需要帮助理解/d 的 perl tr 命令 [英] Need help in understanding perl tr command with /d

问题描述

我在网上遇到了以下 Perl 示例.

I came across the following Perl example on the web.

#!/usr/bin/perl 

$string = 'the cat sat on the mat.';
$string =~ tr/a-z/b/d;

print "$string\n";

结果:

b b   b.

有人可以解释一下吗?

推荐答案

/d 表示 delete.

做这样的 tr 很不寻常，因为它令人困惑.

It's quite unusual to do a tr like that because it's confusing.

tr/a-z//d

将删除所有 'a-z' 字符.

would delete all 'a-z' characters.

tr/a-z/b/ 

会将所有 a-z 字符音译为 b.

would transliterate all a-z characters to b.

这里发生的事情是 - 因为你的音译没有在每一侧映射相同数量的字符 - 任何没有映射的内容都会被删除.

What's happening here though is - because your transliteration doesn't map an equal number of character on each side - anything that doesn't map is deleted.

所以你有效地做的是:

tr/b-z//d;
tr/a/b/;

例如将所有 as 音译为 bs，然后删除其他任何内容(空格和点除外).

E.g. transliterating all the as to bs and then deleting anything else (except spaces and dots).

举例说明:

use strict;
use warnings;
my $string = 'the cat sat on the mat.';
$string =~ tr/the/xyz/d;

print "$string\n";

警告:

Useless use of /d modifier in transliteration operator at line 5.

和打印:

xyz cax sax on xyz max.

如果您将其更改为:

#!/usr/bin/perl 
use strict;
use warnings;
my $string = 'the cat sat on the mat.';
$string =~ tr/the/xy/d;

print "$string\n";

你会得到:

xy cax sax on xy max.

因此:t -> x 和 h -> y.e 刚刚被删除.

And thus: t -> x and h -> y. e just gets deleted.

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