What is the error in this .htaccess? - regex

Question

Using mod_rewrite, I want to look for all requested files in an assets directory, and otherwise fallback to index.php for API calls. This is my .htaccess.
RewriteEngine on
RewriteRule ^.+\.[^/]+$ assets/$1 [L]
RewriteRule .* index.php
Asset files are determined by whether the last last url segment has a file extension, which means a dot without following slashes. I already tested the regex at regexpal and it seems to work fine.
Unfortunately, all request result in a 500 Internal Server Error using these rewrite rules. I know that mod_rewrite is set up correctly, since omitting the second line works as expected.

Answer 1

Try these rules:
RewriteEngine on
# rewrite to assets/file if file exists in assets dir
RewriteCond %{DOCUMENT_ROOT}/assets/$1 -f
RewriteRule ^(.+?\.[^/]+)/?$ assets/$1 [L]
# otherwise rewrite to index.php
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule !^assets/ index.php [NC,L]

Answer 2

You need to add a condition to prevent the rewrite engine from looping:
RewriteEngine on
RewriteCond %{REQUEST_URI} !index\.php
RewriteRule ^.+\.[^/]+$ assets/$1 [L]
RewriteCond %{REQUEST_URI} !index\.php
RewriteRule .* index.php
The rewrite engine loops until the URI stops changing, and since .* matches index.php as well, it'll continue to loop through that rule until the internal recursion limit is reached you get a 500 error.
Additionally, you can add conditions that won't reroute existing files or directories:
RewriteEngine on
RewriteCond %{REQUEST_URI} !index\.php
RewriteRule ^.+\.[^/]+$ assets/$1 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule .* index.php