WordPress at a glance
filter is not described

pre_post_(field) filter-hook . WP 2.3.0

Filters the value of a specific field before saving.

The dynamic portion of the hook name, $field_no_prefix, refers to the post field name.

Usage

add_filter( 'pre_post_(field)', 'filter_function_name_1921' );
function filter_function_name_1921( $value ){
	// filter...

	return $value;
}
$value(mixed)
Value of the post field.

Where the hook is called

sanitize_post_field()
wp-includes/post.php 2221
$value = apply_filters( "pre_post_{$field}", $value );

Where the hook is used (in WP core)

wp-includes/default-filters.php 86
add_filter( 'pre_post_mime_type', 'sanitize_mime_type' );