```ruby
# frozen_string_literal: true

require "abstract_unit"
require "active_support/core_ext/regexp"

class RegexpExtAccessTests < ActiveSupport::TestCase
  def test_multiline
    assert_equal true, //m.multiline?
    assert_equal false, //.multiline?
    assert_equal false, /(?m:)/.multiline?
  end

  # Based on https://github.com/ruby/ruby/blob/trunk/test/ruby/test_regexp.rb.
  def test_match_p
    /back(...)/ =~ "backref"
    # must match here, but not in a separate method, e.g., assert_send,
    # to check if $~ is affected or not.
    assert_equal false, //.match?(nil)
    assert_equal true, //.match?("")
    assert_equal true, /.../.match?(:abc)
    assert_raise(TypeError) { /.../.match?(Object.new) }
    assert_equal true, /b/.match?("abc")
    assert_equal true, /b/.match?("abc", 1)
    assert_equal true, /../.match?("abc", 1)
    assert_equal true, /../.match?("abc", -2)
    assert_equal false, /../.match?("abc", -4)
    assert_equal false, /../.match?("abc", 4)
    assert_equal true, /\z/.match?("")
    assert_equal true, /\z/.match?("abc")
    assert_equal true, /R.../.match?("Ruby")
    assert_equal false, /R.../.match?("Ruby", 1)
    assert_equal false, /P.../.match?("Ruby")
    assert_equal "backref", $&
    assert_equal "ref", $1
  end
end
```

Remove `test_match_p` since Rails 6 requires Ruby 2.4.1 or newer
user
```ruby
# frozen_string_literal: true

require "abstract_unit"
require "active_support/core_ext/regexp"

class RegexpExtAccessTests < ActiveSupport::TestCase
  def test_multiline
    assert_equal true, //m.multiline?
    assert_equal false, //.multiline?
    assert_equal false, /(?m:)/.multiline?
  end
end
```
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