Test for behavior of non-capturing groups, as described in a blog post by Steven Levithan and bug 14931. On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE". PASS /(x)?\1y/.test("y") is true PASS /(x)?\1y/.exec("y") is ["y", undefined] PASS /(x)?y/.exec("y") is ["y", undefined] PASS "y".match(/(x)?\1y/) is ["y", undefined] PASS "y".match(/(x)?y/) is ["y", undefined] PASS "y".match(/(x)?\1y/g) is ["y"] PASS "y".split(/(x)?\1y/) is ["", undefined, ""] PASS "y".split(/(x)?y/) is ["", undefined, ""] PASS "y".search(/(x)?\1y/) is 0 PASS "y".replace(/(x)?\1y/, "z") is "z" PASS "y".replace(/(x)?y/, "$1") is "" PASS "y".replace(/(x)?\1y/, function($0, $1){ return String($1); }) is "undefined" PASS "y".replace(/(x)?y/, function($0, $1){ return String($1); }) is "undefined" PASS "y".replace(/(x)?y/, function($0, $1){ return $1; }) is "undefined" PASS successfullyParsed is true TEST COMPLETE