RegEx bug in MongoDB
There are multiple issues here:


First, your console queries work fine in MongoDB v2.0.2 but don't work in MongoDB v2.2.1. This is probably a regression bug, but the JavaScript parser may have intentionally been changed between versions. The ... after entering a command means the console is waiting for a multi-line command to be completed; I'm not sure why in this specific situation.
Also, the backslash character \  in the $regex string must itself be escaped. (The JavaScript parser consumes the escape first, before the regex engine gets to see it.)



> db.test.find({word:{$regex:"\["}});    // no result
> db.test.find({word:{$regex:"\\["}});   // returns result when escaped properly
{ "_id" : 0, "word" : "[" }
{ "_id" : 1, "word" : "[]" }


MongoDB 2.0.2 console output:


> db.test.drop(); version()
version: 2.0.2
>
> db.test.insert({_id: 0, word: '['});
> db.test.insert({_id: 1, word: '[]'});
> db.test.insert({_id: 2, word: ']'});
> db.test.insert({_id: 9, word: 'no brackets'});
>
> db.test.find({word:/[\[]/});           // could not be run in console
{ "_id" : 0, "word" : "[" }
{ "_id" : 1, "word" : "[]" }
> db.test.find({word:/\[/ });            // could not be run in console
{ "_id" : 0, "word" : "[" }
{ "_id" : 1, "word" : "[]" }
> db.test.find({word:/\]/});             //returns result
{ "_id" : 1, "word" : "[]" }
{ "_id" : 2, "word" : "]" }
> db.test.find({word:/[\]]/});           //returns result
{ "_id" : 1, "word" : "[]" }
{ "_id" : 2, "word" : "]" }
> db.test.find({word:{$regex:"\["}});    //no result
> db.test.find({word:{$regex:"\\["}});   //returns result when escaped properly
{ "_id" : 0, "word" : "[" }
{ "_id" : 1, "word" : "[]" }
> db.test.find({word:{$regex:"\[[]"}});  //returns result
{ "_id" : 0, "word" : "[" }
{ "_id" : 1, "word" : "[]" }


MongoDB 2.2.1 console output:


> db.test.drop(); version()
version: 2.2.1
>
> db.test.insert({_id: 0, word: '['});
> db.test.insert({_id: 1, word: '[]'});
> db.test.insert({_id: 2, word: ']'});
> db.test.insert({_id: 9, word: 'no brackets'});
>
> db.test.find({word:/[\[]/});           // could not be run in console
... db.test.find({word:/\[/ });            // could not be run in console
... db.test.find({word:/\]/});             // returns result
... db.test.find({word:/[\]]/});           // returns result
{ "_id" : 1, "word" : "[]" }
{ "_id" : 2, "word" : "]" }
> db.test.find({word:{$regex:"\["}});    // no result
> db.test.find({word:{$regex:"\\["}});   // returns result when escaped properly
{ "_id" : 0, "word" : "[" }
{ "_id" : 1, "word" : "[]" }
> db.test.find({word:{$regex:"\[[]"}});  // returns result


MongoDB 2.2.1 console output (commented out lines that don't parse as expected):


> db.test.drop(); version()
version: 2.2.1
>
> db.test.insert({_id: 0, word: '['});
> db.test.insert({_id: 1, word: '[]'});
> db.test.insert({_id: 2, word: ']'});
> db.test.insert({_id: 9, word: 'no brackets'});
>
> // db.test.find({word:/[\[]/});        // could not be run in console
> // db.test.find({word:/\[/ });         // could not be run in console
> db.test.find({word:/\]/});             // returns result
{ "_id" : 1, "word" : "[]" }
{ "_id" : 2, "word" : "]" }
> db.test.find({word:/[\]]/});           // returns result
{ "_id" : 1, "word" : "[]" }
{ "_id" : 2, "word" : "]" }
> db.test.find({word:{$regex:"\["}});    // no result
> db.test.find({word:{$regex:"\\["}});   // returns result when escaped properly
{ "_id" : 0, "word" : "[" }
{ "_id" : 1, "word" : "[]" }
> db.test.find({word:{$regex:"\[[]"}});  // returns result
{ "_id" : 0, "word" : "[" }
{ "_id" : 1, "word" : "[]" }
>