Anyone know if this happens in other languages?

Doesn't appear to in JavaScript:

    "੧".match(/\d/); //null

(Incidentally, this may explain the finding from http://stackoverflow.com/a/16622773/172322, as to why adding the RegexOptions.ECMAScript flag in the C# code eliminates the performance gap)

Nor in python:

print re.match(r'\d','੧')

None

it does when using the re.U flag

  re.match(r'\d', u'੧', re.U)
  <_sre.SRE_Match at 0x3070ac0>

  sys.version
  2.7.3 (default, Mar  4 2013, 14:57:34) \n[GCC 4.7.2]

Also, when using Python 3.2 it seems to be the default behavior

  Python 3.2.3 (default, Oct 19 2012, 20:10:41) 
  [GCC 4.6.3] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>> import re
  >>> re.match(r'\d', '੧')
  <_sre.SRE_Match object at 0x7f188f6d4850>

Yes, but not by default.

Not true for Java. Docs even say:

  \d         A digit: [0-9]
  \p{Digit}  A decimal digit: [0-9]

which is actually somewhat depressing. I'd expect the named class to include the full Unicode digit set. It's surprising to see:

  ab1234567890cd matched 1234567890
  ab𝟣𝟤𝟥𝟦𝟧𝟨𝟩𝟪𝟫𝟢cd no match

from code using Pattern.compile("(\\p{Digit}+)");

EDIT: and perhaps more surprising to see in the logs:

  Exception in thread "main" java.lang.NumberFormatException: For input string: "𝟤𝟥𝟦𝟧"
  	at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
  	at java.lang.Integer.parseInt(Integer.java:449)

That'll keep someone guessing for a while...

Not true for Go

http://play.golang.org/p/ls96RxJxpz

I would be reluctant to rely on this until the Go documentation is clearer on the intended behavior. Right now it's very poorly specified. The regex doc[1] talks about "same general syntax" as Perl, but points to [2], which doesn't seem to understand what it's saying, describing '\d' in terms of its "Perl" meaning, but then saying that it's [0-9].

[1] http://golang.org/pkg/regexp/

[2] https://code.google.com/p/re2/wiki/Syntax

As a Perl developer that's been making the switch to Go, I've been caught out a few times with Go's no-so-Perl-like regular expression syntax. In fact I wish I knew about your 2nd link before now, because that could have saved me a few hours over recent months.

Considering go's vocal support for UTF8 I'm surprised at this behavior and curious to the reason for excluding it.

Supporting UTF8 and correctly handling unicode are very, very different beasts. The former is absolutely trivial, the latter is extremely difficult.

Go is vocal about the former, but seems to not give a shit about the latter.

I'm not particularly fond of go, but "correctly handling unicode" can be subjective and case-dependent... I think making only minimal guarantees and punting to the application is often the only sane course.

> "correctly handling unicode" can be subjective and case-dependent...

So is correctly handling integers.

> I think making only minimal guarantees and punting to the application is often the only sane course.

That is completely and utterly crazy, the average developer has neither the knowledge nor the resources to make anything but a mess out of it without proper tools and APIs. Even with these (including a complete implementation of the unicode standard and its technical reports) unicode is already complex enough to deal with.

Of course it's not "completely and utterly crazy."

Not every app needs to deal with the enormous complexities implied by "full unicode support", and given the huge cost of that, there's a real place for a minimalist approach. If all I do with unicode is input strings from the user, store them in a database, and then later spit them out, I don't need to be able to do Turkish case-conversion, and I may not want to pay the cost of making it possible.

Certainly tools and APIs help for those cases where an app needs to do the sort of complicated text-processing that warrants "full" unicode support, but it's not at all clear that the proper place for such support is in the base language libraries. It's quite reasonable for the language implementors to say "if you want to do X, we'll support that, but if you want to do Y and Z, please use external library L."

> Not every app needs to deal with the enormous complexities implied by "full unicode support", and given the huge cost of that, there's a real place for a minimalist approach.

Not sure what point you're trying to make, I never said all applications had to make full use of all possible Unicode APIs, I said the language must expose them. Because if it doesn't, those who should use them will never become aware of them let alone use them.

> If all I do with unicode is input strings from the user, store them in a database, and then later spit them out, I don't need to be able to do Turkish case-conversion, and I may not want to pay the cost of making it possible.

So?

> It's quite reasonable for the language implementors to say "if you want to add numbers, we'll support that, but if you want to subtract or divide them, please use external library L."

Really?

Then again, considering Go's embedded contempt for non-US locales (see: datetime patterns) I'm not even sure why we're having this discussion, and since it's obvious they don't care for a non-US world it make sense that they wouldn't care for processing text.

And at the end of the day, you agree that Go has no provision for unicode handling, you just think it's all fine and dandy.

It happens in Objective-C:

    NSString *pattern = @"\\d", *string = @"੧";
    NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:pattern
                                                                           options:NSRegularExpressionCaseInsensitive
                                                                             error:nil];

    NSUInteger numMatches = [regex numberOfMatchesInString:string
                                                   options:0
                                                     range:NSMakeRange(0, [string length])];

    numMatches ? NSLog(@"%@ found by %@", string, pattern) : NSLog(@"%@ not found", string);

    // 2013-05-20 09:38:42.650 Regexperiment[17848:c07] ੧ found by \d

Happens in PHP only if you enable Unicode regex handling via the /u modifier and are running libpcre 8.10 or later (which corresponds to PHP 5.3.4 and later, assuming you're using the bundled libpcre): http://3v4l.org/QD3k0

If you're using pcre directly from C code, this is controlled by specifying the PCRE_UCP flag to pcre_compile(). By default, \d and friends only match ASCII characters even if the PCRE_UTF8 flag is set.

Exactly what I was thinking.

Doesn't in Ruby:

/\d/.match "੧" #=> nil

Just for reference:

  /\p{Digit}/.match "੧" => #<MatchData "੧">

All the same speed in JavaScrip http://jsperf.com/regexcwm/2

Happens in Perl but not ruby or PostgreSQL.

Doesn't happen in Perl for me:

  pfedor@Pawels-iMac:~$ perl -ne 'print "Digit!\n" if /\d/'
  af
  3
  Digit!
  23fa3
  Digit!
  asdf
  ١٢٣٤٥٦٧٨٩۰۱۲۳۴۵۶۷۸۹
  ৩৪৫৬৭৮৯੦੧੨੩੪੫੬੭੮੯૦૧૨૩૪૫
  ୧୨୩୪୫୬୭୮
  ౨౩౪౫౬౭౮౯೦೧೨೩೪೫೬೭೮೯൦൧൨൩൪൫൬൭൮൯๐๑๒๓๔๕๖๗๘๙໐໑໒໓
  234
  Digit!

(perl from Macports and perl from /usr/bin/perl behave the same in this respect.)

You have to tell to interpret stdin as UTF-8 (flag -C) - then it works: https://news.ycombinator.com/item?id=5734641

Good to know, thanks.

I'd argue that perl gets it right--as the default behavior, this behavior would gravely violate the principle of least surprise, but for the 0.01% of people who want \d to match ੧, there's no harm to making it available as an option you need to specifically request.