Which expert can help write a regular expression

Who can help write a regular expression
Replace URL with Hyper-join

Like what:
Hehe http://www.sina.com.cn sissy haha
After substitution is:
Hehe http://www.sina.com.cn sissy haha

In the example:
China's largest technology forum: www.csdn.net, you can learn a lot of things
After substitution is:
China's largest technology forum: www.csdn.net, you can learn a lot of things

In short, URLs are replaced with hyperlinks. master Help to write. Thank you.

------Solution--------------------

function Enable_url ($STR)
{
$str = Ereg_replace ("([url]www.[ A-za-z0-9@:%_.~#-\?&]+[/url]) "," \\1 ", $str); Automatic conversion of WWW start links
$str = Ereg_replace ("(((F|HT) {1}tp://) [a-za-z0-9@:%_.~#-\?&]+]", "\\1", $str); conversions [url]http://Opening and ftp://start links [/url]
$str = Str_replace (' linkhttp://', ' http://', $STR);
$str = Ereg_replace ("([_\.0-9a-z-]+@ ([0-9a-z][0-9a-z-]+\.)] +[a-z]{2,3}) "," \\1 ", $str); Conversion email Address
return $str;
}
For example
echo Enable_url ("hehe http://www.sina.com.cn sissy haha");

?>

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