darcs-2.16.1: a distributed, interactive, smart revision control system

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.
NOTE: The . (dot) is allowed to be escaped. It has no special meaning if it is not escaped, but the default filename_toks in Darcs.Commands.Replace uses an escaped dot (WHY?).