Edit WordPress post during save - Edit post content just before it is recorded in the database - could be used to add, remove or edit the content.

wp-insert-post-mods.php

<?php
/*
Plugin Name: wp_insert_post() modifications
Plugin URI: https://www.damiencarbery.com/2018/01/edit-wordpress-post-during-save/
Description: Change post data before save - remove #hashtags from post content before save.
Author: Damien Carbery
Version: 0.1
*/


add_filter( 'wp_insert_post_data', 'change_post_data_before_save', 10, 2 );
function change_post_data_before_save( $data, $postarr ) {	
	//error_log( 'Data: ' . var_export( $data, true ) );
	//error_log( 'PostArr: ' . var_export( $postarr, true ) );
	
	if ( 'publish' == $data[ 'post_status' ] ) {
		# Find the first # and extract the content up to that point.
		$first_hashtag = strpos( $data[ 'post_content' ], '#' );
		$before_hashtags = substr( $data[ 'post_content' ], 0, $first_hashtag );
		$data[ 'post_content' ] = $before_hashtags;
	}

	return $data;
}