diff --git a/transforms.go b/transforms.go
index d22a894..6238181 100644
--- a/transforms.go
+++ b/transforms.go
@@ -392,7 +392,7 @@ func stringRegexpTransform(input any, r v1beta1.StringTransformRegexp) (string,
 
 	// Return the entire match (group zero) by default.
 	g := ptr.Deref[int](r.Group, 0)
-	if len(groups) == 0 || g >= len(groups) {
+	if len(groups) == 0 || g < 0 || g >= len(groups) {
 		return "", errors.Errorf(errStringTransformTypeRegexpNoMatch, r.Match, g)
 	}
 
diff --git a/transforms_test.go b/transforms_test.go
index 8414afd..f322cfc 100644
--- a/transforms_test.go
+++ b/transforms_test.go
@@ -977,6 +977,19 @@ func TestStringResolve(t *testing.T) {
 				err: errors.Errorf(errStringTransformTypeRegexpNoMatch, "my-([0-9]+)-string", 2),
 			},
 		},
+		"RegexpNegativeCaptureGroup": {
+			args: args{
+				stype: v1beta1.StringTransformTypeRegexp,
+				regexp: &v1beta1.StringTransformRegexp{
+					Match: "my-([0-9]+)-string",
+					Group: ptr.To[int](-1),
+				},
+				i: "my-1-string",
+			},
+			want: want{
+				err: errors.Errorf(errStringTransformTypeRegexpNoMatch, "my-([0-9]+)-string", -1),
+			},
+		},
 		"ConvertToJSONSuccess": {
 			args: args{
 				stype:   v1beta1.StringTransformTypeConvert,