I have the following assignment:
我有以下任务：
Words of a song are in a file called stairway.txt. Which of the following lines will be printed out after this command:
歌曲的单词位于名为stairway.txt的文件中。在此命令之后将打印以下哪一行：
grep -E '(^.{4})(.{2}).*[ ]\2' stairway.txt
(a) Yes, there are two paths you can go by but in the long run
(b) Its just a spring clean for the May queen.
(c) Don't be alarmed now.
(d) If there's a bustle in your hedgerow.
(e) Theres still time to change the road you're on.
I don't understand what does the \2 at the end mean?
我不明白\ 2到底意味着什么？
1 个解决方案
#1
12
It is a backreference.
这是一个反向引用。
From http://www.selectorweb.com/grep_tutorial.html:
来自http://www.selectorweb.com/grep_tutorial.html：
Backreference is an expression \n where n is a number. It matches the contents of the n'th set of parentheses in the expression.
反向引用是一个表达式\ n，其中n是数字。它匹配表达式中第n组括号的内容。
Also, the answer is (d):
另外，答案是（d）：
$ grep -E '(^.{4})(.{2}).*[ ]\2' test.txt
If there's a bustle in your hedgerow.
#1
12
It is a backreference.
这是一个反向引用。
From http://www.selectorweb.com/grep_tutorial.html:
来自http://www.selectorweb.com/grep_tutorial.html：
Backreference is an expression \n where n is a number. It matches the contents of the n'th set of parentheses in the expression.
反向引用是一个表达式\ n，其中n是数字。它匹配表达式中第n组括号的内容。
Also, the answer is (d):
另外，答案是（d）：
$ grep -E '(^.{4})(.{2}).*[ ]\2' test.txt
If there's a bustle in your hedgerow.