Regex Isn't Hard (2023)(timkellogg.me) |
Regex Isn't Hard (2023)(timkellogg.me) |
* The use of backslash separatores quickly makes a mess, as they tend to need escaping wherever regexes are usefull.
* The uppercase/lowercase is only right if there are no accented characters, so USA. This is bad in western europe in files where they are rare: Your program works for a while, then an accent sneaks in and breaks things.
* The exact meaning of all the specials like \( vs ( .
* Ranges work in most regex dialects but not everywhere.
* A simple regex for an int with a specific range is nasty. If you want a full float, good luck.
Regexes are great as initial filter or quick hack, but you need more in full size programs.
I'd love to see a better regex syntax, too.
Regex is still the best solution I know of for its intended domain.
I love regular expressions but one thing I've learned over the years is the syntax is dense enough that even people who are confident enough to start writing regex tutorials often can't write a regex that matches an IP address.
Syntax error aside (there's an extra ] floating around), it's not even close to correct -- it'll match "999.999.999.000.999." among other things, will never match just one digit (there's a missing ?), and always insists on the trailing dot.
Characters which are sometimes special, depending on context, are one more thing making regexes harder than they appear at first sight.
The author's willingness to publish code without even minimal testing does not inspire confidence.
(
(
25[0-5] # 250-255
|
2[0-4][0-9] # 200-249
|
1[0-9]{2} # 100-199
|
[1-9][0-9] # 10-99
|
[0-9]
)
\.
){3}
(
25[0-5] # 250-255
|
2[0-4][0-9] # 200-249
|
1[0-9]{2} # 100-199
|
[1-9][0-9] # 10-99
|
[0-9]
)
The problem is that expressing “an integer from 0-255” is surprisingly complicated for regex engines to express. And that’s not even accounting for IP addresses that don’t use dots (which is legal as an argument to most software that connects to an IP address), as other commenters have pointed out.
For something like locating IP addresses in text, using a regex to identify candidates is a great idea. But as you show, you don’t want to implement the full validation in it. Use regex to find dotted digit groups, but validate the actual numeric values as a separate step afterwards.
You're right that Ruby has it. Perl also has /x, of course (since most of Ruby regex was "inspired" directly by Perl's syntax), as well as Python (re.VERBOSE). Otherwise, yeah, it's disappointingly rare.
Also, `16843009` is an IP address, try pinging it.
<0-255>\.<0-255>\.<0-255>\.<0-255>
EDIT: formatting
edit: asking partly, because in my current work I occassionally have to convince non-technical users to use one type of entry over other. For that reason, easy to read, simple regex wins over fancy, but convoluted regex.
Sure, I'd take \d+\.\d+\.\d+\.\d+ over... "((2(5[0-5]|[0-4][0-9])|1[0-9]{2}|[1-9]?[0-9])\.){3}(2(5[0-5]|[0-4][0-9])|1[0-9]{2}|[1-9]?[0-9])", assuming that I then validate the results afterwards.
Is it what `inet_addr` accept? In that case, "1", "0x1", "00.01", "00000.01", and more are all ip addresses. `ping` accepts all of em anyway.
Is a valid ipv6 address one with the square brackets around it? Is "::1" a valid ip address? What about "fe80::1%eth2"? ping accepts both of these on my machine (though probably not on yours, since you probably don't have an eth2 interface)
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Someone else then asks the absolute razor of a question: 'What value does this add over just verifying that the input is of the form {something}@{something}.{something}?'
Depends if {something} can contain periods for my email.
name@antispam.mydomain.com
At some point we needed to do some parsing of some strings, and I suggested a simple regex. But apparently a bunch of the streamers he's been watching basically have this attitude that regexes stink, and you should use basically anything else. So we had a conversation, and compared the clarity of coding up the relatively simple regex I'd made, with how you'd have to do it procedurally; I think the regex was a clear winner.
Obviously regexes aren't the right tool for every job, and they can certainly be done poorly; but in the right place at the right time they're the simplest, most robust, easiest to understand solution to the problem.
Just like you wouldn’t copy/paste any random snippet into your source code if you don’t understand exactly what it does.
I see a lot of broken regex at work from people who use regular expressions but don’t understand them (for various reasons).
