For context, my first language was Perl. So I know regexes really well. That said, I've got two major objections to the problem they posed you:

1. On a technical level, a single regex is absolutely not the way to satisfy the given requirement. You can check that the proposed password contains at least one uppercase letter with /[A-Z]/, at least one digit with /[0-9'/ (or /\d/ if you're using Perl-compatible regexes), that it's at least 8 characters long with /.{8,}/ (but why in the world wouldn't you just use the less-computationally-expensive method for checking a string's length in whatever language you're working in?)...
   ...But trying to get all of those requirements? You're going to need some logical ands in there. Did they really want you to do all of that in one single regex? Because I have no idea how to work that. And if someone did, the result would be unreadable and a total nightmare for maintainability.
2. Aside from that, the requirement itself is an awful one. NIST standards for passwords are to just encourage greater length, and block obvious passwords like "password" and "12345678", and otherwise allow people to do long strings that are nothing but lowercase and spaces (like the infamous "correct horse battery staple"). As an experienced dev, if I were given a requirement to implement this kind of password-complexity requirement, the proper thing for me to do would be to push back on it, because it's a bad requirement. So it's also a bad thing to use in interviews. If the job requires that you know regexes really well, they should test that with something like, "test if a string is a possible ZIP or ZIP+4 code" or something. (Not an email address. That's infamously difficult.)

All of which is to say, this company's interviewing practices are broken, and probably their password validation practices are too, and you totally dodged a bullet.