Get the index of the nth occurrence of a string?


Question

Unless I am missing an obvious built-in method, what is the quickest way to get the nth occurrence of a string within a string?

I realize that I could loop the IndexOf method by updating its start index on each iteration of the loop. But doing it this way seems wasteful to me.

1
98
1/12/2017 3:16:54 PM

Accepted Answer

That's basically what you need to do - or at least, it's the easiest solution. All you'd be "wasting" is the cost of n method invocations - you won't actually be checking any case twice, if you think about it. (IndexOf will return as soon as it finds the match, and you'll keep going from where it left off.)

52
10/9/2008 10:26:51 AM

You really could use the regular expression /((s).*?){n}/ to search for n-th occurrence of substring s.

In C# it might look like this:

public static class StringExtender
{
    public static int NthIndexOf(this string target, string value, int n)
    {
        Match m = Regex.Match(target, "((" + Regex.Escape(value) + ").*?){" + n + "}");

        if (m.Success)
            return m.Groups[2].Captures[n - 1].Index;
        else
            return -1;
    }
}

Note: I have added Regex.Escape to original solution to allow searching characters which have special meaning to regex engine.


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