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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
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
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