Capture groups in Bash in Bash Scripting

Introduction

Capture groups let you find and save parts of text that match a pattern. This helps you work with specific pieces of text easily.

Extract a username from an email address.

Get the date parts (year, month, day) from a filename.

Find and save phone number parts from a text.

Split a string into parts based on a pattern.

Check if a string matches a pattern and get details.

Syntax

Bash Scripting

if [[ string =~ regex ]]; then
  echo "${BASH_REMATCH[1]}"
fi

Use double square brackets [[ ]] with =~ to match regex.

Captured groups are saved in BASH_REMATCH array starting at index 1.

Examples

This splits 'hello123' into letters and numbers using capture groups.

Bash Scripting

text="hello123"
if [[ $text =~ ([a-z]+)([0-9]+) ]]; then
  echo "Letters: ${BASH_REMATCH[1]}"
  echo "Numbers: ${BASH_REMATCH[2]}"
fi

Extracts username and domain from an email address.

Bash Scripting

email="[e-mail]"
if [[ $email =~ ([^@]+)@(.+) ]]; then
  echo "User: ${BASH_REMATCH[1]}"
  echo "Domain: ${BASH_REMATCH[2]}"
fi

Sample Program

This script extracts year, month, and day from a filename using capture groups.

Bash Scripting

#!/bin/bash

input="2024-06-15_report.txt"

if [[ $input =~ ([0-9]{4})-([0-9]{2})-([0-9]{2})_report\.txt ]]; then
  year=${BASH_REMATCH[1]}
  month=${BASH_REMATCH[2]}
  day=${BASH_REMATCH[3]}
  echo "Year: $year"
  echo "Month: $month"
  echo "Day: $day"
else
  echo "No match found."
fi

Important Notes

BASH_REMATCH[0] contains the full matched string.

Capture groups start from index 1 in BASH_REMATCH.

Make sure to quote variables when using regex to avoid word splitting.

Summary

Capture groups save parts of matched text for easy use.

Use [[ string =~ regex ]] and BASH_REMATCH array in Bash.

Index 1 and up in BASH_REMATCH hold the captured groups.