Where to search for more specific information about actions used in Siri Shortcuts app? There is a nice user guide on Shortcuts by Apple https://support.apple.com/guide/shortcuts/welcome/ios,  but there should be somewhere  some more datailed information as for example: what Pattern parameters may be used in "Match Text" action? Just some more precise definitions on Actions that may be used in the shortcuts. Anyone can help pointing to the right direction?
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