gsub forwarding with $1

gistfile1.rb

class X < String
  def evil_call(meth, args, block, &wrap_block)
    block.binding.eval(<<-END, __FILE__, __LINE__+1).call(to_str, meth, args, wrap_block)
      lambda do |str, meth, args, block|
        str.send(meth, *args, &block)
      end
    END
  end

  def gsub(*args, &block)
    if block
      X.new(evil_call(:gsub, args, block) {|*x| "<#{block.(*x).downcase}>" })
    else
      X.new(to_str.gsub(*args))
    end
  end
end

require 'minitest/autorun'
class EvilTest < Minitest::Test
  def test_return_type
    assert_equal X, (X.new.gsub('a', 'b')).class
    assert_equal X, (X.new.gsub('a') { 'b' }).class
  end

  def test_normal_string
    assert_equal 'XboXb', 'bob'.gsub(/(b)/) { "X#$1" }
  end

  def test_evil_string
    assert_equal '<xb>o<xb>', X.new('bob').gsub(/(b)/) { "X#$1" }
  end
end