region-extract-function

region-extract-function is a variable defined in `simple.el'.
Its value is

#[128 "\300\301\302#\207"
      [apply rectangle--extract-region
	     #[257 "\300 \205\211\301=\203\300 \302 |\207\303\300 \302 #\207"
		   [region-beginning delete-only region-end filter-buffer-substring]
		   5 "\n\n(fn DELETE)"]
	     nil]
      5 nil]

• This variable may be risky if used as a file-local variable.

Documentation:
Function to get the region's content.
Called with one argument DELETE.
If DELETE is `delete-only', then only delete the region and the return value
is undefined. If DELETE is nil, just return the content as a string.
If anything else, delete the region and return its content as a string.