正则表达式
1.基本正则 ^匹配行首 例：grep '^r' /etc/passwd root:x:0:0:root:/root:/bin/bash
$匹配行未 例：grep 'localhost$' /etc/hosts 127.0.0 localhost.localdomain localhost
[]集合，匹配集合中的任意单个字符 例：grep '[A-Z]' brace.txt abcab CD-ROM
[^]对集合取反 例：grep '[^ab]' brace.txt sdcgfkill
.匹配任意单个字符 例：grep '.grop' brace.txt agrop
*匹配前一个字符任意次数
例:egrep 'stuf*' /etc/rc.local want to do the full Sys V style init stuff.
{n,m}匹配前一个字符n到m次 例：grep '(ab){2,4}' brace.txt dedef abab ghighi
{n}匹配前一个字符n次 例：grep '(ab){3}' brace.txt cdcd ababab
()组合为整体，保留 {n,}匹配前一个字符n次以上 例：grep '(ab){2,}' brace.txt cdcd ababab
2.扩展正则 grep -E 支持扩展正则简写egrep
+最少匹配一次 例：egrep 'f+' /etc/rc.local want to do the full Sys V style init stuff. You can put your own initialization stuff in here if you don't
?最多匹配一次 例：egrep 'i?' /etc/rc.local want to do the full Sys V style in stuff.
{n,m}匹配n到m次 例:egrep '(ab){3}' brace.txt cdcd ababab
|或者 例：grep -E '^root|^daemon' /etc/passwd root:x:0:0:root:/root:/bin/bash daemon:x:2:2:daemon:/sbin:/sbin/nologin
（）组合为整体，保留
\b单词边界 例：egrep '\binit\b' /etc/rc.local want to do the full Sys V style init stuff.
<单词的开头 例：egrep '<init>' /etc/rc.local want to do the full Sys V style init stuff.
>单词的结尾 例：egrep 'll>' /etc/rc.local want to do the full Sys V style init stuff.
