Not a direct reply, but: If you have a copy of "The Camel Book", otherwise known as "Programming Perl" by Larry Wall, Tom Christiansen & John Orwant" pub. O'reilly, or know somebody who has a copy, have a read of Chapter 5 'Pattern Matching' which contains about the clearest explanation of how regexes work and how to write them that I've seen anywhere - and better yet, it describes Perl regexes, which is what SA uses.
FWIW there are also C and Java regex dialects, which are not at all useful in SA rules. https://www.regular-expressions.info/ is also useful. Its worth knowing about these too: https://www.regexplanet.com/advanced/java/index.html https://regex101.com/ They are both pages for testing regexes: both let you type in a regex plus test strings to check whether the regex does what you expect - or not! Martin
