We were talking about EREs, which are an artifact of POSIX, not UTS#18. So the relevant standard for this specific conversation is POSIX. To redirect to UTS#18

We were talking about EREs, which are an artifact of POSIX, not UTS#18. So the relevant standard for this specific conversation is POSIX. To redirect to UTS#18, I don't think UTS#18 subsumes POSIX. UTS#18 doesn't support [[=a=]] for example AFAIK. And UTS#18 more generally doesn't require locale support. UTS#18 Level 3 was actually removed from the spec, which is where "custom tailored" logic for specific locales used to live. On top of that, POSIX also specifies BREs which UTS#18 doesn't touch. So while there is overlap between POSIX and UTS#18, POSIX is not a strict subset of UTS#18. If you're speaking "conceptually" and less precisely, you can maybe say POSIX is subsumed by UTS#18 though. I don't really think about it that way though personally. They serve two different use cases that are still relevant today. I think UTS#18 is a tortured document, but yes, the regex crate supports pretty much all of UTS#18 Level 1: https://github.com/rust-lang/regex/blob/master/UNICODE.md https://github.com/rust-lang/regex/blob/master/UNICODE.md Going beyond Level 1 is difficult.