Abundant regex: Decode next smaller P instead of last P; length-=5

This results in a fairly big slowdown.
Abundant regex: Improve on the previous length optimization; length-=2

The results in a huge slowdown, much bigger than the previous commit.
Abundant regex: Remove an unnecessary test; length-=9
Abundant regex: Add a comment explaining the previous changes a bit better
Abundant regex: Apply an optimization related to the previous changes; length-=7

This results in a speed-up, making the regex faster than it was in commit f014389a123835b585a8a9649a203ceca6396d1c, but still slower than it was in commit 7aaf9f755b6b48f710e147f44fc88e61e61cf58c.
Abundant regex: Apply an optimization related to the previous changes; length-=1
Abundant regex: Fix a comment
Abundant regex: Remove unnecessary prime test; length-=13
[empty commit to get the merge to work]
Abundant regex: Rename this version to reflect its slowness
Merge branch 'slow'