elisp - regex to replace substring with multiple conditions

Question

I want to replace below example

line1 ![img](170319_first.png "test") 
line2 ![img](170319_second.png)
line3 ![img](/assets/170319_second.png) 

to:

line1 ![img](/assets/170319_first.png "test") 
line2 ![img](/assets/170319_second.png)
line3 ![img](/assets/170319_second.png) 

which means:

1. Do not replace line3 case (file name start with "/assets" already)
2. Add "/assets/" to file name for line1 case (end with a space)
3. Add "/assets/" to file name for line2 case also (end with a ')').

I try to use below code but it doesn't work.

(setq str "
line1 ![img](170319_first.png \"test\") 
line2 ![img](170319_second.png)
line3 ![img](/assets/170319_second.png)
")
(with-temp-buffer
  (insert str)
  (goto-char (point-min))
  (setq regex1 "\\!\\[img\\](\\([^/]*?\\)\\([ \\|)]\\)")
  (while (re-search-forward regex1 nil t)
    (setq regex2 (format "![img](/assets/%s" (match-string 1)))
    (replace-match regex2 nil t))
  (print (buffer-string))
  )

But the result not correct:

line1 ![img](/assets/170319_first.png\"test\") 
line2 ![img](/assets/170319_second.png
line3 ![img](/assets/170319_second.png)

Obviously, the ' ' and ')' has been replaced also. but I didn't include it in the match group.

Answer 1

You want to replace SUBEXP (group) 1 only, so you should pass that argument to replace-match and then reduce the replacement string to only what is needed for that group.

I would also suggest not passing LITERAL as you can then simply use a back-reference "\\1" in the replacement string to substitute the original text matched by group 1, which eliminates the need to separately construct a replacement string using (match-string 1).

That leaves us with just:

(while (re-search-forward regex1 nil t)
  (replace-match "/assets/\\1" nil nil nil 1))

Answer 2

Since the condition of 2 and 3 can be simplified to: the character after "![img](" not be '/', below code works for this case:

(setq str "
line1 ![img](170319_first.png \"test\") 
line2 ![img](170319_second.png)
line3 ![img](/assets/170319_second.png)
")
(setq regex1 "\\!\\[img\\](\\([^/]\\)")
(setq str (replace-regexp-in-string regex1 "/assets/\\1" str nil nil 1))
(print str)

Output:

line1 ![img](/assets/170319_first.png \"test\") 
line2 ![img](/assets/170319_second.png)
line3 ![img](/assets/170319_second.png)

Answer 3

(let (
      (str-sample "
line1 ![img](170319_first.png \"test\") 
line2 ![img](170319_second.png)
line3 ![img](/assets/170319_second.png)
")
      )
  (with-temp-buffer
    (insert str-sample)
    ;; just trash
    (insert "foobar\n")
    (insert "line4 ![img](/s/170319_second.png)")
    (insert "\nDone ...\n")
    ;; well
    (goto-char (point-min))
    (while (re-search-forward "\\(.*\\)\\((\\)\\([0-9]+.*$\\)" nil t)
      (replace-match "\\1\\2/assets/\\3")
      )
    (print (buffer-string))
    )
'ok
)