looking-back is a byte-compiled function defined in subr.el.gz.

Signature
(looking-back REGEXP LIMIT &optional GREEDY)

Documentation
Return non-nil if text before point matches regular expression REGEXP.

Like looking-at except matches before point, and is slower.
LIMIT if non-nil speeds up the search by specifying a minimum
starting position, to avoid checking matches that would start
before LIMIT.

If GREEDY is non-nil, extend the match backwards as far as
possible, stopping when a single additional previous character
cannot be part of a match for REGEXP.  When the match is
extended, its starting position is allowed to occur before
LIMIT.

As a general recommendation, try to avoid using looking-back
wherever possible, since it is slow.

View in manual

References
looking-back is unused in subr.el.gz.

Find all references Functions used by looking-back

Debugging
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Disassemble Forget

Aliases
org-looking-back (obsolete since 9.0)

Source Code
;; Defined in /usr/share/emacs/29.4/lisp/subr.el.gz
(defun looking-back (regexp &optional limit greedy)
  "Return non-nil if text before point matches regular expression REGEXP.
Like `looking-at' except matches before point, and is slower.
LIMIT if non-nil speeds up the search by specifying a minimum
starting position, to avoid checking matches that would start
before LIMIT.

If GREEDY is non-nil, extend the match backwards as far as
possible, stopping when a single additional previous character
cannot be part of a match for REGEXP.  When the match is
extended, its starting position is allowed to occur before
LIMIT.

As a general recommendation, try to avoid using `looking-back'
wherever possible, since it is slow."
  (declare
   (advertised-calling-convention (regexp limit &optional greedy) "25.1"))
  (let ((start (point))
	(pos
	 (save-excursion
	   (and (re-search-backward (concat "\\(?:" regexp "\\)\\=") limit t)
		(point)))))
    (if (and greedy pos)
	(save-restriction
	  (narrow-to-region (point-min) start)
	  (while (and (> pos (point-min))
		      (save-excursion
			(goto-char pos)
			(backward-char 1)
			(looking-at (concat "\\(?:"  regexp "\\)\\'"))))
	    (setq pos (1- pos)))
	  (save-excursion
	    (goto-char pos)
	    (looking-at (concat "\\(?:"  regexp "\\)\\'")))))
    (not (null pos))))