What is the syntax for setting 404 fast paths on all Drupal pages in D7 > 7.9 D7.9 introduced 404 fast paths which don't fully bootstrap drupal on 404's for certain file types if it's enabled, thus saving some bandwidth / computation. More info is available here: http://drupal.org/node/1296384

By default this excludes only certain file types such as .jpg, .txt., etc., looks like this:

$conf['404_fast_paths_exclude'] = '/\/(?:styles)\//';
$conf['404_fast_paths'] = '/\.(?:txt|png|gif|jpeg|css|js|ico|swf|flv|cgi|bat|pl|dll|exe|asp)$/i';


I want to use this for all paths. Are there any repercussions to that?

I tried changing it to 

$conf['404_fast_paths'] = '/*$/i'; 


However, this returns a warning:


  Warning: preg_match() Compilation failed: nothing to repeat at offset 0 in drupal_fast_404() (line 2484 of /cms/main/includes/bootstrap.inc).


So what should the syntax look like up there to both work and not generate a warning?

Alternatively, if there isn't a solution to this specific question, is there a way I can speed up 404's for all Drupal pages, would it better to use the Fast 404 contributed module for instance?
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."