The warning comes from the fact that Perl can't tell if (?{ ... }) matches any characters or not, so it's possible you're putting a quantifier on a zero-width assertion.
Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart

In reply to Re^3: Using a capture in /(?{...})/ by japhy
in thread Using a capture in /(?{...})/ by ikegami

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