What is the syntax for setting 404 fast paths on all Drupal pages in D7 > 7.9
You used "/*$/i" as regular expression, but there is nothing before the asterisk; in other words, you are not saying what can be repeated zero or more times.
Valid regular expressions expressions such as "/.*/i", "/[a-zA-Z]*/i", "/(foo|bar)*/i". 

drupal_fast_404() is called by drupal_deliver_html_page() when the result of menu_execute_active_handler() is MENU_NOT_FOUND, which means Drupal is asked to look for a path without any menu callback associated.
drupal_fast_404() first checks that $_GET['q'] doesn't match the regular expression contained in the "404_fast_paths_exclude" Drupal variable; if it doesn't match that regular expression, then it checks $_GET['q'] matches the regular expression contained in the "404_fast_paths" Drupal variable. The default value for that variable is '/\.(?:txt|png|gif|jpe?g|css|js|ico|swf|flv|cgi|bat|pl|dll|exe|asp)$/i' which means the fast 404 page is returned when the request is for a file with the following extensions:


.txt
.png, .gif, .jpg, .jpeg
.css, .js
.ico, .swf, .flv
.cgi, .bat, .pl, .dll, .exe, .asp


If you want the fast 404 pages be used in any case, I would not touch the value of the "404_fast_paths_exclude" Drupal variable, and change the content of the "404_fast_paths" Drupal variable to '/.*/i', which would match any path that causes a 404 error, excluding the ones containing "styles."