In response to a different post he read on a postal/zip code validation topic, Ken Guest has points out other resources that can be used to accomplish the same sort of thing and already exist.

I drew attention to two things. The first is that there are Validation packages in PEAR, including the main Validate class and all the Validate_xx subclasses such as Validate_US, Validate_CA and some 22 others). The second item I drew Jamie's attention to is that his validation code counts a zip code of "00000' as valid, when the USPS zip code look up tool correctly (and they should know!) identifies that code as invalid.

He wholeheartedly opposes the "Not Invented Here" syndrome that developers (with a hefty dose of framework developers included) seem to have.