It used to come with a “found this on stackoverflow”-excuse, but mostly now it’s “AI told me to use this” instead.
totally definitively
This is absolutely horrible, pattern are fairly readable if they follow the syntax logic. Matching "everything but that random character that will not appear" is absurd. Also the idea that a . (dot) behaves arbitrary in different languages shows a sever lack up understanding about regex syntax. Ofc you can't write a proper pattern if you don't know which syntax is used. If anything you would force override the behavior of the . (dot) with the appropriate flag to ensure it works the same with different compatible regex engines.
Even as much as I like regex, I wouldn’t recommend this post. One reason is the code style is too close to regular text:
> a matches a single character, always lowercase a.
That sentence uses “a” three times, two of them as code and once as an indefinite article, but it’s not immediately obvious to eye. VoiceOver completely fumbles it, especially considering the sentence immediately after.
A more important reason against recommending the article is that I find a bunch of the arguments to be unhelpful. If you’re trying to convince people to give regular expressions a chance, telling them to ignore `.` and use `[^%]` is going to bite them. That’s not super common (important when trying to learn more from other sources) and even an experienced regexer must do a double take to figure out “is there a reason this specific character must not be matched?” Furthermore, no new learner is going to remember that four character incantation, and neither are they going to understand what’s happening when their code doesn’t work because there was a `%` in their text. People need to learn about `.` (possibly the most common character in regex) if only because they also need to learn to escape it and not ignore it when there is a literal period in the text. Don’t tell people to ignore repetition ranges either, they aren’t difficult to reason about and are certainly simpler to read than the same blob of intractable text multiple times.
Also you can use . with the dotAll /s
Everyone tries to create the platonic ideal regex that does everything in one line.
The hardest part is remembering how you struggled with it when you started.
Crafting regexes is story of its own. The other commentor has described it. Just to summarize, regexes are fine for simple patterns. But their complexity explode as soon as you need to handle a lot of corner cases.
https://jimbly.github.io/regex-crossword/
See also: Are Regex Crosswords NP-hard?
https://cs.stackexchange.com/questions/30143/are-regex-cross...
One can construct a regex that matches a number between x and y by enumerating all the digit patterns that fit the criteria. For example, the following pattern matches a number between 1 and 255: ^([1-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])$
This can be extended to match a version less than or equal to x.z.y by enumerating all the patterns across the different fields. The following pattern matches any version less than or equal to 2.14.0: ^([0-1]\.\d+\.\d+)|(2\.[0-9]\.\d+|(2\.1[0-3]\.\d+))$
Basically, I wrote a Java method that would generate a regex with all the patterns to match a version greater than or equal to a lower bound, which was then fed to MongoDB queries to exclude documents too old to process based on the version field. It was a stupid solution to a dumb problem, but it worked flawlessly.
But not because of the regex idea itself.
It is quoting.
The reason people don't properly learn how to use a regex is because they are insulated from it by whatever language they are using.
It's literally like those surgeons who do heart surgery starting at a vein in your leg.
I use regexes all the time, in emacs, python, perl, bash, sed, awk, grep and more...
and just about every time the regex syntax is mixed with single quotes, double quotes, backslashes, $variable names and more from the "enclosing language or tool".
If I have a parenthesis or $, I'm always wondering if it is part of the enclosing language, or the matching pattern, or the literal. Also, the kind of regex adds to the confusion (basic or extended regex?)
I think it would be nice to have a syntax highlighter that would help with this, independent of language. green for variable or other language construct, red for regex pattern, white for matching literal.
Escaping/quoting is such a mud pile everywhere because it's in-band communication, but nobody would tolerate all out-of-band because it's too tedious. At least newer languages are getting better with things like 'raw' strings or Rust's arbitrarily long delimeters, but I'd still like more control.
I'm surprised I never see languages adopt directed delimeters like {my string} or something, since it lets you avoid escaping in the very common case of balanced internal delimeters.
Every time I need to write even the simplest regex, I can't seem to get it right the first time. I always need to struggle with it for a long time. Sometimes even using online tools takes me time to get it right. This happens every.single.time.
It baffles me to no end. I'm a pretty quick learner of pretty much everything I get into. I write the most sophisticated Typescript code you can imagine; I've written a small toy language; I've written biometric authentication drivers; I've written my own functional UI lib. But, I cannot master regex.
You can give me all the arguments about what is good about regex, but in my experience (which you can't argue with), it is a VERY badly designed API, and nothing will convince me otherwise. Regex is probably the worst thing ever in programming.
