The "content_edit_pre" filter is part of a group of dynamic filters. This filter runs in wp-admin when loading a post for editing. The content that shows in the wp-editor will be passed as parameters a string and the id of the post.
When the 'content_edit_pre' filter is called, it is passed two parameters: the content and the id of the post to be edited
function filter_function_name( $content, $post_id ) {
// Process content here
return $content;
}
add_filter( 'content_edit_pre', 'filter_function_name', 10, 2 );
Where 'filter_function_name' is the function WordPress should call when filter is run. Note that the filter function must return a string after it is finished processing or the content will be empty.
filter_function_name should be unique function name. It cannot match any other function name already declared.