Ok, I tried with one of your strings and I just get blank; not True.
You see "True" only when the formula doesn't end with a variable name or has nothing to return. In this case we do have the fin_string variable at the end so it shouldn't say True.
If it doesn't find the 3 letters and 6 numbers combination, it will just return nothing and when it does, it returns the combination.
What do you wish to show when the string does not contain the combination?
For eg: "This is SCN". What should such a string return? The string itself or nothing?
-Abhilash