Despite widespread use, the complexity of the matching problem for modern regular expressions languages remains unclear. Previous work proved that an idealized regular expression language with backreferences and lookarounds had PSPACE-complete matching. We extend this work to a real-world regex language by proving that JavaScript regex matching with expanded lower-bounded quantifiers is PSPACE-complete. We then generalize the result: we show that PSPACE-hardness survives the removal of negative lookarounds, and that removing all lookarounds leads to an OptP-complete parsing problem. Our core arguments are formalized in Rocq.

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