Enclose a region in an org-mode block (e.g. #+BEGIN_SRC), or insert a structure template with completion

org-insert-structure-template-or-enclose-region.el

(defun org-read-structure-template ()
  "Read org-mode structure template with completion.  Returns template string."
  (let* ((templates (map 'list 'second org-structure-template-alist))
         (prefixes (map 'list (lambda (tp)
                                ;; Get template and pre-whitespace prefix for completion
                                (reverse (s-match (rx (group
                                                       (1+ (not (any "\n" space))))
                                                      (1+ anything))
                                                  tp)))
                        templates))
         (prefix (completing-read "Template: " prefixes nil t))
         (template (second (assoc prefix prefixes))))
    template))

(defun org-insert-structure-template-or-enclose-region ()
  "Insert structure block template.  When region is active, enclose region in block."
  (require 's)
  (interactive)
  (let* ((template (ap/org-read-structure-template))
         (text "")
         enclosed-text)
    (when (use-region-p)
      (setq text (buffer-substring-no-properties (region-beginning) (region-end)))
      (delete-region (region-beginning) (region-end)))
    (setq enclosed-text (s-replace "?" text template))
    (insert enclosed-text)
    (backward-char (- (length enclosed-text) (length (s-shared-start enclosed-text template))))))