Split a String at every 3rd comma in Java

Question 1

NOTE: while solution using split may work (last test on Java 17) it is based on bug since look-ahead in Java should have obvious maximum length. This limitation should theoretically prevent us from using + but somehow \G at start lets us use + here. In the future this bug may be fixed which means that split will stop working.

Safer approach would be using Matcher#find like

String data = "0,0,1,2,4,5,3,4,6";
Pattern p = Pattern.compile("\\d+,\\d+,\\d+");//no look-ahead needed
Matcher m = p.matcher(data);
List<String> parts = new ArrayList<>();
while(m.find()){
    parts.add(m.group());
}
String[] result = parts.toArray(new String[0]);

You can try to use split method with (?<=\\G\\d+,\\d+,\\d+), regex

Demo

String data = "0,0,1,2,4,5,3,4,6";
String[] array = data.split("(?<=\\G\\d+,\\d+,\\d+),"); //Magic :) 
// to reveal magic see explanation below answer
for(String s : array){
    System.out.println(s);
}

output:

0,0,1
2,4,5
3,4,6

Explanation

\\d means one digit, same as [0-9], like 0 or 3
\\d+ means one or more digits like 1 or 23
\\d+, means one or more digits with comma after it, like 1, or 234,
\\d+,\\d+,\\d+ will accept three numbers with commas between them like 12,3,456
\\G means last match, or if there is none (in case of first usage) start of the string
(?<=...), is positive look-behind which will match comma , that has also some string described in (?<=...) before it
(?<=\\G\\d+,\\d+,\\d+), so will try to find comma that has three numbers before it, and these numbers have aether start of the string before it (like ^0,0,1 in your example) or previously matched comma, like 2,4,5 and 3,4,6.

Also in case you want to use other characters then digits you can also use other set of characters like

\\w which will match alphabetic characters, digits and _
\\S everything that is not white space
[^,] everything that is not comma
... and so on. More info in Pattern documentation

By the way, this form will work with split on every 3rd, 5th, 7th, (and other odd numbers) comma, like split("(?<=\\G\\w+,\\w+,\\w+,\\w+,\\w+),") will split on every 5th comma.

To split on every 2nd, 4th, 6th, 8th (and rest of even numbers) comma you will need to replace + with {1,maxLengthOfNumber} like split("(?<=\\G\\w{1,3},\\w{1,3},\\w{1,3},\\w{1,3}),") to split on every 4th comma when numbers can have max 3 digits (0, 00, 12, 000, 123, 412, 999).

To split on every 2nd comma you can also use this regex split("(?<!\\G\\d+),") based on my previous answer

Question 2

Obligatory Guava answer:

String input = "0,0,1,2,4,5,3,4,6";
String delimiter = ",";
int partitionSize = 3;

for (Iterable<String> iterable : Iterables.partition(Splitter.on(delimiter).split(s), partitionSize)) {
    System.out.println(Joiner.on(delimiter).join(iterable));
}

Outputs:

0,0,1
2,4,5
3,4,6

Question 3

Try something like the below:

public String[] mySplitIntoThree(String str) 
{
    String[] parts = str.split(",");

    List<String> strList = new ArrayList<String>();

    for(int x = 0; x < parts.length - 2; x = x+3) 
    {
        String tmpStr = parts[x] + "," + parts[x+1] + "," + parts[x+2];

        strList.add(tmpStr);
    }

    return strList.toArray(new String[strList.size()]);
}

(You may need to import java.util.ArrayList and java.util.List)

Question 4

Nice one for the coding dojo! Here's my good old-fashioned C-style answer:

If we call the bits between commas 'parts', and the results that get split off 'substrings' then:

n is the amount of parts found so far, i is the start of the next part, startIndex the start of the current substring

Iterate over the parts, every third part: chop off a substring.

Add the leftover part at the end to the result when you run out of commas.

List<String> result = new ArrayList<String>();
int startIndex = 0;
int n = 0;
for (int i = x.indexOf(',') + 1; i > 0; i = x.indexOf(',', i) + 1, n++) {
    if (n % 3 == 2) {
        result.add(x.substring(startIndex, i - 1));
        startIndex = i;
    }
}
result.add(x.substring(startIndex));