This bit of code will help you if you need to replace a piece of text with another piece of text.
The Regular Expression action allows you to build complex expressions so that not only do you replace a piece of text, but you can find any number of matching sequences of text or replace only one piece or just from the beginning or from the end. The number of expressions you can build is almost infinite.
Nintex Workflow 2010
Nintex Workflow 2013
SharePoint Server 2010
SharePoint Server 2013
If you want to learn more, take a look at this site, which is one that I go to often to if I need a refresher - RegExr: Learn, Build, & Test RegEx
I hope this explains a little on how to use Regular Expressions in your Nintex Workflows. If you have any questions, please free to post them in the comments section.