Make Uppercase the Text Surrounded By Tags

using System;
using System.Collections.Generic;

class MakeUppercaseTextSurroundedByTags
{
  //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.

  static List<int> FindInTextIndices(string text, string substrToSearch)//Returns a list with the indices of the searchhed substring in a text
  {
    List<int> tagsPositions = new List<int>();
    int lastSubstrFoundIndex = text.LastIndexOf(substrToSearch);

    for (int index = 0; index < lastSubstrFoundIndex; )
    {
      index = text.ToLowerInvariant().IndexOf(substrToSearch, index);
      tagsPositions.Add(index);
      index += substrToSearch.Length;
    }

    return tagsPositions;
  }

  static string MakeUppercaseSubstring(string text, List<int> startIndices, List<int> endIndices)//Makes the text between the tags Uppercase
  {
    for (int i = 0; i < startIndices.Count; i++)
            {
        string substring = text.Substring(startIndices[i], endIndices[i] - startIndices[i] + 1).ToUpper();
        text = text.Replace((text.Substring(startIndices[i], endIndices[i] - startIndices[i] + 1)), substring);
            }
    return text;
  }

  static string RemoveTags(string text, string tag, List <int> tagIndices)//Removes the tags from the text
  {
    for (int index = tagIndices.Count-1; index >=0; index--)
    {
      text = text.Remove(tagIndices[index], tag.Length);
    }

    return text;
  }

  static void Main()
  {
    string text = "We are living in a <upcase>yellow submarine</upcase>. We don't have <upcase>anything</upcase> else.";

    string openingTag = "<upcase>",
           closingTag = "</upcase>";
    List<int> openingTagIndices = FindInTextIndices(text,openingTag);//Find the indices of the opening tag
    List<int> closingTagIndices = FindInTextIndices(text, closingTag);//Find the indices of the closing tag

    if (openingTagIndices == null || closingTagIndices == null)//If one of the arrays is null
    {
      Console.WriteLine("At least one of the tags haven't been found in the text!");
    }
    else
    {
      string result = MakeUppercaseSubstring(text, openingTagIndices, closingTagIndices);//Make uppercase the text between the tags
      result = RemoveTags(result, openingTag, openingTagIndices);//Remove the opening tags

      closingTagIndices.Clear();//Clear the array with the end tag indices because they have changed
      closingTagIndices = FindInTextIndices(result, closingTag);//Find the new indices of the closing tags
      result = RemoveTags(result, closingTag, closingTagIndices);//Remove the closing tags

      Console.WriteLine(result);//Print the result
    }
  }
}