• ^ for the beginning of the line (so nothing before that)
• d meaning a number (decimbal)
• {16} to mean \"exactly 16\"
• $ for the ending of the line (so nothing after that)

<form method=\"post\">
    <input name=\"pub\" maxlength=\"16\" type=\"text\" />
    <input type=\"submit\" value=\"go\" />
</form>

if (isset($_POST[\'pub\'])) {
    if (preg_match(\"/^\\d{16}$/\", $_POST[\'pub\'])) {
         echo \'ok its good\' ;  	
    }else{
        echo \'its not real pub code\';       
    }    	
}