一、定义：元字符字符是这样一类字符，它们表达的是不同字面本身的含义

二、分类：

        1、基本正则表达式元字符

#    ^ 行首定位
[root@localhost ~]# grep root  /etc/passwd
root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin
[root@localhost ~]# grep ^root /etc/passwd
root:x:0:0:root:/root:/bin/bash

#    $ 行尾定位
[root@localhost ~]# grep love$ 1.txt 
love

#    . 匹配任意单个字符
[root@localhost ~]# grep a.c 1.txt 
abc
adc

#    * 匹配前导符0次到多次
[root@localhost ~]# grep "abcde*" 1.txt 
abcd
abcde
abcdef
abcdefj

#    .* 任意多个字符
[root@localhost ~]# grep ".*" 1.txt 
qian
yoa
huang
kai
kong

#    [] 匹配指定范围内的一个字符
[root@localhost ~]# grep "[lL]ove" 1.txt 
love
love
Love

#    [-] 匹配指定范围内的一个字符，连续的范围
		[root@localhost ~]# grep "[a-z]ove" 1.txt 
love
love

#    [^] 匹配不在指定组内的字符
[root@localhost ~]# grep "[^lL]ove" 1.txt 
1ove
|ove

#    \ 用来转义元字符('' "" \),脱意符
[root@localhost ~]# grep "l\.ve" 1.txt 
l.ve

#    \< 词首定位符
[root@localhost ~]# grep "\<love" 1.txt 
love
love

#    \> 词尾定位符
		[root@localhost ~]# grep "love\>" 1.txt 
love
love

#    () 匹配稍后使用的字符的标签
:1,2 s/(.*\)/#\1/      #加注释

#    x\{m\} 字符x重复出现m次
[root@localhost ~]# grep "o\{3\}" 1.txt 
|oooove

#    x\{m,\} 字符x重复出现m次以上
[root@localhost ~]# grep "o\{3,\}" 1.txt 
|oooove
looove
loooooooove

#    x\{m,n\} 字符x重复出现m到n次
[root@localhost ~]# grep "o\{3,4\}" 1.txt 
|oooove
looove
loooooooove

 

 

 

 

 

        2、扩展正则表达式元字符

#    + 匹配1-n个前导字符
[root@localhost ~]# egrep lo+ve 1.txt 
love
love
looove
loooooooove

#    ？ 匹配0-1个前导字符
[root@localhost ~]# egrep lo?ve 1.txt 
love
love

#    a | b 匹配a或b
[root@localhost ~]# egrep "o|v" 1.txt 
yoa
kong
duo
gou
love
love

#    () 组字符
[root@localhost ~]# egrep "love(able|rs)" 1.txt 
loveable
lovers

[root@localhost ~]# egrep "loveable|rs" 1.txt 
loveable
lovers