Is a value a valid new site number? i.e. 5 capital letters followed by 4 digits

isNewSiteNum.R

isNewSiteNum <- function(x) {
  
  # returns true if value is missing, or a site number i.e. 5 capitals follows by 4 digits
  # otherwise returns false
  
  if(is.na(x)) return(TRUE)
  if(!is.character(x)) return(FALSE)
  
  if(str_count(x) != 9) return(FALSE)
  str_detect(x, "^[A-Z]{5}[:digit:]{4}") # 5 capitals followed by 4 digits
  
}