to line things up

^\({0,1}
   (
    ((ix)|(iv))
    |(x{0,3}((ix)|(iv)))
    |(x{0,3}(v{0,1}i{0,3}))
   ){1,1}(
          (\)\. )
          |(\))
          |(\. )
          |( )
          ){1,1}
[download]

|(x{0,3}(v{0,1}i{0,3})) matches nothing (the zeros) 1 times

Had to look hard

Edit: see also http://stackoverflow.com/questions/267399/how-do-you-match-only-valid-roman-numerals-with-a-regular-expression