markdown-it/lib/rules_inline/mark.js

// Process ==highlighted text==

'use strict';

module.exports = function del(state, silent) {
  var found,
      pos,
      stack,
      max = state.posMax,
      start = state.pos,
      lastChar,
      nextChar;

  if (state.src.charCodeAt(start) !== 0x3D/* = */) { return false; }
  if (silent) { return false; } // don't run any pairs in validation mode
  if (start + 4 >= max) { return false; }
  if (state.src.charCodeAt(start + 1) !== 0x3D/* = */) { return false; }
  if (state.level >= state.options.maxNesting) { return false; }

  lastChar = start > 0 ? state.src.charCodeAt(start - 1) : -1;
  nextChar = state.src.charCodeAt(start + 2);

  if (lastChar === 0x3D/* = */) { return false; }
  if (nextChar === 0x3D/* = */) { return false; }
  if (nextChar === 0x20 || nextChar === 0x0A) { return false; }

  pos = start + 2;
  while (pos < max && state.src.charCodeAt(pos) === 0x3D/* = */) { pos++; }
  if (pos !== start + 2) {
    // sequence of 3+ markers taking as literal, same as in a emphasis
    state.pos += pos - start;
    if (!silent) { state.pending += state.src.slice(start, pos); }
    return true;
  }

  state.pos = start + 2;
  stack = 1;

  while (state.pos + 1 < max) {
    if (state.src.charCodeAt(state.pos) === 0x3D/* = */) {
      if (state.src.charCodeAt(state.pos + 1) === 0x3D/* = */) {
        lastChar = state.src.charCodeAt(state.pos - 1);
        nextChar = state.pos + 2 < max ? state.src.charCodeAt(state.pos + 2) : -1;
        if (nextChar !== 0x3D/* = */ && lastChar !== 0x3D/* = */) {
          if (lastChar !== 0x20 && lastChar !== 0x0A) {
            // closing '=='
            stack--;
          } else if (nextChar !== 0x20 && nextChar !== 0x0A) {
            // opening '=='
            stack++;
          } // else {
            //  // standalone ' == ' indented with spaces
            // }
          if (stack <= 0) {
            found = true;
            break;
          }
        }
      }
    }

    state.parser.skipToken(state);
  }

  if (!found) {
    // parser failed to find ending tag, so it's not valid emphasis
    state.pos = start;
    return false;
  }

  // found!
  state.posMax = state.pos;
  state.pos = start + 2;

  if (!silent) {
    state.push({ type: 'mark_open', level: state.level++ });
    state.parser.tokenize(state);
    state.push({ type: 'mark_close', level: --state.level });
  }

  state.pos = state.posMax + 2;
  state.posMax = max;
  return true;
};