一.正则表达式
1.1. 什么是正则表达式
正则表达式是处理字符串的方法，以行为单位，通过一些特殊符号的辅助，让用户可以轻易进行查找、删除、替换某特定字符串的操作。
1.2. 正则表达式与通配符的区别
网友看法，有些道理，直接摘抄了：
通配符是系统level的，通配符多用在文件名上，比如查找find，ls，cp，等等；
而正则表达式需要相关工具的支持: egrep, awk, vi, perl。在文本过滤工具里，都是用正则表达式，比如像awk，sed等，是针对文件的内容的。不是所有工具（命令）都支持正则表达式。
说白了就是有些命令支持正则表达式，一些不支持。
1.3. 语系对正则表达式的影响
不通语系，对字符的翻译规则不通，例如
LANG=C, 顺序为：0,1,2,3,4....A,B,C,D......Za,b,c,d....z
LANG=zh_CN,顺序为：0,1,2,3,4....a,A,b,B,c,C,d,D......z,Z
1.4. 一些特殊符号
特殊符号可以规避语系的影响，一些常用的特殊符号：
[:alnum:] 所有的字母和数字，0-9，A-Z,a-z
[:alpha:] 所有的字母，A-Z,a-z
[:blank:] 所有呈水平排列的空白字符，空格和TAB
[:cntrl:] 所有的控制字符，CR,LF,TAL,DEL等
[:digit:] 所有的数字，0-9
[:graph:] 所有的可打印字符，不包括空格（空格和TAB）外的所有按键
[:lower:] 所有的小写字母，,a-z
[:print:] 所有的可打印字符，包括空格
[:punct:] 所有的标点字符
[:space:] 所有呈水平或垂直排列的空白字符
[:upper:] 所有的大写字母，A-Z
[:xdigit:] 所有的十六进制数，0-9，A-Z,a-z的数字与字符
二.基础正则表达式
2.1.练习，使用grep
2.1.1. grep的高级功能
grep [-A] [-B] ‘搜索字符串’ filename
-A: after + 数字n，除了该行，列出后面的n行，-An，无空格
-B：before + 数字n，除了该行，列出前面的n行，-Bn，无空格
:/$ dmesg | grep -n 'eth'
:[ 2.427478] e1000 ::01.0 eth0: (PCI:66MHz:-bit) :0c::::
:[ 2.427489] e1000 ::01.0 eth0: Intel(R) PRO/ Network Connection
:[ 2.433153] e1000 ::01.0 ens33: renamed from eth0
:/$ dmesg | grep -n -A3 -B2 'eth' #-A和-B紧接数字，没有空格
-[ 2.364718] Console: switching to colour frame buffer device 100x37
-[ 2.395386] [drm] Initialized vmwgfx 2.9. for ::0f. on minor
:[ 2.427478] e1000 ::01.0 eth0: (PCI:66MHz:-bit) :0c::::
:[ 2.427489] e1000 ::01.0 eth0: Intel(R) PRO/ Network Connection
-[ 2.427823] ahci ::05.0: version 3.0
-[ 2.428781] ahci ::05.0: AHCI 0001.0300 slots ports Gbps 0x3fffffff impl SATA mode
-[ 2.428784] ahci ::05.0: flags: 64bit ncq clo only
:[ 2.433153] e1000 ::01.0 ens33: renamed from eth0
-[ 2.445075] scsi host3: ahci
-[ 2.445243] scsi host4: ahci
-[ 2.445375] scsi host5: ahci
2.1.2. 基础正则表达式练习
使用鸟哥的例子，regular_express.txt
2.1.2.1 查找特定字符串和反向选取
:~/test$ grep -n 'the' regular_express.txt
:I can't finish the test.^M
:the symbol '*' is represented as start.
:You are the best is mean you are the no. .
:The world <Happy> is the same with "glad".
:google is the best tools for search keyword.
:~/test$ grep -vn 'the' regular_express.txt #反向选取
:"Open Source" is a good mechanism to develop programs.
:apple is my favorite food.
:Football game is not use feet only.
:this dress doesn't fit me.
:However, this dress is about $ dollars.^M
:GNU is free air not free beer.^M
:Her hair is very beauty.^M
:Oh! The soup taste good.^M
:motorcycle is cheap than car.
:This window is clear.
:Oh! My god!
:The gd software is a library for drafting programs.^M
:I like dog.
:goooooogle yes!
:go! go! Let's go.
2.1.2.2 利用[]查找字符集合
:~/test$ grep -n 't[ae]st' regular_express.txt #[ae]表示1个字符，a或者e
:I can't finish the test.^M
:Oh! The soup taste good.^M
:~/test$ grep -n 'oo' regular_express.txt
1:"Open Source" is a good mechanism to develop programs.
2:apple is my favorite food.
3:Football game is not use feet only.
9:Oh! The soup taste good.^M
18:google is the best tools for search keyword.
19:goooooogle yes!
