Question

How to write a function where whenever a variable is found, it returns t (in order to allow a loop):

(setq x 1)
(while ("backward search for regexp "%x" equals true") ;where x is variable
  (setq x (+ x 1))
(insert (concat "%" (int-to-string x)))
)

Example: If %1 (x=1) is found, it will add 1 to x. If %2 (x=2) is found, it will add 1 to x.
Let’s say %3 is not found in a backward search, the while loop stops and “%” + “3” is inserted (%3).

I just don’t understand the how to return true on a backward-search.

Answer 1

search-backward takes an optional third argument which, when non-nil, tells it to return nil in case the search was unsuccessful:

(setq x 1)
(while (search-backward (format "%%%d" x) nil t)
  (setq x (1+ x)))
(insert (format "%%%d" x))

---

Now, if I try to understand what you really want to do (something like inserting at point the first %d string which doesn’t appear before), then you might want to wrap the search inside a save-excursion form to avoid moving the point:

(setq x 1)
(while (save-excursion (search-backward (format "%%%d" x) nil t))
  (setq x (1+ x)))
(insert (format "%%%d" x))