Invalid oniguruma nested lookarounds

[RedCMD]

Oniguruma does not support every combination of nested lookarounds

(?=), (?!), (?<!) and (?~) are not valid inside a lookbehind (?<=)

(?=), (?!) and () capture groups are not valid inside a negative lookbehind (?<!)

interesting JS does allow them

should be invalid

[slevithan]

Thanks for the report/details!

This is currently low priority since the JS handling is more permissive, so all valid Oniguruma lookbehinds should be handled correctly. But I agree it would be good to error in the same cases that Oniguruma would.

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^{Note to self: Previously removed code in 696dcab might be adaptable for the new checks.}

[slevithan]

low priority

I take this back, since using oniguruma-to-es to identify invalid Oniguruma regexes (as opposed to converting known-valid Oniguruma regexes) is an important use case that this library has always supported.

(?=), (?!), (?<!) and (?~) are not valid inside a lookbehind (?<=)

Re: (?~), only the absent stopper (?~|…) and range clear (?~|) forms are invalid within lookbehind (noted in the Oniguruma docs). Absent repeaters (?~…) and absent expressions (?~|…|…) are allowed within lookbehind (noted in #13 [for supporting absence operators], and I've just verified in Oniguruma).