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 "\\\