Its like SQL - everyone has a dialect. For most things where a SQL/regex engine/parser isn't the core of what they do, it will never be a priority. The best approach IMO is something like this in priority order:

1. Stick to using the lowest common denominator like you did for case insensitivity.

2. If that becomes too cumbersome, then consider whether regex is the right tool for the job. Maybe you can use e.g Python/your favorite language with a known regex standard.

3. If there are no other tools and you're stuck with whatever flavor of regex one particular thing supports, only then invest time in learning the details. There is probably a book out there with the details even if there's no webpage.

Then pray you never get to step 3 :)