Ticket #62523: 62523.2.patch

src/wp-includes/class-wp-block-templates-registry.php

@@ -50 +50 @@
                 } elseif ( preg_match( '/[A-Z]+/', $template_name ) ) {
                         $error_message = __( 'Template names must not contain uppercase characters.' );
                         $error_code    = 'template_name_no_uppercase';
-                } elseif ( ! preg_match( '/^[a-z0-9-]+\/\/[a-z0-9-]+$/', $template_name ) ) {
+                } elseif ( ! preg_match( '/^[a-z0-9_\-]+\/\/[a-z0-9_\-]+$/', $template_name ) ) {
                         $error_message = __( 'Template names must contain a namespace prefix. Example: my-plugin//my-custom-template' );
                         $error_code    = 'template_no_prefix';
                 } elseif ( $this->is_registered( $template_name ) ) {