fix: preserve single regex values in converted ServiceMonitors

api/operator/v1beta1/vmextra_types.go

@@ -1021,19 +1021,17 @@ func (m *StringOrArray) UnmarshalJSON(data []byte) error {
 	}
 }
 
-// MarshalJSON implements json.Marshaller interface
-func (m *StringOrArray) MarshalJSON() ([]byte, error) {
-	if m == nil {
-		return nil, nil
-	}
-	ms := *m
+// MarshalJSON implements json.Marshaller interface.
+// Use a value receiver so nested struct fields of type StringOrArray
+// are serialized as strings when they contain a single element.
+func (m StringOrArray) MarshalJSON() ([]byte, error) {
 	switch {
-	case len(ms) == 0:
+	case len(m) == 0:
 		return json.Marshal("")
-	case len(ms) == 1:
-		return json.Marshal(ms[0])
+	case len(m) == 1:
+		return json.Marshal(m[0])
 	default:
-		return json.Marshal([]string(ms))
+		return json.Marshal([]string(m))
 	}
 }
 

api/operator/v1beta1/vmextra_types_test.go

@@ -102,6 +102,13 @@ func TestStringOrArrayMarshal(t *testing.T) {
 `)
 	f(&StringOrArray{}, yaml.Marshal, `""
 `)
+	got, err := json.Marshal(struct {
+		Match StringOrArray `json:"match"`
+	}{
+		Match: StringOrArray{"1"},
+	})
+	assert.NoError(t, err)
+	assert.Equal(t, `{"match":"1"}`, string(got))
 
 }
 

internal/controller/operator/factory/converter/apis_test.go

@@ -2,6 +2,7 @@ package converter
 
 import (
 	"context"
+	"encoding/json"
 	"testing"
 
 	promv1 "github.com/prometheus-operator/prometheus-operator/pkg/apis/monitoring/v1"
@@ -224,6 +225,33 @@ func TestServiceMonitor(t *testing.T) {
 	})
 }
 
+func TestServiceMonitorSingleRegexMarshalsAsJSONScalar(t *testing.T) {
+	ctx := context.TODO()
+	got := ServiceMonitor(ctx, &promv1.ServiceMonitor{
+		Spec: promv1.ServiceMonitorSpec{
+			Endpoints: []promv1.Endpoint{{
+				MetricRelabelConfigs: []promv1.RelabelConfig{{
+					Action:       "drop",
+					Regex:        "^karpenter_cloudprovider_instance.*",
+					SourceLabels: []promv1.LabelName{"__name__"},
+				}},
+				RelabelConfigs: []promv1.RelabelConfig{{
+					Action: "replace",
+					Regex:  "^(.*)$",
+					SourceLabels: []promv1.LabelName{"__meta_kubernetes_pod_node_name"},
+					TargetLabel:  "nodename",
+				}},
+			}},
+		},
+	}, &config.BaseOperatorConf{})
+
+	data, err := json.Marshal(got)
+	assert.NoError(t, err)
+	assert.Contains(t, string(data), `"regex":"^karpenter_cloudprovider_instance.*"`)
+	assert.Contains(t, string(data), `"regex":"^(.*)$"`)
+	assert.NotContains(t, string(data), `"regex":["`)
+}
+
 func TestConvertPodEndpoints(t *testing.T) {
 	type opts struct {
 		pe   []promv1.PodMetricsEndpoint