Basic regex in grep in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the command grep 'pattern' filename do?

It searches the file named filename for lines that match the given pattern and prints those lines.

beginner

What is a regular expression (regex) in the context of grep?

A regex is a special text pattern that describes a set of strings. grep uses regex to find matching text in files.

beginner

How do you use grep to search for lines that start with the word 'Hello'?

Use the caret symbol ^ to match the start of a line: grep '^Hello' filename.

beginner

What does the dot . mean in a regex pattern?

The dot . matches any single character except a newline.

intermediate

How can you search for lines containing either 'cat' or 'dog' using grep?

Use the alternation symbol \| inside quotes: grep 'cat\|dog' filename.

Which symbol in regex matches the start of a line?

A *

B $

C .

D ^

The caret symbol ^ matches the start of a line in regex.

What does the regex pattern grep 'a.b' filename match?

A Lines containing 'a' followed by any character, then 'b'

B Lines containing 'ab' only

C Lines starting with 'a' and ending with 'b'

D Lines containing 'a' or 'b'

The dot '.' matches any single character, so 'a.b' matches 'a' + any character + 'b'.

How do you tell grep to treat the pattern as a basic regex?

A Use <code>grep -E</code>

B Use <code>grep -P</code>

C Use <code>grep -G</code>

D Use <code>grep -F</code>

The -G option tells grep to use basic regular expressions (default behavior).

Which grep option allows using extended regex with alternation | without escaping?

A -E

B -F

C -P

D -G

The -E option enables extended regex, allowing alternation with | without escaping.

What does the regex symbol $ match?

A Start of a line

B End of a line

C Any character

D Zero or more repetitions

The dollar sign $ matches the end of a line in regex.

Explain how to use grep with a basic regex to find lines starting with a specific word.

Think about how to anchor your search to the line start.

Describe the difference between basic and extended regex in grep and when to use each.

Consider how you write alternation patterns.

Practice

1. What does the caret symbol ^ mean in a basic regex pattern used with grep?

easy

A. It matches the end of a line

B. It matches the start of a line

C. It matches any single character

D. It matches zero or more repetitions

Solution

1. Step 1: Understand caret usage in regex

   The caret ^ is used to anchor the pattern to the start of a line in basic regex.

2. Step 2: Compare with other anchors

   The dollar sign $ matches the end of a line, so caret is specifically for the start.

3. Final Answer:

   It matches the start of a line -> Option B

4. Quick Check:

   ^ means start of line [OK]

Hint: Caret (^) always means line start in grep regex [OK]

Common Mistakes:

• Confusing ^ with $ (end of line)
• Thinking ^ matches any character
• Using ^ to mean OR condition

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

easy

A. grep 'cat\|dog' file.txt

B. grep 'cat|dog' file.txt

C. grep 'cat\dog' file.txt

D. grep 'cat/dog' file.txt

Solution

1. Step 1: Understand OR in basic regex

   In basic regex with grep, the OR operator is written as \|, not just |.

2. Step 2: Check each option

   grep 'cat|dog' file.txt uses | without escape, which is invalid in basic regex. grep 'cat\|dog' file.txt correctly escapes it as \|. Options C and D use wrong characters.

3. Final Answer:

   grep 'cat\|dog' file.txt -> Option A

4. Quick Check:

   OR in basic grep regex = \| [OK]

Hint: Use \| for OR in basic grep regex [OK]

Common Mistakes:

• Using | without escaping in basic grep
• Confusing \| with \dog or /
• Forgetting to escape special characters

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

cat
caterpillar
dog
catalog
dogma

What is the output of grep '^cat$' animals.txt?

medium

A. cat
caterpillar
catalog

B. caterpillar
catalog

C. cat

D. No output

Solution

1. Step 1: Understand the regex pattern

   The pattern ^cat$ matches lines that start and end exactly with "cat" (no extra characters).

2. Step 2: Check each line against pattern

   Only the line "cat" matches exactly. "caterpillar" and "catalog" have extra characters, so they don't match.

3. Final Answer:

   cat -> Option C

4. Quick Check:

   ^cat$ matches exact line 'cat' [OK]

Hint: Use ^ and $ to match exact whole line [OK]

Common Mistakes:

• Assuming partial matches with ^ and $
• Ignoring line boundaries in regex
• Confusing grep default substring match with anchored match

4. You want to find lines containing either "apple" or "banana" in fruits.txt using basic regex with grep. Which command uses incorrect syntax for the OR operation?

medium

A. grep 'apple|banana' fruits.txt

B. grep 'apple\|banana$' fruits.txt

C. grep '^apple\|banana$' fruits.txt

D. grep 'apple\|banana' fruits.txt

Solution

1. Step 1: Identify correct OR syntax in basic grep

   Basic regex requires escaping the OR operator as \|. Using | alone treats it as a literal | character, not as an OR operator.

2. Step 2: Check each option

   grep 'apple|banana' fruits.txt uses | without escape, treating | as literal, not OR. Options A, B, and C correctly escape OR operator.

3. Final Answer:

   grep 'apple|banana' fruits.txt -> Option A

4. Quick Check:

   Unescaped | in basic grep regex = literal |, not OR [OK]

Hint: Escape | as \| in basic grep regex [OK]

Common Mistakes:

• Not escaping | in basic regex
• Mixing extended regex syntax without -E flag
• Misplacing anchors with OR operator

5. You want to find lines in data.txt that start with "error" or end with "fail" using basic regex with grep. Which command correctly does this?

hard

A. grep '^error\|fail' data.txt

B. grep '^error|fail$' data.txt

C. grep '^error$\|fail$' data.txt

D. grep '^error\|fail$' data.txt

Solution

1. Step 1: Understand the pattern requirements

   We want lines starting with "error" OR lines ending with "fail". So pattern is ^error OR fail$.

2. Step 2: Use correct OR syntax in basic regex

   Basic regex OR is \|. So pattern is ^error\|fail$. grep '^error\|fail$' data.txt matches this exactly.

3. Step 3: Check other options

   grep '^error|fail$' data.txt misses escape on OR, causing error. grep '^error$\|fail$' data.txt incorrectly anchors "error" with $. grep '^error\|fail' data.txt misses anchor on "fail".

4. Final Answer:

   grep '^error\|fail$' data.txt -> Option D

5. Quick Check:

   Start with ^error OR end with fail$ using \| [OK]

Hint: Combine ^ and $ with \| for OR in basic grep regex [OK]

Common Mistakes:

• Not escaping | in basic regex
• Misplacing anchors with OR operator
• Using $ with start anchor or vice versa