Regex – Should I use \d or [0-9] to match digits in a Perl regex

perlregex

Having read a number of questions/answers over the past few weeks, I have seen the use of \d in perl regular expressions commented on as incorrect. As in the later versions of perl \d is not the same as [0-9], as \d will represent any Unicode character that has the digit attribute, and that [0-9] represents the characters '0', '1', '2', …, '9'.

I appreciate that in some contexts [0-9] will be the correct thing to use, and in others \d will be. I was wondering which people feel is the correct default to use?

Personally I find the \d notation very succinct and expressive, whereas in comparison [0-9] is somewhat cumbersome. But I have little experience of doing multi-language code, or rather code for languages that do not fit into the ASCII character range, and therefore may be being naive.

I notice

$find /System/Library/Perl/5.8.8/ -name \*pm | xargs grep '\\d' | wc -l
  298
$find /System/Library/Perl/5.8.8/ -name \*pm | xargs grep '\[0-9\]' | wc -l
  26

Best Answer

It seems to me very dangerous to use \d, It is a poor design decision in the language, as in most cases you want [0-9]. Huffman-coding would dictate the use of \d for ASCII numbers.

Most of the previous posters have already highlighted why you should use [0-9], so let me give you a bit more data:

  • If I read the unicode charts correctly '۷۰' is a number (70 in indic, don't take my word for it).

  • Try this: 

    $ perl -le '$one = chr 0xFF11; print "$one + 1 = ", $one+1;'
１ + 1 = 1

  • Here is a partial list of valid numbers (which may or may not show up properly in your browser, depending on the fonts you use), for each number, only the first of those being interpreted as a number when doing arithmetics with Perl, as shown above:

     ZERO:  0٠۰߀०০੦૦୦௦౦೦൦๐໐０
 ONE:   1١۱߁१১੧૧୧௧౧೧൧๑໑１
 TWO:   2٢۲߂२২੨૨୨௨౨೨൨๒໒２
 THREE: 3٣۳߃३৩੩૩୩௩౩೩൩๓໓３
 FOUR:  4٤۴߄४৪੪૪୪௪౪೪൪๔໔４
 FIVE:  5٥۵߅५৫੫૫୫௫౫೫൫๕໕５
 SIX:   6٦۶߆६৬੬૬୬௬౬೬൬๖໖６
 SEVEN: 7٧۷߇७৭੭૭୭௭౭೭൭๗໗７
 EIGHT: 8٨۸߈८৮੮૮୮௮౮೮൮๘໘８
 NINE:  9٩۹߉९৯੯૯୯௯౯೯൯๙໙９��

Are you still not convinced?

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