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A general description is here.
textConnection() gains an 'encoding' argument to determine how input strings with marked encodings will be handled.
"\uxxxx" and "\Uxxxxxxxx" escapes can now be parsed to a UTF-8 encoded string even in non-UTF-8 locales (this has been implemented on Windows since R 2.7.0). (Use of such escapes requires Unicode wide characterThe semantics have been changed slightly: a string containing such escapes is always stored in UTF-8 (and hence is suitable for portably including Unicode text in packages). Use of such escape requires MBCS support.)