Is it correct to say "have I had known"? Thank you all.
The correct form is, "Had I known (about the lack of security), ...". It requires the same tense as the if clause "If I had known (about the lack of security), ..."
This type of conditional structure is known as the third conditional or conditional III. It is used to refer to a hypothetical situation that could have occurred in the past.
Your sentence is incorrect, it should be stated as
Had I had known (about it, I would not have done that).
Had I known, (I would not have done that).
If I had known (what would happen, I would not have done that).
to show regret for something which might have been avoided.
No, your sentence is not correct. The pattern in question here is:
had I done something, something else would or would not have happened.
This pattern is cast in stone and cannot be changed or manipulated in any way. We use it to talk about situations that are not real and only exist in our imagination. Oftentimes, this structure is employed when we want to talk about things we wish we had done in the past, but unfortunately the ship has sailed and there is nothing we can do about it now.
Example #1 (wish):
Had I had a million dollars, I would have bought the best car in the world!
Example #2 (regret):
Had I studied harder, I would have passed the exam!
Example #3 (regret):
Had I known that earlier, I would not have made this stupid mistake!