After upgrading from Debian Stretch to Buster, opening some Dokuwiki pages show the following error message:
A PCRE internal error occured. This might be caused by a faulty plugin
These pages were working correctly on Debian Stretch. I didn't change anything.
After looking into /var/log/apache2/error.log, I narrowed down to the error message:
preg_match(): Compilation failed: invalid range in character class at offset 3552 in /.../inc/parser/lexer.php on line 118
Further research using google, I found this github issue about the lexer.php that is not compatible with PHP 7.3. I was indeed using Frusterick Manners version of Dokuwiki.
To resolve this issue, I upgraded my Dokuwiki version to Greebo. However, at the same time, I also need to upgrade my plugins to the latest version. Otherwise, it is showing a lot of errors. So, before upgrading to Greebo, make sure that your plugins work with that version.