{
 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;