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

test/mjsunit/regexp.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 704 matching lines @@
 // Test that RegExp.prototype.toString() throws TypeError for
 // incompatible receivers (ES5 section 15.10.6 and 15.10.6.4).
 assertThrows("RegExp.prototype.toString.call(null)", TypeError);
 assertThrows("RegExp.prototype.toString.call(0)", TypeError);
 assertThrows("RegExp.prototype.toString.call('')", TypeError);
 assertThrows("RegExp.prototype.toString.call(false)", TypeError);
 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.