正则存在的根本就是对数据的匹配，而数据基本上都是有字符组成的，而正则表达式提供了非常多的字符匹配表达式，常见的表达式模式有：

单字符匹配
	.   匹配任意单个字符，当然包括汉字的匹配
	[]  匹配指定范围内的任意单个字符
		- 示例：[shuji]、[0-9]、[a-z]、[a-zA-Z]
	[^] 匹配指定范围外的任意单个字符
		- 示例：[^shuji] 
	 |  匹配管道符左侧或者右侧的内容

[root@localhost ~]# cat keepalived.conf
! Configuration File for keepalived

global_defs {
   router_id kpmaster
}

vrrp_instance VI_1 {
    state MASTER
    interface ens33
    virtual_router_id 50
    nopreempt
    priority 100
    advert_int 1
    virtual_ipaddress {
        192.168.8.100
    }
}

.过滤单个字符
[root@localhost ~]# grep 'st..e' keepalived.conf
ftp:x:14:50:FTP User:/var/ftp:/sbin/nologin
[root@localhost ~]# grep 'ens..' keepalived.conf
    interface ens33

[] 过滤范围字符
[root@localhost ~]# grep 'i[a-z]t' keepalived.conf
    interface ens33
    virtual_router_id 50
    advert_int 1
    virtual_ipaddress {
[root@localhost ~]# grep 'i[a-n]t' keepalived.conf
    interface ens33
    advert_int 1
[root@localhost ~]# grep '[b-c]' keepalived.conf
global_defs {
vrrp_instance VI_1 {
    interface ens33
[root@localhost ~]# egrep '[x-z]' keepalived.conf
    priority 100

只要包括的内容，都不要显示
[root@localhost ~]# grep '[^a-Z_ }{0-5]' keepalived.conf
! Configuration File for keepalived
        192.168.8.100

[root@localhost ~]# egrep 'state|priority' keepalived.conf
    state MASTER
    priority 100
    
[root@localhost ~]# egrep 'st|pri' keepalived.conf
   router_id kpmaster
vrrp_instance VI_1 {
    state MASTER
    priority 100