diff -r 000000000000 -r 6474c204b198 js/src/tests/js1_5/Regress/regress-256798.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/tests/js1_5/Regress/regress-256798.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,36 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//-----------------------------------------------------------------------------
+var BUGNUMBER = 256798;
+var summary = 'regexp zero-width positive lookahead';
+var actual = '';
+var expect = '';
+
+printBugNumber(BUGNUMBER);
+printStatus (summary);
+
+var status;
+
+status = summary + ' ' + inSection(1);
+expect = 'aaaa,a';
+actual = /(?:(a)+)/.exec("baaaa") + '';
+reportCompare(expect, actual, status);
+
+status = summary + ' ' + inSection(2);
+expect = ',aaa';
+actual = /(?=(a+))/.exec("baaabac") + '';
+reportCompare(expect, actual, status);
+
+status = summary + ' ' + inSection(3);
+expect = 'b,aaa';
+actual = /b(?=(a+))/.exec("baaabac") + '';
+reportCompare(expect, actual, status);
+
+// XXXbc revisit this
+status = summary + ' ' + inSection(4);
+expect = 'null';
+actual = /b(?=(b+))/.exec("baaabac") + '';
+reportCompare(expect, actual, status);