How do I use Java Regex to find all repeating character sequences in a string?

Question

Parsing a random string looking for repeating sequences using Java and Regex.

Consider strings:

aaabbaaacccbb

I'd like to find a regular expression that will find all the matches in the above string:

aaabbaaacccbb
^^^  ^^^

aaabbaaacccbb
   ^^      ^^

What is the regex expression that will check a string for any repeating sequences of characters and return the groups of those repeating characters such that group 1 = aaa and group 2 = bb. Also note that I've used an example string but any repeating characters are valid: RonRonJoeJoe ... ... ,, ,,...,,

Answer 1

This does it:

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Test {
    public static void main(String[] args) {
        String s = "aaabbaaacccbb";
        find(s);
        String s1 = "RonRonRonJoeJoe .... ,,,,";
        find(s1);
        System.err.println("---");
        String s2 = "RonBobRonJoe";
        find(s2);
    }

    private static void find(String s) {
        Matcher m = Pattern.compile("(.+)\\1+").matcher(s);
        while (m.find()) {
            System.err.println(m.group());
        }
    }
}

OUTPUT:

aaa
bb
aaa
ccc
bb
RonRonRon
JoeJoe
....
,,,,
---

Answer 2

The below should work for all requirements. It is actually a combination of a couple of the answers here, and it will print out all of the substrings that are repeated anywhere else in the string.

I set it to only return substrings of at least 2 characters, but it can be easily changed to single characters by changing "{2,}" in the regex to "+".

public static void main(String[] args)
{
  String s = "RonSamJoeJoeSamRon";
  Matcher m = Pattern.compile("(\\S{2,})(?=.*?\\1)").matcher(s);
  while (m.find())
  {
    for (int i = 1; i <= m.groupCount(); i++)
    {
      System.out.println(m.group(i));
    }
  }
}

Output:
Ron
Sam
Joe