[regexp] break recursion in mutually recursive capture/back references.

test/mjsunit/regexp.js

Index: test/mjsunit/regexp.js
diff --git a/test/mjsunit/regexp.js b/test/mjsunit/regexp.js
index 00bbfb962d2a700b8e07febb6b4d43235e585180..6ff47500b72ba93c505dc1276c8a2490b3d5d55f 100644
--- a/test/mjsunit/regexp.js
+++ b/test/mjsunit/regexp.js
@@ -722,3 +722,7 @@ assertThrows("RegExp.prototype.toString.call(true)", TypeError);
 assertThrows("RegExp.prototype.toString.call([])", TypeError);
 assertThrows("RegExp.prototype.toString.call({})", TypeError);
 assertThrows("RegExp.prototype.toString.call(function(){})", TypeError);
+
+// Test mutually recursive capture and backreferences.
+assertEquals(["b", "", ""], /(\2)b(\1)/.exec("aba"));
+assertEquals(["a", "", ""], /(\2).(\1)/.exec("aba"));

[erikcorry, 2015/12/15 08:09:19]

Can we also test:

/(.\2).(\1)/

and

/w(.\2)x(\1)$/

which should match "foo....wyxy"

This tests cases where there is mutual recursion, but the max length is not
zero.