Regular expression of Linux command rollup

Regular expressions are divided into two categories: the basic Regular Expression (BRE) and the extended regular expression (ERE).

1. the meta-character meaning of the basic regular expression:

   1.: matches any single character

   3, [^]: matches a single character outside the specified range

   4, *: matches any number of characters preceding it

   5, \?: Matches its previous 0 or 1 times

   6, \+: Matches its previous 1 or more

   7, \{m\}: Exact match m times

   8, \{m,n\}: matches at least m times, up to N times

   9,. *: matches any character of any length

   10, ^: Beginning of line anchoring,

   11, $: End of line anchoring

   12, \<: First anchor (can also use \b)

   13, \>: Suffix anchor (can also use \b)

   14, ^$: matching blank lines

   15, \ (\): used in conjunction with \ n (numbers) to match the number of repetitions of a single or multiple character combination

The meta-character meaning of the extended regular expression:

1.: Match any single character

2, []: matches a single character within the specified range

3, [^]: matches a single character outside the specified range

4, *: matches any number of characters preceding it

5,?: Match its front 0 or 1 times

6, +: match its front 1 or more

7, {m}: exact match m times

8, {M,n}: matches at least m times, up to N times

9. *: Matches any character of any length

10, ^: anchor at the beginning of the line,

11. $: End of line anchoring

12, \<: The first anchor (can also be used \b)

13, \>: Suffix anchor (can also use \b)

14. ^$: Match blank line

15, (): Used with \ n (n is a number) to match the number of repetitions of a single or multiple character combinations

16, |: Match | characters before or after the symbol

Three, the command that can match the regular expression is: (here only grep command)

1. Grep:grep matches the line of text with the basic regular expression.

Format: grep [OPTIONS] PATTERN [FILE ...]

grep [OPTIONS] [-E PATTERN |-f file] [FILE ...]

Options:

-e: Match with extended regular expressions (equivalent to egrep)

-F: Equivalent to Fgrep

-I: Ignore case when matching

-V: Show rows that do not match the pattern

-O: Show only rows that match

-Q: Silent mode

--color=auto: Highlighting matched characters

Iv. Examples:

1. Display the line in the/etc/passwd file ending with bash;

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2. Display the two-digit or three-digit number in the/etc/passwd file;

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3. Display lines with ' LISTEN ' followed by 0, one or more whitespace characters in the ' Netstat-tan ' command result

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4, add user Bash,testbash, basher and Nologin user (Nologin the user's shell is/sbin/nologin), and then find the/etc/passwd file in the user name with the shell name of the line;

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5. Display the default shell and UID of root, CentOS or User1 user on the current system;

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6, find a word in the/etc/rc.d/init.d/functions file (the middle of the word can be underlined) followed by a set of parentheses line

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7, use echo to output a path, and then egrep find its path base name;

Further: Use Egrep to remove its directory name

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8. Find out the number between 1-255 in the result of ifconfig command execution;

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Regular expression of Linux command rollup