Custom syntax extension

highlight.lua

--[[
Add support for a custom inline syntax.

This pandoc Lua filter allows to add a custom markup syntax
extension. It is designed to be adjustable; it should not be
necessary to modify the code below the separator line.

The example here allows to add highlighted text by enclosing the
text with `==` on each side. Pandoc supports this for HTML output
out of the box. Other outputs will need additional filters.

Copyright: © 2022 Albert Krewinkel
License: MIT
]]

-- Lua pattern matching the opening markup string.
local opening = [[==]]

-- Lua pattern matching the closing markup string.
local closing = [[==]]

-- Toggle whether the opening markup may be followed by whitespace.
local nospace = true

-- Function converting the enclosed inlines to their internal pandoc
-- representation.
local function markup_inlines (inlines)
  return pandoc.Span(inlines, {class="mark"})
end

------------------------------------------------------------------------

local function is_space (inline)
  return inline and
    (inline.t == 'Space' or
     inline.t == 'LineBreak' or
     inline.t == 'SoftBreak')
end

function Inlines (inlines)
  local result = pandoc.Inlines{}
  local markup = nil
  local start = nil
  for i, inline in ipairs(inlines) do
    if inline.tag == 'Str' then
      if not markup then
        local first = inline.text:match('^' .. opening .. '(.*)')
        if first then
          start = inline -- keep element around in case the
                         -- markup is not closed. Check if the
                         -- closing pattern is already in this
                         -- string.
          local selfclosing = first:match('(.*)' .. closing .. '$')
          if selfclosing then
            result:insert(markup_inlines{pandoc.Str(selfclosing)})
          elseif nospace and first == '' and is_space(inlines[i+1]) then
            -- the opening pattern is followed by a space, but the
            -- config disallows this.
            result:insert(inline)
          else
            markup = pandoc.Inlines{pandoc.Str(first)}
          end
        else
          result:insert(inline)
        end
      else
        local last = inline.text:match('(.*)' .. closing .. '$')
        if last then
          markup:insert(pandoc.Str(last))
          result:insert(markup_inlines(markup))
          markup = nil
        else
          markup:insert(inline)
        end
      end
    else
      local acc = markup or result
      acc:insert(inline)
    end
  end

  -- keep unterminated markup
  if markup then
    markup:remove(1) -- the stripped-down first element
    result:insert(start)
    result:extend(markup)
  end
  return result
end

Thanks a lot for sharing your work! I would love to make this work for obsidian compatibility but keep running into the following error message:

Error: Command failed: C:/Users/danie/AppData/Local/Pandoc/pandoc.exe --from=markdown-raw_tex+tex_math_single_backslash --to=html --katex --citeproc --bibliography=Bibliography.bib --csl=Chicago-origdate.csl --lua-filter=highlights.lua
Error running filter highlights.lua:
highlights.lua:41: attempt to call a nil value (field 'Inlines')
stack traceback:
highlights.lua:41: in function 'Inlines'

Could you take a moment to explain to me what I may be doing wrong? Thanks!

The most likely cause is an outdated pandoc version: the pandoc.Inlines function was added in pandoc 2.17, I think. Try updating to the latest version.

@tarleb
Thanks for this!! What would I have to add so the highlighting filter also triggers if the == is before a period or any other symbols like commas, apostrophes, etc.? so: ==highlighting==. or ==highlighting==,
As is it seems that this only works if it starts and stops with a space.
Thanks!

I don't have time to look into this right now, sorry. But why not try the latest pandoc nightly which includes this?

Btw, since the last Pandoc update it just worked, thanks very much!

I don't have time to look into this right now, sorry. But why not try the latest pandoc nightly which includes this?

As the link is down, I could not see this. But my guess is: you were talking about pandoc --from=markdown+mark, which enables highlighted text without the need of a Lua script.

Yes, exactly.