In an Office Hours session a couple of months back, I covered an important change that comes to regular expressions once you upgrade to 12c Release 2. You can see the video covering the issue here:
but for the TL;DR brigade reading this post: Regular expressions are not deterministic when you take NLS settings into account and thus cannot be used in constraints and/or function-based indexes.
This is just a post to quickly revisit the topic for anyone thinking of upgrading from an earlier release to 12c Release 2. An AskTOM question came in asking what would happen to such constraints during the upgrade process.
The sad answer is … nothing. For example, if you successfully (and by strict definition, incorrectly) created a constraint with a regular expression in 11g, then after the upgrade, that constraint will still be present in your new 12c Release 2 system. It will continue to work as it did in 11g, and even if you disable/enable it, or put it through a validate command to exercise the data, it will work as it did before.
To be honest, I do not like this, because it can become what I call a “sleeper” problem. If, 6 months after you upgrade, you happen to drop and recreate that constraint you’ll be most distressed to find that it cannot be added, and you will have most probably long forgotten that it was caused by an event that occurred 6 months ago, namely the upgrade. And perhaps worse, you now have an index or constraint that could potentially be corrupted by innocent tinkering with session NLS settings.
So before you upgrade, definitely take a cursory glance through your constraint definitions and take remedial action if needed.