I think I am using an older version of E10 the screens appear different (10.1.400.38)
Also your screenshots show “In-Transaction” - I am using “Pre-Processing” ~ does this matter ?
I have created the set Argument/variable Task, created the variable “Regex” but when I try to add the C# expression it thows an error:
The error mentions a using directive which I can see in your screenshots bit do not know where it is in my Epicor version