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


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

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?