Extended regex (grep -E) in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of grep -E with alternation

What is the output of this command when run on a file containing these lines:

apple
banana
apricot
blueberry
blackberry

Command:

grep -E 'ap|bl' filename

Bash Scripting

grep -E 'ap|bl' filename

A

apple
apricot
blueberry
blackberry

B

apple
banana
apricot
blueberry
blackberry

C

banana
blueberry
blackberry

D

apple
apricot

💡 Hint

Look for lines containing either 'ap' or 'bl' anywhere in the line.

The pattern 'ap|bl' matches lines containing 'ap' or 'bl'. 'apple' and 'apricot' contain 'ap', 'blueberry' and 'blackberry' contain 'bl'. 'banana' does not match.

💻 Command Output

intermediate

Using grep -E with character classes

Given a file with these lines:

cat
cot
cut
cit
cet

What is the output of:

grep -E 'c[aeiou]t' filename

Bash Scripting

grep -E 'c[aeiou]t' filename

A

cat
cot
cut

B

cat
cot
cut
cet

C

cat
cot
cut
cit
cet

D

cat
cit
cet

💡 Hint

The character class [aeiou] matches any vowel.

The pattern matches lines starting with 'c', then a vowel (a,e,i,o,u), then 't'. All lines have 'c' + vowel + 't', so all lines match.

💻 Command Output

advanced

Output of grep -E with quantifiers

What is the output of this command on a file with these lines:

aaa
aa
aaaa
a
aaaaa

Command:

grep -E 'a{3,4}' filename

Bash Scripting

grep -E 'a{3,4}' filename

A

aaa
aaaa
aaaaa

B

aaa
aaaa

C

aaaa
aaaaa

D aaa

💡 Hint

The pattern matches sequences of 'a' repeated 3 or 4 times anywhere in the line.

Lines with at least 3 consecutive 'a's match. 'aaa', 'aaaa', and 'aaaaa' contain such sequences. 'aa' and 'a' do not.

💻 Command Output

advanced

grep -E with anchors and groups

Given a file with these lines:

foo123
123foo
foo456
bar123foo
foo

What is the output of:

grep -E '^foo[0-9]+$' filename

Bash Scripting

grep -E '^foo[0-9]+$' filename

A

foo123
123foo
foo456

B

foo123
foo456

C

foo123
foo456
foo

D

foo
foo123
foo456

💡 Hint

The pattern matches lines starting with 'foo' and ending with one or more digits.

Only lines starting with 'foo' and followed by digits until end match: 'foo123' and 'foo456'. 'foo' alone has no digits, others don't start with 'foo' or have extra chars.

💻 Command Output

expert

Complex pattern with grep -E and lookalike syntax

What error or output results from running this command on any file?

grep -E '(?<=foo)bar' filename

Bash Scripting

grep -E '(?<=foo)bar' filename

A No output, no error

B Prints lines containing 'foobar'

C Prints lines containing 'bar' only if preceded by 'foo'

D Syntax error: invalid regular expression

💡 Hint

grep -E does not support lookbehind assertions like (?<=...).

The '(?<=foo)' is a lookbehind assertion not supported by grep -E, causing a regex syntax error.