How to check if a substring is repeated without using regex?

Question

I want to check if a String contains repeated substrings.

For example, how do I check (bc)* without using regex library in Java?

Answer 1

You could simply use this recursive algorithm:

public static boolean repeatedString(String str, String repeat, int lastIndex) {
    int next = str.indexOf(repeat, lastIndex+repeat.length());

    if(next == -1) return false;
    else if(next-lastIndex == repeat.length()) return true;
    else return repeatedString(str, repeat, next);
}

Call repeatedString(str, "bc", -1) , It's essentially checking if any two occurences of repeat are consecutive.

Answer 2

An easy way would be to filter through each character and check to see if it starts with the substring that you are looking for.

public static int countSubstringOccurences(String st, String substring) {
    int count = 0;
    for(int i = 0; i < st.length(); i++) {
        if(st.substring(i).startsWith(substring)) {
            count++;
        }
    }
    return count;
}

This will method will test each substring of the given String to see if it starts with the given substring, and increase the count by one each time it finds a match.