Querying large Index definition (Custom Analyzers)

I have been scratching my head over this all day and scoured MongoDB entirely, in the hope of learning a way to solve the example below.

The following example index definition uses a custom analyzer named keywordTokenizingIndex. It uses the keyword tokenizer and a regular expression token filter that redacts email addresses. You can also find the data for this index definition in the Usage section of the " custom analyzers" page.

I would love it if someone could help me to solve this using aggregation, below a link to the example index definition.