Hello Andrew,
You are definitely on the right path, as using regex is exactly how I would recommend doing that. If the output is not showing in the activity it is because it's not finding a match. This is just a guess but one thing that's easy to overlook is the formatting of the email. What the regex actually looks at is the code behind the email, so that includes formatting things like html tags and whatnot.
I would double check the output of your trigger step to see exactly what the regex needs to be compared against and see if that requires any change in your regex formula.
If this isn't what's going on or you would like a hand at this, please open a support case with your CSR ID# and Pipeline ID and link to this article, the agent will escalate it up to me and I'll be happy to take a look into it for you.
Hope that helps,
