You should probably point out that your expression with match simple
numbers such as '1', '12', '76', '45', and so on. It will also match
any trailing space along with those numbers. For example, '69 '
might get matched.

The space should be allowed only when noon/am/pm is encountered. If one
of these are not encountered than the :\d{2} should be required. This
should be a simple fix.

-Robert