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<h1><center>shell正则</center></h1>
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作者：行癫（盗版必究）
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------
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## 一：正则
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#### 1.正则介绍
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正则表达式（regular expression, RE）是一种字符模式，用于在查找过程中匹配指定的字符
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在大多数程序里，正则表达式都被置于两个正斜杠之间；例如/l[oO]ve/就是由正斜杠界定的正则表达式
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它将匹配被查找的行中任何位置出现的相同模式。在正则表达式中，元字符是最重要的概念
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重要的文本处理工具：vim、sed、awk、grep
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重要的应用软件：mysql、oracle、php、python、Apache、Nginx ...
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案例：
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```shell
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匹配数字: ^[0-9]+$ 123 456 +表示前面的内容出现多次
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匹配Mail： [a-z0-9_]+@[a-z0-9]+\.[a-z]+ xingdian131420@126.com
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匹配IP： [0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3} {1,3}数字出现1-3次
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[root@xingdian ~]# egrep '[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}' /etc/sysconfig/network-scripts/ifcfg-eth0
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IPADDR=172.16.100.1
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NETMASK=255.255.255.0
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GATEWAY=172.16.100.25
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```
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#### 2.元字符
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元字符是这样一类字符，它们表达的是不同于字面本身的含义
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shell元字符(也称为通配符)
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正则表达式元字符
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案例：
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```shell
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[root@xingdian ~]# rm -rf *.pdf
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[root@xingdian ~]# grep 'abc*' /etc/passwd
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abrt:x:173:173::/etc/abrt:/sbin/nologin
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```
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#### 3.正则表达式元字符
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基本正则表达式元字符：
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```shell
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元字符 功能 示例
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^ 行首定位符 ^love
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$ 行尾定位符 love$
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. 匹配单个字符 l..e
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* 匹配前导符0到多次 ab*love
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.* 任意多个字符
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[] 匹配指定范围内的一个字符 [lL]ove
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[ - ] 匹配指定范围内的一个字符 [a-z0-9]ove
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[^] 匹配不在指定组内的字符 [^a-z0-9]ove
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\ 用来转义元字符 love\.
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\< 词首定位符 \<love
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\> 词尾定位符 love\>
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\(..\) 匹配稍后使用的字符的标签 :% s/172.16.130.1/172.16.130.5/
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:% s/\(172.16.130.\)1/\15/
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:% s/\(172.\)\(16.\)\(130.\)1/\1\2\35/
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:3,9 s/\(.*\)/#\1/
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x\{m\} 字符x重复出现m次 o\{5\}
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x\{m,\} 字符x重复出现m次以上 o\{5,\}
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x\{m,n\} 字符x重复出现m到n次 o\{5,10\}
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```
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扩展正则表达式元字符：
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```shell
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+ 匹配一个或多个前导字符 [a-z]+ove
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? 匹配零个或一个前导字符 lo?ve
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a|b 匹配a或b love|hate
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() 组字符 loveable|rs love(able|rs) ov+ (ov)+
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(..)(..)\1\2 标签匹配字符 (love)able\1er
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x{m} 字符x重复m次 o{5}
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x{m,} 字符x重复至少m次 o{5,}
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x{m,n} 字符x重复m到n次 o{5,10}
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```
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#### 4.正则匹配示例
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```shell
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/love/
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/^love/
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/love$/
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/l.ve/
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/lo*ve/
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/[Ll]ove/
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/love[a-z]/
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/love[^a-zA-Z0-9]/
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/.*/
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/^$/
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/^[A-Z]..$/
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/^[A-Z][a-z ]*3[0-5]/
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/[a-z]*\./
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/^[A-Z][a-z][a-z]$/
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/^[A-Za-z]*[^,][A-Za-z]*$/
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/\<fourth\>/
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/\<f.*th\>/
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/5{2}2{3}\./
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5{2}
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空行
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/^$/
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```
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#### 5.正则案例
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```shell
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#!/bin/bash
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read -p "please input number:" num
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if [[ ! "$num" =~ ^[0-9]+$ ]]
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then
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echo "error number!"
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else
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echo "is number!"
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fi
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```
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