Improve Scanno regex & fix bug in S/R replacement

[windymilla]

If regex had nested lookahead/lookbehind at the end of the string, the code failed to remove it properly, so the replacement didn't work.

Also, stealth scanno regexes 18 & 19 were wrong - they matched the correct string, but didn't fix them correctly.

And the original purpose of the work - regex 18 now doesn't match if the line below is actually lowercase markup for poetry, center, etc., e.g. p/

[windymilla]

Testing notes:

Example file:

/p
It totters when. she licks it with her tongue.
It sha'n't be gone long.--You come too.
p/

/r
Robert Frost.
z/

1. In master regexes 18 & 19 will find the incorrect commas/periods, but will not correct them properly
2. In this branch, regex 18 should only find "Frost.", not "too." (because it's followed by p/
3. Also in this branch, the replacement should work