1. 正則表達(dá)式的定義

正則表達(dá)式又稱正規(guī)表達(dá)式、常規(guī)表達(dá)式。在代碼中常簡寫為 regex、regexp或 RE；正則表達(dá)式是使用單個(gè)字符串來描述、匹配一系列符合某個(gè)句法規(guī)則的字符串簡單來說， 是一種匹配字符串的方法，通過一些特殊符號(hào)，實(shí)現(xiàn)快速查找、刪除、替換某個(gè)特定字符串。

正則表達(dá)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(shù)字、標(biāo)點(diǎn)符號(hào)及一些其他符號(hào)，元字符則是指那些在正則表達(dá)式中具有特殊意義的專用字符，可以用來規(guī)定其前導(dǎo)字符（即位于元字符前面的字符）在目標(biāo)對象中的出現(xiàn)模式。

正則表達(dá)式一般用于腳本編程與文本編輯器中。

2.正則表達(dá)式用途

正則表達(dá)式對于系統(tǒng)管理員來說是非常重要的，系統(tǒng)運(yùn)行過程中會(huì)產(chǎn)生大量的信息，這些信息有些是非常重要的，有些則僅是告知的信息。身為系統(tǒng)管理員如果直接看這么多的信息數(shù)據(jù)，無法快速定位到重要的信息，如“用戶賬號(hào)登錄失敗”“服務(wù)啟動(dòng)失敗”等信息。這時(shí)可以通過正則表達(dá)式快速提取“有問題”的信息。如此一來，可以將運(yùn)維工作變得更加簡單、方便。

基礎(chǔ)正則表達(dá)式

正則表達(dá)式的字符串表達(dá)方法根據(jù)不同的嚴(yán)謹(jǐn)程度與功能分為基本正則表達(dá)式與擴(kuò)展正則表達(dá)式?；A(chǔ)正則表達(dá)式是常用的正則表達(dá)式的最基礎(chǔ)的部分。在 Linux 系統(tǒng)中常見的文件處理工具中grep 與 sed 支持基礎(chǔ)正則表達(dá)式，掌握基礎(chǔ)正則表達(dá)式的使用方法，首先必須了解基本正則表達(dá)式所包含的元字符的含義，下面通過 grep 命令以舉例的方式逐個(gè)介紹。

1.基礎(chǔ)正則表達(dá)式示例

下面的操作我這邊復(fù)制一份httpd配置文件作為測試使用。

[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
...//省略部分內(nèi)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(hào)、“-i”表示不區(qū)分大小寫

[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
...//省略部分內(nèi)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
...//省略部分內(nèi)容...

若反向選擇，如查找不包含“the”字符的行，則需要通過 grep 命令的“-vn”選項(xiàng)實(shí)現(xiàn)。

[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
...//省略部分內(nèi)容...

2） 利用中括號(hào)“[]”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(gè)字符串時(shí)，可以發(fā)現(xiàn)這兩個(gè)字符串均包含“sh” 與“rt”。此時(shí)執(zhí)行以下命令即可同時(shí)查找到“shirt”與“short”這兩個(gè)字符串。“[]”中無論有幾個(gè)字符，都僅代表一個(gè)字符，也就是說“[io]”表示匹配“i”或者“o”。

[root@localhost opt]# vim httpd.txt 
...//省略部分內(nèi)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

若要查找包含重復(fù)單個(gè)字符“oo”時(shí)，只需要執(zhí)行以下命令即可。

[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，只需要通過集合字符的反向選擇“[^]”來實(shí)現(xiàn)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(zhí)行結(jié)果中發(fā)現(xiàn)“woooood”也符合匹配規(guī)則，上述結(jié)果中可以得知，“oo”前面的“o”是符合匹配規(guī)則的。若不希望“oo”前面存在小寫字母，可以使用“grep –n‘[^a-z]oo’httpd.txt”命令實(shí)現(xiàn)，其中“a-z”表示小寫字母，大寫字母則通過“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(shù)字的行可以通過“grep –n‘[0-9]’ httpd.txt”命令來實(shí)現(xiàn).

[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(chǔ)正則表達(dá)式包含兩個(gè)定位元字符：“^”（行首）與“$”（行尾）。若想查找 “Ser”字符串為行首的行，則可以通過“^”元字符來實(shí)現(xiàn)。

[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可以通過“^[a-z]”規(guī)則來過濾，查詢大寫字母開頭的行則使用“^[A-Z]”規(guī)則，若查詢不以字母開頭的行則使用“^[^a-zA-Z]”規(guī)則。“^”符號(hào)在元字符集合“[]”符號(hào)內(nèi)外的作用是不一樣的，在“[]”符號(hào)內(nèi)表示反向選擇，在“[]”符號(hào)外則代表定位行首。

[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.  
...//省略部分內(nèi)容...

