为什么这些正则表达式不同？ [英] Why are these regular expressions different?

问题描述

我正在寻找有撇号的线条，并尝试了一些看起来与我完全相同的表情。然而，有些人工作，其他人没有。为什么我得到以下行为：

  egrep \'file＃按预期工作
 
 egrep \'file＃似乎返回所有行
 
 egrep[\']file＃按预期工作
 
 egrep'\''file＃似乎等待更多输入
  

解决方案

单双引号在逃避时有不同之处。把 echo 放在前面，看看实际发送到 egrep 的内容：

  $ echo egrep \'文件
 egrep'文件
 $ echo egrep\'文件
 egrep \'文件
 $ echo egrep[\']file 
 egrep [\'] file 
 $ echo egrep'\''file 
> 
  

最后一种情况是提示输入更多信息，因为您仍在单引号表达式中：而不是单引号中的转义引用（ \'）（因为这不是您如何转义单引号的方式）。它是单引号之间的反斜杠， 。

顺便说一句，为了避免单引号字符串中的单引号，请使用如下构造：

  $ echo'foo'\''bar'
 foo'bar 
  

实际做的是在两个单引号字符串之间放置一个裸字面引号（ \'）。这些都是隐式连接在一起的。

我不知道为什么\'匹配所有行但它的确如此。）

I am looking for lines that have an apostrophe in them and tried a few expressions that seemed identical to me; however, some worked and others didn't. Why did I get the following behavior:

egrep \' file      # works as expected

egrep "\'" file    # seems to return all lines

egrep "[\']" file  # works as expected

egrep '\'' file    # seems to be waiting for more input

解决方案

Single vs. double quotes have differences when it comes to escaping. Put echo in front to see what actually gets sent to egrep:

$ echo egrep \' file
egrep ' file
$ echo egrep "\'" file
egrep \' file
$ echo egrep "[\']" file
egrep [\'] file
$ echo egrep '\'' file
>

The last case is prompting for more input because you're still within a single-quoted expression: it's not an escaped quote (\') in single quotes (since that isn't how you escape single quotes.) It's a backslash between single quotes, with a trailing opening quote.

Incidentally, to escape a single quote in a single-quoted string, use a construction like this:

$ echo 'foo'\''bar'
foo'bar

What this is actually doing is putting a naked literal quote(\') between two single-quoted strings. These are then all implicitly concatenated together.

I have no idea why "\'" matches all lines (but it indeed seems to.)

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