Regex in [[ ]] with =~ in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the variable 'text' contains the word 'hello' using regex.

Bash Scripting

if [[ $text [1] "hello" ]]; then
  echo "Match found"
fi

Drag options to blanks, or click blank then click option'

A=

B==

C=~

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '=~' for regex matching.

Using single brackets [ ] which do not support '=~'.

The operator '=~' is used inside [[ ]] to test if a string matches a regex pattern.

2fill in blank

medium

Complete the code to check if the variable 'input' starts with a digit using regex.

Bash Scripting

if [[ $input [1] ^[0-9] ]]; then
  echo "Starts with a digit"
fi

Drag options to blanks, or click blank then click option'

A!=

B==

C=

D=~

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '=~' for regex.

Not escaping special characters when needed.

The '=~' operator is used to match regex patterns inside [[ ]]. '^' means start of string.

3fill in blank

hard

Fix the error in the code to correctly check if 'var' contains only letters.

Bash Scripting

if [[ $var [1] ^[a-zA-Z]+$ ]]; then
  echo "Only letters"
fi

Drag options to blanks, or click blank then click option'

A=~

B==

C=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '=~' causes the regex to be treated as a string.

Forgetting to quote the variable can cause errors if it is empty.

Use '=~' to match regex. '^[a-zA-Z]+$' matches strings with only letters.

4fill in blank

hard

Fill both blanks to check if 'filename' ends with '.txt' using regex.

Bash Scripting

if [[ $filename [1] \.[2]$ ]]; then
  echo "Text file"
fi

Drag options to blanks, or click blank then click option'

A=~

Btxt

Ctxt$

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '=~' for regex.

Not escaping the dot before 'txt'.

Use '=~' for regex matching. '\.txt$' matches strings ending with '.txt'.

5fill in blank

hard

Fill all three blanks to create a regex that matches a string with exactly 3 digits.

Bash Scripting

if [[ $code [1] ^[2][3]$ ]]; then
  echo "Three digits"
fi

Drag options to blanks, or click blank then click option'

A=~

B[0-9]

C{3}

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '=~'.

Forgetting to use curly braces for exact count.

Use '=~' for regex matching. '^[0-9]{3}$' matches exactly three digits.

Practice

1. What does the expression [[ "hello123" =~ [a-z]+[0-9]+ ]] check in a bash script?

easy

A. If the string is exactly 'hello123'

B. If the string contains letters followed by numbers

C. If the string contains only numbers

D. If the string starts with a number

Solution

1. Step 1: Understand the regex pattern

   The pattern [a-z]+[0-9]+ means one or more lowercase letters followed by one or more digits.

2. Step 2: Apply the pattern to the string

   The string "hello123" starts with letters "hello" and ends with digits "123", so it matches the pattern.

3. Final Answer:

   If the string contains letters followed by numbers -> Option B

4. Quick Check:

   Regex matches letters then digits = A [OK]

Hint: Look for letter-digit sequence in string with regex [OK]

Common Mistakes:

• Thinking quotes are needed around regex inside [[ ]]
• Confusing exact match with pattern match
• Assuming regex matches anywhere without order

2. Which of the following is the correct syntax to test if variable var matches regex ^[0-9]+$ in bash?

easy

A. [[ $var =~ "^[0-9]+$" ]]

B. [[ "$var" =~^[0-9]+$ ]]

C. [[ $var =~ ^[0-9]+$ ]]

D. [[ "$var" =~ "^[0-9]+$" ]]

Solution

1. Step 1: Recall regex syntax in [[ ]]

   Inside [[ ]], the regex pattern should NOT be quoted to work correctly.

2. Step 2: Check each option

   [[ $var =~ ^[0-9]+$ ]] uses [[ $var =~ ^[0-9]+$ ]] without quotes around regex, which is correct syntax.

3. Final Answer:

   [[ $var =~ ^[0-9]+$ ]] -> Option C

4. Quick Check:

   No quotes around regex pattern = B [OK]

Hint: Do not quote regex pattern inside [[ ]] [OK]

Common Mistakes:

• Putting quotes around regex pattern causing syntax errors
• Quoting variable but not regex or vice versa
• Using single brackets [ ] instead of [[ ]]

3. What will be the output of this bash script snippet?

text="abc123"
if [[ $text =~ ^[a-z]+[0-9]+$ ]]; then
  echo "Match"
else
  echo "No match"
fi

medium

A. Match

B. No match

C. Syntax error

D. Empty output

Solution

1. Step 1: Analyze the regex pattern

   The pattern ^[a-z]+[0-9]+$ means the string must start with one or more lowercase letters and end with one or more digits, with nothing else.

2. Step 2: Check the string against the pattern

   The string "abc123" starts with letters "abc" and ends with digits "123", so it matches the pattern exactly.

3. Final Answer:

   Match -> Option A

4. Quick Check:

   String matches regex pattern = D [OK]

Hint: Check start ^ and end $ anchors carefully [OK]

Common Mistakes:

• Ignoring anchors ^ and $ in regex
• Assuming partial match is enough
• Quoting regex pattern causing failure

4. Identify the error in this bash snippet:

input="2023-04-01"
if [[ "$input" =~ "^[0-9]{4}-[0-9]{2}-[0-9]{2}$" ]]; then
  echo "Valid date"
else
  echo "Invalid date"
fi

medium

A. Quotes around regex cause the match to fail

B. Variable input is not quoted

C. Regex pattern is invalid syntax

D. Missing semicolon after if condition

Solution

1. Step 1: Check quoting rules for regex in [[ ]]

   Regex pattern inside [[ ]] with =~ must NOT be quoted; quotes make it a string literal, breaking regex matching.

2. Step 2: Identify the error in the snippet

   The snippet quotes the regex pattern, causing the condition to always fail and print "Invalid date".

3. Final Answer:

   Quotes around regex cause the match to fail -> Option A

4. Quick Check:

   No quotes around regex pattern = C [OK]

Hint: Never quote regex pattern inside [[ ]] [OK]

Common Mistakes:

• Quoting regex pattern inside [[ ]]
• Forgetting to quote variable when needed
• Assuming regex syntax errors without checking quotes

5. You want to check if a variable filename ends with either ".jpg" or ".png" using regex in bash. Which condition correctly tests this?

hard

A. [[ "$filename" =~ "\.(jpg|png)$" ]]

B. [[ $filename =~ "\.(jpg|png)$" ]]

C. [[ "$filename" =~\.(jpg|png)$ ]]

D. [[ $filename =~ \.(jpg|png)$ ]]

Solution

1. Step 1: Understand regex for file extensions

   The pattern \.(jpg|png)$ matches a dot followed by 'jpg' or 'png' at the end of the string.

2. Step 2: Apply correct quoting rules

   Regex pattern must NOT be quoted inside [[ ]], and variable can be unquoted if it has no spaces. [[ $filename =~ \.(jpg|png)$ ]] correctly uses unquoted regex and variable.

3. Final Answer:

   [[ $filename =~ \.(jpg|png)$ ]] -> Option D

4. Quick Check:

   Unquoted regex with alternation = A [OK]

Hint: Use unquoted regex with alternation inside [[ ]] [OK]

Common Mistakes:

• Quoting regex pattern causing failure
• Not escaping dot before extension
• Quoting variable unnecessarily