grep with regex (-E) in Linux CLI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to search for lines containing the word 'error' using extended regex.

Linux CLI

grep [1] 'error' logfile.txt

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

A-i

B-c

C-v

D-E

Attempts:

3 left

💡 Hint

Common Mistakes

Using -i instead of -E, which only ignores case.

Using -v which inverts the match.

Using -c which counts matches instead of showing them.

The -E option enables extended regular expressions in grep, allowing more complex patterns.

2fill in blank

medium

Complete the code to find lines that start with 'Error' or 'Warning' using extended regex.

Linux CLI

grep [1] '^(Error|Warning)' logfile.txt

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

A-c

B-v

C-E

D-i

Attempts:

3 left

💡 Hint

Common Mistakes

Using -v which inverts the match.

Using -i which ignores case but doesn't enable extended regex.

Using -c which counts matches instead of showing them.

The -E option allows using the alternation (|) in the regex pattern.

3fill in blank

hard

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

Linux CLI

grep -E 'cat[1]dog' animals.txt

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

A|

B*

C.

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' which means zero or more of the previous character.

Using '.' which matches any single character.

Using '+' which means one or more of the previous character.

The '|' symbol is used in extended regex to mean 'or' between patterns.

4fill in blank

hard

Fill both blanks to search for lines containing either 'apple' or 'banana' and ignore case.

Linux CLI

grep [1] [2] '(apple|banana)' fruits.txt

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

A-E

B-i

C-v

D-c

Attempts:

3 left

💡 Hint

Common Mistakes

Using -v which inverts the match.

Using -c which counts matches instead of showing them.

Use -E for extended regex and -i to ignore case when searching.

5fill in blank

hard

Fill all three blanks to count lines that do NOT contain 'error' or 'fail' using extended regex and ignoring case.

Linux CLI

grep [1] [2] [3] '(error|fail)' logfile.txt

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

A-E

B-i

C-v

D-c

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up -v and -c options.

Forgetting to use -E for extended regex (though not asked here).

Use -c to count, -i to ignore case, and -v to invert match (exclude lines with the pattern).

Practice

1. What does the -E option do when used with the grep command?

easy

A. Ignores case when searching

B. Searches only for exact string matches without regex

C. Prints line numbers along with matching lines

D. Enables extended regular expressions for more complex pattern matching

Solution

1. Step 1: Understand grep options

   The -E option tells grep to use extended regular expressions, which allow special symbols like |, +, and ().

2. Step 2: Compare with other options

   Other options like -i ignore case, and -n print line numbers, but -E specifically enables extended regex.

3. Final Answer:

   Enables extended regular expressions for more complex pattern matching -> Option D

4. Quick Check:

   grep -E = extended regex [OK]

Hint: Remember: -E means extended regex for flexible patterns [OK]

Common Mistakes:

• Confusing -E with -i (ignore case)
• Thinking -E disables regex
• Mixing -E with line number option -n

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 regex for alternatives

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

2. Step 2: Check other regex symbols

   + means one or more of previous character, ? means zero or one, and . means any single character, so they don't match the OR condition.

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 + or ? instead of | for OR
• Not quoting the pattern
• Confusing . with |

3. Given a file data.txt with these lines:

apple
banana
applepie
pineapple
banana split

What is the output of grep -E '^apple$|banana$' data.txt?

medium

A. applepie
banana

B. apple
banana split

C. apple
banana

D. apple
banana
banana split

Solution

1. Step 1: Analyze the regex pattern

   The pattern ^apple$|banana$ means lines exactly matching "apple" OR lines ending with "banana".

2. Step 2: Match lines in the file

   "apple" matches ^apple$. "banana" matches banana$. "applepie" does not match ^apple$ due to extra characters after "apple". "pineapple" matches neither. "banana split" does not end with "banana".

3. Final Answer:

   apple banana -> Option C

4. Quick Check:

   ^apple$ or banana$ matches apple and banana [OK]

Hint: Remember ^ means start, $ means end in regex [OK]

Common Mistakes:

• Assuming banana split matches banana$
• Matching applepie because it contains apple
• Ignoring anchors ^ and $

4. You run grep -E '(foo|bar)+' file.txt but get an error: grep: invalid regular expression. What is the likely cause?

medium

A. You forgot to quote the pattern

B. The parentheses are not escaped and your grep version does not support unescaped ()

C. The + symbol is not valid in extended regex

D. The file.txt does not exist

Solution

1. Step 1: Check regex syntax and grep version

   Some grep versions require escaping parentheses even with -E, so unescaped () cause invalid regex error.

2. Step 2: Verify other options

   Quoting is done, + is valid in extended regex, and file existence error would be different.

3. Final Answer:

   The parentheses are not escaped and your grep version does not support unescaped () -> Option B

4. Quick Check:

   Escape () if grep -E errors [OK]

Hint: Escape parentheses if grep -E gives regex error [OK]

Common Mistakes:

• Assuming + is invalid in extended regex
• Not quoting pattern causing shell errors
• Confusing file not found with regex error

5. You want to find lines in log.txt that contain a date in format YYYY-MM-DD or YYYY/MM/DD using grep -E. Which pattern correctly matches both formats?

hard

A. grep -E '[0-9]{4}[-/][0-9]{2}[-/][0-9]{2}' log.txt

B. grep -E '[0-9]{4}-|/[0-9]{2}-|/[0-9]{2}' log.txt

C. grep -E '\d{4}[-/]\d{2}[-/]\d{2}' log.txt

D. grep -E '[0-9]{4}[-|/][0-9]{2}[-|/][0-9]{2}' log.txt

Solution

1. Step 1: Understand character classes and alternation

   Character class [-/] matches either '-' or '/'. Using [0-9] matches digits. Curly braces specify exact counts.

2. Step 2: Analyze each option

   grep -E '\d{4}[-/]\d{2}[-/]\d{2}' log.txt uses \d which is not supported by grep regex. grep -E '[0-9]{4}-|/[0-9]{2}-|/[0-9]{2}' log.txt uses ungrouped -|/ causing incorrect alternation that splits the pattern. grep -E '[0-9]{4}[-|/][0-9]{2}[-|/][0-9]{2}' log.txt uses [-|/] which includes '|' as a character, incorrect. grep -E '[0-9]{4}[-/][0-9]{2}[-/][0-9]{2}' log.txt correctly uses character classes [-/] without extra characters.

3. Final Answer:

   grep -E '[0-9]{4}[-/][0-9]{2}[-/][0-9]{2}' log.txt -> Option A

4. Quick Check:

   Use [0-9]{n} and [-/] for digits and separators [OK]

Hint: Use [0-9] and [-/] for digits and dash or slash [OK]

Common Mistakes:

• Using \d which grep does not support
• Including | inside character class incorrectly
• Using parentheses without escaping in some grep versions