A macro in Tera (for Zola) to only permit a subset of markdown to become HTML

filter_md.html

{#
  This macro is for removing desired html tags from markdown. You pass it literal text that is markdown, and it will remove the desired tags. There's an optional `allow_html` parameter that defaults to `false`. This controls whether to permit html that is mixed with markdown.

  Note: you MUST store the result as a variable, before evaluating it as an expression, i.e. with the {{ }} syntax.
  Otherwise, you will always get a string representation of the HTML. For some reason, saving the string, then evaluating
  the expression doesn't have that issue.

  TODO: file a bug. Case to reproduce is {{ filter_md(md, ["h1"]) }} does not work, but saving as a variable and then doing it does.
#}
{% macro filter_md(md, allow_html=false, disallow) %}

{% if not allow_html %}
{% set html_esc  = md | striptags | markdown %}
{% else %}
{% set html_esc  = md | markdown %}
{% endif %}

{% set_global result = html_esc %}
{% for tag in disallow %}
{% set_global result = self::strip_html(html_esc=result, tag=tag) %}
{% endfor %}

{% set_global result = result %}
{{ result }}
{% endmacro  %}

{% macro strip_html(html_esc, tag) %}

{% set tokens = html_esc | split(pat="<" ~ tag) %}
{% set result = [] %}
{% for t in tokens | slice(start=1) %}
{% set t_2 = t | split(pat=">") | slice(start=1) | join(sep=">") %}
{% set_global result = result | concat(with=t_2) %}
{% endfor %}

{% set result = result | join(sep="") | split(pat="</" ~ tag ~ ">" ) | join(sep="" ) %}
{% set result = tokens[0] ~ result %}

{{ result }}

{% endmacro  %}