How not to check the validity of an email address

« Back to blog :: September 2, 2013

How not to check the validity of an email address

Another update, after this post somehow hit #1 on Hacker News and garnered my website more visitors in one hour than it usually gets in a year: I got a lot of feedback on this post. While a lot of it was positive, there was also some valid criticism about the amount of flaming in this post. While at first my response was along the lines of “h8rs gunna h8 lol”, after some thought I realised that while posts like this can be fun to write, they can also be counterproductive and even harmful to the developer community. I know that if someone had found some of my code and decided to publicly rip it apart, I would almost certainly feel hurt — even if it were deserved (like that time I wrote a custom CMS and made it vulnerable to SQL injection).

I was young and foolish when I wrote this post. Admittedly, I’m less than a week older now, but hopefully much less foolish. Although I don’t regret writing this post, I don’t think I will be making posts of a similar nature in the future. While I do believe that bad practices in the developer community should be called out, there are undoubtedly ways of getting the same message across without resorting to vitriol or an overly snarky tone — even if the end result runs the risk of being less entertaining. Posts like this have a tendency to exacerbate a culture that is already mired in toxicity, and while recognising this fact cannot completely absolve me, I hope that it will at least deter others from behaving similarly.

I considered taking this post down, but that seemed silly considering that 99% of people who will ever read this post have, in all likelihood, already read it. In addition, I am no longer afraid of being sued for defamation by the vendor, since they seem to have taken it in stride and — amazingly enough — have even expressed an interest in interviewing me for a developer position as a result of this blog post. If the original programmer ever reads this post, I apologise for the public ridicule at your expense. All I can say is that I hope you no longer write code like this. Though if you do, please send it to me and I will do my utmost to write a sincere, snark-free analysis of it. <3

As a sidenote, one thing I found rather surprising is the fact that a significant number of readers seemed to interpret this post as an ode to my hatred of drugs. This would have been ideal if I ever intended to run for office, but since I don’t, it’s just perplexing; I would have thought the tone of the post made it obvious enough, but since it clearly wasn’t, I will clarify that I merely took an over-the-top approach and ran with it, in the interest of writing an article that would be more interesting to read than “Here’s some code I found”. It is a fairly lazy literary technique, though, and probably not one I will be using again anytime soon. (Hey, I never claimed this post was a literary masterpiece. I promise it will get better, though.)

Post-update update (December 2016): the original list of categories for this post, which was written back when the primary purpose of the category system was for self-amusement, included “desire2learnhow2code” – a reference to the fact that the vendor in this story is called Desire2Learn. That category has been removed, partly because I am now using the category system for its intended purpose, and partly because it’s really quite a mean thing to say, even if I still find it somewhat amusing.

Pretend, for a moment, that you’re a newly hired programmer working on a popular learning management system called Hot4Learning. Your predecessor had been working on adding email functionality to the system, so that any user would be able to send an email to any other user at the school via a web interface. Unfortunately, your predecessor was recently hit by a bus and never managed to complete his magnum opus. Your task is to finish it by adding an email validation feature, to ensure that an email is sent only if the recipient is a valid email address associated with the school.

For example: if Bob is a student at McGill University, then he should be able to send an email to any valid or email address. So if his friend Jane’s school email is, then Bob should be able to send an email to that address. On the other hand, he should not be able to send an email to Nor should he be able to send an email to

So your job is to write this feature, assuming you have access to a list of valid emails for the school.

I hope you’re thinking, “Please, that’s easy. First, I’ll create a string by joining the emails in the list with the string '|****|', then I’ll pass that to the client by assigning the string to a variable within a <script> tag. Next, in the Javascript, I’ll instantiate an Array object and assign it to a variable endearingly named temp, then throw away the aforementioned Array object and set temp to be the array created by splitting the string on '|****|'. Whenever a user enters an email, I’ll convert it to lowercase and store it in a variable with the beguiling name curForwardUserName, and I’ll create a variable called validUserName which I will set to be the string '0', because why use a boolean when you can use a string? Then, I’ll loop through the entirety of temp; for each email address in temp, I’ll convert that to lowercase, then check if it’s equal to curForwardUserName. If so, I’ll set validUserName to the string '1', and break. Finally, if validUserName equals the string ‘1’, I’ll consider curForwardUserName to be a valid email address; otherwise, it’s invalid.

