Why does this expression parse? '4 days 10:10:21'

[Erotemic]

I use pint to parse simple time-delta patterns, and in my code I made the (wrong) assumption that it will error on weirder input that isn't of the form .

However when I run

import pint
ureg = pint.UnitRegistry()
ureg.parse_expression('4 days 10:10:21')

It returns 8400 <Unit('day')>, which is 8400 = 21 * 10 * 10 * 4, so it makes some sense in that it is multiplying all of the numbers, but why isn't it choking on those colons?

This expression is supposed to represent 4 days, 10 hours, 10 minutes, and 21 seconds, although I don't expect pint to parse that, but I did expect that it wouldn't accept it. I'm curious what parser rules are allowing this to pass, and if they were made more strict would that break other use cases?