Quantifiers (*, +, ?) in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to match zero or more 'a' characters in the string.

Bash Scripting

echo 'aaaab' | grep -E 'a[1]b'

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

A+

B*

C?

D{2}

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' which requires at least one 'a'.

Using '?' which matches zero or one 'a' only.

The '*' quantifier matches zero or more occurrences of the preceding character.

2fill in blank

medium

Complete the code to match one or more digits in the input.

Bash Scripting

echo '12345' | grep -E '[0-9][1]'

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

A+

B*

C?

D{0,}

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' which allows zero digits, so it matches empty strings.

Using '?' which matches zero or one digit only.

The '+' quantifier matches one or more occurrences of the preceding pattern.

3fill in blank

hard

Complete the code to match zero or one 'a' followed by 'b'.

Bash Scripting

echo 'ab' | grep -E 'a[1]b'

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

A?

B*

C+

D{1,2}

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' which requires at least one 'a'.

Using '*' which allows any number including many 'a's.

The '?' quantifier matches zero or one occurrence of the preceding character.

4fill in blank

hard

Fill both blanks to match a string starting with one or more 'x' followed by zero or one 'y'.

Bash Scripting

echo 'xxxy' | grep -E '^x[1]y[2]$'

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

A+

B*

C?

D{2}

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' for 'x' which allows zero 'x's, but we want at least one.

Using '+' for 'y' which requires at least one 'y'.

Use '+' to match one or more 'x' and '?' to match zero or one 'y'.

5fill in blank

hard

Fill all three blanks to match a string with zero or more 'a's, followed by one or more 'b's, then zero or one 'c'.

Bash Scripting

echo 'aaabbc' | grep -E '^a[1]b[2]c[3]$'

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

A*

B+

C?

D{3}

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' for 'a' which requires at least one 'a'.

Using '*' for 'b' which allows zero 'b's but we need at least one.

Using '+' for 'c' which requires at least one 'c'.

Use '*' for zero or more 'a's, '+' for one or more 'b's, and '?' for zero or one 'c'.

Practice

1. In bash scripting, what does the quantifier * mean when used in a regular expression?

easy

A. Match zero or more occurrences of the preceding element

B. Match exactly one occurrence of the preceding element

C. Match one or more occurrences of the preceding element

D. Match zero or one occurrence of the preceding element

Solution

1. Step 1: Understand the meaning of * in regex

   The * quantifier matches zero or more occurrences of the character or group before it.

2. Step 2: Compare with other quantifiers

   Unlike + which requires at least one match, * allows zero matches too.

3. Final Answer:

   Match zero or more occurrences of the preceding element -> Option A

4. Quick Check:

   * means zero or more matches [OK]

Hint: Remember * means zero or more times, including none [OK]

Common Mistakes:

• Confusing * with + which requires at least one match
• Thinking * matches exactly one occurrence
• Mixing * with ? which means zero or one

2. Which of the following is the correct way to use the + quantifier in grep to match one or more digits?

easy

A. grep '[0-9]+' file.txt

B. grep -P '[0-9]*' file.txt

C. grep '[0-9]*' file.txt

D. grep -E '[0-9]+' file.txt

Solution

1. Step 1: Recognize that + needs extended regex

   The + quantifier is not supported by default in basic grep; it requires -E or -P option.

2. Step 2: Identify correct grep syntax

   Using grep -E '[0-9]+' enables extended regex and matches one or more digits.

3. Final Answer:

   grep -E '[0-9]+' file.txt -> Option D

4. Quick Check:

   Use -E or -P for + quantifier [OK]

Hint: Use grep -E or -P to enable + quantifier [OK]

Common Mistakes:

• Using + without -E or -P causes literal + match
• Confusing * with + quantifier
• Using -P without knowing if Perl regex is supported

3. What will be the output of the following command if file.txt contains the lines:

apple
apples
ap
ape

Command:

grep -E 'ap?le' file.txt

medium

A. apple

B. (no output)

C. apple
aple

D. apple
aple
ape

Solution

1. Step 1: Understand the regex ap?le

   The p? means zero or one 'p' character. So it matches 'ale' or 'aple'.

2. Step 2: Check which lines match

   Lines: 'apple' (has two p's, so no), 'apples' (extra chars, no), 'ap' (too short), 'ape' (does not match 'ap?le' because 'l' is missing). Actually 'apple' has two 'p's, so 'ap?le' matches 'ale' or 'aple' but 'apple' has 'pp', so no match. 'ape' does not match 'ap?le' because 'l' is missing. So only 'apple' matches partially but not fully.

3. Step 3: Re-examine lines carefully

   Actually, 'apple' is 'a p p l e'. The regex 'ap?le' means 'a' followed by zero or one 'p', then 'l', then 'e'. So 'apple' has two 'p's, so it does not match. 'ape' does not match 'ap?le' because 'l' is missing. So no output.

4. Final Answer:

   apple -> Option A

5. Quick Check:

   Regex matches zero or one p, so 'apple' no match [OK]

Hint: p? means zero or one p, so two p's won't match [OK]

Common Mistakes:

• Assuming 'apple' matches when it has two p's
• Ignoring that ? allows zero or one occurrence only
• Confusing 'ap?le' with 'ap*le'

4. You wrote this grep command to find lines with one or more digits:

grep '[0-9]+' file.txt

But it returns no matches even though the file has digits. What is the likely problem?

medium

A. The regex should use * instead of +

B. The + quantifier needs -E or -P option to work

C. Digits are not matched by [0-9]

D. The file.txt is empty

Solution

1. Step 1: Understand grep default regex

   By default, grep uses basic regex where + is treated as a literal plus sign, not a quantifier.

2. Step 2: Identify correct usage for + quantifier

   To use + as 'one or more', grep needs -E (extended regex) or -P (Perl regex) option.

3. Final Answer:

   The + quantifier needs -E or -P option to work -> Option B

4. Quick Check:

   Use -E or -P for + quantifier [OK]

Hint: Add -E or -P to grep when using + quantifier [OK]

Common Mistakes:

• Using + without enabling extended regex
• Thinking * and + are interchangeable always
• Assuming [0-9] does not match digits

5. You want to write a bash script that uses grep to find lines containing words that start with 'cat' followed by zero or one 's' (to match 'cat' or 'cats'). Which grep command correctly uses quantifiers to do this?

hard

A. grep -E '\bcat?s\b' file.txt

B. grep '\bcat?s\b' file.txt

C. grep -E '\bcats?\b' file.txt

D. grep '\bcats?\b' file.txt

Solution

1. Step 1: Understand the pattern needed

   We want to match 'cat' or 'cats'. The 's' is optional, so use s? quantifier.

2. Step 2: Use word boundaries and extended regex

   Use \b for word boundaries and -E to enable ? quantifier.

3. Step 3: Check options

   grep -E '\bcats?\b' file.txt uses grep -E '\bcats?\b' which matches 'cat' or 'cats' correctly. Others either lack -E or misplace the quantifier.

4. Final Answer:

   grep -E '\bcats?\b' file.txt -> Option C

5. Quick Check:

   Use -E and s? to match zero or one s [OK]

Hint: Use -E and s? to match optional s after cat [OK]

Common Mistakes:

• Forgetting -E option for ? quantifier
• Placing ? after wrong character
• Not using word boundaries causing partial matches