Other considerations might be whether you want to search for PO or P.O. within the first 4 or 5 characters of the address or anywhere in the address. For example:
P.O. Box 66
P O 222
1234 Rumpole St
4567 Popopo Blvd.
You would probably want the first 3, but not the last 2 (if you're looking for PO boxes). In which case, the SUBSTRING function might help limit the search to the first 4 or 5 characters of the address.