Nashorn rejects extended RegExp syntax accepted by all major JS engines

From https://mail.mozilla.org/pipermail/es-discuss/2010-December/012289.html :

It's far from the only extension to RegExp syntax that is common to most
implementations. In fact, the extensions are both extensive and consistent
across browsers. A quick check through the possible syntax errors show
the following:

// Invalid ControlEscape/IdentityEscape character treated as literal.
   /\z/; // Invalid escape, same as /z/
// Incomplete/Invalid ControlEscape treated as either "\\c" or "c"
   /\c/; // same as /c/ or /\\c/
   /\c2/; // same as /c2/ or /\\c2/
// Incomplete HexEscapeSequence escape treated as either "\\x" or "x".
   /\x/; // incomplete x-escape
   /\x1/; // incomplete x-escape
   /\x1z/; // incomplete x-escape
// Incomplete UnicodeEscapeSequence escape treated as either "\\u" or "u".
   /\u/; // incomplete u-escape
   /\uz/; // incomplete u-escape
   /\u1/; // incomplete u-escape
   /\u1z/; // incomplete u-escape
   /\u12/; // incomplete u-escape
   /\u12z/; // incomplete u-escape
   /\u123/; // incomplete u-escape
   /\u123z/; // incomplete u-escape
// Bad quantifier range:
   /x{z/; // same as /x\{z/
   /x{1z/; // same as /x\{1z/
   /x{1,z/; // same as /x\{1,z/
   /x{1,2z/; // same as /x\{1,2z/
   /x{10000,20000z/; // same as /x\{10000,20000z/
// Notice: It needs arbitrary lookahead to determine the invalidity,
// except Mozilla that limits the numbers.

// Zero-initialized Octal escapes.
   /\012/; // same as /\x0a/

// Nonexisting back-references treated as octal escapes:
   /\5/; // same as /\x05/

// Invalid PatternCharacter accepted unescaped
   /]/;
   /{/;
   /}/;

// Bad escapes also inside CharacterClass.
   /[\z]/;
   /[\c]/;
   /[\c2]/;
   /[\x]/;
   /[\x1]/;
   /[\x1z]/;
   /[\u]/;
   /[\uz]/;
   /[\u1]/;
   /[\u1z]/;
   /[\u12]/;
   /[\u12z]/;
   /[\u123]/;
   /[\u123z]/;
   /[\012]/;
   /[\5]/;
// And in addition:
   /[\B]/;
   /()()[\2]/; // Valid backreference should be invalid.

Many of these regular expressions currently throw syntax errors in Nashorn.