Sorry, I can't answer that without fuller definitions of "legitimate" and "cannot be better expressed" - which does my previous example fall foul of?

Any pattern that repeats a "non-simple" expression will consume C-stack space on each repetition. At least 3 of the 4 bugs linked to the metabug #24274 arose from people solving real world tasks, and there are more in the bugs database that should also be linked to the metabug - more recent ones often involve searching for particular structures in a genome.

Here's another common fragment that invokes the problem:

  $_ = sprintf q{"%s"}, "a" x 32768;
  /"((?:\\.|[^"])+)"/;
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Hugo