markdown-it/lib/rules_inline/sub.js

// Process ~subscript~

'use strict';

// same as UNESCAPE_MD_RE plus a space
var UNESCAPE_RE = /\\([ \\!"#$%&'()*+,.\/:;<=>?@[\]^_`{|}~-])/g;

module.exports = function sub(state, silent) {
  var found,
      content,
      max = state.posMax,
      start = state.pos;

  if (state.src.charCodeAt(start) !== 0x7E/* ~ */) { return false; }
  if (silent) { return false; } // don't run any pairs in validation mode
  if (start + 2 >= max) { return false; }
  if (state.level >= state.options.maxNesting) { return false; }

  state.pos = start + 1;

  while (state.pos < max) {
    if (state.src.charCodeAt(state.pos) === 0x7E/* ~ */) {
      found = true;
      break;
    }

    state.parser.skipToken(state);
  }

  if (!found || start + 1 === state.pos) {
    state.pos = start;
    return false;
  }

  content = state.src.slice(start + 1, state.pos);

  // don't allow unescaped spaces/newlines inside
  if (content.match(/(^|[^\\])(\\\\)*\s/)) {
    state.pos = start;
    return false;
  }

  // found!
  state.posMax = state.pos;
  state.pos = start + 1;

  if (!silent) {
    state.push({
      type: 'sub',
      level: state.level,
      content: content.replace(UNESCAPE_RE, '$1')
    });
  }

  state.pos = state.posMax + 1;
  state.posMax = max;
  return true;
};