As others have already pointed out, it's quite clear that POSIX EREs do not support back-references.
The rationale given in the OpenGroup Base Specifications Issue 7 for not adding back-references to EREs is given as:
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.
Quoted from: Rationale: Base Definitions: Extended Regular Expressions
The primary reason for this limitation is to allow POSIX EREs to be converted to a deterministic finite automata (DFA), and indeed the original implementation of EREs in Unix was done as a DFA. The use of a DFA allows guarantees to be made about the performance of the implementation. Pattern matching with (an unbounded number of) back references is an NP-hard problem, and perhaps even an NP-complete problem. Consensus in the POSIX standards committee could never have been reached if back-references were proposed for EREs because that would force all the companies using the original Unix implementation to change their code to a non-deterministic implementation and to drop their performance guarantees, and some of those companies had members on the committee.
It has also been noted that back-references in REs are not intuitive for either users or implementors, and indeed they've caused extreme confusion more often than now. See for example the examples given in RE-Interpretation: The Dark Corners
NOTE: back-references in REs are not the same as references to sub-patterns in substitution text in tools such as sed.
