$ten_dang_nhap = 'nguyenvana';
$bieu_thuc = '/^[a-zA-Z0-9_]{5,30}$/';
if(!preg_match($bieu_thuc, $ten_dang_nhap)){
echo "Ten dang nhap khong hop le";
}else{
echo "Ten dang nhap OK";
}
$ten_dang_nhap = 'nguyenvana';
$bieu_thuc = '/^[a-zA-Z0-9_]{5,30}$/';
if(!preg_match($bieu_thuc, $ten_dang_nhap)){
echo "Ten dang nhap khong hop le";
}else{
echo "Ten dang nhap OK";
}