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))
+- Honor COMPLEMENT for Regexp queries ([#18640](https://github.com/opensearch-project/OpenSearch/pull/18640))
 
 
 ### Security

gradle/run.gradle

@@ -52,6 +52,7 @@ testClusters {
         }
       }
     }
+    setting 'logger.level', 'DEBUG'
   }
 
 }

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;