How do Regular Expressions work in JavaScript? | JavaScript Interview Questions

What You'll Learn

Creating regular expressions
Common patterns
Methods for matching

Creating RegExp

code.jsJavaScript

// Literal syntax
const regex1 = /pattern/flags;

// Constructor
const regex2 = new RegExp('pattern', 'flags');

// Flags
// g - global (find all)
// i - case insensitive
// m - multiline

Common Patterns

Pattern	Meaning
.	Any character
\d	Digit [0-9]
\w	Word character [a-zA-Z0-9_]
\s	Whitespace
^	Start of string
$	End of string
*	0 or more
+	1 or more
?	0 or 1
{n}	Exactly n times
[abc]	Character class
(x)	Capturing group

Methods

code.jsJavaScript

const text = 'The quick brown fox';
const regex = /quick/;

// test - returns boolean
regex.test(text); // true

// match - returns matches
text.match(/o/g); // ['o', 'o']

// replace
text.replace(/fox/, 'dog'); // 'The quick brown dog'

// exec - detailed match info
/quick/.exec(text);
// ['quick', index: 4, input: 'The quick brown fox']

Practical Examples

code.jsJavaScript

// Email validation
const email = /^[\w.-]+@[\w.-]+\.\w+$/;
email.test('user@example.com'); // true

// Phone number
const phone = /^\d{3}-\d{3}-\d{4}$/;
phone.test('123-456-7890'); // true

// Extract numbers
'Price: $99.99'.match(/\d+\.?\d*/g); // ['99.99']

// Capture groups
const date = /^(\d{4})-(\d{2})-(\d{2})$/;
const [, year, month, day] = date.exec('2024-01-15');