I don't think "alphanumeric sort" means what you think it means.
In any case, looks like you want mixedsort, part of gtools.
> install.packages('gtools')
[...]
> require('gtools')
Loading required package: gtools
> n
[1] "abc21" "abc2" "abc1" "abc01" "abc4" "abc201" "1b" "1a"
> mixedsort(n)
[1] "1a" "1b" "abc1" "abc01" "abc2" "abc4" "abc21" "abc201"
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