regex - 为什么这个正则表达式返回的组比它应该返回的多？

Question

我正在浏览一本关于正则表达式的流行书籍，发现了这段正则表达式，它应该从包含逗号分隔值的行中挑选出值。

这应该处理双引号，"" 被视为转义双引号(序列 "" 允许在另一对双引号内)

这是我为此编写的 perl 脚本:

$str = "Ten Thousand,10000, 2710 ,,\"10,000\",\"It's \"\"10 Grand\"\", baby\",10K";
#$regex = qr"(?:^|,)(?:\"((?:[^\"]|\"\")+)\"|([^\",]+))*";
$regex = qr!
        (?: ^|,)
        (?: 
            "
                ( (?: [^"] | "" )+ )
            "
            |
            ( [^",]+ )
        )
    !x;

@matches = ($str =~ m#$regex#g);
print "\nString : $str\n";
if (scalar(@matches) > 0 ) {
    print "\nMatches\n";
    print "\nNumber of groups: ", scalar(@matches), "\n";
    for ($i=0; $i < scalar(@matches); $i++) {
        print "\nGroup $i - |$matches[$i]|\n";
    }
}
else {
    print "\nDoesnt match\n";
}

这是我期望的输出(据我所知，这也是作者所期望的):

String : Ten Thousand,10000, 2710 ,,"10,000","It's ""10 Grand"", baby",10K
   Matches
   Number of groups: 7
   Group 0 - |Ten Thousand|
   Group 1 - |10000|
   Group 2 - | 2710 |
   Group 3 - |10,000|
   Group 4 - ||
   Group 5 - |It's ""10 Grand"", baby|
   Group 6 - |10K|

这是我实际得到的输出:

String : Ten Thousand,10000, 2710 ,,"10,000","It's ""10 Grand"", baby",10K
   Matches
   Number of groups: 12
   Group 0 - ||
   Group 1 - |Ten Thousand|
   Group 2 - ||
   Group 3 - |10000|
   Group 4 - ||
   Group 5 - | 2710 |
   Group 6 - |10,000|
   Group 7 - ||
   Group 8 - |It's ""10 Grand"", baby|
   Group 9 - ||
   Group 10 - ||
   Group 11 - |10K|

有人可以解释为什么实际输出中有空组(除了 10,000 之前的组，这是预期的)？我直接从书中复制了正则表达式，所以我做错了什么吗？

TIA

Answer 1

该正则表达式有 2 个捕获组和几个非捕获组。当您将正则表达式应用于字符串时，您使用了 g 修饰符告诉它继续匹配尽可能多的次数。在这种情况下，模式匹配 6 次，每次返回数组中总共 12 个元素的 2 个捕获组。

The regular expression:

(?-imsx:!
        (?: ^|,)

        (?:

            "

                ( (?: [^"] | "" )+ )

            "

            |

            ( [^",]+ )
        )
    !x)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  !                        '!\n        '
----------------------------------------------------------------------
  (?:                      group, but do not capture:
----------------------------------------------------------------------
                             ' '
----------------------------------------------------------------------
    ^                        the beginning of the string
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    ,                        ','
----------------------------------------------------------------------
  )                        end of grouping
----------------------------------------------------------------------
                           '\n\n        '
----------------------------------------------------------------------
  (?:                      group, but do not capture:
----------------------------------------------------------------------
                  "          '\n\n            "\n\n                '
----------------------------------------------------------------------
    (                        group and capture to \1:
----------------------------------------------------------------------
                               ' '
----------------------------------------------------------------------
      (?:                      group, but do not capture (1 or more
                               times (matching the most amount
                               possible)):
----------------------------------------------------------------------
                                 ' '
----------------------------------------------------------------------
        [^"]                     any character except: '"'
----------------------------------------------------------------------
                                 ' '
----------------------------------------------------------------------
       |                        OR
----------------------------------------------------------------------
         ""                      ' "" '
----------------------------------------------------------------------
      )+                       end of grouping
----------------------------------------------------------------------
                               ' '
----------------------------------------------------------------------
    )                        end of \1
----------------------------------------------------------------------
                  "          '\n\n            "\n\n            '
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
                             '\n\n            '
----------------------------------------------------------------------
    (                        group and capture to \2:
----------------------------------------------------------------------
                               ' '
----------------------------------------------------------------------
      [^",]+                   any character except: '"', ',' (1 or
                               more times (matching the most amount
                               possible))
----------------------------------------------------------------------
                               ' '
----------------------------------------------------------------------
    )                        end of \2
----------------------------------------------------------------------
                             '\n        '
----------------------------------------------------------------------
  )                        end of grouping
----------------------------------------------------------------------
       !x                  '\n    !x'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

TLP 已经提到您还可以使用 Text::CSV 模块。这是那个例子。

#!/usr/bin/perl

use strict;
use warnings;
use Text::CSV_XS;
use Data::Dumper;

my $csv = Text::CSV_XS->new({binary => 1, eol => $/, allow_whitespace => 1});

while (my $row = $csv->getline (*DATA)) {
    print Dumper $row;
}

__DATA__
Ten Thousand,10000, 2710 ,,"10,000","It's ""10 Grand"", baby",10K;

输出:

$VAR1 = [
          'Ten Thousand',
          '10000',
          '2710',
          '',
          '10,000',
          'It\'s "10 Grand", baby',
          '10K;'
        ];