ISBN 13 to 10 conversion, via shortcode

functions.php

add_shortcode('isbn-13to10', 'ISBN13toISBN10_shortcode');
// Converts ISBN-13 to ISBN-10
// Leaves ISBN-10 numbers (or anything else not matching 13 consecutive numbers) alone
function ISBN13toISBN10_shortcode($atts) {
	extract( shortcode_atts( array(
	'isbn' => ''
	), $atts ) );
    if (preg_match('/^\d{3}(\d{9})\d$/', $isbn, $m)) {
        $sequence = $m[1];
        $sum = 0;
        $mul = 10;
        for ($i = 0; $i < 9; $i++) {
            $sum = $sum + ($mul * (int) $sequence[$i]);
            $mul--;
        }
        $mod = 11 - ($sum%11);
        if ($mod == 10) {
            $mod = "X";
        }
        else if ($mod == 11) {
            $mod = 0;
        }
        $isbn = $sequence.$mod;
    }
    return $isbn;
}