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Regular Expression

Regular Expression also calls Regexp. There are two classes regular expressions: Basic Regexp (BRE) and Extended Regexp (ERE). Regexp should be put in double quotation marks.

It can be categorized to:

Strings


. - Can be any character.



Sample: #cat /etc/fstab | grep "U..D"

It returns the lines contains U, D and whatever 2 characters between its.

[] - Any specified single character.



Sample: #cat /etc/fstab | grep "U[UAB]ID"

It returnes the lines contains U, I, D and U or A or B.

[^] - Any character except specified single character.



Sample: #cat /etc/fstab | grep "[^AB]ID"

It returnes the lines contains I and D but no A or B in left.

[[:digit:]] - All digital characters.


[[:lower:]] - All lower case characters.


[[:upper:]] - All upper case characters.


[[:alpha:]] - All alphabet characters.


[[:alnum:]] - All alphnumeric characters.


[[:space:]] - All space.


[[:print:]] - All visible characters and space.


[[:blank:]] - Space and tab.


[[:punct:]] - Punctuation and symbols.



Counting


 

Location


 

Grouping


 

 

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