password setting for web page

(?=^.{8,}$)
This part means has 8 more more characters, and the match starts at the start of input.

((?=.*\d)
Means contains a digit.

|(?=.*\W+))
Or contains something that is neither a letter or a digit

(?![.\n])
not starting with a dot or UNIX newline.

(?=.*[A-Z])
Contains at least one capital letter.

(?=.*[a-z])
Contains at least one lowercase letter

.*$
Consists entirely of non-newline characters and the matched group will contain the entire string.

For alternatives, have a look:
Here[^]
Here[^]
and Here[^]