User script that warns if the text of a clicked link looks like a domain that does not match the href attribute.

url_scam_detection.js

// ==UserScript==
// @name         URL Scam Detection
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Warns if the text of a clicked link looks like a domain that does not match the href attribute.
// @author       vctls
// @match        *
// @grant        none
// ==/UserScript==

function warnIfLinkNotMatching(e){
    const el = e.target;
    if (!el.nodeName === 'A') {
        return;
    }
    const href = el.href;
    const text = el.text;
    // This should match most domain names, and probably give some false positives too.
    const regex = /(((?!-))(xn--|_{1,1})?[a-z0-9-]{0,61}[a-z0-9]{1,1}\.)*(xn--)?([a-z0-9-]{1,30}\.[a-z]{2,})/;
    // TODO We're only looking for something that looks like a domain name, not a full URL.
    //  This will obviously break window.open() if the full text doesn't correspond exactly to a URL.
    //  Use a different regex to get the full link instead.
    const textMatches = text.match(regex);
    const hrefMatches = href.match(regex);
    if (
        textMatches !== null
        && textMatches[0] !== hrefMatches[0]
    ){
        e.preventDefault();
        const selection = prompt(
            "The link you clicked looks like a domain name, but doesn't match the underlying URL. "
            + "Which domain would you like to follow?"
            + `\nA: ${textMatches[0]}\nB: ${hrefMatches[0]}`
                                );
        if (selection === 'B') {
            window.open(href, '_blank').focus();
        }
        if (selection === 'A') {
            window.open(text, '_blank').focus();
        }
    }
}

(function() {
    'use strict';

    document.addEventListener('click', warnIfLinkNotMatching);
})();