正则表达式

基本正则表达式
基本正则表达式（ Basic Regular Expression ， BRE ），又称为标准正则表达式，是最早制订的正则表达
式规范，仅支持最基本的元字符集。基本正则表达式是 POSIX 规范制订的两种正则表达式语法标准之
一，另外一种语法标准称为扩展正则表达式。

[root@shell shell] # grep ^a /etc/passwd
[root@shell shell] # grep h$ /etc/passwd
[root@shell shell] # grep . /etc/passwd
[root@shell shell] # grep ro* /etc/passwd
[root@shell shell] # grep "ro\{1,2\}" /etc/passwd
[root@shell shell] # grep "ro\{1\}" /etc/passwd
[root@shell shell] # grep [a-zA-Z] /etc/passwd
[root@shell shell] # grep [a-Z] /etc/passwd
[root@shell shell] # grep [0-9] /etc/passwd
[root@shell shell] # grep [0-9a-Z] /etc/passwd 或者 grep [[:alnum:]]
/etc/passwd
[root@shell shell] # grep [[:space:]] /etc/passwd
[root@shell shell] # grep [^a] /etc/passwd
[root@shell shell] # grep ^[ab] /etc/passwd
[root@shell shell] # grep "\." file
[root@shell shell] # grep "\(root\).*\1" file
[root@shell shell] # grep "ly\>" file
[root@shell shell] # grep "\<lo" file
[root@shell shell] # grep "\<love\>" file
love
[root@shell shell] # grep "\blove\b" file
love
扩展正则

[root@shell shell] # grep -E ro? /etc/passwd
[root@shell shell] # grep -E ro+ /etc/passwd
[root@shell shell] # grep -E "(root|adm|ssh)" /etc/passwd