Steps to reproduce:

1. add a global regular expression that Expression is set to like "\q"
2. test the expression, and it does not detect errors
3. add a some log[] item that uses the global regex

Result:
A regex error occurs in Zabbix agent like:

  5522:20231215:175524.212 active check "log[/var/log/messages,@wrong regex,,,skip]" is not supported: Invalid regular expression: unrecognized character follows \, position 1, flags:0x2408

Expected:
I think regex errors should be detected in regex test in Zabbix frontend.

Cause:
In PHP (< 8.0), BAD_ESCAPE_IS_LITERAL flag is set in preg_match() by default.

So wrong expressions like "\q" are not detected as errors.

Fix:
Removing BAD_ESCAPE_IS_LITERAL flag by adding "X" flag in reglar expression resolves this issue.

Here is a patch.

--- include/classes/validators/CRegexValidator.php.bak  2023-12-19 10:02:40.033397199 +0900
+++ include/classes/validators/CRegexValidator.php      2023-12-19 10:04:51.666395320 +0900
@@ -60,7 +60,7 @@
                        }
                });
-               preg_match('/'.$value.'/', '');
+               preg_match('/'.$value.'/X', '');
                restore_error_handler();

Additional information:
In PHP 8.0, BAD_ESCAPE_IS_LITERAL flag is changed to not be used by default.

https://github.com/php/php-src/pull/4430

So this issue does not occur in RHEL 9 like system.