library(magrittr)

gtsummary::tbl_summary(mtcars, statistic = all_continuous() ~ "\\makecell{{{mean}}}")
#> Error in `gtsummary::tbl_summary()`:
#> ! Error processing `statistic` argument.
#> ! Caused by error in `all_continuous()`: ! could not find function
#>   "all_continuous"
#> ℹ Select among columns "mpg", "cyl", "disp", "hp", "drat", "wt", "qsec", "vs",
#>   "am", "gear", and "carb"

.extract_glue_elements <- function(x) {
  regmatches(x, gregexpr("\\{([^\\}]*)\\}", x)) |>
    unlist() %>%
    {substr(., 2, nchar(.) - 1)} # styler: off
}

.extract_glue_elements("\\makecell{mean}")     # should return `c('mean')`
#> [1] "mean"
.extract_glue_elements("\\makecell{{mean}}")   # should return `character(0L)`
#> [1] "{mean"
.extract_glue_elements("\\makecell{{{mean}}}") # should return `c('mean')`
#> [1] "{{mean"

# these will eventually be processed with glue()
glue::glue("{letters[1]}")
#> a
glue::glue("{{letters[1]}}")
#> {letters[1]}
glue::glue("{{{letters[1]}}}")
#> {a}