11 Advanced Linux ‘Grep’ Commands on Character Classes and Bracket Expressions

Have you ever been into a situation where you need to search for a string, word or pattern inside a file? if yes, then the grep utility comes handy in such situation. grep is a command...

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Have you ever been into a situation where you need to search for a string, word or pattern inside a file? if yes, then the grep utility comes handy in such situation.

grep is a command line utility for searching plain-text data for lines which matching a regular expression. If you will divide the word grep like g/re/p then the meaning of grep is (globally search a regular expression and print) which search pattern from the file and print the line on the screen i.e. standard output.

Suggested Read: 12 Basic Practical Examples of Linux grep Command

In this article I will be going to explain advanced commands on grep for the Character Classes in Linux and Unix like operating system.

Here I have considered tecmint.txt is the base file where we will search pattern with the help of grep command in this article for explanation.

1. Search Alphanumeric Characters


If you have thousands of lines in a file and wanted to search a line which will start from only A-Z, a-z & 0-9 (Alphanumeric Characters).

$ grep "^[[:alnum:]]" tecmint.txt
Grep - Search Alphanumeric Characters in File

Grep – Search Alphanumeric Characters in File

2. Search Alpha Characters

Similar options like if you want to search line which will start from only [A-Z & a-z] i.e. Alpha Characters.

$ grep "^[[:alpha:]]" tecmint.txt
Grep - Search Alpha Characters in File

Grep – Search Alpha Characters in File

3. Search Blank Characters

Another options like if you want to search line which will start from [Tab & Space] i.e. Blank Characters.

$ grep "^[[:blank:]]" tecmint.txt
Grep - Search for Spaces or Tabs in File

Grep – Search for Spaces or Tabs in File

4. Search Digit Characters

The digit option for grep is also very useful to search line which will start from digit [0-9] i.e. Digit Characters.

$ grep "^[[:digit:]]" tecmint.txt
Grep - Search Number Characters in File

Grep – Search Number Characters in File

5. Search Lower Letters

Another option for grep is to search line which will start from lower letters i.e [a-z] (Lower Letters).

$ grep "^[[:lower:]]" tecmint.txt
Grep - Search Lower Letters or Words in File

Grep – Search Lower Letters or Words in File

6. Search Punctuation Characters

The Punctuation characters for grep is to search line which will start from [! ” # $ % & ‘ ( ) * + , – . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~. ] i.e. Punctuation Characters.

$ grep "^[[:punct:]]" tecmint.txt
Grep - Search Punctuation Characters in File

Grep – Search Punctuation Characters in File

7. Search Graphical Characters

The grep is also used to search a line which will start from Alphanumeric & Punctuation Characters called as Graphical Characters.

$ grep "^[[:graph:]]" tecmint.txt
Grep - Search Graphical Characters in File

Grep – Search Graphical Characters in File

8. Search Printable Characters

Similarly like Graphical Characters, grep is useful to search a line which will start from Alphanumeric, Punctuation and space characters.

$ grep "^[[:print:]]" tecmint.txt
Grep - Search Printable Characters in File

Grep – Search Printable Characters in File

9. Search Space Characters

The grep has also a functionality to search a line which will start from [tab, newline, vertical tab, form feed, carriage return, and space] i.e. Space Characters.

$ grep "^[[:space:]]" tecmint.txt
Grep - Search Space Characters in File

Grep – Search Space Characters in File

10. Search Uppercase Letters

Another option in the grep is also used to search a line which will start from [A-Z] i.e Upper-case Letters.

$ grep "^[[:upper:]]" tecmint.txt
Grep - Search Uppercase Letters in File

Grep – Search Uppercase Letters in File

11. Search Hexadecimal Digits

The grep searches a line which will start from [0-9, A-F and a-f] i.e Hexadecimal Digits.

$ grep "^[[:xdigit:]]" tecmint.txt
Grep - Search Hexadecimal Digits in File

Grep – Search Hexadecimal Digits in File

I have explained the advanced functionality of grep which is very strong and powerful tool to search the pattern in a File. Grep is also an important tool for shell scripting and programmers to search the pattern in the programs. It is worth to be familiar with other options and syntax to save the time.

Suggested Read: What’s Difference Between Grep, Egrep and Fgrep in Linux?

In case any issues on the commands which is explained in the article, you can post your comment in the comment section below.

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