Multiline regex replace within elisp function

Question

I would like to write a elisp function that corrects a few formatting issues in my LaTeX code. Precisely, I would like to transform the following text

Some useless text
\begin{equation}
    some math stuff
\end{equation}
Further text.

into

Some useless text
%
\begin{equation}
    some math stuff
\end{equation}
%
Further text.

i.e. I want that equation environments that are not surrounded by comments get an empty comment line before and after the environment. My idea was to write a small elisp function that replaces all such occurences with some regex. With lots of trial and error I found the following regex that seems to find the \begin{equations} that are not preceeded by % in the line before.

^[[:space:]]*[^%].*\n[[:space:]]*\\\\begin{equation}

I also found the following snippet on the internet to do search an replace:

(defun foo ()
  (interactive)
  (save-excursion
    (beginning-of-buffer)
    (while (re-search-forward "foo\nbar" nil t)
      (replace-match "bar\nfoo" nil nil))))

My guess is the a clever combination of these things will give me my result, but so far all my attempts have failed. Can anybody give me some hints on how I achieve the sought substitution?

Update:

After reading: Emacs regexp groups in regex-replace I came up with the following:

(defun foo ()
  (interactive)
  (save-excursion
    (beginning-of-buffer)
    (while (re-search-forward "\\(^[[:space:]]*[^%].*\n\\)\\([[:space:]]*\\)\\(\\\\begin{equation}\\)" nil t)
    (replace-match "\\1\\2%\n\\2\\3" nil nil))))

which seems to work for the few test cases that I tried so far. It fails when \begin{equation} terms are nested immediately after each other, but since this is not allowed in LaTeX anyway its not a critical issue.

Answer 1

Here's something quick I came up with. It looks like it can catch situations when equations follow each other, but do tell if I missed some corner cases:

(defun my/wrap-equation-with-% ()
  (interactive)
  (save-excursion
    (beginning-of-buffer)
    (let ((last ""))
      (while (not (eobp))
        (search-forward-regexp "^.*$" nil t)
        (let ((line (match-string 0)))
          (cond
           ((and (not (string-equal last "%"))
                 (s-starts-with-p "\\begin{equation" (s-trim-left line)))
            (move-beginning-of-line 0)
            (forward-line)
            (insert "%\n"))
           ((and (s-starts-with-p "\\end{equation" (s-trim-left last))
                 (not (string-equal line "%")))
            (move-end-of-line 0)
            (insert "\n%")
            (forward-line)))
          (setf last line)
          (forward-line))))))

Ah, this also uses s-starts-with from https://github.com/magnars/s.el