shell腳本之正則表達式（一）

時間 2019-11-05

原文原文鏈接

正則表達式概述

基礎正則表達式

擴展正則表達式

正則表達式概述

1. 正則表達式的定義html

正則表達式又稱正規表達式、常規表達式。在代碼中常簡寫爲 regex、regexp或 RE；正則表達式是使用單個字符串來描述、匹配一系列符合某個句法規則的字符串簡單來講，是一種匹配字符串的方法，經過一些特殊符號，實現快速查找、刪除、替換某個特定字符串。web

正則表達式是由普通字符與元字符組成的文字模式，普通字符包括大小寫字母、數字、標點符號及一些其餘符號，元字符則是指那些在正則表達式中具備特殊意義的專用字符，能夠用來規定其前導字符（即位於元字符前面的字符）在目標對象中的出現模式。正則表達式

正則表達式通常用於腳本編程與文本編輯器中。apache

2.正則表達式用途編程

正則表達式對於系統管理員來講是很是重要的，系統運行過程當中會產生大量的信息，這些信息有些是很是重要的，有些則僅是告知的信息。身爲系統管理員若是直接看這麼多的信息數據，沒法快速定位到重要的信息，如「用戶帳號登陸失敗」「服務啓動失敗」等信息。這時能夠經過正則表達式快速提取「有問題」的信息。如此一來，能夠將運維工做變得更加簡單、方便。vim

基礎正則表達式

正則表達式的字符串表達方法根據不一樣的嚴謹程度與功能分爲基本正則表達式與擴展正則表達式。基礎正則表達式是經常使用的正則表達式的最基礎的部分。在 Linux 系統中常見的文件處理工具中grep 與 sed 支持基礎正則表達式，掌握基礎正則表達式的使用方法，首先必須瞭解基本正則表達式所包含的元字符的含義，下面經過 grep 命令以舉例的方式逐個介紹。app

1.基礎正則表達式示例

下面的操做我這邊複製一份httpd配置文件做爲測試使用。運維

[root@localhost ~]# cp /etc/httpd/conf/httpd.conf  /opt/httpd.txt
[root@localhost ~]# cd /opt
[root@localhost opt]# ls
httpd.txt  rh
[root@localhost opt]# cat httpd.txt 
#
# This is the main Apache HTTP server configuration file.  It contains the
# configuration directives that give the server its instructions.
# See <URL:http://httpd.apache.org/docs/2.4/> for detailed information.
# In particular, see 
# <URL:http://httpd.apache.org/docs/2.4/mod/directives.html>
# for a discussion of each configuration directive.
#
# Do NOT simply read the instructions in here without understanding
# what they do.  They're here only as hints or reminders.  If you are unsure
# consult the online docs. You have been warned.  
#
# Configuration and logfile names: If the filenames you specify for many
# of the server's control files begin with "/" (or "drive:/" for Win32), the
# server will use that explicit path.  If the filenames do *not* begin
# with "/", the value of ServerRoot is prepended -- so 'log/access_log'
# with ServerRoot set to '/www' will be interpreted by the
# server as '/www/log/access_log', where as '/log/access_log' will be
# interpreted as '/log/access_log'.

#
# ServerRoot: The top of the directory tree under which the server's
# configuration, error, and log files are kept.
#
# Do not add a slash at the end of the directory path.  If you point
...//省略部份內容...

1）查找特定字符

使用grep命令查找特定字符，其中「-n」表示顯示行號、「-i」表示不區分大小寫編輯器

[root@localhost opt]# grep -n "the" httpd.txt 
2:# This is the main Apache HTTP server configuration file.  It contains the
3:# configuration directives that give the server its instructions.
9:# Do NOT simply read the instructions in here without understanding
10:# what they do.  They're here only as hints or reminders.  If you are unsure
11:# consult the online docs. You have been warned.  
13:# Configuration and logfile names: If the filenames you specify for many
14:# of the server's control files begin with "/" (or "drive:/" for Win32), the
15:# server will use that explicit path.  If the filenames do *not* begin
16:# with "/", the value of ServerRoot is prepended -- so 'log/access_log'
17:# with ServerRoot set to '/www' will be interpreted by the
22:# ServerRoot: The top of the directory tree under which the server's
25:# Do not add a slash at the end of the directory path.  If you point
26:# ServerRoot at a non-local disk, be sure to specify a local disk on the
27:# Mutex directive, if file-based mutexes are used.  If you wish to share the
35:# ports, instead of the default. See also the <VirtualHost>
47:# To be able to use the functionality of a module which was built as a DSO you
48:# have to place corresponding `LoadModule' lines at this location so the
49:# directives contained in it are actually available _before_ they are used.
62:# User/Group: The name (or #number) of the user/group to run httpd as.
71:# The directives in this section set up the values used by the 'main'
74:# any <VirtualHost> containers you may define later in the file.
76:# All of these directives may appear inside <VirtualHost> containers,
77:# in which case these default settings will be overridden for the
...//省略部份內容...

