Recently I noticed errors on my Ubuntu 18.04 machine in /var/log/apache2/errors.log that look as below. These may also occur on any other Linux server running Apache and ModSecurity.
These would show multiple times for any requested page.
According to the Atomicorp ModSecurity page here - https://support.atomicorp.com/hc/en-us/articles/360000188468-Rule-execution-error-PCRE-limits-exceeded-8-null- you should increase a couple of limits.
I have edited /etc/modsecurity/modsecurity.conf and set these as recommended below.
I restarted Apache (sudo systemctl restart apache2) and I have no more warnings in my Apache error log.