GNU sed，^和$与|当第一个/最后一个字符匹配时 [英] GNU sed, ^ and $ with | when first/last character matches

问题描述

在REGEXP sed中进行包含类似 ^ |.之类的替换时，如果第一个字符匹配，则不匹配模式空间开头的空字符串.如果最后一个字符匹配，它也不匹配结尾.为什么会这样?

When doing a substitution that includes something like ^|. in the REGEXP sed doesn't match the null string at beginning of the pattern space if the first character matches. It also doesn't match the end if the last character matches. Why is that?

以下是使用 123 作为输入(带有 -r 选项)的一些示例:

Here are some examples using 123 as input (with the -r option):

substitution    expected output     actual output   comments
s/^/x/g         x123                x123            works as expected
s/$/x/g         123x                123x            works as expected
s/^|$/x/g       x123x               x123x           works as expected
s/^|./x/g       xxxx                xxx             didn't match the very begining
s/.|$/x/g       xxxx                xxx             didn't match the very end
s/^|1/x/g       xx23                x23             didn't match the very begining
s/^|2/x/g       x1x3                x1x3            this time it did match the begining

使用 \`而不是 ^ 时，我得到相同的结果.
我已经尝试了GNU sed版本4.2.1和4.2.2

I get the same results when using \` instead of ^.
I've tried GNU sed version 4.2.1 and 4.2.2

在线试玩！

推荐答案

AFAIK sed将尝试匹配轮流中最长的匹配项.

AFAIK sed will try to match the longest match in an alternation.

因此，如果可以将模式空间开头的空字符串与 1 在同一位置进行匹配.选择了 1 ，因为它是最长的匹配项.

So when the null string at the beginning of the pattern space can be matched vs. 1 at the same position. 1 is chosen as it's the longest match.

请考虑以下内容:

$ sed 's/12\|123/x/g' <<< 123
x
$ sed 's/123\|12/x/g' <<< 123
x
$ sed 's/^1\|12/x/g' <<< 123
x3

到达终点时同样适用.让 sed's/.\ | $/x/g'<<<123 向下:

The same applies when reaching the end. Lets break sed 's/.\|$/x/g' <<< 123 down:

123
^
. matches and replace with x
x23
 ^
 . matches and replace with x
xx3
  ^
  . matches and replace with x
xxx
   ^
   Out of pattern space $ will not match.

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