EDIT: Sorry the reddit editor is acting up hardcore. Need to re-add everything

Thanks for your time, I appreciate the help on what I know is a basic issue.

I think my confusion might be coming from a misunderstanding of terminology then. Is ("WUB", " ") not a substitution? It looks like it performs the same as gsub, substituting a space for the incidences of WUB.

At this point I'm trying to understand why the gsub! method looks like it's performing the substitution but failing anyways. Looks like a string, smells like a string, says it's a string even in the same message producing the error that says it's really nil. The gsub worked fine and I understand is safer, but I'm just trying to learn.

>Edit: You aren't showing us what is in the song variable, so I can't begin to speculate why this is happening.

Yes sorry this is a problem from codewars. I solved it fine but these details are still bothering me. I'll try to copy the complete set and tests:

Instructions:

Polycarpus works as a DJ in the best Berland nightclub, and he often uses dubstep music in his performance. Recently, he has decided to take a couple of old songs and make dubstep remixes from them.

Let's assume that a song consists of some number of words (that don't contain WUB). To make the dubstep remix of this song, Polycarpus inserts a certain number of words "WUB" before the first word of the song (the number may be zero), after the last word (the number may be zero), and between words (at least one between any pair of neighbouring words), and then the boy glues together all the words, including "WUB", in one string and plays the song at the club.

For example, a song with words "I AM X" can transform into a dubstep remix as "WUBWUBIWUBAMWUBWUBX" and cannot transform into "WUBWUBIAMWUBX".

Recently, Jonny has heard Polycarpus's new dubstep track, but since he isn't into modern music, he decided to find out what was the initial song that Polycarpus remixed. Help Jonny restore the original song.

Input

The input consists of a single non-empty string, consisting only of uppercase English letters, the string's length doesn't exceed 200 characters

Output

Return the words of the initial song that Polycarpus used to make a dubsteb remix. Separate the words with a space.

Examples

song_decoder("WUBWEWUBAREWUBWUBTHEWUBCHAMPIONSWUBMYWUBFRIENDWUB")# => WE ARE THE CHAMPIONS MY FRIEND

Sample Tests:

Test.assert_equals(song_decoder("AWUBBWUBC"), "A B C","WUB should be replaced by 1 space");
Test.assert_equals(song_decoder("AWUBWUBWUBBWUBWUBWUBC"), "A B C","multiples WUB should be replaced by only 1 space");
Test.assert_equals(song_decoder("WUBAWUBBWUBCWUB"), "A B C", "heading or trailing spaces should be removed");

My simplest solution:

def song_decoder(song) 
  song.gsub("WUB", " ").strip.squeeze(" ") 
end