Rewrite rule in .htacess from URL with equal sign and short folder name - .htaccess

Question

I need a rewrite rule in my .htaccess for the following example URL:
https://www.example.com/book=18ABCDEFG
This is the resulting URL that I need:
https://www.example.com/books/2018/ABCDEFG.pdf
I have spent a couple of hours googling and trying to solve this, but I am really stuck. If there is a rewrite wizard out there, I would really appreciate the help.
EDITED:
This is what i have come up with so far, but the only result is a 404 (not found):
RewriteEngine on
RewriteCond %{QUERY_STRING} ^book=18(.*)$
RewriteRule ^(.*)book=(.*)$ http://www.examplesite.com/books/2018/$1.pdf [R=301,L]
I was hoping that the $1 should reference the string after "18" since the only parenthesised group in the condition contains that string, but so far I haven't found the right syntax.
I should explain about the "18" too. Now the URLs are different and there will never be any other year than 2018 for book URLs with this pattern. So it can be hard coded.
But how do I reference the string after "18" in the rewrite rule?

Answer 1

This is what i have come up with so far, but the only result is a 404 (not found):
RewriteEngine on
RewriteCond %{QUERY_STRING} ^book=18(.*)$
RewriteRule ^(.*)book=(.*)$ http://www.examplesite.com/books/2018/$1.pdf [R=301,L]
As mentioned in comments, this is trying to match two different URLs at the same time: one where the information is contained in a query string (after an "imaginary" ?) and the other where the information is contained in the URL-path. So, it's probably not doing anything; hence the 404.
I was hoping that the $1 should reference the string after "18" since the only parenthesised group in the condition contains that string
$1 refers to the first parenthesised group in the RewriteRule pattern (of which there are two). If you want to match the first subpattern in the last matched condition then you need to use a backreference of the form %1.
However, your example does not contain a ? and therefore there is no query string. The information is contained in the URl-path instead. (Unless that is a typo in your question?! It looks like a typo, since the = is superfluous otherwise. But that would also completely change your question.)
To redirect the URL example.com/book=18ABCDEFG (ie. information in the URL-path) then your would need something like the following near the top of your .htaccess file in the document root:
RewriteEngine on
RewriteRule ^book=18([^/]+)$ /books/2018/$1.pdf [R=302,L]
If the code can only be specific characters then the regex should be appropriately specific. As it stands, it matches pretty much anything.
Test with 302 (temporary) redirects and only change to 301 (permanent) when you are sure it's working OK (if this is intended to be a permanent redirect and cached by the browser).
You will need to clear your browser cache before testing.