Tweet Hash Tags

Can someone explain to me ho开发者_Python百科w Hash tags get anchors after being posted. I want to work on a similar implementation in php. The logic behind that operation and links will be useful. thanks

$html = preg_replace("/#([a-z0-9_]+)/", '<a href="hashtag/${1}">#${1}</a>', htmlspecialchars($tweet_contents));

something like http://www.snipe.net/2009/09/php-twitter-clickable-links/