Suitability of (Lua) Filters or YAML specification of pre- and post-processing?

[iwelch]

I am learning filters for manipulating md input files. My impression is that the filters are very good in manipulating the AST components, but I can't figure out whether they are also intended for the related tasks of more global operations.

In particular, I am pondering whether a "replace anywhere" need is best addressed via a filter or via a completely external filter:

1. I would like to have an entity (like &TODAY; or $TODAY$) be replaced with the day (15 Mar 2025).
2. I would want to add new quasi-markup whose output depends on the format: think ==highlight me== turns into \hl{highlight me} when the output is latex or beamer, and into 'highlight me' when the output is html or epub.
3. I would like $END$ to stop processing, so I can keep comments beyond.

I would want this to be executed document wide, even in the YAML. These are not difficult outside pandoc (e.g., s/==(.*?)==/\hl{$1}/g ) and thus may be better kept outside pandoc in a completely external filter. Maybe the nicest way to do this would be to have YAML commands inside md files that invoke external commands:

---
author: "me"
title: "example"
date: "&TODAY"
prefilter: "/usr/local/bin/prefilter.pl"   ## would have to pass output format
optprefilter: "./prefilter.pl"                  ## would have to ignore if file does not exist
postfilter: "/usr/local/bin/postfilter.pl"
optpostfilter: "./postfilter.pl"
---

which would first run /usr/local/bin/prefilter -FORMAT < mdfile etc.

Or am I thinking about my problems altogether incorrectly?

[jgm]

1. You could use a filter that looks for this in Str elements.
2. Have you tried -f markdown+mark?
3. You could achieve this with a filter.

[iandol]

Pandocomatic is a really cool Pandoc runner that allows you to create recipes that include setup/preproccing/postprocessing/teardown scripts to run as part of a pandoc compile (which still can run filters). You then have the flexibility of just about any operation anywhere in the pipleine...