You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: test_regexp.rb
    path of file: ./repos/artichoke/artichoke-backend/vendor/ruby/test/ruby
    
    the code of the file until where you have to start completion: # coding: US-ASCII
# frozen_string_literal: false
require 'test/unit'

c
    
  def test_to_s
    assert_equal '(?-mix:\x00)', Regexp.new("\0").to_s

    str = "abcd\u3042"
    [:UTF_16BE, :UTF_16LE, :UTF_32BE, :UTF_32LE].each do |es|
      enc = Encoding.const_get(es)
      rs = Regexp.new(str.encode(enc)).to_s
      assert_equal("(?-mix:abcd\u3042)".encode(enc), rs)
      assert_equal(enc, rs.encoding)
    end
  end