[root@localhost opt]# grep -ni "the" httpd.txt 
2:# This is the main Apache HTTP server configuration file.  It contains the
3:# configuration directives that give the server its instructions.
9:# Do NOT simply read the instructions in here without understanding
10:# what they do.  They're here only as hints or reminders.  If you are unsure
11:# consult the online docs. You have been warned.  
13:# Configuration and logfile names: If the filenames you specify for many
14:# of the server's control files begin with "/" (or "drive:/" for Win32), the
15:# server will use that explicit path.  If the filenames do *not* begin
16:# with "/", the value of ServerRoot is prepended -- so 'log/access_log'
17:# with ServerRoot set to '/www' will be interpreted by the
22:# ServerRoot: The top of the directory tree under which the server's
25:# Do not add a slash at the end of the directory path.  If you point
26:# ServerRoot at a non-local disk, be sure to specify a local disk on the
27:# Mutex directive, if file-based mutexes are used.  If you wish to share the
35:# ports, instead of the default. See also the <VirtualHost>
47:# To be able to use the functionality of a module which was built as a DSO you
48:# have to place corresponding `LoadModule' lines at this location so the
49:# directives contained in it are actually available _before_ they are used.
62:# User/Group: The name (or #number) of the user/group to run httpd as.
71:# The directives in this section set up the values used by the 'main'
73:# <VirtualHost> definition.  These values also provide defaults for
74:# any <VirtualHost> containers you may define later in the file.
76:# All of these directives may appear inside <VirtualHost> containers,
77:# in which case these default settings will be overridden for the
82:# ServerAdmin: Your address, where problems with the server should be
89:# ServerName gives the name and port that the server uses to identify itself.
98:# Deny access to the entirety of your server's filesystem. You must
99:# explicitly permit access to web content directories in other
...//省略部份內容...

若反向選擇，如查找不包含「the」字符的行，則須要經過 grep 命令的「-vn」選項實現。ide

[root@localhost opt]# grep -nv "the" httpd.txt 
1:#
4:# See <URL:http://httpd.apache.org/docs/2.4/> for detailed information.
5:# In particular, see 
6:# <URL:http://httpd.apache.org/docs/2.4/mod/directives.html>
7:# for a discussion of each configuration directive.
8:#
12:#
18:# server as '/www/log/access_log', where as '/log/access_log' will be
19:# interpreted as '/log/access_log'.
20:
21:#
23:# configuration, error, and log files are kept.
24:#
28:# same ServerRoot for multiple httpd daemons, you will need to change at
29:# least PidFile.
30:#
31:ServerRoot "/etc/httpd"
32:
33:#
34:# Listen: Allows you to bind Apache to specific IP addresses and/or
36:# directive.
37:#
38:# Change this to Listen on specific IP addresses as shown below to 
39:# prevent Apache from glomming onto all bound IP addresses.
40:#
41:#Listen 12.34.56.78:80
42:Listen 80
43:
44:#
45:# Dynamic Shared Object (DSO) Support
46:#
50:# Statically compiled modules (those listed by `httpd -l') do not need
51:# to be loaded here
...//省略部份內容...

