Shell Regular Expression

What is a regular

　　A regular is a way to describe a character or string by combining symbols with special meanings, called regular expressions. Or, the regular is the rule used to describe a class of things.

There are regular things in life:

For example we describe: 4 legs

You might think of a four-legged animal or a table, a chair, etc.

Continue description: 4 legs, live

It's just a four-legged animal.

In Linux, wildcard characters are interpreted by the shell, and regular expressions are interpreted by commands, so let's introduce three types of text processing Tools/commands: grep, sed, awk, all of which can be interpreted as regular.

Two grep

Parameters

-N: Show line numbers

-O: Show only matching content

-Q: Silent mode, no output, you have to use $? To determine the success of the execution, that is, there is no filtering to the desired content

-L: If the match succeeds, only the file name is printed, the failure is not printed, usually-rl together, grep-rl ' root '/etc

-A: If the match is successful, the matching row and the subsequent n rows are printed together

-B: If the match succeeds, the matching row and its first n rows are printed together

-C: If the match succeeds, the matching row and its n rows are printed together

-C: If the match succeeds, the number of rows to match is printed

-e: Equals Egrep, extended

-I: Ignore case

-V: Inverse, mismatch
-W: Match word

[[email protected] ~]# cat a.txt root123root asdfroot_123rootssroot 123[[email protected] ~]# grep-i "root" a.txt root123 Root asdfroot_123rootssroot 123[[email protected] ~]# grep-w "root" a.txt root 123

grep type
Grep
Fgrep
Pgrep
Egrep

The regular introduction

^ Beginning of the line
$ End of line
. Any single character other than the line break
* 0 or more of the leading characters
. * All Characters
[] Any character within a group of characters
[^] reverse each character within a character group (does not match each character in a group of characters)
^[^] lines that begin with characters within a non-character group
[A-z] lowercase letter
[A-z] capital letters
[A-z] lowercase and uppercase letters
[0-9] Number
\< words are usually separated by spaces or special characters, and successive strings are treated as words.
\> Word Tail

Extended regular sed plus-r parameter or escape
grep plus-E or egrep or escaped
AWK directly supports but does not contain {n,m}
You can use--posix support
[[email protected] ~]# awk '/ro{1,3}/{print} '/etc/passwd
[[email protected] ~]# awk--posix '/ro{1,3}/{print} '/etc/passwd

Sed-n '/roo\?/p '/etc/passwd
Sed-rn '/roo?/p '/etc/passwd
? A leading character 0 or one
+ Leading character one or more
ABC|DEF ABC or DEF
A (Bc|de) f ABCF or Adef
x\{m\} X appears m times
x\{m,\} X appears m times to multiple times (at least m times)
x\{m,n\} X appears m times to N times

POSIX-defined character classification

[: Alnum:] alphanumeric characters. The
Match range is [a-za-z0-9]
[: Alpha:] alphabetic characters.
The match range is [a-za-z]
[: blank:] Space or tab characters.
The match range is a space and the TAB key
[: Cntrl:] Control characters.
Match control keys such as ^m to press CTRL + V and press ENTER to output
[:d igit:] Numeric characters.
Match all numbers [0-9]
[: graph:] characters that is both printable and visible. (A space is the print-
able, but not visible, while an A is both.)
Match all visible characters without spaces and tab is all the symbols you can see with your eyes on your keyboard in a text document
[: Lower:] lower-case alphabetic characters.
lowercase [A-z]
[:p rint:] Printable characters (characters that is not control characters.)
Match all visible characters including spaces and tab
to all symbols printed on the paper
[:p UNCT:] punctuation characters (characters that is not letter, digits, con-< Br>trol characters, or space characters).
Special input symbol +-=) (*&^%$#@!~ ' |\ "' {}[]:;? />.<,
Note that it does not contain spaces and tab
This collection does not equal ^[a-za-z0-9]
[: space:] space characters (such as space, tab, and FormFeed, to Name a few).

[: Upper:] Upper-case alphabetic characters.
uppercase [A-z]
[: Xdigit:] Characters that is hexadecimal digits.
16 binary number [0-f]

grep job

Shell Regular Expression