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My favorite regex of all time

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Re: My favorite regex of all time

#61

Post by andrewflnr »

csense wrote: post #57
lambada wrote: post #2

Are people seriously still deliberately using ASCII-reliant code?

Every time I've had to deal with Unicode and internationalization, it's been a problem. For example, a few years ago I grabbed a source tarball from somewhere, I forget what or where. It had the author's name in a comment, which included an O with dots over it. That was the only non-ASCII character in the source code. No matter what I did, both Eclipse and command-line javac refused to compile the source. Finally I w…

What? You suffered from other peoples' bad internationalization, which implies that people shouldn't care about internationalization?

Re: My favorite regex of all time

#62

Post by cjfont »

hnriot wrote: post #45
citricsquid wrote: post #20

Earlier quoted context omitted.

Are you saying people should google regular expressions? in my experience (correct me if I'm wrong) that doesn't work, I've never been able to get google to return relevant results even with quotation marks.

I'm saying that usually comments are either wrong or out of date, developers code one regex, comment it, then fix a bug later and don't, then there's a discrepancy between the comment and the code. It's nearly always easier to just google the code and see what it does, if (as in this case) it's not obvious.

Your response doesn't address what citricsquid said, googling for a regex will almost never return helpful results.

Re: My favorite regex of all time

#63

Post by boyter »

lubutu wrote: post #60
boyter wrote: post #30

My favorite regex is the following, /^1?$|^(11+?)\1+$/ Which finds prime numbers. Although, I can't for the life of me think of a reason for using it. http://stackoverflow.com/questions/3296050/how-does-this-reg...

I do dislike people calling that expression a "regex", because it isn't: regular expressions cannot contain backreferences, and must be computable in linear time, whereas primality tests are polynomial.

While I agree I believe this comment by _delirium sums this up rather well,

http://news.ycombinator.com/item?id=1486502

full comment thread here http://news.ycombinator.com/item?id=1486158

Re: My favorite regex of all time

#64

Post by boyter »

jervisfm wrote: post #59

Can anyone explain how this regex [- ~] matches ASCII characters ?

It's pretty simple. Assuming you know regex... Im going to assume you don't since you are asking.

The bracket expression [ ] defines single characters to match, however you can have more then 1 character inside which all will match.

  [a] matches a
  [ab] matches either a or b
  [abc] matches either a or b or c
  [a-c] matches either a or b or c.
The - allows us to define the range. You can just as easily use [abc] but for long sequences such as [a-z] consider it short hand.

In this case [ -~] it means every character between and , which just happens to be all the ASCII printable characters (see chart in the article). The only bit you need to keep in mind is that is a character as well, and hence you can match on it.

You could rewrite the regex like so (note I haven't escaped or anything in this so its probably not valid)

  [ !"#$%&'()*+,-./0123456789:;?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~]
but that's not quite as clever or neat.

Re: My favorite regex of all time

#65

Post by lubutu »

boyter wrote: post #63
lubutu wrote: post #60

Earlier quoted context omitted.

I do dislike people calling that expression a "regex", because it isn't: regular expressions cannot contain backreferences, and must be computable in linear time, whereas primality tests are polynomial.

While I agree I believe this comment by _delirium sums this up rather well, http://news.ycombinator.com/item?id=1486502 full comment thread here http://news.ycombinator.com/item?id=1486158

I agree more with philh's response that there is no alternative term for the true meaning of "regular expression" — a regular language, as suggested by _delirium, is not the same thing.

I suppose I could accept "regex" as not being a regular expression as such, but the two are used so interchangeably that maintaining a distinction isn't very realistic. I'd personally rather a regular expression described a regular language, and "PCRE" (or so) used for the Turing-complete expressions with a similar syntax.

Re: My favorite regex of all time

#67

Post by wpietri »

logn wrote: post #37
missing_cipher wrote: post #19

Earlier quoted context omitted.

Search "[ -~]" (with or without quotes) to see how good Google's comment is.

The only ambiguous thing about this regex is knowing what's between space and tilde. Otherwise this is a pretty ordinary regex.

The only thing ambiguous about it is most of it?

Re: My favorite regex of all time

#68

Post by CountHackulus »

rmc wrote: post #24
CountHackulus wrote: post #15

Earlier quoted context omitted.

Thank you for that. You have no idea how annoying it is to port perl scripts from ASCII to EBCDIC when they do that kind of thing.

It's not an ASCII v EBCDIC thing, its an ASCII vs Unicode thing.

It's not just Unicode either. I just mentioned EBCDIC because that particular regex has bit me before when I was translating perl scripts from Linux to zOS USS. Take a look at the code page for EBCDIC, you'll see quickly why it's a massive pain to sort through regexes like that.

Re: My favorite regex of all time

#69

Post by Shorel »

noonespecial wrote: post #7

As someone who makes much of his living rehabilitating old perl scripts, please, if you must use such things, use them like this: [ -~] #match only printable characters It takes 5 seconds longer and with regexes, just knowing what the damn thing is trying to do is half the battle. When you use a regex, use a comment. Its the civil thing to do.

In this case, the entire post was the comment.

You are right anyway.

Re: My favorite regex of all time

#70

Post by [deleted] »

noonespecial wrote: post #7

As someone who makes much of his living rehabilitating old perl scripts, please, if you must use such things, use them like this: [ -~] #match only printable characters It takes 5 seconds longer and with regexes, just knowing what the damn thing is trying to do is half the battle. When you use a regex, use a comment. Its the civil thing to do.

[deleted]
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