「\ 2」在正則表達式結尾處的含義是什麼

Question

我有以下任務：

歌曲的單詞在名爲stairway.txt的文件中。

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. 

我不明白什麼是\2在到底是什麼意思：以下哪個行會此命令後，打印出來？

Answer 1

這是反向引用。

從http://www.selectorweb.com/grep_tutorial.html：

向引用是一種表達\ n，其中n是一個數字。它與表達式中第n組圓括號的內容相匹配。

而且，答案是（d）：

$ grep -E '(^.{4})(.{2}).*[ ]\2' test.txt 
If there's a bustle in your hedgerow.