Allow short-form license header

Author: srchase

config/checkstyle/checkstyle.xml

@@ -1,17 +1,7 @@
 <?xml version="1.0"?>
 <!--
-  ~ Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
-  ~
-  ~ Licensed under the Apache License, Version 2.0 (the "License").
-  ~ You may not use this file except in compliance with the License.
-  ~ A copy of the License is located at
-  ~
-  ~  http://aws.amazon.com/apache2.0
-  ~
-  ~ or in the "license" file accompanying this file. This file is distributed
-  ~ on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
-  ~ express or implied. See the License for the specific language governing
-  ~ permissions and limitations under the License.
+  ~ Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+  ~ SPDX-License-Identifier: Apache-2.0
   -->
 
 <!DOCTYPE module PUBLIC
@@ -29,10 +19,10 @@
         <property name="fileNamePattern" value="(module\-info\.java$|Test.java$|src/test/java)"/>
     </module>
 
-    <!-- Files must contain a copyright header. -->
+    <!-- Files must contain a copyright header, with or without a year. -->
     <module name="RegexpHeader">
         <property name="header"
-                  value="/\*\n \* Copyright 20(19|20|21|22) Amazon\.com, Inc\. or its affiliates\. All Rights Reserved\.\n"/>
+                  value="/\*\n \* Copyright( 20(19|20|21|22)|) Amazon\.com, Inc\. or its affiliates\. All Rights Reserved\.\n"/>
         <property name="fileExtensions" value="java"/>
     </module>