regChars returns a filter function that tells if a char is a member
of the regChar expression or not. The regChar expression is basically a
set of chars, but it can contain ranges with use of the
- (dash), and
it can also be specified as a complement set by prefixing with
(caret). The dash and caret, as well as the backslash, can all be
escaped with a backslash to suppress their special meaning.
. (dot) is allowed to be escaped. It has no special meaning
if it is not escaped, but the default
Darcs.Commands.Replace uses an escaped dot (WHY?).