Grep歧义嵌套方括号 [英] Grep ambiguity nested square bracket

问题描述

sample.txt contains

  abcde 
 abde 
  

任何人都可以解释下列命令的输出 -

1. grep'[[ab]]'sample.txt - 无输出
   
2. - 无输出


3. grep'[ab []'sample .txt - 输出为 abcde ， abde


4. grep'[ab]]'sample.txt - no output

[（ab）] 和 [^（ab）] 是什么意思？它与 [ab] 和 [^ ab] ？

* 将匹配 * ，并不意味着 0或1 重复。同样，（）会匹配（和），将不会创建捕获组。

现在，如果] 在一个字符类中找到，它会自动关闭字符类，而另一个字符不会是该字符类的一部分。现在，让我们了解上面发生的事情：

---

在 1 ， 2 和 4 ，你的角色类在第一个结尾] 。因此，最后一个括号 - ] 不是字符类的一部分。它必须分开配对。所以，你的模式将匹配这样的东西：

 '[[ab]]'与'（[| a | b）（]）'//最后一个`]`必须匹配。 
'[ab []]'与'（a | b | [）（]）'相同//最后一个`]`必须匹配。 
'[ab]]'与'（a | b |]）（'）''//相同，最后一个`]`必须匹配。 
 ^ 
 ^ ----字符类在这里关闭。 
  

现在，因为在这两个字符串中都没有]


然而，在第三种模式中，您的角色类只能由最后一个 。因此所有东西都进入了字符类。

pre $ '[ab []'表示包含'a'的匹配字符串， b'或'['


这是完全有效的并且与字符串匹配。

---

[（ab）]
$ [^ > [（ab）] 表示匹配任何（， a ， b ，）。请记住，在字符类中，正则表达式的元字符没有任何特殊含义。所以，你不能在角色类中创建组。

[^（ab）] 表示 [（ab）] 。它匹配任何不包含任何指定字符的字符串。

---

它是与 [ab] 和 [^ ab] ？

$相同b
$ b

没有。这两个不包含（和）。因此他们有点不同。

sample.txt contains

abcde
abde

Can anybody explain the output of following commands -

1. grep '[[ab]]' sample.txt - no output
2. grep '[ab[]]' sample.txt - no output
3. grep '[ab[]' sample.txt - output is abcde , abde
4. grep '[ab]]' sample.txt - no output

And what does [(ab)] and [^(ab)] mean? Is it the same as [ab] and [^ab] ?

解决方案

First thing to understand is, inside a character class, none of the meta-characters of regex has any special meaning. They are matched literally. For e.g., an * will match a * and will not mean 0 or 1 repetition. Similarly, () will match ( and ), and will not create a capture group.

Now, if a ] is found in a character class, that automatically closes the character class, and the further character won't be the part of that character class. Now, let's understand what is happening above:

---

In 1, 2, and 4, your character class ends at the first closing ]. So, the last closing bracket - ], is not the part of character class. It has to be matched separately. So, your pattern will match something like this:

'[[ab]]' is same as '([|a|b)(])'  // The last `]` has to match.
'[ab[]]' is same as '(a|b|[)(])'  // Again, the last `]` has to match.
'[ab]]'  is same as '(a|b|])(])'  // Same, the last `]` has to match.
    ^
    ^---- Character class closes here.

Now, since in both the string, there is no ] at the end, hence no match is found.

Whereas, in the 3rd pattern, your character class is closed only by the last ]. And hence everything comes inside the character class.

'[ab[]' means match string that contains 'a', or 'b', or '['

which is perfectly valid and match both the string.

---

And what does [(ab)] and [^(ab)] mean?

[(ab)] means match any of the (, a, b, ). Remember, inside a character class, no meta-character of regex has any special meaning. So, you can't create groups inside a character class.

[^(ab)] means exact opposite of [(ab)]. It matches any string which does not contain any of those characters specified.

---

Is it the same as [ab] and [^ab] ?

No. These two does not include ( and ). Hence they are little different.

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