You are right indeed. Looked at the specification again and indeed there is no back-reference in POSIX ERE.

Quoting from <https://pubs.opengroup.org/onlinepubs/9699919799.2008edition...>:

> It was suggested that, in addition to interval expressions, back-references ( '\n' ) should also be added to EREs. This was rejected by the standard developers as likely to decrease consensus.

Updated my comment to present a better example that avoids back-references. Thanks!

That’s because POSIX EREs are actual regular expressions thank god.