KAFKA-19891: Bump group epoch when member regex subscription transitions from non-empty to empty

group-coordinator/src/main/java/org/apache/kafka/coordinator/group/GroupMetadataManager.java

@@ -3314,6 +3314,8 @@ private UpdateRegularExpressionsResult maybeUpdateRegularExpressions(
                         updateRegularExpressionsResult = UpdateRegularExpressionsResult.REGEX_UPDATED_AND_RESOLVED;
                     }
                 }
+            } else if (isNotEmpty(oldSubscribedTopicRegex)) {
+                updateRegularExpressionsResult = UpdateRegularExpressionsResult.REGEX_UPDATED_AND_RESOLVED;
             }
         }
 

group-coordinator/src/test/java/org/apache/kafka/coordinator/group/GroupMetadataManagerTest.java

@@ -21009,6 +21009,87 @@ fooTopicName, computeTopicHash(fooTopicName, metadataImage)
             tasks
         );
     }
+
+    @Test
+    public void testConsumerGroupMemberClearsRegex() {
+        String groupId = "fooup";
+        String memberId1 = Uuid.randomUuid().toString();
+
+        Uuid fooTopicId = Uuid.randomUuid();
+        String fooTopicName = "foo";
+
+        CoordinatorMetadataImage metadataImage = new MetadataImageBuilder()
+            .addTopic(fooTopicId, fooTopicName, 6)
+            .buildCoordinatorMetadataImage(12345L);
+
+        MockPartitionAssignor assignor = new MockPartitionAssignor("range");
+        assignor.prepareGroupAssignment(new GroupAssignment(Map.of()));
+        GroupMetadataManagerTestContext context = new GroupMetadataManagerTestContext.Builder()
+            .withConfig(GroupCoordinatorConfig.CONSUMER_GROUP_ASSIGNORS_CONFIG, List.of(assignor))
+            .withMetadataImage(metadataImage)
+            .withConsumerGroup(new ConsumerGroupBuilder(groupId, 10)
+                .withMember(new ConsumerGroupMember.Builder(memberId1)
+                    .setState(MemberState.STABLE)
+                    .setMemberEpoch(10)
+                    .setPreviousMemberEpoch(10)
+                    .setClientId(DEFAULT_CLIENT_ID)
+                    .setClientHost(DEFAULT_CLIENT_ADDRESS.toString())
+                    .setRebalanceTimeoutMs(5000)
+                    .setSubscribedTopicRegex("foo*")
+                    .setServerAssignorName("range")
+                    .setAssignedPartitions(mkAssignment(
+                        mkTopicAssignment(fooTopicId, 0, 1, 2, 3, 4, 5)))
+                    .build())
+                .withAssignment(memberId1, mkAssignment(
+                    mkTopicAssignment(fooTopicId, 0, 1, 2, 3, 4, 5)))
+                .withAssignmentEpoch(10))
+            .build();
+
+        // Member 1 updates its new regular expression.
+        CoordinatorResult<ConsumerGroupHeartbeatResponseData, CoordinatorRecord> result = context.consumerGroupHeartbeat(
+            new ConsumerGroupHeartbeatRequestData()
+                .setGroupId(groupId)
+                .setMemberId(memberId1)
+                .setMemberEpoch(10)
+                .setRebalanceTimeoutMs(5000)
+                .setSubscribedTopicRegex("")
+                .setServerAssignor("range")
+                .setTopicPartitions(List.of()));
+
+        assertResponseEquals(
+            new ConsumerGroupHeartbeatResponseData()
+                .setMemberId(memberId1)
+                .setMemberEpoch(11)
+                .setHeartbeatIntervalMs(5000)
+                .setAssignment(new ConsumerGroupHeartbeatResponseData.Assignment()
+                    .setTopicPartitions(List.of())
+                ),
+            result.response()
+        );
+
+        ConsumerGroupMember expectedMember1 = new ConsumerGroupMember.Builder(memberId1)
+            .setState(MemberState.STABLE)
+            .setMemberEpoch(11)
+            .setPreviousMemberEpoch(10)
+            .setClientId(DEFAULT_CLIENT_ID)
+            .setClientHost(DEFAULT_CLIENT_ADDRESS.toString())
+            .setRebalanceTimeoutMs(5000)
+            .setSubscribedTopicRegex("")
+            .setServerAssignorName("range")
+            .build();
+
+        List<CoordinatorRecord> expectedRecords = List.of(
+            GroupCoordinatorRecordHelpers.newConsumerGroupMemberSubscriptionRecord(groupId, expectedMember1),
+            // previous expression is deleted
+            GroupCoordinatorRecordHelpers.newConsumerGroupRegularExpressionTombstone(groupId, "foo*"),
+            GroupCoordinatorRecordHelpers.newConsumerGroupEpochRecord(groupId, 11, 0),
+            GroupCoordinatorRecordHelpers.newConsumerGroupTargetAssignmentRecord(groupId, memberId1, Map.of()),
+            GroupCoordinatorRecordHelpers.newConsumerGroupTargetAssignmentEpochRecord(groupId, 11),
+            GroupCoordinatorRecordHelpers.newConsumerGroupCurrentAssignmentRecord(groupId, expectedMember1)
+        );
+
+        assertRecordsEquals(expectedRecords, result.records());
+    }
 
     @Test
     public void testConsumerGroupMemberJoinsWithUpdatedRegex() {