Hello,
I also got this error 500 "PHP regular expression limit reached (pcre.backtrack_limit)" on my website Top iPhone Apps created in Joomla 1.7.3
This occurred when I created so many menu items. After searching forums, I was able to fix it.
How? Here is the solution:
This is some kind of php restriction and you will need to override this php restriction by modifying index.php on your root.
Add following two lines in index.php and you don't need to modify any other file:
ini_set('pcre.backtrack_limit',1000000);
ini_set('pcre.recursion_limit',1000000);
Once it is modified, clear your cache and reopen your website. Its working fine.