Question

i think I found a bug in clojure, can anyone explain why the backslash is missing from the output?

(clojure.string/replace "The color? is red." #"[?.]"  #(str "zzz\\j" %1 %1))

=> "The colorzzzj?? is redzzzj.."

Answer 1

This isn’t a bug. The string returned by the function in the 3rd parameter is parsed for escape sequences so that you can do things like this:

(clojure.string/replace "The color? is red." #"([?.])" "\\$1$1")
; => "The color$1? is red$1."

Notice how the first $ is escaped by the backslash, whereas the second serves as the identifier for a capture group. Change your code to use four backslashes and it works:

(clojure.string/replace "The color? is red." #"[?.]"  #(str "zzz\\\\j" %1 %1))