FTML100

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Text.RegularExpressions;
using System.IO;

namespace FTML
{
    class FTML
    {
        static string ReverseOneLineString(string str)
        {
            StringBuilder output = new StringBuilder();
            for (int i = str.Length - 1; i >= 0; i--)
            {
                output.Append(str[i]);
            }

            return output.ToString();
        }
        static StringBuilder Reverse(StringBuilder str)
        {
            if (str.ToString().Contains(Environment.NewLine.ToString()))
            {
                StringBuilder result = new StringBuilder();
                string[] substrings = str.ToString().Split(new string[] { Environment.NewLine.ToString() }, StringSplitOptions.RemoveEmptyEntries);
                for (int i = substrings.Length - 1; i >= 0; i--)
                {
                    result.Append(ReverseOneLineString(substrings[i]));
                    if (i > 0)
                    {
                        result.Append(Environment.NewLine.ToString());
                    }
                }

                return result;
            }
            else
            {
                return new StringBuilder( ReverseOneLineString(str.ToString()));
            }
        }
        static StringBuilder Delete(StringBuilder str)
        {
            return new StringBuilder("");
        }
        static StringBuilder Toggle(StringBuilder str)
        {

            string s = str.ToString();
            StringBuilder result = new StringBuilder(str.Length);
            for (int i = 0; i < s.Length; i++)
            {
                if (char.IsUpper(s[i]))
                {
                    result.Append(s[i].ToString().ToLower());
                }
                else
                {
                    result.Append(s[i].ToString().ToUpper());
                }
            }
            return result;
        }
        static StringBuilder ToUpper(StringBuilder str)
        {
            string s = str.ToString().ToUpper();
            return new StringBuilder(s);
        }
        static StringBuilder ToLower(StringBuilder str)
        {
            string s = str.ToString().ToLower();
            return new StringBuilder(s);
        }
        static void Main(string[] args)
        {
            string pattern = @"(?<currentTag>[<]\w+[>])(?<content>[^<]*)(?<closingTag>[<]{1}[//]{1}\w+[>]{1})";
            int n = int.Parse(Console.ReadLine());
            StringBuilder text = new StringBuilder();
            for (int i = 0; i < n; i++)
            {
                text.AppendLine(Console.ReadLine());
            }
            //for (int i = 0; i < n; i++)
            {

                Match match = Regex.Match(text.ToString(), pattern);
                while (match.Success)
                {
                    StringBuilder replacement = new StringBuilder();
                    if (match.Groups["currentTag"].ToString() == "<rev>")
                    {
                        replacement = Reverse(new StringBuilder(match.Groups["content"].ToString()));
                    }
                    else if (match.Groups["currentTag"].ToString() == "<upper>")
                    {
                        replacement = ToUpper(new StringBuilder(match.Groups["content"].ToString()));
                    }
                    else if (match.Groups["currentTag"].ToString() == "<lower>")
                    {
                        replacement = ToLower(new StringBuilder(match.Groups["content"].ToString()));
                    }
                    else if (match.Groups["currentTag"].ToString() == "<toggle>")
                    {
                        replacement = Toggle(new StringBuilder(match.Groups["content"].ToString()));
                    }
                    else if (match.Groups["currentTag"].ToString() == "<del>")
                    {
                        replacement = Delete(new StringBuilder(match.Groups["content"].ToString()));
                    }
                    text.Replace(match.Groups["currentTag"].ToString() + match.Groups["content"].ToString() + match.Groups["closingTag"].ToString(), replacement.ToString());
                    match = Regex.Match(text.ToString(), pattern);
                    //match = match.NextMatch();
                }
            }
            Console.WriteLine(text);
        }
    }
}