LINUX

LINUX – Regular Expresssions

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

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

[] –Range of character

[^char] –negate of occurrence of a character set

<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

bash code
grep 'dead$' filename

How about finding empty lines in a file?

bash code
grep '^$' filename

REGULAR EXPRESSION

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

bash code
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

bash code
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

bash code
 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

bash code
   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

bash code

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

bash code
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
  • [[email protected]#$%^] — Match’s any ! or @ or # or $ or % or ^ character.

 

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About the author

Venkatesan Prabu

Venkatesan Prabu

Wikitechy Founder, Author, International Speaker, and Job Consultant. My role as the CEO of Wikitechy, I help businesses build their next generation digital platforms and help with their product innovation and growth strategy. I'm a frequent speaker at tech conferences and events.

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