:~/test$ grep -n '[^g]oo' regular_express.txt #[^g]不是g，查找oo且前面不是g的，第1行和第9行没有了
:apple is my favorite food.
:Football game is not use feet only.
:google is the best tools for search keyword.
:goooooogle yes!
:~/test$ grep -n '[^a-z]oo' regular_express.txt #找oo且前面不是小写字符的
:Football game is not use feet only.
:~/test$ grep -n '[^[:lower:]]oo' regular_express.txt #[:lower:]小写字符的另一种写法
3:Football game is not use feet only.
:~/test$ grep -n '[0-9]' regular_express.txt #找数字
:However, this dress is about $ dollars.^M
:You are the best is mean you are the no. .
:~/test$ grep -n '[[:digit:]]' regular_express.txt #数字的另一种写法[:digit:]
:However, this dress is about $ dollars.^M
:You are the best is mean you are the no. .
2.1.2.3 行首^与行尾$字符
注意：[^]代表反向选取， 在括号外面^[]表示行首
:~/test$ grep -n '^the' regular_express.txt #找行首是the的
:the symbol '*' is represented as start.
:~/test$ grep -n '^[a-z]' regular_express.txt #行首是小写字符的
:apple is my favorite food.
:this dress doesn't fit me.
:motorcycle is cheap than car.
:the symbol '*' is represented as start.
:google is the best tools for search keyword.
:goooooogle yes!
:go! go! Let's go.
:~/test$ grep -n '^[^a-zA-Z]' regular_express.txt #行首不是字符的
:"Open Source" is a good mechanism to develop programs.
:~/test$ grep -n '\.$' regular_express.txt #行尾是点.的，点前加了转义字符，以为.本身是特殊字符
20:go! go! Let's go.
2.1.2.3 任意一个字符.与重复字符*
*：重复前一个字符到无穷多次的意思，例如a*,代表 “空~无穷多个a”。与通配符不同，通配符中*表示0到多个字符，a*表示a或者“a若干字符”
. : 一定有一个任意字符
:~/test$ grep -n 'g..d' regular_express.txt # g..d表示g和d之间一定有2个任意字符
:"Open Source" is a good mechanism to develop programs.
:Oh! The soup taste good.^M
:The world <Happy> is the same with "glad". :~/test$ grep -n 'ooo*' regular_express.txt #ooo*，表示有~无穷多个o
:"Open Source" is a good mechanism to develop programs.
:apple is my favorite food.
:Football game is not use feet only.
:Oh! The soup taste good.^M
:google is the best tools for search keyword.
:goooooogle yes! :~/test$ grep -n 'g.*g' regular_express.txt # g.*g，找g开头，g结尾的字符，“.*”可以理解成0个或人一多个字符，与通配符中的*相当了
1:"Open Source" is a good mechanism to develop programs.
14:The gd software is a library for drafting programs.^M
18:google is the best tools for search keyword.
19:goooooogle yes!
20:go! go! Let's go.
:~/test$ grep -n 'g*g' regular_express.txt # g*g，不一定是g开头，g结尾，因为g*表示0~无穷多个g，
1:"Open Source" is a good mechanism to develop programs.
3:Football game is not use feet only.
9:Oh! The soup taste good.^M
13:Oh! My god!
14:The gd software is a library for drafting programs.^M
16:The world <Happy> is the same with "glad".
17:I like dog.
18:google is the best tools for search keyword.
19:goooooogle yes!
20:go! go! Let's go.
2.1.2.4 限定连续重复的字符范围{}
:~/test$ grep -n 'go\{2,5\}g' regular_express.txt # \{2,5\},2到5个o
:google is the best tools for search keyword.
:~/test$ grep -n 'go\{2\}g' regular_express.txt # \{2\},2个o
:google is the best tools for search keyword.
:~/test$ grep -n 'go\{2,\}g' regular_express.txt # \{2,\},2个及以上个o
:google is the best tools for search keyword.
:goooooogle yes!:~/test$ grep -n 'go\{,5\}g' regular_express.txt # \{,5\},5个及以下个o
:google is the best tools for search keyword.
:~/test$ grep -n 'go\{,10\}g' regular_express.txt # \{,10\},10个及以下个o
:google is the best tools for search keyword.
:goooooogle yes!
2.基础正则表达式字符总结
^word: word在行首
word$: word在行尾
. ：一定有一个任意字符
\ ：转义
* ： 0~无穷多个前一字符
[list] ：在list中的1个字符
[n1-n2] ：在字符范围内的1个字符
[^lish] ：反向选取
\{n1,n2\} ： 连续n1到n2个前一字符
3.sed命令
管道命令，可以进行数据替换、删除、新增、选取特定行等。
4.awk命令
按字段处理
5.扩展的正则表达式
+ 重复1个或1个以上前一个字符
？ 0个或1个前一个字符
| 或 'glad|good'
（） 分组 g(la|oo)d
（）+ 1个或多个重复组