Where to search for more specific information about actions used in Siri Shortcuts app?
Here’s a good resource page on Siri shortcuts...

https://www.reddit.com/r/shortcuts/comments/9k0sup/siri_shortcuts_resources/

And here’s the link you are looking for...

http://userguide.icu-project.org/strings/regexp

Also a great site for learning and testing regexp

https://regex101.com

And a great beginners video to get started

https://youtu.be/sa-TUpSx1JA