gistfile1.rb

 class String
       def do_certain_parts_contain_a_number?
           self.match /regex/ do |match|
               'its a number!' if match.contains_a_number? # HOW TO I MOCK contains_a_number? HERE?!
           end
       end
    end
 
    RSpec.decribe String do
        describe '#do_certain_parts_contain_a_number?'
            it 'returns ' do
                expect{ match }.to_receive(:contains_a_number?).and_return true # OR SOMETHING?!
                expect{ 'acertainstring43'.do_certain_parts_contain_a_number? }.to eq 'its a number!'
            end
        end
    end
 
 
     NoMethodError:
       undefined method `contains_a_number?' for "acertainstring43":String