diff --git a/ch04/4.2/4.2.md b/ch04/4.2/4.2.md
index 22472c49..4d0cb837 100644
--- a/ch04/4.2/4.2.md
+++ b/ch04/4.2/4.2.md
@@ -113,7 +113,9 @@ substring.
 
 1、
 
-    S -> (0?1)*
+   S -> XY
+   X -> 1X | ε
+   Y -> 01X | ε
 
 2、