Use regex to match external links

tildes/static/js/behaviors/external-links-new-tabs.js

@@ -1,7 +1,17 @@
 $.onmount('[data-js-external-links-new-tabs]', function() {
-    // Make links in the text open in a new tab
-    $(this).find(
-        'a[href^="http"]' +  // Select external links
-        ':not([href*="tildes.net"])'  // But not links to tildes
-    ).attr('target', '_blank');
+    /* Open external links in topic, comment, and message text in
+     * new tabs.
+     *
+     * The regex matches links starting with "/" or "tildes.net", which are
+     * "internal" links to Tildes pages (posts, topics, users, etc). All
+     * other links are considered "external" and open in a new tab.
+     */
+    let re = /^((?!\/).)$|^https?:\/\/((?!tildes\.net).).*$/gm;
+    // Loop through links
+    $(this).find('a').each(function() {
+        // Check if href is external
+        if (re.exec(this.href) !== null) {
+            $(this).attr('target', '_blank');
+        }
+    });
 });