Actually, no. That’s a lie. I really hope that’s not what you’re thinking. I hope what you actually thought was along the lines of “Well, obviously I’d do the validation on the server side. I can store the valid emails in a data structure that allows O(1) membership testing, then check if the input email is contained in there.” Because, well, that’s the right way to do it.

This is your brain

In Python, the right way to do it would look something like this:

if input_email in valid_emails_set:
    send_email(input_email, another_param, etc)

This is your brain on drugs

On the other hand, if you’re working on a product called Hot4Learning, you probably hate your job and need to consume a staggering amount of drugs in order to keep coming to work. This is what your drug-addled brain might come up with:

Warning: NSFWUYWOH (Not Safe For Work Unless You Work On Hot4Learning)

// [some other code here]
var userNamesStr = '|****| |****|
(pretend there are 70,000 more emails here)|****|';
var temp = new Array();
temp = userNamesStr.split('|****|');
var validUserName = '0';
// [more code here]
for( i =0; i< temp.length; i++){
if( curForwardUserName == temp[i].toLowerCase()) {
validUserName = '1';
// [some last bits of wisdom before we go]

UPDATE: Yes, the above code does involve sending all 70,000+ McGill email addresses to the client upon each page load. The source code of the page was over 2.5MB in size. As in, 2.5MB of pure text.

Since your reviewer would likely be as stoned as you, you would get a quick LGTM and the above code would soon find its way into production.

The actual story

Last year, the IT department at McGill University replaced the old LMS (Blackboard) with a new system (which is not actually called Hot4Learning, but the actual moniker is no less trite or gimmicky so I’ll spare you). Since the old system was riddled with vulnerabilities and suffered from general all-around shittiness, I was excited to see what the new system would be like. That excitement soon died, just like brain cells do when exposed to copious amounts of drugs.

Upon clicking the “email” tab, I was greeted with 10 seconds of white screen as my browser tried to load the page. “Why on Earth is this taking so long?” I asked myself. “It can’t be, like, trying to load every single valid email address at McGill, right? Lol. Haha. Hahaha. Hahahaha. Wait. ohgod”

Words cannot express the horror I felt when I looked at the source of the page and found the code that I have reproduced above. The only time I’ve been more terrified was when I dreamt that I slept through my cryptography exam and then began to suffocate (but that turned out to just be the blanket).

If you want to take a look at this code in the wild, well, you can’t anymore. I reported it as a data leak vulnerability and the vendor actually fixed it within two weeks. However, the code as it appeared then is essentially what you see above, shitty whitespace and all; I just removed some code irrelevant to the functionality at hand and also removed the actual list of emails. Apologies to Zamboni Man if you exist and are a student at McGill.

I would really like to know the combination and quantities of drugs consumed that resulted in this code. Do you know? Can you hook me up? Send me a message on Twitter @dellsystem.

I hereby claim that the above snippet of code is fair dealing according to section 29 of the Copyright Act of Canada, which allows for usage for the purpose of research, private study, education, parody, satire, criticism or review and news reporting. In fact, this snippet was used for nearly all of these purposes:


I stumbled upon this snippet while doing research for my B.S. thesis, “Not even once: Prolonged drug use and its effect on code quality”. It will be the perfect B.S. thesis, complete with B.S. graphs and footnotes in which I beg the reader to take me on as a grad student.

Private study

I saved the source code of the page because I wanted to privately study the correlation between prolonged drug use and code quality.


I would like to educate the public on the deleterious effects of prolonged drug use on code quality.


I wish I could say that the code above is a parody of the actual source code I encountered. I really do. But I can’t.


The code above was used as part of a scathing satirical indictment of the modern programmer’s tragic penchant for drugs. Also, society.

Criticism or review

3/10 would not bang

News reporting

BREAKING: Prolonged drug usage can result in terrible code. CNN had better link back to me when they pick up this story, I could use the ad revenue.