[Sheryld]

примерно так:

preg_match("/(http:\/\/)?(([a-zA-Z0-9]+\.)?([a-zA-Z0-9]+)\.[a-zA-Z]{2,4})/i","http://www.php1.net/index.html", $matches); 
	echo $matches[4]; echo "<BR>";

	preg_match("/(http:\/\/)?(([a-zA-Z0-9]+\.)?([a-zA-Z0-9]+)\.[a-zA-Z]{2,4})/i","http://php2.net/index.html", $matches); 
	echo $matches[4]; echo "<BR>";

	preg_match("/(http:\/\/)?(([a-zA-Z0-9]+\.)?([a-zA-Z0-9]+)\.[a-zA-Z]{2,4})/i","http://sub1.3php.net/index.html", $matches); 
	echo $matches[4]; echo "<BR>";

	preg_match("/(http:\/\/)?(([a-zA-Z0-9]+\.)?([a-zA-Z0-9]+)\.[a-zA-Z]{2,4})/i","http://1sub2.mydomen12.info/index.html", $matches); 
	echo $matches[4]; echo "<BR>";

	preg_match("/(http:\/\/)?(([a-zA-Z0-9]+\.)?([a-zA-Z0-9]+)\.[a-zA-Z]{2,4})/i","http://23eaitswork1.info/index.html", $matches); 
	echo $matches[4]; echo "<BR>";

результаты у меня получились такие:

php1
php2
3php
mydomen12
23eaitswork1