Find all indexes of substring, duplicates

        public static IEnumerable<int> FindAll(this string str, string sub)
        {
            if (sub.Length < 1)
                yield break;
            var index = 0;
            while (true)
            {
                index = str.IndexOf(sub, index + 1, StringComparison.InvariantCultureIgnoreCase);
                if (index == -1)
                    break;
                yield return index;
            }
        }