egrep (grep -E) supports extended metacharacters for regular expressions
1. Use regular rules to determine the need [[ ]]
num1=1
[[ $num1 =~ ^[0-9] + $ ]] & amp; & amp; echo "yes" || echo "no"
num3=1b1
[[ $num3 =~ ^[0-9] + $ ]] & amp; & amp; echo "yes" || echo "no"
2, * 0 or more
useradd abrt
grep 'abc*' /etc/passwd
3. \Word-ending positioning symbol
cat a.txt | egrep '\<[Jj]ack\>'
Extension: ” :% s/\<[Jj]ack\>/123/g “
vim jack.txt
Jack JACK JAck jackly
:% s/\<[Jj]ack\>/123/g
4. What does ^ begin with?
grep '^root' /etc/passwd
5. What does $ end with?
grep 'bash$' /etc/passwd
6. . Matches a single character
grep 'r..t' /etc/passwd
grep 'r.t' /etc/passwd
7, .* any number of characters
grep 'r.*t' /etc/passwd
8. [ ] matches any character in square brackets
grep 'Root' /etc/passwd
grep '[Rr]oot' /etc/passwd
9. [ – ] matches a character within the specified range
grep [a-z]oot /etc/passwd
10. [ ^ ] matches characters that are not in the specified group, reverse the meaning
^ inside [] means negation, ^ outside [] means something to start with.
grep '[^0-9]oot' /etc/passwd
grep '[^0-9A-Z]oot' /etc/passwd
grep '[^0-9A-Za-z]oot' /etc/passwd
grep '[a-z]oot' /etc/passwd
grep '^[rc]oot' /etc/passwd
11. Tags after \( \) matching
vim file1.txt
IPADDR=192.168.1.123
GATEWAY=192.168.1.1
NETMASK=255.255.255.0
DNS=114.114.114.114
:% s#\(192.168.1.\)123#\12#
:% s#\(192.\)\(168.\)\(1.\)2#\1\2\35#
:% s#\(192.\)\(168.\)\(1.\)\(5\)#\1\26.\4#
12. + matches one or more leading characters
egrep 'ro + t' /etc/passwd
13.? Matches zero or one leading character
egrep 'ro?t' /etc/passwd
14. a|b matches a or b
netstat -anlp|egrep ':80|:22'
cat /etc/passwd | egrep 'root|alice'
15, x{m} character x repeated m times
[root@localhost ~]# cat a.txt
love
love.
loove
looooove
cat a.txt | egrep 'o{2}' a.txt
cat a.txt | egrep 'o{2,}' a.txt
cat a.txt | egrep 'o{6,7}' a.txt
16. Extended regular expression metacharacters
+ matches one or more leading characters
[a-z] + ove
? matches zero or one leading character
lo?ve
a|b matches a or b
love|hate
(..)(..)\1\2 label matching characters
(love)able\1er
x{m} character x repeated m times
o{5
x{m,} character x repeated at least m times
o{5,}
x{m,n} character x is repeated m to n times
o{5,10}
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