Merge pull request #995 from prometheus/relabel-replace

retrieval: Don't include unmatched source of regex in replacement.

retrieval/relabel.go

@@ -47,12 +47,13 @@ func relabel(labels clientmodel.LabelSet, cfg *config.RelabelConfig) (clientmode
 			return nil, nil
 		}
 	case config.RelabelReplace:
+		indexes := cfg.Regex.FindStringSubmatchIndex(val)
 		// If there is no match no replacement must take place.
-		if !cfg.Regex.MatchString(val) {
+		if indexes == nil {
 			break
 		}
-		res := cfg.Regex.ReplaceAllString(val, cfg.Replacement)
-		if res == "" {
+		res := cfg.Regex.ExpandString([]byte{}, cfg.Replacement, val, indexes)
+		if len(res) == 0 {
 			delete(labels, cfg.TargetLabel)
 		} else {
 			labels[cfg.TargetLabel] = clientmodel.LabelValue(res)

retrieval/relabel_test.go

@@ -90,6 +90,25 @@ func TestRelabel(t *testing.T) {
 			},
 			output: nil,
 		},
+		{
+			input: clientmodel.LabelSet{
+				"a": "abc",
+			},
+			relabel: []*config.RelabelConfig{
+				{
+					SourceLabels: clientmodel.LabelNames{"a"},
+					Regex:        &config.Regexp{*regexp.MustCompile("(b)")},
+					TargetLabel:  clientmodel.LabelName("d"),
+					Separator:    ";",
+					Replacement:  "$1",
+					Action:       config.RelabelReplace,
+				},
+			},
+			output: clientmodel.LabelSet{
+				"a": "abc",
+				"d": "b",
+			},
+		},
 		{
 			input: clientmodel.LabelSet{
 				"a": "foo",