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).