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A general description is here.
‘grep(perl = TRUE)’ and friends can now make use of PCRE's Just-In-Time mechanism, for PCRE >= 8.20 on platforms where JIT is supported. It is used by default whenever the ‘pattern’ is studied (see the previous item). (Based on a patch from Mikko Korpela.)
This is controlled by a new option ‘PCRE_use_JIT’.
Note that in general this makes little difference to the speed, and may take a little longer: its benefits are most evident on strings of thousands of characters. As a side effect it reduces the chances of C stack overflow in the PCRE library on very long strings (millions of characters, but see next item).
There is a new option ‘PCRE_limit_recursion’ for ‘grep(perl = TRUE)’ and friends to set a recursion limit taking into account R's estimate of the remaining C stack space (or 10000 if that is not available). This reduces the chance of C stack overflow, but because it is conservative may report a non-match with a warning in examples that matched before. By default it is enabled if any input string has at least 1000 bytes. (PR#16757)
The ‘deriv()’ and similar functions now can compute derivatives of ‘log1p()’, ‘sinpi()’ and similar one-argument functions, thanks to a contribution by Jerry Lewis.