filter-buffer-substring-function

filter-buffer-substring-function is a variable defined in `simple.el'.
Its value is

buffer-substring--filter

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

Documentation:
Function to perform the filtering in `filter-buffer-substring'.
The function is called with the same 3 arguments (BEG END DELETE)
that `filter-buffer-substring' received. It should return the
buffer substring between BEG and END, after filtering. If DELETE is
non-nil, it should delete the text between BEG and END from the buffer.