[Aloso]

(Author of rulex here)

It's a middle ground, actually: When compiling a rulex, you specify the target regex flavor. Some features (e.g. lookaround and backreferences) aren't supported in every flavor. Some features are polyfilled by rulex, so they work even when the targeted regex engine doesn't. However, there are also features supported by PCRE that aren't available in rulex yet. Notably, atomic groups, recursion, character class intersection/subtraction and some mode modifiers aren't supported yet. Eventually I'd like to support all of PCRE's features.