Find all occurrences of substring in string in Java

Question

I\'m trying to find all occurrences of a substring in a string in Java.

For example: searching \"ababsdfasdfhelloasdf\" for \"asdf\" would return [8,17] since there

Answer 1

Using a regex is definitely an overly heavy solution for finding substrings, and it'll especially be a problem if your substring contains special regex characters like .. Here's a solution adapted from this answer:

String str = "helloslkhellodjladfjhello";
String findStr = "hello";
int lastIndex = 0;
List<Integer> result = new ArrayList<Integer>();

while(lastIndex != -1) {

    lastIndex = str.indexOf(findStr,lastIndex);

    if(lastIndex != -1){
        result.add(lastIndex);
        lastIndex += 1;
    }
}

Answer 2

You can use capturing inside a positive look-ahead to get all overlapping matches and use Matcher#start to get the indices of the captured substrings.

As for the regex, it will look like

(?=(aa))

In Java code:

String s = "aaaaaa";
Matcher m = Pattern.compile("(?=(aa))").matcher(s);
List<Integer> pos = new ArrayList<Integer>();
while (m.find())
{
    pos.add(m.start());
}
System.out.println(pos);

Result:

[0, 1, 2, 3, 4]

See IDEONE demo