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Sorry for the tags, but otherwise I would have had to respond to all your comments individually.

I also wanted to read on, so I searched for the book and found a page where it was possible to ‘read a preview’

You can use backreferences \1 \2 etc. but you can also give them names explicitly.
it looks like this: (?<name>inner-regex)
Some flavors support it, kotlins doesn’t apparently.

I don’t actually know whether POSIX grep would support named groups :o