Tips for using regular expressions in Linux

This time I will bring you tips on using regular expressions in Linux, and 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: have special meaning in regular expressions
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, removing the special meaning of the symbol, (), {}, etc. are available in the shell Special meaning] The difference between
. and and.:

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

2.1. .: Matches any single character (except null, which cannot be Empty)

[root@localhost ~]# grep -n "." test.txt                                                                  :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 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 following regular expression The formula ends 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 starting with # and Empty line



2.6, []: Match any character in square brackets

(such as [sS], match s or match S), hyphens can be used (-) specifies the range of hyphens (such as [(0-9)], matching any character from 0-9); [^0-9] If the ^ symbol appears in the first position of the square bracket, it means that the match is not any character 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 in POSIX ERE (extended regular)

4.1, {n,m} : Same function as BRE's {n,m}

[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 of the previous regular expressions Times

[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 # MySQL
ncube-lm 1521/tcp                                                                                                                                                                         ##[root@localhost ~]



4.4, ( ): group filtering, backward reference

Group filtering

[root@localhost ~]# echo "glad" >> test.txt

[root@localhost ~]# egrep "(la|oo)" test.txt

good

goood
glad



() back 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. 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        
does
[root@localhost tmp]# grep "\bdo\b" test
do
[root@localhost tmp]



#5.2. \B: Match non-word boundaries, the opposite of \b

[root@localhost tmp]# grep "do\B" test

does

doxy

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



5.3, \d: Match 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, Underscore is equivalent to [A-Za-z0-9_]

There are many metacharacters, so I won’t list them one by one 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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