[WordPress] Search Post Metadata in the WordaPress Admin Area

00-pre-get-posts.php

<?php
add_filter('pre_get_posts', function ($query) {

	// Make sure we're in the admin, an administrator, and on the edit page.
	if (!is_admin() || !current_user_can('manage_options') || 'edit' !== get_current_screen()->base) {
		return;
	}

	// Determine if the user is running a search.
	$search = filter_input(INPUT_GET, 's', FILTER_SANITIZE_STRING);
	if (empty($search)) {
		return;
	}

	$query->set(
		'meta_query',
		[
			[
				'key'     => 'headline_notes',
				'value'   => esc_sql(preg_replace('/\s+?/', '%', $search)), // For flexible search, make sure to replace all white space with % for LIKE.
				'compare' => 'LIKE',
			],
		]
	);
});

01-posts-where.php

<?php
add_filter( 'posts_where', function($where, $query) {

	// Determine if 'headline_notes' appears in the WHERE clause.
	$position = strpos($where, 'acme_meta_key');
	if (false === $position) {
		return $where;
	}

	// Look for the 'AND' clasue and replace it with 'OR' in the WHERE clause.
  	$regularExpress = '//mi'; // Your regular expression.
  	$replacement = ''; // Your string to replace.
  
  	return preg_replace(
		$regularExpression,
		$replacement,
		$where
	);
}, 10, 2);

What is acme_meta_key ?

Thanks!