Basically something like:
const string start = @"(\p{Lu}|\p{Ll}|\p{Lt}|\p{Lm}|\p{Lo}|\p{Nl})";
const string extend = @"(\p{Mn}|\p{Mc}|\p{Nd}|\p{Pc}|\p{Cf})";
Regex ident = new Regex(string.Format("{0}({0}|{1})*", start, extend));
s = s.Normalize();
return ident.IsMatch(s);