Update changelog

Signed-off-by: Prudhvi Godithi <pgodithi@amazon.com>

Author: prudhvigodithi

CHANGELOG.md

@@ -36,6 +36,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - [Autotagging] Fix delete rule event consumption in InMemoryRuleProcessingService ([#18628](https://github.com/opensearch-project/OpenSearch/pull/18628))
 - Cannot communicate with HTTP/2 when reactor-netty is enabled ([#18599](https://github.com/opensearch-project/OpenSearch/pull/18599))
 - Fix the visit of sub queries for HasParentQuery and HasChildQuery ([#18621](https://github.com/opensearch-project/OpenSearch/pull/18621))
+- Fix the backward compatibility regression with COMPLEMENT for Regexp queries introduced in OpenSearch 3.0 ([#18640](https://github.com/opensearch-project/OpenSearch/pull/18640))
 
 
 ### Security

server/src/main/java/org/opensearch/index/query/RegexpQueryBuilder.java

@@ -38,6 +38,7 @@
 import org.apache.lucene.search.RegexpQuery;
 import org.apache.lucene.util.automaton.Operations;
 import org.apache.lucene.util.automaton.RegExp;
+import org.opensearch.common.logging.DeprecationLogger;
 import org.opensearch.common.lucene.BytesRefs;
 import org.opensearch.common.xcontent.LoggingDeprecationHandler;
 import org.opensearch.core.ParseField;
@@ -60,6 +61,9 @@
  * @opensearch.internal
  */
 public class RegexpQueryBuilder extends AbstractQueryBuilder<RegexpQueryBuilder> implements MultiTermQueryBuilder {
+
+    private static final DeprecationLogger deprecationLogger = DeprecationLogger.getLogger(RegexpQueryBuilder.class);
+
     public static final String NAME = "regexp";
 
     public static final int DEFAULT_FLAGS_VALUE = RegexpFlag.ALL.value();
@@ -294,6 +298,18 @@ protected Query doToQuery(QueryShardContext context) throws QueryShardException,
                     + "] index level setting."
             );
         }
+
+        // Check if COMPLEMENT flag is being used
+        // The COMPLEMENT flag maps to Lucene's DEPRECATED_COMPLEMENT which is marked for removal in Lucene 11
+        // This deprecation warning helps users migrate their queries before the feature is completely removed
+        if ((syntaxFlagsValue & RegexpFlag.COMPLEMENT.value()) != 0) {
+            deprecationLogger.deprecate(
+                "regexp_complement_operator",
+                "The complement operator (~) for arbitrary patterns in regexp queries is deprecated and will be removed in a future version. "
+                    + "Consider rewriting your query to use character class negation [^...] or other query types."
+            );
+        }
+
         MultiTermQuery.RewriteMethod method = QueryParsers.parseRewriteMethod(rewrite, null, LoggingDeprecationHandler.INSTANCE);
 
         int matchFlagsValue = caseInsensitive ? RegExp.ASCII_CASE_INSENSITIVE : 0;

server/src/test/java/org/opensearch/index/query/RegexpQueryBuilderTests.java

@@ -55,7 +55,15 @@ protected RegexpQueryBuilder doCreateTestQueryBuilder() {
             List<RegexpFlag> flags = new ArrayList<>();
             int iter = randomInt(5);
             for (int i = 0; i < iter; i++) {
-                flags.add(randomFrom(RegexpFlag.values()));
+                // Exclude COMPLEMENT from random selection to avoid deprecation warnings
+                RegexpFlag[] availableFlags = {
+                    RegexpFlag.INTERSECTION,
+                    RegexpFlag.EMPTY,
+                    RegexpFlag.ANYSTRING,
+                    RegexpFlag.INTERVAL,
+                    RegexpFlag.NONE,
+                    RegexpFlag.ALL };
+                flags.add(randomFrom(availableFlags));
             }
             query.flags(flags.toArray(new RegexpFlag[0]));
         }
@@ -162,4 +170,32 @@ public void testParseFailsWithMultipleFields() throws IOException {
         e = expectThrows(ParsingException.class, () -> parseQuery(shortJson));
         assertEquals("[regexp] query doesn't support multiple fields, found [user1] and [user2]", e.getMessage());
     }
+
+    // Test that COMPLEMENT flag triggers deprecation warning
+    public void testComplementFlagDeprecation() throws IOException {
+        RegexpQueryBuilder query = new RegexpQueryBuilder("field", "a~bc");
+        query.flags(RegexpFlag.COMPLEMENT);
+        QueryShardContext context = createShardContext();
+        Query luceneQuery = query.toQuery(context);
+        assertNotNull(luceneQuery);
+        assertThat(luceneQuery, instanceOf(RegexpQuery.class));
+        assertWarnings(
+            "The complement operator (~) for arbitrary patterns in regexp queries is deprecated "
+                + "and will be removed in a future version. Consider rewriting your query to use character class negation [^...] or other query types."
+        );
+    }
+
+    // Separate test for COMPLEMENT flag Cacheability
+    public void testComplementFlagCacheability() throws IOException {
+        RegexpQueryBuilder queryBuilder = new RegexpQueryBuilder("field", "pattern");
+        queryBuilder.flags(RegexpFlag.COMPLEMENT);
+        QueryShardContext context = createShardContext();
+        QueryBuilder rewriteQuery = rewriteQuery(queryBuilder, new QueryShardContext(context));
+        assertNotNull(rewriteQuery.toQuery(context));
+        assertTrue("query should be cacheable: " + queryBuilder, context.isCacheable());
+        assertWarnings(
+            "The complement operator (~) for arbitrary patterns in regexp queries is deprecated "
+                + "and will be removed in a future version. Consider rewriting your query to use character class negation [^...] or other query types."
+        );
+    }
 }