How do fix this error

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C#6.0

C#7.0

gives this error

System.ArgumentOutOfRangeException: 'Index and count must refer to a location within the string.'

string Intermeddiate = //Literally any news websites articles html
string Titlestart = $"<title>";
string Titleend = $"";

var matches = Regex.Matches(Intermeddiate.ToString(), "<title>(.*?)</title>");
           foreach (var item in matches)
           {


               if (TitlestartPresnt && TitleendPresent)
               {

                   string titleImpure = item.ToString();
                   string removedfront = titleImpure.Remove(0, 7);
                   string removedback = removedfront.Remove(removedfront.IndexOf(Titleend), removedfront.IndexOf(Titleend)+7);
                   Title = removedback;
                   Console.WriteLine(Title);
                  // int Titlestartindex = item.IndexOf(Titlestart);
                  //int Titleemdindex = Intermeddiate.IndexOf(Titleend);
                  // if (Titlestartindex+50 > Titleemdindex -50)
                  // {



                   //     int titlestart = Intermeddiate.IndexOf(Titlestart) + Titlestart.Length;
                   //     int titleend = Intermeddiate.IndexOf(Titleend) - 10; //make so works properly
                   //     string PureTexttitle = Intermeddiate.Substring(titlestart, titleend - titlestart);

                   //     Console.WriteLine("Title-" + PureTexttitle);

                   //     TitlesCList.Add(PureTexttitle);
                   // }
                   // else
                   // {
                   //     Console.WriteLine("No title for this article");
                   // }
               }

           }

TitlestartPresent
can be multiple occurences of titlestart and titlened
the stuff in the comments does the same thing and gives th esame error really

tried doing a check to make sure that the indexes of titlestart and titleend overlap incase there is more than 1 titlestart and titleend in the entire html

wanna cry

What I have tried:

more or less everything I can think of