Extreme minification of shortest possible PSR-0 compliant autoloader, 5 lines !

l.php

<?php
//set_include_path(get_include_path() . PATH_SEPARATOR . __DIR__); // optional
spl_autoload_register(function ($class) {
	$file = preg_replace('#\\\|_(?!.+\\\)#','/', $class) . '.php';
	if (stream_resolve_include_path($file))
		require $file;
});

Why not use require_once? Or does spl_autoload_register automatically prevent loading files multiple times?

Yes, autoloading happens only if the class definition is not yet included. But may be require_once is safer ;)

This regex is reliable?
Some tests indicate an error in this part "(?!".

For the perfect, please add braces to if statement :))
"The body of each structure MUST be enclosed by braces." (c) PSR-2