[8.17](backport #41903) Add regex pattern matching to add_kubernetes_metadata processor

CHANGELOG.next.asciidoc

@@ -177,6 +177,10 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 - Update to Go 1.22.7. {pull}41018[41018]
 - Replace Ubuntu 20.04 with 24.04 for Docker base images {issue}40743[40743] {pull}40942[40942]
 - Reduce memory consumption of k8s autodiscovery and the add_kubernetes_metadata processor when Deployment metadata is enabled
+- Add `lowercase` processor. {issue}22254[22254] {pull}41424[41424]
+- Add `uppercase` processor. {issue}22254[22254] {pull}41535[41535]
+- Replace `compress/gzip` with https://github.com/klauspost/compress/gzip library for gzip compression {pull}41584[41584]
+- Add regex pattern matching to add_kubernetes_metadata processor {pull}41903[41903]
 
 *Auditbeat*
 

libbeat/processors/add_kubernetes_metadata/docs/indexers_and_matchers.asciidoc

@@ -74,6 +74,27 @@ processors:
           lookup_fields: ['destination.ip', 'server.ip']
 -------------------------------------------------------------------------------
 
+It's also possible to extract the matching key from fields using a regex pattern.
+The optional `regex_pattern` field can be used to set the pattern. The pattern
+*must* contain a capture group named `key`, whose value will be used as the matching key.
+
+For example, the following configuration uses the `container` indexer to identify
+containers by their id, and extracts the matching key from the cgroup id field added
+to system process metrics. This field has the form `cri-containerd-<id>.scope`, so
+we need a regex pattern to obtain the container id.
+
+[source,yaml]
+-------------------------------------------------------------------------------
+processors:
+  - add_kubernetes_metadata:
+      indexers:
+        - container:
+      matchers:
+        - fields:
+            lookup_fields: ['system.process.cgroup.id']
+            regex_pattern: 'cri-containerd-(?P<key>[0-9a-z]+)\.scope'
+-------------------------------------------------------------------------------
+
 ifdef::has_kubernetes_logs_path_matcher[]
 ===== `logs_path`
 

libbeat/processors/add_kubernetes_metadata/matchers.go

@@ -19,6 +19,8 @@ package add_kubernetes_metadata
 
 import (
 	"fmt"
+	"regexp"
+	"slices"
 
 	"github.com/elastic/beats/v7/libbeat/beat"
 	"github.com/elastic/beats/v7/libbeat/common/fmtstr"
@@ -32,6 +34,7 @@ import (
 const (
 	FieldMatcherName       = "fields"
 	FieldFormatMatcherName = "field_format"
+	regexKeyGroupName      = "key"
 )
 
 // Matcher takes a new event and returns the index
@@ -87,42 +90,67 @@ func (m *Matchers) MetadataIndex(event mapstr.M) string {
 }
 
 func (m *Matchers) Empty() bool {
-	if len(m.matchers) == 0 {
-		return true
-	}
-
-	return false
+	return len(m.matchers) == 0
 }
 
 type FieldMatcher struct {
 	MatchFields []string
+	Regexp      *regexp.Regexp
 }
 
 func NewFieldMatcher(cfg config.C) (Matcher, error) {
-	config := struct {
+	matcherConfig := struct {
 		LookupFields []string `config:"lookup_fields"`
+		RegexPattern string   `config:"regex_pattern"`
 	}{}
 
-	err := cfg.Unpack(&config)
+	err := cfg.Unpack(&matcherConfig)
 	if err != nil {
-		return nil, fmt.Errorf("fail to unpack the `lookup_fields` configuration: %s", err)
+		return nil, fmt.Errorf("fail to unpack the fields matcher configuration: %w", err)
 	}
 
-	if len(config.LookupFields) == 0 {
+	if len(matcherConfig.LookupFields) == 0 {
 		return nil, fmt.Errorf("lookup_fields can not be empty")
 	}
 
