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A general description is here.
‘grep(perl = TRUE)’ and friends set a maximal recursion limit, taking into account R's estimate of the remaining C stack space. This reduces the chance of C stack overflow, but because it is conservative may return a non-match with a warning in examples that succeeded before. (PR#16757)
The deprecated support for PCRE versions older than 8.20 will be removed in R 3.4.1. (Versions 8.20-8.31 will still be accepted but deprecated.)