.htaccess - how to refrence the request strings in the rewrite conditions code example

Example: htaccess rewrite rule

  RewriteEngine On
  RewriteBase /
  RewriteCond %{REQUEST_FILENAME} !-d
  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteRule ^(.*)$ index.php?url=$1 [QSA,L]