nitpicks
Correct 5.20 regression: '"X" !~ /[x]/i'

This problem occurs only when the pattern is UTF-8, contains a single ASCII
lowercase letter.  It does not match its uppercase counterpart.

For RT #122655

[Note from committer: This is a manual back-port of b6e093f25d
by James E Keenan <jkeen@verizon.net>.]