RegEx to link Twitter Usernames and Hashtags (e.g. in WordPress)

/*
 * Filter the_content() to make hashtags and usernames clickable
 */
function make_twitter_clickable($text) { // use /u unicode flag and options to match non-latin characters
    $text = preg_replace("/@([\p{L}\p{N}_]{1,15})/u", '<a class="tweet-action" href="https://twitter.com/$1">@$1</a>', $text); 
    $text = preg_replace("/#([\p{L}]+[\p{N}]*)/u", '<a class="tweet-action" href="https://twitter.com/search?q=%23$1">#$1</a>',$text); 
    return $text;
}
add_filter( 'the_content', 'make_twitter_clickable', 12 );

Dropping this in to functions.php in a WordPress theme will detect all Twitter usernames and hashtags and link them to appropriate pages on Twitter. Customise the class on the link if you want styling control.