Regular Expression learning notes series ---- 1

A regular expression is a character pattern. When matching the content of a text file, it uses a special set of symbols to match the content we want.

Why do I use regular expressions?

Search (MATCH) the content we want to see.

Metacharacters of the regular expression:

Metacharacter function example

^ The first line locator/^ root/matches the line starting with Root

$ Line tail locator/bash $/match the line ending with Bash

. Match a single character/L. ve/match love, live, etc. There is a character between l and V

* Leading character/LO * SE/matching LSE, lose, loose, etc.

Match 0 or more characters before it (string)

/L */matches 0 l, or l, ll, and so on.

/Ll */match l, ll, lll, etc.

[] Match any one of a group of characters/L [OI] VE/match love, live

[X-y] indicates a range of/[A-C] OVE/matches aove, Bove, Cove

Where X and Y are letters or numbers/[0-9] 2/match,..., 92

[^] Indicates reverse/[^ TT] om/matches

\ Indicates escape/2 \. 0/matches 2.0. At this time,. is a common character, no longer a Metacharacter

/Love./match love.

\ <Start separator/\ <love/matches loved and lovely

\> The ending separator/love \>/matches inlove and so on

X \ {M \} X characters repeated m times/A \ {5 \}/matching aaaaa

The characters x \ {M, \} X must be repeated at least m times/A \ {5, \}/matches AAAAA, aaaaaa, aaaaaaa, and so on

X \ {M, n \} the preceding X contains M-N // \ <[A-Za-Z] \ {5, 7 \} \> or/A \ {5, 7 \}/matches AAAAA, aaaaaa, aaaaaaa

(3-5) indicates three or four or five matches (the range of M and N is 0-255 .)

. * Any 0 or multiple characters

\ (... \) Label lovely \ (love \) ly \ 1ly

How to Learn:

Let's look at the example first, convert it, modify it, and change it to yourself.

Syntax format:

Command "Regular Expression" Text File

Grep "^ root" passwd

Examples of Regular Expressions:

1 ).

[[Email protected] TMP] # Cat test2.txt

Lovely

Livl

Levllaeq

Lcveroop

Woeijfdklkiod

Love Mo forerver

Hahahh lilylilei

Ku Hou Haha Xixi

Doqiadddoqiia

[[Email protected] TMP] # grep "C. E" test2.txt

Lcveroop

[[Email protected] TMP] # grep "L. L" test2.txt

Hahahh lilylilei

[[Email protected] TMP] # grep "E. J" test2.txt

Woeijfdklkiod

######################################## ######################################## ######################################## ########

2 )*

[[Email protected] TMP] # Cat test2.txt

Lovely

Livl

Levllaeq

Lcveroop

Woeijfdklkiod

Love Mo forerver

Hahahh lilylilei

Ku Hou Haha Xixi

Zxysypr

Love Mo forervez

[[Email protected] TMP] # grep "lo *" test2.txt

Lovely

Livl

Levllaeq

Lcveroop

Woeijfdklkiod

Love Mo forerver

Hahahh lilylilei

Love Mo forervez

[[Email protected] TMP] # grep "lo * I" test2.txt

Livl

Hahahh lilylilei

[[Email protected] TMP] # grep "lo * o" test2.txt

Lovely

Love Mo forerver

Love Mo forervez

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