How to find and replace every even-numbered appearance of a match in BASH? - perl

Question

I am using sed -i 's/AAA/ZZZ/g' filename to replace every occurance of "AAA" with "ZZZ" in a file. I need to instead replace every even-numbered appearance of "AAA" with "ZZZ", e.g.:
This is a AAA sentence. AAA
This is another AAA sentence.
This is yet AAA another AAA sentence.
This is AAA stillAAA AAA yet AAA another AAA sentence.
This would become:
This is a AAA sentence. ZZZ
This is another AAA sentence.
This is yet ZZZ another AAA sentence.
This is ZZZ stillAAA ZZZ yet AAA another ZZZ sentence.
How to replace every even-numbered appearance of a match?

Answer 1

Here is a short gnu awk version
awk '{ORS=NR%2==0?"ZZZ":RS}1' RS="AAA" file
This is a AAA sentence. ZZZ
This is another AAA sentence.
This is yet ZZZ another AAA sentence.
This is ZZZ stillAAA ZZZ yet AAA another ZZZ sentence.

Answer 2

awk is better tool for this than sed. Consider this awk command:
awk -F 'AAA' '{for (i=1; i<NF; i++) {OFS=c%2?"ZZZ":FS; printf "%s%s", $i, OFS; c++}
print $NF}' file
This is a AAA sentence. ZZZ
This is another AAA sentence.
This is yet ZZZ another AAA sentence.
This is ZZZ stillAAA ZZZ yet AAA another ZZZ sentence.
This awk sets the input field separator as AAA and and toggles output field separator between AAA and ZZZ depending upon a counter is odd or even. Every time counter is even OFS is set to AAA and when it is odd OFS is set to ZZZ

Answer 3

Here is a perl solution:
$ cat inp
This is a AAA sentence. AAA
This is another AAA sentence.
This is yet AAA another AAA sentence.
This is AAA stillAAA AAA yet AAA another AAA sentence.
$ perl -pe 'my $line = "" ; while(<>){ $line=$line.$_} $line =~ s/(.*?AAA.*?)AAA/\1ZZZ/mgs; print $line;' < inp
This is another AAA sentence.
This is yet ZZZ another AAA sentence.
This is ZZZ stillAAA ZZZ yet AAA another ZZZ sentence.
Here, first I accumulate entire file in a variable $line. & Then, I replace every alternate occurrence of AAA with ZZZ; using non-greedy matching.

Answer 4

Perl:
perl -wpe 'BEGIN{$/="AAA"} $.%2 or s/AAA/ZZZ/' foo.txt

Answer 5

You can do it with sed too:
sed -n -e '1,$ {
:oddline s/AAA/\n/g; :odd s/\n/AAA/m; t even ;p;N;s/.*\n//;b oddline ;
:evenline s/AAA/\n/g; :even s/\n/ZZZ/m; t odd ; p;N;s/.*\n//;b evenline ;
}' << _END_
This is a AAA sentence. AAA
This is another AAA sentence.
This is yet AAA another AAA sentence.
This is AAA stillAAA AAA yet AAA another AAA sentence.
_END_
The sed script loops through all lines and remembers odd/even replacements (across lines). In the pattern space, all AAAs are first replaced by newlines and then replaced one at a time by either AAA or ZZZ. In order to switch to the next line it is first appended (N) and then the previous one is deleted (s/.*\n//).

Answer 6

sed "1 h;1 !H;$ {x;l;s/=/=e/g;s/²/=c/g;s/AAA/²/g;s/²\([^²]\{1,\}\)²/²\1ZZZ/g;s/²/AAA/g;s/=c/²/g;s/=e/=/g;}" YourFile
Using substitution (due to AAA that could be inside a .*) insurring that even with substitute char is inside it work with the double translation before and after

Answer 7

This might work for you (GNU sed):
sed -r ':a;$!{N;ba};/\x00/q1;s/AAA/\x00/g;s/(\x00)([^\x00]*)\1/AAA\2ZZZ/g' file
This slurps the file into memory and then replaces all occurences of AAA with a unique character. Then every odd and even occurence of the unique character is replaced by AAA and ZZZ respectively.
N.B. If the unique character is not unique, no change is made to the file and an error code of 1 is set.
This second method is more long-winded but can be used to change the N'th value and does not rely on an unique value:
sed -r 's/AAA/\n&/g;/\n/!b;G;:a;s/$/#/;s/#{2}$//;/\n$/s/\nAAA/\nZZZ/;s/\n//;/\n.*\n/ba;P;s/^.*\n//;h;d' file
It stores the number of occurences of the required pattern in the hold space and retrieves it when encounters a line with such a pattern.