delim.js

var markdownIt = require('markdown-it')

function delim_plugin(md) {
  // Insert each marker as a separate text token, and add it to delimiter list
  //
  function tokenize(state, silent) {
    var i, scanned, token, len, ch,
        start = state.pos,
        marker = state.src.charCodeAt(start);

    if (silent) { return false; }

    if (marker !== 0x40/* @ */) { return false; }

    scanned = state.scanDelims(state.pos, true);
    len = scanned.length;
    ch = String.fromCharCode(marker);

    if (len < 3) { return false; }

    while (len % 3) {
      token         = state.push('text', '', 0);
      token.content = ch;
      len--;
    }

    for (i = 0; i < len; i += 3) {
      token         = state.push('text', '', 0);
      token.content = ch.repeat(3);

      state.delimiters.push({
        marker: marker,
        jump:   i,
        token:  state.tokens.length - 1,
        level:  state.level,
        end:    -1,
        open:   scanned.can_open,
        close:  scanned.can_close
      });
    }

    state.pos += scanned.length;

    return true;
  }


  // Walk through delimiter list and replace text tokens with tags
  //
  function postProcess(state) {
    var i, j,
        startDelim,
        endDelim,
        token,
        loneMarkers = [],
        delimiters = state.delimiters,
        max = state.delimiters.length;

    for (i = 0; i < max; i++) {
      startDelim = delimiters[i];

      if (startDelim.marker !== 0x40/* @ */) {
        continue;
      }

      if (startDelim.end === -1) {
        continue;
      }

      endDelim = delimiters[startDelim.end];

      token         = state.tokens[startDelim.token];
      token.type    = 'ques_delim_open';
      token.tag     = 'quesDelim';
      token.nesting = 1;
      token.markup  = '===';
      token.content = '';

      token         = state.tokens[endDelim.token];
      token.type    = 'ques_delim_close';
      token.tag     = 'quesDelim';
      token.nesting = -1;
      token.markup  = '===';
      token.content = '';

      if (state.tokens[endDelim.token - 1].type === 'text' &&
          state.tokens[endDelim.token - 1].content === '=') {

        loneMarkers.push(endDelim.token - 1);
      }
    }

    // If a marker sequence has an odd number of characters, it's splitted
    // like this: `~~~~~` -> `~` + `~~` + `~~`, leaving one marker at the
    // start of the sequence.
    //
    // So, we have to move all those markers after subsequent s_close tags.
    //
    while (loneMarkers.length) {
      i = loneMarkers.pop();
      j = i + 1;

      while (j < state.tokens.length && state.tokens[j].type === 'ques_delim_close') {
        j++;
      }

      j--;

      if (i !== j) {
        token = state.tokens[j];
        state.tokens[j] = state.tokens[i];
        state.tokens[i] = token;
      }
    }
  }

  md.inline.ruler.before('emphasis', 'quesDelim', tokenize);
  md.inline.ruler2.before('emphasis', 'quesDelim', postProcess);
}

const md = markdownIt().use(delim_plugin)

const html = md.render('@@@The extension does **not** seem to be working@@@')
console.log(html)