Ah ok. Im not familiar with the finer points of the POSIX engine, but PCRE and Perl's engine, and most similar engines are not true regular expression engines and thus benefit *greatly* from atomic matching if it is available. Like the difference between heat-death performance (or stack overflow), and running instantly.
You can almost always fix the regex to avoid this, by ensuring that whenever you have (...)+ (or *) a suffix of the subexpression cannot be a prefix of the subexpression too. This is what my patch did -- changing a bad regex to a nicely behaving one.
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