RegExp.prototype is an ordinary object.

src/js/regexp.js

 // Copyright 2012 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.
 
 (function(global, utils) {
 
 %CheckIsBootstrapping();
 
 // -------------------------------------------------------------------
 // Imports
 
 var FLAG_harmony_regexps;
 var FLAG_harmony_tolength;
 var FLAG_harmony_unicode_regexps;
+var GlobalObject = global.Object;
 var GlobalRegExp = global.RegExp;
 var InternalPackedArray = utils.InternalPackedArray;
 var MakeTypeError;
 
 utils.ImportFromExperimental(function(from) {
   FLAG_harmony_regexps = from.FLAG_harmony_regexps;
   FLAG_harmony_tolength = from.FLAG_harmony_tolength;
   FLAG_harmony_unicode_regexps = from.FLAG_harmony_unicode_regexps;
 });
 
@@ skipping 304 matching lines @@
     var matchStart = RegExpLastMatchInfo[CAPTURE(index)];
     var matchEnd = RegExpLastMatchInfo[CAPTURE(index + 1)];
     if (matchStart == -1 || matchEnd == -1) return '';
     return %_SubString(LAST_SUBJECT(RegExpLastMatchInfo), matchStart, matchEnd);
   };
 }
 
 // -------------------------------------------------------------------
 
 %FunctionSetInstanceClassName(GlobalRegExp, 'RegExp');
+%FunctionSetPrototype(GlobalRegExp, new GlobalObject());
 %AddNamedProperty(
     GlobalRegExp.prototype, 'constructor', GlobalRegExp, DONT_ENUM);
 %SetCode(GlobalRegExp, RegExpConstructor);
 
 utils.InstallFunctions(GlobalRegExp.prototype, DONT_ENUM, [
   "exec", RegExpExecJS,
   "test", RegExpTest,
   "toString", RegExpToString,
   "compile", RegExpCompileJS
 ]);
@@ skipping 71 matching lines @@
 // Exports
 
 utils.Export(function(to) {
   to.RegExpExec = DoRegExpExec;
   to.RegExpExecNoTests = RegExpExecNoTests;
   to.RegExpLastMatchInfo = RegExpLastMatchInfo;
   to.RegExpTest = RegExpTest;
 });
 
 })