Regex in [[ ]] with =~ in Bash Scripting

Introduction

You use regex inside [[ ]] with =~ to check if a string matches a pattern easily in bash scripts.

Checking if a user input contains only numbers.

Finding if a filename ends with .txt before processing it.

Validating if an email address looks correct in a script.

Detecting if a log line contains a specific error code.

Syntax

Bash Scripting

[[ string =~ regex_pattern ]]

The regex pattern is written without quotes inside [[ ]].

If the string matches the pattern, the condition is true.

Examples

Checks if 'hello123' has letters followed by numbers.

Bash Scripting

[[ "hello123" =~ [a-z]+[0-9]+ ]]

Checks if the variable input contains only digits.

Bash Scripting

[[ "$input" =~ ^[0-9]+$ ]]

Checks if the string ends with '.txt'.

Bash Scripting

[[ "file.txt" =~ \.txt$ ]]

Sample Program

This script asks for a word and checks if it has only letters using regex inside [[ ]] with =~.

Bash Scripting

#!/bin/bash

read -p "Enter a word: " word

if [[ $word =~ ^[a-zA-Z]+$ ]]; then
  echo "Only letters detected."
else
  echo "Contains other characters."
fi

Important Notes

Do not quote the regex pattern inside [[ ]] when using =~, or it won't work as expected.

Regex matching with =~ is case sensitive by default.

You can use parentheses in regex for grouping, but remember to escape special characters if needed.

Summary

Use [[ string =~ regex ]] to test patterns in bash scripts.

The condition is true if the string matches the regex.

Do not put quotes around the regex pattern inside [[ ]].