help with regular expression

[John Livermore]

I am not sure if this is the correct newsgroup, so if this post
belongs somewhere else, I would appreciate the pointer.

Here's my question:

I have the following regular expression I use to match against user
entered US zip codes. However, no entry is valid as well (the zip
code can be blank). Can I specify that with one regular expression?

Here is the one I am using to validate zip codes:

\d{5}(-\d{4})?