# The following small library lets us create regex-matchers
    # and inverted regex-matchers.

    sub make_regex_matcher {
        my $regex = shift;
        return sub {
            local $_ = $_[0];
            /$regex/g;
        }
    }

    sub invert_regex_matcher {
        my $matcher = shift;
        sub { wantarray
              ? die "inverted matchers are only for scalar context"
              : ! $matcher->(@_)
        }
    }