When adding a new source, specifying only a filter word does not do anything besides outputting an error to the log.

I haven't been able to find an explanation of how it works, but I would assume the filter field has a required syntax for it to work.

Can anyone point me in the right direction? Preferably a direction that doesn't require digging into the code.

You have to use the RegEx-Syntax for PHP.

Example:
If you want to use a filte to include only entrys with the word "comic" in it, you have to put
/comic/
in the filter.