oracle异常

declare
v_str varchar2(50):='00xxx';
v_char varchar2(1):=null;
v_count number(10):=0;
begin
  v_count:=length(v_str);
  for i in 1..v_count loop
    begin
    v_char:=substr(v_str,0,i);
    dbms_output.put_line(v_char);
    exception
      when others then
        null;
    end;
  end loop;
end;