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JS TutorialLesson #94

Regular Expression Assertions

Assertions matches Boundaries and Lookarounds:

RegExp Assertions

Assertions matches Boundaries and Lookarounds:

String Boundaries and Word Boundaries.

Lookarounds: Lookaheads and Lookbehinds.

// Match beginning of string const pattern = /^EA Academy/;

// Match end of string const pattern = /EA Academy$/;

JavaScript Regex Assertions

Revised July 2025

Syntax	Name	Description
^	String boundary	Matches the beginning of a string
$	String boundary	Matches the end of a string
\b	Word boundary	Matches the beginning or end of a word
\B	Word boundary	Matches NOT the beginning or end of a word
(?=...)	Lookahead	Matches the subsequent string
(?!...)	Lookahead	Matches NOT the subsequent string

| (?

RegExp ^ Metacharacter

The ^ metacharacter matches the beginning of a string.

Examples

Example Code

javascript

Try it Yourself

const pattern = /^EA Academy/;

let text = "EA Academy Tutorial";

let result = pattern.test(text); // true

RegExp $ Metacharacter

The $ metacharacter matches the end of a string.

Example

Example Code

javascript

Try it Yourself

const pattern = /EA Academy$/;

let text = "Hello EA Academy";

let result = pattern.test(text); // true

The \b Metacharacter

The \b metacharacter matches the beginning of a word or the end of a word.

Examples

Example Code

javascript

Try it Yourself

let text = "HELLO, LOOK AT YOU!";

let result = text.search(/\bLO/);

RegExp Lookahead x(?=y)

x(?=y) matches "x" if "x" is followed by "y".

Example

Example Code

javascript

Try it Yourself

let text = "EA Academy Tutorials";

let pattern = /EA Academy(?= Tutorials)/;

let result = pattern.test(text);

Negative Lookahead x(?!y)

x(?!y) matches "x" if "x" is NOT followed by "y".

Example

Example Code

javascript

Try it Yourself

let text = "EA Academy Tutorials";

let pattern = /EA Academy(?! Tutorials)/;

let result = pattern.test(text);

Explanation

EA Academy matches the literal word.

(?! Tutorials) asserts that what follows is not " Tutorials".

If text = "EA Academy Tutorials", the test returns false.

If text = "EA Academy Website", the test returns true.

RegExp Lookbehind (? See Also: > JavaScript RegExp Tutorial > JavaScript RegExp Flags > JavaScript RegExp Character Classes > JavaScript RegExp Meta Characters > JavaScript RegExp Groups > JavaScript RegExp Quantifiers > JavaScript RegExp Patterns > JavaScript RegExp Objects > JavaScript RegExp Methods

Regular Expression Methods

Regular Expression Search and Replace can be done with different methods.

These are the most common:

String Methods

Method	Description
match(regex)	Returns an Array of results
matchAll(regex)	Returns an Iterator of results
replace(regex)	Returns a new String
replaceAll(regex)	Returns a new String
search(regex)	Returns the index of the first match
split(regex)	Returns an Array of results

RegExp Methods

Method	Description
regex.exec()	Returns an Iterator of results
regex.test()	Returns true or false