[Kafka output] removed regex validation to allow dynamic topic

CHANGELOG.next.asciidoc

@@ -191,6 +191,7 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 - elasticsearch output now supports `idle_connection_timeout`. {issue}35616[35615] {pull}36843[36843]
 - Enable early event encoding in the Elasticsearch output, improving cpu and memory use {pull}38572[38572]
 - The environment variable `BEATS_ADD_CLOUD_METADATA_PROVIDERS` overrides configured/default `add_cloud_metadata` providers {pull}38669[38669]
+- Kafka output allows dynamic topic in `topic` field {pull}40415[40415]
 
 *Auditbeat*
 

libbeat/outputs/kafka/config.go

@@ -22,7 +22,6 @@ import (
 	"fmt"
 	"math"
 	"math/rand"
-	"regexp"
 	"strings"
 	"time"
 
@@ -109,11 +108,6 @@ var compressionModes = map[string]sarama.CompressionCodec{
 	"snappy": sarama.CompressionSnappy,
 }
 
-// validTopicRegExp is used to validate the topic contains only valid characters
-// when running under Elastic-Agent. The regexp is taken from:
-// https://github.com/apache/kafka/blob/a126e3a622f2b7142f3543b9dbee54b6412ba9d8/clients/src/main/java/org/apache/kafka/common/internals/Topic.java#L33
-var validTopicRegExp = regexp.MustCompile("^[a-zA-Z0-9._-]+$")
-
 func defaultConfig() kafkaConfig {
 	return kafkaConfig{
 		Hosts:              nil,
@@ -193,10 +187,6 @@ func (c *kafkaConfig) Validate() error {
 		if len(c.Topics) != 0 {
 			return errors.New("'topics' is not supported when running under Elastic-Agent")
 		}
-
-		if !validTopicRegExp.MatchString(c.Topic) {
-			return fmt.Errorf("topic '%s' is invalid, it must match '[a-zA-Z0-9._-]'", c.Topic)
-		}
 	}
 
 	return nil

libbeat/outputs/kafka/config_test.go

@@ -142,25 +142,10 @@ func TestConfigUnderElasticAgent(t *testing.T) {
 			expectError: true,
 		},
 		{
-			name: "topic cannot contain invalid characters",
+			name: "valid topic with dynamic topic selection",
 			cfg: mapstr.M{
-				"topic": "foo bar",
+				"topic": "%{[event.field]}",
 			},
-			expectError: true,
-		},
-		{
-			name: "topic with invalid characters",
-			cfg: mapstr.M{
-				"topic": "foo + bar",
-			},
-			expectError: true,
-		},
-		{
-			name: "topic with invalid characters from dynamic topic selection",
-			cfg: mapstr.M{
-				"topic": "%{event.field}",
-			},
-			expectError: true,
 		},
 
 		// The default config does not set `topic` not `topics`.

libbeat/outputs/kafka/kafka.go

@@ -23,7 +23,6 @@ import (
 	"github.com/Shopify/sarama"
 
 	"github.com/elastic/beats/v7/libbeat/beat"
-	"github.com/elastic/beats/v7/libbeat/management"
 	"github.com/elastic/beats/v7/libbeat/outputs"
 	"github.com/elastic/beats/v7/libbeat/outputs/codec"
 	"github.com/elastic/beats/v7/libbeat/outputs/outil"
@@ -91,21 +90,11 @@ func makeKafka(
 // running under Elastic-Agent based on cfg.
 //
 // When running standalone the topic selector works as expected and documented.
-// When running under Elastic-Agent, dynamic topic selection is not supported,
-// so a constant selector using the `topic` value is returned.
+// When running under Elastic-Agent, dynamic topic selection is also supported
 func buildTopicSelector(cfg *config.C) (outil.Selector, error) {
-	topicCfg := struct {
-		Topic string `config:"topic" yaml:"topic"`
-	}{}
 
-	if err := cfg.Unpack(&topicCfg); err != nil {
-		return outil.Selector{}, fmt.Errorf("cannot unpack Kafka config to read the topic: %w", err)
-	}
-
-	if management.UnderAgent() {
-		exprSelector := outil.ConstSelectorExpr(topicCfg.Topic, outil.SelectorKeepCase)
-		selector := outil.MakeSelector(exprSelector)
-		return selector, nil
+	if cfg == nil {
+		return outil.Selector{}, fmt.Errorf("Kafka config cannot be nil")
 	}
 
 	return outil.BuildSelectorFromConfig(cfg, outil.Settings{

libbeat/outputs/kafka/kafka_test.go

@@ -42,7 +42,7 @@ func TestBuildTopicSelector(t *testing.T) {
 		{
 			name:       "dynamic topic under agent",
 			topic:      "%{[foo]}",
-			expected:   "%{[foo]}",
+			expected:   "bar",
 			underAgent: true,
 		},
 		{