I would like to perform a "contains" query on a single text field in a case insensitive and diacritic insensitive way. I would like the query to have the performance of the field being in a text index by itself (assuming that this gives the maximum speed).

It is usually the case that the text index already exists for multiple fields. This means that a text search then has to be combined with a complicated (to make it diacritic insensitive) regex expression to find the correct results.

https://developer.mongodb.com/community/forums/t/query-for-field-containing-a-string-case-in-sensitive-and-diacritic-in-sensitive/14814/2/