grep and sed support basic regular expressions in common file processing tools on Linux systems.

grep - Regularly expresses commands to filter file contents.

Example (in the output after the command executes, what is red is what is found):

[root@localhost ~]# dmesg | grep 'Ebtables'    #dmesg lists the core information and then filters out rows containing the'Ebtables'character
[   18.440389] 'Ebtables' v2.0 registered
[root@localhost ~]# dmesg | grep -n -A3 -B2 'Ebtables'  
#This is the line number that will be filtered and lists the'Ebtables'line along with the next three and the first two lines
1773-[    7.850479] NET: Registered protocol family 40
1774-[   18.203047] ip6_tables: (C) 2000-2006 Netfilter Core Team
1775:[   18.440389] 'Ebtables' v2.0 registered
1776-[   18.510067] nf_conntrack version 0.5.0 (16384 buckets, 65536 max)
1777-[   18.714192] bridge: automatic filtering via arp/ip/ip6tables has been deprecated. Update your scripts to load br_netfilter if you need this.
1778-[   18.783253] IPv6: ADDRCONF(NETDEV_UP): ens33: link is not ready
[root@localhost ~]# grep -in 'bash' /etc/passwd  
#Find the "bash" character in the file, and prompt: All users who have the "bash" character can log on to the system.
1:root:x:0:0:root:/root:/bin/bash
43:lisi:x:1000:1000:lisi:/home/lisi:/bin/bash
[root@localhost ~]# Grep-in'pr[i o]'/etc/passwd "pr" followed by either I or O lines
14:systemd-bus-'pro'xy:x:999:997:systemd Bus 'Pro'xy:/:/sbin/nologin
28:rtkit:x:172:172:RealtimeKit:/'pro'c:/sbin/nologin
41:sshd:x:74:74:'Pri'vilege-separated SSH:/var/empty/sshd:/sbin/nologin
[root@localhost /]# grep -n '^the' test.txt    #Find the line in the file that begins with the and display the line number
4:'the'  tongue is boneless bu it breaks  bones.12!
[root@localhost /]# grep -n '^[a-z]' test.txt     #Search for lines in a file that begin with a lowercase letter
1:he was short and  fat.
4:the  tongue is boneless bu it breaks  bones.12!
5:google is the best  tools for search keyword.
8:a wood cross!
[root@localhost ~]# grep -n ^[^a-zA-Z] test.txt     #Search for lines that do not begin with letters
11:#woood #
12:#woooooood #
[root@localhost ~]# grep -n "\.$" test.txt     #Search for rows ending in.
1:he was short and  fat.
2:He was  wearing a blue  polo shirt with  black pants.
3:The  home  of Football  on BBC  Sport  online.
5:google is the best  tools for search keyword.
6:The year  ahead will  test  our political  establishment to the limit.
15:Misfortunes  never come  alone/single.
16:I shouldn't have lett  so tast.
[root@localhost ~]# grep -n "^$" test.txt     #Show blank lines
10:
17:

Special symbols in regular expressions and their meaning (most can be expressed in another way, here is just a record for future reference):

Basic Regular Expression - Decimal Points and Asterisks

Examples of use:

[root@localhost ~]# grep -n 'sh..t' test.txt    #Query begins with sh and ends with t, with at least two characters in between
1:he was 'short' and  fat.
2:He was  wearing a blue  polo 'shirt' with  black pants.
[root@localhost ~]# grep -n 'wo*' test.txt    #Find rows that start with w and end with 0 or infinite o
1:he 'w'as short and  fat.
2:He 'w'as  'w'earing a blue  polo shirt 'w'ith  black pants.
5:google is the best  tools for search key'wo'rd.
6:The year  ahead 'w'ill  test  our political  establishment to the limit.
8:a 'woo'd cross!
9:Actions speak louder  than  'wo'rds
11:#'wooo'd #
12:#'wooooooo'd #

Since the above display is not friendly, I'll also flag the filter results in red, so I'll take a screenshot next.

Look at the following results for yourself:

Summary of grep commands - Character Summary of Basic Regular Expressions

egrep command - extended regular expression

Generally speaking, basic regular expressions are sufficient for us to use, but if you want to simplify the entire instruction, you can use extended regular expressions. If you use extended regular expressions, you need egrep or awk commands. Common metacharacters of extended regular expressions include the following:

Extended Regular Expression Character Summary: