I have a set of product pages that obey the following htaccess rule:
RewriteCond %{REQUEST_FILENAME} !-s
RewriteCond %{REQUEST_URI} ^/([0-9]+)\\-(.+)\\.html
Add the QSA flag to pass along existing query string params
RewriteRule . /product/index.php?prod=%1-%2 [QSA,L]
Also edited match as . and ^.*$ are equivalent in this case
Your looking for QSA, Query String Append
RewriteCond %{REQUEST_FILENAME} !-s
RewriteCond %{REQUEST_URI} ^/([0-9]+)\-(.+)\.html
RewriteRule ^(.*)$ /product/index.php?prod=%1-%2 [L,QSA]