No, that doesn't work. 
$users_regexp = qr/^./s;
'abc,v' = /$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.

[download]

Solution: 

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

[download]
which simplifies to 
/^(?!.*,v$).*?$users_regex/s

[download]

Of course, this is not nearly as readable as the original.

Updated to show more examples where the parent doesn't work.

In reply to Re^2: RegEx - match pattern not followed by literal by ikegami
in thread RegEx - match pattern not followed by literal by Melly

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