Emacs: use font-lock to add unit conversion to temperatures

emacs-font-lock-unit-conversion.el

;; -*- lexical-binding: t; -*-

(defun add-C-to-F-highlighting ()
  "demo: add highlighting to current buffer"
  (interactive)
  (font-lock-add-keywords
   nil
   `((,(rx (group (seq bow (+ digit) (optional (or "." "/")) (* digit))) (optional "°") (group "C"))
     (0 'bold)
     (2
      (let* ((temperature-C (* 1.0 (string-to-number (match-string 1))))
             (temperature-F (+ 32 (* 9 (/ temperature-C 5)))))
        (list
         'face nil
         'display
         (concat "C"
                 (propertize (format " (%.1f°F)" temperature-F) 'face 'italic)))))))))

;; need to repeat that for F to C conversion