I have a line as follows

 // Testing this

I am trying to use a sed command to replace the // with a /* (sentence or word in between) */ on each line.

so it should look something like this

/* Testing this */

The first part is easy with calling

sed 's#//#/*#'

however, with the second part, I tried this solution

Appending word at the end of line with sed [duplicate]

I tried with -e which gave me an error of 'unterminated s command'. Then, I tried with a semi colon to make it one so it was something along the lines of


but all that seems to do is the first part (replacing // with /*)and not the second (putting a */ at the end of the same line).

What am I not doing correctly? Any pointers would be appreciated.

  • Are you trying to convert C++-style comments to C-style comments ? Sep 23, 2018 at 21:34
  • @don_crissti yes but I want to know in general. I have other files where I have to replace phone numbers and add things to the end of them so I am trying to figure out how to do it in general. Sep 23, 2018 at 21:48

2 Answers 2


How about

$ cat testfile
// Testing this
$ sed 'sx//\(.*\)x/*\1 */x' testfile
/* Testing this */
  • sx// : search for lines containing //
  • \(.*\)x : place the remaining part of the line into capture group 1
  • /*\1 */x : replace the remaining part of the line with /* (start of comment, C-style) followed by the contents of the capture group 1 (referenced as \1), followed by */ (end of comment, C-style)
  • 1
    Thank you for the explanation. Do you mind me asking, what is the (.*) is for in this case? I realize the " ( " and " ) " are escape sequences for the parenthesis, but what are we doing with the " .* " ? PS: I don't know why the backslash character is not showing up. Sep 24, 2018 at 2:48
  • Sure, elaborated.
    – steve
    Sep 25, 2018 at 21:35

Another approach: make the search and replace conditional on the comment being matched at all:

sed '\!//! {s!!/*!; s!$! */!}' file

That reads:

  • if the line contains the pattern //, then
    • replace the matched text with /*
    • and then replace the empty string at end of line with */

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