The previous regex was using the [ABC..D]*[ABC] pattern, resulting in a lot of
backtracking. The new regex is stopping the matching at the first space or end
of text (and removes the trailing `.` should one be present).
The backtracking was taking around 50% of the CPU time spent in atom.Parse
- Simplify a switch-case by moving a common condition above it.
- Remove a superfluous error-check: `strconv.ParseInt` returns `0` when passed
an empty string.
