ZIP codes are a type of postal code used by the U.S. Postal Service to route mail. First introduced by the Post Office Department in 1963, ZIP stands for zone improvement plan, and was initially established to help the USPS deliver packages. Today it is used to ship almost anything to an address.

A ZIP+4 Code is a 4-digit code appended to a standard 5-digit ZIP that helps the USPS sort, route, and deliver mail more efficiently. The last 4 digits of these 9-digit ZIP Codes usually represent 6-20 delivery points on a USPS carrier route.

Introduced on July 1, 1963, the basic format comprised five digits. In 1983, an extended code was introduced named ZIP+4 ; it included the five digits of the ZIP Code, followed by a hyphen and four digits that designated a more specific location.

A United States postal code, for example, might be either five digits or five digits followed a hyphen (dash) and another four digits (12345-1234). If you are validating US postal codes, allow for both conditions. The following rule validates a field that can be five digits, five digits + dash + 4 digits, or blank.