Perfect email regex finally found • Hacker News Board

Re: Perfect email regex finally found

#71

Post by alnayyir » Fri, May 28, 2010, 11:15 PM UTC

adamc wrote: post #7

alnayyir wrote: post #3

Earlier quoted context omitted.

Regex is the only real sensible way to validate strings until something better is found. Even if you just wrote code to do it manually, you'd really just be writing a verbose and poorly implemented finite state machine that globbed symbols together, which in the end, would just be inferior to writing a well tested Regex string. Regex can be easier to read if you have something do a graphical expansion for you. Otherw…

That's just silly. Depending on what's in the string, writing a parser might be much better than a regex. Lots of parser libraries already out there, too.

By all means, use a CFG or a PEG for parsing emails.

While you do that, I'll do something that doesn't involve tweezers and code.

Re: Perfect email regex finally found

#72

Post by JadeNB » Fri, May 28, 2010, 11:24 PM UTC

huherto wrote: post #59

JadeNB wrote: post #46

Earlier quoted context omitted.

> Regex is the only real sensible way to validate strings until something better is found. There are plenty of ‘better’ (in the sense of ‘more powerful’) string-validation techniques. For example, lots of grammars are expressed in BNF; the languages that can be so expressed are (if I remember my Chomsky hierarchy correctly) the context-free grammars, a strictly larger class than the regular languages. The extra power…

> Given this, why would we use regular expressions? If I can get a way with a simple regexp I will use it. If it starts getting complicated I would use something else.(e.g. top down parser or a combination)

> If I can get a way with a simple regexp I will use it. If it starts getting complicated I would use something else.(e.g. top down parser or a combination)

Isn't that what I said?

Re: Perfect email regex finally found

#73

Post by twopoint718 » Fri, May 28, 2010, 11:28 PM UTC

_delirium wrote: post #29

aphyr wrote: post #26

Earlier quoted context omitted.

Especially when the spec itself is in EBNF.

True, though it's EBNF with a bunch of explanatory text and annotations. What would be interesting, but I can't find with some googling: Has someone implemented a parser-generator based on the spec? The ideal would be that the parser specification looks a lot like the RFC, since then you'd have more confidence it was actually correct (and it'd be easier to maintain for future changes).

There was a long post on the topic of 'regex-vs-parser' for email on reddit a while back. I hope I'm pointing to the correct person, but he wrote a parser in Haskell that validates against RFC5322: http://hackage.haskell.org/package/email-validate

Re: Perfect email regex finally found

#74

Post by curio » Fri, May 28, 2010, 11:38 PM UTC

pornel wrote: post #12

deutronium wrote: post #5

Earlier quoted context omitted.

One nice looking trick I've seen is checking the domain of the email address for an MX record. (e.g. http://php.dzone.com/news/php-email-validator-email-mx-d ) Possibly this approach is too slow to use by itself without a regex. Also maybe there are other problems with this method I'm not aware of?

MX record is not required. You should allow hosts with only A records too. http://tools.ietf.org/html/rfc5321#section-5.1

practically speaking, most respectable email services will have an MX record. i tested it on about 30,000 emails & a simple MX lookup caught about 40 gmial's and only 1 false positive.

Re: Perfect email regex finally found

#75

Post by nfnaaron » Fri, May 28, 2010, 11:49 PM UTC

"Perfect email regex finally found"

Based on the HN title I thought it was going to be an article describing a post-it found on Fermat's dressing table mirror.

Instead it's a list of mostly correct regexes. As Miracle Max might have observed, mostly correct is partly imperfect, and partly imperfect is Not Perfect.

Re: Perfect email regex finally found

#76

Post by sjs » Sat, May 29, 2010, 12:14 AM UTC

mhartl wrote: post #63

jeff18 wrote: post #42

Earlier quoted context omitted.

I would estimate about 0.25% of people will make a typo like "@homail.com" or "@gmial.com" Multiply that by say, 130,000 people, and you are dealing with 325 people who don't receive their download, etc. and are not happy! I think what would be really awesome is a regex that catches these common typos and warns the user immediately.

True, but you have to balance that against the small but nonzero number of people put off by an extra text field. Plus, I would find email repetition more annoying if I didn't always do Cmd-A/Cmd-C/Tab/Cmd-V, and in this case the repeated field won't catch any errors.

The fact that you know the shortcuts for select all, copy, and paste puts you in the top percentile of users. Most people don't even know that's possible, and certainly not with keyboard shortcuts.

(The point being that for most users the faster approach that requires less thinking is to type it twice. Sometimes I do things the "slow" or "long" way when coding because it doesn't require a mental shift from the task at hand.)

Re: Perfect email regex finally found

#77

Post by mrb » Sat, May 29, 2010, 12:47 AM UTC

NOT perfect, FAILS to accept valid addresses

Another know-it-all web developer guy who thinks he got his regex right... These are valid addresses that he rejects: user@ua (.ua = Ukraine) user@km (.km = Comoros) user@ne (.ne = Niger) Many ccTLDs have MX or A records pointing to real MTAs.

Re: Perfect email regex finally found

#78

Post by davidu » Sat, May 29, 2010, 12:57 AM UTC

jrockway wrote: post #41

jbr wrote: post #28

Earlier quoted context omitted.

Along those lines, I've settled on the following overly permissive regex: /^[^\s@]+@[^\s@]+\.[^\s@]{2,}$/ -- it makes sure it looks something like an email address (a@b.cd)

What if I want to receive mail directly at my TLD?

Except it's not allowed...

http://tools.ietf.org/html/rfc5321#section-2.3.5

RFC 5321 section 2.3.5 specifically prohibits TLDs from receiving email. Other RFCs back that up, actually including RFC5322 (not directly, but in wire format).

That said, we see some TLDs run MX's. I think at least some portion of them sell the mail received there to spammers. Seriously.

Re: Perfect email regex finally found

#79

Post by mhartl » Sat, May 29, 2010, 1:11 AM UTC

sjs wrote: post #76

mhartl wrote: post #63

Earlier quoted context omitted.

True, but you have to balance that against the small but nonzero number of people put off by an extra text field. Plus, I would find email repetition more annoying if I didn't always do Cmd-A/Cmd-C/Tab/Cmd-V, and in this case the repeated field won't catch any errors.

The fact that you know the shortcuts for select all, copy, and paste puts you in the top percentile of users. Most people don't even know that's possible, and certainly not with keyboard shortcuts. (The point being that for most users the faster approach that requires less thinking is to type it twice. Sometimes I do things the "slow" or "long" way when coding because it doesn't require a mental shift from the task a…

That's true. My main point was the first one. The second point was mainly to illustrate that annoyance isn't hypothetical; I'm annoyed by an extra text box.

Re: Perfect email regex finally found

#80

Post by jemfinch » Sat, May 29, 2010, 2:22 AM UTC

How in the world did this get 172 points?

After this I almost stopped paying attention: "It's my philosophy that it's better to accept a few invalid addresses than reject any valid ones, so I'm shooting for 0 false-positives and as few false-negatives as possible."

But then I looked at the regexps and they miss an absolutely trivial fact: valid email addresses can end in a dot. "jemfinch@supybot.com." is just as valid (more so, in fact) than "jemfinch@supybot.com".