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.
Table Of Content
- Basically regular expressions are divided in to 3 types for better understanding.
- Basic Regular expressions
- Interval Regular expressions (Use option -E for grep and -r for sed)
- 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–Escape character
[ad type=”banner”]$ REGULAR EXPRESSION
- Match all the files which ends with sh[pastacode lang=”bash” manual=”%20%20ls%20-l%20%7C%20grep%20sh%24%0A” message=”bash code” highlight=”” provider=”manual”/]
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
How about finding empty lines in a file?
[pastacode lang=”bash” manual=”grep%20’%5E%24’%20filename%0A” message=”bash code” highlight=”” provider=”manual”/]REGULAR EXPRESSION
Example : Match all files which have a word twt, twet, tweet etc in the file name.
[pastacode lang=”bash” manual=”ls%20-l%20%7C%20grep%20’twe*t’%0A” message=”bash code” highlight=”” provider=”manual”/] [ad type=”banner”]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
[pastacode lang=”bash” manual=”grep%20’ap*le’%20filename%0A” message=”bash code” highlight=”” provider=”manual”/]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
[pastacode lang=”bash” manual=”%20ls%20%7C%20grep%20%20’%5B%5Eabc%5D’%0A” message=”bash code” highlight=”” provider=”manual”/]This will give output all the file names except files which contain a or b or c.
REGULAR EXPRESSION
Example: Search for a word abc, for example I should not get abcxyz or readabc in my output
[pastacode lang=”bash” manual=”%20%20%20grep%20’%3Cabc%3E’%20filename%0A” message=”bash code” highlight=”” provider=”manual”/]ESCAPE REGULAR EXPRESSION
Example : Find files which contain [ in it’s name, as [ is a special charter we have to escape it
[pastacode lang=”bash” manual=”%0Agrep%20%22%5B%22%20filename%0A%0Aor%0A%0Agrep%20’%5B%5B%5D’%20filename%0A” message=”bash code” highlight=”” provider=”manual”/] [ad type=”banner”][] SQUARE BRACES/BRACKETS REGULAR EXPRESSION
Example : Find all the files which contains a number in the file name between a and x
[pastacode lang=”bash” manual=”ls%20-l%20%7C%20grep%20’a%5B0-9%5Dx’%0A%0AThis%20will%20find%20all%20the%20files%20which%20is%0Aa0xsdf%0Aasda1xsdfas%0A..%0A..%0Aasdfdsara9xsdf%0Aetc.%0A” message=”bash code” highlight=”” provider=”manual”/]- 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.
