Example:
> db.stuff.save({\"foo\":\"bar\"});
> db.stuff.find({\"foo\":\"bar\"}).count();
1
> db.stuff.find({\"foo\":\"BAR\"}).count();
0
Keep in mind that the previous example:
db.stuff.find( { foo: /bar/i } );
will cause every entries containing bar to match the query ( bar1, barxyz, openbar ), it could be very dangerous for a username search on a auth function ...
You may need to make it match only the search term by using the appropriate regexp syntax as:
db.stuff.find( { foo: /^bar$/i } );
See http://www.regular-expressions.info/ for syntax help on regular expressions