The Lisp family of languages is relatively new to me. I learned both Clojure and Emacs Lisp at the same time, as Emacs is a popular Clojure editor. Learning these two lisps side by side has made me wonder about the subtle differences between the two, in particular how they approach passing functions as arguments to other functions (first class functions). It turns out this boils down to Clojure being a lisp-1 and Emacs Lisp a lisp-2.
Sometimes you need to validate phone numbers, googlei18n/libphonenumber is a Java library for doing just that (and more). Thanks to Clojure's great java interop using this library in your project is straightforward.