Match last occuring enclosing outer brackets

I tried now for three hours to construct the following regex match without much success. I have the following two strings:

This is a test string to illustrate the problem (example) in complex matching logic (Work / not working (in this case) to match this last occurring bracket closure)

and

Simpler version of the string (Matchable in any easy way)

I would like to define a str.match() that matches this last part of the strings above. Resulting in:

Work / not working (in this case) to match this last occurring bracket closure

and

Matchable in any easy way

Any good way to achieve this? Sadly the data is highly volatile that a strong Regex is much rather preferred instead of long functional logic. Thanks so much!