Pull out links, usernames and hashtags from a plaintext tweet. I imagine there is a cleaner way (regex is not my thing) but it works well at getting what's needed.

find_tweet_links.php

<?php

// Find normal links, wrap with anchor tags and add href to link
$tweet = preg_replace('/(https?:\/\/[^\s"<>]+)/','<a href="$1">$1</a>',$tweet);

// Find usernames, wrap with anchor and add href to their profile
$tweet = preg_replace('/(^|[\n\s])@([^\s"\t\n\r<:]*)/is', '$1<a href="http://twitter.com/$2">@$2</a>', $tweet);

// Find hashtags, wrap with anchor and add href to twitter search for that tag
$tweet = preg_replace('/(^|[\n\s])#([^\s"\t\n\r<:]*)/is', '$1<a href="http://twitter.com/search?q=%23$2">#$2</a>', $tweet);

echo $tweet;