正则表达式是一种字符模式，用于在查找过程中匹配制定的字符。

元字符通常在Linux中分为两类：

• Shell元字符，由Linux Shell进行解析；

• 正则表达式元字符，由vi/grep/sed/awk等文本处理工具进行解析；

正则表达式一般以文本行进行处理，在进行下面实例之前，先为grep命令设置--color参数：

$ alias grep='grep --color=auto'

这样每次过滤出来的字符串都会带色彩了。

在开始之前还需要做一件事情，就是创建一个测试用的re-file文件，内容如下：

$ cat re-file
I had a lovely time on our little picnic.
Lovers were all around us. It is springtime. Oh
love, how much I adore you. Do you know
the extent of my love? Oh, by the way, I think
I lost my gloves somewhere out in that field of
clover. Did you see them?  I can only hope love.
is forever. I live for you. It's hard to get back in the
groove.

文件内容摘录自<<UNIX/SHELL范例精解第四版>>

$ cat linux.txt
Linux is a good
god assdxw bcvnbvbjk
greatttttt  wexcvxc
operaaaating  dhfghfvx
gooodfs awrerdxxhkl
gdsystem awxxxx
glad
good

正则表达式元字符

• 特殊的元字符

• 扩展的正则表达式

实操

• 匹配以love开头的所有行

$ grep '^love' re-file
love, how much I adore you. Do you know

• 匹配love结尾的所有行

$ grep 'love$' re-file
clover. Did you see them?  I can only hope love.

• 匹配以l开头，中间包含两个字符，结尾是e的所有行

$ grep 'l..e' re-file
I had a lovely time on our little picnic.
love, how much I adore you. Do you know
the extent of my love? Oh, by the way, I think
I lost my gloves somewhere out in that field of
clover. Did you see them?  I can only hope love.
is forever. I live for you. It's hard to get back in the

• 匹配0个或多个空行，后面是love的字符

$ grep ' *love' re-file
I had a lovely time on our little picnic.
love, how much I adore you. Do you know
the extent of my love? Oh, by the way, I think
I lost my gloves somewhere out in that field of
clover. Did you see them?  I can only hope love.

• 匹配love或Love

$ grep '[Ll]ove' re-file  # 对l不区分大小写
I had a lovely time on our little picnic.
Lovers were all around us. It is springtime. Oh
love, how much I adore you. Do you know
the extent of my love? Oh, by the way, I think
I lost my gloves somewhere out in that field of
clover. Did you see them?  I can only hope love.

• 匹配A-Z的字母，其次是ove

$ grep '[A-Z]ove' re-file
Lovers were all around us. It is springtime. Oh

• 匹配不在A-Z范围内的任何字符行，所有的小写字符

$ grep '[^A-Z]' re-file
I had a lovely time on our little picnic.
Lovers were all around us. It is springtime. Oh
love, how much I adore you. Do you know
the extent of my love? Oh, by the way, I think
I lost my gloves somewhere out in that field of
clover. Did you see them?  I can only hope love.
is forever. I live for you. It's hard to get back in the
groove.

• 匹配love.

$ grep 'love\\.' re-file
clover. Did you see them?  I can only hope love.

• 匹配空格

$ grep '^$' re-file

• 匹配任意字符

$ grep '.*' re-file
I had a lovely time on our little picnic.
Lovers were all around us. It is springtime. Oh
love, how much I adore you. Do you know
the extent of my love? Oh, by the way, I think
I lost my gloves somewhere out in that field of
clover. Did you see them?  I can only hope love.
is forever. I live for you. It's hard to get back in the
groove.

• 前面o字符重复2到4次

$ grep 'o\\{2,4\\}' re-file
groove.

• 重复o字符至少2次

$ grep 'o\\{2,\\}' re-file
groove.

• 重复0字符最多2次

$ grep 'o\\{,2\\}' re-file
I had a lovely time on our little picnic.
Lovers were all around us. It is springtime. Oh
love, how much I adore you. Do you know
the extent of my love? Oh, by the way, I think
I lost my gloves somewhere out in that field of
clover. Did you see them?  I can only hope love.
is forever. I live for you. It's hard to get back in the
groove.

• 重复前一个字符一个或一个以

$ egrep "go+d" linux.txt
Linux is a good
god assdxw bcvnbvbjk
gooodfs awrerdxxhkl
good

• 0个或者一个字符

ansheng@Ubuntu:/tmp$ egrep "go?d" linux.txt
god assdxw bcvnbvbjk
gdsystem awxxxx

• 或，查找多个字符串

$ egrep "gd|good" linux.txt
Linux is a good
gdsystem awxxxx
good

• 分组过滤匹配

$ egrep "g(la|oo)d" linux.txt
Linux is a good
glad
good