diff --git a/model/labels/regexp.go b/model/labels/regexp.go
index 9a9d846fd1..11fadc687c 100644
--- a/model/labels/regexp.go
+++ b/model/labels/regexp.go
@@ -389,9 +389,8 @@ func optimizeConcatRegex(r *syntax.Regexp) (prefix, suffix string, contains []st
 	}
 
 	// If contains any literal which is not a prefix/suffix, we keep track of
-	// all the ones which are case sensitive.
+	// all the ones which are case-sensitive.
 	for i := 1; i < len(sub)-1; i++ {
-		// TODO if it's case insensitive we should return an contains list or is it safe to keep searching for case sensitive ones?
 		if sub[i].Op == syntax.OpLiteral && (sub[i].Flags&syntax.FoldCase) == 0 {
 			contains = append(contains, string(sub[i].Rune))
 		}
diff --git a/model/labels/regexp_test.go b/model/labels/regexp_test.go
index 0a75841c98..400b5721b7 100644
--- a/model/labels/regexp_test.go
+++ b/model/labels/regexp_test.go
@@ -84,11 +84,12 @@ var (
 		// Concat of literals and wildcards.
 		".*-.*-.*-.*-.*",
 		"(.+)-(.+)-(.+)-(.+)-(.+)",
-		"((.*))-((.*))-((.*))-((.*))-((.*))",
+		"((.*))(?i:f)((.*))o((.*))o((.*))",
+		"((.*))f((.*))(?i:o)((.*))o((.*))",
 	}
 	values = []string{
 		"foo", " foo bar", "bar", "buzz\nbar", "bar foo", "bfoo", "\n", "\nfoo", "foo\n", "hello foo world", "hello foo\n world", "",
-		"FOO", "Foo", "OO", "Oo", "\nfoo\n", strings.Repeat("f", 20), "prometheus", "prometheus_api_v1", "prometheus_api_v1_foo",
+		"FOO", "Foo", "fOo", "foO", "OO", "Oo", "\nfoo\n", strings.Repeat("f", 20), "prometheus", "prometheus_api_v1", "prometheus_api_v1_foo",
 		"10.0.1.20", "10.0.2.10", "10.0.3.30", "10.0.4.40",
 		"foofoo0", "foofoo", "😀foo0",