2）利用中括號「[]」來查找集合字符

在httpd.txt測試文件中添加字符串shirt、short、wd、wod、wood、woooood。想要查找「shirt」與「short」這兩個字符串時，能夠發現這兩個字符串均包含「sh」 與「rt」。此時執行如下命令便可同時查找到「shirt」與「short」這兩個字符串。「[]」中不管有幾個字符，都僅表明一個字符，也就是說「[io]」表示匹配「i」或者「o」。

[root@localhost opt]# vim httpd.txt 
...//省略部份內容...
# Supplemental configuration
#
# Load config files in the "/etc/httpd/conf.d" directory, if any.
IncludeOptional conf.d/*.conf
shirt
short
wd                                                                          
wod                                                                          
wood                                                                            
woooood                                                                          
:wq

[root@localhost opt]# grep -n 'sh[io]rt' httpd.txt 
354:shirt
355:short

若要查找包含重複單個字符「oo」時，只須要執行如下命令便可。

[root@localhost opt]# grep -n 'oo' httpd.txt 
16:# with "/", the value of ServerRoot is prepended -- so 'log/access_log'
17:# with ServerRoot set to '/www' will be interpreted by the
22:# ServerRoot: The top of the directory tree under which the server's
26:# ServerRoot at a non-local disk, be sure to specify a local disk on the
28:# same ServerRoot for multiple httpd daemons, you will need to change at
31:ServerRoot "/etc/httpd"
54:# LoadModule foo_module modules/mod_foo.so
60:# httpd as root initially and it will switch.  
63:# It is usually good practice to create a dedicated user and group for
86:ServerAdmin root@localhost
115:# DocumentRoot: The directory out of which you will serve your
119:DocumentRoot "/var/www/html"
130:# Further relax access to the default document root:
226:    # Redirect permanent /foo http://www.example.com/bar
230:    # access content that does not live under the DocumentRoot.
332:#ErrorDocument 500 "The server made a boo boo."
358:wood
359:woooood

若查找「oo」前面不是「w」的字符串，只須要經過集合字符的反向選擇「[^]」來實現該目的

[root@localhost opt]# grep -n '[^w]oo' httpd.txt 
16:# with "/", the value of ServerRoot is prepended -- so 'log/access_log'
17:# with ServerRoot set to '/www' will be interpreted by the
22:# ServerRoot: The top of the directory tree under which the server's
26:# ServerRoot at a non-local disk, be sure to specify a local disk on the
28:# same ServerRoot for multiple httpd daemons, you will need to change at
31:ServerRoot "/etc/httpd"
54:# LoadModule foo_module modules/mod_foo.so
60:# httpd as root initially and it will switch.  
63:# It is usually good practice to create a dedicated user and group for
86:ServerAdmin root@localhost
115:# DocumentRoot: The directory out of which you will serve your
119:DocumentRoot "/var/www/html"
130:# Further relax access to the default document root:
226:    # Redirect permanent /foo http://www.example.com/bar
230:    # access content that does not live under the DocumentRoot.
332:#ErrorDocument 500 "The server made a boo boo."
359:woooood

在上述命令的執行結果中發現「woooood」也符合匹配規則，上述結果中能夠得知，「oo」前面的「o」是符合匹配規則的。若不但願「oo」前面存在小寫字母，可使用「grep –n‘[^a-z]oo’httpd.txt」命令實現，其中「a-z」表示小寫字母，大寫字母則經過「A-Z」表示。

[root@localhost opt]# grep -n '[^a-z]oo' httpd.txt 
16:# with "/", the value of ServerRoot is prepended -- so 'log/access_log'
17:# with ServerRoot set to '/www' will be interpreted by the
22:# ServerRoot: The top of the directory tree under which the server's
26:# ServerRoot at a non-local disk, be sure to specify a local disk on the
28:# same ServerRoot for multiple httpd daemons, you will need to change at
31:ServerRoot "/etc/httpd"
115:# DocumentRoot: The directory out of which you will serve your
119:DocumentRoot "/var/www/html"
230:    # access content that does not live under the DocumentRoot.

查找包含數字的行能夠經過「grep –n‘[0-9]’ httpd.txt」命令來實現.

[root@localhost opt]# grep -n '[0-9]' httpd.txt 
4:# See <URL:http://httpd.apache.org/docs/2.4/> for detailed information.
6:# <URL:http://httpd.apache.org/docs/2.4/mod/directives.html>
14:# of the server's control files begin with "/" (or "drive:/" for Win32), the
41:#Listen 12.34.56.78:80
42:Listen 80
95:#ServerName www.example.com:80
141:    # http://httpd.apache.org/docs/2.4/mod/core.html#options
311:# interpretation of all content as UTF-8 by default.  To use the 
312:# default browser choice (ISO-8859-1), or to allow the META tags
316:AddDefaultCharset UTF-8
329:# 1) plain text 2) local redirects 3) external redirects
332:#ErrorDocument 500 "The server made a boo boo."
333:#ErrorDocument 404 /missing.html
334:#ErrorDocument 404 "/cgi-bin/missing_handler.pl"
335:#ErrorDocument 402 http://www.example.com/subscription_info.html

