Detailed explanation of the use of regular expressions in Linux

This time I will bring you a detailed explanation of the use of regular expressions in Linux. What are the precautions for using regular expressions in Linux? The following is a practical case. Let’s take a look. .

1. Composition

Ordinary characters: ordinary strings, no special meaning
Special characters: Special characters in regular expressions The meaning of
Common meta characters [special characters] in regular expressions

2. Meta characters found in POSIX BRE [Basic] and ERE [Extended]

\: Usually used to turn on or off the special meaning of subsequent characters, such as (...) [\ is an escape character, which removes the special meaning of the symbol. (), {}, etc. have special meanings in the shell 】
.The difference between and:

[root@localhost ~]# cat -n test.txt
1 gd
2 god
3
4 good
5 goood
6 goad
7
8 gboad

2.1, .: Match any single character (except null, which cannot be empty)

[root@localhost ~]# grep -n "." test.txt        
1:gd
2:god
4:good
5:goood
6 :goad
8:gboad
[root@localhost ~]# grep -n "go.d" test.txt
4:good
6:goad

2.2. : Match the preceding character any number of times, such as o, which can be no o, one o, or multiple o

[root@localhost ~]# grep -n "*" test .txt
[root@localhost ~]# grep -n "o*" test.txt
1:gd
2:god
3:
4:good
5: goood
6:goad
7:
8:gboad
[root@localhost ~]# echo "gbad" >>test.txt
[root@localhost ~]# echo "pbad" >>test.txt
[root@localhost ~]# echo "kgbad" >>test.txt
[root@localhost ~]# echo "poad" >>test .txt
[root@localhost ~]# grep -n "go*" test.txt [o can be omitted, the g before o must match]
1:gd
2:god
4:good
5:goood
6:goad
8:gboad
9:gbad
11:kgbad

*2.3, .: Match any character (match all), can be empty**

[root@localhost ~]# grep -n ".*" test.txt
1:gd
2:god
3:
4:good
5:goood
6:goad
7:
8:gboad
9:gbad
10:pbad
11:kgbad
12:poad
[root@localhost ~]# grep -n "go.*" test.txt
2:god
4:good
5:goood
6: goad
[root@localhost ~]# grep -n "po.*" test.txt
12:poad
[root@localhost ~]# echo "pgoad" >>test.txt
[root@localhost ~]# grep -n "go.*" test.txt [Any character exists after matching go, can be empty]
2:god
4:good
5:goood
6:goad
13:pgoad
[root@localhost ~]
#[root@localhost ~]# grep -n "o.*" test.txt
2:god
4:good
5:goood
6:goad
8:gboad
12:poad

2.4, ^: Match the following regular expression , starting with...

[root@localhost tmp]# grep "^root" /etc/passwd
root:x:0:0:root:/root:/bin /bash
[root@localhost tmp]

#2.5, $: Match the immediately preceding regular expression, ending with...

[root @localhost tmp]# grep "bash$" /etc/passwd | head -1
root:x:0:0:root:/root:/bin/bash
[root@localhost tmp]
#^$: means a blank line
"#|^$": matches comment lines and blank lines starting with

#2.6, []: matches lines in square brackets Any character

(such as [sS], matches s or matches S), where a hyphen (-) can be used to specify the range of hyphens (such as [(0-9)], matches any 0-9 character); [^0-9] If the ^ symbol appears in the first position of the square bracket, it means matching any character that is not in the list.

[root@localhost tmp]# cat hosts
192.168.200.1
192.168.200.3
a.b.123.5
23.c.56.1
1456.1.2.4
12.4 .5.6.8
[root@localhost tmp]# grep -E '([0-9]{1,3}\.){3}[0-9]{1,3}' hosts
192.168.200.1
192.168.200.3
1456.1.2.4
12.4.5.6.8
[root@localhost tmp]# grep -E '^([0-9]{1,3}\ .){3}[0-9]{1,3}$' hosts
192.168.200.1
192.168.200.3
[root@localhost tmp]

#2.7 ,?: Match zero or more times of the previous character

[root@localhost ~]# grep -E "go?d" test.txt
gd
god
[root@localhost ~]
#[root@localhost tmp]# cat test
do
does
doxy
[root@localhost tmp]# grep -E "do(es)? " test
do
does
doxy
[root@localhost tmp]

#3. Characters only found in POSIX BRE (Basic Regular)

{n,m}: Interval expression, matching the single character before it repeats [repeat, the following single character such as https{0,1}, that is, repeat s 0-1 times . {n} refers to matching n times; {n,m} refers to matching n to m times, {n,} refers to matching at least n times, and {,m} matches at most m times. [\escape characters]

4. Characters only found in POSIX ERE (extended regular)

4.1. {n,m}: {n with BRE ,m} has the same function

[root@localhost tmp]# grep -E '^([0-9]{1,3}\.){3}[0-9]{1 ,3}$' hosts
192.168.200.1
192.168.200.3

4. 2. : Match one or more times of the previous regular expression

[root@localhost ~]# egrep "go d" test.txt
god
good
goood
[root@localhost ~]

#4.3, | : Indicates matching multiple strings [or relationship]

[root@localhost ~]# grep -E "3306|1521" /etc/services
mysql 3306/tcp # MySQL
mysql 3306/udp 3306/udp # MySQL
ncube-lm 1521/tcp # nCube License Manager
ncube-lm 1521/udp             # nCube License Manager
[root@localhost ~]

#4.4, (): Group filtering, Back reference

Group filtering

[root@localhost ~]# echo "glad" > > test.txt
[root@localhost ~]# egrep "(la|oo)" test.txt
good
goood
glad

() Direct reference; when the previous matching part uses parentheses, the content of the first bracket can be output with \1 in the later part; and so on.

[root@localhost tmp]# ifconfig |sed -rn 's#.*addr:(.*)(B.*)$#\1#gp'
192.168.4.27

5. The metacharacters of regular expressions

5. 1. \b: Match a word boundary

[root@localhost tmp]# cat test
do
does
doxy
agdoeg
[root@localhost tmp]# grep "do\b" test
do
[root@localhost tmp]# grep "\bdo" test        
do
does
doxy
[root@localhost tmp]# grep "\bdoes" test                                                               ##[root@localhost tmp]# grep "\bdo\b" test
do
[root@localhost tmp]

#5. 2. \B: Match non-words Boundary, opposite to \b

does

doxy
agdoeg
[root@localhost tmp] # grep "do\b" test
do
[root@localhost tmp]
#

5.3, \d: Matches a numeric character, equivalent to [0-9]

5.4, \D: Matches a non-numeric character, equivalent to [^0-9]

5.5, \w: Matches letters, numbers, and underscores, equivalent to [A-Za-z0-9_]

There are many metacharacters, so I won’t list them all here

Case: Streamlined startup

[root@localhost ~]# chkconfig --list| egrep -v "crond|network|rsyslog|sshd|sysstat" | awk '{print " chkconfig",$1,"off"}'|bash

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the PHP Chinese website!

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