Nintex Workflow - Regular expression: extract after third equality sign (=)

Assuming that your layout will always be like this, why not Split the string around the '=' into a collection and take the 5th element - then replace the ' promocode' at the end of the string with nothing.

This is going to be so much easier to understand than a regex

if your concern is realy to get a value after 3rd equal sign, following pattern might work

(?<=((?:[^=]+=)[^=]+(?=(s|$))){2}[^=]+=).*(?=s)

if the value you want to get is always prefixed with 'projectnr=' then it can be simplified as follows and then it doesn't matter on what position key-value pair appears

(?<=projectnr=).*(?=s)