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
