diff --git a/7. Deep Dive into GraphQL Type System.md b/7. Deep Dive into GraphQL Type System.md
index 4dc336c..f319ba2 100644
--- a/7. Deep Dive into GraphQL Type System.md	
+++ b/7. Deep Dive into GraphQL Type System.md	
@@ -386,6 +386,11 @@ var ValidateStringType = (params) => {
       return value;
     },
     parseValue: value => {
+      if(params.regex !== null) {
+        if(!params.regex.test(value)) {
+          throw new GraphQLError(`Query error: Not a valid ${params.name}`);
+        }
+      }
       return value;
     },
     parseLiteral: ast => {