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