grep with regex (-E) in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of grep -E with alternation

Given a file colors.txt with the following content:

red
blue
green
yellow
purple
orange

What is the output of this command?

grep -E 'red|blue|pink' colors.txt

A

red
blue
pink
purple

B

red
blue

C

red
blue
pink

D

blue
green

💡 Hint

The -E option allows using extended regex. The pattern matches lines containing 'red' or 'blue' or 'pink'.

The command searches for lines containing either 'red', 'blue', or 'pink'. Since 'pink' is not in the file, only 'red' and 'blue' lines are printed.

💻 Command Output

intermediate

Using grep -E with character classes and quantifiers

Consider a file words.txt containing:

cat
cot
cut
cit
cet
citizen
cute

What is the output of:

grep -E '^c[aeiou]t$' words.txt

A

citizen
cute

B

cat
cot
cut
cit
cet
citizen
cute

C

cat
cot
cut
cit
cet

D

cat
cot
cut

💡 Hint

The pattern matches lines starting with 'c', followed by a vowel, then 't', and nothing else.

The regex ^c[aeiou]t$ matches exactly three-letter words starting with 'c', followed by a vowel (a,e,i,o,u), and ending with 't'. Matches: 'cat' (a), 'cot' (o), 'cut' (u), 'cit' (i), 'cet' (e). 'citizen' and 'cute' have additional characters.

💻 Command Output

advanced

Output of grep -E with grouping and quantifiers

Given a file numbers.txt with:

123
1123
11123
111123
1111123

What is the output of:

grep -E '^1{3}23$' numbers.txt

A 11123

B

11123
111123

C

123
1123
11123

D

111123
1111123

💡 Hint

The pattern matches lines starting with exactly three '1's followed by '23' and nothing else.

The regex ^1{3}23$ matches lines that start with exactly three '1's then '23'. Only '11123' matches exactly. '111123' has four '1's, so it does not match.

💻 Command Output

advanced

grep -E with optional groups and anchors

File animals.txt contains:

cat
cats
dog
dogs
caterpillar
catalog

What is the output of:

grep -E '^cat(s)?$' animals.txt

A

cat
cats

B

cat
cats
caterpillar
catalog

C

cats
caterpillar

D cat

💡 Hint

The pattern matches lines starting with 'cat' and optionally ending with 's', and nothing else.

The regex ^cat(s)?$ matches 'cat' or 'cats' exactly. 'caterpillar' and 'catalog' have extra characters, so they do not match.

💻 Command Output

expert

Complex grep -E regex with nested groups and alternation

File log.txt contains:

error123
warning456
info789
error456
warning123
info123

What is the output of:

grep -E '^(error|warning)(123|456)$' log.txt

A

error456
warning456
info123

B

error123
warning456
info123

C

error123
warning123
info789

D

error123
error456
warning123
warning456

💡 Hint

The pattern matches lines starting with 'error' or 'warning' followed by '123' or '456' exactly.

The regex ^(error|warning)(123|456)$ matches lines that start with either 'error' or 'warning' and end with either '123' or '456'. So 'error123', 'error456', 'warning123', and 'warning456' match. 'info789' and 'info123' do not match.