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<pre>
(defun org-replace-region-by-html (beg end)
"Assume the current region has org-mode syntax, and convert it to HTML.
This can be used in any buffer. For example, you could write an
itemized list in org-mode syntax in an HTML buffer and then use this
command to convert it."
(interactive "r")
(let (reg html buf pop-up-frames)
(save-window-excursion
(if (derived-mode-p 'org-mode)
(setq html (org-export-region-as-html
beg end t 'string))
(setq reg (buffer-substring beg end)
buf (get-buffer-create "*Org tmp*"))
(with-current-buffer buf
(erase-buffer)
(insert reg)
(org-mode)
(setq html (org-export-region-as-html
(point-min) (point-max) t 'string)))
(kill-buffer buf)))
(delete-region beg end)
(insert html)))
</pre>