I have this text:
$string = \"this is my friend\'s website http://example.com I think it is coll\";
How can I extract the link into another
Probably the safest way is using code snippets from WordPress. Download the latest one (currently 3.1.1) and see wp-includes/formatting.php. There's a function named make_clickable which has plain text for param and returns formatted string. You can grab codes for extracting URLs. It's pretty complex though.
This one line regex might be helpful.
preg_match_all('#\bhttps?://[^\s()<>]+(?:\([\w\d]+\)|([^[:punct:]\s]|/))#', $string, $match);
But this regex still can't remove some malformed URLs (ex. http://google:ha.ckers.org
).
See also: How to mimic StackOverflow Auto-Link Behavior