Regex in [[ ]] with =~ in Bash Scripting - Mini Project: Build & Apply

Using Regex in Bash with [[ ]] and =~

📖 Scenario: You work in a small company where you need to check if user input matches certain patterns. You want to automate this check using a Bash script.

🎯 Goal: Build a Bash script that uses the [[ ]] test with the =~ operator to check if a string matches a regular expression pattern.

📋 What You'll Learn

Create a variable with a specific string value

Create a regex pattern variable to match a pattern

Use the [[ ]] test with =~ to check if the string matches the regex

Print a message showing if the string matches or not

💡 Why This Matters

🌍 Real World

Checking if user input or file names match expected patterns is common in scripts that automate tasks like validation or filtering.

💼 Career

Many IT and DevOps roles require writing Bash scripts that validate data formats using regex to automate system checks and data processing.

1

Create a string variable

Create a variable called input_string and set it to the exact value "hello123".

Bash Scripting

# Create the variable input_string with value "hello123"
# Your code here

Hint

Use variable_name="value" syntax to create a string variable in Bash.

2

Create a regex pattern variable

Create a variable called pattern and set it to the regex ^[a-z]+[0-9]+$ which matches strings starting with letters followed by numbers.

Bash Scripting

input_string="hello123"
# Create the variable pattern with regex ^[a-z]+[0-9]+$
# Your code here

Hint

Use single quotes to store the regex pattern as a string in Bash.

3

Use [[ ]] with =~ to test the regex

Write an if statement using [[ $input_string =~ $pattern ]] to check if input_string matches pattern. Inside the if, set a variable result to "Match", else set result to "No match".

Bash Scripting

input_string="hello123"
pattern='^[a-z]+[0-9]+$'
# Use if [[ $input_string =~ $pattern ]] to check match
# Set result="Match" if true, else result="No match"
# Your code here

Hint

Use if [[ $var =~ $regex ]]; then ... else ... fi syntax in Bash.

4

Print the result

Write a echo command to print the value of the result variable.

Bash Scripting

input_string="hello123"
pattern='^[a-z]+[0-9]+$'
if [[ $input_string =~ $pattern ]]; then
    result="Match"
else
    result="No match"
fi
# Print the result variable
# Your code here

Hint

Use echo "$result" to print the variable's value.

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