正則表達式是一種字符模式，用於在查找過程中匹配製定的字符。

元字符通常在Linux中分爲兩類：

1. Shell元字符，由Linux Shell進行解析；

2. 正則表達式元字符，由vi/grep/sed/awk等文本處理工具進行解析；

正則表達式一般以文本行進行處理，在進行下面實例之前，先爲grep命令設置—color參數：

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

這樣每次過濾出來的字符串都會帶色彩了。

在開始之前還需要做一件事情，就是創建一個測試用的re-file文件，內容如下：

$ 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.

正則表達式元字符

• 特殊的元字符

• 擴展的正則表達式

實操

• 匹配以love開頭的所有行

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

• 匹配love結尾的所有行

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

• 匹配以l開頭，中間包含兩個字符，結尾是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個或多個空行，後面是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  # 對l不區分大小寫
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範圍內的任何字符行，所有的小寫字符

$ 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字符重複2到4次

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

• 重複o字符至少2次

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

• 重複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.

• 重複前一個字符一個或一個以

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

• 0個或者一個字符

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

• 或，查找多個字符串

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

• 分組過濾匹配

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

來源：公衆號 IT 釋然雜談

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