How to get possibly overlapping matches in a string

In Ruby you could achieve the expected result using:

str = "abcadc"
[/(a[^c]*c)/, /(a.*c)/].flat_map{ |pattern| str.scan(pattern) }.reduce(:+)
# => ["abc", "adc", "abcadc"]

Whether this way works for you is highly dependent on what you really want to achieve.

I tried to put this into a single expression but I couldn't make it work. I would really like to know if there is some scientific reason this cannot be parsed by regular expressions or if I just don't know enough about Ruby's parser Oniguruma to do it.

def matching_substrings(string, regex)
  string.size.times.each_with_object([]) do |start_index, maching_substrings|
    start_index.upto(string.size.pred) do |end_index|
      substring = string[start_index..end_index]
      maching_substrings.push(substring) if substring =~ /^#{regex}$/
    end
  end
end

matching_substrings('abcadc', /a.*c/) # => ["abc", "abcadc", "adc"]
matching_substrings('foobarfoo', /(\w+).*\1/) 
  # => ["foobarf",
  #     "foobarfo",
  #     "foobarfoo",
  #     "oo",
  #     "oobarfo",
  #     "oobarfoo",
  #     "obarfo",
  #     "obarfoo",
  #     "oo"]
matching_substrings('why is this downvoted?', /why.*/)
  # => ["why",
  #     "why ",
  #     "why i",
  #     "why is",
  #     "why is ",
  #     "why is t",
  #     "why is th",
  #     "why is thi",
  #     "why is this",
  #     "why is this ",
  #     "why is this d",
  #     "why is this do",
  #     "why is this dow",
  #     "why is this down",
  #     "why is this downv",
  #     "why is this downvo",
  #     "why is this downvot",
  #     "why is this downvote",
  #     "why is this downvoted",
  #     "why is this downvoted?"]