do_shortcode.php

<?php
function do_shortcode($content) {
	global $shortcode_tags;

	if (empty($shortcode_tags) || !is_array($shortcode_tags))
		return $content;

	$pattern = get_shortcode_regex();
	return preg_replace_callback('/'.$pattern.'/s', 'do_shortcode_tag', $content);
}

get_shortcode.php

<?php
function get_shortcode_regex() {
	global $shortcode_tags;
	$tagnames = array_keys($shortcode_tags);
	$tagregexp = join( '|', array_map('preg_quote', $tagnames) );

	// WARNING! Do not change this regex without changing do_shortcode_tag() and strip_shortcodes()
	return '(.?)\[('.$tagregexp.')\b(.*?)(?:(\/))?\](?:(.+?)\[\/\2\])?(.?)';
}