No, I'm not talking about Dr. Regex' emulated variable-length lookbehinds, which frankly make my head hurt. Beginning with Perl 5.29.9, Perl has honest-to-heaven, really truly variable-length lookbehinds.

Now, there is at least one restriction. No lookbehind assertion can be more than 255 characters long. This limit has been around, as nearly as I can tell, ever since lookaround assertions were introduced in 5.005. But it has been lightly documented until now. This restriction means you can not use …