>>338

irb(main):001:0> xs = "121112111"
=> "121112111"
irb(main):002:0> (0..(xs.size - 1)).map { |i| xs[i, xs.size - i].scan /1+21+/ }.flatten.uniq
=> ["12111", "1112111", "112111"]
irb(main):003:0>