How to put a check on a cell so that only 8 digits are written there?

Question

Google Sheets

In the cell you need to write 8 digits, no more and no less.
It can start from scratch, if it is important for the formula.
In fact, this is the date of birth DDMMYYYY, only without spaces, dots, slashes, etc., only numbers.
Link to the spreadsheet:
https://docs.google.com/spreadsheets/d/1vqKnsN_8qQ...
Is it possible to solve this problem without taking into account to write like this: DD.MM.YYYY?
That is, if you wrote 7 or 9 digits in this cell, then some kind of warning popped up in any form, the main thing is that the person entering the numbers understands that he did not enter the numbers correctly.
Maybe there is a solution different from mine, but I just don't see it.

Answer 1

You can use data validation. Menu - Data - Data Validation

For example, for a cell '8 ЦИФР'!D9
, you can use the formula
=REGEXMATCH(B9;"^\d{8}$")