Lesson 1.3: Use grep and regular expressions to analyze text

The grep command is one of the most useful commands in a Linux terminal environment. The name grep stands for “global regular expression print”. This means that you can use grep to check whether the input it receives matches a specified pattern.

[ ]: 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.

Example File:

[sanjeeb@client ~]$ cat example_file 
Newa
Newb
Newc
Newd
Newe
Newf
Newg
Newh

Example 1: [ ] : Matches any one of a set characters

[sanjeeb@client ~]$ grep "New[a,b,c]" example_file 
Newa
Newb
Newc

[sanjeeb@client ~]$ grep "New[h][d]" example_file 
Newhdef
[sanjeeb@client ~]$ grep "New[h][def]" example_file 
Newhdef
[sanjeeb@client ~]$ grep "New[h][defghi]" example_file 
Newhdef
Newhghi

Example 2: Use [ ] with hyphen: Matches any one of a range characters

[sanjeeb@client ~]$ grep "New[0-1][d-o]" example_file 
New0def
New0ghi
New0jkl
New0mno
New1def
New1ghi
New1jkl
New1mno

Example 3: Use ^: The pattern following it must occur at the beginning of each line

[sanjeeb@client ~]$ grep "^old" example_file 
old1file
old2image
old3audio
old4video
old5readme
 
[sanjeeb@serverB test]$ cat testfile
new1
new2
new3
new4
new5
new6
new7
new8
new9
This is new file1
This is new file2
 
# This is comment 1
# This is comment 2
 
[sanjeeb@serverB test]$ grep "^[#]"  testfile
# This is comment 1
# This is comment 2
[sanjeeb@serverB test]$ grep "^[^#]"  testfile
new1
new2
new3
new4
new5
new6
new7
new8
new9
This is new file1
This is new file2

Example 4: Use ^ with [ ]: The pattern must not contain any character in the set specified

[sanjeeb@client ~]$ grep "^[a-d]" example_file 
a boy
bad boy
cute puppy
dead plant

Example 5: Use $: The pattern preceding it must occur at the end of each line

[sanjeeb@client ~]$ grep "y$" example_file 
a boy
bad boy
cute puppy
jersey
lonely
money

[sanjeeb@client ~]$ grep "oy$" example_file 
a boy
bad boy

Example 6: Use . (dot): Matches any one character

[sanjeeb@client ~]$ grep "b.y$" example_file 
a boy
bad boy
bengal bay

Example 7: Use \ (backslash): Ignores the special meaning of the character following it

[sanjeeb@client ~]$ grep "New.pqr" example_file 
Newhpqr
New2pqr
New.pqr

We can see that the special character is not ignored so prints both h and 2

[sanjeeb@client ~]$ grep "New\.\pqr" example_file 
New.pqr

By adding Backslash

Example 8: Use *: zero or more occurrences of the previous character

[sanjeeb@client ~]$ grep "kni*" example_file 
knife 
[sanjeeb@client ~]$ grep "kni*f*" example_file 
knife

Example 9: Use (dot).*: Nothing or any numbers of characters.

[sanjeeb@client ~]$ grep ".*abc" example_file 
Newhabc
New0abc
New1abc
New$abc
[sanjeeb@client ~]$ grep ".*boy" example_file 
a boy
bad boy