Introduction to regular expressions of shell, general matching, special characters: ^, $,., * Character range (brackets): [], special characters: \, matching mobile phone number

1. Getting started with regular expressions

Regular expressions are described using a single string 、 Match a series of strings that conform to a certain syntax rule . In many text editors , Regular expressions are often used for retrieval 、 Replace the text that matches a pattern . stay Linux in ,grep,sed,awk And other text processing tools support pattern matching through regular expressions .

1.1 Regular match

A string of regular expressions without special characters matches itself , for example ：

[[email protected] ~]# cat /etc/passwd | grep 00:
It will match all that contain 00: The line of .

1.2 Special characters are often used

1.2.1 Special characters ：^

^ Match the beginning of a line , for example ：

It will match all of them with a Beginning line

1.2.2 Special characters ：$

$ Match the end of a line , for example

It will match all of them with h The line at the end

• reflection ：^$ Match what ？

Match blank line

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1.2.3 Special characters ：.

Match an arbitrary character , for example

1.2.4 Special characters ：*

“*” Not used alone , He used it with the last character , Means to match the previous character 0 Times or times , for example

[[email protected] ~]# cat /etc/passwd | grep ro*t

Will match rt, rot, root, rooot, roooot Wait for all the lines

reflection ：.* Match what ？
Match any character , You can talk to ^$ Continuous use , Put it among them , You can implement the following code , With a start ,login The line at the end

[[email protected] ~]# cat /etc/passwd | grep ^a.*login$

[[email protected] ~]# cat /etc/passwd | grep ^a.*var.*login$

1.2.5 Character range （ brackets ）：[ ]

[ ] Means to match a character in a range , for example
[6,8]------ matching 6 perhaps 8
[0-9]------ Match one 0-9 The number of
[0-9]------ Matches any length of numeric string
[a-z]------ Match one a-z Characters between
[a-z] ------ Matches an alphabetic string of any length
[a-c, e-f]- matching a-c perhaps e-f Any character between

[[email protected] ~]# cat /etc/passwd | grep r[a-z]*t

1.2.6 Special characters ：\

\ To signify an escape , It will not be used alone . Because all special characters have their own matching patterns , When we want to match a particular character itself （ for example , I want to find out all the things that contain ‘$’ The line of ）, There will be difficulties . At this point, we need to use escape characters with special characters , To represent the special character itself , for example

It will match all that contain $ The line of . Note that you need to use single quotes to cause the expression to .

1.2.7 Match cell phone number

[[email protected] scripts]# echo “13812345678” | grep ^1[34578][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]$

[[email protected] scripts]# echo “13812345678” | grep -E ^1[34578][0-9]{9}$

Remember to capitalize -E, Because at present grep I won't support it {9} Extended regular , So we need to add -E, Express regular expressions that support extensions