brainfreeze.rb

triangular_numbers = Enumerator.new do |yielder| number = 0
  count = 1
  loop do
    number += count
    count  += 1
    yielder.yield number
  end
end

class Enumerator
  def infinite_select(&block)
    Enumerator.new do |yielder| self.each do |value|
      yielder.yield(value) if block.call(value) 
     end
    end
  end
end

p triangular_numbers
   .infinite_select {|val| val % 10 == 0}
   .infinite_select {|val| val.to_s =~ /3/ }
   .first(5)

Here’s an example that returns the first five triangular numbers that are multiples of 10 and that have the digit 3 in them.
Output:
[300, 630, 1830, 3160, 3240]