grep“？”与有效匹配不符 [英] grep "?" does not match valid matches

问题描述

我想匹配文件中的标签（带有可选的括号）...容易的人会认为...正则表达式类似于 ^ \ [？MyTag \]？。但是... Grep不喜欢它。

有趣的部分是：如果我替换？用 * （所以零到无限匹配，而不是零或一）它匹配所有它应该的东西，但真的这意味着功能被破坏，我不

任何输入？

在Windows上使用grep（GNU grep）2.22。

$ b $ PS：so grep就是这样...

pre $ grep -e^ \？MyTag\]？ file.txt

和我的测试文件是这样的

  [MyTag] hello 
 NotMyTag ugly 
 [NotMyTag] dumb 
 MyTag world 
  

显然应该会显示第一行和第四行，但不会显示任何内容。

解决方案 div> ？不是grep支持的基本正则表达式的一部分。 GNU grep支持它们作为扩展名，但你必须逃避它们：

  $ grep'^ \ [\？MyTag \] \？'file.txt 
 [MyTag] hello 
 MyTag world 
  

或者，正如指出的那样，使用 grep -E 来启用扩展正则表达式。

对于GNU grep ， grep 和 grep -E 之间的唯一区别，即使用基本和扩展正则表达式，就是你所拥有的

• 基本正则表达式
  
  
  
  
  • 捕获组和量化必须被转义： \（\）和 \ {\}
  
  
  • 一个或多个（ + ）和替换（ | ）不是BRE的一部分，但由GNU grep作为扩展支持（但需要转义： \？\ + \ | b
$ b
  • 扩展正则表达式


  （）和 {}
  
  • 捕获组和量化不必转义$ c $ ？， +
  

  
支持并且不需要转义
|

I want to match tags in files (with optional brackets) ... easy one would think ... the regex is something like ^\[?MyTag\]?. But ... Grep doesn't like it. None of the lines that would be valid matches are actually matched.

The interesting part is: if I replace the ? with a * (so zero to infinite matches, not zero or one) it matches everything like it should, but really that would mean the feature is broken and I don't believe that.

Any input?

Using grep (GNU grep) 2.22 on Windows.

PS: so grep is like this ...

grep -e "^\[?MyTag\]?" file.txt

and my test file is like this

[MyTag] hello
NotMyTag ugly
[NotMyTag] dumb
MyTag world

which obviously should result in 1st and 4th line showing but shows nothing.

解决方案

? is not part of the basic regular expressions, which grep supports. GNU grep supports them as an extension, but you have to escape them:

$ grep '^\[\?MyTag\]\?' file.txt
[MyTag] hello
MyTag world

Or, as pointed out, use grep -E to enable extended regular expressions.

For GNU grep, the only difference between grep and grep -E, i.e., using basic and extended regular expressions, is what you have to escape and what not.

• Basic regular expressions
  • Capture groups and quantifying have to be escaped: \( \) and \{ \}
  • Zero or one (?), one or more (+) and alternation (|) are not part of BRE, but supported by GNU grep as an extension (but need to be escaped: \? \+ \|)
• Extended regular expressions
  • Capture groups and quantifying don't have to be escaped: ( ) and { }
  • ?, + and | are supported and don't need be be escaped

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