FTML

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

class Program
{
    static void Main()
    {
        StringBuilder text = new StringBuilder();
        int count = int.Parse(Console.ReadLine());
        string code = null;

        while (count > 0)
        {
            code += Console.ReadLine() ;
            code += "\r\n";
            count--;
        }

        // regular expressions that match the tags but not nested tags
        string upper = @"(<upper>([a-zA-Z0-9\s!?""]+)</upper>)";
        string lower = @"(<lower>([a-zA-Z0-9\s!?""]+)</lower>)";
        string rev = @"(<rev>([a-zA-Z0-9\s!?""]+)</rev>)";
        string toggle = @"(<toggle>([a-zA-Z0-9\s!?""]+)</toggle>)";

        // the cycle continues while there is a match
        while (Regex.IsMatch(code, toggle) || Regex.IsMatch(code, rev)|| Regex.IsMatch(code, lower)||Regex.IsMatch(code, upper))
        {
            code = Regex.Replace(code, upper, m => m.Groups[2].Value.ToUpper()); // sets upper case and deletes tags
            code = Regex.Replace(code, lower, m => m.Groups[2].Value.ToLower()); // sets lower and del tags

            if (Regex.IsMatch(code, toggle)) //match toggle
            {
                Match strTog = Regex.Match(code, toggle); // takes the match
                string[] split = strTog.ToString().Split(new char[] { '>', '<' }, StringSplitOptions.RemoveEmptyEntries); // splits the match
                char[] letters = split[1].ToCharArray(); //splits  element[1] from the string because it holds the text after the first split
                string finalTog = null;
                foreach (var ch in letters)
                {
                    finalTog += (char.IsLower(ch) ? char.ToUpper(ch) : char.ToLower(ch)); // toggles the chars in the text
                }
                Regex regx = new Regex(toggle);
                code = regx.Replace(code,finalTog,1); // replaces the old text with toggled text and deletes the tags
            }
            if (Regex.IsMatch(code, rev)) //match reverse
            {
                Match strRev = Regex.Match(code, rev, RegexOptions.Singleline); // takes the match
                string[] local = strRev.ToString().Split(new char[] { '>', '<' }, StringSplitOptions.RemoveEmptyEntries); // splits the match
                char[] localLetters = local[1].ToCharArray(); //splits  element[1] from the string because it holds the text after the first split
                Array.Reverse(localLetters); //reverse array
                string finalRev = null;
                foreach (var ch in localLetters)
                {
                    finalRev += ch;    //build new string with reversed letters
                }
                Regex rgx = new Regex(rev);
                code = rgx.Replace(code, finalRev, 1); // replase the old text with reversed and delete the tags
            }
        }
        Console.WriteLine(code);
    }
}