Try something like this.

$pattern = "/http:\/\/(www\.)?([^.]+\.[^.\s]+\.?[^.\s]*)/i";
$replace = "<a href='http://\\1\\2'>http://\\1\\2</a>";
$string = [url=http://us2.php.net/preg-replace]preg_replace[/url]($pattern,$replace,$string);
echo $string;

Where $string is the text. Just a couple notes about this pattern:
1) The URI MUST have "http://" before it
2) It works for URIs formatted in the following ways
>>>http://www.example.foo
>>>http://www.example.foo.foo #having two
>>>http://example.foo
>>>http://example.foo.foo #having two
>>>http://foo.exmaple.foo #pretty much the same as above

http://www.webmasterworld.com/php/3268855.htm

Try something like this.

$pattern = "/http:\/\/(www\.)?([^.]+\.[^.\s]+\.?[^.\s]*)/i";
$replace = "<a href='http://\\1\\2'>http://\\1\\2</a>";
$string = [url=http://us2.php.net/preg-replace]preg_replace[/url]($pattern,$replace,$string);
echo $string;

Where $string is the text. Just a couple notes about this pattern:
1) The URI MUST have "http://" before it
2) It works for URIs formatted in the following ways
>>>http://www.example.foo
>>>http://www.example.foo.foo #having two
>>>http://example.foo
>>>http://example.foo.foo #having two
>>>http://foo.exmaple.foo #pretty much the same as above

URLs in Text, and Parsing them into Hyperlinked HTML urls

PHP