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

namespace LettersToUpper
{
    class LettersToUpper
    {
        static string ToUpper(string text)
        {
            string pattern = @"(?'OpenTag'<upcase>)(?'TagContent'.*?)(?'CloseTag'</upcase>)";
            StringBuilder resultString = new StringBuilder(text.Length, text.Length);
            int LastIndex = 0;

            Regex regex = new Regex(pattern);

            Match match = regex.Match(text);

            while (match.Success)
            {
                string patch = text.Substring(LastIndex, match.Index - LastIndex);
                string upperCasePatch = match.Groups["TagContent"].ToString();

                resultString.Append(patch);
                resultString.Append(upperCasePatch.ToUpper());

                LastIndex = match.Index + match.Length;
                match = match.NextMatch();
            }

            resultString.Append(text.Substring(LastIndex));

            return resultString.ToString();
        }

        static void Main(string[] args)
        {
        }
    }
}