showdown/src/subParsers/strikethrough.js

showdown.subParser('strikethrough', function (text, options, globals) {
  'use strict';

  if (options.strikethrough) {
    text = globals.converter._dispatch('strikethrough.before', text, options, globals);
    text = text.replace(/(?:~T){2}([\s\S]+?)(?:~T){2}/g, '<del>$1</del>');
    text = globals.converter._dispatch('strikethrough.after', text, options, globals);
  }

  return text;
});