showdownjs/src/subParsers/makemarkdown/txt.js

showdown.subParser('makeMarkdown.txt', function (node) {
  'use strict';

  var txt = node.nodeValue;

  // multiple spaces are collapsed
  txt = txt.replace(/ +/g, ' ');

  // replace the custom ¨NBSP; with a space
  txt = txt.replace(/¨NBSP;/g, ' ');

  // ", <, > and & should replace escaped html entities
  txt = showdown.helper.unescapeHTMLEntities(txt);

  // escape markdown magic characters
  // emphasis, strong and strikethrough - can appear everywhere
  // we also escape pipe (|) because of tables
  // and escape ` because of code blocks and spans
  txt = txt.replace(/([*_~|`])/g, '\\$1');

  // escape > because of blockquotes
  txt = txt.replace(/^(\s*)>/g, '\\$1>');

  // hash character, only troublesome at the beginning of a line because of headers
  txt = txt.replace(/^#/gm, '\\#');

  // horizontal rules
  txt = txt.replace(/^(\s*)([-=]{3,})(\s*)$/, '$1\\$2$3');

  // dot, because of ordered lists, only troublesome at the beginning of a line when preceded by an integer
  txt = txt.replace(/^( {0,3}\d+)\./gm, '$1\\.');

  // +, * and -, at the beginning of a line becomes a list, so we need to escape them also (asterisk was already escaped)
  txt = txt.replace(/^( {0,3})([+-])/gm, '$1\\$2');

  // images and links, ] followed by ( is problematic, so we escape it
  txt = txt.replace(/]([\s]*)\(/g, '\\]$1\\(');

  // reference URIs must also be escaped
  txt = txt.replace(/^ {0,3}\[([\S \t]*?)]:/gm, '\\[$1]:');

  return txt;
});