Try using this expression in the Pattern field: /[^/]*$
The steps to configure the Regular Expression action are in my blog post: Remove a Forward Slash at the End of a URL String in Nintex Workflow
Which version of Nintex Workflow are you using? I ran this in both Nintex Workflow 2013 and Nintex Workflow for O365, with both returning just the URL from your expression. If you're using the Nintex Workflow 2013, try using the Run Now option to see what you get. Below is how I configured the Regular Expression action.