Disallow creating indices starting with '-' or '+'

core/src/main/java/org/elasticsearch/cluster/metadata/MetaDataCreateIndexService.java

@@ -144,7 +144,7 @@ public MetaDataCreateIndexService(Settings settings, ClusterService clusterServi
         this.activeShardsObserver = new ActiveShardsObserver(settings, clusterService, threadPool);
     }
 
-    public void validateIndexName(String index, ClusterState state) {
+    public static void validateIndexName(String index, ClusterState state) {
         if (state.routingTable().hasIndex(index)) {
             throw new IndexAlreadyExistsException(state.routingTable().index(index).getIndex());
         }
@@ -157,8 +157,8 @@ public void validateIndexName(String index, ClusterState state) {
         if (index.contains("#")) {
             throw new InvalidIndexNameException(index, "must not contain '#'");
         }
-        if (index.charAt(0) == '_') {
-            throw new InvalidIndexNameException(index, "must not start with '_'");
+        if (index.charAt(0) == '_' || index.charAt(0) == '-' || index.charAt(0) == '+') {
+            throw new InvalidIndexNameException(index, "must not start with '_', '-', or '+'");
         }
         if (!index.toLowerCase(Locale.ROOT).equals(index)) {
             throw new InvalidIndexNameException(index, "must be lowercase");

core/src/main/java/org/elasticsearch/snapshots/RestoreService.java

@@ -257,7 +257,7 @@ public ClusterState execute(ClusterState currentState) {
                             if (currentIndexMetaData == null) {
                                 // Index doesn't exist - create it and start recovery
                                 // Make sure that the index we are about to create has a validate name
-                                createIndexService.validateIndexName(renamedIndexName, currentState);
+                                MetaDataCreateIndexService.validateIndexName(renamedIndexName, currentState);
                                 createIndexService.validateIndexSettings(renamedIndexName, snapshotIndexMetaData.getSettings());
                                 IndexMetaData.Builder indexMdBuilder = IndexMetaData.builder(snapshotIndexMetaData).state(IndexMetaData.State.OPEN).index(renamedIndexName);
                                 indexMdBuilder.settings(Settings.builder().put(snapshotIndexMetaData.getSettings()).put(IndexMetaData.SETTING_INDEX_UUID, UUIDs.randomBase64UUID()));

core/src/test/java/org/elasticsearch/cluster/metadata/MetaDataCreateIndexServiceTests.java

@@ -191,7 +191,9 @@ public void testValidateIndexName() throws Exception {
 
         validateIndexName("index#name", "must not contain '#'");
 
-        validateIndexName("_indexname", "must not start with '_'");
+        validateIndexName("_indexname", "must not start with '_', '-', or '+'");
+        validateIndexName("-indexname", "must not start with '_', '-', or '+'");
+        validateIndexName("+indexname", "must not start with '_', '-', or '+'");
 
         validateIndexName("INDEXNAME", "must be lowercase");
 
@@ -201,7 +203,7 @@ public void testValidateIndexName() throws Exception {
 
     private void validateIndexName(String indexName, String errorMessage) {
         InvalidIndexNameException e = expectThrows(InvalidIndexNameException.class,
-            () -> getCreateIndexService().validateIndexName(indexName, ClusterState.builder(ClusterName.CLUSTER_NAME_SETTING
+            () -> MetaDataCreateIndexService.validateIndexName(indexName, ClusterState.builder(ClusterName.CLUSTER_NAME_SETTING
                 .getDefault(Settings.EMPTY)).build()));
         assertThat(e.getMessage(), endsWith(errorMessage));
     }

docs/reference/migration/migrate_5_0/index-apis.asciidoc

@@ -49,3 +49,11 @@ CPU usage can be obtained from `OsStats.Cpu#getPercent`.
 
 Suggest stats exposed through `suggest` in indices stats has been merged
 with `search` stats. `suggest` stats is exposed as part of `search` stats.
+
+==== Creating indices starting with '-' or '+'
+
+Elasticsearch no longer allows indices to be created started with '-' or '+', so
+that the multi-index matching and expansion is not confused. It was previously
+possible (but a really bad idea) to create indices starting with a hyphen or
+plus sign. Any index already existing with these preceding characters will
+continue to work normally.