using System;

using System.Linq;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var str = "Bob hit a ball,  the hit BALL flew far after it was hit.";

        var forbidden = new [] {"hit"};

        var matches = new Regex(@"\b(\w+)\b" ).Matches(str).Select(x => x.Value.ToLowerInvariant());

        var allowedMatches = matches.Where(s => !forbidden.Contains(s)); //you would think you can use Except here but it dedupes, which we dont want

        var counts = allowedMatches.ToLookup(x => x).ToDictionary(g => g.Key, g => g.Count());

        FiddleHelper.WriteTable(counts);

        var mostCommon = counts.OrderByDescending(kv => kv.Value).Select(kv => kv.Key).FirstOrDefault();

        Console.WriteLine(mostCommon);

    }

}