Finding a repeated pattern in a string

Question

How can I find a repeated pattern in a string? For example, if the input file were

AAAAAAAAA
ABABAB
ABCAB
ABAb

it would output:

A
AB
ABCAB
ABAb

Answer 1

If you use regex, you only need one line:

String repeated = str.replaceAll("(.+?)\\1+", "$1");

Breaking down the regex (.+?)\1:

• (.+?) means "at least one character, but as few as possible, captured as group 1"
• \1 means "the same character(s) as group 1

---

Here's some test code:

String[] strs = {"AAAAAAAAA", "ABABAB", "ABCAB", "ABAb"};
for (String str : strs) {
    String repeated = str.replaceAll("(.+?)\\1+", "$1");
    System.out.println(repeated);
}

Output:

A
AB
ABCAB
ABAb

Answer 2

This outputs what you ask for - the regex can probably be improved to avoid the loop but I can't manage to fix it...

public static void main(String[] args) {
    List<String> inputs = Arrays.asList("AAAAAAAAA", "ABABAB", "ABCAB", "ABAb");
    for (String s : inputs) System.out.println(findPattern(s));
}

private static String findPattern(String s) {
    String output = s;
    String temp;
    while (true) {
        temp = output.replaceAll("(.+)\\1", "$1");
        if (temp.equals(output)) break;
        output = temp;
    }
    return output;
}