$users_regexp = qr/^./s;
'abc,v' = /$users_regex(?!,v$)/;  # Matches, but it shouldn't.

$users_regexp = qr/^.*$/s;
'abc,v' = /$users_regex(?!,v$)/;  # Matches, but it shouldn't.

$users_regexp = qr/,/;
'abc,v' = /$users_regex(?!,v$)/;  # Matches, but it shouldn't.

##</code><code>##

/^(?=.*?$users_regex)(?!.*?,v$)/s

##</code><code>##

/^(?!.*,v$).*?$users_regex/s