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'.

Practice

1. What does the BASH_REMATCH array hold after a successful regex match in Bash?

easy

A. The full matched string at index 0 and captured groups from index 1 onwards

B. Only the captured groups without the full match

C. The regex pattern used for matching

D. The exit status of the last command

Solution

1. Step 1: Understand BASH_REMATCH usage

   When using [[ string =~ regex ]], Bash stores the full matched text in BASH_REMATCH[0].

2. Step 2: Identify captured groups storage

   Captured groups from parentheses in the regex are stored starting at BASH_REMATCH[1] onwards.

3. Final Answer:

   The full matched string at index 0 and captured groups from index 1 onwards -> Option A

4. Quick Check:

   BASH_REMATCH = full match + groups [OK]

Hint: Remember index 0 is full match, groups start at 1 [OK]

Common Mistakes:

• Thinking BASH_REMATCH holds only groups
• Confusing BASH_REMATCH with exit status
• Expecting regex pattern in BASH_REMATCH

2. Which of the following is the correct syntax to test if a string matches a regex and capture groups in Bash?

easy

A. match $string $regex

B. if ($string =~ $regex); then

C. [[ $string =~ $regex ]]

D. test $string =~ $regex

Solution

1. Step 1: Recall Bash regex match syntax

   Bash uses the conditional expression [[ string =~ regex ]] to test regex matches.

2. Step 2: Identify correct syntax among options

   Only [[ $string =~ $regex ]] uses the correct Bash syntax with double brackets and the =~ operator.

3. Final Answer:

   [[ $string =~ $regex ]] -> Option C

4. Quick Check:

   Use [[ ]] with =~ for regex match [OK]

Hint: Use double brackets [[ ]] with =~ for regex matching [OK]

Common Mistakes:

• Using single brackets [ ] which don't support =~
• Using parentheses () like in other languages
• Using commands like test or match incorrectly

3. What will be the output of this Bash script?

string="user:1234"
if [[ $string =~ ([a-z]+):([0-9]+) ]]; then
  echo "Name=${BASH_REMATCH[1]}, ID=${BASH_REMATCH[2]}"
fi

medium

A. Name=user, ID=1234

B. Name=user:1234, ID=

C. Name=, ID=

D. Syntax error

Solution

1. Step 1: Analyze regex and string

   The regex ([a-z]+):([0-9]+) matches letters before colon and digits after colon in the string "user:1234".

2. Step 2: Check captured groups in BASH_REMATCH

   BASH_REMATCH[1] captures "user" and BASH_REMATCH[2] captures "1234".

3. Final Answer:

   Name=user, ID=1234 -> Option A

4. Quick Check:

   Captured groups print correct values [OK]

Hint: Captured groups appear in BASH_REMATCH starting at index 1 [OK]

Common Mistakes:

• Expecting full string in BASH_REMATCH[1]
• Forgetting to use double brackets [[ ]]
• Misreading regex pattern

4. Identify the error in this Bash snippet:

string="abc123"
if [[$string =~ ([a-z]+)([0-9]+)]]; then
  echo "${BASH_REMATCH[1]} and ${BASH_REMATCH[2]}"
fi

medium

A. Regex pattern is invalid

B. Missing spaces inside [[ and ]]

C. BASH_REMATCH is not set by =~ operator

D. Using single brackets instead of double brackets

Solution

1. Step 1: Check syntax of conditional expression

   The conditional expression [[ ... ]] requires spaces after [[ and before ]]. The snippet misses spaces.

2. Step 2: Confirm regex and BASH_REMATCH usage

   The regex is valid and BASH_REMATCH is set correctly when using =~ inside [[ ]].

3. Final Answer:

   Missing spaces inside [[ and ]] -> Option B

4. Quick Check:

   Spaces needed inside [[ and ]] [OK]

Hint: Always put spaces after [[ and before ]] [OK]

Common Mistakes:

• Omitting spaces inside [[ ]]
• Confusing single [ ] with [[ ]]
• Assuming regex pattern is wrong

5. You want to extract the domain and path from a URL string in Bash using capture groups. Which regex and code snippet correctly captures domain and path?

url="https://example.com/path/to/page"
if [[ $url =~ https://([^/]+)(/.*) ]]; then
  echo "Domain: ${BASH_REMATCH[1]}"
  echo "Path: ${BASH_REMATCH[2]}"
fi

hard

A. Captures domain and path but misses https:// prefix

B. BASH_REMATCH does not support multiple groups

C. Fails because regex does not escape dots

D. Correctly captures domain as example.com and path as /path/to/page

Solution

1. Step 1: Analyze regex pattern

   The regex https://([^/]+)(/.*) matches 'https://' literally, then captures domain (any chars except '/') and path (slash followed by anything).

2. Step 2: Check BASH_REMATCH usage

   BASH_REMATCH[1] holds domain, BASH_REMATCH[2] holds path, correctly extracted from the URL.

3. Final Answer:

   Correctly captures domain as example.com and path as /path/to/page -> Option D

4. Quick Check:

   Regex groups extract domain and path correctly [OK]

Hint: Use ([^/]+) for domain and (/.*) for path [OK]

Common Mistakes:

• Not escaping dots in regex (not needed inside character class)
• Assuming BASH_REMATCH can't hold multiple groups
• Missing literal https:// in regex