How to check for repeating sequence in an integer

Question

I have an alpha-numeric string and I want to check for pattern repetition in it just for the integers. And they should be continuous.

Example

1. 12341234qwe should tell me 1234 is repeated.
2. 1234qwe1234 should NOT tell me that 1234 is repeated since its not continuous.
3. 12121212 should be treated as 12 being repeated as that is the first set which would be found being repeated. But if there is an algorithm which would find 1212 as the repeated set before 12 then I guess it has to perform the steps again on 1212.

What I thought was I can store the integer part by iterating and comparing it with ( <= '0' && >= '9') in a different StringBuilder. Then I read about performing FFT on the string and it shows the repeated patterns. But I have no idea on how to perform FFT in Java and look for the results, also, I was hoping to try to do this without going to Signal Processing. I read about KMP pattern matching but that only works with a given input. Is there any other way to do this?

Answer 1

You can take help of regex to solve this I think. Consider code like this:

String arr[] = {"12341234abc", "1234foo1234", "12121212", "111111111", "1a1212b123123c12341234d1234512345"};
String regex = "(\\d+?)\\1";
Pattern p = Pattern.compile(regex);
for (String elem : arr) {
    boolean noMatchFound = true;
    Matcher matcher = p.matcher(elem);
    while (matcher.find()) {
        noMatchFound = false;
        System.out.println(elem + " got repeated: " + matcher.group(1));
    }
    if (noMatchFound) {
        System.out.println(elem + " has no repeation");
    }
}

OUTPUT:

abc12341234abc got repeated: 1234
1234foo1234 has no repeation
12121212 got repeated: 12
12121212 got repeated: 12
111111111 got repeated: 1
111111111 got repeated: 1
111111111 got repeated: 1
111111111 got repeated: 1
1a1212b123123c12341234d1234512345 got repeated: 12
1a1212b123123c12341234d1234512345 got repeated: 123
1a1212b123123c12341234d1234512345 got repeated: 1234
1a1212b123123c12341234d1234512345 got repeated: 12345

Explanation:

Regex being used is (\\d+?)\\1 where

\\d        - means a numerical digit
\\d+       - means 1 or more occurrences of a digit
\\d+?      - means reluctant (non-greedy) match of 1 OR more digits
( and )    - to group the above regex into group # 1
\\1        - means back reference to group # 1
(\\d+?)\\1 - repeat the group # 1 immediately after group # 1