Allow $not to be applied to $regex (currently only /regex/ syntax is allowed)

Executing the following queries I was expecting the same result but apparently they are dealt differently:

2 scenarios

• matching

  nair(mongod-2.7.0-pre-) test> db.system.namespaces.find({ name:/\$/} )

  {

  "name": "test.data.$_id_"

  }

  {

  "name": "test.difference.$_id_"

  }

  {

  "name": "test.posts.$_id_"

  }

  {

  "name": "test.kunal.$_id_"

  }

  {

  "name": "test.maha_example.$_id_"

  }

  has the same result has using $regex operator

  nair(mongod-2.7.0-pre-) test> db.system.namespaces.find({ name:{ $regex: "\\$"}} )

  {

  "name": "test.data.$_id_"

  }

  {

  "name": "test.difference.$_id_"

  }

  {

  "name": "test.posts.$_id_"

  }

  {

  "name": "test.kunal.$_id_"

  }

  {

  "name": "test.maha_example.$_id_"

  }

• not matching query

  nair(mongod-2.7.0-pre-) test> db.system.namespaces.find({ name: {$not: /\$/}} )

  {

  "name": "test.system.indexes"

  }

  {

  "name": "test.data"

  }

  {

  "name": "test.difference"

  }

  {

  "name": "test.posts"

  }

  {

  "name": "test.kunal"

  }

  {

  "name": "test.maha_example"

  }

  {

  "name": "test.system.profile",

  "options": {

  "capped": true,

  "size": 1048576

  }

  }

  But using $regex operator fails

  nair(mongod-2.7.0-pre-) test> db.system.namespaces.find({ name: {$not: { $regex: "\\$"}}} )

  error: {

  "$err": "Can't canonicalize query: BadValue $not cannot have a regex",

  "code": 17287

  }

Is this the expected behaviour ? and if so why ?

Looking deeper in the code I can also see the following comment:
mongo/db/matcher/expression_parser_tree.cpp:103

...

         // TODO: this seems arbitrary?

         // tested in jstests/not2.js

         for ( unsigned i = 0; i < theAnd->numChildren(); i++ )

             if ( theAnd->getChild(i)->matchType() == MatchExpression::REGEX )

                 return StatusWithMatchExpression( ErrorCodes::BadValue, "$not cannot have a regex" );

...

and looking int the jstests it makes even less sense:

jstests/core/not2.js

  49 check( {i:{$not:/a/}}, "b" );

  50 check( {i:{$not:/(a|b)/}}, "", 0 );

  51 check( {i:{$not:/a/,$regex:"a"}}, "", 0 );

  52 check( {i:{$not:/aa/}}, "a", 2 );

  53 fail( {i:{$not:{$regex:"a"}}} );

  54 fail( {i:{$not:{$options:"a"}}} );