I was able to fix this myself, so I'm posting this here for reference. If this belongs somewhere else or there's something I should change, please let me know. This assumes that you are using WHM / cPanel and Apache on a dedicated or virtual private server, and that you have SSH and root access to said server. I received this error during the initial install of xF, specifically at the admin account creation step: Code: xenforo preg_match(): Compilation failed: unknown option bit(s) set at offset 0 I found out that it was being caused by mismatched PCRE versions in my server configuration. Connect to your server via SSH and log in as root. To confirm if your PCRE versions are mismatched or not, run: Code: pcretest -C PCRE | grep version && /opt/pcre/bin/pcretest -C PCRE | grep version The output of that command should resemble the following: Code: PCRE version 6.7 2006-12-07 PCRE version 8.21 2011-12-12 If the versions do not match like in the example above, continue to the next step. Run: Code: echo "/opt/pcre/lib/" >> /etc/ld.so.conf && echo "--with-pcre-regex=/opt/pcre" >> /var/cpanel/easy/apache/rawopts/all_php5 The following will cause your website to be temporarily inaccessible! Apache and friends need to be rebuilt so they can make use of this new, correct configuration. Run: Code: /scripts/easyapache --build Once it finishes, run the following to make sure Apache successfully restarted: Code: service httpd status Run the command in step 2 again. The versions should now match: Code: PCRE version 8.21 2011-12-12 PCRE version 8.21 2011-12-12 You should now be able to successfully install xenForo.