JavaScript RegExp Basics

Creating Regular Expressions

There are two ways to create a regular expression in JavaScript:

Literal Notation

const pattern = /hello/i;  // 'i' flag for case-insensitive

Constructor Function

const pattern = new RegExp('hello', 'i');

Common Flags

• i - Case-insensitive matching
• g - Global search (find all matches)
• m - Multiline mode
• s - Dot matches newline characters

Basic Methods

test() - Check if pattern exists

const regex = /hello/i;
console.log(regex.test('Hello World'));  // true
console.log(regex.test('Goodbye'));      // false

exec() - Find match details

const regex = /(\w+)@([\w.]+)/;
const match = regex.exec('contact@example.com');
console.log(match[0]);  // 'contact@example.com'
console.log(match[1]);  // 'contact'
console.log(match[2]);  // 'example.com'

String Methods with RegExp

match() - Find all matches

const text = 'cat, dog, cat';
const matches = text.match(/cat/g);
console.log(matches);  // ['cat', 'cat']

replace() - Replace matches

const text = 'Hello World';
const result = text.replace(/world/i, 'JavaScript');
console.log(result);  // 'Hello JavaScript'

split() - Split by pattern

const text = '2025-02-15';
const parts = text.split(/-/);
console.log(parts);  // ['2025', '02', '15']

Common Patterns

Pattern	Matches
.	Any character except newline
\d	Digit (0-9)
\w	Word character (a-z, A-Z, 0-9, _)
\s	Whitespace
[abc]	Any of a, b, or c
[^abc]	Anything except a, b, or c
a*	Zero or more a’s
a+	One or more a’s
a?	Zero or one a
a{3}	Exactly 3 a’s

Practical Example

Email validation pattern:

const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
console.log(emailRegex.test('user@example.com'));    // true
console.log(emailRegex.test('invalid.email'));       // false