test_that("regroup_ works", {
    set.seed(1)
    int <- sample(-3:50, 100L, replace = TRUE)
    dates <- as.Date("2018-01-31") + int
    group_1 <- sample(letters[1:3], length(dates), replace = TRUE)
    group_2 <- sample(letters[1:3], length(dates), replace = TRUE)
    dat <- data.frame(dates, group_1, group_2)
    dat$dates <- as_epiweek(dat$dates)
    x <- incidence(dat, date_index = "dates", groups = c("group_1", "group_2"))

    # regroup to know groups
    expected <- incidence(dat, date_index = "dates")
    expect_equal(regroup_(x), expected)

    # regroup to one group
    expected <- incidence(dat, date_index = "dates", groups = "group_1")
    expect_equal(regroup_(x, group_1), expected)

    # no groups
    expected <- incidence(dat, date_index = "dates")
    expect_equal(regroup_(expected), expected)

    # regroup none-incidence object
    expect_error(regroup_("test"), "`x` must be an <incidence2> object.")
})