I tried to use REGEXP_SUBSTR in an Actionable Query and kept getting errors. I used SQL Tracker to see what was being submitted to the database and the pattern was in uppercase. This caused \d (only digits) to become \D (anything but digits) which broken the code. Code works as expected when run directly from Toad.
I got it working by using a different pattern but this removes all the POSIX classes and all the lower case Perl-Influenced Extensions from use.