[Страдалецъ]

Примерно так:

Код:

Var
  S: String;
  i,n: Integer;
begin
 S := '123 456abc#78%90';
 n := Length(S);
 for i := n downto 1
 do if not (S[i] in ['0'..'9','a'..'z'])
     then Delete(S,i,1);
end;