if (preg_match("/^[a-zA-Z0-9]+$/", $_POST['username']) == false) {
$errors[] = 'Your account name can only contain characters a-z, A-Z and 0-9.';
if (preg_match("/^[0-9]+$/", $_POST['username']) == false) {
$errors[] = 'Your account name can only contain numbers.';
system/load.compat.php
find
function check_account_name($name)
and change
$temp = strspn("$name", "QWERTYUIOPASDFGHJKLZXCVBNM0123456789");
to
$temp = strspn("$name", "0123456789");
then
pages/ajax_check_account.php
echo '<font color="red">Invalid account name format. Use only A-Z and numbers 0-9.</font>';
to
echo '<font color="red">Invalid account name format. numbers 0-9.</font>';