Steps to reproduce. crate a collection with collation that has a "caseLevel" : false db.createCollection("comments", { "collation": { "locale": "de", "caseLevel": false, "caseFirst": "off", "strength": 2, "numericOrdering": false, "alternate": "non-ignorable", "maxVariable": "punct", "normalization": false, "backwards": false, } } ) Insert two documents db.comments.insert({"text":"Abcdefghij"}) db.comments.insert({"text":"abcdefghijXyz"}) db.comments.find() // returns { "_id" : ObjectId("59c5161c3f1fff167ea2a5fc"), "text" : "Abcdefghij" } { "_id" : ObjectId("59c5161c3f1fff167ea2a5fd"), "text" : "abcdefghijXyz" } Run a simple regex query: > db.comments.find({"text": /cde/}) // returns { "_id" : ObjectId("59c5161c3f1fff167ea2a5fc"), "text" : "Abcdefghij" } { "_id" : ObjectId("59c5161c3f1fff167ea2a5fd"), "text" : "abcdefghijXyz" } The result makes sence, since the collection has a collation which is case insensitive, so the sever "complement" the regex to be case insensitive. Run another query that has two expressions: db.comments.find({"text": /cde/, "text": /jxy/}) // returns no documents Run a "modified" query that includes a case insensitive flag on the second expression: db.comments.find({"text": /cde/, "text": /jxy/i}) // returns: { "_id" : ObjectId("59c5161c3f1fff167ea2a5fd"), "text" : "abcdefghijXyz" } It looks like that when the sever evaluates the second expression the collation of the collection does not come into consideratin. This behavior was tested on windows 64 bit versions 3.4.6 and 3.4.9