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 

Answer 2

This should work for matches that might overlap:

public static void main(String[] args) {     String input = "aaaaaaaa";     String regex = "aa";     Pattern pattern = Pattern.compile(regex);     Matcher matcher = pattern.matcher(input);     int from = 0;     int count = 0;     while(matcher.find(from)) {         count++;         from = matcher.start() + 1;     }     System.out.println(count); }