I found the following RegEx string to validate a zip code: "^\d{5}(-\d{4})?$"

The following two string pass the test: 90210, 90210-1234. However, I need to make it accept the following string: 90210-.

Any help would be greatly appreciated. From the little I know about regular expressions, I think there needs to be another optional clause added. Thanks!!!

Edit:
I changed it to the following and it seems to work: "^\d{5}(-)?(-\d{4})?$". Can somebody validate that I didn't do something silly? Thanks!