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

List of filter functions for `buffer-substring--filter'.
Each function must accept a single argument, a string, and return a string.
The buffer substring is passed to the first function in the list,
and the return value of each function is passed to the next.
As a special convention, point is set to the start of the buffer text
being operated on (i.e., the first argument of `buffer-substring--filter')
before these functions are called.