From an LRUG thread. See https://gist.github.com/knaveofdiamonds/7155189

letter_match.rb

module Refines
  refine Array do
    def strictly_increasing?
      each_cons(2).all? {|x, y| x < y }
    end
  end

  refine String do
    def char_indexes
      each_char
        .with_index
        .with_object(Hash.new { |h, k| h[k] = [] }) { |(char, idx), hsh|
          hsh[char] << idx
        }
    end
  end
end

using Refines

def letter_match(target, str)
  first_set, *rest = target.char_indexes.values_at(*str.split(//))

  first_set.product(*rest).select { |arr| arr.strictly_increasing? }
end

describe "letter_match" do

  let(:a) { "hello world" }

  specify "single match" do
    expect(letter_match(a, "e")).to eq [[1]]
  end

  specify "single letter with multiple matches" do
    expect(letter_match(a, "l")).to eq [[2],[3],[9]]
  end

  specify "multiple matches, example 1" do
    expect(letter_match(a, "el")).to eq [[1,2], [1,3], [1,9]]
  end

  specify "multiple matches, example 2" do
    expect(letter_match(a, "lo")).to eq [[2,4], [2,7], [3,4], [3,7]]
  end

  specify "multiple matches, example 3" do
    expect(letter_match(a, "lod")).to eq [[2,4,10], [2,7,10], [3,4,10], [3,7,10]]
  end
end

# $ rspec ordered_matches_lrug.rb 
# /home/luca/Dropbox/snippets/ordered_matches_lrug.rb:2: warning: Refinements are experimental, and the behavior may change in future versions of Ruby!
# .....
#
# Finished in 0.00109 seconds
# 5 examples, 0 failures