$regex, $in and $sort with index returns too many results

My index is : { "v" : 1, "key" : { "channelId" : 1, "searchField" : 1 }, "name" : "channelId_1_searchField_1", "ns" : "search.searchSeriesOnChannel1" } Here's a working query hinting not to use the index which returns 2 results : db.searchSeriesOnChannel1.find({$and : [{ "searchField" : { "$regex" : "_dr" , "$options" : "i"}, "channelId" : { "$in" : [ "CTVTo" , "CTVHT"]}}]}).sort({searchField:1}).hint({$natural:1}).explain() { "cursor" : "BasicCursor", "isMultiKey" : false, "n" : 2, "nscannedObjects" : 21743, "nscanned" : 21743, "nscannedObjectsAllPlans" : 21743, "nscannedAllPlans" : 21743, "scanAndOrder" : true, "indexOnly" : false, "nYields" : 169, "nChunkSkips" : 0, "millis" : 59, "server" : "fonsemongo:27017", "filterSet" : false } Using the index, it returns 62 elements because it discards the searchField $regex filter. db.searchSeriesOnChannel1.find({$and : [{ "searchField" : { "$regex" : "_dr" , "$options" : "i"}, "channelId" : { "$in" : [ "CTVTo" , "CTVHT"]}}]}).sort({searchField:1}).explain() { "clauses" : [ { "cursor" : "BtreeCursor channelId_1_searchField_1", "isMultiKey" : false, "n" : 62, "nscannedObjects" : 62, "nscanned" : 62, "scanAndOrder" : false, "indexOnly" : false, "nChunkSkips" : 0, "indexBounds" : { "channelId" : [ [ "CTVHT", "CTVHT" ] ], "searchField" : [ [ "", {   } ], [ /_dr/i, /_dr/i ] ] } }, { "cursor" : "BtreeCursor channelId_1_searchField_1", "isMultiKey" : false, "n" : 0, "nscannedObjects" : 0, "nscanned" : 0, "scanAndOrder" : false, "indexOnly" : false, "nChunkSkips" : 0, "indexBounds" : { "channelId" : [ [ "CTVTo", "CTVTo" ] ], "searchField" : [ [ "", {   } ], [ /_dr/i, /_dr/i ] ] } } ], "cursor" : "QueryOptimizerCursor", "n" : 62, "nscannedObjects" : 62, "nscanned" : 62, "nscannedObjectsAllPlans" : 62, "nscannedAllPlans" : 62, "scanAndOrder" : false, "nYields" : 1, "nChunkSkips" : 0, "millis" : 0, "server" : "fonsemongo:27017", "filterSet" : false }