3）查找行首「^」與行尾字符「$」

基礎正則表達式包含兩個定位元字符：「^」（行首）與「$」（行尾）。若想查找 「Ser」字符串爲行首的行，則能夠經過「^」元字符來實現。

[root@localhost opt]# grep -n '^Ser' httpd.txt 
31:ServerRoot "/etc/httpd"
86:ServerAdmin root@localhost

查詢以小寫字母開頭的行能夠經過「^[a-z]」規則來過濾，查詢大寫字母開頭的行則使用「^[A-Z]」規則，若查詢不以字母開頭的行則使用「^[^a-zA-Z]」規則。「^」符號在元字符集合「[]」符號內外的做用是不同的，在「[]」符號內表示反向選擇，在「[]」符號外則表明定位行首。

[root@localhost opt]# grep -n '^[a-z]' httpd.txt 
354:shirt
355:short
356:wd
357:wod
358:wood
359:woooood

[root@localhost opt]# grep -n '^[A-Z]' httpd.txt 
31:ServerRoot "/etc/httpd"
42:Listen 80
56:Include conf.modules.d/*.conf
66:User apache
67:Group apache
86:ServerAdmin root@localhost
119:DocumentRoot "/var/www/html"
182:ErrorLog "logs/error_log"
189:LogLevel warn
316:AddDefaultCharset UTF-8
348:EnableSendfile on
353:IncludeOptional conf.d/*.conf

[root@localhost opt]# grep -n '^[^a-zA-Z]' httpd.txt 
1:#
2:# This is the main Apache HTTP server configuration file.  It contains the
3:# configuration directives that give the server its instructions.
4:# See <URL:http://httpd.apache.org/docs/2.4/> for detailed information.
5:# In particular, see 
6:# <URL:http://httpd.apache.org/docs/2.4/mod/directives.html>
7:# for a discussion of each configuration directive.
8:#
9:# Do NOT simply read the instructions in here without understanding
10:# what they do.  They're here only as hints or reminders.  If you are unsure
11:# consult the online docs. You have been warned.  
...//省略部份內容...

若想查找以某一特定字符結尾的行則可使用「$」定位符。例如，執行如下命令便可實現查詢以小數點(.)結尾的行。由於小數點（.） 在正則表達式中也是一個元字符，因此在這裏須要用轉義字符「\」將具備特殊意義的字符轉化成普通字符。

...//省略部份內容...[root@localhost opt]# grep -n '\.$' httpd.txt 
3:# configuration directives that give the server its instructions.
4:# See <URL:http://httpd.apache.org/docs/2.4/> for detailed information.
7:# for a discussion of each configuration directive.
19:# interpreted as '/log/access_log'.
23:# configuration, error, and log files are kept.
29:# least PidFile.
36:# directive.
...//省略部份內容...

當查詢空白行時，執行「grep –n ‘^$’ httpd.txt」命令便可。

[root@localhost opt]# grep -n '^$' httpd.txt 
20:
32:
43:
57:
68:
80:
87:
96:
...//省略部份內容...

4）查找任意一個字符「.」與重複字符「*」

在正則表達式中小數點（.）也是一個元字符，表明任意一個字符。例如，執行如下命令就能夠查找「w??d」的字符串，即共有四個字符，以w 開頭 d 結尾。

[root@localhost opt]# grep -n 'w..d' httpd.txt 
108:# Note that from this point forward you must specifically allow
148:    # It can be "All", "None", or any combination of the keywords:
358:wood

在上述結果中，「wood」字符串「w..d」匹配規則。若想要查詢o、oo、ooooo 等資料，則須要使用星號（*）元字符。但須要注意的是，「*」表明的是重複零個或多個前面的單字符。「o*」表示擁有零個（即爲空字符）或大於等於一個「o」的字符，由於容許空字符，因此執行「grep –n‘o*’ httpd.txt」命令會將文本中全部的內容都輸出打印。若是是「oo*」，則第一個 o 必須存在，第二個 o 則是零個或多個 o，因此凡是包含 o、oo、ooooo，等的資料都符合標準。同理，若查詢包含至少兩個 o 以上的字符串，則執行「grep –n‘ooo*’ httpd.txt」命令便可。

