Fix pattern replace by making flag optional as on api

CHANGELOG.md

@@ -65,6 +65,7 @@ This section is for maintaining a changelog for all breaking changes for the cli
 
 ### Fixed
 - Fix missing properties on UpdateOperation ([#744](https://github.com/opensearch-project/opensearch-java/pull/744))
+- Fix pattern replace by making flag optional as on api  ([#895](https://github.com/opensearch-project/opensearch-java/pull/895))
 
 ### Security
 

java-client/src/main/java/org/opensearch/client/opensearch/_types/analysis/PatternReplaceCharFilter.java

@@ -34,6 +34,7 @@
 
 import jakarta.json.stream.JsonGenerator;
 import java.util.function.Function;
+import javax.annotation.Nullable;
 import org.opensearch.client.json.JsonpDeserializable;
 import org.opensearch.client.json.JsonpDeserializer;
 import org.opensearch.client.json.JsonpMapper;
@@ -46,6 +47,7 @@
 
 @JsonpDeserializable
 public class PatternReplaceCharFilter extends CharFilterBase implements CharFilterDefinitionVariant {
+    @Nullable
     private final String flags;
 
     private final String pattern;
@@ -57,7 +59,7 @@ public class PatternReplaceCharFilter extends CharFilterBase implements CharFilt
     private PatternReplaceCharFilter(Builder builder) {
         super(builder);
 
-        this.flags = ApiTypeHelper.requireNonNull(builder.flags, this, "flags");
+        this.flags = builder.flags;
         this.pattern = ApiTypeHelper.requireNonNull(builder.pattern, this, "pattern");
         this.replacement = ApiTypeHelper.requireNonNull(builder.replacement, this, "replacement");
 
@@ -76,8 +78,9 @@ public CharFilterDefinition.Kind _charFilterDefinitionKind() {
     }
 
     /**
-     * Required - API name: {@code flags}
+     * API name: {@code flags}
      */
+    @Nullable
     public final String flags() {
         return this.flags;
     }
@@ -100,8 +103,11 @@ protected void serializeInternal(JsonGenerator generator, JsonpMapper mapper) {
 
         generator.write("type", "pattern_replace");
         super.serializeInternal(generator, mapper);
-        generator.writeKey("flags");
-        generator.write(this.flags);
+
+        if (this.flags != null) {
+            generator.writeKey("flags");
+            generator.write(this.flags);
+        }
 
         generator.writeKey("pattern");
         generator.write(this.pattern);
@@ -118,16 +124,17 @@ protected void serializeInternal(JsonGenerator generator, JsonpMapper mapper) {
      */
 
     public static class Builder extends CharFilterBase.AbstractBuilder<Builder> implements ObjectBuilder<PatternReplaceCharFilter> {
+        @Nullable
         private String flags;
 
         private String pattern;
 
         private String replacement;
 
         /**
-         * Required - API name: {@code flags}
+         * API name: {@code flags}
          */
-        public final Builder flags(String value) {
+        public final Builder flags(@Nullable String value) {
             this.flags = value;
             return this;
         }

java-client/src/test/java/org/opensearch/client/opensearch/_types/analysis/PatternReplaceCharFilterTest.java

@@ -0,0 +1,53 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.client.opensearch._types.analysis;
+
+import static org.junit.Assert.assertEquals;
+
+import jakarta.json.stream.JsonParser;
+import java.io.StringReader;
+import org.junit.Test;
+import org.opensearch.client.json.jackson.JacksonJsonpMapper;
+
+public class PatternReplaceCharFilterTest {
+    @Test
+    public void testCreatePatternReplaceCharFilter() {
+        PatternReplaceCharFilter patternReplaceCharFilter = new PatternReplaceCharFilter.Builder().pattern("pattern")
+            .replacement("replacement")
+            .build();
+        assertEquals("pattern", patternReplaceCharFilter.pattern());
+        assertEquals("replacement", patternReplaceCharFilter.replacement());
+    }
+
+    @Test
+    public void testCreatePatternReplaceCharFilterWithFlags() {
+        PatternReplaceCharFilter patternReplaceCharFilter = new PatternReplaceCharFilter.Builder().pattern("pattern")
+            .replacement("replacement")
+            .flags("flags")
+            .build();
+        assertEquals("pattern", patternReplaceCharFilter.pattern());
+        assertEquals("replacement", patternReplaceCharFilter.replacement());
+        assertEquals("flags", patternReplaceCharFilter.flags());
+    }
+
+    @Test
+    public void testDeserializePatternReplaceCharFilter() {
+        String jsonString =
+            "{\"type\": \"pattern_replace\", \"pattern\": \"pattern\", \"replacement\": \"replacement\", \"flags\": \"flags\"}";
+
+        StringReader reader = new StringReader(jsonString);
+        JacksonJsonpMapper mapper = new JacksonJsonpMapper();
+        JsonParser parser = mapper.jsonProvider().createParser(reader);
+
+        PatternReplaceCharFilter patternReplaceCharFilter = PatternReplaceCharFilter._DESERIALIZER.deserialize(parser, mapper);
+        assertEquals("pattern", patternReplaceCharFilter.pattern());
+        assertEquals("replacement", patternReplaceCharFilter.replacement());
+        assertEquals("flags", patternReplaceCharFilter.flags());
+    }
+}