src/third_party/mozjs-45/js/src/jit-test/tests/basic/regexp-sticky-undef-capture.js

var re = /abc(WHOO!)?def/y;
var input = 'abcdefabcdefabcdef';
var count = 0;
while ((match = re.exec(input)) !== null) {
    print(count++);
    assertEq(match[0], 'abcdef');
    assertEq(match[1], undefined);
}