In my package, one of my functions uses
#' Group_by with regex based column selection
#' Similar to `group_by` but grouping columns are selected with a regex.
#' @importFrom dplyr group_by
#' @importFrom base grep
group_at <- function(df, pattern)
group_by_(df, .dots=grep(names(df), pattern=pattern, value=T))
From the Writing R Extensions manual sec.1.1.3:
It makes no sense to declare a dependence on R without a version specification, nor on the package base: this is an R package and package base is always available.