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

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.