Construct the regex pattern for DATE_PERIOD values
reg_date_period(flatten = TRUE)
A logical value which determines whether a single regex string should be returned (flatten = TRUE) or if a vector of them should be returned (flatten = FALSE). The vector output is used if the regexes need to be combined with other regexes. If they do not, then they are anchored with ^ and $ and separated with | (OR).
Either a single regex string or a vector of them