Custom REGEXP Validation

I need help with a custom REGEXP validation requiring users to enter a valid U.S.A. Postal (ZIP) Code when they register. I've tried:
'/ \d{5}(?\d{4})? / i'.

Thanks in advance!

If that's the exact REGEXP you're using remove the spaces and the single quotes. You can use the below site to help create and test REGEXP .

regex101.com/

It needs to be a valid REGEXP in PCRE2 (PHP) and ECMAScript (JavaScript). There's also a library of already made REGEXP that you can search below.

regex101.com/library
regexlib.com/

Thank you very, very much, Kyle!