SQL & PL/SQL :: Exception Handler For ORA-1722 Invalid Number

I have to ask a pretty basic question (my pl/sql skills need serious work). I need to validate that a string can be converted to a number, and if it can't be, substitute something else. This works:orcl> create or replace function jw_to_number(str varchar2) return number is

2 n number;
3 begin
4 n := to_number(str);
5 return n;
6 exception when others then return 0;
7 end;
8 /

Function created.

orcl> select jw_to_number('a') from dual;
JW_TO_NUMBER('A')
-----------------
0
orcl>

but of course I don't want to use WHEN OTHERS. So I tried this:orcl> create or replace function jw_to_number(str varchar2) return number is

2 n number;
3 begin
4 n := to_number(str);
5 return n;
6 exception when INVALID_NUMBER then return 0;
7 end;
8 /

Function created.

orcl> select jw_to_number('a') from dual;
select jw_to_number('a') from dual
*
ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error: character to number conversion error
ORA-06512: at "SCOTT.JW_TO_NUMBER", line 4
orcl>I

thought this would replace the standard error handler with my own, but it doesn't. I tried redeclaring the pre-defined exception, same result.