Regular Expressions in JavaScript — Interview Questions & Answers

Ques: What is a Regular Expression in JavaScript?

Ans: A Regular Expression (RegExp) is an object used for matching text patterns in strings. It helps you search, replace, or validate string data efficiently.

Example:

const pattern = /hello/;
console.log(pattern.test("hello world")); // true

Ques: How can you create a Regular Expression in JavaScript?

Ans: There are two ways:

Literal syntax:

const regex = /abc/;

Constructor function:

const regex = new RegExp("abc");

Ques: What are RegExp flags in JavaScript?

Ans: Flags modify how the regular expression behaves.

Flag	Name	Description
g	Global	Match all occurrences
i	Ignore Case	Case-insensitive match
m	Multiline	Match across multiple lines
s	Dotall	. matches newline (\\n)
u	Unicode	Enables full Unicode support
y	Sticky	Matches from lastIndex position

Example:

const re = /hello/gi;

Ques: What are some common RegExp methods in JavaScript?

Method	Used On	Description
test()	RegExp	Returns true/false
exec()	RegExp	Returns match details
match()	String	Returns match array
matchAll()	String	Returns all matches (iterator)
replace()	String	Replaces matched text
search()	String	Returns index of first match
split()	String	Splits string by regex pattern

Ques: How do you perform a case-insensitive match?

Ans: Use the i flag.

const re = /hello/i;
console.log(re.test("HELLO")); // true

Ques: What is the difference between String.match() and String.matchAll()?

• match() returns an array (or null)
• matchAll() returns an iterator (ES2020)

const text = "cat bat mat";
console.log(text.match(/.at/g));     // ['cat', 'bat', 'mat']
console.log([...text.matchAll(/.at/g)]);

Ques: What are anchors in Regular Expressions?

Ans: Anchors are used to match positions, not characters.

Anchor	Description
^	Start of string
$	End of string
\\b	Word boundary
\\B	Non-word boundary

Example:

/^Hello/.test("Hello JS"); // true
/JS$/.test("I love JS");   // true

Ques: What are character classes?

Ans: They define sets of characters to match.

Pattern	Matches
[abc]	a, b, or c
[^abc]	any char except a, b, or c
[a-z]	lowercase letters
[A-Z]	uppercase letters
[0-9]	digits 0–9

Example:

/[A-Z]/.test("Hello"); // true

Ques: What are predefined character classes?

Pattern	Meaning
\\d	Digit (0–9)
\\D	Non-digit
\\w	Word char (A–Z, a–z, 0–9, _)
\\W	Non-word char
\\s	Whitespace
\\S	Non-whitespace
.	Any character except newline

Example:

/\\d\\d/.test("Year2025"); // true

Ques: What are quantifiers in RegExp?

Ans: They define how many times a pattern should appear.

Quantifier	Description	Example
+	One or more	/a+/ → "aa"
*	Zero or more	/go*/ → "g", "gooo"
?	Zero or one	/colou?r/ → "color" or "colour"
{n}	Exactly n times	/\\d{4}/
{n,}	At least n times	/\\d{2,}/
{n,m}	Between n and m	/\\d{2,4}/

Ques: Example of using quantifiers

const re = /\\d{3}-\\d{2}-\\d{4}/;
console.log(re.test("123-45-6789")); // true

Ques: What is the purpose of parentheses ( ) in RegExp?

Ans: Parentheses are capturing groups — they store matched substrings.

const re = /(\\d{2})-(\\d{2})-(\\d{4})/;
const result = re.exec("27-10-2025");
console.log(result[1]); // "27"

Ques: How do you create a non-capturing group?

Ans: Use (?: ) to group without capturing.

const re = /(?:Mr|Ms)\\.?\\s[A-Z]/;

Ques: What is the use of alternation (|)?

Ans: The | operator means “OR”.

const re = /cat|dog/;
console.log(re.test("I have a dog")); // true

Ques: What does \\b mean in regex?

Ans: \\b matches word boundaries, ensuring complete word matches.

/\\bcat\\b/.test("catfish"); // false
/\\bcat\\b/.test("my cat");  // true

Ques: Example: Validate an Email Address

const emailRegex = /^[\\w.-]+@[a-zA-Z_-]+?\\.[a-zA-Z]{2,}$/;
console.log(emailRegex.test("[email protected]")); // true

Ques: Example: Extract Numbers from String

const str = "Order 45 has 3 items";
console.log(str.match(/\\d+/g)); // ['45', '3']

Ques: How can you replace matched text using regex?

Ans: Use String.replace() with a pattern.

const str = "Hello 2025";
console.log(str.replace(/\\d+/, "World")); // "Hello World"

Ques: How do you use backreferences in regex?

Ans: Backreferences recall previously captured groups using \\1, \\2, etc.

const re = /(\\w+)\\s\\1/;
console.log(re.test("bye bye")); // true

Ques: What are lookaheads and lookbehinds?

Ans: They allow matching without consuming characters.

Type	Syntax	Description
Positive Lookahead	X(?=Y)	Match X if followed by Y
Negative Lookahead	X(?!Y)	Match X if not followed by Y
Positive Lookbehind	(?<=Y)X	Match X if preceded by Y
Negative Lookbehind	(?<!Y)X	Match X if not preceded by Y

Example:

/\\d(?=px)/.test("10px"); // true
/\\d(?!px)/.test("10em"); // true

Ques: Example: Match a password pattern

const password = /^(?=.*[A-Z])(?=.*\\d)(?=.*[@$!%*?&]).{8,}$/;
console.log(password.test("MyPass@123")); // true

Ques: How to escape special characters in regex?

Ans: Use a backslash \\.

const re = /\\./; // matches literal dot

Ques: How to split a string using regex?

const text = "HTML, CSS; JS";
console.log(text.split(/[,;]\\s*/)); // ['HTML', 'CSS', 'JS']

Ques: Can regex be used for Unicode characters?

Ans: Yes, using the u flag.

const re = /\\u{1F600}/u; // ?

Ques: What are greedy and lazy quantifiers?

Type	Example	Description
Greedy	/".+"/	Matches as much as possible
Lazy	/".+?"/	Matches as little as possible

Example:

const str = '"one" "two"';
console.log(str.match(/".+?"/g)); // ["one", "two"]

Ques: How do you debug or visualize regex patterns?

Use online tools like regex101.com or regexr.com for explanations and testing.

Ques: Can regex work across multiple lines?

Ans: Yes, using the m flag.

/^start/m.test("line1\nstart line2"); // true