Get WordPress Post Meta by RegExp, a regular expression variant of get_post_meta

get_post_meta_regexp.php

<?php

/**
 * Retrieve post meta field for a post, by Regular Expression match on the key.
 *
 * @since 0.0.1
 *
 * @param int    $post_id             Post ID.
 * @param string $regexp              The regexp to match the meta key against. One capture-group 
 *                                    allowed for key-ing the results. Full meta_key is used if no 
 *                                    capture-groups in the expression
 * @param bool   $single_sub_values   Optional. Whether to return an array of single values. Default false.
 * @return WP_Error|array(mixed)      Will be an array with either scalars or arrays based on $single_sub_values.
 *                                    contains the individual meta_value results of get_post_meta. 
 *                                    Will be WP_Error in case the regular expression fails to validate.
 */

function get_post_meta_regexp($post_id, $regexp, $single_sub_values = false) {
	global $wpdb;
	if (preg_match($regexp, null) === false) {
		return new WP_Error('regexp_invalid', 'Please supply a valid regexp, the current expression is missing boundaries', array($regexp));
	}
	$regexp_boundary = substr($regexp, 0, 1);
	$regexp_boundary_end = strrpos($regexp, $regexp_boundary); // position of closing boundary
	$regexp_options = substr($regexp, $regexp_boundary_end + 1); // just the part between the boundaries
	$trimmed_regexp = substr($regexp, 1, $regexp_boundary_end - 1); // just the part between the boundaries

	// notes:
	// MYSQL regexp is always case insensitive, so in case of no /i flag, MYSQL might report too many rows, these are then filtered by PHP,
	// flags m and s are for multi-line matching. meta_keys are never multiline, do, we can ignore those
	// flag x modified the way whitepsace is handled, sinde meta_keys cannot have whitespace, we ignore those too
	// flag e (eval) makes no sense whatsoever in this context, do we ignore that as well.
	$regexp_options = (false !== strpos($regexp_options, 'i')) ? 'i' : '';
	$regexp = $regexp_boundary . $trimmed_regexp . $regexp_boundary . $regexp_options;

	$meta_keys = $wpdb->get_col( $wpdb->prepare("SELECT meta_key FROM {$wpdb->postmeta} WHERE meta_key REGEXP %s AND post_id = %d", $trimmed_regexp, $post_id ) );
	$meta = array();
	foreach ($meta_keys as $key) {
		if (preg_match($regexp, $key, $m)) {
			$meta[ isset($m[1]) ? $m[1] : $m[0] ] = get_post_meta($post_id, $m[0], $single_sub_values);
		}
	}

	return $meta;
}