Copy and store the contents in parentheses with Notepad++

Solution 1:

Try this for the match:

^.*\((\d+)\)$

And replace with:

$1

This works because:

Match from the beginning of the line with ^
Match everything until a paren .*\(, the paren is escaped because it is special
Use a group () to match some text and save for later.
Match one or more digits \d+
The digits are followed by a close paren \)
... which is at the end of the line $

Then the replacement $1 is the first group match in the regex, everything inside of the un-escaped parens.