WordEncounter_100

namespace Word_encounter
{
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text.RegularExpressions;

    public class WordEncounter
    {
        public static void Main()
        {
            var filter = Console.ReadLine();
            var letter = filter[0];
            var neededCount = int.Parse(filter[1].ToString());
            var filteredWords = new List<string>();

            var sentence = Console.ReadLine();

            while (sentence != "end")
            {
                string patternValidSentence = @"^[A-Z].*[.!?]$";
                var validSentence = new Regex(patternValidSentence);

                if (validSentence.IsMatch(sentence))
                {
                    var regex = new Regex(@"\w+");

                    var words = regex.Matches(sentence);

                    foreach (Match item in words)
                    {
                        var word = item.Groups[0].Value;

                            var count = 0;
                            foreach (var character in word)
                            {
                                if (character== letter)
                                {
                                    count++;
                                }
                            }

                            if (count >= neededCount)
                            {
                                filteredWords.Add(word);
                            }
                    }
                }

                sentence = Console.ReadLine();
            }

            Console.WriteLine(string.Join(", ", filteredWords));
        }
    }
}