If you know even a little about how an LLM works it's obvious why regex is basically impossible for it. I suspect perl has similar problems, but no one is capable of actually validating that.
They aren't context aware, it's using statistical probability. It can replicate things it's seen a lot of like a tutorial regex. It can't apply that to make a more complicated one. Regex in the wild isn't really standard at all, because it's rarely used to solve common problems. It has a bunch of random regexs from code it analyzed and will spit something out that looks similar.