Any time you’re turning a string of input into something else, what you are doing is parsing.
Even if the word “parser” never appears in your code, the act of interpreting a string as structured data is parsing, and the code that does parsing is a parser.
Programmers write parsers quite a lot, and many of the parsers they write are ad-hoc, ill-specified, bug-ridden, and can’t tell you why your input didn’t parse right.
Writing a parser without realizing you’re writing a parser, usually leads to writing a bad parser. Bad parsers do things like accepting malformed input that causes security holes. When bad parsers do reject malformed input, they rarely emit useful error messages about why it’s malformed. Bad parsers are often written using regex and duct tape.
Try not to write bad parsers. If you need to parse something, consider writing a grammar and using a parser library. (If you’re very ambitious, try a parser combinator library.) But at least try to recall something about parsers you learned once way back in a CS class, before throwing regex at the problem and calling it a day.
(And now the word “parser” no longer makes sense, because of semantic satiation.)
By the way, please don’t write regex to try to validate email addresses. Seriously. There are libraries for that; some of them are even good. When people write their own regex to match email addresses, they do things like forget that the hyphen is a valid character in domain names.
By the way, please don’t write regex to try to validate email addresses. Seriously.
Amen.
There are libraries for that; some of them are even good.
Spoiler alert: Few of them are good, and those few are so simple that you might as well not use a library.
The only way to correctly validate an email address is to send a message to it, and verify that it arrived.
You can use a regex to do basic validation. That regex is
.+@.+. Anything beyond that is a waste of time.That rejects valid emails
Which ones? In RFC 5322 every address contains an addr-spec at some point, which in turn must include an @. RFC 6854 does not seem to change this. Or did I misread something?
EDIT: I’M DUMB I THOUGHT YOU MEANT A LITERAL “.” , that regex is indeed correct.
Original:
This is a valid address:
user.name@[IPv6:2001:db8:1ff::a0b:dbd0]Relevant spec: https://www.rfc-editor.org/rfc/rfc5321#section-4.1.3
And you all doubted me… :P
Edit: Wait, I also don’t think local-part even needs a “.” in it? user@domain.com should be valid as well.
And it’s matched by
.+@.+as it contains an @.Remember, we’re taking about regular expressions here so
.+means “a sequence of one or more arbitrary characters”. It does not imply that an actual dot is present.(And I overlooked the edit. Oops.)
There are also cases where you want to have a disallow list of known bad email providers. That’s also part of the parsing and validating.
It’s a valid need, but a domain blacklist isn’t part of email parsing and if you conflate the two inside your program then you’re mixing concerns.
Why is the domain blacklist even in your program? It should be a user configurable file or a list of domains in the database.
We were discussing validation, not parsing. There’s no parsing in an email. You might give it a type once it passes validation, but an email is just a string with an
in it (and likely some . because you want at least 1 TLD but even that I’m not sure).You are right in that it isn’t (or shouldn’t be) part of the parsing, but the program has to check the blacklist even if it’s in a database.
fuck any website that requires an account to just READ it’s stupid content and at the same time blocks guerrillamail/10minutemail (looking at you, Glassdoor,I don’t want to get fucking spam just so that I can check approximate salary in a company)
Sounds like your gripe is with people requiring accounts for reading public content, and not with preventing usage of automated email creation and trying to limit bots on your website.
disallow list of known bad email providers.
Imagine giving someone your phone number, and having them say you have to get a different one because they don’t like some of the digits in it.
I have seen this nonsense more times than I care to remember. Please don’t build systems this way.
If you’re trying to do bot detection or the like, use a different approach. Blacklisting email addresses based on domain or any other pattern does a poor job of it and creates an awful user experience.
(And if it prevents people from using spam-fighting tools like forwarding services, then it’s directly user-hostile, and makes the world a worse place.)
The only way to correctly validate an email address is to send a message to it, and verify that it arrived.
If you’re accepting email addresses as user input (e.g. from a web form), it might be nice to check that what’s to the right of the rightmost
sign is a domain name with an MX or A record. That way, if a user enters a typo’d address, you have some chance of telling them that instead of handing an email touseroruser@gmailc.omto your MTA.But the validity of the local-part (left of the rightmost
) is up to the receiving server.Checking MX in your application means you needlessly fail on transient outages, like when a DNS server is rebooting or a net link hiccups. When it happens, the error flag your app puts on the user’s email address is likely to confuse or frustrate them, will definitely waste their time, and may drive them away and/or generate support calls.
