How to add text before the Nth occurrence of a text using sed only?

Question

I searched on previous questions but they were only with appending so adding after the occurrence. i need it to be before:

 world world
 world world
 world world

So sed must add "hello" for example at the start of the Nth occurrence of a specific text. in this case im adding it to the fourth world:

world world
world hello world
world world

Answer 1

If you absolutely must use sed, you may try the following, but as the comments already made clear the sed route will be rickety.

sed -e '
  H;1h;g
  s/world/HELLO &/4;tn
  $!d;:n;n;bn
' yourfile

Answer 2

sed is the wrong tool for this job. Use awk or perl instead. e.g.

$ perl -pe 's/world/++$i == 4 ? "hello world" : $&/ge' input.txt 
world world
world hello world
world world

Note that this uses the /e perl regex modifer, which causes the replacement portion of the s/// substitute operator to be executed as perl code rather than be interpreted as a string.

That code ++$i == 4 ? "hello world" : $& pre-increments a counter variable ($i) on every match, and if it's equal to 4, replace the match with "hello world", otherwise replace the match with itself ($&).

Answer 3

I know you said "using sed" but if you ever have to do something like this in the real world, here's how using any awk in any shell on every Unix box and doing a full-word string comparison (see how-do-i-find-the-text-that-matches-a-pattern for why that matters):

$ awk -v n=4 '{
    for (i=1;i<=NF;i++) {
        if ( ($i == "world") && (++cnt == n) ) {
            $i = "hello " $i
        }
    }
    print
}' file
world world
world hello world
world world

Imagine your input was:

$ cat file
google.com mygoole.comedy
googleycom google.com
google.com google.com

and you wanted to put "hello" before the 4th google.com (which is now the last one in the input). With the above awk script you just change $i=="world" to $i=="google.com":

$ cat file
awk -v n=4 '{
    for (i=1;i<=NF;i++) {
        if ( ($i == "google.com") && (++cnt == n) ) {
            $i = "hello " $i
        }
    }
    print
}' file
google.com mygoole.comedy
googleycom google.com
google.com hello google.com

Now try to do the same with a sed script (especially if you only use POSIX syntax and no GNU extensions). Now try using in&out as the replacement text instead of hello and you'll find more problems with the sed script.

Answer 4

sed 'H;1h;$!d;x;s/world/hello &/4'