Workflow regular expression: replace url part after 3rd slash

I've got a regular expression action in Nintex Workflow where I need to replace everything after the third slash of a url.

- This is the url: http://www.testurl.com/this-part/needs-to/be-replaced

- This part should be replaced: this-part/needs-to/be-replaced

- With this: become-this

So the final url is like this: http://www.testurl.com/become-this 

What Regex pattern should I use for this?