(undef, $userid) = split(qr{(?:/|\\)}, $ENV{'REMOTE_USER'});

##</code><code>##

# Alternation with non-capturing clustering (?:foo)
(undef, $userid) = split(qr{(?:/|\\)}, $ENV{'REMOTE_USER'});

# Alternation without clustering
(undef, $userid) = split(qr{\\|/}, $ENV{'REMOTE_USER'});

# Character class including '\' and '/'
(undef, $userid) = split(qr{[\\/]}, $ENV{'REMOTE_USER'});