From: gabriel.birke@...
Date: 2006-09-16T07:25:58+09:00
Subject: No regex backreference with four backslashes

Consider the following test case:

require 'test/unit'
class RegexTest < Test::Unit::TestCase
  def test_escaping
    numbers = "12345"
    assert_equal "12345", numbers.gsub(/(2|4)/, '\1')
    assert_equal "12345", numbers.gsub(/(2|4)/, "\\1")
    assert_equal "1\\ 23\\ 45", numbers.gsub(/(2|4)/, '\\ \1')
    assert_equal "1\\ 23\\ 45", numbers.gsub(/(2|4)/, "\\ \\1")
    assert_equal "1\\23\\45", numbers.gsub(/(2|4)/, '\\\1')
    assert_equal "1\\23\\45", numbers.gsub(/(2|4)/, "\\\\1")
  end
end
require 'test/unit/ui/console/testrunner'
Test::Unit::UI::Console::TestRunner.run(RegexTest)

The last two assertions fail (With the message <"1\\23\\45"> expected
but was <"1\\13\\15">.) - but why?

Is this a bug in the regex implementation or is there something wrong
with my regular expression or substitution string?