Proof of concept: Nested liquid templates

nested_liquid.rb

require 'liquid'

class Button < Liquid::Tag
  def initialize(tag_name, args, tokens)
     super
     @color = args.split(' ')[0]
     @url = args.split(' ')[1]
     @text = args.split(' ')[2..-1].join(' ')
  end

  def render(context)
    %Q{<a href="#{@url}" style="background: #{@color}; color: white; padding: 20px;">#{@text}</div>}
  end
end

class FacebookShareUrl < Liquid::Tag
  def initialize(tag_name, url, tokens)
     super
     @url = url
  end

  def render(context)
    "https://www.facebook.com/sharer.php?u=#{CGI.escape(@url)}"
  end
end

Liquid::Template.register_tag('button', Button)
Liquid::Template.register_tag('facebook_share_url', FacebookShareUrl)

# Find every {% that has another {% before the first  %}, and store the index
def get_mask_indexes(str, forward = true)
  if forward
    open_tokens = ['{%', '{{']
    open_pattern = /\{%|{{/
    close_pattern = /%}|}}/
  else
    open_tokens = ['}%', '}}']
    open_pattern = /\}%|}}/
    close_pattern = /%{|{{/
  end

  indexes = []
  s = StringScanner.new(str)

  loop do
    break if s.scan_until(open_pattern).nil?

    meanwhile_content = s.scan_until(close_pattern)
    if open_tokens.any? { |tok| meanwhile_content&.include?(tok) }
      s.unscan
      indexes << s.pos - 2
    end

    break if s.eos?
  end

  indexes.sort
end

p get_mask_indexes('{% {{ {% {{ }} %} }} %}')

def masked_non_leaf_tags(str)
  replace_forward = get_mask_indexes(str)
  replace_backward = get_mask_indexes(str.reverse, false).map { |x| str.size - x - 2 }

  new_str = str
  offset = 0
  tokens_to_replace = ['{%', '{{', '}}', '%}']

  # It's important to sort the indexes so we replace them in order. Otherwise offsets won't work
  (replace_forward + replace_backward).sort.each do |i|
    token_index = tokens_to_replace.find_index(new_str[i+offset..i+offset+1])
    new_str[i+offset..i+offset+1] = "__TOKEN#{token_index}__"
    offset += 8 # The token moves all subsequent tokens along by 8 characters
  end

  new_str
end

def unmask_tags(str)
  new_str = str
  tokens_to_replace = ['{%', '{{', '}}', '%}']
  tokens_to_replace.each_with_index do |token, i|
    new_str.gsub!("__TOKEN#{i}__", token)
  end
  new_str
end

def render_nested(str, context)
  rendered_str = str
  loop do
    masked_template = masked_non_leaf_tags(rendered_str)
    rendered_str = Liquid::Template.parse(masked_template, :error_mode => :strict).render(context)
    break if !masked_template.include?('__TOKEN') # If nothing was masked for that render, we're done
    rendered_str = unmask_tags(rendered_str)
  end

  rendered_str
end

str = <<~HTML
  Dear {{ first_name }},<br />
  <br />
  {% button blue {% facebook_share_url https://www.example.com?tracking_param={{ guid }} %} Share Now {{ first_name | upcase }} %}<br />
  <br />
  {% button #ff00ff {% facebook_share_url https://www.example.com?tracking_param={{ guid }} %} Share Again {{ first_name }} %}
HTML

puts render_nested(str, { 'first_name' => 'Hiemanshu', 'guid' => 'absdf324rsd' })
puts render_nested(str, { 'first_name' => 'Frances', 'guid' => 'kkk234jsdf09' })