you shouldn't restrict anything - just give the users enough hints to be more informed. I would highlite the background of any character that is not from the u

you shouldn't restrict anything - just give the users enough hints to be more informed. I would highlite the background of any character that is not from the users codepage in red. for eg. if your local settings are us-en, any character not from that codepage will have a red background (or even in italics, some way to signify that the character is 'foreign')