[kiwidude]

Originally Posted by cybmole

I look at regex syntax reference page here
http://www.regular-expressions.info/reference.html

but there's no definition of the \1 operation ?

do I need a better reference page or book ???

You just needed to click on the "Advanced syntax" page link (that is the "Basic syntax" page).
http://www.regular-expressions.info/refadv.html

Originally Posted by cybmole

what's a good book to learn from ( intending to use more regex in both sigil and calibre ). I'd prefer an ebook that I can put onto my kindle.

I can't name any books, perhaps someone else can. That website is very good, and between the basic and advanced page you have the "cheat sheet" for most of what you need to know to refresh your memory.