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A general description is here.
On platforms using ‘configure’ option ‘---with-internal-tzcode’, additional values ‘"internal"’ and (on macOS only) ‘"macOS"’ are accepted for the environment variable ‘TZDIR’.
On macOS, ‘"macOS"’ is used by default if the system timezone database is a newer version than that in the R installation.
The code mitigating stack overflow with PCRE regexps on very long strings is enabled for PCRE2 < 10.30 also when JIT is enabled, since stack overflows have been seen in that case.