Error 500 on RewriteRule - .htaccess

Question

I want the following behaviour:
http://www.example.com/mysubdir -> show /mysubdir/index.htm
http://www.example.com/mysubdir/first -> show /mysubdir/redirect.php?token=first
http://www.example.com/mysubdir/first/second -> show /mysubdir/redirect.php?token=first&action=second
http://www.example.com/mysubdir/first/second/third -> show /mysubdir/redirect.php?token=first&action=second&param=third
therefore I created the following .htaccess (inside mysubdir)
RewriteEngine on
RewriteBase /mysubdir/
RewriteRule ^([^\/]+)\/([^\/]+)\/([^\/]+)\/? ./redirect.php?token=$1&action=$2&param=$3 [L]
RewriteRule ^([^\/]+)\/([^\/]+)\/? ./redirect.php?token=$1&action=$2 [L]
RewriteRule ^([^\/]+)\/? ./redirect.php?token=$1 [L]
This causes an error 500
If I remove the last line, however, the first two rules work as expected. I can't see any visible difference between these rules that should cause such an error.

Answer 1

You're getting 500 (internal server error) because your rules are looping infinitely due to presence of [^/]+ in your last rule that will match rewritten URI redirect.php also.
You can fix it using this modified code:
RewriteEngine on
RewriteBase /mysubdir/
# skip all files and directories from rewrite rules below
RewriteCond %{REQUEST_FILENAME} -d [OR]
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule ^ - [L]
RewriteRule ^([^/]+)/([^/]+)/([^/]+)/?$ redirect.php?token=$1&action=$2&param=$3 [L,QSA]
RewriteRule ^([^/]+)/([^/]+)/?$ redirect.php?token=$1&action=$2 [L,QSA]
RewriteRule ^([^/]+)/?$ redirect.php?token=$1 [L,QSA]
Few more changes made are:
Use QSA to preserve any existing query string in URLs.
You should use anchor $ in your regex patterns.
No need to escape / in regex patterns.