模式分组/y(.)(.)\2\1/里的向后引用，"\+数字"部分必须有吗？

bequan

小骆驼第五版讲模式分组的时候，关于向后引用（back reference）部分有这样一个例子：

$_ = "yabba dabba doo";
if (/y(.)(.)\2\1/) { # matches 'abba'
    print "It matched the same after y and d!\n";
}

我对这一节里的向后引用理解得不好。可以理解单独的/y(.)\1/就是要匹配两个重复的字符，也可以理解/(.)(.)\1/可以匹配形如aba这样的轴对称字符。但是对于这个模式里，为什么往(.)\1中间插入(.)\2后就有了匹配形如abba的回文字符的功能呢？

[ 本帖最后由 bequan 于 2009-8-30 21:09 编辑 ]

Perl_Er

这又什么问题吗，就是这样啊.

bequan

()后面跟一个“\+数字”，我不太明白“\+数字”的作用

Perl_Er

Backreferences

Closely associated with the matching variables $1 , $2 , ... are the backreferences \1 , \2 ,... Backreferences are simply matching variables that can be used inside a regexp. This is a really nice feature -- what matches later in a regexp is made to depend on what matched earlier in the regexp. Suppose we wanted to look for doubled words in a text, like 'the the'. The following regexp finds all 3-letter doubles with a space in between:

   1. /\b(\w\w\w)\s\1\b/;

The grouping assigns a value to \1, so that the same 3 letter sequence is used for both parts.

A similar task is to find words consisting of two identical parts:

   1. % simple_grep '^(\w\w\w\w|\w\w\w|\w\w|\w)\1$' /usr/dict/words
   2. beriberi
   3. booboo
   4. coco
   5. mama
   6. murmur
   7. papa

The regexp has a single grouping which considers 4-letter combinations, then 3-letter combinations, etc., and uses \1 to look for a repeat. Although $1 and \1 represent the same thing, care should be taken to use matched variables $1 , $2 ,... only outside a regexp and backreferences \1 , \2 ,... only inside a regexp; not doing so may lead to surprising and unsatisfactory results.

bequan

明白了，back reference里的"\+数字"就是用来代替数字所对应的第几个分组要在此处出现。

iLRainyday

发错了~~

[ 本帖最后由 iLRainyday 于 2009-8-31 10:37 编辑 ]

blackold

先看正则基础吧。

freedos520

bequan 发表于 2009-08-30 22:47
明白了，back reference里的"\+数字"就是用来代替数字所对应的第几个分组要在此处出现。

正解！