Regular Expressions/ Regex Unix

Basic  Regular Expressions Unix:

Expression             Use

[ ]:                                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 of Basic  regular expressions with grep command:

1. 1.  [ ] 

• • Matches any one of a set characters in the text.
  • The character set to be used in the matching process is enclosed in brackets.
  • A range of text characters is indicated by a dash (-). [a-d]

Example:

$grep  “Test[abc]”  filename

Search pattern will be:

Testa , Testb or Testc

      2. Use [ ] with hyphen/ dash

• • Matches any one of a range characters

Example:

$grep  “Test[1-5]” filename

It specifies the search pattern as

Test1 , Test2, Test3 , Test4, Test5

     3.  Use ^

• • The pattern following it must occur at the beginning of each line

$grep  “^Test”  filename

Search lines beginning with Test. 

Search pattern will be like:

Testing ,Testers, Testable, Tester etc.

$ls –l |grep  “^d” 

Display list of directories only

$ls –l |grep  “^-” 

Display list of regular files only

     4. Use ^ in [ ]

• • The pattern must not contain any character in the set specified

$grep  “Test[^a-c]”  filename

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

     5. Use $

• • The pattern preceding it must occur at the end of each line

$grep "ing$" filename

Search pattern will be like:

Testing ,eating, sitting etc.

     6.  Use . (dot)

• • Matches any one character

$grep ".est" filename

     7. Use \ (backslash)

• • Ignores the special meaning of the character following it

$grep "Test\.\[abc\]" file.txt

It specifies the search pattern as Test.[abc]

$grep "M\.A" file.txt

It specifies the search pattern as

M.A

    8. Use *

• • zero or more occurrences of the previous character

$grep "[aA]gg*[ar]wal" filename

   9.  Use (dot).*

• • Nothing or any numbers of characters.

$grep "M.*A" filename

Interval Regular Expressions Unix

• tell us about the number of occurrences of a character in a string.

Expression                    Use
{n}                                     Matches the preceding character appearing ‘n’ times exactly
{n,m}                                Matches the preceding character or expression immediately before appearing ‘n’ times but not more than m
{n, }                                  Matches the preceding character only when it appears ‘n’ times or more
\+                                       Matches previous character one or more times
\?                                       Matches previous character 0(zero) or one time only

1. Use {n}

• • Matches the preceding character appearing ‘n’ times exactly

$ cat test.txt
Sitting
Eating
Rating

$cat test.txt | grep -E t\{2}
Output: Sitting

2. Use {n,m}

• • Matches the preceding character appearing ‘n’ times  but not appearing more than m times

$ cat test.txt
Sitttting
fitting
Rating

$cat test.txt | grep -E t\{2,3}
Output: fitting

2. Use \+

• • Matches previous character one or more times

$ cat test.txt
Sitting
fitting
Rating

filter out lines where character ‘i’ precedes character ‘t’

$ cat test.txt | grep "i\+t"
Output: 
Sitting
fitting

Some practical example for regex:

1. Matching a word by ignoring its case

$ grep "[Tt]utor"

2. Matching mobile number using regex with grep
3. Match email-address

$ grep -E "[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,6}"

4. Finding files with a particular extension

$ ls -l | grep '.csv$'

For more on regex refer here

• Recommended Articles:

  • • Basic Unix Commands
    • Unix File System Commands
    • Regular Expressions
    • Text Processing Unix Commands