STRING:
vvvvvvvvvvvv0wwwwwwwwww

REGEX:
(?<a>(?:v)?)
(?<b>(?:vvvvvvvvvvv)?)
0
(?{ print "trying " . Dumper \%+})
(?=
(?(?{ $1 })ww|)
(?(?{ $2 })wwwwwwwww|)
)

OUTPUT:
trying $VAR1 = {
          'a' => 'v',
          'b' => 'vvvvvvvvvvv'
        };
trying $VAR1 = {
          'a' => '',
          'b' => 'vvvvvvvvvvv'
        };