Character classes ([a-z], [0-9]) in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to match all lowercase letters in the string.

Bash Scripting

echo "abc123" | grep -o '[[1]]'

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

AA-Z

Ba-z

C0-9

DA-Za-z

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase letters range [A-Z] instead of lowercase.

Using digits range [0-9] which matches numbers, not letters.

The character class [a-z] matches all lowercase letters from a to z.

2fill in blank

medium

Complete the code to match all digits in the string.

Bash Scripting

echo "abc123" | grep -o '[[1]]'

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

A0-9

Ba-z

CA-Z

Da-zA-Z

Attempts:

3 left

💡 Hint

Common Mistakes

Using letter ranges like [a-z] instead of digits.

Using uppercase letter ranges [A-Z] which do not match digits.

The character class [0-9] matches all digits from 0 to 9.

3fill in blank

hard

Fix the error in the code to match lowercase letters only.

Bash Scripting

echo "Hello123" | grep -o '[[1]]'

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

AA-Z

BA-Za-z

Ca-z

D0-9

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase letters range [A-Z] which matches uppercase letters.

Using combined ranges like [A-Za-z] which matches both cases.

To match lowercase letters only, use the range [a-z].

4fill in blank

hard

Fill both blanks to match lowercase letters and digits separately.

Bash Scripting

echo "a1b2c3" | grep -o '[[1]]' | sort | uniq && echo "a1b2c3" | grep -o '[[2]]' | sort | uniq

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

Aa-z

B0-9

CA-Z

Da-zA-Z

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing uppercase letters in the ranges.

Using the same range for both blanks.

Use [a-z] to match lowercase letters and [0-9] to match digits.

5fill in blank

hard

Fill all three blanks to create a grep pattern matching lowercase letters, digits, and uppercase letters.

Bash Scripting

echo "aA1bB2cC3" | grep -o '[[1][2][3]]'

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

Aa-z

B0-9

CA-Z

D_

Attempts:

3 left

💡 Hint

Common Mistakes

Using underscores or other characters that do not match letters or digits.

Separating ranges into multiple character classes instead of combining.

Combine ranges [a-z], [0-9], and [A-Z] inside one character class to match all lowercase letters, digits, and uppercase letters.

Practice

1. What does the character class [a-z] match in bash scripting?

easy

A. All lowercase letters from a to z

B. All uppercase letters from A to Z

C. All digits from 0 to 9

D. All special characters

Solution

1. Step 1: Understand character class notation

   The notation [a-z] means any character from 'a' to 'z' in lowercase.

2. Step 2: Identify what [a-z] matches

   This matches all lowercase letters, not uppercase letters, digits, or special characters.

3. Final Answer:

   All lowercase letters from a to z -> Option A

4. Quick Check:

   [a-z] = lowercase letters [OK]

Hint: Remember: a-z means lowercase letters only [OK]

Common Mistakes:

• Confusing lowercase with uppercase letters
• Thinking it matches digits
• Assuming it matches special characters

2. Which of the following is the correct syntax to match any digit using a character class in bash?

easy

A. [9-0]

B. [a-z]

C. [A-Z]

D. [0-9]

Solution

1. Step 1: Identify digit matching syntax

   The character class [0-9] matches any digit from 0 to 9.

2. Step 2: Check other options

   [a-z] matches lowercase letters, [A-Z] matches uppercase letters, and [9-0] is invalid because ranges must go from lower to higher.

3. Final Answer:

   [0-9] -> Option D

4. Quick Check:

   [0-9] = digits [OK]

Hint: Digits always use [0-9], not reversed ranges [OK]

Common Mistakes:

• Using reversed ranges like [9-0]
• Confusing letters with digits
• Using uppercase letter ranges for digits

3. What will be the output of this command?

echo -e "a1 b2 c3" | grep -o '[a-z][0-9]'

medium

A. a1 b2 c3

B. a1
b2
c3

C. a b c

D. 1 2 3

Solution

1. Step 1: Understand the grep command with -o and pattern

   The pattern [a-z][0-9] matches a lowercase letter followed by a digit. The -o option prints each match on a new line.

2. Step 2: Apply pattern to input string

   Input "a1 b2 c3" contains matches: "a1", "b2", and "c3". Each will be printed on its own line.

3. Final Answer:

   a1 b2 c3 -> Option B

4. Quick Check:

   Pattern matches pairs, -o prints each on new line [OK]

Hint: Use -o to print each match on a separate line [OK]

Common Mistakes:

• Expecting all matches on one line
• Matching letters or digits alone
• Ignoring the -o option effect

4. Identify the error in this grep command:

grep '[a-z-9]' file.txt

medium

A. Should use double brackets [[ ]] instead of single brackets

B. Missing quotes around the pattern

C. The character class is invalid because of the dash placement

D. No error, command is correct

Solution

1. Step 1: Analyze the character class syntax

   The pattern [a-z-9] is intended to match letters and digits, but the dash is misplaced between 'z' and '9'.

2. Step 2: Understand dash usage in character classes

   Dash (-) defines a range if placed between two characters. Here, 'z-9' is invalid because 'z' comes after '9' in ASCII order. Dash should be first or last to be literal.

3. Final Answer:

   The character class is invalid because of the dash placement -> Option C

4. Quick Check:

   Dash in ranges must go from lower to higher [OK]

Hint: Put dash at start or end to avoid range errors [OK]

Common Mistakes:

• Placing dash between characters in wrong order
• Confusing quotes usage
• Using wrong bracket types

5. You want to extract all words from a file that contain only lowercase letters and digits. Which grep pattern correctly matches such words?

grep -E '\b[?]+\b' file.txt

Choose the correct pattern to replace ?.

hard

A. [a-z0-9]+

B. [a-z]+[0-9]+

C. [a-z][0-9]

D. [0-9a-z]{2,}

Solution

1. Step 1: Understand the requirement for matching words

   We want words with only lowercase letters and digits, any order, any length (at least one character).

2. Step 2: Analyze each pattern

   [a-z0-9]+ matches one or more lowercase letters or digits in any order.
   [a-z]+[0-9]+ requires letters followed by digits only.
   [a-z][0-9] matches exactly two characters: one letter then one digit.
   [0-9a-z]{2,} matches two or more letters/digits but order is not enforced; this is valid but less clear than [a-z0-9]+.

3. Final Answer:

   [a-z0-9]+ -> Option A

4. Quick Check:

   Character class with + matches any length words [OK]

Hint: Use [a-z0-9]+ to match letters and digits in any order [OK]

Common Mistakes:

• Forcing letters then digits order
• Matching only two characters
• Ignoring word boundaries