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A general description is here.
R now builds by default against a PCRE2 library for Perl-like regular expressions if available at build time on non-Windows platforms (otherwise PCRE1 is used as in older versions of R).
PCRE2 is stricter in some cases and reports errors for some regular expressions that were accepted by PCRE1. A hyphen now has to be escaped in a character class to be interpreted as a literal (unless first or last in the class definition). ‘\R’, ‘\B’ and ‘\X’ are no longer allowed in character classes (PCRE1 treated these as literals).
Option ‘PCRE_study’ is no longer used, and reported as ‘FALSE’.
The version of PCRE used can be obtained via ‘extSoftVersion()’, PCRE1 has versions < 10, PCRE2 versions >= 10.
Added an argument ‘add_datalist(*, small.size = 0)’ to allow the creation of a ‘data/datalist’ file even when the total size of the data sets is small.
The deprecated support for specifying C++98 in package installation has been removed.
Certain ‘Ops’ (notably comparison such as ‘==’) now also work for 0-length data frames, after reports by Hilmar Berger.