Also, MX records are not required. Edit to clarify: So checking MX in your application means you fail 100% of the time on some perfectly valid email domains. Good luck to the users and support staff who have to troubleshoot that, because there’s nothing wrong with the email address or domain; the problem is your application doing something it should not.
Better to just hand the verification message off to your mail server, which knows how to handle these things. You can flag the address if your outgoing mail server refuses to accept it.
If DNS is transiently down, the most common mail domains are still in local resolver cache. And if you’re parsing live user requests, that means the IP network itself is not in transient failure at the moment. So it takes a pretty narrow kind of failure to trigger a problem… And the outcome is the app tells the user to recheck their email address, they do, and they retry and it works.
If DNS is having a worse problem, it’s probably down for your mail server too, which means an email would at least sit in the outbound mail spool for a bit until DNS comes back. Meanwhile the user is wondering where their confirmation email is, because people expect email delivery in seconds these days.
So yeah … yay, tradeoffs!
(Confirmation emails are still important for closed-loop opt-in, to make sure the user isn’t signing someone else up for your marketing department’s spam, though.)
By the way, please don’t write regex to try to validate email addresses. Seriously.
Speaking of things you can’t parse with regex…
You can’t parse [X]HTML with regex. Because HTML can’t be parsed by regex. Regex is not a tool that can be used to correctly parse HTML. As I have answered in HTML-and-regex questions here so many times before, the use of regex will not allow you to consume HTML. Regular expressions are a tool that is insufficiently sophisticated to understand the constructs employed by HTML. HTML is not a regular language and hence cannot be parsed by regular expressions. Regex queries are not equipped to break down HTML into its meaningful parts. so many times but it is not getting to me. Even enhanced irregular regular expressions as used by Perl are not up to the task of parsing HTML. You will never make me crack. HTML is a language of sufficient complexity that it cannot be parsed by regular expressions. Even Jon Skeet cannot parse HTML using regular expressions. Every time you attempt to parse HTML with regular expressions, the unholy child weeps the blood of virgins, and Russian hackers pwn your webapp. Parsing HTML with regex summons tainted souls into the realm of the living. HTML and regex go together like love, marriage, and ritual infanticide. The <center> cannot hold it is too late. The force of regex and HTML together in the same conceptual space will destroy your mind like so much watery putty. If you parse HTML with regex you are giving in to Them and their blasphemous ways which doom us all to inhuman toil for the One whose Name cannot be expressed in the Basic Multilingual Plane, he comes. HTML-plus-regexp will liquify the nerves of the sentient whilst you observe, your psyche withering in the onslaught of horror. Rege̿̔̉x-based HTML parsers are the cancer that is killing StackOverflow it is too late it is too late we cannot be saved the transgression of a chi͡ld ensures regex will consume all living tissue (except for HTML which it cannot, as previously prophesied) dear lord help us how can anyone survive this scourge using regex to parse HTML has doomed humanity to an eternity of dread torture and security holes using regex as a tool to process HTML establishes a breach between this world and the dread realm of c͒ͪo͛ͫrrupt entities (like SGML entities, but more corrupt) a mere glimpse of the world of regex parsers for HTML will instantly transport a programmer’s consciousness into a world of ceaseless screaming, he comes, the pestilent slithy regex-infection will devour your HTML parser, application and existence for all time like Visual Basic only worse he comes he comes do not fight he com̡e̶s, ̕h̵is un̨ho͞ly radiańcé destro҉ying all enli̍̈́̂̈́ghtenment, HTML tags lea͠ki̧n͘g fr̶ǫm ̡yo͟ur eye͢s̸ ̛l̕ik͏e liquid pain, the song of re̸gular expression parsing will extinguish the voices of mortal man from the sphere I can see it can you see ̲͚̖͔̙î̩́t̲͎̩̱͔́̋̀ it is beautiful the final snuffing of the lies of Man ALL IS LOŚ͖̩͇̗̪̏̈́T ALL IS LOST the pon̷y he comes he c̶̮omes he comes the ichor permeates all MY FACE MY FACE ᵒh god no NO NOO̼OO NΘ stop the an*̶͑̾̾̅ͫ͏̙̤g͇̫͛͆̾ͫ̑͆l͖͉̗̩̳̟̍ͫͥͨe̠̅s ͎a̧͈͖r̽̾̈́͒͑e not rè̑ͧ̌aͨl̘̝̙̃ͤ͂̾̆ ZA̡͊͠͝LGΌ ISͮ̂҉̯͈͕̹̘̱ TO͇̹̺ͅƝ̴ȳ̳ TH̘Ë͖́̉ ͠P̯͍̭O̚N̐Y̡ H̸̡̪̯ͨ͊̽̅̾̎Ȩ̬̩̾͛ͪ̈́̀́͘ ̶̧̨̱̹̭̯ͧ̾ͬC̷̙̲̝͖ͭ̏ͥͮ͟Oͮ͏̮̪̝͍M̲̖͊̒ͪͩͬ̚̚͜Ȇ̴̟̟͙̞ͩ͌͝S̨̥̫͎̭ͯ̿̔̀ͅ
Have you tried using an XML parser instead?
