Check cell if following specific pattern

So it seems like you are literally looking for [BOOKS] and this not just a reference to a book title, so having [This is a title] instead of [Books] should generate an error, correct?
And the comma delimited, is that between the set of values, or can one set of values have a comma, like [BOOKS],123 - is this a fail because there's a comma between [BOOKS] and 123?

That's right, literally looking for "[BOOKS]" spelled out, followed by lowercase "b" then 3 random digits.
[BOOKS],[BOOKS]b123 --> Should fail because it's missing the bxxx following.

What about the question about the comma? Is this pass or fail?

[BOOKS],b123

Also, I haven't heard of version 2020. Is that the same as MS365? I thought the latest besides 365 was 2019.

Lastly, is there a limit to the # of times [BOOKS]b123,[BOOKS]b321,... can be repeated?

Apologies, you are correct this is Excel for Mac 365 version 16.41

Comma after [BOOKS] would be a fail.
Each "value" should follow the "[BOOKS]" + "b" + 3 digit number e.g. [BOOKS]b222
and it should be comma delimited. If there are any values that don't follow the pattern above e.g. Uppercase "B", More than 3 digits, Missing "["... --> it would be an automatic fail.

We didn't apply a limit to the number of books that can be listed in a cell, but max number of books we have are 200 if that helps.

Well, this might not be the most elegant solution, but at least for what you have stated it seems to work:

=IF(AND(12*(LEN(A1)-LEN(SUBSTITUTE(A1,"[BOOKS]b","")))/LEN("[BOOKS]b")-1=LEN(A1),ISNUMBER(SUBSTITUTE(A1,"[BOOKS]b",1)*1)),"Pass","Fail")

Greg, Thank you so so much! This "not the most elegant solution" was WAY more elegant than my multiple SEARCH calls that I had strung together.
Appreciate it and this seems to work well for all my test cases!

Glad it helped. Thanks for the rep!

This part:
(LEN(A2) - LEN(SUBSTITUTE(A2, "[STORE]r", ""))) / LEN("[STORE]r")

tells you how many times the text string of "[STORE]r" is in the cell. Since the total string also includes 3 digits and a comma (12 characters in all), the total amount of characters in the cell would be:
12*the number of text strings minus 1 because the last string will not include a comma. So if this doesn't match the total length of the characters in the cell, it means that there are either too many or not enough numbers after the "r" (or "b", as it were).

The other parts assures that everything else besides [BOOKS]b is a number

THANK YOU! Learned something new and definitely makes sense. Thank you thank you!