-	return &FieldMatcher{MatchFields: config.LookupFields}, nil
+	if len(matcherConfig.RegexPattern) == 0 {
+		return &FieldMatcher{MatchFields: matcherConfig.LookupFields}, nil
+	}
+	regex, err := regexp.Compile(matcherConfig.RegexPattern)
+	if err != nil {
+		return nil, fmt.Errorf("invalid regex: %w", err)
+	}
+
+	captureGroupNames := regex.SubexpNames()
+	if !slices.Contains(captureGroupNames, regexKeyGroupName) {
+		return nil, fmt.Errorf("regex missing required capture group `key`")
+	}
+
+	return &FieldMatcher{MatchFields: matcherConfig.LookupFields, Regexp: regex}, nil
 }
 
 func (f *FieldMatcher) MetadataIndex(event mapstr.M) string {
 	for _, field := range f.MatchFields {
-		keyIface, err := event.GetValue(field)
-		if err == nil {
-			key, ok := keyIface.(string)
-			if ok {
-				return key
-			}
+		fieldIface, err := event.GetValue(field)
+		if err != nil {
+			continue
+		}
+		fieldValue, ok := fieldIface.(string)
+		if !ok {
+			continue
+		}
+		if f.Regexp == nil {
+			return fieldValue
+		}
+
+		matches := f.Regexp.FindStringSubmatch(fieldValue)
+		if matches == nil {
+			continue
+		}
+		keyIndex := f.Regexp.SubexpIndex(regexKeyGroupName)
+		key := matches[keyIndex]
+		if key != "" {
+			return key
 		}
 	}
 
@@ -140,7 +168,7 @@ func NewFieldFormatMatcher(cfg config.C) (Matcher, error) {
 
 	err := cfg.Unpack(&config)
 	if err != nil {
-		return nil, fmt.Errorf("fail to unpack the `format` configuration of `field_format` matcher: %s", err)
+		return nil, fmt.Errorf("fail to unpack the `format` configuration of `field_format` matcher: %w", err)
 	}
 
 	if config.Format == "" {

libbeat/processors/add_kubernetes_metadata/matchers_test.go

@@ -20,6 +20,8 @@ package add_kubernetes_metadata
 import (
 	"testing"
 
+	"github.com/stretchr/testify/require"
+
 	"github.com/stretchr/testify/assert"
 
 	"github.com/elastic/elastic-agent-libs/config"
@@ -35,6 +37,7 @@ func TestFieldMatcher(t *testing.T) {
 	assert.NoError(t, err)
 	matcher, err := NewFieldMatcher(*fieldCfg)
 	assert.Error(t, err)
+	assert.Nil(t, matcher)
 
 	testCfg["lookup_fields"] = "foo"
 	fieldCfg, _ = config.NewConfigFrom(testCfg)
@@ -58,13 +61,55 @@ func TestFieldMatcher(t *testing.T) {
 	assert.Equal(t, out, "")
 }
 
+func TestFieldMatcherRegex(t *testing.T) {
+	testCfg := map[string]interface{}{
+		"lookup_fields": []string{"foo"},
+		"regex_pattern": "(?!)",
+	}
+	fieldCfg, err := config.NewConfigFrom(testCfg)
+	assert.NoError(t, err)
+	matcher, err := NewFieldMatcher(*fieldCfg)
+	assert.ErrorContains(t, err, "invalid regex:")
+	assert.Nil(t, matcher)
+
+	testCfg["regex_pattern"] = "(?P<invalid>.*)"
+	fieldCfg, _ = config.NewConfigFrom(testCfg)
+
+	matcher, err = NewFieldMatcher(*fieldCfg)
+	assert.ErrorContains(t, err, "regex missing required capture group `key`")
+	assert.Nil(t, matcher)
+
+	testCfg["regex_pattern"] = "bar-(?P<key>[^-]+)-suffix"
+	fieldCfg, _ = config.NewConfigFrom(testCfg)
+
+	matcher, err = NewFieldMatcher(*fieldCfg)
+	require.NoError(t, err)
+	require.NotNil(t, matcher)
+
+	input := mapstr.M{
+		"foo": "bar-keyvalue-suffix",
+	}
+
+	out := matcher.MetadataIndex(input)
+	assert.Equal(t, out, "keyvalue")
+
+	nonMatchInput := mapstr.M{
+		"not": "match",
+		"foo": "nomatch",
+	}
+
+	out = matcher.MetadataIndex(nonMatchInput)
+	assert.Equal(t, out, "")
+}
+
 func TestFieldFormatMatcher(t *testing.T) {
 	testCfg := map[string]interface{}{}
 	fieldCfg, err := config.NewConfigFrom(testCfg)
 
 	assert.NoError(t, err)
 	matcher, err := NewFieldFormatMatcher(*fieldCfg)
 	assert.Error(t, err)
+	assert.Nil(t, matcher)
 
 	testCfg["format"] = `%{[namespace]}/%{[pod]}`
 	fieldCfg, _ = config.NewConfigFrom(testCfg)