Change on the fly the permastructure for a content.

rewrite-slug.php

<?php namespace BEA\CPT_Section;

/*
 * /!\ Warnings /!\
 * - Only working with https://github.com/BeAPI/hm-rewrite class which must be loaded
 * - PHP5.6+
 * - The custom "Post" class is loaded as model and used, so it would break your site if you don't delete the related code
 * - There is a namesapce declared, you might not use it, so simply delete it
 */

use \BEA\Post_Customize\Models\Post;

class Rewrites {
	/** Use the trait */
	use Singleton;

	protected $rewrite_struct = 'r/%s';
	protected $rewrite_tag;

	protected function init() {
		// Init tag with cpt name
		$this->rewrite_tag = sprintf( '%%%s%%', BEA_CPT_SECTION_NAME );

		add_filter( 'post_link', array( $this, 'rewrite_permalink' ), 10, 3 );
		add_action( 'init', array( $this, 'add_rewrites' ) );
		add_filter( 'bea_create_site_filter', [ $this, 'create_site_filter' ] );
	}

	/**
	 * Rewrite post permalink to include the section relationship
	 *
	 * @param string   $link
	 * @param \WP_Post $object
	 * @param string   $leave_name
	 *
	 * @author Maxime CULEA
	 *
	 * @return mixed
	 */
	public function rewrite_permalink( $link, \WP_Post $object, $leave_name ) {
		if ( wp_is_post_revision( $object ) ) {
			return $link;
		}

		$bea_post = new Post( $object );
		$section  = $bea_post->p2p_get_section();

		global $wp_rewrite;

		/** Not connected to a section, delete rewriting tag without replacing it */
		if ( ! is_a( $section, 'BEA\CPT_Section\Models\Section' ) || ! in_array( $this->rewrite_tag, $wp_rewrite->rewritecode ) ) {
			return str_replace( $this->rewrite_tag . '/', '', $link );
		}

		$slug = $section->post_name;
		/** Error on section, delete rewriting tag without replacing it */
		if ( empty( $slug ) ) {
			return str_replace( $this->rewrite_tag . '/', '', $link );
		}

		/**
		 * While connected to a section, replace rewriting tag with 'r/{section_slug}'
		 * Then add the #{post_id} to the end for animation facilities
		 */
		return sprintf(
				'%s#%s',
				str_replace(
					$this->rewrite_tag,
					sprintf( $this->rewrite_struct, $slug ),
					$link
				),
				$bea_post->get_ID()
			);
	}

	/**
	 * On init, add rewrite rule to match new structured url for posts with the wanted php template
	 * As we are almost rewriting same as pages, we redeclare page's rewrite after custom one
	 *
	 * @author Maxime CULEA
	 *
	 * @return void
	 */
	public function add_rewrites() {
		// Custom rewrite : prefix/section-slug/post-slug
		hm_add_rewrite_rule( array(
				'regex'               => '^'.sprintf( $this->rewrite_struct, '[^/]+' ) . '/([^/]+)/?$',
				'query'               => 'index.php?name=$matches[1]&post_type=post',
				'template'            => 'single.php',
				'body_class_callback' => function ( $classes = array() ) {
					$classes[] = 'section-post';
					return $classes;
				},
				'disable_canonical'   => true,
			)
		);
		// Page rewrite
		hm_add_rewrite_rule( array(
				'regex'               => '^([^/]+)/?$',
				'query'               => 'pagename=$matches[1]&post_type=page',
				'template'            => 'page.php',
				'disable_canonical'   => true,
			)
		);
	}

	/**
	 * Change default option (permastructure) for site creation
	 *
	 * @param $options
	 *
	 * @author Maxime CULEA
	 *
	 * @return mixed
	 */
	public function create_site_filter( $options ) {
		$options['permalink_structure'] = apply_filters(
			'BEA\CPT_Section\Sites\permalink_structure',
			sprintf(
				'/%s/%s',
				$this->rewrite_tag,
				'%postname%'
			)
		);
		return $options;
	}
}