Question

I have a file like this:

Sed eleifend orci eget odio
consequat. Sed sagittis ipsum
eget pulvinar. Sed ut lacus
Sed luctus sollicitudin ligula
varius neque. Sed tincidunt
Sed mauris egestas eget. Sed
Curae; Sed aliquam enim Sed,
Sed dictum quis sem. Sed
volutpat tincidunt. Sed lacus.

I want to convert it to:

Sed eleifend orci eget odio
consequat. Sed sagittis ipsum
eget pulvinar. Sed Sed ut lacus
Sed luctus sollicitudin ligula
varius neque. Sed tincidunt
Sed Sed mauris egestas eget. Sed
Curae; Sed aliquam enim Sed Sed,
Sed dictum quis sem. Sed
volutpat tincidunt. Sed Sed lacus.

Answer 1

A Perl one-liner can achieve this by use of the /e modifier, which allows for logic-based substitution:

$ perl -pi.bak -e 'BEGIN{ $str = "Sed"; } s/(?<=$str)/ ++$cnt % 3 ? "" : " $str" /ge' file.txt

Explanation

• -pi.bak

  Line-by-line in-place editing of the file. Backup stored in file.txt.bak

• BEGIN block

  Specify value of $str, executes once only

• s/PATTERN/REPLACEMENT/ge

  Regex substitution on all matches in $_, line-by-line. REPLACEMENT evaluated as Perl code.

• (?<=$str)

  Fixed-length look-behind assertion

• ++$cnt % 3 ? "" : " $str"

  Every third match, append " $str", otherwise append nothing