JavaScript Regular Expressions (RegEx)


Regular Expressions (RegEx) are patterns used to match, search, or replace characters in strings. In JavaScript, RegEx is powerful for validating input, searching data, and replacing text.



1. Creating a RegEx Pattern


Two ways to define a RegEx:

const pattern1 = /hello/; // Literal notation
const pattern2 = new RegExp("hello"); // RegExp constructor


2. Basic Methods

Method Description
test() Returns true if pattern is found
exec() Returns match result or null
match() Returns an array of matches
replace() Replaces matched string
search() Returns index of match or -1
split() Splits string based on RegEx


3. Common Patterns & Syntax

Pattern Description
. Any character (except newline)
\d Digit (0–9)
\w Word character (a-z, A-Z, 0–9, _)
\s Whitespace
^ Start of string
$ End of string
+ One or more
* Zero or more
? Zero or one
[abc] Match a, b, or c
(abc) Capture group
` `


4. Example: Validate Email

const email =
"test@example.com";
const pattern = /^[\w.
-]+@[\w.
-]+\.\w+$/;
console.log(pattern.test(email)); // true


5. Example: Extract Numbers

const str = "Price: $100 and $250";
const numbers = str.match(/\d+/g);
console.log(numbers); // ['100', '250']


6. Example: Replace Words

const text = "JavaScript is awesome!";
const newText = text.replace(/awesome/, "powerful");
console.log(newText); // "JavaScript is powerful!"


7. Case-Insensitive & Global Flags

Flag Meaning
g Global (match all)
i Case-insensitive
m Multi-line mode
const sentence = "Hello hello HELLO";
const result = sentence.match(/hello/gi);
console.log(result); // ['Hello', 'hello', 'Hello']


8. Replace All Digits with *1

const input = "My pin is 1234";
const masked = input.replace(/\d/g,"*");
console.log(masked); // "My pin is ****"


Real-World Use Cases

  • Form validation (email, phone, password)
  • Search and highlight text
  • Text processing (cleaning, formatting)
  • Data extraction (like numbers or dates)