diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index 2fb320e..9b20478 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -1062,6 +1062,7 @@
 *c_no_cformat*		don't highlight %-formats in strings
 *c_no_c99*		don't highlight C99 standard items
 *c_no_c11*		don't highlight C11 standard items
+*c_no_c23*		don't highlight C23 standard items
 *c_no_bsd*		don't highlight BSD specific types
 *c_functions*		highlight function calls and definitions
 *c_function_pointers*	highlight function pointers definitions