Perl - 不能去除空行 [英] Perl - can't strip blank lines

问题描述

比方说，我有一个这样的文件(它不是实际内容，而是十六进制转储):

Let's say, I have a file like this (it's not the actual content but the hexdump):

0000000  \r  \n  \r  \n   T   h   i   s       i   s       a       f   i
0000010   l   e  \r  \n                              \r  \n   H   e   r
0000020   e   '   s       s   o   m   e       t   e   x   t  \r  \n
000002f 

如果我运行以下:

#!/usr/bin/perl
use strict;
use warnings;
use File::Slurp;
$_ = read_file("file.txt");

s/^\s*$//mg;
print;

产生的输出是:

0000000  \n   T   h   i   s       i   s       a       f   i   l   e  \r
0000010  \n  \n   H   e   r   e   '   s       s   o   m   e       t   e
0000020   x   t  \r  \n

显然，空白行没有被剥离.

Apparently, the blank lines aren't stripped.

谁能指出我做错了什么?

Can anyone point out what I'm doing wrong?

推荐答案

在正则表达式中，$ 断言可能有点令人困惑.根据文档，它匹配 [es] 行的末尾(或末尾的换行符之前)".所以它的行为大致类似于

In regexes, the $ assertion can be a bit confusing. According to the docs, it "Match[es] the end of the line (or before newline at the end)". So it behaves roughly like

(?=\n\z)|\z

使用 /m 修饰符，这会变成

With the /m modifier, this changes to

(?=\n)|\z

这意味着 \n 不包含在匹配的子字符串中.你想要:

This means that the \n is not included in the matched substring. You want:

s/^\s*\n//mg;

现在您的代码中仍有一些问题需要解决.主要是，一次读入整个文件并在其上运行正则表达式几乎没有意义.相反，我会这样做:

Now there remain some points in your code that should be addressed. Mainly, it makes little sense to read in the whole file at once, and run a regex over it. Rather, I'd do:

use strict; use warnings; use autodie;

open my $fh, "<", "file.txt";
while (<$fh>) {
  print if /\S/; # print if this line contains at least one non-space character
                 # this elegantly skips whitespace-only lines.
}

这假设行尾完全由空白字符组成，并以 \n 结尾.这适用于 \r\n 和 \n 行结尾.否则，分配自定义行结尾，如

This assumes that line endings consist entirely of whitespace characters and end with \n. This holds for both \r\n and \n line endings. Else, assign custom line endings like

local $/ = local $\ = "\r\n"; # input and output line endings
while (<$fh>) {
  chomp;  # remove line endings
  print if /\S/; # print adds the line ending again.
}

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