Regex in [[ ]] with =~ in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this Bash script using regex with [[ ]]?

Consider the following Bash script snippet. What will it print when run?

Bash Scripting

input="hello123"
if [[ $input =~ ^hello[0-9]+$ ]]; then
  echo "Match"
else
  echo "No match"
fi

A Runtime error

B No match

C Syntax error

D Match

💡 Hint

The regex checks if the string starts with 'hello' followed by one or more digits.

The input string 'hello123' matches the regex ^hello[0-9]+$ because it starts with 'hello' and ends with digits only.

💻 Command Output

intermediate

What error does this Bash regex condition produce?

What happens when you run this Bash snippet?

Bash Scripting

input="test"
if [[ $input =~ (test ]]; then
  echo "Yes"
else
  echo "No"
fi

A Syntax error

B No

C Yes

D Runtime error

💡 Hint

Look carefully at the regex pattern syntax inside [[ ]].

The regex pattern '(test' is missing a closing parenthesis, causing a syntax error.

🚀 Application

advanced

Which option correctly checks if a variable contains only lowercase letters using [[ ]] and =~?

You want to check if the variable 'word' contains only lowercase letters a-z. Which Bash snippet does this correctly?

A if [[ $word =~ ^[A-Z]+$ ]]; then echo "Valid"; fi

B if [[ $word =~ [a-z]+ ]]; then echo "Valid"; fi

C if [[ $word =~ ^[a-z]+$ ]]; then echo "Valid"; fi

D if [[ $word =~ ^[a-z]*$ ]]; then echo "Valid"; fi

💡 Hint

The regex should match the whole string with only lowercase letters.

Option C uses ^ and $ anchors with [a-z]+ to ensure the entire string is lowercase letters only.

🔧 Debug

advanced

Why does this Bash regex condition always fail?

Examine this code snippet: input="abc123" if [[ $input =~ "^[a-z]+[0-9]+$" ]]; then echo "Match" else echo "No match" fi Why does it print "No match" even though the input fits the pattern?

A Because the regex is quoted, it is treated as a string, not a regex pattern

B Because the input variable is not quoted

C Because =~ does not support character classes

D Because the regex is missing anchors

💡 Hint

Check how quoting affects regex matching in [[ ]]

Quoting the regex disables regex matching and treats it as a fixed string, so the match fails.

🧠 Conceptual

expert

What is the value of BASH_REMATCH after this regex match?

Given this Bash code: text="[e-mail]" if [[ $text =~ ([a-z]+)@([a-z]+)\.com ]]; then echo "${BASH_REMATCH[0]}" echo "${BASH_REMATCH[1]}" echo "${BASH_REMATCH[2]}" fi What will be printed?

A

[e-mail]
user@example
com

B

[e-mail]
user
example

C

[e-mail]
example
user

D Syntax error

💡 Hint

BASH_REMATCH[0] is the full match; subsequent indices are capture groups.

BASH_REMATCH[0] holds the full matched string, [1] the first group (user), [2] the second group (example).