using System;
using System.Text.RegularExpressions;
 
public class Test
{
public static void Main()
{
string str = "We are living in a <upcase>yellow submarine</upcase>. We don't have <upcase>anything</upcase> else.";
string result = Regex.Replace(str, "(?<=<upcase>).*?(?=</upcase>)",  m => m.ToString().ToUpper());
Console.WriteLine(Regex.Replace(result, "</?upcase>", ""));
Console.ReadLine();
}
}