The follwing javascript expression /^(?=a)?b$/.test("b") evaluates to false since changeset 39164. It should give true, both according to the ECMAScript specification and, e.g., what Firefox does.
<rdar://problem/6471394>
Created attachment 26785 [details] patch
Comment on attachment 26785 [details] patch r=me, although you should also include the Bugzilla URL in the ChangeLogs.
Committed revision 39963.