[root@localhost opt]# grep -n 'o*' httpd.txt 
...//省略部份內容...
353:IncludeOptional conf.d/*.conf
354:shirt
355:short
356:wd
357:wod
358:wood
359:woooood

[root@localhost opt]# grep -n 'oo*' httpd.txt
...//省略部份內容...
353:IncludeOptional conf.d/*.conf
355:short
357:wod
358:wood
359:woooood

[root@localhost opt]# grep -n 'ooo*' httpd.txt 
16:# with "/", the value of ServerRoot is prepended -- so 'log/access_log'
17:# with ServerRoot set to '/www' will be interpreted by the
22:# ServerRoot: The top of the directory tree under which the server's
26:# ServerRoot at a non-local disk, be sure to specify a local disk on the
28:# same ServerRoot for multiple httpd daemons, you will need to change at
31:ServerRoot "/etc/httpd"
54:# LoadModule foo_module modules/mod_foo.so
60:# httpd as root initially and it will switch.  
63:# It is usually good practice to create a dedicated user and group for
86:ServerAdmin root@localhost
115:# DocumentRoot: The directory out of which you will serve your
119:DocumentRoot "/var/www/html"
130:# Further relax access to the default document root:
226:    # Redirect permanent /foo http://www.example.com/bar
230:    # access content that does not live under the DocumentRoot.
332:#ErrorDocument 500 "The server made a boo boo."
358:wood
359:woooood

查詢以 w 開頭 d 結尾，中間包含至少一個 o 的字符串，執行如下命令便可實現。

[root@localhost opt]# grep -n 'woo*d' httpd.txt 
357:wod
358:wood
359:woooood

查詢以 w 開頭 d 結尾，中間的字符無關緊要的字符串。

[root@localhost opt]# grep -n 'w.*d' httpd.txt 
...//省略部份內容...
342:# be turned off when serving from networked-mounted 
356:wd
357:wod
358:wood
359:woooood

查詢任意數字所在行

[root@localhost opt]# grep '[0-9][0-9]*' httpd.txt 
# See <URL:http://httpd.apache.org/docs/2.4/> for detailed information.
# <URL:http://httpd.apache.org/docs/2.4/mod/directives.html>
# of the server's control files begin with "/" (or "drive:/" for Win32), the
#Listen 12.34.56.78:80
Listen 80
#ServerName www.example.com:80
    # http://httpd.apache.org/docs/2.4/mod/core.html#options
# interpretation of all content as UTF-8 by default.  To use the 
# default browser choice (ISO-8859-1), or to allow the META tags
AddDefaultCharset UTF-8
# 1) plain text 2) local redirects 3) external redirects
#ErrorDocument 500 "The server made a boo boo."
#ErrorDocument 404 /missing.html
#ErrorDocument 404 "/cgi-bin/missing_handler.pl"
#ErrorDocument 402 http://www.example.com/subscription_info.html

5）查找連續字符範圍「{}」

在上面的示例中，咱們使用「.」與「*」來設定零個到無限多個重複的字符，若是想要限制一個範圍內的重複的字符串，這個時候就須要使用基礎正則表達式中的限定範圍的字符「{}」，由於「{}」在 Shell 中具備特殊意義，因此在使用「{}」字符時，須要利用轉義字符「\」，將「{}」字符轉換成普通字符。

(1）查詢兩個 o 的字符。

[root@localhost opt]# grep -n 'o\{2\}' httpd.txt 
16:# with "/", the value of ServerRoot is prepended -- so 'log/access_log'
17:# with ServerRoot set to '/www' will be interpreted by the
22:# ServerRoot: The top of the directory tree under which the server's
26:# ServerRoot at a non-local disk, be sure to specify a local disk on the
28:# same ServerRoot for multiple httpd daemons, you will need to change at
31:ServerRoot "/etc/httpd"
54:# LoadModule foo_module modules/mod_foo.so
60:# httpd as root initially and it will switch.  
63:# It is usually good practice to create a dedicated user and group for
86:ServerAdmin root@localhost
115:# DocumentRoot: The directory out of which you will serve your
119:DocumentRoot "/var/www/html"
130:# Further relax access to the default document root:
226:    # Redirect permanent /foo http://www.example.com/bar
230:    # access content that does not live under the DocumentRoot.
332:#ErrorDocument 500 "The server made a boo boo."
358:wood
359:woooood

