Grep basic Regular Expression and egrep extension Regular Expression

I. Introduction

Grep is used to display the part.) match the rows in string format.

Family: grep egrep fgrp

Ii. How to Use grep

Usage: grep [options] PATTERN file ..

Options:

-I case-insensitive

-- Color Matching highlighted

-O: only show Matching Parts

-N: displays the matched rows and rows.

-V Inversion

-E: use an extended regular expression.

-A n: displays the matched rows and the content of the last N rows.

-B N: displays the matched rows and the content of the N rows before the matched rows.

-C n: display the content of the matched rows and the content of the plus N rows before and after the matched rows.

Basic Regular Expression: REGularEXPression

① Metacharacters:

. Any single character

[] A single character in the specified range

[^] Except for a single character in the specified range

② Number of Characters

* Match any character before

. * Any character of any length

\? Match the first character 0 or 1 time

\ {M, n \} matches the previous character at least m times, at most n times

\ {3 \} matches the previous character three times

\ {1, \} matches the previous character at least once

③ Positioning

^ Beginning of Line

$ End of line

^ $ Blank line

\ <Or \ B any character after it must appear as the first part of the word

\> Or \ B, the character before it must end with the end of the word

\ <Root \> match the word root

Note: You need to add "" when using it.

④ Group

\ (Root \) takes root as a whole

\ (Root \) * root can appear as a whole for any number of times

Group backward reference:

\ (Root \). * \ 1 is equivalent\ (Root \). * \ (root \)

\ 1 reference content in the first one)

\ 2 reference content in second)

....

Extended regular expression:

Use grep-E or egrep

① Metacharacters:

Same as the basic Regular Expression

② Number of Characters

*
?
+ At least once
{M, n} is the same as a basic regular expression, but the escape symbol \

③ Positioning

^
$
\ <
\>

④ Tuples are basically the same as basic regular expressions, but only escape characters are missing)

()
\ 1
\ 2

⑤ Or "|"
Dog | cat dog or cat

Exercise: capture the IP address x. x in ifconfig)

Ideas: x [1-9] | [1-9] [0-9] | [1] [0-9] [0-9] | [2] [0-5] [0-5]

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