The best validation for a valid email address is always sending a verification mail. I’ve rejected countless MRs that contained ad-hoc regex email validation copied from the internet. I’d allow a check for it to contain an
.What if it only contains @?
If they are also sending a validation email, it would fail, so no issue.
Programmers write parsers quite a lot
Speak for yourself. I’ve done it exactly once. It didn’t work, and never shipped. Learned my lesson and always use a parser that someone else wrote. Usually a big team of at least thousands of people (how many people have worked on JSON? Millions? What about UTF8? Those are the main two I use).
So you have never iterated over command line arguments and tried to identify options? Or taken a string input field?
Yeah! And integers do way too many things as well. Counters, indexes, number of orange slices in an orange, there’s just no end to the wacky things people try to make integers do, and it’s impossible to keep track of it all when looking at code. And floats? Don’t get me started on floats. Angles, probabilities, weights, heights, degrees of separation from Kevin Bacon … I’m getting dizzy just listing all these different things that floats do.
It’s a big problem, because there isn’t an easy way to fix it in every programming language known to man, and someone needs to write more articles about this to get more hits for their sites.
Wait till you hear what strings are under the hood. Integers in a trench coat!
And the boolean is revealed as the archvillain working behind the scenes, pulling the strings of all its minions.
You thought it was true but it was I, One!
It’s integers all the way down.
I think the article makes a good point, but perhaps I’m over-interpreting. It’s not that we should stop using strings. It’s that we should use the type system to separate different kinds of strings and enlist the compiler’s help to detect incorrect mingling of them. So for example a symbol type would only permit strings that contain ASCII letters, underscore and digits, and concatenation with / conversion to plain strings would be limited.
It’s definitely a rule that can be taken so far that it is counterproductive, but I think it’s good practice to thbk about how I could use something other than a raw string ( even if it’s just a constant defined somewhere )
What’s the meaning of a fractional “Degree of Kevin Bacon”?
The analytic continuation of KB(x) to the complex plane subject to a superconvexivity constraint is unique but doesn’t necessarily have a straightforward geometric interpretation
You joke, but Rails actually does make Integer do too many things lol. I’d argue they’re useful things, but it does so by patching the core Ruby Integer class :p
As a physicist, I agree ;)
Strings became ubiquitously used for a reason, they map really clearly to the way we think as humans. Most importantly, when you’re debugging, seeing string data is much friendlier than whatever data your symbols map to (usually integers, from enum structures)
No, obviously it’s not the most efficient thing in the world, but it hardly matters, and you’re not getting anyone to stop because you’re “technically right”.
Symbols display with friendly string-y names in a number of languages. Clojure, for example, has a symbol type.
And a number of languages display friendly strings for enumy things - Scala, Haskell, and Rust spring to mind.
The problem with strings over enums with a nice debugging display is that the string type is too wide. Strings don’t tell you what values are valid, strings don’t catch typos at compile time, and they’re murder when refactoring.
Clojure symbols are good at differentiation between symbolly things and strings, though they don’t catch typos.
The other problem the article mentions is strings over a proper struct/adt/class hierarchy is that strings don’t really have any structure to them. Concatenating strings is brittle compared to building up an AST then rendering it at the end.
Edit: autocorrect messed a few things up I didn’t catch.
Despite all my rage I’m still just some bytes in a page
Man, stop overthinking stuff.
Sounds like skill issues and depends on the context. Is this article about JavaScript? I recommend trying out Rust. Strings do only too many things, if you do it. Create your own types and enums. An example missing in the article are file or url paths. Use the standard libraries and types that are given to you.
Unless you’re using assembly, strings do everything, since the code files are also strings.
…and that’s why it is wise to use monadic parsers at the input boundary.
Is that your theory?
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