I think you have to use a MatchEvaluator.

// demonstration calling code
// assuming:
// using System.Text.RegularExpressions
string s = "abc{def}";
string t = Regex.Replace(s, "{.*}", new MatchEvaluator(MatchToUpper));

// somewhere else
static string MatchToUpper(Match m)
{
   return m.Value.ToUpper();
}