diff --git a/src/pkg/regexp/all_test.go b/src/pkg/regexp/all_test.go
index 3596573..9c4d64f 100644
--- a/src/pkg/regexp/all_test.go
+++ b/src/pkg/regexp/all_test.go
@@ -196,6 +196,10 @@
 	{"a+", "${oops", "aaa", "${oops"},
 	{"a+", "$$", "aaa", "$"},
 	{"a+", "$", "aaa", "$"},
+
+	// Substitution when subexpression isn't found
+	{"(x)?", "$1", "123", "123"},
+	{"abc", "$1", "123", "123"},
 }
 
 var replaceLiteralTests = []ReplaceTest{