'20131007'.cut 3,5 => ['2013','10','07']

gistfile1.rb

require 'test/unit'

class String
    def cut *arg 
        raise TypeError unless arg.is_a? Array and arg.first.is_a? Fixnum
        start = 0 
        arg.map do |n| 
          r = start..n
          start = n + 1 
          r   
        end.push(start..-1).map do |r| 
          self[r]
        end 
    end 
end

class TestString < Test::Unit::TestCase
  def test_cut
    substrs = '20131007'.cut 3,5 
    assert_equal ['2013','10','07'],substrs
  end 
end