Case Insensitive search with $in

You can use $elemMatch with regular expressions search, e.g. let's search for "blue" color in the following collection:

db.items.save({ 
  name : 'a toy', 
  colors : ['red', 'BLUE'] 
})

> ok

db.items.find({
  'colors': {
    $elemMatch: { 
      $regex: 'blue', 
      $options: 'i' 
    }
  }
})

>[ 
  {   
    "name": "someitem",
    "_id": { "$oid": "4fbb7809cc93742e0d073aef"},   
    "colors": ["red", "BLUE"]
   }
]

Use $in with the match being case insensitive:

Data example:

{ 
    name : "...Event A",
    fieldX : "aAa" 
},
{ 
  name : "...Event B",
  fieldX : "Bab" 
},
{ 
  name : "...Event C",
  fieldX : "ccC" 
},
{ 
  name : "...Event D",
  fieldX : "dDd" 
}

And we want documents were "fieldX" is contained in any value of the array (optValues):

var optValues = ['aaa', 'bbb', 'ccc', 'ddd'];

var optRegexp = [];
optValues.forEach(function(opt){
        optRegexp.push(  new RegExp(opt, "i") );
});

db.collection.find( { fieldX: { $in: optRegexp } } );

This works for $all either.

I hope this helps!

p.s.: This was my solution to search by tags in a web application.

This works for me perfectly.

From code we can create custom query like this:

{
  "first_name":{
    "$in":[
      {"$regex":"^serina$","$options":"i"}, 
      {"$regex":"^andreW$","$options":"i"}
    ]
  }
}

This will transform to following in mongo after query:

db.mycollection.find({"first_name":{"$in":[/^serina$/i, /^andreW$/i]}})

Same for "$nin".