To strictly answer you question:
^(123-45-6789|XXX-XX-XXXX)$
should work. 😉
If you read the section “Valid SSNs” on Wikipedia`s SSN article then it becomes clear that a regex for SSN validation is a bit more complicated.
Accordingly a little bit more accurate pure SSN regex would look like this:
^(?!(000|666|9))\d{3}-(?!00)\d{2}-(?!0000)\d{4}$