Regular Expressions in Linux Explained with Examples

Regular expressions (Regexp) is one of the advanced concept we require to write efficient shell scripts and for effective system administration.

• Basically regular expressions are divided in to 3 types for better understanding.

1. Basic Regular expressions
2. Interval Regular expressions (Use option -E for grep and -r for sed)
3. Extended Regular expressions (Use option -E for grep and -r for sed)

• What is a Regular expression?

A regular expression is a concept of matching a pattern in a given string.

• Which commands/programming languages support regular expressions?

vi, tr, rename, grep, sed, awk, perl, python etc.

BASIC REGULAR EXPRESSIONS

Basic regular expressions:

• This set includes very basic set of regular expressions which do not require any options to execute.
• This set of regular expressions are developed long time back.

^ –Caret/Power symbol to match a starting at the beginning of line.

$ –To match end of the line

* –0 or more occurrence of the previous character.

. –To match any character

<word> –Actual word finding

–Escape character

$ REGULAR EXPRESSION

• Match all the files which ends with sh
  bash code

    ls -l | grep sh$

  As $ indicates end of the line, the above command will list all the files whose names end with sh.

  how about finding lines in a file which ends with dead

grep 'dead$' filename

How about finding empty lines in a file?

grep '^$' filename

REGULAR EXPRESSION

Example : Match all files which have a word twt, twet, tweet etc in the file name.

ls -l | grep 'twe*t'

How about searching for apple word which was spelled wrong in a given file where apple is misspelled as ale, aple, appple, apppple, apppppple etc. To find all patterns

grep 'ap*le' filename

Readers should observe that the above pattern will match even ale word as * indicates 0 or more of the previous character occurrence.

[^CHAR] REGULAR EXPRESSION

Example: Match all the file names except a or b or c in it’s filenames

 ls | grep  '[^abc]'

This will give output all the file names except files which contain a or b or c.

<WORD> REGULAR EXPRESSION

Example: Search for a word abc, for example I should not get abcxyz or readabc in my output

   grep '<abc>' filename

ESCAPE REGULAR EXPRESSION

Example : Find files which contain [ in it’s name, as [ is a special charter we have to escape it

grep "[" filename

or

grep '[[]' filename

[] SQUARE BRACES/BRACKETS REGULAR EXPRESSION

Example : Find all the files which contains a number in the file name between a and x

ls -l | grep 'a[0-9]x'

This will find all the files which is
a0xsdf
asda1xsdfas
..
..
asdfdsara9xsdf
etc.

• So where ever it finds a number it will try to match that number.
• Some of the range operator examples for  you.
• [a-z] –Match’s any single char between a to z.
• [A-Z] –Match’s any single char between A to Z.
• [0-9] –Match’s any single char between 0 to 9.
• [a-zA-Z0-9] – Match’s any single character either a to z or A to Z or 0 to 9
• [!@#$%^] — Match’s any ! or @ or # or $ or % or ^ character.