(2）查詢以 w 開頭以 d 結尾，中間包含 2～5 個 o 的字符串。

[root@localhost opt]# grep -n 'o\{2,5\}' httpd.txt 
16:# with "/", the value of ServerRoot is prepended -- so 'log/access_log'
17:# with ServerRoot set to '/www' will be interpreted by the
22:# ServerRoot: The top of the directory tree under which the server's
26:# ServerRoot at a non-local disk, be sure to specify a local disk on the
28:# same ServerRoot for multiple httpd daemons, you will need to change at
31:ServerRoot "/etc/httpd"
54:# LoadModule foo_module modules/mod_foo.so
60:# httpd as root initially and it will switch.  
63:# It is usually good practice to create a dedicated user and group for
86:ServerAdmin root@localhost
115:# DocumentRoot: The directory out of which you will serve your
119:DocumentRoot "/var/www/html"
130:# Further relax access to the default document root:
226:    # Redirect permanent /foo http://www.example.com/bar
230:    # access content that does not live under the DocumentRoot.
332:#ErrorDocument 500 "The server made a boo boo."
358:wood
359:woooood

(3）查詢以 w 開頭以 d 結尾，中間包含 2 以上 o 的字符串。

[root@localhost opt]# grep -n 'wo\{2\}' httpd.txt 
358:wood
359:woooood

2. 元字符總結

元字符	做用
`^`	匹配輸入字符串的開始位置。除非在方括號表達式中使用，表示不包含該字符集合。要匹配`「^」`字符自己，請使用`「\^」`
`$`	匹配輸入字符串的結尾位置。若是設置了 `RegExp` 對象的 `Multiline` 屬性，則`「$」`也匹配`‘\n’`或`‘\r’`。要匹配`「$」`字符自己，請使用`「\$」`
`.`	匹配除`「\r\n」`以外的任何單個字符
`\`	將下一個字符標記爲特殊字符、原義字符、向後引用、八進制轉義符。例如，`‘n’`匹配字符`「n」`。 `‘\n’`匹配換行符。序列`‘\\’`匹配`「\」`，而`‘\(’`則匹配`「(」`
`*`	匹配前面的子表達式零次或屢次。要匹配`「」`字符，請使用`「\」`
`[]`	字符集合。匹配所包含的任意一個字符。例如，`「[abc]」`能夠匹配`「plain」`中的`「a」`
`[^]`	賦值字符集合。匹配未包含的一個任意字符。例如，`「[^abc]」`能夠匹配`「plain」`中`「plin」`中的任何一個字母
`[n1-n2]`	字符範圍。匹配指定範圍內的任意一個字符。例如，`「[a-z]」`能夠匹配`「a」`到`「z」`範圍內的任意一個小寫字母字符。注意：只有連字符`（-）`在字符組內部，而且出如今兩個字符之間時，才能表示字符的範圍；若是出如今字符組的開頭，則只能表示連字符自己
`{n}`	`n` 是一個非負整數，匹配肯定的`n` 次。例如，`「o{2}」`不能匹配`「Bob」`中的`「o」`，可是能匹配`「food」`中的兩個`o`
`{n,}`	`n` 是一個非負整數，至少匹配`n` 次。例如，`「o{2,}」`不能匹配`「Bob」`中的`「o」`，但能匹配`「foooood」`中的全部 `o`。`「o{1,}」`等價於`「o+」`。`「o{0,}」`則等價於`「o*」`
`{n,m}`	`m` 和`n` 均爲非負整數，其中 `n<=m`，最少匹配 `n` 次且最多匹配 `m` 次

擴展正則表達式

一般狀況下會使用基礎正則表達式就已經足夠了，但有時爲了簡化整個指令，須要使用範圍更廣的擴展正則表達式，例如，使用基礎正則表達式查詢除文件中空白行與行首爲「#」以外的行（一般用於查看生效的配置文件），執行「grep –v ‘^$’ httpd.txt | grep –v ‘^#’」便可實現。這裏須要使用管道命令來搜索兩次。若是使用擴展正則表達式，能夠簡化爲「egrep –v ‘^$|^#’ httpd.txt」，其中，單引號內的管道符號表示或者（or）。

[root@localhost opt]# grep -v '^$' httpd.txt | grep -v "^#"
ServerRoot "/etc/httpd"
Listen 80
Include conf.modules.d/*.conf
User apache
Group apache
ServerAdmin root@localhost
<Directory />
    AllowOverride none
    Require all denied
</Directory>
...//省略部份內容...

