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A general description is here.
A number of non-API declarations and macro definitions have been moved from the installed header ‘Rinternals.h’ to the internal header ‘Defn.h’. Packages that only use entry points and definitions documented to be part of the API as specified in ‘Writing R Extensions’ §6 should not be affected.
New function ‘gregexec()’ which generalizes ‘regexec()’ to find _all_ disjoint matches and all substrings corresponding to parenthesized subexpressions of the given regular expression. (Contributed by Brodie Gaslam.)