tag-to-link.php

<?php
/*
Plugin Name: Convert Comment tag to link
Description: Convert #tag to links in the comment_text area using a filter
Author: Jan Dembowski
*/

add_filter( 'comment_text', 'mh_commenttaglink' , 50 );

function mh_commenttaglink( $text ) {

        // RegEx to find #tag, #hyphen-tag with letters and numbers
        $mh_regex = "/\ #[a-zA-Z0-9-]+/";

        // Use that RegEx and populate the hits into an array
        preg_match_all( $mh_regex , $text , $mh_matches );

        // If there's any hits then loop though those and replace those hits with a link
        for ( $mh_count = 0; $mh_count < count( $mh_matches[0] ); $mh_count++ )
                {
                        $mh_old = $mh_matches[0][$mh_count];
                        $mh_old_lesshash = str_replace( ' #' , '' , $mh_old );
                        $mh_new = str_replace( $mh_old , '<a href="' . get_bloginfo( url ) . '/tag/' . $mh_old_lesshash . '"/ rel="tag">' . $mh_old . '</a>' , $mh_matches[0][$mh_count] );
                        $text = str_replace( $mh_old  , $mh_new , $text );
                }
        // Return any substitutions
        return $text;
}