$ 和 Perl 的全局正则表达式修饰符 [英] $ and Perl's global regular expression modifier

问题描述

我终于想出了如何将文本附加到文件中每一行的末尾:

I finally figured out how to append text to the end of each line in a file:

perl -pe 's/$/addthis/' myfile.txt

但是，当我尝试学习 Perl 以经常使用正则表达式时，我无法弄清楚为什么以下 perl 命令将文本addthis"添加到末尾并开始每行:

However, as I'm trying to learn Perl for frequent regex use, I can't figure out why is it that the following perl command adds the text 'addthis' to the end and start of each line:

perl -pe 's/$/addthis/g' myfile.txt

我认为无论正则表达式匹配使用什么修饰符，$"都会匹配行尾，但我猜这是错误的?

I thought that '$' matched the end of a line no matter what modifier was used for the regex match, but I guess this is wrong?

推荐答案

总结:对于你正在做的事情，删除 /g 以便它只匹配 before 换行符./g 告诉它在换行符之前匹配 和 字符串末尾(在换行符之后).

Summary: For what you're doing, drop the /g so it only matches before the newline. The /g is telling it to match before the newline and at the end of the string (after the newline).

如果没有 /m 修饰符，$ 将匹配换行符之前(如果它出现在字符串的末尾)或字符串的末尾.例如，对于 "foo" 和 "foo\n"，$ 将在 foo 之后匹配.但是，对于 "foo\nbar"，它会在 bar 之后匹配，因为 embedded 换行符不在字符串的末尾.

Without the /m modifier, $ will match either before a newline (if it occurs at the end of the string) or at the end of the string. For instance, with both "foo" and "foo\n", the $ would match after foo. With "foo\nbar", though, it would match after bar, because the embedded newline isn't at the end of the string.

使用 /g 修饰符，您将获得 $ 匹配的所有位置 -- 所以

With the /g modifier, you're getting all the places that $ would match -- so

s/$/X/g;

将像 "foo\n" 这样的一行变成 "fooX\nX".

would take a line like "foo\n" and turn it into "fooX\nX".

侧边栏:/m 修饰符将允许 $ 匹配出现在字符串末尾之前的换行符，以便

Sidebar: The /m modifier will allow $ to match newlines that occur before the end of the string, so that

s/$/X/mg;

会将 "foo\nbar\n" 转换为 "fooX\nbarX\nX".

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