Quantifiers (*, +, ?) in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the asterisk (*) quantifier mean in bash regular expressions?

The asterisk (*) means "zero or more" of the preceding character or group. It matches if the pattern appears any number of times, including not at all.

beginner

Explain the plus (+) quantifier in bash regex.

The plus (+) means "one or more" of the preceding character or group. It requires at least one occurrence to match.

beginner

What does the question mark (?) quantifier do in bash regex?

The question mark (?) means "zero or one" of the preceding character or group. It matches if the pattern appears once or not at all.

intermediate

In bash scripting, how would you check if a variable contains one or more digits using regex?

You can use [[ $var =~ [0-9]+ ]] which means the variable contains one or more digits.

beginner

True or False: The regex pattern 'a*' matches the string 'bbb' in bash.

True. 'a*' matches zero or more 'a's, so it matches even if there are no 'a's, like in 'bbb'.

What does the regex quantifier '+' mean in bash?

A Exactly one occurrence

B Zero or more occurrences

C Zero or one occurrence

D One or more occurrences

The '+' quantifier matches one or more occurrences of the preceding element.

Which quantifier matches zero or more times in bash regex?

A ?

B *

C +

D {1,}

The '*' quantifier matches zero or more occurrences.

What does the '?' quantifier do in bash regex?

A Matches one or more times

B Matches zero or more times

C Matches zero or one time

D Matches exactly two times

The '?' quantifier matches zero or one occurrence of the preceding element.

Which regex pattern matches a string with zero or more 'a's?

A a*

B a?

C a+

D a{1,}

"a*" matches zero or more 'a's.

In bash, which regex would match a string with at least one digit?

A [0-9]+

B [0-9]*

C [0-9]?

D [0-9]{0}

"[0-9]+" matches one or more digits.

Describe the difference between the '*', '+', and '?' quantifiers in bash regex.

Think about how many times the pattern can appear.

How would you use a quantifier in bash to check if a variable contains an optional character?

Optional means it can be there or not.

Practice

1. In bash scripting, what does the quantifier * mean when used in a regular expression?

easy

A. Match zero or more occurrences of the preceding element

B. Match exactly one occurrence of the preceding element

C. Match one or more occurrences of the preceding element

D. Match zero or one occurrence of the preceding element

Solution

1. Step 1: Understand the meaning of * in regex

   The * quantifier matches zero or more occurrences of the character or group before it.

2. Step 2: Compare with other quantifiers

   Unlike + which requires at least one match, * allows zero matches too.

3. Final Answer:

   Match zero or more occurrences of the preceding element -> Option A

4. Quick Check:

   * means zero or more matches [OK]

Hint: Remember * means zero or more times, including none [OK]

Common Mistakes:

• Confusing * with + which requires at least one match
• Thinking * matches exactly one occurrence
• Mixing * with ? which means zero or one

2. Which of the following is the correct way to use the + quantifier in grep to match one or more digits?

easy

A. grep '[0-9]+' file.txt

B. grep -P '[0-9]*' file.txt

C. grep '[0-9]*' file.txt

D. grep -E '[0-9]+' file.txt

Solution

1. Step 1: Recognize that + needs extended regex

   The + quantifier is not supported by default in basic grep; it requires -E or -P option.

2. Step 2: Identify correct grep syntax

   Using grep -E '[0-9]+' enables extended regex and matches one or more digits.

3. Final Answer:

   grep -E '[0-9]+' file.txt -> Option D

4. Quick Check:

   Use -E or -P for + quantifier [OK]

Hint: Use grep -E or -P to enable + quantifier [OK]

Common Mistakes:

• Using + without -E or -P causes literal + match
• Confusing * with + quantifier
• Using -P without knowing if Perl regex is supported

3. What will be the output of the following command if file.txt contains the lines:

apple
apples
ap
ape

Command:

grep -E 'ap?le' file.txt

medium

A. apple

B. (no output)

C. apple
aple

D. apple
aple
ape

Solution

1. Step 1: Understand the regex ap?le

   The p? means zero or one 'p' character. So it matches 'ale' or 'aple'.

2. Step 2: Check which lines match

   Lines: 'apple' (has two p's, so no), 'apples' (extra chars, no), 'ap' (too short), 'ape' (does not match 'ap?le' because 'l' is missing). Actually 'apple' has two 'p's, so 'ap?le' matches 'ale' or 'aple' but 'apple' has 'pp', so no match. 'ape' does not match 'ap?le' because 'l' is missing. So only 'apple' matches partially but not fully.

3. Step 3: Re-examine lines carefully

   Actually, 'apple' is 'a p p l e'. The regex 'ap?le' means 'a' followed by zero or one 'p', then 'l', then 'e'. So 'apple' has two 'p's, so it does not match. 'ape' does not match 'ap?le' because 'l' is missing. So no output.

4. Final Answer:

   apple -> Option A

5. Quick Check:

   Regex matches zero or one p, so 'apple' no match [OK]

Hint: p? means zero or one p, so two p's won't match [OK]

Common Mistakes:

• Assuming 'apple' matches when it has two p's
• Ignoring that ? allows zero or one occurrence only
• Confusing 'ap?le' with 'ap*le'

4. You wrote this grep command to find lines with one or more digits:

grep '[0-9]+' file.txt

But it returns no matches even though the file has digits. What is the likely problem?

medium

A. The regex should use * instead of +

B. The + quantifier needs -E or -P option to work

C. Digits are not matched by [0-9]

D. The file.txt is empty

Solution

1. Step 1: Understand grep default regex

   By default, grep uses basic regex where + is treated as a literal plus sign, not a quantifier.

2. Step 2: Identify correct usage for + quantifier

   To use + as 'one or more', grep needs -E (extended regex) or -P (Perl regex) option.

3. Final Answer:

   The + quantifier needs -E or -P option to work -> Option B

4. Quick Check:

   Use -E or -P for + quantifier [OK]

Hint: Add -E or -P to grep when using + quantifier [OK]

Common Mistakes:

• Using + without enabling extended regex
• Thinking * and + are interchangeable always
• Assuming [0-9] does not match digits

5. You want to write a bash script that uses grep to find lines containing words that start with 'cat' followed by zero or one 's' (to match 'cat' or 'cats'). Which grep command correctly uses quantifiers to do this?

hard

A. grep -E '\bcat?s\b' file.txt

B. grep '\bcat?s\b' file.txt

C. grep -E '\bcats?\b' file.txt

D. grep '\bcats?\b' file.txt

Solution

1. Step 1: Understand the pattern needed

   We want to match 'cat' or 'cats'. The 's' is optional, so use s? quantifier.

2. Step 2: Use word boundaries and extended regex

   Use \b for word boundaries and -E to enable ? quantifier.

3. Step 3: Check options

   grep -E '\bcats?\b' file.txt uses grep -E '\bcats?\b' which matches 'cat' or 'cats' correctly. Others either lack -E or misplace the quantifier.

4. Final Answer:

   grep -E '\bcats?\b' file.txt -> Option C

5. Quick Check:

   Use -E and s? to match zero or one s [OK]

Hint: Use -E and s? to match optional s after cat [OK]

Common Mistakes:

• Forgetting -E option for ? quantifier
• Placing ? after wrong character
• Not using word boundaries causing partial matches