how to modify a fen string in java?

So i have this string StringBuilder fenToString = new StringBuilder("1P111Pr1")

now how can i change it to a = "1P3Pr1"?

i tried this

int fenNum = 0;
for(int i = 0; i < fenToString.length(); i++){
        if(Character.isDigit(fenToString.charAt(i))){
            fenNum += 1;
            fenToString.setCharAt(i, (char)(fenNum+'0'));
        }else{
            fenNum = 0;
        }
    }

i get "1P123Pr1" instead of "1P3Pr1"

So to explicitly state the task: You want to convert a sequence of two or more digits into the sum of those digits. The sum is guaranteed to be 8 or less, i.e. to also be a single digit.

There are many ways to do that, but the closest to what you're trying would likely be:

static String normalizeFen(String fen) {
    StringBuilder buf = new StringBuilder(fen);
    for (int i = 1; i < buf.length(); i++) {
        if (Character.isDigit(buf.charAt(i)) && Character.isDigit(buf.charAt(i - 1))) {
            // Found 2 consecutive digits, so sum them
            int sum = Character.digit(buf.charAt(i - 1), 10)
                    + Character.digit(buf.charAt(i), 10);
            buf.setCharAt(i - 1, Character.forDigit(sum, 10));
            buf.deleteCharAt(i); // Remove second digit
            i--; // Go back to reprocess the same index position again
        }
    }
    return buf.toString();
}

Tests

System.out.println(normalizeFen("1P111Pr1"));
System.out.println(normalizeFen("rnbqkbnr/pp1ppppp/11111111/11p11111/1111P111/11111111/PPPP1PPP/RNBQKBNR"));

Output

1P3Pr1
rnbqkbnr/pp1ppppp/8/2p5/4P3/8/PPPP1PPP/RNBQKBNR