正则的零宽断言的一个冷门知识点
一般来说’(?<’用在左边，’(?’用在右边,但是也有人反着用(倒不如说可以反着用)
下面现象看例子
[[email protected] temp]# cat -n txt
1 123,asdasd,23sfsdf,
2 123,1nd.
3 ,addfsdf,hjgfj
4 1,23t.fdgdfg,dfdg
5 a,5ggh
6 .asf,先来一个正常的,匹配一个字符后面逗号的到结尾
[[email protected] temp]# grep -Pon '.(?=,).+' txt
1:3,asdasd,23sfsdf,
2:3,1nd.
3:f,hjgfj
4:1,23t.fdgdfg,dfdg
5:a,5ggh
6:f,
然
[[email protected] temp]# grep -Pon '(?=,).+' txt
1:,asdasd,23sfsdf,
2:,1nd.
3:,addfsdf,hjgfj
4:,23t.fdgdfg,dfdg
5:,5ggh
6:,
可以理解为前面有字符后去掉前面的,或者理解为前面0个字符从这个字符开始,也就是’(?<=正则)’但是保留这个正则部分,不能理解成(?=,).+可以转换为,.+
原因看下面例子
[[email protected] temp]# cat txt
1 ,23sfsdf,
2 1nd,
3 ,addfsdf,hjgfj
4 1,23t.fdgdfg,dfdg
5 a,5ggh
6 .asf,
[[email protected] temp]# grep -Pon '(?=,).+' txt
1:,23sfsdf,
2:,
3:,addfsdf,hjgfj
4:,23t.fdgdfg,dfdg
5:,5ggh
6:,
[[email protected] temp]# grep -Pon ',.+' txt
1:,23sfsdf,
3:,addfsdf,hjgfj
4:,23t.fdgdfg,dfdg
5:,5ggh
总之(?=)用在左边比较鸡肋
说下(?!)用在左边
[[email protected] temp]# cat -n txt
1 123,asdasd,23sfsdf,
2 123,1nd.
3 ,addfsdf,hjgfj
4 1,23t.fdgdfg,dfdg
5 aa,5ggh
6 .as.f,
7 1al,sd.
8 ,wewqe
[[email protected] temp]# grep -Pon '.(?!,).+' txt
1:123,asdasd,23sfsdf,
2:123,1nd.
3:,addfsdf,hjgfj
4:,23t.fdgdfg,dfdg
5:aa,5ggh
6:.as.f,
7:1al,sd.
8:,wewqe
[[email protected] temp]# grep -Pon '(?!,).+' txt
1:123,asdasd,23sfsdf,
2:123,1nd.
3:addfsdf,hjgfj
4:1,23t.fdgdfg,dfdg
5:aa,5ggh
6:.as.f,
7:1al,sd.
8:wewqe正则默认贪婪的,所以一般这样匹配到的是很多(?!)一般用在开头(原
上面就是开头的逗号,可以理解为无论开头有没有逗号都去掉开头的逗号
下面同
[[email protected] temp]# cat -n txt
1 123,asdasd,23sfsdf,
2 123,1nd.
3 ,addfsdf,hjgfj
4 1,23t.fdgdfg,dfdg
5 aa,5ggh
6 .as.f,
7 1al,sd.
8 ,wewqe
[[email protected] temp]# grep -Pon '.+(?<=,).' txt
1:123,asdasd,2
2:123,1
3:,addfsdf,h
4:1,23t.fdgdfg,d
5:aa,5
7:1al,s
8:,w
[[email protected] temp]# grep -Pon '.+(?<=,)' txt
1:123,asdasd,23sfsdf,
2:123,
3:,addfsdf,
4:1,23t.fdgdfg,
5:aa,
6:.as.f,
7:1al,
8:,同理(?<!)表示无论结尾有没有正则部分,都去掉这部分
[[email protected] temp]# cat txt
123,asdasd,23sfsdf,
123,1nd.
,addfsdf,hjgfj
1,23t.fdgdfg,dfdg
aa,5ggh
.as.f,
1al,sd.
,wewqe
[[email protected] temp]# grep -Pon '.+(?<!,).' txt
1:123,asdasd,23sfsdf,
2:123,1nd.
3:,addfsdf,hjgfj
4:1,23t.fdgdfg,dfdg
5:aa,5ggh
6:.as.f,
7:1al,sd.
8:,wewqe
[[email protected] temp]# grep -Pon '.+(?<!,)' txt
1:123,asdasd,23sfsdf
2:123,1nd.
3:,addfsdf,hjgfj
4:1,23t.fdgdfg,dfdg
5:aa,5ggh
6:.as.f
7:1al,sd.
8:,wewqe
总结的话就是
(?=)和(?<=)用在相反的方向都很鸡肋
(?!)和(?<!)用在相反方向一般是最左边和最右边这样反着用来去