Fake Text Markup Language v2

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

namespace Tests
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            string pattern = @"<(?<tag>.*?)>(?<text>[^<]*?)</\k<tag>>";
            string lines = string.Empty;

            for (int count = 0; count < n; count++) lines += "\n" + Console.ReadLine();

            Match match = Regex.Match(lines, pattern);
            while (match.Success)
            {
                lines = lines.Replace(match.Value, Change(match.Groups["tag"].Value, match.Groups["text"].Value));
                match = Regex.Match(lines, pattern);
            }
            Console.WriteLine(lines);
        }

        static string Change(string action, string text)
        {
            switch (action)
            {
                case "upper":
                    {
                        return text.ToUpper();
                    }
                case "lower":
                    {
                        return text.ToLower();
                    }
                case "toggle":
                    {
                        char[] result = text.ToCharArray();
                        for (int i = 0; i < result.Length; i++)
                        {
                            if (char.IsLower(result[i])) result[i] = char.ToUpper(result[i]);
                            else if (char.IsUpper(result[i])) result[i] = char.ToLower(result[i]);
                        }
                        return (new string(result));
                    }
                case "rev":
                    {
                        return new string(text.Reverse().ToArray());
                    }
            }
            return string.Empty;
        }
    }
}