diff --git a/core/lib/Drupal/Component/Utility/Xss.php b/core/lib/Drupal/Component/Utility/Xss.php
index 60c3c08..f6488a9 100644
--- a/core/lib/Drupal/Component/Utility/Xss.php
+++ b/core/lib/Drupal/Component/Utility/Xss.php
@@ -346,7 +346,7 @@ protected static function filterProtocol($name, $value) {
     // If the value matches the typical namespace:value pattern used in RDFa,
     // return it directly. Otherwise, filter it.
     if (in_array($name, static::$rdfaAttributes)) {
-      return preg_match('/[a-zA-Z0-9]+\:[a-zA-Z0-9]+$/', $value) ? $value : UrlHelper::stripDangerousProtocols($value);
+      return preg_match('/^[a-zA-Z0-9]+\:[a-zA-Z0-9]+$/', $value) ? $value : UrlHelper::stripDangerousProtocols($value);
     }
     else {
       return UrlHelper::filterBadProtocol($value);