Regular Expression in grep (original) (raw)

Last Updated : 30 Jan, 2019

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*