RegExp(/xyzzy/) & RegExp(/xyzzy/, undefined) will return a reference to the exact same /xyzzy/ object...

...instead of creating a new copy

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 {

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)