I think outsourcing the problem is the best bet: send it to the Google (or Yahoo) geocoder. The geocoder returns not only the lat/long (which aren't of interest here), but also a rich parsing of the address, with fields filled in that you didn't send (including ZIP+4 and county).
For example, parsing "1600 Amphitheatre Parkway, Mountain View, CA" yields
{
"name": "1600 Amphitheatre Parkway, Mountain View, CA, USA",
"Status": {
"code": 200,
"request": "geocode"
},
"Placemark": [
{
"address": "1600 Amphitheatre Pkwy, Mountain View, CA 94043, USA",
"AddressDetails": {
"Country": {
"CountryNameCode": "US",
"AdministrativeArea": {
"AdministrativeAreaName": "CA",
"SubAdministrativeArea": {
"SubAdministrativeAreaName": "Santa Clara",
"Locality": {
"LocalityName": "Mountain View",
"Thoroughfare": {
"ThoroughfareName": "1600 Amphitheatre Pkwy"
},
"PostalCode": {
"PostalCodeNumber": "94043"
}
}
}
}
},
"Accuracy": 8
},
"Point": {
"coordinates": [-122.083739, 37.423021, 0]
}
}
]
}
Now that's parseable!