RegExp.prototype is an ordinary object.

test/mjsunit/es6/regexp-prototype.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// ES6 21.2.4.1
+var proto_desc = Object.getOwnPropertyDescriptor(RegExp, "prototype");
+assertFalse(proto_desc.writable);
+assertFalse(proto_desc.enumerable);
+assertFalse(proto_desc.configurable);
+
+// ES6 21.2.5.1
+var proto = proto_desc.value;
+assertFalse(proto instanceof RegExp);
+assertEquals(undefined, Object.getOwnPropertyDescriptor(proto, "valueOf"));
+assertEquals(proto.valueOf, Object.prototype.valueOf);
+var proto_constr = Object.getOwnPropertyDescriptor(proto, "constructor");
+assertEquals(RegExp, proto_constr.value);