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A general description is here.
strsplit() gains a 'useBytes' argument.
The algorithm used by strsplit() has been reordered to batch by elements of 'split': this can be much faster where regexps are used (as multiple compilation is avoided).
A different regular expression engine for basic and extended regexps and also for approximate matching. This based on the TRE library of Ville Laurikari, a modifed copy of which is included in the R sources.
This is often faster, especially in a MBCS locale.
Known differences are that it is less tolerant of invalid inputs in MBCS locales, and conforms more strictly to the POSIX standard in its interpretation of incorrect regexps such as "^*".
This engine may in future be used in 'literal' mode for fixed = TRUE.
[g]sub use a single-pass algorithm instead of matching twice and so is usually faster.
identical() failed to take the encoding of character strings into account, so identical byte patterns are not necessarily identical strings, and similarly Latin-1 and UTF-8 versions of the same string differ in byte pattern.