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.

Practice

1. What does the grep -E command do in bash scripting?

easy

A. Runs grep with extra debugging information

B. Enables extended regular expressions for more powerful pattern matching

C. Executes grep in encrypted mode

D. Searches only for exact string matches without regex

Solution

1. Step 1: Understand grep basic function

   grep searches text for patterns. By default, it uses basic regex.

2. Step 2: Role of -E option

   The -E option enables extended regex, allowing special characters like +, ?, | without backslashes.

3. Final Answer:

   Enables extended regular expressions for more powerful pattern matching -> Option B

4. Quick Check:

   grep -E = extended regex enabled [OK]

Hint: Remember: -E means extended regex, more pattern power [OK]

Common Mistakes:

• Confusing -E with debugging or encryption
• Thinking -E disables regex
• Assuming -E is for exact string match only

2. Which of the following is the correct syntax to search for lines containing either "cat" or "dog" using grep -E?

easy

A. grep -E 'cat|dog' filename

B. grep -E 'cat&dog' filename

C. grep -E 'cat?dog' filename

D. grep -E 'cat+dog' filename

Solution

1. Step 1: Understand the OR operator in extended regex

   The vertical bar | means OR, matching either pattern on left or right.

2. Step 2: Check each option's pattern

   'cat|dog' matches lines with 'cat' or 'dog'. Others (&, ?, +) do not mean OR.

3. Final Answer:

   grep -E 'cat|dog' filename -> Option A

4. Quick Check:

   Use | for OR in grep -E [OK]

Hint: Use | inside quotes for OR patterns with grep -E [OK]

Common Mistakes:

• Using & instead of | for OR
• Confusing ? or + as OR
• Not quoting the pattern

3. Given a file data.txt with the following lines:

apple
apples
applepie
appl
apply

What is the output of grep -E 'apple(s|pie)?' data.txt?

medium

A. applepie

B. apple
apples
applepie
appl
apply

C. apples
applepie

D. apple
apples
applepie

Solution

1. Step 1: Understand the regex pattern 'apple(s|pie)?'

   This matches 'apple' followed optionally by either 's' or 'pie'. So it matches 'apple', 'apples', or 'applepie'.

2. Step 2: Check each line against the pattern

   'apple' matches exactly.
   'apples' matches 'apple' + 's'.
   'applepie' matches 'apple' + 'pie'.
   'appl' and 'apply' do not match because they lack the full 'apple' base.

3. Final Answer:

   apple apples applepie -> Option D

4. Quick Check:

   Optional group (s|pie)? matches these three lines [OK]

Hint: Remember ? means optional, (a|b) means either a or b [OK]

Common Mistakes:

• Assuming partial matches like 'appl' count
• Ignoring the optional group ?
• Confusing | with + or ?

4. You run the command grep -E colou?r file.txt but it returns no matches, even though the file contains the word "color". What is the likely problem?

medium

A. The pattern is missing the optional character syntax

B. The file does not contain the word "color"

C. The regex should be quoted to prevent shell expansion

D. The pattern is correct but the file encoding is wrong

Solution

1. Step 1: Check the pattern and shell quoting

   The pattern 'colou?r' uses ? to make 'u' optional, matching 'color' or 'colour'.

2. Step 2: Identify quoting issue

   If the pattern is not quoted, the shell may interpret ?, causing grep to receive a wrong pattern and no matches.

3. Final Answer:

   The regex should be quoted to prevent shell expansion -> Option C

4. Quick Check:

   Always quote regex patterns to avoid shell issues [OK]

Hint: Always quote regex patterns to avoid shell interpreting special chars [OK]

Common Mistakes:

• Not quoting regex patterns
• Assuming file lacks the word
• Blaming file encoding without checking

5. You want to find lines in log.txt that contain either "error", "fail", or "warning" using grep -E. Which command correctly matches these words as whole words only?

hard

A. grep -E '\b(error|fail|warning)\b' log.txt

B. grep -E '(error|fail|warning)' log.txt

C. grep -E '\<(error|fail|warning)\>' log.txt

D. grep -E '[error|fail|warning]' log.txt

Solution

1. Step 1: Understand word boundaries in extended regex

   \b marks word boundaries, ensuring the pattern matches whole words only.

2. Step 2: Analyze each option

   grep -E '\b(error|fail|warning)\b' log.txt uses \b around the group, correctly matching whole words.
   grep -E '(error|fail|warning)' log.txt matches substrings inside words.
   grep -E '\<(error|fail|warning)\>' log.txt uses \< and \> which are not standard in grep -E.
   grep -E '[error|fail|warning]' log.txt uses character class [] incorrectly, matching any single character from the set.

3. Final Answer:

   grep -E '\b(error|fail|warning)\b' log.txt -> Option A

4. Quick Check:

   Use \b for whole word matching in grep -E [OK]

Hint: Use \b to match whole words in grep -E patterns [OK]

Common Mistakes:

• Using character classes [] instead of grouping ()
• Not using word boundaries \b
• Using non-standard boundary markers like \< or \>