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PCRE has been upgraded to version 8.21: as well as bug fixes and greater Perl compatibility, this adds a JIT pattern compiler, about which PCRE's news says ‘large performance benefits can be had in many situations’. This is supported on most but not all R platforms.
Function ‘compactPDF()’ in package ‘tools’ now takes the default for argument ‘gs_quality’ from environment variable ‘GS_QUALITY’: there is a new value ‘"none"’, the ultimate default, which prevents GhostScript being used in preference to ‘qpdf’ just because environment variable ‘R_GSCMD’ is set. If ‘R_GSCMD’ is unset or set to ‘""’, the function will try to find a suitable GhostScript executable.