Add test case for default regex explicitly provided

Signed-off-by: Liam Howe <liam.howe@maersk.com>

model/relabel/relabel_test.go

@@ -874,6 +874,16 @@ separator: ;
 target_label: __meta_kubernetes_pod_container_port_number
 replacement: $1
 action: keepequal
+`,
+		},
+		{
+			name: "Default regex provided",
+			inputYaml: `source_labels: [__meta_kubernetes_pod_annotation_prometheus_io_port]
+separator: ;
+regex: (.*)
+target_label: __meta_kubernetes_pod_container_port_number
+replacement: $1
+action: replace
 `,
 		},
 	}