Open in appropriate browser based on URL

open-custom.el

(setq
 browse-url-chromium-program "/opt/homebrew-cask/Caskroom/google-chrome/latest/Google Chrome.app/Contents/MacOS/Google Chrome"
 browse-url-firefox-program "/opt/homebrew-cask/Caskroom/firefox/41.0.1/Firefox.app/Contents/MacOS/firefox")

(defun my-browse-url-maybe-privately (url &optional new-window)
  "Ask whether URL should be browsed in a private browsing window."
  (interactive "sURL: ")
  (if (y-or-n-p "Private Browsing? ")
      (my-browse-url-chrome-privately url)
    (browse-url-default-browser url new-window)))

(defun my-browse-url-chrome-privately (url &optional new-window)
  "Make firefox open URL in private-browsing window."
  (interactive (browse-url-interactive-arg "URL: "))
  (let ((process-environment (browse-url-process-environment)))
    (apply 'start-process
           (concat "chrome " url)
           nil
           browse-url-chromium-program
           (list "--incognito" url))))

(setq browse-url-browser-function
      '(("^https?://t\\.co" . my-browse-url-chrome-privately)
        ("^https?://instagram\\.com" . my-browse-url-chrome-privately)
        ("^https?://github\\.com/PrivateRepo" . browse-url-default-browser)
        ("^https?://github\\.com" . my-browse-url-maybe-privately)
        ("." . my-browse-url-maybe-privately)))