MongoDB regular expression with indexed field
"cursor" : "BtreeCursor A_1 multi",
    "nscanned" : 500001,
    "nscannedObjects" : 10,
    "n" : 10,
    "millis" : 956,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "isMultiKey" : false,
    "indexOnly" : false,
    "indexBounds" : {
            "A" : [
                    [
                            "",
                            {

                            }
                    ],
                    [
                            /BLABLA!25500[0-9]/,
                            /BLABLA!25500[0-9]/
                    ]
            ]
    }

"cursor" : "BasicCursor",
    "nscanned" : 500002,
    "nscannedObjects" : 500002,
    "n" : 10,
    "millis" : 531,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "isMultiKey" : false,
    "indexOnly" : false,
    "indexBounds" : {

    }