Clojure Tricks: Replace in String

Today I saw a clever bit of Clojure code involving clojure.string/replace, that reminded me how powerful the Clojure standard library is. I guess pretty much everyone knows that replace is normally used to replace some part of a string using a regular expression to describe what exactly to replace:

(str/replace "OCaml rocks!" #"([Hh]askell)|([Oo][Cc]aml)" "Clojure")
;; => "Clojure rocks!"

;; we can refer to the match groups in the replacement string
(str/replace "Haskell rocks!" #"([Hh]askell)|([Oo][Cc]aml)" "$1 is nice, but Clojure")
;; => "Haskell is nice, but Clojure rocks!"

Pretty useful and pretty straightforward. But wait, there’s more! I had forgotten you can actually use a function for the replacement, which allows us to do more sophisticated things. Here’s how we can capitalize every word with 5 or more letters in a string:

(str/replace "master of Clojure is pulling the strings" #"\w{5,}" str/upper-case)
;; => "MASTER of CLOJURE is PULLING the STRINGS"

If you’ve got more match groups in your regular expression you can use all of them in the function that you’re using to generate the replacements:

(str/replace "pom kon sor" #"(.)o(.)" (fn [[_ a b]] (str (str/upper-case a) "-o-" (str/upper-case b))))
"P-o-M K-o-N S-o-R"

Note here the use of deconstructing to account that each match is essentially a vector of the full match and each group match (e.g. ["pom" "p" "m"]).

That’s I have for you today! Feel free to share with me more fun usages of replace!