Ограничение ввода в текстовое поле

{ 
  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;

Взято с сайта: https://www.swissdelphicenter.ch