Linux: Regular Expression in grep

Prerequisite: Grep

Basic Regular Expression

Regular Expression provides an ability to match a “string of text” in a very flexible and concise manner. A “string of text” can be further defined as a single character, word, sentence or particular pattern of characters.

Like the shell’s wild–cards which match similar filenames with a single expression, grep uses an expression of a different sort to match a group of similar patterns.

◉ [ ]: Matches any one of a set characters
◉ [ ] with hyphen: Matches any one of a range characters
◉ ^: The pattern following it must occur at the beginning of each line
◉ ^ with [ ] : The pattern must not contain any character in the set specified
◉ $: The pattern preceding it must occur at the end of each line
◉ . (dot): Matches any one character
◉ \ (backslash): Ignores the special meaning of the character following it
◉ *: zero or more occurrences of the previous character
◉ (dot).*: Nothing or any numbers of characters.

Examples

(a) [ ] : Matches any one of a set characters

1. $grep  “New[abc]”  filename

It specifies the search pattern as :

Newa , Newb or Newc

2. $grep  “[aA]g[ar][ar]wal”  filename

It specifies the search pattern as

Agarwal , Agaawal , Agrawal , Agrrwal

agarwal , agaawal , agrawal , agrrwal

(b) Use [ ] with hyphen: Matches any one of a range characters

1. $grep  “New[a-e]” filename

It specifies the search pattern as

Newa , Newb or Newc , Newd, Newe

2. $grep  “New[0-9][a-z]”  filename

It specifies the search pattern as: New followed by a number and then an alphabet.

New0d, New4f etc

(c ) Use ^: The pattern following it must occur at the beginning of each line

1. $grep  “^san”  filename

Search lines beginning with san. It specifies the search pattern as

sanjeev ,sanjay, sanrit , sanchit , sandeep etc.

2. $ls –l |grep  “^d”

Display list of directories only

3. $ls –l |grep  “^-”

Display list of regular files only

(d) Use ^ with [ ]: The pattern must not contain any character in the set specified

1. $grep  “New[^a-c]”  filename

It specifies the pattern containing the word “New” followed by any character other than an ‘a’,’b’, or ‘c’

2. $grep  “^[^a-z A-Z]”  filename

Search lines beginning with an non-alphabetic character

(e) Use $: The pattern preceding it must occur at the end of each line

$ grep "vedik$" file.txt

(f) Use . (dot): Matches any one character

$ grep "..vik" file.txt
$ grep "7..9$" file.txt

(g) Use \ (backslash): Ignores the special meaning of the character following it

1. $ grep "New\.\[abc\]" file.txt

It specifies the search pattern as New.[abc]

2. $ grep "S\.K\.Kumar" file.txt

It specifies the search pattern as

S.K.Kumar

(h) Use *: zero or more occurrences of the previous character

$ grep "[aA]gg*[ar][ar]wal" file.txt

(i) Use (dot).*: Nothing or any numbers of characters.

$ grep "S.*Kumar" file.txt