若想查找以某一特定字符結(jié)尾的行則可以使用“$”定位符。例如，執(zhí)行以下命令即可實(shí)現(xiàn)查詢以小數(shù)點(diǎn)(.)結(jié)尾的行。因?yàn)樾?shù)點(diǎn)（.） 在正則表達(dá)式中也是一個(gè)元字符，所以在這里需要用轉(zhuǎn)義字符“\\\\”將具有特 殊意義的字符轉(zhuǎn)化成普通字符。

...//省略部分內(nèi)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.
...//省略部分內(nèi)容...

當(dāng)查詢空白行時(shí)，執(zhí)行“grep –n ‘^$’ httpd.txt”命令即可。

[root@localhost opt]# grep -n \\\'^$\\\' httpd.txt 
20:
32:
43:
57:
68:
80:
87:
96:
...//省略部分內(nèi)容...

4） 查找任意一個(gè)字符“.”與重復(fù)字符“*”

在正則表達(dá)式中小數(shù)點(diǎn)（.）也是一個(gè)元字符，代表任意一個(gè)字符。例如， 執(zhí)行以下命令就可以查找“wd”的字符串，即共有四個(gè)字符，以w 開頭 d 結(jié)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(jié)果中，“wood”字符串“w..d”匹配規(guī)則。若想要查詢o、oo、ooooo 等資料，則需要使用星號(hào)（*）元字符。但需要注意的是，“*”代表的是重復(fù)零個(gè)或多個(gè)前面的單字符。“o*”表示擁有零個(gè)（即為空字符）或大于等于一個(gè)“o”的字符，因?yàn)樵试S空字符，所以執(zhí)行“grep –n‘o*’ httpd.txt”命令會(huì)將文本中所有的內(nèi)容都輸出打印。如果是“oo*”， 則第一個(gè) o 必須存在，第二個(gè) o 則是零個(gè)或多個(gè) o，所以凡是包含 o、oo、ooooo，等的資料都符合標(biāo)準(zhǔn)。同理，若查詢包含至少兩個(gè) o 以上的字符串，則執(zhí)行“grep –n‘ooo*’ httpd.txt”命令即可。

[root@localhost opt]# grep -n \\\'o*\\\' httpd.txt 
...//省略部分內(nèi)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
...//省略部分內(nèi)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(jié)尾，中間包含至少一個(gè) o 的字符串，執(zhí)行以下命令即可實(shí)現(xiàn)。

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

查詢以 w 開頭 d 結(jié)尾，中間的字符可有可無的字符串。

[root@localhost opt]# grep -n \\\'w.*d\\\' httpd.txt 
...//省略部分內(nèi)容...
342:# be turned off when serving from networked-mounted 
356:wd
357:wod
358:wood
359:woooood

查詢?nèi)我鈹?shù)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(xù)字符范圍“{}”

在上面的示例中，我們使用“.”與“*”來設(shè)定零個(gè)到無限多個(gè)重復(fù)的字符，如果想要限制一個(gè)范圍內(nèi)的重復(fù)的字符串，這個(gè)時(shí)候就需要使用基礎(chǔ)正則表達(dá)式中的限定范圍的字符“{}”，因?yàn)?code>“{}”在 Shell 中具有特殊 意義，所以在使用“{}”字符時(shí)，需要利用轉(zhuǎn)義字符“\\\\”，將“{}”字符轉(zhuǎn)換成普通字符。

(1）查詢兩個(gè) 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(jié)尾，中間包含 2～5 個(gè) 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(jié)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(jié)

^
匹配輸入字符串的開始位置。除非在方括號(hào)表達(dá)式中使用，表示不包含該字符集合。要匹配“^”字符本身，請使用“\\\\^”

$
匹配輸入字符串的結(jié)尾位置。如果設(shè)置了 RegExp 對象的 Multiline 屬性，則“$”也匹配‘\\\\n’或‘\\\\r’。要匹配“$”字符本身，請使用“\\\\$”

.
匹配除“\\\\r\\\\n”之外的任何單個(gè)字符

\\\\
將下一個(gè)字符標(biāo)記為特殊字符、原義字符、向后引用、八進(jìn)制轉(zhuǎn)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(dá)式零次或多次。要匹配“*”字符，請使用“\\\\*”

