// Automatically link Twitter usernames (i.e. any text in the form of @username)
add_filter( 'the_content', 'typewheel_link_twitter_username' );
add_filter( 'the_excerpt', 'typewheel_link_twitter_username' );
add_filter( 'comment_text', 'typewheel_link_twitter_username' );
function typewheel_link_twitter_username( $content ) {
return preg_replace( '/([^a-zA-Z0-9-_&])@([0-9a-zA-Z_]+)/', '$1<a href="https://twitter.com/$2" target="_blank" rel="nofollow">@$2</a>', $content );
}