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[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 "o*" test.txt
1:gd2:god[root@localhost ~]# grep -n ".*" test.txt3:
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**
1:gd
2:god3:[root@localhost tmp]# grep "^root" /etc/passwd4: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: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
#^$: means a blank line"#|^$": matches comment lines starting with # and Empty line
(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.
2.6, []: Match any character in square brackets
[root@localhost tmp]# cat hosts
192.168.200.1
a.b.123.5[root@localhost ~]# grep -E "go?d" test.txt23.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}$' hosts
192.168.200.1
192.168.200.3
[root@localhost tmp]
#2.7, ?: Match zero or more times of the previous character
gd
god[root@localhost ~]#[root@localhost tmp]# cat test
do
does
doxy
[root@localhost tmp]# grep -E "do(es)?" test
do
does
doxy
[root@localhost tmp]
#3、 Characters only found in POSIX BRE (Basic Regular)
{n,m}: Interval expression, matching the single character before it repeats [repeat, the following single character such as https{0,1}, that is, repeat s 0-1 times. {n} refers to matching n times; {n,m} refers to matching n to m times, {n,} refers to matching at least n times, and {,m} matches at most m times. [\escape characters]
4, characters only in POSIX ERE (extended regular)
4.1, {n,m} : Same function as BRE's {n,m}
[root@localhost tmp]# grep -E '^([0-9]{1,3}\.){3}[ 0-9]{1,3}$' 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/servicesGroup filteringmysql 3306/tcp # MySQL
mysql 3306/udp # MySQL
ncube-lm 1521/tcp ##[root@localhost ~]
4.4, ( ): group filtering, backward reference
[root@localhost ~]# echo "glad" >> test.txt
goodgoood
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'
5. Metacharacters of regular expressions
5. 1.\b: Match a word boundary
[root@localhost tmp]# cat test
dodoesdoxy
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
doesdoxy5.4, \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 hereCase: Streamlined startupagdoeg
[root@localhost tmp]# grep "do\b" test
do
[root@localhost tmp]
5.3, \d: Match a numeric character, Equivalent to [0-9]
[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!
How to use scss in Angular projects
The above is the detailed content of Tips for using regular expressions in Linux. For more information, please follow other related articles on the PHP Chinese website!
![[Web front-end] Node.js quick start](https://img.php.cn/upload/course/000/000/067/662b5d34ba7c0227.png)
