diff --git a/promql/functions.go b/promql/functions.go
index cb6135e0e..80de10efb 100644
--- a/promql/functions.go
+++ b/promql/functions.go
@@ -616,7 +616,7 @@ func funcLabelReplace(ev *evaluator, args Expressions) model.Value {
 		regexStr = ev.evalString(args[4]).Value
 	)
 
-	regex, err := regexp.Compile(regexStr)
+	regex, err := regexp.Compile("^(?:" + regexStr + ")$")
 	if err != nil {
 		ev.errorf("invalid regular expression in label_replace(): %s", regexStr)
 	}
diff --git a/promql/testdata/functions.test b/promql/testdata/functions.test
index 6a679917d..f203b24a0 100644
--- a/promql/testdata/functions.test
+++ b/promql/testdata/functions.test
@@ -100,11 +100,16 @@ load 5m
   testmetric{src="source-value-10",dst="original-destination-value"} 0
   testmetric{src="source-value-20",dst="original-destination-value"} 1
 
-# label_replace does a substring match and replace.
-eval instant at 0m label_replace(testmetric, "dst", "destination-value-$1", "src", "value-(.*)")
+# label_replace does a full-string match and replace.
+eval instant at 0m label_replace(testmetric, "dst", "destination-value-$1", "src", "source-value-(.*)")
   testmetric{src="source-value-10",dst="destination-value-10"} 0
   testmetric{src="source-value-20",dst="destination-value-20"} 1
 
+# label_replace does not do a sub-string match.
+eval instant at 0m label_replace(testmetric, "dst", "destination-value-$1", "src", "value-(.*)")
+  testmetric{src="source-value-10",dst="original-destination-value"} 0
+  testmetric{src="source-value-20",dst="original-destination-value"} 1
+
 # label_replace works with multiple capture groups.
 eval instant at 0m label_replace(testmetric, "dst", "$1-value-$2", "src", "(.*)-value-(.*)")
   testmetric{src="source-value-10",dst="source-value-10"} 0
@@ -129,7 +134,7 @@ eval instant at 0m label_replace(testmetric, "dst", "value-$1", "src", "non-matc
   testmetric{src="source-value-20",dst="original-destination-value"} 1
 
 # label_replace drops labels that are set to empty values.
-eval instant at 0m label_replace(testmetric, "dst", "", "dst", "")
+eval instant at 0m label_replace(testmetric, "dst", "", "dst", ".*")
   testmetric{src="source-value-10"} 0
   testmetric{src="source-value-20"} 1