Weirdness with gsub

Question

I was trying to use gsub to remove non word characters in a string in a rails app. I used the following code:

somestring.gsub(/[\W]/i, '')  #=> ""

but it is actually incorrect, it will remove letter k as well. The correct one should be:

somestring.gsub(/\W/i, '')  #=> "kkk"

But my problem is that the unit test of a rails controller which contains the above code using rspec does not work, the unit test actually passes. So I created a pretty extreme test case in rspec

it "test this gsub" do
  'kkk'.gsub(/[\W]/i, '').should == 'kkk'
end

the above test case should fail, but it actually passes. What is the problem here? Why would the test pass?

Answer 1

Ruby 1.9 switched to a different regular expression engine (Oniguruma), which accounts for the behavior change. This seems like a bug in it.

For your example, you can get around the issue by not specifying a case insensitive match:

irb(main):001:0> 'kkk'.gsub(/[\W]/i, '')
=> ""
irb(main):002:0> 'kkk'.gsub(/[\W]/, '')
=> "kkk"
irb(main):004:0> 'kkk'.gsub(/\W/i, '')
=> "kkk"
irb(main):003:0> 'kkk'.gsub(/\W/, '')
=> "kkk"

Update: It looks like removing the character group is another approach. It might be that negated matches like that aren't necessarily valid in a character group?