I direct your attention to perlre, the section named Repeated patterns matching zero-length substring. This complicated section can be summarized by this extract: "the following match after a zero-length match is prohibited to have a length of zero."

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Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a (from-home) job
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;