This repository contains various examples of Pandoc filters, using the pandocfilters Python module. So far, most of the filters are just Python implementations of the Haskell examples on the scripting page.
When writing filters, printing out the abstract syntax tree using -t native
or -t json on Pandoc is helpful. All a filter does it manipulate that JSON
file to get another JSON file, which feeds back into Pandoc. The trick is to
not mess up the JSON, because if you do Pandoc won't like it.
For Lua filters, the same trick works, but there is no intermediate JSON step, so it takes more imagination to see what's going on. Even more unfortunately, Lua lacks a native way to print tables (its list/dictionary data structure), and since the Lua library is embedded in the Pandoc executable, it doesn't seem easy to download a library like Penlight to dump tables for inspection. Lua filters are faster and don't require GHC/Python though.