строка

using System;
using System.Text;
using System.Text.RegularExpressions;

class MainClass
{
  public static void Main(string[] args)
  {
    string pattern = @"[A-Z]";
    string to_delete = @"[^a-z0-9 \f\n\r\t\v]";
    string to_delete_word = @"(\b([a-z0-9]){1,3}\b)";
    string to_delete_space = @"\s{2, }";
    string first_space = @"^\s";
    string str;

    while((str = Console.ReadLine())!= null)
    {
      str = Regex.Replace(str, pattern, m => m.ToString().ToLower());
      str = Regex.Replace(str, to_delete, String.Empty);
      str = Regex.Replace(str, to_delete_word, String.Empty);
      str = Regex.Replace(str, to_delete_space, @"\s");
      Console.WriteLine(Regex.Replace(str, first_space, String.Empty));
    }
}
}