[]
字符集合。匹配所包含的任意一個(gè)字符。例如，“[abc]”可以匹配“plain”中的“a”

[^]
賦值字符集合。匹配未包含的一個(gè)任意字符。例如，“[^abc]”可以匹配“plain”中“plin”中的任何一個(gè)字母

[n1-n2]
字符范圍。匹配指定范圍內(nèi)的任意一個(gè)字符。例如，“[a-z]”可以匹配“a”到“z”范圍內(nèi)的任意一個(gè)小寫字母字符。注意：只有連字符（-）在字符組內(nèi)部，并且出現(xiàn)在兩個(gè)字符之間時(shí)，才能表示字符的范圍；如果出現(xiàn)在字符組的開頭，則只能表示連字符本身

{n}
n 是一個(gè)非負(fù)整數(shù)，匹配確定的n 次。例如，“o{2}”不能匹配“Bob”中的“o”，但是能匹配“food”中的兩個(gè)o

{n,}
n 是一個(gè)非負(fù)整數(shù)，至少匹配n 次。例如，“o{2,}”不能匹配“Bob”中的“o”，但能匹配“foooood”中的所有 o。“o{1,}”等價(jià)于“o ”。“o{0,}”則等價(jià)于“o*”

{n,m}
m 和n 均為非負(fù)整數(shù)，其中 n<=m，最少匹配 n 次且最多匹配 m 次

擴(kuò)展正則表達(dá)式

通常情況下會(huì)使用基礎(chǔ)正則表達(dá)式就已經(jīng)足夠了，但有時(shí)為了簡化整個(gè)指令，需要使用范圍更廣的擴(kuò)展正則表達(dá)式，例如，使用基礎(chǔ)正則表達(dá)式查詢除文件中空白行與行首為“#”之外的行（通常用于查看生效的配置文件），執(zhí)行“grep –v ‘^$’ httpd.txt | grep –v ‘^#’”即可實(shí)現(xiàn)。這里需要使用管道命令來搜索兩次。如果使用擴(kuò)展正則表達(dá)式，可以簡化為“egrep –v ‘^$|^#’ httpd.txt”，其中，單引號(hào)內(nèi)的管道符號(hào)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>
...//省略部分內(nèi)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>
...//省略部分內(nèi)容...

grep 命令僅支持基礎(chǔ)正則表達(dá)式，如果使用擴(kuò)展正則表達(dá)式，需要使用 egrep 或 awk 命令。awk 命令在后面進(jìn)行講解，這里我們直接使用 egrep 命令。egrep命令與 grep 命令的用法基本相似。egrep 命令是一個(gè)搜索文件獲得模式，使用該命令可以搜索文件中的任意字符串和符號(hào)，也可以搜索一個(gè)或多個(gè)文件的字符串，一個(gè)提示符可以是單個(gè)字符、一個(gè)字符串、一個(gè)字或一個(gè)句子。

與基礎(chǔ)正則表達(dá)式類型相同，擴(kuò)展正則表達(dá)式也包含多個(gè)元字符，常見的擴(kuò)展正則表達(dá)式的元字符主要包括以下幾個(gè)：

重復(fù)一個(gè)或者一個(gè)以上的前一個(gè)字符

？
零個(gè)或者一個(gè)的前一個(gè)字符

|
使用或者（or）的方式找出多個(gè)字符

()
查找“組”字符串

()
辨別多個(gè)重復(fù)的組

示例

執(zhí)行“egrep -n \\\'wo d\\\' httpd.txt”命令，即可查詢wod、wood、woooood等字符串。

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

執(zhí)行“egrep -n \\\'wo?d\\\' httpd.txt”命令，即可查詢“wd”“wod”這兩個(gè)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(zhí)行“egrep -n \\\'of|is|on\\\' httpd.txt”命令即可查詢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
...//省略部分內(nèi)容...

“egrep -n \\\'sh(i|o)rt\\\' httpd.txt”。`“shirt”與“short”因?yàn)檫@兩個(gè)單詞的“sh”與“rt”是重復(fù)的，所以將“i”與“o列于“()”符號(hào)當(dāng)中，并以“|”分隔，即可查詢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(jié)尾是C，中間有一個(gè)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 
...//省略部分內(nèi)容...
woooood
AxyzC
AxyzxyzC
~                                                                             
~                                                                             
:wq     
[root@localhost opt]# egrep -n \\\'A(xyz) C\\\' httpd.txt 
360:AxyzC
361:AxyzxyzC

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