function check_for_symbols($string)
{
     $len=strlen($string);
     $allowed_chars="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
     for($i=0;$i<$len;$i++)if(!strstr($allowed_chars,$string[$i]))
         return TRUE;
     return FALSE;
}

[A-Za-zа-Я0-9]+