Select all found RegEx results in Atom
Pressing Alt + Enter will select all instances matching your Find results. This works for RegEx search also.
Source: https://github.com/atom/find-and-replace/pull/290
On Atom 1.57 (nov 2021) clicking on "find all" selects all matching items, esc to exit the search box, then copy :)
Alternatively you can select all whit the menu -> find > select all.
On mac also selects all by pressing ^ + cmd + G.