正則表達式入門

看了一點Ellie Quigley的《UNIX Shell 範例精解》，學了一點正則表達式入門的知識。總結一下：

 

正則表達式是一種字符模式，用於在查找過程中匹配相同的字符。在大多數程序裏，正則表達式都被擴在2個正斜槓“/”之間。

 

（1）正則表達式元字符

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元字符                      功能                                     示例                               解釋

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   ^                     行首定位符                           /^love/                        匹配以love開頭的行

   $                     行尾定位符                           /love$/                        匹配以love結尾的行

   .                      匹配一個字符                       /l..e/                            匹配包含l，後跟2個任意字符，再跟一個e的行

   *                     匹配0或任意個前一字符  /o*ve/                         匹配包含任意個o後跟ve的行

  [ ]                    匹配集合中任意一個         /[Ll]ove/                      匹配Love或者love

[x-y]                  匹配範圍中任意一個         /[a-z]/                           匹配任意一個小寫字符

 [^]                     匹配不在指定組內字符    /[^a-z]/                         匹配非小寫字母的一個字符

   \                      用於轉義字符                     /love\./                         匹配包含love後跟一個句號的行

（以下是多數使用正則表達式元字符的UNIX程序都支持的元字符）：

 \<                      詞首定位符                         /\<love/                       匹配以love開頭的詞的行(vi and grep)

 \>                      詞尾定位符                         /love\>/                       匹配以love結尾的詞的行

(vi and grep)

\(..\)                   匹配標籤                            /\(love\)\1er/                love模式被保存爲標籤1，此後\1就等價於love

x\{m\}               字符x的m次重複出現       a\{3\}  

x\{m,\}              字符x至少出現m次          a\{3,\}

x\{m,n\}            字符x至少出現m次，至多n次 a\{2,8\}  

 

（2）組合正則表達式元字符

元字符可以組合成爲更爲複雜一些的表達式，例如：

2.1 /^[A-Z]..$/

查找的行以大寫字母開頭，後跟2個任意字符，後跟換行符

2.2/^[A-Z][a-z]*3[0-8]/

查找的行以大寫字母開頭，後跟0或任意個小寫字母，後跟數字3，後跟一個0-8之間的一個數字

2.3/[a-z]*\./

查找的行包含0個或者多個小寫字母后跟一個句點

2.4/*[A-Z][a-z][a-z]$/

查找的行以若干空格開頭，後跟一個大寫字母2個小寫字母，後跟換行

2.5/^[A-Za-z]*[^,][A-Za-z]*$/

查找的行以0或多個大小寫字母開頭，後面不是句號，後跟0或多個大小寫字母，然後換行

 

（3）使用舉例

可以找到多個匹配項時，用n

3.1 關於^和[]

I had a lovely time on our little picnic.
Lovers were all around us.It is springtime.Oh
love,how much I adore you.Oh,buy 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.

~

~

~

/^[Ll]ove/

說明：匹配以love或者Love開頭

 

3.2 關於.

I had a lovely time on our little picnic.
Lovers were all around us.It is springtime.Oh
love,how much I adore you.Oh,buy 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.

~

~

~

/l.ve/

說明：匹配包含l後跟任意字符+ve的行

 

3.3 關於*

I had a lovely time on our little picnic.
Lovers were all around us.It is springtime.Oh
love,how much I adore you.Oh,buy 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*ve/

說明：*是與前面字符緊密聯繫的，如果是/*a/則是表示匹配包含任意個空格之後含a的行

 

3.4 關於[^]

I had a lovely time on our little picnic.
Lovers were all around us.It is springtime.Oh
love,how much I adore you.Oh,buy 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.

~

~

~

/ove[^a-zA-Z0-9]/

說明:空格可以被匹配到，但是換行符不能被匹配到。這裏是找ove後面跟一個非大消息字母及數字的行。

 

3.5 關於$和[]

I had a lovely time on our little picnic.
Lovers were all around us.It is springtime.Oh
love,how much I adore you.Oh,buy 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.

~

~

~

/lov[a-z]$/