So the solution is to replace any lines that use the mb_ereg function so they use the ereg counterpart. You can’t just update the system setting.
For example, see how I’ve commented out the mg_ereg line and force the $condition to use the ereg function:
// $condition = $this->config['use_multibyte'] ? @mb_ereg($pattern,$value) : @ereg($pattern, $value);
$condition = @ereg($pattern, $value);
Once I did that, it worked.
Gotta file a bug / feature request for this weird scenario.
I think I need to recompile PHP with the "--disable-mbregex" option omitted.