diff --git a/src/JsonSchema/Constraints/FormatConstraint.php b/src/JsonSchema/Constraints/FormatConstraint.php
index 9ed4df9d..c38488dd 100644
--- a/src/JsonSchema/Constraints/FormatConstraint.php
+++ b/src/JsonSchema/Constraints/FormatConstraint.php
@@ -210,7 +210,8 @@ protected function validateHostname($host)
             return true;
         }
 
-        $hostnameRegex = '/^(([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\-]*[a-zA-Z0-9])\.)*([A-Za-z0-9]|[A-Za-z0-9][A-Za-z0-9\-]*[A-Za-z0-9])$/i';
+        // RFC 1035: labels are max 63 chars (1 start + 0-61 middle + 1 end)
+        $hostnameRegex = '/^(?!-)(?!.*?[^A-Za-z0-9\-\.])(?:(?!-)[A-Za-z0-9](?:[A-Za-z0-9\-]{0,61}[A-Za-z0-9])?\.)*(?!-)[A-Za-z0-9](?:[A-Za-z0-9\-]{0,61}[A-Za-z0-9])?$/';
 
         return preg_match($hostnameRegex, $host);
     }