regex - 行尾字符 ($) 在方括号内不起作用

Question

将 $ 放在方括号内对于 grep 不起作用。

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

这是 grep 中的错误还是我做错了什么？

Answer 1

这就是它应该做的。

[$]

...定义匹配一个字符 $ 的字符类。

因此，这将匹配包含 hello$ 的行。

<小时/>

参见the POSIX RE Bracket Expression definition对于要求如此的正式规范。引用完整的定义:

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.

因此，任何括号表达式都匹配单个元素。

<小时/>

此外，在 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.

因此 - 从 BRE 开始，grep 默认情况下识别不带参数的正则表达式格式 - 如果 $ 不在表达式末尾，则它是不需要被识别为 anchor 。