Redirect - Keep string. This JavaScript will redirect a visitor to a different URL but keeps everything up until a defined part of the URL.

redirect-keep-substring.js

/* _optimizely_redirect=http://www.example.com */
var toKeep = "/[TO_KEEP]";
var addition = "/[NEW_ADDITION]";
var _optly = {redir: document.createElement("a")};
_optly.redir = {protocol: "https:" == document.location.protocol ? "https://" : "http://",
                domain: window.location.hostname,
                first: window.location.href.split(toKeep)[0],
                query: window.location.search
               };
_optly.redir.href = _optly.redir.first + toKeep + addition + _optly.redir.query;
window.location.replace(_optly.redir.href);