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A general description is here.
PCRE has been upgraded to version 8.20 with 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’. (It is supported on most but not all platforms that run R.)
‘summary()’ behaves slightly differently (or more precisely, its ‘print()’ method does). For numeric inputs, the number of NAs is printed as an integer and not a real. For dates and datetimes, the number of NAs is included in the printed output (the latter being the wish of PR#14720).
The ‘"data.frame"’ method is more consistent with the default method: in particular it now applies ‘zapsmall()’ to numeric/complex summaries.
Where multiple sub-architectures are to be tested, ‘R CMD check’ now runs the examples and tests for all the sub-architectures even if one fails.
Use of ‘library.dynam()’ without specifying all the first three arguments is now disallowed.
Use of an argument ‘chname’ in ‘library.dynam()’ including the extension ‘.so’ or ‘.dll’ (which was never allowed according to the help page) is defunct. This also applies to ‘library.dynam.unload()’ and to ‘useDynLib’ directives in ‘NAMESPACE’ files.
The internal functions (but still used by some packages) ‘.readRDS()’ and ‘.saveRDS()’ are now defunct.
The ‘quartz()’ device now does tilde expansion on its ‘file’ argument.
‘read.dcf()’ removes trailing blanks: unfortunately on some platforms this included ‘\xa0’ (non-breaking space) which is the trailing byte of a UTF-8 character. It now only considers ASCII space and tab to be ‘blank’.