leopard.user.js

// ==UserScript==
// @name s/keyboard/leopard/g
// @version 3.0
// @description Replaces the word "keyboard" with "leopard".
// @match *://*/*
// @license MIT License
// @updateURL http://userscripts.org/scripts/source/128626.meta.js
// @downloadURL https://userscripts.org/scripts/source/128626.user.js
// ==/UserScript==

//which letters in "keyboard" get replaced with which in "leopard"
var leopard_subs = {
  'k': 'l', 'K': 'L',
  'y': 'o', 'Y': 'O',
  'b': 'p', 'B': 'P'
};

//Transform all instances of 'keyboard' in a string into 'leopard'
function leopardize(str) {
  return str.replace(/(k)(e)(y)(b)o(ard)/ig, function(match,k,e,y,b,ard){
    return leopard_subs[k] + e +  leopard_subs[y] + leopard_subs[b] + ard
  })
}

var TEXT_NODE = Node.TEXT_NODE || 3
var replacingContent = false

function replaceTextContent(node) {
  //flag that content is being replaced so the event it generates
  //won't trigger another replacement
  replacingContent = true
  node.textContent = leopardize(node.textContent)
  replacingContent = false
}

function changeTextNodes(node) {
  var length, childNodes
  //If this is a text node, leopardize it
  if (node.nodeType == TEXT_NODE) {
    replaceTextContent(node)
  //If this is anything other than a text node, recurse any children
  } else {
    childNodes = node.childNodes
    length = childNodes.length
    for(var i=0; i<length; ++i){
      changeTextNodes(childNodes[i])
    }
  }
}

function insertion_listener(event) {
  //change any new text nodes in a node that is added to the body
  changeTextNodes(event.target)
}

function cdm_listener(event) {
  //avoid infinite loop by ignoring events triggered by replacement
  if(!replacingContent){
    replaceTextContent(event.target)
  }
}

changeTextNodes(document.body)
document.title = leopardize(document.title)
document.body.addEventListener ("DOMNodeInserted", insertion_listener, false)
document.body.addEventListener ("DOMCharacterDataModified", cdm_listener, false)