Filter Gutenburg block before display - Search and replace some tags in a block to dynamically insert the title and excerpt https://www.damiencarbery.com/2019/11/filter-gutenburg-block-before-display/

filter-block-before-render.php

<?php
/*
Plugin Name: Filter Gutenburg block before display
Plugin URI: https://www.damiencarbery.com/2019/11/filter-gutenburg-block-before-display/
Description: Search and replace some tags in a block to dynamically insert the title and excerpt.
Author: Damien Carbery
Version: 0.1
*/

add_filter( 'render_block', 'dcwd_block_tag_substitution', 10, 2 );
function dcwd_block_tag_substitution( $block_content, $block ) {
	if ( is_singular() ) {
		$title = get_the_title();
		// Get excerpt from post object otherwise there is an infinite loop using
		// get_the_excerpt() as it will call 'render_block' and around and around
		global $post;
		$excerpt = $post->post_excerpt;
		
		$tags = array( '~TITLE~', '~TAGLINE~' );
		//$replacements = array( get_the_title(), get_the_excerpt() );
		$replacements = array( $title, $excerpt );
		
		return str_replace( $tags, $replacements, $block_content );
	}

	return $block_content;
}