SQL & PL/SQL

I am having problems safely converting text values to numbers with the TO_NUMBER function.

select to_number('BAD') from dual

Result: ORA-01722: invalid number. This makes sense.

select to_number('6E9547') from dual

Result: ORA-01722: invalid number. This makes sense too considering that the E is treated as an exponent marker.

So I create a function to test if a number is valid:

CREATE OR REPLACE FUNCTION is_number( str IN VARCHAR2 ) RETURN NUMBER IS
    BEGIN
     RETURN TO_NUMBER(str);
    EXCEPTION
     WHEN OTHERS THEN RETURN NULL;
    END;

Now it is time for a retest:

select is_number('BAD') from dual

Result: NULL as expected

select is_number('6E9547') from dual

Result: 1E126
This last one is unexpected. I expect NULL but get a 'random' number?

All this on 10.2.0.4 (patch set 20) on Windows 2003 server.