Regular expression pattern matching of things like C<qr/[^I<a>]/> is
significantly sped up, where I<a> is any ASCII character.  Other classes
can get this speed up, but which ones is complicated and depends on the
underlying bit patterns of those characters, so differs between ASCII
and EBCDIC platforms, but all case pairs, like C<qr/[Gg]/> are included,
as is C<[^01]>.