WordEncounter

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace _03.WordEncounter
{
    public class WordEncounter
    {
        public static void Main()
        {
            var words = new List<string>();

            var filter = Console.ReadLine();
            var symbol = filter[0];
            var digitString = string.Empty;

            for (int i = 1; i < filter.Length; i++)
            {
                digitString += filter[i];
            }

            var number = int.Parse(digitString);

            var listOfSentences = Console.ReadLine();

            var pattern = @"[A-Z](.*?)(\.|\?|\!)";
            var regex = new Regex(pattern);

            while (true)
            {
                if (listOfSentences == "end")
                {
                    break;
                }

                var isValidSentence = regex.IsMatch(listOfSentences);

                if (isValidSentence)
                {
                    var matches = regex.Matches(listOfSentences);

                    foreach (Match match in matches)
                    {
                        var sentenceToString = match.Value;
                        var wordsInSentence = sentenceToString.Split(new char[] { ' ', ',', '.', '!', '?', ';', ':', '(', ')', '-' }, StringSplitOptions.RemoveEmptyEntries);

                        foreach (var word in wordsInSentence)
                        {
                            var count = 0;
                            for (int i = 0; i < word.Length; i++)
                            {
                                if (word[i] == symbol)
                                {
                                    count++;
                                }
                            }

                            if (count == number)
                            {
                                words.Add(word);
                            }
                        }
                    }
                }

                listOfSentences = Console.ReadLine();
            }

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