(require 'yasnippet)
(defvar yas-original-buffer nil)
(defun yas-new-snippet--with-region (&optional no-template)
  "Pops a new buffer for writing a snippet.

Expands a snippet-writing snippet, unless the optional prefix arg
NO-TEMPLATE is non-nil."
  (interactive "P")
  (let ((guessed-directories (yas--guess-snippet-directories)))

    (setq yas-original-buffer (current-buffer))
    (switch-to-buffer "*new snippet*")
    (erase-buffer)
    (kill-all-local-variables)
    (snippet-mode)
    (yas-minor-mode 1)
    (set (make-local-variable 'yas--guessed-modes) (mapcar #'(lambda (d)
                                                              (yas--table-mode (car d)))
                                                          guessed-directories))
    (when (and (not no-template) yas-new-snippet-default)
      (save-excursion (insert (yas-snippet-from-region)))
      (yas-expand-snippet yas-new-snippet-default))))
(defun yas-snippet-from-region ()
  "Initial snippet content from region."
  (or (with-current-buffer yas-original-buffer
        (if (region-active-p)
            (replace-regexp-in-string
             "[\\$]" "\\\\\\&"
             (buffer-substring-no-properties (region-beginning) (region-end)))))
      ""))

(advice-add 'yas-new-snippet :override 'yas-new-snippet--with-region)