assert: report no match for a typed nil *regexp.Regexp instead of panicking

assert/assertions.go

@@ -1711,6 +1711,11 @@ func ErrorContains(t TestingT, theError error, contains string, msgAndArgs ...in
 func matchRegexp(rx interface{}, str interface{}) bool {
 	var r *regexp.Regexp
 	if rr, ok := rx.(*regexp.Regexp); ok {
+		// A typed nil passes the type assertion, and MatchString would then
+		// dereference it. There is no pattern, so nothing can match.
+		if rr == nil {
+			return false
+		}
 		r = rr
 	} else {
 		r = regexp.MustCompile(fmt.Sprint(rx))

assert/assertions_test.go

@@ -3634,6 +3634,21 @@ func TestNeverFailQuickly(t *testing.T) {
 	False(t, Never(mockT, condition, 100*time.Millisecond, time.Second))
 }
 
+// A typed nil *regexp.Regexp passes the type assertion in matchRegexp and is
+// then dereferenced, so Regexp and NotRegexp panic instead of reporting a
+// result.
+func TestRegexpNilTypedRegexp(t *testing.T) {
+	t.Parallel()
+
+	var rx *regexp.Regexp
+	mockT := new(testing.T)
+
+	NotPanics(t, func() {
+		False(t, Regexp(mockT, rx, "anything"))
+	})
+	True(t, NotRegexp(t, rx, "anything"))
+}
+
 func Test_validateEqualArgs(t *testing.T) {
 	t.Parallel()