Extended regex (grep -E) in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to search for lines containing either 'cat' or 'dog' in the file animals.txt using extended regex.

Bash Scripting

grep -E '[1]' animals.txt

Drag options to blanks, or click blank then click option'

Acat|dog

Bcat*dog

Ccat?dog

Dcat.dog

Attempts:

3 left

💡 Hint

Common Mistakes

Using '.' which matches any character instead of '|' for OR.

Using '*' or '?' which are quantifiers, not OR operators.

The pipe symbol '|' in extended regex means OR, so 'cat|dog' matches lines with 'cat' or 'dog'.

2fill in blank

medium

Complete the code to find lines starting with 'Error' followed by a space and a number in log.txt using extended regex.

Bash Scripting

grep -E '^[1] [0-9]+' log.txt

Drag options to blanks, or click blank then click option'

AError*

BError

CError?

DError+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' or '+' after 'Error' which changes the meaning.

Using '?' which makes 'Error' optional.

The caret '^' anchors the start of the line, so '^Error ' matches lines starting with 'Error ' exactly.

3fill in blank

hard

Fix the error in the code to match lines containing either 'cat', 'dog', or 'bird' in pets.txt using extended regex.

Bash Scripting

grep -E 'cat[1]dog[2]bird' pets.txt

Drag options to blanks, or click blank then click option'

A|

B.

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '.' which matches any character instead of OR.

Using '*' or '+' which are quantifiers, not separators.

The '|' symbol is used to separate alternatives in extended regex, so 'cat|dog|bird' matches any of these words.

4fill in blank

hard

Fill both blanks to create a pattern that matches lines with a word starting with 'a' followed by one or more digits in data.txt.

Bash Scripting

grep -E '[1][0-9][2]' data.txt

Drag options to blanks, or click blank then click option'

Aa+

B+

Da

Attempts:

3 left

💡 Hint

Common Mistakes

Putting '+' after 'a' which means one or more 'a's, not digits.

Not using '+' after digits, so only one digit matches.

The pattern 'a[0-9]+' matches 'a' followed by one or more digits. The '+' after [0-9] means one or more digits.

5fill in blank

hard

Fill all three blanks to create a pattern that matches lines containing either 'cat', 'dog', or 'bird' followed by a space and a number in pets.txt.

Bash Scripting

grep -E '[1][2][3] [0-9]+' pets.txt

Drag options to blanks, or click blank then click option'

Acat

B|

Cdog|bird

Dbird

Attempts:

3 left

💡 Hint

Common Mistakes

Putting all animals in one option without '|'.

Missing the space before the number.

The pattern 'cat|dog|bird [0-9]+' matches any of the three words followed by a space and one or more digits.

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 \>