showdown/src/subParsers/encodeAmpsAndAngles.js

/**
 * Smart processing for ampersands and angle brackets that need to be encoded.
 */
showdown.subParser('encodeAmpsAndAngles', function (text) {
  'use strict';
  // Ampersand-encoding based entirely on Nat Irons's Amputator MT plugin:
  // http://bumppo.net/projects/amputator/
  text = text.replace(/&(?!#?[xX]?(?:[0-9a-fA-F]+|\w+);)/g, '&');

  // Encode naked <'s
  text = text.replace(/<(?![a-z\/?\$!])/gi, '&lt;');

  return text;
});