The Expression Language is loosely based off of JavaScript expressions, so you could use JavaScript resources to try and learn some of them better. The official documentation for the language is here and we try to give some examples, although maybe they could be flushed out more. There is also a section of this community dedicated to learning it, but there isn’t a whole lot of content yet.
Here’s another great resource related to learning the Expression Language, where @koryknick goes over his starter pack in a video tutorial.
Try and validate if values are getting to the filter. Additionally, it would be helpful if you can take a screen shot of the maper after the filter.
