String Helpers

        private static readonly Regex NonSymbols = new Regex("([a-zA-Z0-9]+)", RegexOptions.Compiled); // Words without symbols.
    //private static readonly Regex NonSymbols = new Regex("(\S+)", RegexOptions.Compiled); // Any words.

        static IEnumerable<string> GetUniqueWords(string input)
        {
            return AllWord(input).Distinct();
        }

        static IEnumerable<string> AllWord(string input)
        {
            return NonSymbols.Matches(input.ToLower()).OfType<Match>().Select(s => s.Value);
        }

        static long LargestSingleWordCount(string input)
        {
            IEnumerable<string> words = AllWord(input);
            return words.Distinct().Max(s => words.Count(t => t == s));
        }