Backreferencing inside of a regex (which is a way to find repeat occurrences of a substring) is a very controversial feature that some regex engines intentionally omit, while most others support it only grudgingly. Supporting backreferences opens up a big can of worms: Most regex engines will fall back to a slow, poorly optimised algorithm if they ever encounter backreferences.

Also, it's important for people not to confuse backreferences with capture groups. They look a bit similar, but capture groups are actually fine.

In fact, backreferencing is such a footgun that I don't see it as a possible regex feature. It's a PCRE feature - perhaps even the core distinguishing feature. While all regexes are PCREs, not all PCREs are regexes. It's simply not possible to create a consistently performant implementation of PCRE.