Quantifiers (*, +, ?) in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this grep command using the * quantifier?

Consider a file named data.txt with the following content:

apple
appple
aple
appppple

What will be the output of the command grep -E 'ap*le' data.txt?

Bash Scripting

grep -E 'ap*le' data.txt

A

aple
appple

B

apple
aple

C

apple
appple
appppple

D

apple
appple
aple
appppple

💡 Hint

Remember, * means zero or more of the previous character.

The pattern 'ap*le' matches 'a' followed by zero or more 'p's, then 'le'. All lines match: 'apple' (two 'p's), 'appple' (three 'p's), 'aple' (one 'p'), 'appppple' (four 'p's).

💻 Command Output

intermediate

What does the + quantifier do in this grep command?

Given a file words.txt with content:

cat
caat
ct
caaat

What will be the output of grep -E 'ca+t' words.txt?

Bash Scripting

grep -E 'ca+t' words.txt

A

cat
caat
caaat

B

cat
ct

C

cat
caat
ct
caaat

D

caat
caaat

💡 Hint

The + quantifier means one or more of the previous character.

The pattern 'ca+t' matches 'c' followed by one or more 'a's, then 't'. So 'cat', 'caat', and 'caaat' match. 'ct' does not match because it has zero 'a's.

💻 Command Output

advanced

What is the output of this grep command using the ? quantifier?

File input.txt contains:

color
colour
colr
colouur

What will grep -E 'colou?r' input.txt output?

Bash Scripting

grep -E 'colou?r' input.txt

A

color
colour
colouur

B

colour
colouur

C

color
colour

D

color
colr

💡 Hint

The ? quantifier means zero or one of the previous character.

The pattern 'colou?r' matches 'colo' followed by zero or one 'u', then 'r'. So 'color' (zero 'u') and 'colour' (one 'u') match. 'colr' (missing 'o') and 'colouur' (two 'u's) do not match.

🔧 Debug

advanced

Why does this grep command fail to match expected lines?

You run grep -E 'ab+c*' file.txt on a file with lines:

abc
abbc
ac
abcc

But it matches 'abc', 'abbc', and 'abcc'. Why doesn't it match 'ac'?

Bash Scripting

grep -E 'ab+c*' file.txt

A Because the pattern requires at least one 'b' and zero or more 'c's, so 'ac' is missing 'b'.

B Because + quantifier is not supported in grep -E.

C Because c* matches zero or more 'c's, so 'ac' does not match as 'b+' requires at least one 'b'.

D Because the pattern matches only lines ending with 'c', so 'abcc' is excluded.

💡 Hint

Check what each quantifier means and what the pattern requires.

The pattern 'ab+c*' means 'a' followed by one or more 'b's, then zero or more 'c's. 'ac' has zero 'b's, so it does not match. 'abcc' matches because it has 'a', 'b', and 'cc'.

🧠 Conceptual

expert

How many lines will match this grep pattern with mixed quantifiers?

File test.txt contains:

aaab
ab
aab
b
aaaab

How many lines will match grep -E 'a+b?a*' test.txt?

Bash Scripting

grep -E 'a+b?a*' test.txt

A 5

B 4

C 2

D 3

💡 Hint

Break down the pattern: a+ then b? then a*.

The pattern 'a+b?a*' means one or more 'a's, optionally one 'b', then zero or more 'a's. Matching lines: 'aaab', 'ab', 'aab', 'aaaab'. 'b' does not match (no initial a+). So 4 lines match (option B).

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