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: gsub_spec.rb
    path of file: ./repos/natalie/spec/core/string
    
    the code of the file until where you have to start completion: # -*- encoding: utf-8 -*-
require_relative '../../spec_helper'
require_relative 'fixtures/classes'

describe :string_gsub_named_capture, shared: true do
  it "replaces \\k named backreferences with the regexp's corresponding capture" do
    str = "hello"

    NATFIXME 'Unknown backslash reference: \k', exception: SpecFailedException do
      str.gsub(/(?<foo>[aeiou])/, '<\k<foo>>').should == "h<e>ll<o>"
      str.gsub(/(?<foo>.)/, '\k<foo>\k<foo>').should == "hheelllloo"
    end
  end
end

describe "String#gsub with pattern and replacement" do
  it "inserts the replacement around every character when the pattern c
    
    def replacement.to_s() "hello_replacement" end

    "hello".gsub(/hello/) { replacement }.should == "hello_replacement"

    obj = mock('ok')
    def obj.to_s() "ok" end

    "hello".gsub(/.+/) { obj }.should == "ok"
  end

  it "uses the compatible encoding if they are compatible" do
    s  = "hello"
    s2 = "#{195.chr}#{192.chr}#{195.chr}"

    NATFIXME 'uses the compatible encoding if they are compatible', exception: SpecFailedException do
      s.gsub(/l/) { |bar| 195.chr }.encoding.should == Encoding::BINARY
      s2.gsub("#{192.chr}") { |bar| "hello" }.encoding.should == Encoding::BINARY
    end
  end

  it "raises an Encoding::CompatibilityError if the encodings are not compatible" do
    s = "hllëllo"
    s2 = "hellö"

    NATFIXME 'Raise Encoding::CompatibilityError', exception: SpecFailedException do
      -> { s.gsub(/l/) { |bar| "Русский".force_encoding("iso-8859-5") } }.should raise_error(Encoding::CompatibilityError)
      -> { s2.gsub(/l/) { |bar| "Русский".force_encoding("iso-8859-5") } }.should raise_error(Encoding::CompatibilityError)
    end
  end

  it "replaces the incompatible part properly even if the encodings are not compatible" do
    s = "hllëllo"

    NATFIXME 'Not preserving encoding properly', exception: SpecFailedException do
      s.gsub(/ë/) { |bar| "Русский".force_encoding("iso-8859-5") }.encoding.should == Encoding::ISO_8859_5
    end
  end

  not_supported_on :opal do
    it "raises an ArgumentError if encoding is not valid" do
      x92 = [0x92].pack('C').force_encoding('utf-8')
      NATFIXME 'raises an ArgumentError if encoding is not valid', exception: SpecFailedException do
        -> { "a#{x92}b".gsub(/[^\x00-\x7f]/u, '') }.should raise_error(ArgumentError)
      end
    end
  end
end