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Re^4: (Ab)using the Regex Engine

by jo37 (Pilgrim)
on May 26, 2020 at 17:55 UTC ( #11117297=note: print w/replies, xml ) Need Help??

in reply to Re^3: (Ab)using the Regex Engine
in thread (Ab)using the Regex Engine

Probably my statement in Re^2: (Ab)using the Regex Engine about "use" vs. "abuse" was unclear and I should have quoted the relevant section from perlre:

(*FAIL) (*F) (*FAIL:arg)
This pattern matches nothing and always fails. It can be used to force the engine to backtrack. It is equivalent to (?!), but easier to read. In fact, (?!) gets optimised into (*FAIL) internally. You can provide an argument so that if the match fails because of this FAIL directive the argument can be obtained from $REGERROR. It is probably useful only when combined with (?{}) or (??{}).
My point was that I realized that
(?{CODE})(?!) (?{CODE})(*F)
are documented as equivalently forcing the engine to backtrack and are just what I was looking for. I don't call this "abuse", but YMMV.

The example with a character class was just historical and the one with (??{CODE}) was a result of my own ignorance.



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