Need SQL number conversion

I need to find the max numeric value in a field that contains some non numeric data, such as

31528
63595
X2187
77141
35-28

records with non numeric data can be ignored.

I've tried:

SELECT MAX(TO_NUMBER(SerialNumber)) FROM XYZ.COUPON;

but the TO_Number stumbles on the non numeric data.

I would grateful for any help.

--
Randy Harris