What's the easiest way to tell if a character is a letter or not?

[Justin Mclean]

Hi,

Anyone know how to work out if a character, including unicode characters, is a 
letter or not?

This is not the right way of doing it:

            if ("a" <= letter && letter <= "z" ||
                "A" <= letter && letter <= "Z")

From the DateFormatter class for the curious.

Thanks,
Justin