行尾字符 ($) 在方括号内不起作用 [英] End of line char ($) doesn't work inside square brackets

问题描述

将 $ 放在方括号内对 grep 不起作用.

~ $ echo -e "hello
there" >例子.txt~ $ grep "hello$" example.txt你好~ $ grep "hello[$]" example.txt~$

这是 grep 中的错误还是我做错了什么?

解决方案

这就是它应该做的.

[$]

...定义一个匹配一个字符的字符类，$.

因此，这将匹配包含 hello$ 的行.

<小时>

见POSIX RE 括号表达式定义规范要求如此.引用完整定义:

<块引用>

括号表达式(用方括号括起来的表达式，[]")是一个 RE，它应匹配括号表达式表示的非空整理元素集中包含的单个整理元素.

因此，任何括号表达式都匹配单个元素.

<小时>

此外，在 BRE 锚定表达式定义中:

<块引用>

2. 美元符号 ( '$' ) 在用作整个 BRE 的最后一个字符时应作为锚点.当用作子表达式的最后一个字符时，实现可以将美元符号视为锚点.美元符号应将表达式(或可选的子表达式)锚定到匹配字符串的末尾；美元符号可以说是匹配最后一个字符之后的字符串结尾.

因此——从 BRE 开始，grep 默认识别的正则表达式格式不带参数——如果 $ 不在表达式的末尾，它是不需要被识别为锚点.

Putting $ inside square brackets doesn't work for grep.

~ $ echo -e "hello
there" > example.txt
~ $ grep "hello$" example.txt 
hello
~ $ grep "hello[$]" example.txt 
~ $ 

Is this a bug in grep or am I doing something wrong?

解决方案

That's what it's supposed to do.

[$]

...defines a character class that matches one character, $.

Thus, this would match a line containing hello$.

---

See the POSIX RE Bracket Expression definition for the formal specification requiring that this be so. Quoting from that full definition:

A bracket expression (an expression enclosed in square brackets, "[]" ) is an RE that shall match a single collating element contained in the non-empty set of collating elements represented by the bracket expression.

Thus, any bracket expression matches a single element.

---

Moreover, in the BRE Anchoring Expression definition:

2. A dollar sign ( '$' ) shall be an anchor when used as the last character of an entire BRE. The implementation may treat a dollar sign as an anchor when used as the last character of a subexpression. The dollar sign shall anchor the expression (or optionally subexpression) to the end of the string being matched; the dollar sign can be said to match the end-of-string following the last character.

Thus -- as of BRE, the regexp format which grep recognizes by default with no arguments -- if $ is not at the end of the expression, it is not required to be recognized as an anchor.

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