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A general description is here.
There is a new utility program untgz.exe to be used by untar() when 'tar' is not available.
New function untar() to list or unpack tar archives, possibly compressed.
available.packages() by default only returns information on the latest versions of packages whose version requirements are satisified by the currently running R: use 'duplicates = TRUE' to return information on all available versions.
Basic regular expressions (extended = FALSE) are deprecated in strsplit, grep and friends. There is a precise POSIX standard for them, but it is not what recent RE engines implement, and it seems that in almost all cases package authors intended fixed = TRUE when using extended = FALSE.