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.

正则表达式元字符

实操

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

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

$ 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

$ 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.

$ 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.

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

$ 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.

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

$ grep '^$' re-file

$ 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.

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

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

$ 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.

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

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

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

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

来源：公众号 IT 释然杂谈

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