Perl 正则表达式 'e' (eval) 修饰符，带 s/// [英] Perl Regex 'e' (eval) modifier with s///

问题描述

我在理解/e 正则表达式修饰符的这种简单用法时有点困难.

I'm having a little trouble comprehending this simple use of the /e regex modifier.

my $var = 'testing';
$_ = 'In this string we are $var the "e" modifier.';

s/($w+)/$1/ee;

print;

返回:在这个字符串中，我们正在测试e"修饰符."

Returns: "In this string we are testing the "e" modifier."

我不明白为什么需要两个e"修饰符.据我所知， $1 应该从字符串中捕获 '$var' 并且一个 'e' 修饰符应该能够用它的值替换变量.然而，我一定是误解了一些东西，因为仅使用一个 'e' 修饰符来尝试上述代码并不能明显替换字符串中的任何内容.

I cannot see why two 'e' modifiers are required. As far as I can see, $1 should capture '$var' from the string and a single 'e' modifier should then be able to replace the variable with its value. I must be misunderstanding something however, since trying the above code with just one 'e' modifier does not visibly replace anything in the string.

不好意思问这么简单的问题！

Excuse me for asking such a simple question!

谢谢.

推荐答案

这不是一个简单"的问题，所以不要自责.

It’s not exactly a "simple" question, so don’t beat yourself up.

问题在于，对于单个 /e，RHS 被理解为其 eval 的结果用于替换的代码.

The issue is that with a single /e, the RHS is understood to be code whose eval’d result is used for the replacement.

那个 RHS 是什么?它是 $1.如果您计算 $1，您会发现它包含 字符串 $var.它不包含所述变量的内容，只是 $ 后跟一个 v 后跟一个 a 后跟一个 r.

What is that RHS? It’s $1. If you evaluated $1, you find that contains the string $var. It does not contain the contents of said variable, just $ followed by a v followed by an a followed by an r.

因此你必须对它求值两次，一次将 $1 变成 $var，然后再将 $var 的前一个结果变成字符串 "testing".您可以通过在 ee 修饰符来做到这一点>s 运算符.

Therefore you must evaluate it twice, once to turn $1 into $var, then again to turn the previous result of $var into the string "testing". You do that by having the double ee modifier on the s operator.

您可以通过使用一个 /e 与其中两个来运行它来轻松检查这一点.这是一个演示 a both，以及使用符号解引用的第三种方法 - 因为它引用包符号表，所以仅适用于包变量.

You can check this pretty easily by running it with one /e versus with two of them. Here’s a demo a both, plus a third way that uses symbolic dereferencing — which, because it references the package symbol table, works on package variables only.

use v5.10;

our $str = q(In this string we are $var the "e" modifier.);
our $var = q(testing);

V1: {
    local $_ = $str; 
    s/($w+)/$1/e;
    say "version 1: ", $_;

}

V2: {
    local $_ = $str;
    s/($w+)/$1/ee;
    say "version 2: ", $_;
}

V3: {
    no strict "refs";
    local $_ = $str;
    s/$(w+)/$$1/e;
    say "version 3: ", $_;
}

运行时，会产生:

version 1: In this string we are $var the "e" modifier.
version 2: In this string we are testing the "e" modifier.
version 3: In this string we are testing the "e" modifier.

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