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