1.基本元字符

字符	功能	示例
^	行首定位符	^love
$	行尾定位符	love$
.	匹配单个字符	l…e
*	匹配前导符0到多次	ab*love
.*	匹配任意多个字符（贪婪匹配）	ab.*love
[]	匹配方括号中任意一个字符	[lL]ove
[-]	匹配指定范围内的一个字符	[a-zA-Z0-9]ove
[^]	匹配不在指定组里的字符	[^a-z0-9]ove
\	用来转义元字符	love\.
\<	词首定位符	\<love
\>	词尾定位符	love>\

练习：

[root@localhost ~]# cat test.txt
Love
love
loveyou
aaabbblove
a123love
www.love.com
i love you
do you love me//以lo开头
[root@localhost ~]# grep  ^lo test.txt 
love
loveyou
[root@localhost ~]# grep -i ^lo test.txt    //-i不区分大小写
Love
love
loveyou//以me结尾
[root@localhost ~]# grep  me$ test.txt
do you love me//匹配单个字符，一个.代表一个字符
[root@localhost ~]# grep  y.u test.txt
loveyou
i love you
do you love me
[root@localhost ~]# grep  l.e test.txt
[root@localhost ~]# grep  l..e test.txt
love
loveyou
aaabbblove
a123love
www.love.com
i love you
do you love me//匹配前导符0到多次；前导符是"a" 即*前面a有0次到多次
[root@localhost ~]# grep a*l test.txt
love
loveyou
aaabbblove
a123love
www.love.com
i love you
do you love me//贪婪匹配；匹配任意多个字符；即*的前导符是"a." 但是"."可以匹配任意单个字符
[root@localhost ~]# grep a.*l test.txt
aaabbblove
a123love//匹配中括号中任意一个字符
[root@localhost ~]# grep a[0-9]l*  test.txt
a123love//匹配指定范围内的一个字符
[root@localhost ~]# grep a[0-9a-z]l*  test.txt
aaabbblove
a123love//匹配不在指定组里的字符
[root@localhost ~]# grep a[^0-9]  test.txt
aaabbblove//转义匹配
[root@localhost ~]# grep www\.  test.txt
www.love.com
[root@localhost ~]# cat >>./test.txt<<eof
l*o*v*e
eof
[root@localhost ~]# grep * test.txt
[root@localhost ~]# grep \* test.txt
[root@localhost ~]# l*o*v*e
//单引号和双引号这里作用相同
[root@localhost ~]# grep "*" test.txt
[root@localhost ~]# l*o*v*e
[root@localhost ~]# grep '*' test.txt
[root@localhost ~]# l*o*v*e

2.拓展元字符

（示例在grep正则过滤）

字符	功能	示例
+	匹配一次或多次前导字符	[a-z]+ove
?	匹配零次或一次前导字符	lo?ve
a|b	匹配a或b	love|hate
x{m}	字符x重复m次	o{5}
x{m,}	字符x重复至少m次	o{5,}
x{m,n}	字符x重复m到n次	o{5,10}
()	字符组

3.正则判断=~

[root@localhost ~]# num1=1 
//运用正则，判断需要[[ ]]
//注意：^在[]内表示取反，^在[]外表示以什么开头
//=~ 用于正则表达式匹配
[root@localhost ~]# [[ $num1 =~ ^[0-9] ]] && echo "yes" || echo "no"
yes
[root@localhost ~]# [[ $num1 =~ [^0-9] ]] && echo "yes" || echo "no"
no[root@localhost ~]# num3=1b1 
[root@localhost ~]# [[ $num3 =~ ^[0-9] ]] && echo "yes" || echo "no"
yes
[root@localhost ~]# [[ $num3 =~ ^[0-9]+$ ]] && echo "yes" || echo "no"
no    //num3是纯数字，这里才会是yes
[root@localhost ~]# [[ $num3 =~ [^0-9] ]] && echo "yes" || echo "no"
yes
[root@localhost ~]# [[ $num3 =~ ^[^0-9] ]] && echo "yes" || echo "no"
no
[root@localhost ~]# [[ $num3 =~ ^[0-9][a-z][0-9]+$ ]] && echo "yes" || echo "no"
yes//或：
if [[ $num3 =~ ^[0-9]+$ ]];then  echo "yes" ;else echo 'no'; fi[root@localhost ~]# num=1.6
[root@localhost ~]# [[ $num =~ ^[0-9]\.[0-9]+$ || $num =~ ^[0-9]+$ ]] && echo "yes" || echo "no"       //输入的只能是数字(包括小数)
yes
[root@localhost ~]# [[ $num =~ . && echo "yes" || echo "no"
yes    //1.6作为字符串被匹配