sed: how to add text to first x occurrences

Question

I am trying to add text to the end of a line the first x times it occurs. I know how to do it globally and for the n occurrence. I can't figure out how to do it for the first nth occurrences. An example would be a text.txt file that contains:

This is a test
junk
This is a test
More junk
This is a test
This is a test
This is a test

And, I want to add a '.' at the end of the first three times that "This is a test" occurs. The output I am trying to get is:

This is a test.
junk
This is a test.
More junk
This is a test.
This is a test
This is a test

Answer 1

This.*test is the correct regex. The asterisk means "0 or more times the previous character" so This*test wouldn't match any of your lines.

Now, Sed is bad at Arithmetics. For something elegant I suggest Awk:

awk '/This.*test/{c++};{print $0 (c<4 ? "." : "")}' file

I think it suffices to say that c, as any unset variable in Awk, is treated as zero, but let me know if you need further clarification.

Answer 2

Another variant that avoids doing the regexp matching after all 3 occurrences have already been found:

awk -v n=3 'n && /This is a test/ {n--; $0 = $0 "."}; {print}'

With sed specifically, you could do something like:

sed '
  1 {
    x
    s/^/.../
    x
  }
  /This is a test/ {
    s/$/./
    x
    s/.//
    /./ {
      x
      b
    }
    g
    :1
    $! {
      n
      b 1
    }
  }'

Where we track the number of .s to append as corresponding number of .s in the hold space.

It goes without saying that sed is a lot less appropriate for this kind of task. If the reason for wanting sed is for the -i extension for in-place editing found on a few implementations (borrowed from perl), note that the GNU implementation of awk can also do it with -i inplace, or you could use the real thing:

perl -lpi -e '
  if ($n < 3 && /This is a test/) {
    $n++;
    $_ .= ".";
  }' your-file

If you wanted to add a . after each occurrence of This is a test as opposed to all lines that contain at least one occurrence of This is a test, perl would also be the best choice:

perl -pi -e 's{This is a test\K}{$n++ < 3 ? "." : ""}ge' your-file

Answer 3

With perl we could do as shown

perl -lpe '
  $_ = $k == 3 ? next : s/This is a test(?{$k++}).*\K/./r;
' file

---

Elephants can dance too, albeit the simple steps. Using GNU sed writing in its extended regex mode -E we can store the count as number of newlines in the hold.

K=3
sed -Ee '
  /This is a test/!b
  G
  /(.*\n){'"$K"'}.*\n/!{
    s/\n+/./p;z;H;d
  }
  s/\n+//
  :a;n;ba
' file