Filtra con il driver C# regex MongoDB

Puoi usare Builders.Filter.Regex .

public async Task<Entity> GetEntityIdByOriginalEmail(string originalEmail)
{
    var collection = GetCollection();
    var filter = Builders<Entity>.Filter.Regex("x", new BsonRegularExpression(originalEmail, "i"));
    return await collection.Find(filter).FirstOrDefaultAsync();
}