How to get Nth IndexOf a string

using System.Text.RegularExpressions;

public class Utility

{

    public static int GetNthIndexOf(string str, string match, int n)

    {

        Match m = Regex.Match(str, "((" + match + ").*?){" + n + "}");

        if (m.Success)

            return m.Groups[2].Captures[n - 1].Index;

        else

            return -1;

    }

    public static int GetNthIndexOf(string str, string match, int n)

    {

        int i = 1;

        int index = -1;

        while (((index = str.IndexOf(match, index + 1)) != -1) && (i <= n))

        {

            if (i == n) return index;

            i++;

        }

        return -1;

    }

}