Issue
As a simplified example I have printf "abc\ndef\nghi\n" | grep -oPz '\w{2}(?=c|f|i)\n' and that does not match anything, however grep -oPz '\w{2}(?=(c|f|i)\n)' does. Why is that? I need the newline in the output.
Desired output is to match the rows and output them without the lookahead part eg. ab\n
Solution
ab means a followed by b, and continue matching after the b.
a(?=b) means a followed by b, and continue matching after the a.
So, for (?=c|f|i)\n to match, there must be a position that matches both c and \n, both f and \n, or by both i and \n. That's impossible.
Answered By - ikegami