Use case insensitive matching sparingly.
Use case insensitive matching sparingly.

When compiled under debug mode, regex compilation of case folded
character classes can be quite slow. I've patched `regex` to be a
bit faster, but it's not quite fast enough.

As a work-around, only use case insensitivity on literals. Leave
the character classes alone.

zsiciarz/docopt.rs,brandonson/docopt.rs,zsiciarz/docopt.rs,zsiciarz/docopt.rs,birkenfeld/docopt.rs,birkenfeld/docopt.rs,brandonson/docopt.rs,brandonson/docopt.rs,birkenfeld/docopt.rs