[PetBest]

The regular expression:

(\.(?!kobo|adobe).*?|Books\..+)

can be broken down as follows:

Breakdown:

(\.(?!kobo|adobe).*?

\.: Matches a literal dot (.).

(?!kobo|adobe): Negative lookahead — ensures that what follows the dot is not "kobo" or "adobe".

.*?: Lazily matches any characters (as few as possible).

|Books\..+

Alternation (|) — matches either the previous part or this one.

Books\.: Matches the literal string "Books.".

.+: Matches one or more of any characters following "Books.".

Summary:

This regex matches:

A string that starts with a dot not followed by "kobo" or "adobe", and then has any characters.

OR a string that starts with "Books." followed by at least one more character.

Example Matches:

.kindle

.somethingelse

Books.MyBook

Books.test.pdf

Example Non-Matches:

.kobo

.adobe