parse_arg

# File lib/optparse.rb, line 324
    def parse_arg(arg)
      pattern or return nil, [arg]
      unless m = pattern.match(arg)
        yield(InvalidArgument, arg)
        return arg, []
      end
      if String === m
        m = [s = m]
      else
        m = m.to_a
        s = m[0]
        return nil, m unless String === s
      end
      raise InvalidArgument, arg unless arg.rindex(s, 0)
      return nil, m if s.length == arg.length
      yield(InvalidArgument, arg) # didn't match whole arg
      return arg[s.length..-1], m
    end