Grep 正则表达式没有按预期工作 [英] Grep regular expression not working as expected

问题描述

我有一个简单的 grep 命令，试图只获取 CSV 文件的第一列，包括逗号.事情是这样的……

I have a simple grep command trying to get only the first column of a CSV file including the comma. It goes like this...

grep -Eo '^[^,]+,' some.csv

所以在我的脑海中，这读起来就像只获取该行的匹配部分，其中每行至少以一个不是逗号的字符开头，后跟一个逗号."

So in my head, that reads like "get me only the matching part of the line where each line starts with at least one character that is not a comma, followed by a single comma."

在一个文件 some.csv 上，看起来像这样:

So on a file, some.csv, that looks like this:

column1,column2,column3,column4
column1,column2,column3,column4
column1,column2,column3,column4

我期待这个输出:

column1,
column1,
column1,

但我得到这个输出:

column1,
column2,
column3,
column1,
column2,
column3,
column1,
column2,
column3,

这是为什么?我的 grep/regex 遗漏了什么?我的预期输出不正确吗?

Why is that? What am I missing from my grep/regex? Is my expected output incorrect?

如果我删除正则表达式中尾随逗号的要求，该命令将按预期工作.

If I remove the requirement of the trailing comma in the regex, the command works as I expect.

grep -Eo '^[^,]+' some.csv

给我:

column1
column1
column1

注意:我在 macOS High Sierra 上使用 grep 版本:grep (BSD grep) 2.5.1-FreeBSD

NOTE: I'm on macOS High Sierra with grep version: grep (BSD grep) 2.5.1-FreeBSD

推荐答案

BSD grep 通常有问题.请参阅以下相关帖子:

BSD grep is buggy in general. See the following related posts:

• 为什么这个 BSD grep 结果不同来自 GNU grep?
• 用单字母词grep奇怪的行为
• 如何让 BSD grep 尊重开始-of-line 锚

上面的最后一个链接提到了您的情况:当使用 -o 选项时，grep 会出于某种原因忽略 ^ 锚点.此问题也在 FreeBSD 错误中描述:

That last link above mentions your case: when -o option is used, grep ignores the ^ anchor for some reason. This issue is also described in a FreeBSD bug:

我注意到相同版本的 grep 存在更多问题.我不知道它们是否相关，但我现在将它们附加在这里.

I've noticed some more issues with the same version of grep. I don't know whether they're related, but I'll append them here for now.

<代码>$ printf abc |grep -o '^[a-c]'

应该只打印 'a'，而是针对每个字母给出 3 次点击传入文本.

should just print 'a', but instead gives three hits, against each letter of the incoming text.

作为一种解决方法，安装 GNU grep 按预期工作.

As a workaround, it might be a better idea to just install GNU grep that works as expected.

或者，使用带有 BRE POSIX 模式的 sed:

Or, use sed with a BRE POSIX pattern:

sed -i '' 's/^([^,]*,).*/1/' file

模式匹配的地方

• ^ - 一行的开始
• ([^,]*,) - 第 1 组(稍后使用来自 RHS 的 1 反向引用引用):
  • [^,]* - 除 ,
  之外的零个或多个字符
  • , - 一个 , 字符
  • ^ - start of a line
  • ([^,]*,) - Group 1 (later referred to with 1 backreference from the RHS):
    • [^,]* - zero or more chars other than ,
    • , - a , char

    请注意，-i 将就地更改文件内容.如果需要，请使用 -i.bak 创建备份文件(然后，您将不需要下一个空的 '').

    Note that -i will change the file contents inplace. Use -i.bak to create a backup file if needed (then, you wouldn't need the next empty '' though).

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