(use irregex)
(irregex-search "[\\]d" "1 \\dupe!")

;; Here's what I got when I ran the above. I was hoping to match "\\d"

;; Error: incomplete char set
;; "[\\]d"
;; 4
;; 4

;; 	Call history:

;; 	<syntax>	  (irregex-search "[\\]d" "1 \\dupe!")
;; 	<eval>	  (irregex-search "[\\]d" "1 \\dupe!")	<--