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A general description is here.
agrep() now makes use of the TRE library of Ville Laurikari rather than apse, and so has full support for MBCS locales (and enables help.search() to use approximate matching characterwise rather than bytewise).
There is a different regular expression engine for basic and extended regexps, based on the TRE library. This is often faster, especially in a MBCS locale, and it allows us to implement the same set of options as for fixed = TRUE and perl = TRUE, including 'useBytes' and full support for UTF-8 encoded strings in all locales. [Currently experimental, selected by the configure option --with-TRE.]