/^
(?: ' \s* )?
($positive_number){1}+ \s* (?: \( \s* ($stdev) \s* \) )?
\s* ($units_1)?
(
    \s* ($positive_number) \s* (?: \( \s* ($stdev) \s* \) )?
    \s* ($units_2)
)?
(?: \s* ' )?
$/x