diff --git a/builtin_regexp.go b/builtin_regexp.go
index f9533f7..88f0322 100644
--- a/builtin_regexp.go
+++ b/builtin_regexp.go
@@ -7,7 +7,14 @@ import (
 // RegExp
 
 func builtinRegExp(call FunctionCall) Value {
-	return toValue(call.runtime.newRegExp(call.Argument(0), call.Argument(1)))
+	pattern := call.Argument(0)
+	flags := call.Argument(1)
+	if object := pattern._object(); object != nil {
+		if object.class == "RegExp" && flags.IsUndefined() {
+			return pattern
+		}
+	}
+	return toValue(call.runtime.newRegExp(pattern, flags))
 }
 
 func builtinNewRegExp(self *_object, _ Value, argumentList []Value) Value {
diff --git a/regexp_test.go b/regexp_test.go
index 26c11b1..46e05a7 100644
--- a/regexp_test.go
+++ b/regexp_test.go
@@ -121,6 +121,18 @@ func TestRegExp_zaacbbbcac(t *testing.T) {
 	}
 }
 
+func TestRegExpCopying(t *testing.T) {
+	Terst(t)
+
+	test := runTest()
+	test(`
+        abc = /xyzzy/i;
+        def = RegExp(abc);
+        abc.indicator = 1;
+        [ abc.indicator, def.indicator ];
+    `, "1,1")
+}
+
 func TestRegExp_multiline(t *testing.T) {
 	Terst(t)