mod_rewrite help when .htaccess is located in subdirectory - regex

Question

I have a simple rewrite rule that checks if a *.gz version of the requesting file exists, and if so returns that file to the browser (it's based around this technique for serving compressed CSS & JS: https://blog.jcoglan.com/2007/05/02/compress-javascript-and-css-without-touching-your-application-code/).
Assume that my CSS & JS reside in a /cache subdirectory of my site, e.g.:
http://domain.com/cache/app.css
http://domain.com/cache/app.css.gz
http://domain.com/cache/app.js
http://domain.com/cache/app.js.gz
When the following directives are placed within the .htaccess file at webroot, everything works as expected:
AddEncoding gzip .gz
RewriteCond %{HTTP:Accept-encoding} gzip
RewriteCond %{HTTP_USER_AGENT} !Safari
RewriteCond %{REQUEST_FILENAME}.gz -f
RewriteRule ^(.*)$ $1.gz [QSA,L]
However I'd prefer to place these rules within a separate .htaccess file that sits within that /cache directory. But pasted as-is, the rules no longer work. From what I can tell, the last RewriteCond is failing, e.g. the condition which checks if the requested filename exists.
I've tried adding RewriteBase /cache/ but that didn't seem to work. I've removed my webroot .htaccess in case there was another directive there that was conflicting. I've also tried changing the RewriteRule to:
RewriteRule ^(.*)$ /cache/$1.gz [QSA,L]
And while I think that might be correct in the end, it still doesn't work - like I said because from what I can tell it's the RewriteCond which is failing.
So I'm stumped! Any help?

Answer 1

Following rule should work from /cache/.htaccess:
AddEncoding gzip .gz
RewriteEngine On
# Determine the RewriteBase automatically/dynamically
RewriteCond $0#%{REQUEST_URI} ^([^#]*)#(.*)\1$
RewriteRule ^.*$ - [E=BASE:%2]
RewriteCond %{HTTP:Accept-encoding} gzip
RewriteCond %{HTTP_USER_AGENT} !Safari
RewriteCond %{DOCUMENT_ROOT}%{ENV:BASE}/$1\.gz -f [NC]
RewriteRule ^(.+?)/?$ $1.gz [L]