[root@localhost opt]# egrep -v '^$|^#' httpd.txt 
ServerRoot "/etc/httpd"
Listen 80
Include conf.modules.d/*.conf
User apache
Group apache
ServerAdmin root@localhost
<Directory />
    AllowOverride none
    Require all denied
</Directory>
DocumentRoot "/var/www/html"
<Directory "/var/www">
    AllowOverride None
    # Allow open access:
    Require all granted
</Directory>
...//省略部份內容...

grep 命令僅支持基礎正則表達式，若是使用擴展正則表達式，須要使用 egrep 或 awk 命令。awk 命令在後面進行講解，這裏咱們直接使用 egrep 命令。egrep命令與 grep 命令的用法基本類似。egrep 命令是一個搜索文件得到模式，使用該命令能夠搜索文件中的任意字符串和符號，也能夠搜索一個或多個文件的字符串，一個提示符能夠是單個字符、一個字符串、一個字或一個句子。

與基礎正則表達式類型相同，擴展正則表達式也包含多個元字符，常見的擴展正則表達式的元字符主要包括如下幾個：

元字符	做用
`+`	重複一個或者一個以上的前一個字符
`？`	零個或者一個的前一個字符
`\|`	使用或者（or）的方式找出多個字符
`()`	查找「組」字符串
`()+`	辨別多個重複的組

示例

執行「egrep -n 'wo+d' httpd.txt」命令，便可查詢"wod、wood、woooood"等字符串。

[root@localhost opt]# egrep -n 'wo+d' httpd.txt 
357:wod
358:wood
359:woooood

執行「egrep -n 'wo?d' httpd.txt」命令，便可查詢「wd」「wod」這兩個字符串。

[root@localhost opt]# egrep -n 'wo?d' httpd.txt 
168:# The following lines prevent .htaccess and .htpasswd files from being 
356:wd
357:wod

執行「egrep -n 'of|is|on' httpd.txt」命令便可查詢"of"或者"if"或者"on"字符串。

[root@localhost opt]# egrep -n 'if|is|on' httpd.txt 
2:# This is the main Apache HTTP server configuration file.  It contains the
3:# configuration directives that give the server its instructions.
4:# See <URL:http://httpd.apache.org/docs/2.4/> for detailed information.
7:# for a discussion of each configuration directive.
9:# Do NOT simply read the instructions in here without understanding
10:# what they do.  They're here only as hints or reminders.  If you are unsure
11:# consult the online docs. You have been warned.  
13:# Configuration and logfile names: If the filenames you specify for many
14:# of the server's control files begin with "/" (or "drive:/" for Win32), the
16:# with "/", the value of ServerRoot is prepended -- so 'log/access_log'
23:# configuration, error, and log files are kept.
26:# ServerRoot at a non-local disk, be sure to specify a local disk on the
27:# Mutex directive, if file-based mutexes are used.  If you wish to share the
...//省略部份內容...

「egrep -n 'sh(i|o)rt' httpd.txt」。`「shirt」與「short」由於這兩個單詞的「sh」與「rt」是重複的，因此將「i」與「o"列於「()」符號當中，並以「|」分隔，便可查詢"shirt"或者"short"字符串。

[root@localhost opt]# egrep -n 'sh(i|o)rt' httpd.txt 
354:shirt
355:short

「egrep -n 'A(xyz)+C' httpd.txt」。該命令是查詢開頭的"A"結尾是"C"，中間有一個以上的"xyz"字符串的意思，在httpd.txt文件中添加字符串AxyzC、AxyzxyzC。

[root@localhost opt]# vim httpd.txt 
...//省略部份內容...
woooood
AxyzC
AxyzxyzC
~                                                                             
~                                                                             
:wq     
[root@localhost opt]# egrep -n 'A(xyz)+C' httpd.txt 
360:AxyzC
361:AxyzxyzC