What you want to do is match on a RawBlock element.
Something like

function RawBlock(el)
  if el.text:match("<!--") then
    content = -- fill this in based on el.text
    return pandoc.Header(2, content)
  end
end


Peter Matulis <pmatulis-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> writes:

> Hi. I'm new to Pandoc & Lua but my present issue is straightforward. I am
> converting a Markdown file to HTML with pandoc like this:
>
> pandoc -L filter.lua -f gfm -t html -o file.html file.md
>
> Where file.md contains a line with this MD comment (the MD file was
> obtained by converting from RST where I had to seed that file with an RST
> comment to use as a placeholder for what I'm doing here):
>
> <!-- blah -->
>
> I simply want to convert this into an actual (second-level) MD header:
>
> ## blah
>
> From what I've read, I would ideally specify an "element" that will be
> acted upon, but what is a comment considered? Anyway, I think I can specify
> `Pandoc` to mean "check the entire document", and that's what I've done
> here (I will eventually be iterating through multiple MD files so I should
> really not be doing it like this; please advise!):
>
> function Pandoc (value)
>   comment = "<!-- blah -->"
>   comment = comment:gsub("%<%!%-%- blah %-%-%>", "## blah")
>   return value
> end
>
> I've run the main logic through Lua on the terminal and it works:
>
> $ lua
> Lua 5.3.3  Copyright (C) 1994-2016 Lua.org, PUC-Rio
>> comment = "<!-- blah -->"
>> comment = comment:gsub("%<%!%-%- blah %-%-%>", "## blah")
>> print (comment)
> ## blah
>
> But I'm failing to integrate it with Pandoc (no change is registered in
> file.html). It's probably due to my misunderstanding of functions (not a
> programmer!).
>
> TIA.
>
> /pm
>
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