Rdoc is here:

~/NetBeansProjects/jruby $ ri MatchData#select
------------------------------------------------------- MatchData#select
      mtch.select([index]*)   => array
------------------------------------------------------------------------
      Uses each _index_ to access the matching values, returning an array
      of the corresponding matches.

         m = /(.)(.)(\d+)(\d)/.match("THX1138: The Movie")
         m.to_a               #=> ["HX1138", "H", "X", "113", "8"]
         m.select(0, 2, -2)   #=> ["HX1138", "X", "113"]

But the example given doesn't run, and the arity of select is -1 instead 
of zero as would be indicated by the resulting error.

- Charlie