I had to eat that elephant one bite at a time....and I'm nowhere close to being as good as some of the reg-fu masters around here... but I can get most of what I want done with a few of the basics like Karellen mentioned.
This website has a basic description that can get you started understanding what regex is about
https://www.developer.com/languages/...ssions-primer/
but that website's examples are used more for programmers. Once you have a basic understanding then you can use this website to learn the specific flavor of regex that Sigil uses (PCRE):
https://www.regular-expressions.info/pcre.html
And here is a basic memory aid that helps until you just memorize them from using them all the time:
http://marvin.cs.uidaho.edu/~heckend...uts/regex.html
Once you have a basic idea of what is involved, then the regex sticky is where people go to share (or ask) how to do some specific things.