Help with `replace-regexp-in-string`, to replace only first instance in every line

Question

I have the following string:

// const asdf = 'asdf'
// // comment
// const asdf2 = asdf

And I want to run a regexp replace to make it look as follows:

const asdf = 'asdf'
// comment
const asdf2 = asdf

Basically, I'm trying to make a regexp that will comment out the first instance of // on every line. Here is my current implementation:

(setq comment "// const asdf = 'asdf'\n// // comment\n// const asdf2 = asdf")
(setq comment (replace-regexp-in-string "\\(//\\)" "" comment)))

But note that it is removing all matches to //, which gives the following output:

const asdf = 'asdf'
comment
const asdf2 = asdf

But I want to leave the second instance of // on the // // comment line. Any suggestions?

If your are curious, I am trying to solve this issue: https://github.com/fxbois/web-mode/issues/980 at this place in the code of web-mode: https://github.com/fxbois/web-mode/blob/master/web-mode.el#L9973

Answer 1

1. You are likely looking for this:

   (setq comment (replace-regexp-in-string "^// " "" comment))
   

2. But are you sure you want to create a string and then act on it? A guess is that you really want to act on the text in a buffer directly, using re-search-forward followed by replace-match. Something like this:

   (defun foo ()
     (interactive)
     (save-excursion
       (goto-char (point-min))
       (while (re-search-forward "^// " nil t)
         (replace-match "")
         (forward-line 1))))
   

Answer 2

C-hf replace-regexp-in-string actually covers this exact requirement (or it does in Emacs 25.3, at any rate).

To replace only the first match (if any), make REGEXP match up to \'
and replace a sub-expression, e.g.
 (replace-regexp-in-string "\\(foo\\).*\\'" "bar" " foo foo" nil nil 1)
   => " bar foo"

To replace the first match (if any) in each line of the string, you would use $ (which matches the end of a line) in place of \' (which matches the end of the text), like so:

(replace-regexp-in-string "\\(foo\\).*$" "bar"
                          " foo foo\nbaz\nbar foo foo"
                          nil nil 1)
 => " bar foo\nbaz\nbar bar foo"

Answer 3

You could look for the pattern "beginning of line, spaces, //, spaces" and replace that with the empty string:

(replace-regexp-in-string "^\\s-*//\\s-*" "" comment)