This is a greasemonkey script that replaces the word "cloud" with "butt" on websites.

cloud_to_butt.js

// ==UserScript==
// @name      Cloud to Butt
// @namespace https://3nt3.de
// @include   *
// @version   1
// ==/UserScript==

(function () {
  "use strict";
  var words = {
    cloud: "butt",
    Cloud: "Butt",
  };
  ///////////////////////////////////////////////////////////////////////////////
  var regexs = [],
    replacements = [],
    tagsWhitelist = [
      "PRE",
      "BLOCKQUOTE",
      "CODE",
      "INPUT",
      "BUTTON",
      "TEXTAREA",
    ],
    rIsRegexp = /^\/(.+)\/([gm]+)?$/,
    word,
    text,
    texts,
    i,
    userRegexp;
  // prepareRegex by JoeSimmons
  // used to take a string and ready it for use in new RegExp()
  function prepareRegex(string) {
    return string.replace(/([\[\]\^\&\$\.\(\)\?\/\\\+\{\}\|])/g, "\\$1");
  }
  // function to decide whether a parent tag will have its text replaced or not

  function isTagOk(tag) {
    return tagsWhitelist.indexOf(tag) === -1;
  }
  // convert the 'words' JSON object to an Array
  for (word in words) {
    if (typeof word === "string" && words.hasOwnProperty(word)) {
      userRegexp = word.match(rIsRegexp);
      // add the search/needle/query
      if (userRegexp) {
        regexs.push(new RegExp(userRegexp[1], "g"));
      } else {
        regexs.push(
          new RegExp(
            prepareRegex(word).replace(/\\?\*/g, function (fullMatch) {
              return fullMatch === "\\*" ? "*" : "[^ ]*";
            }),
            "g"
          )
        );
      }
      // add the replacement

      replacements.push(words[word]);
    }
  }
  // do the replacement

  texts = document.evaluate(
    '//body//text()[ normalize-space(.) != "" ]',
    document,
    null,
    6,
    null
  );
  for (i = 0; (text = texts.snapshotItem(i)); i += 1) {
    if (isTagOk(text.parentNode.tagName)) {
      regexs.forEach(function (value, index) {
        text.data = text.data.replace(value, replacements[index]);
      });
    }
  }
})();