Regex Quiz & Skill Assessment

Question 1

What is a regular expression?

Accepted Answer

A sequence of characters that defines a search pattern for matching text

Answer

A compiled binary format for storing images

Answer

A special type of database query language unrelated to text

Answer

A CSS selector used to style HTML elements

Question 2

What will `/cat/.test('concatenate')` return in JavaScript?

Accepted Answer

true

Answer

false

Answer

'cat'

Answer

null

Question 3

Which of the following is NOT a valid way to create a regex object in JavaScript?

Accepted Answer

Regex.compile('abc')

Answer

/abc/

Answer

new RegExp('abc')

Answer

RegExp('abc')

Question 4

Which of the following characters must be escaped with a backslash to match it literally (outside a character class)?

Accepted Answer

the dot '.'

Answer

the letter 'a'

Answer

the digit '5'

Answer

a plain space character

Question 5

What does \d match in a JavaScript regex (without the u flag)?

Accepted Answer

Any digit character 0-9

Answer

Any word character including letters and digits

Answer

Any whitespace character

Answer

Any character that is not a digit

Question 6

Which shorthand character class matches any 'word character' (letters, digits, or underscore)?

Accepted Answer

\w

Answer

\s

Answer

\b

Answer

\W

Question 7

What does the character class [^aeiou] match?

Accepted Answer

Any single character that is NOT one of a, e, i, o, or u

Answer

Any vowel character

Answer

The literal string '^aeiou'

Answer

The start of a string followed by a vowel

Question 8

Which pattern correctly matches a single digit from 3 to 7 inclusive?

Accepted Answer

[3-7]

Answer

[3,7]

Answer

{3-7}

Answer

(3-7)

Question 9

Which quantifier matches zero or one occurrence of the preceding token?

Accepted Answer

?

Answer

*

Answer

+

Answer

{1,}

Question 10

What matched substring does /a{2,4}/.exec('aaaaa')[0] return?

Accepted Answer

'aaaa'

Answer

'aa'

Answer

'aaaaa'

Answer

'a'

Question 11

Which quantifier syntax matches exactly 3 to 5 repetitions of the preceding token?

Accepted Answer

{3,5}

Answer

{3-5}

Answer

{3,,5}

Answer

[3,5]

Question 12

What does /a.*?b/.exec('a1b2b')[0] return?

Accepted Answer

'a1b'

Answer

'a1b2b'

Answer

'b2b'

Answer

null

Question 13

What does the ^ anchor represent when used without the m (multiline) flag?

Accepted Answer

The start of the entire input string

Answer

The start of any line within the string

Answer

A word boundary

Answer

The end of the string

Question 14

What does \b match in a regular expression?

Accepted Answer

A word boundary: a position between a word character and a non-word character

Answer

A literal backspace character

Answer

The beginning of the string only

Answer

Any whitespace character

Question 15

With the m (multiline) flag enabled, what does $ additionally match beyond the end of the whole string?

Accepted Answer

The end of each line within the string

Answer

The end of each word within the string

Answer

The start of each line within the string

Answer

Nothing extra; m has no effect on $

Question 16

A developer wants /^Error/ to match any line beginning with 'Error' in a multi-line log string, but it only matches when 'Error' is at the very start of the whole string. What is wrong?

Accepted Answer

The m flag is missing, so ^ only anchors to the start of the entire string, not each line

Answer

Nothing; ^ already matches the start of every line by default

Answer

^ should be replaced with \b to match line starts

Answer

The pattern needs a $ at the end to work at all

Question 17

What is the key difference between (abc) and (?:abc) in a regex?

Accepted Answer

(abc) creates a capturing group whose match is saved for later reference, while (?:abc) groups without capturing

Answer

There is no difference; they behave identically

Answer

(?:abc) is capturing and (abc) is not

Answer

(abc) only works with the g flag

Question 18

Which syntax creates a named capturing group in a JavaScript regular expression?

Accepted Answer

(?<name>...)

Answer

(?name...)

Answer

(%name...)

Answer

{name:...}

Question 19

What does /(\w)\1/.test('hello') return, and why?

Accepted Answer

true, because \1 backreferences the first group and matches the repeated 'l' in 'hello'

Answer

false, because backreferences are not supported in JavaScript

Answer

true, because 'h' matches the pattern by itself

Answer

false, because \1 refers to a second group that does not exist

Question 20

After a successful match with a named group (?<year>\d{4}), how do you access the captured value on the resulting match object?

Accepted Answer

match.groups.year

Answer

match.group('year')

Answer

match[1].year

Answer

match.named.year

Question 21

Which syntax represents a positive lookahead in a JavaScript regex?

Accepted Answer

(?=...)

Answer

(?!...)

Answer

(?<=...)

Answer

(?<!...)

Question 22

Which syntax represents a negative lookbehind in a JavaScript regex?

Accepted Answer

(?<!...)

Answer

(?<=...)

Answer

(?!...)

Answer

(?=...)

Question 23

What does /\d+(?= dollars)/.exec('50 dollars')[0] return?

Accepted Answer

'50'

Answer

'50 dollars'

Answer

' dollars'

Answer

null, because the lookahead prevents any match

Question 24

What is the defining characteristic of lookaround assertions (lookahead and lookbehind)?

Accepted Answer

They assert that a pattern does or doesn't exist nearby without consuming any characters

Answer

They consume and capture characters just like a normal group

Answer

They only function when the g flag is set

Answer

They automatically capture their contents into numbered groups

Question 25

What does the i flag do when applied to a JavaScript regex?

Accepted Answer

Makes the match case-insensitive

Answer

Allows the pattern to span multiple lines

Answer

Enables searching for all matches instead of just the first

Answer

Allows Unicode code point matching

Question 26

A regex with the global flag g is reused across multiple .exec() calls on the same string. What determines where the search for the next match begins?

Accepted Answer

The regex object's lastIndex property, which advances after each successful match

Answer

It always restarts searching from index 0

Answer

The g flag is ignored when using exec(), so behavior is unaffected

Answer

It is chosen randomly by the engine

Question 27

Which flag makes the dot (.) metacharacter also match newline characters?

Accepted Answer

s (dotAll)

Answer

m (multiline)

Answer

u (unicode)

Answer

y (sticky)

Question 28

What is 'catastrophic backtracking' in the context of regex engines?

Accepted Answer

An exponential blow-up in matching time caused by ambiguous, overlapping quantifiers trying many combinations

Answer

A syntax error that prevents a regex from compiling

Answer

A guaranteed crash of the JavaScript runtime whenever backreferences are used

Answer

A performance optimization that makes regex matching faster

Question 29

Which of the following patterns is most likely to trigger catastrophic backtracking on certain long, non-matching input?

Accepted Answer

(a+)+b

Answer

[a-z]+

Answer

^\d{3}$

Answer

\bword\b

Question 30

What is a common, effective way to avoid catastrophic backtracking in a regex?

Accepted Answer

Rewrite ambiguous nested quantifiers (like (a+)+) into more specific, non-overlapping patterns

Answer

Add more capturing groups so the engine tracks progress more precisely

Answer

Always add the global (g) flag to the regex

Answer

Avoid using character classes such as \d and \w

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