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[1] Press: Ctrl + H
[2] Check the regex icon: .*
[3] In the find field, enter the following (replace N with the number of lines to select: 2 for every 2nd, 10 for every 10th):
(.*\n){N}
example: (.*\n){5} for selecting every 5th line
[4] Press: Alt + Enter
[5] Move the multiple cursors with your arrow keys (and/or type what you want)
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// mac OS shortcut
alt + (click on the line, then click the next line etc)
// note: you can click on the cursor to remove it if you've placed it by mistake