Code:

{

In this exemple, the only keys allowed are the

same allowed in e-mail adresses.

}

procedure TForm1.Edit1KeyPress(Sender: TObject;

  var Key: Char);

const

  AllowedChars: string = 'abcdefghijklmnopq' +

    'rstuvwxyz01234567_.@';

var

  i: Integer;

  Ok: Boolean;

begin

  i  := 0;

  Ok := False;

  { If you erase next line, user won't be able to type backspace }

  if Key = #8 then Ok := True;

  repeat

    i := i + 1;

    if Key = AllowedChars[i] then Ok := True;

  until (Ok) or (i = Length(AllowedChars));

  if not Ok then Key := #0;

end;