using System;
using System.Text.RegularExpressions;

class ChangeTextBetweenTagsWithUppercase
{
    static void Main()
    {
        string str = "We are living in a <upcase>yellow submarine</upcase>. We don't have <upcase>anything</upcase> else.";
        Console.WriteLine("Insert string with tags <upcase>...</upcase>");
        //string str = Console.ReadLine();
        Regex regex = new Regex (@"(<\s*upcase\s*>)(?<tag>.*?)(<\s*/upcase\s*>)");
        MatchCollection matches = regex.Matches(str);

        foreach (Match match in matches)
        {
            str = regex.Replace(str, match.Groups["tag"].Value.ToUpper(),1);
        }
        Console.WriteLine(str);
    }

}