基礎正規表示法

• 重要特殊字元(characters)

  RE字符         意義與範例
  ^word    待搜尋的字串(word)在行首。
           #範例：grep -n '^#' re.txt
           搜尋行首為 # 開始的那一行。
  word$    待搜尋的字串(word)在行尾。
           #範例：grep -n '!$' re.txt
           將行尾為 ! 的那一行列印出來。
    .      代表『任意一個』字符，一定是一個任意字符。
           #範例：grep -n 'e.e' re.txt
           搜尋的字串可以是 (eve) (eae) (eee) (e e)， 但不能僅有 (ee)。
           亦即 e 與 e 中間『一定』僅有一個字元，而空白字元也是字元。
    \      跳脫字符，將特殊符號的特殊意義去除。
           #範例：grep -n \' re.txt
           搜尋含有單引號 ' 的那一行。
    *      重複零個或多個的前一個 RE 字符
           #範例：grep -n 'ess*' re.txt
           找出含有 (es) (ess) (esss) 等等的字串。
  \{n,m\}  連續 n 到 m 個的『前一個 RE 字符』
           若為 \{n\} 則是連續 n 個的前一個 RE 字符，
           若是 \{n,\} 則是連續 n 個以上的前一個 RE 字符。
           #範例：grep -n 'go\{2,3\}g' re.txt
           在 g 與 g 之間有 2 個到 3 個的 o 存在的字串，亦即 (goog)(gooog)
    [ ]    在 [ ] 當中『謹代表一個待搜尋的字元』
           #範例：grep -n 'g[ld]' re.txt
           搜尋含有 (gl) 或 (gd) 的那一行
           #範例：grep -n '[0-9]' re.txt
           搜尋含有任意數字的那一行。
           在字元集合 [ ] 中的減號 - 是代表兩個字元之間的所有連續字元。
           [^]：^ 在 [ ] 內時， 代表的意義是『反向選擇』
           #範例：grep -n 'oo[^t]' re.txt
           搜尋的字串可以是 (oog) (ood) 但不能是 (oot)。
  

• 以 grep 擷取字串
  1. 編輯 re.txt 純文字檔

     [root@test root]# vi re.txt
      "Open Source" is a good mechanism to develop programs.
      apple is my favorite food.
      Football game is not use feet only.
      this dress doesn't fit me.
      However, this dress is about $ 3183 dollars. 
      GNU is free air not free beer. 
      Her hair is very beauty. 
      I can’t finish the test. 
      Oh! The soup taste good. 
      motorcycle is cheap than car.
      This window is clear.
      the symbol '*' is represented as start.
      Oh! My god!
      The gd software is a library for drafting programs. 
      You are the best is mean you are the no. 1.
      The world  is the same with "glad".
      I like dog.
      google is the best tools for search keyword.
      goooooogle yes!
      go! go! Let's go.
      # I am csie
     

  2. 搜尋特定字串：從檔案 re.txt 中取得特定字串 the

     [root@test root]# grep -n 'the' re.txt
     8:I can't finish the test.
     12:the symbol '*' is represented as start.
     15:You are the best is mean you are the no. 1.
     16:The world  is the same with "glad".
     18:google is the best tools for search keyword.
     

  3. 該行沒有字串 the 時，才顯示在螢幕上：

     [root@test root]# grep -vn 'the' re.txt
     # 螢幕上出現的行列為除了 8,12,15,16,18 五行之外的其他行列。
     

  4. 取得不論大小寫的字串 the：

     [root@test root]# grep -in 'the' re.txt
     8:I can't finish the test.
     9:Oh! The soup taste good.
     12:the symbol '*' is represented as start.
     14:The gd software is a library for drafting programs.
     15:You are the best is mean you are the no. 1.
     16:The world  is the same with "glad".
     18:google is the best tools for search keyword.
     

  5. 利用 [ ] 來搜尋集合字元

     [root@test root]# grep -n 't[ae]st' re.txt
     8:I can't finish the test.
     9:Oh! The soup taste good.
     # [ ] 裡面不論有幾個字元，都謹代表某『一個』字元。
     

  6. 搜尋有 oo 的字元：

     [root@test root]# grep -n 'oo' re.txt
     1:"Open Source" is a good mechanism to develop programs.
     2:apple is my favorite food.
     3:Football game is not use feet only.
     9:Oh! The soup taste good.
     18:google is the best tools for search keyword.
     19:goooooogle yes!
     

  7. 不想要 oo 前面有 g ，可以利用在集合字元的反向選擇 [^]

     [root@test root]# grep -n '[^g]oo' re.txt
     2:apple is my favorite food.
     3:Football game is not use feet only.
     18:google is the best tools for search keyword.
     19:goooooogle yes!
     

  8. oo 前面不想要有小寫字元：

     [root@test root]# grep -n '[^a-z]oo' re.txt
     3:Football game is not use feet only.
     

  9. 要求字串是數字與英文：[a-zA-Z0-9]

     [root@test root]# grep -n '[0-9]' re.txt
     5:However, this dress is about $ 3183 dollars.
     15:You are the best is mean you are the no. 1.
     

  10. 行首與行尾字元 ^ $：

      [root@test root]# grep -n '^the' re.txt
      12:the symbol '*' is represented as start.
      
      [root@test root]# grep -n '^[a-z]' re.txt
      2:apple is my favorite food.
      4:this dress doesn't fit me.
      10:motorcycle is cheap than car.
      12:the symbol '*' is represented as start.
      18:google is the best tools for search keyword.
      19:goooooogle yes!
      

