unit_from_string produces some unexpected results when parsing strings with periods or dashes. E.g. all of the following are equivalent to "m*s":

"m.s"
"m. s"
"m.s."
"m. s."
"m- s"

But spacing matters and "m-s" is parsed as milliseconds.

This came up for us with "a.u." which was intended as 'arbitrary units' but was parsed as "1.66054e-25*m**2*kg**1".

Is it a design choice to interpret . as * and drop - when there are no numbers?