 0;
int i = 0;
while (matcher.find(i)) {
    count++;
    i = matcher.start() + 1;
}

System.out.println(count);    // prints 3