diff --git a/apps/theming/lib/Controller/ThemingController.php b/apps/theming/lib/Controller/ThemingController.php
index 1638f5bd95799..b9e71996a9541 100644
--- a/apps/theming/lib/Controller/ThemingController.php
+++ b/apps/theming/lib/Controller/ThemingController.php
@@ -203,8 +203,8 @@ public function updateStylesheet($setting, $value) {
 	 * Check that a string is a valid http/https url
 	 */
 	private function isValidUrl(string $url): bool {
-		return ((strpos($url, 'http://') === 0 || strpos($url, 'https://') === 0) &&
-			filter_var($url, FILTER_VALIDATE_URL) !== false);
+		// Regex copied from https://stackoverflow.com/a/206087 but without ftp support
+		return preg_match('#(https?://(\S*?\.\S*?))([\s)\[\]{},;"\':<]|\.\s|$)#i', $url);
 	}
 
 	/**