Regular Expressions

Question 1

What does the . (dot) metacharacter match in regex?

Answer 1

• Matches any single character except newline
• Example: gr.p matches grep, grip, grab, etc.

Question 2

What do ^ and $ anchors do in regex?

Answer 2

• ^ matches the beginning of a line
• $ matches the end of a line
• Example: ^error matches lines starting with ‘error’, log$ matches lines ending with ‘log’

Question 3

How do square brackets [] work in regex?

Answer 3

• Define a character class - matches any single character from the set
• Example: [aeiou] matches any vowel, [0-9] matches any digit

Question 4

What does [^] mean inside square brackets?

Answer 4

• Negates the character class - matches any character NOT in the set
• Example: [^0-9] matches any non-digit character

Question 5

What do * + and ? quantifiers mean?

Answer 5

• • means zero or more of the preceding character
• • means one or more of the preceding character
• ? means zero or one of the preceding character (optional)

Question 6

How do curly braces {} work as quantifiers?

Answer 6

• {n} matches exactly n occurrences
• {n,} matches n or more occurrences
• {n,m} matches between n and m occurrences
• Example: [0-9]{3} matches exactly 3 digits

Question 7

What does \ (backslash) do in regex?

Answer 7

• Escapes special characters to match them literally
• Example: . matches a literal dot, $ matches literal dollar sign

Question 8

What are \d, \w, and \s shortcuts for?

Answer 8

• \d matches any digit (equivalent to [0-9])
• \w matches word characters (letters, digits, underscore)
• \s matches whitespace characters (space, tab, newline)

Question 9

How do parentheses () work in regex?

Answer 9

• Create groups for applying quantifiers to multiple characters
• Example: (abc)+ matches abc, abcabc, abcabcabc

Question 10

What does the pipe | symbol do in regex?

Answer 10

• Acts as logical OR - matches either pattern before or after
• Example: cat|dog matches either ‘cat’ or ‘dog’

Question 11

What do < and > represent in regex?

Answer 11

• < matches beginning of a word boundary
• > matches end of a word boundary
• Example: <cat> matches ‘cat’ but not ‘category’ or ‘concatenate’

Question 12

How does .* differ from .+ in regex?

Answer 12

• .* matches zero or more of any character (can match empty string)
• .+ matches one or more of any character (requires at least one character)

Question 13

What does \b represent in regex?

Answer 13

• Word boundary - matches position between word and non-word character
• Example: \bcat\b matches ‘cat’ as whole word only

Question 14

How do you match a literal special character in regex?

Answer 14

• Use backslash to escape: ., *, +, \?, [, ], (, )
• Example: price$10 matches ‘price$10’ literally

Question 15

What does [a-zA-Z0-9] match?

Answer 15

• Matches any single alphanumeric character
• Includes lowercase letters, uppercase letters, and digits

Question 16

How do you make regex case-insensitive in grep?

Answer 16

• Use grep -i flag for case-insensitive matching
• Example: grep -i ‘error’ file.txt matches Error, ERROR, error

Question 17

What does \n represent in regex?

Answer 17

• Matches a newline character
• Useful in multiline patterns or text processing

Question 18

How do you match the beginning and end of entire string?

Answer 18

• Use \A for absolute beginning of string
• Use \Z for absolute end of string

Question 19

What is a greedy vs non-greedy quantifier?

Answer 19

• Greedy quantifiers (*, +, {}) match as much as possible
• Non-greedy (*?, +?, {}?) match as little as possible\
• Example: ‘a.b’ vs ‘a.?b’ in string ‘aXbYb’ - greedy matches ‘aXbYb’, non-greedy matches ‘aXb’

Question 20

How do you match one or more whitespace characters?

Answer 20

• \s+ matches one or more whitespace (spaces, tabs, newlines)
• Example: [ ]+ matches one or more spaces only