  11. 不要開頭是英文字母：

      [root@test root]# grep -n '^[^a-zA-Z]' re.txt
      1:"Open Source" is a good mechanism to develop programs.
      20:# I am csie
      

  12. 行尾結束為小數點 (.) 的那一行：

      [root@test root]# grep -n '\.$' re.txt
      1:"Open Source" is a good mechanism to develop programs.
      2:apple is my favorite food.
      3:Football game is not use feet only.
      4:this dress doesn't fit me.
      10:motorcycle is cheap than car.
      11:This window is clear.
      12:the symbol '*' is represented as start.
      15:You are the best is mean you are the no. 1.
      16:The world  is the same with "glad".
      17:I like dog.
      18:google is the best tools for search keyword.
      # 小數點具有其他意義，必須要使用跳脫字元(\)來加以解除其特殊意義。
      # 第 5~9 行最後面也是 .？
      

  13. Windows 的 notepad 會主動加上 ^M 作為斷行的判斷。

      [root@test root]# cat -A re.txt
      However, this dress is about $ 3183 dollars.^M$
      

  14. 找出哪一行是『空白行』？

      [root@test root]# grep -n '^$' re.txt
      21:
      
      [root@test root]# cat /etc/syslog.conf
      [root@test root]# grep -v '^$' /etc/syslog.conf | grep -v '^#'
      

  15. 任意一個字元 . 與重複字元 *

      #『.』代表『絕對有一個任意字元』 
      [root@test root]# grep -n 'g..d' re.txt
      1:"Open Source" is a good mechanism to develop programs.
      9:Oh! The soup taste good.
      16:The world  is the same with "glad".
      # 『*』代表的是『重複 0 個或多個前面的 RE 字符』
      [root@test root]# grep -n 'ooo*' re.txt
      1:"Open Source" is a good mechanism to develop programs.
      2:apple is my favorite food.
      3:Football game is not use feet only.
      9:Oh! The soup taste good.
      18:google is the best tools for search keyword.
      19:goooooogle yes!
      

  16. 字串開頭與結尾都是 g，但是兩個 g 之間僅能存在至少一個 o

      # 亦即 gog, goog, gooog.... 等。 
      [root@test root]# grep -n 'goo*g' re.txt
      18:google is the best tools for search keyword.
      19:goooooogle yes!
      

  17. 找出 g 開頭與 g 結尾的字串，當中的字元可有可無。(錯誤找法)

      [root@test root]# grep -n 'g*g' re.txt
      1:"Open Source" is a good mechanism to develop programs.
      3:Football game is not use feet only.
      9:Oh! The soup taste good.
      13:Oh!  My god!
      14:The gd software is a library for drafting programs.
      16:The world  is the same with "glad".
      17:I like dog.
      18:google is the best tools for search keyword.
      19:goooooogle yes!
      # g*g 的內容是 g, gg, ggg, gggg。
      

  18. 找出 g 開頭與 g 結尾的字串，當中的字元可有可無。

      [root@test root]# grep -n 'g.*g' re.txt
      1:"Open Source" is a good mechanism to develop programs.
      14:The gd software is a library for drafting programs.
      18:google is the best tools for search keyword.
      19:goooooogle yes!
      

  19. 要找出『任意數字』的行列：

      [root@test root]# grep -n '[0-9][0-9]*' re.txt
      5:However, this dress is about $ 3183 dollars.
      15:You are the best is mean you are the no. 1.
      # 使用 grep -n '[0-9]' re.txt 也可以得到相同的結果。
      

  20. 限定連續 RE 字符範圍 {}：找到兩個 o 的字串。

      [root@test root]# grep -n 'o\{2\}' re.txt
      1:"Open Source" is a good mechanism to develop programs.
      2:apple is my favorite food.
      3:Football game is not use feet only.
      9:Oh! The soup taste good.
      18:google is the best tools for search keyword.
      19:goooooogle yes!
      

  21. 找出 g 後面接 2 到 5 個 o ，然後再接一個 g 的字串：

      [root@test root]# grep -n 'go\{2,5\}g' re.txt
      18:google is the best tools for search keyword.
      

  22. 找出 2 個 o 以上的 goooo....g？除了可以是 gooo*g ，也可以是：

      [root@test root]# grep -n 'go\{2,\}g' re.txt
      18:google is the best tools for search keyword.
      19:goooooogle yes!
      

  23. 找出 2 個 o 以上的 goooo....g，當找到一筆即停止。

      [root@test root]# grep -m 1 'go\{2,\}g' re.txt
      18:google is the best tools for search keyword.
      19:goooooogle yes!
      

• 『正規表示法的特殊字元』與『萬用字元』之差異：
  

  字元或代表意義	萬用字元	正規表示法的特殊字元
  *	0 至無限多個字元	重複 0 到多個的前一個 RE 字符
  反向選擇	[!range]	[^range]

  例題：不支援正規表示法的 ls 工具中
  『ls -l * 』：代表列出任意檔名的檔案；
  『ls -l a* 』：代表列出以 a 為開頭的任何檔名的檔案。
  在正規表示法中要找到含有以 a 為開頭的檔案，必須搭配支援正規表示法的工具 grep：
  ls | grep -n '^a.*'
  

• 例題：以正規表示法顯示行號方式找出目錄 /etc 以下符合下列條件之檔案。
  1. 包含 boot 或 root 字串的檔案。
  2. 包含 b 開頭 t 結尾之字串的檔案。
  3. b 開頭 t 結尾且中間 1 個 o 以上之字串的檔案。
  4. 包含 b 開頭 t 結尾且中間 2 至 4 個 o 之字串的檔案。
  5. 包含行首為 root 之字串的檔案。
  6. 包含行尾為 root 之字串的檔案。
• 例題：想要查出來檔案中含有 ! 與 > 的字行( ! 在正規表示法中並不是特殊字元)：

  grep -n '[!>]' re.txt