I am searching for the regex url spinning records for .htaccess - particularly, I would like a method to re-write:

/site/this-is-my-slug/

to

/site/?slug=this-is-my-slug

Is possible?

Do this:

RewriteRule ^site/(.*)$ site/?slug=$1 [L,QSA]

The QSA flag will copy over any query string out of your original URL request so take it off if you do not want this functionality.