Strip port numbers from root page domains (see contao#2553)

Description
-----------

| Q                | A
| -----------------| ---
| Fixed issues     | Fixes contao#2214
| Docs PR or issue | -

Tested against https://regex101.com/r/jU0OrQ/2

Commits
-------

52e85cd Strip port numbers from root page domains
48a0c3c Improve the regex

Co-authored-by: Martin Auswöger <martin@auswoeger.com>

core-bundle/src/Resources/contao/dca/tl_page.php

@@ -1282,7 +1282,16 @@ public function loadDns($varValue)
 	 */
 	public function checkDns($varValue)
 	{
-		return Idna::encode(preg_replace('#^(?:[a-z]+://)?([\pN\pL[\].:_-]+).*$#iu', '$1', $varValue));
+		if (!$varValue)
+		{
+			return '';
+		}
+
+		// The first part will match IPv6 addresses in square brackets. The
+		// second part will match domain names and IPv4 addresses.
+		preg_match('#^(?:[a-z]+://)?(\[[0-9a-f:]+]|[\pN\pL._-]*)#ui', $varValue, $matches);
+
+		return Idna::encode($matches[1]);
 	}
 
 	/**