Fake Text Markup Language

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

class FakeMarkup
{
    static void Main(string[] args)
    {
        int n = int.Parse(Console.ReadLine());
        string pattern = "<(.*?)>";
        string lines = string.Empty;

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

        Match prev = Regex.Match(lines, pattern);
        while (prev.Success)
        {
            Match next = prev.NextMatch();
            if (next.Groups[1].Value == ("/" + prev.Groups[1].Value))
            {
                string original = lines.Substring(prev.Index, next.Index + next.Length - prev.Index);

                int start = prev.Index + prev.Length;
                int end = next.Index - start;
                string affected = lines.Substring(start, end);

                lines = lines.Replace(original, Change(prev.Value, affected));
                prev = Regex.Match(lines, pattern);
            }
            else prev = next;
        }
        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;
    }
}