gistfile1.rb

require 'awesome_print'

hash = {
  "key_one" => {
    "sub_key_one_a" => {
      "sub_sub_key_one_a" => "hello I'm sub sub key one a"
      },
    "sub_key_one_b" => {
      "sub_sub_key_one_b" => "hello I'm sub sub key one b", 
      "sub_sub_sub_key_one_b" => {
        "sub_sub_sub_sub_key_one_b" => "hello I'm deep man."
      }
      }
  }, 
  "key_two" => {
    "sub_key_two" => {
      "sub_sub_key_two" => "hello I'm sub sub key two"
      } 
  },
  "key_three" => {
    "sub_key_three_a" => [
      {
        "sub_sub_key_three_a" => "hello from sub sub key three a",
        "sub_sub_key_three_b" => "hello from sub sub key three b",
        "sub_sub_key_three_c" => "hello from sub sub key three c"
      },
      "sub_key_three_a_a" => [
        "sub_sub_key_three_a_a" => "hello from sub sub key three a a",        
      ]
    ]
  },
  "key_four" => [
    {
      "sub_key_four_a" => [
        {
          "sub_sub_key_four_a" => "hello from sub sub key four a",
          "sub_sub_key_four_b" => "hello from sub sub key four b",
          "sub_sub_key_four_c" => "hello from sub sub key four c"
        },
        "sub_key_four_a_a" => [
          "sub_sub_key_four_a_a" => "hello from sub sub key four a a",        
        ]
      ]
    }
  ]
}

def find_key(node, search="")
  if node.is_a?(Hash)
    node.select do |k, v|
      @key = search if k == search
      find_key( node[k], search )
    end
  elsif node.is_a?(Array)
    node.each do |elm|
       if elm[search] == search && !search.empty?
         @key = elm
       else
         result = find_key(elm, search)
         @key = result if result
       end
     end
  elsif node.is_a?(String)
    # do nothing
  end
  @key
end

ap find_key(hash, "key_one") == "key_one"
ap find_key(hash, "sub_key_one_a") == "sub_key_one_a"
ap find_key(hash, "sub_sub_key_one_a") == "sub_sub_key_one_a"
ap find_key(hash, "sub_key_one_b") == "sub_key_one_b"
ap find_key(hash, "sub_sub_key_one_b") == "sub_sub_key_one_b"
ap find_key(hash, "key_two") == "key_two"
ap find_key(hash, "sub_key_two") == "sub_key_two"
ap find_key(hash, "sub_sub_key_two") == "sub_sub_key_two"
ap find_key(hash, "sub_sub_sub_key_one_b") == "sub_sub_sub_key_one_b"
ap find_key(hash, "sub_sub_sub_sub_key_one_b") == "sub_sub_sub_sub_key_one_b"
ap find_key(hash, "sub_sub_key_three_a") == "sub_sub_key_three_a"
ap find_key(hash, "sub_sub_key_three_b") == "sub_sub_key_three_b"
ap find_key(hash, "sub_sub_key_three_c") == "sub_sub_key_three_c"
ap find_key(hash, "sub_sub_key_three_a_a") == "sub_sub_key_three_a_a"
ap find_key(hash, "sub_key_four_a_a") == "sub_key_four_a_a"
ap find_key(hash, "sub_sub_key_four_a_a") == "sub_sub_key_four_a_a"