From e1d5eb52f2524d10e370fc5c65528ec4fc484326 Mon Sep 17 00:00:00 2001
From: Brian Brazil <brian.brazil@robustperception.io>
Date: Sun, 16 Aug 2015 23:39:39 +0100
Subject: [PATCH] retrieval: Don't include unmatched source of regex in
 replacement.

ReplaceAllString only replaces the matching part of the regex,
the unmatched bits around it are left in place. This is not the
expected or desired behaviour as the replacement string should
be everything.

This may break users dependant on this behaviour, but
what they're doing is still possible.
---
 retrieval/relabel.go      |  7 ++++---
 retrieval/relabel_test.go | 19 +++++++++++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/retrieval/relabel.go b/retrieval/relabel.go
index ec6af2eee..f63486889 100644
--- a/retrieval/relabel.go
+++ b/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)
diff --git a/retrieval/relabel_test.go b/retrieval/relabel_test.go
index 984fbc187..42a6509b2 100644
--- a/retrieval/relabel_test.go
+++ b/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",