Honest question, why would you read a book about Regex? I personally find it a lot easier to learn how it works just by going to sites like https://regex101.com/ and trying things out while reading their docs.
Because it was highly recommended to me by a sysadmin I respected who was extremely proficient with regex. Sure, there are sites too, but after reading the book, the sites weren't really needed. I personally got more value from the book. It's subjective, of course.
