Capture groups in Bash in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this Bash script using capture groups?

Consider the following Bash script that uses a regex with capture groups. What will it print?

Bash Scripting

text="User: alice, ID: 42"
if [[ $text =~ User: ([a-z]+), ID: ([0-9]+) ]]; then
  echo "Name=${BASH_REMATCH[1]}, Number=${BASH_REMATCH[2]}"
else
  echo "No match"
fi

A Name=User, Number=alice

B Name=alice, Number=42

C No match

D Name=, Number=

💡 Hint

Remember that BASH_REMATCH stores the full match at index 0, and capture groups start at index 1.

The regex matches the text, capturing 'alice' and '42' in groups 1 and 2. BASH_REMATCH[1] and BASH_REMATCH[2] hold these values.

📝 Syntax

intermediate

Which Bash regex syntax correctly captures two groups?

Which option correctly uses capture groups in a Bash regex inside [[ ]]?

A [[ $var =~ /(foo)(bar)/ ]]

B [[ $var =~ \(foo\)\(bar\) ]]

C [[ $var =~ (foo)(bar) ]]; then

D [[ $var =~ (foo)(bar) ]]

💡 Hint

Bash regex inside [[ ]] does not use slashes or escaped parentheses.

In Bash, regex is written without slashes and parentheses are used directly for capture groups.

🔧 Debug

advanced

Why does this Bash script fail to capture groups?

Given this script, why does it print 'No match' even though the text looks correct? text="abc123" if [[ $text =~ abc([0-9]+) ]]; then echo "Number: ${BASH_REMATCH[1]}" else echo "No match" fi

A The variable text is not quoted in the regex

B The regex is missing anchors ^ and $

C The script is run with sh, not bash

D BASH_REMATCH is not set outside functions

💡 Hint

Check which shell runs the script; sh does not support BASH_REMATCH.

BASH_REMATCH is a Bash feature. Running the script with sh causes the regex match and capture groups to fail.

🚀 Application

advanced

Extract domain and path from URL using Bash capture groups

You want to extract the domain and path from this URL using Bash regex capture groups: url="https://example.com/path/to/page" Which option correctly extracts domain and path into variables domain and path?

A if [[ $url =~ https://([^/]+)/(.*) ]]; then domain=${BASH_REMATCH[1]}; path=${BASH_REMATCH[2]}; fi

B if [[ $url =~ https://(.*)/(.*) ]]; then domain=${BASH_REMATCH[1]}; path=${BASH_REMATCH[2]}; fi

C if [[ $url =~ https://([^/]+)(/.*) ]]; then domain=${BASH_REMATCH[1]}; path=${BASH_REMATCH[2]}; fi

D if [[ $url =~ https://([^/]+)(.*) ]]; then domain=${BASH_REMATCH[1]}; path=${BASH_REMATCH[2]}; fi

💡 Hint

The domain ends before the first slash after https://, and path starts after the slash.

Option A correctly captures the domain as all characters except slash, then the path as the rest after the slash.

🧠 Conceptual

expert

How many capture groups are in this Bash regex?

In the Bash regex used inside [[ ]]: regex='(a(b)c)(d(e)f)' How many capture groups does this regex have, and what does BASH_REMATCH[3] contain if matched against 'abcdef'?

A 4 groups; BASH_REMATCH[3] = 'def'

B 3 groups; BASH_REMATCH[3] = 'e'

C 5 groups; BASH_REMATCH[3] = 'd'

D 5 groups; BASH_REMATCH[3] = 'b'

💡 Hint

Count all parentheses pairs, including nested ones.

There are 4 capture groups, numbered by opening parenthesis order: group 1 '(a(b)c)' matches 'abc', group 2 '(b)' matches 'b', group 3 '(d(e)f)' matches 'def', group 4 '(e)' matches 'e'. Therefore, BASH_REMATCH[3] contains 'def'.