The popular idea of them being write-only is obviously a joke, but it has some truth to it. On the good side, small code that needs to be rewritten is often better than large code that needs to be maintained.
Doesn't this go against the "literals are enclosed in quotes" idea? In this case, you have a special character (`-`) inside a quoted string. IMO this would be more consistent: `['0'-'9''a'-'f'']`, maybe even have comma separation like `['0'-'9','a'-'f'']`. This would also allow you to include the character classes like `[d,'a'-'f'']` although that might be a little confusing if you're used to normal regex.
> Doesn't this go against the "literals are enclosed in quotes" idea?
Sure, one could argue that other changes would also be useful, but then it would be less concise. I think the main reasons why people like regex are: (a) powerful, (b) concise.
For my V2 proposal, the new rule is: "literals are enclosed in quotes", the rule isn't "_only_ literals are enclosed in quotes" :-) In this case, I think `-` can be quoted as well. I wanted to keep the v2 syntax as close as possible to the existing syntax.
He’s also strangely worried about portability. If you are really concerned about portability, you are moving between languages and you probably aren’t some novice who should be frightened by complexity.
I don’t think about portability at all, ever. And I do maintain code in Perl, Python, and Javascript.
But yeah, just as in all programming languages, you can get by with knowing about a 20% subset of all it can do.
This pattern ([0-9][0-9]?[0-9]][.])+ matches one, two or three digits followed by a . and also matches repeated patterns of this. This wold match an IP address (albeit not strictly).
[0] https://en.wikipedia.org/wiki/Regular_expression#Formal_lang...
So I don’t think it’s easy, but I do agree that they are very useful.
Involves much less thinking!
What Rust feature is this referring to?
This might work for you, but in general the amount of bugs is proportional to the amount of code. The regex engine is alredy throughly tested by someone else while a custom implementation in procedural code will probably have bugs and be a lot more work to maintain if the pattern changes.
https://regex101.com can explain your regex back to you, and allows you to test it with more inputs.
Though I’m not trying to convince you to always use regular expressions, I agree with GP:
> Obviously regexes aren't the right tool for every job, and they can certainly be done poorly; but in the right place at the right time they're the simplest, most robust, easiest to understand solution to the problem.
but they’re not an excuse to avoid regex. Similarly git has many warts but there’s no getting around it. Same with CSS
If you want to run with the herd though you need to know these things, even enjoy them.
You can rely on tooling and training wheels like Python VERBOSE but you’re never going to get away from the fact that the “rump” of the population works with them.
Easier to bite the bullet and get practised. I’ve no doubt you have the intellect - you only need be convinced it’s a good use of your time.
I don't incorporate a lot of regular expressions into my code. But where I do like them is for search and replace. So I do treat them as mostly disposable.
Calling the extra ] a syntax error was a slight exaggeration on my behalf, but that was clearly an unintended extra character -- there's no way the author thinks "123].45].67].89]" is a valid IP address. But yes, it does compile and is interpreted as a valid regex, albeit not a useful one in this context.
The out-of-range values are not ideal but can be fixed with post-validation in code (which is cleaner than writing unnecessarily complicated regex, anyways). The missing ? leads to a bunch of false negatives, and the trailing . causes even more problems.
‘Only’ in the most commonly used character encodings. In EBCDIC (https://en.wikipedia.org/wiki/EBCDIC), the [a-z] range includes more than 26 characters.
That’s one of the reasons POSIX has character classes (https://en.wikipedia.org/wiki/Regular_expression#Character_c...). [:lower:] always gets you the lowercase characters in the encoding that the program uses.
If you wanted to look for cases which serve as an exception to this rule, code relying on regexes would be an excellent place to start.
I also prefer procedural code instead of regexes.
Regexes can certainly be hard to read - the solution is to use formatting and comments to make them easier to understand - not to drown the logic in reams of boilerplate code.
I don't think I fully agree with this, and I don't see a basis for why this should be true. If I have a very specific implementation, it could have very little incidental complexity, it could be fully targeted to the use case. Whereas with regular expressions there is incidental complexity of the regex engine itself by definition.
The relevant complexity for using a regex is the complexity of the pattern itself and the complexity of invoking the regex. Any custom procedural solution will be more complex unless it is literally something as simple as checking whether a string contain a given literal string.