Mongo.Ecto.Helpers.regex

You're seeing just the function regex, go back to Mongo.Ecto.Helpers module for more information.
Link to this function

regex(pattern, options \\ "")

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Specs

Creates proper regex object that can be passed to the database.

Usage in queries

from p in Post,
  where: fragment(title: ^regex("elixir", "i"))

For supported options please see Mongo.Ecto.Regex module documentation.