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：
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
| 元字符 | 功能 | |
|---|---|---|
^ |
以什么开头 | |
$ |
以什么结尾 | |
. |
匹配一个字符 | |
* |
匹配0个或多个 | |
[] |
匹配集合中的 | |
[x-y] |
匹配集合范围内的 | |
[^ ] |
匹配不在集合中的 | |
\ |
转义 | 'love\.' |
| 元字符 | 功能 | 实例 | 怎么匹配 |
|---|---|---|---|
\< |
以什么开头 | '\<love' |
匹配以love开头的所有行 |
\> |
以什么结尾 | 'love\>' |
匹配love结尾的所有行 |
\(..\) |
标签匹配以后使用的字符 | '\(love\)able \1er' |
用位置\1\2引导前面做好的标签，最大支持9个 |
x\{m\} or x\{m,\} or x\{m,n\} |
重复字符x，m次，至少m次，至少m且不超过n次 | o\{5,10\} |
o字符重复5到10次的行 |
| 元字符 | 说明 |
|---|---|
+ |
重复前一个字符一个或一个以上 |
？ |
0个或者一个字符 |
| |
表示或，查找多个字符串 |
() |
分组过滤匹配 |
$ grep '^love' re-file
love, how much I adore you. Do you know
$ grep 'love$' re-file
clover. Did you see them? I can only hope love.
l开头，中间包含两个字符，结尾是e的所有行$ 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
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.
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.
A-Z的字母，其次是ove$ grep '[A-Z]ove' re-file
Lovers were all around us. It is springtime. Oh
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.
love.$ 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.
o字符重复2到4次$ grep 'o\{2,4\}' re-file
groove.
o字符至少2次$ grep 'o\{2,\}' re-file
groove.
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.
$ 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