One Regular Expression to validate US and Canada ZIP / Postal Code

Question

I am developing a stationery program. Customers have choice to pick their region either US or Canada. When they enter address they have to enter ZIP/Postal code. I am trying to

Answer 1

Not knowing what language you're using, I will not use any abbreviations for character classes:

^[0-9]{5}$|^[A-Z][0-9][A-Z] ?[0-9][A-Z][0-9]$

Depending on your language, you might be able to abbreviate this to

^([0-9]{5}|[A-Z][0-9][A-Z] ?[0-9][A-Z][0-9])$

or

^(\d{5}|[A-Z]\d[A-Z] ?\d[A-Z]\d)$

To support ZIP+4:

^(\d{5}(-\d{4})?|[A-Z]\d[A-Z] ?\d[A-Z]\d)$

And if you want to get really picky about your Canada codes:

^(\d{5}(-\d{4})?|[A-CEGHJ-NPRSTVXY]\d[A-CEGHJ-NPRSTV-Z] ?\d[A-CEGHJ-NPRSTV-Z]\d)$