Perl s///g 中发生了多少次替换? [英] How many substitutions took place in a Perl s///g?

问题描述

小例子:

perl -e '$s="aaabbcc";$c=()=$s=~/a/g;print"$c\n$s\n"' (m//g) 输出

3
aaabbcc

而 perl -e '$s="aaabbcc";$c=()=$s=~s/a/x/g;print"$c\n$s\n"' (s///g) 输出

whereas perl -e '$s="aaabbcc";$c=()=$s=~s/a/x/g;print"$c\n$s\n"' (s///g) outputs

1
xxxbbcc

我想同时做这两件事，而不必先匹配:替换并知道替换的次数.显然， s///g 不会返回标量上下文中的替换次数——与 m//g 在匹配时所做的不同.这可能吗?如果是，如何?

I'd like to do both things at once without having to match first: substitute and know the number of substitutions. Obviously a s///g does not return the number of substitutions in scalar context--unlike m//g does with matches. Is this possible? If yes, how?

perlre、perlvar 和 perlop 没有提供任何帮助(或者我就是找不到).

perlre, perlvar and perlop provided no help (or I just couldn't find it).

推荐答案

s/// 确实返回在标量上下文中进行的替换次数.来自 perlop(强调):

s/// does return the number of substitutions made in scalar context. From perlop (emphasis added):

s/PATTERN/REPLACEMENT/msixpogce
在字符串中搜索模式，如果找到，则替换该模式带有替换文本的模式，返回数量替换.否则它返回 false(特别是，空字符串).

s/PATTERN/REPLACEMENT/msixpogce
Searches a string for a pattern, and if found, replaces that pattern with the replacement text and returns the number of substitutions made. Otherwise it returns false (specifically, the empty string).

你的问题是你没有在标量上下文中调用 s/// .您在列表上下文中调用它，然后在标量上下文中评估 赋值(到一个空列表).标量上下文中的列表赋值返回表达式右侧生成的元素数.由于 s/// 返回单个值(在列表和标量上下文中)，即使 s/// 没有做任何事情，元素的数量也总是一.

Your problem is that you didn't call s/// in scalar context. You called it in list context and then evaluated the assignment (to an empty list) in scalar context. A list assignment in scalar context returns the number of elements produced by the right-hand side of the expression. Since s/// returns a single value (in both list and scalar context) the number of elements is always one even if the s/// didn't do anything.

perl -E "$s='aaabbcc'; $c=()=$s=~s/x/y/g; say qq'$c-$s'"  # prints "1-aaabbcc"

要在标量上下文中调用 s///，请省略 =()= 伪运算符.

To call s/// in scalar context, omit the =()= pseudo-operator.

perl -E "$s='aaabbcc'; $c=$s=~s/a/x/g; say qq'$c-$s'"  # prints "3-xxxbbcc"

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