character length two; default c("~", "~"); space separator between variable and units, and space separator between two different units
group
character length two; start and end group, may be two empty strings, a parenthesis pair, or square brackets.
negative_power
logical, default FALSE; should denominators have negative power, or follow a division symbol?
parse
logical, default TRUE; should the units be made into an expression (so we get subscripts)? Setting to FALSE may be useful if parse fails, e.g. if the unit contains symbols that assume a particular encoding