SQL & PL/SQL

I am on 11g.

I need to remove the alpha characters from a string, leaving only numbers, but I am getting unexpected results:
SQL> SELECT TRANSLATE('3N', 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', NULL) a FROM DUAL;
A
-

I thought this would leave the 3 from the 3N, but it is returning an empty string. For my application, the string '3N' could be any length, will only contain letters and numbers, and the letters will always come at the end, but there could be more than one letter
VALID INPUT samples:
4
25
11F
361NG
8ABC

Is there a better way?