Strings/Task5

//You are given a text. Write a program that changes the text in all regions surrounded by the tags <upcase> and </upcase> to uppercase. The tags cannot be nested.
//Example: We are living in a <upcase>yellow submarine</upcase>. We don't have <upcase>anything</upcase> else.
//The expected result: We are living in a YELLOW SUBMARINE. We don't have ANYTHING else.

using System;
using System.Collections.Generic;
using System.Text;
class TextInTagsToUpper
{
    static string ChangeTextInTagsToUpper(string str)
    {
        string openTag = "<upcase>";
        string closeTag = "</upcase>";
        int startIndex = str.IndexOf(openTag, 0);

        while (startIndex != -1)
        {
            int endIndex = str.IndexOf(closeTag, startIndex + openTag.Length);
            string textInTagsAndTags = str.Substring(startIndex, endIndex - startIndex + closeTag.Length);
            string textInTags = str.Substring(startIndex + openTag.Length, endIndex - (startIndex + openTag.Length));

            str = str.Replace(textInTagsAndTags, textInTags.ToUpper());
            startIndex = str.IndexOf(openTag, startIndex + 1);
        }

        return str;
    }

    static void Main()
    {
        string input = "We are living in a <upcase>yellow submarine</upcase>. We don't have <upcase>anything</upcase> else.";
        string result = ChangeTextInTagsToUpper(input);
        Console.WriteLine("Result: " + result);
    }
}