Summary
When using regex conditions (see How to use pattern matching conditioning), there are substitution variables available for any capture group specified in your regex statement. These can be used and provide an indirect way of doing string manipulation. Even if you don't need the condition specifically for determining whether to create an issue or not (just make sure the condition statement is always matches).
Example
This was created to answer this question.
Easier way to get the project key
%original_project% is a simple way of getting the project key, so the above technique is not needed if this is all you need.