get-variable-cache-key.php

<?php
/**
 * Retrieve a variable cache key via RegEx.
 *
 * A good use for this would be where an incrementer is part of the key and you can't
 * necessarily predict what it is, though you can predict the format it will take.
 */
function get_variable_cache_key() {
	/**
	 * @global WP_Object_Cache $wp_object_cache
	 */
	global $wp_object_cache;

	if ( ! is_object( $wp_object_cache ) ) {
		return;
	}

	$matches = $cleaned_matches = array();
	$group   = 'whatever';

	if ( ! empty( $wp_object_cache->group_ops[ $group ] ) ) {
		$matches = preg_grep( 'pattern-here', $wp_object_cache->group_ops[ $group ] );
	}
	
	if ( ! empty( $matches ) ) {
		foreach ( $matches as $match )  {
			$cleaned_matches[] = str_replace( 'get ', '', $match );
		}
	}

	// Do stuff.
}
add_action( 'wp_after_admin_bar_render', 'get_variable_cache_key', 100 )