I have a field for estimations, "T-shirt size", where the developers can estimate the amount of work before they actually start. This is a text field, with these options right now:

As a goal, I want to extract the larger numeric value for each entry and use that for later calculation in automation rules.
I found several substring-functions and started testing with them to figure out how to parse the string, but so far these returns nothing for me 
I have a manually triggered test rule that simply logs output for now:

Now, running this on a subtask with T-shirt size field set to "M (3-5)" I get this output in audit log:

What am I doing wrong?
I'm running a Team-managed Jira software project in Cloud
As final result, I want to extract the "5" and convert it to a number, but neither of my substring seems to do anything useful.