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Regular Expressions in JavaScript (Very Very Important)

Introduction

Regular Expressions (RegEx) are special patterns used to search, match, validate, and manipulate strings in JavaScript.

They are widely used for:

  • Form Validation
  • Email Validation
  • Password Validation
  • Mobile Number Validation
  • Search Operations
  • Replace Operations
  • Pattern Matching

Regular Expressions are one of the most important topics in JavaScript and are frequently asked in interviews.

Definition

A Regular Expression is a sequence of characters that forms a search pattern.

It is used to:

  • Search text
  • Match patterns
  • Validate user input
  • Replace characters
  • Extract information from strings

Syntax

var regex = /pattern/;

Or

var regex = new RegExp("pattern");

Example 1: Matching "hello"

var regex = /hello/;

var str = "hello world";

console.log(regex.test(str));

Output

true

Explanation

  • /hello/ searches for the word "hello".
  • The string contains "hello".
  • Therefore test() returns:
true

Example 2: Matching "world"

var regex = /world/;

var str = "hello world";

console.log(regex.test(str));

Output

true

Explanation

The word world exists in the string.

Hence:

true

Example 3: Matching "hi"

var regex = /hi/;

var str = "hello world";

console.log(regex.test(str));

Output

false

Explanation

The word hi is not present inside the string.

Therefore:

false

test() Method

The test() method checks whether the given pattern exists in the string.

Syntax

regex.test(string)

Return Type

  • true → Pattern Found
  • false → Pattern Not Found

Special Characters in Regular Expressions

1. Dot (.)

The dot matches any single character.

Example

var regex = /h.llo/;

var str = "hello world";

console.log(regex.test(str));

Output

true

Explanation

The dot (.) replaces one character.

h . l l o

e replaces .

Hence:

hello → Match Found

2. Asterisk (*)

The asterisk means:

Zero or More Occurrences

Example

var regex = /h*llo/;

var str = "hllo world";

console.log(regex.test(str));

Output

true

Explanation

h*

means:

h → 0 times
h → 1 time
h → many times

So:

hllo
hello
hhllo
hhhllo

All are valid matches.

3. Plus (+)

The plus means:

One or More Occurrences

Example

var regex = /h+llo/;

var str = "hhllo world";

console.log(regex.test(str));

Output

true

Explanation

h+

means:

hllo
hhllo
hhhllo
hhhhllo

At least one h is mandatory.

Regular Expression Modifiers

1. i Modifier (Case Insensitive)

Example

var regex = /hello/i;

console.log(regex.test("HELLO"));

Output

true

Explanation

The i modifier ignores uppercase and lowercase differences.

2. g Modifier (Global Search)

Example

var str = "hello hello hello";

var regex = /hello/g;

console.log(str.match(regex));

Output

["hello","hello","hello"]

Explanation

The global modifier searches all occurrences.

Important RegEx Symbols

Symbol Meaning
. Any one character
* Zero or more occurrences
+ One or more occurrences
? Zero or one occurrence
^ Starts with
$ Ends with
[] Character set
[a-z] Lowercase letters
[A-Z] Uppercase letters
[0-9] Digits
\d Any digit
\D Non-digit
\w Word character
\W Non-word character
\s White space
\S Non white space

Example: Email Validation

var email = "abc@gmail.com";

var regex = /^[a-zA-Z0-9]+@[a-z]+\.[a-z]{2,3}$/;

console.log(regex.test(email));

Output

true

Example: Mobile Number Validation

var mobile = "9876543210";

var regex = /^[0-9]{10}$/;

console.log(regex.test(mobile));

Output

true

Example: Password Validation

var password = "Java@123";

var regex = /^(?=.*[A-Z])(?=.*[a-z])(?=.*[0-9]).{8,}$/;

console.log(regex.test(password));

Output

true

Conditions

Password must contain:

  • One Uppercase letter
  • One Lowercase letter
  • One Number
  • Minimum 8 Characters

Difference Between * and +

* Operator + Operator
Zero or More One or More
Empty value allowed Empty value not allowed
h*llo matches "llo" h+llo does not match "llo"
Optional occurrences Minimum one occurrence

Applications of Regular Expressions

Regular Expressions are used in:

  1. Email Validation
  2. Password Validation
  3. Mobile Number Validation
  4. Username Validation
  5. Search Functionality
  6. Find and Replace
  7. Data Extraction
  8. Form Validation
  9. Parsing Text Files
  10. Input Sanitization

Interview Questions

Q1. What is Regular Expression?

Answer:

Regular Expression is a sequence of characters used to create search patterns for matching, validating, and manipulating strings.

Q2. Which method is commonly used for matching a pattern?

Answer:

test()

It returns:

  • true
  • false

Q3. What does "." represent in RegEx?

Answer:

It matches any one character.

Example:

/h.llo/

Matches:

hello
hallo
hxllo

Q4. Difference between * and + ?

Answer:

  • * → Zero or More Occurrences
  • + → One or More Occurrences

Q5. What does the i modifier do?

Answer:

It performs a case-insensitive search.

Example:

/hello/i

Matches:

HELLO
Hello
hello

Summary

Regular Expressions are powerful tools used for pattern matching, validation, and string manipulation in JavaScript. They simplify complex searching operations and are extensively used in real-world applications such as form validation, email verification, password checking, and data extraction. Mastering Regular Expressions is an important skill for every JavaScript developer and a frequently asked topic in technical interviews.