--- app/code/DCKAP/Checkout/Controller/Index/Index.php
+++ app/code/DCKAP/Checkout/Controller/Index/Index.php
@@ -69,7 +69,7 @@
         }
           /* code to maintain invoice transaction log*/
         $data=$this->getRequest()->getParam('data');
-        if ( $data && !preg_match('/^[0-9_-]+$/',  $data)) {
+        if ($data && !preg_match('/^[a-zA-Z0-9_-]+$/', $data)) {
             // Show error and redirect
              /** @var \Magento\Framework\Controller\Result\Forward $resultForward */
             $resultForward = $this->resultForwardFactory->create();