Shell programming regular expressions and metacharacters

Catalog

Regular expressions

^ : First locator

$ : End of line locator

. : Match any single character

* ： Match leader 0 To many times ; Can only be used later

.* : Any number of characters

[ ] : Match characters in the specified range

[^] Matches characters that are not in the specified group

\ ： Used to escape metacharacters (''  ""   \), Disjunctive sign

\<  Initial locators

\> Suffix locators

()    A label that matches the characters used later

x\{m\}  character x Recurring m Time

x\{m,n\} character x Recurring m To n Time

Extended regular expression metacharacter

+  matching 1~n Leading characters

？ matching 0~1 Leading characters

a|b  matching a perhaps b

---

Regular expressions

Metacharacters

Definition ： Metacharacters are such characters , They express different literal meanings

^ : First locator

[[email protected] ~]# grep "root" /etc/passwd
root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin

[[email protected] ~]# grep "^root" /etc/passwd
root:x:0:0:root:/root:/bin/bash

$ : End of line locator

. : Match any single character

[[email protected] ~]# cat 1.txt
abc
adc
bac
[[email protected] ~]# grep abc 1.txt
abc
[[email protected] ~]# grep a.c 1.txt
abc
adc
[[email protected] ~]# grep ..c 1.txt
abc
adc
bac
[[email protected]ost ~]# grep .bc 1.txt
abc
[[email protected] ~]# grep ab. 1.txt
abc

* ： Match leader 0 To many times ; Can only be used later

[[email protected] ~]# grep "abc*" 1.txt
abc
abcdffe
abcddfgh

.* : Any number of characters

[[email protected] ~]# grep "ab.*" 1.txt
abc
abcdffe
abcddfgh
 You are in  /var/spool/mail/root  New message in 
[[email protected] ~]# grep "ab.*h" 1.txt
abcddfgh
[[email protected] ~]# grep "ab.*g" 1.txt
abcddfgh

[ ] : Match characters in the specified range

[[email protected] ~]# grep [iL]ove 1.txt
Love

[[email protected] ~]# cat 1.txt
abc
adc
bac
abcdffe
abcddfgh
ggg

love
LOve
Love
i love
i love you
hhhh

[[email protected] ~]# grep [lL]ove 1.txt
love
Love
i love
i love you

[^] Matches characters that are not in the specified group

similar ！ stay “[]” ^  Mismatch

  

 \ ： Used to escape metacharacters (''  ""   \), Disjunctive sign

[[email protected] ~]# grep "l." 1.txt
love
i love
i love you
l.ve
[[email protected] ~]# grep "l\." 1.txt
l.ve

\<  Initial locators

\> Suffix locators

()    A label that matches the characters used later

 

 x\{m\}  character x Recurring m Time

[[email protected] ~]# grep o 1.txt
#love
Love
i love
i love you

[[email protected] ~]# grep "h\{3\}" 1.txt
hhhh
abcdefhhhgggff

 x\{m,n\} character x Recurring m To n Time

[[email protected] ~]# grep "h\{3,5\}" 1.txt
hhhh
abcdefhhhgggff

Extended regular expression metacharacter

+  matching 1~n Leading characters

[[email protected] ~]# egrep lo+ve   1.txt 
#love
i love
i love you

？ matching 0~1 Leading characters

lo?ve:?  Ahead o Yes or no , Will do ！

[[email protected] ~]# egrep lo?ve   1.txt 
#love
i love
i love you
lve
love

a|b  matching a perhaps b

 （） Group character