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A general description is here.
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’.
Packages which define S4 generics for ‘plot()’ should be re-installed and package code using such generics from other packages needs to ensure that they are imported rather than rely on their being found on the search path (as in a namespace, the base namespace has priority over the search path).
‘.Machine’ now contains new ‘longdouble.*’ entries (when R internally uses long double).