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Regular Expression Assertions

RegExp Assertions

Assertions matches Boundaries and Lookarounds:

String Boundaries and Word Boundaries.

Lookarounds: Lookaheads and Lookbehinds.

// Match beginning of string
const pattern = /^W3Schools/;

// Match end of string
const pattern = /W3Schools$/;

JavaScript Regex Assertions

Revised July 2025

SyntaxNameDescription
^String boundaryMatches the beginning of a string
$String boundaryMatches the end of a string
\bWord boundaryMatches the beginning or end of a word
\BWord boundaryMatches NOT the beginning or end of a word
(?=...)LookaheadMatches the subsequent string
(?!...)LookaheadMatches NOT the subsequent string
(?<=...)LookbehindMatches the previous string
(?<!...)LookbehindMatches NOT the previous string

RegExp ^ Metacharacter

The ^ metacharacter matches the beginning of a string.

Examples

Test if a string starts with W3Schools:

const pattern = /^W3Schools/;
let text = "W3Schools Tutorial";

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

Try it Yourself »

const pattern = /^W3Schools/;
let text = "Hello W3Schools";

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

Try it Yourself »


RegExp $ Metacharacter

The $ metacharacter matches the end of a string.

Test if a string ends with W3Schools:

const pattern = /W3Schools$/;
let text = "Hello W3Schools";

let result = pattern.test(text); // true
Try it Yourself »
const pattern = /W3Schools$/;
let text = "W3Schools tutorial";

let result = pattern.test(text); // false
Try it Yourself »

The \b Metacharacter

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

Examples

Search for the characters "LO" at the beginning of a word:

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

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

Try it Yourself »

Search for the characters "LO" at the end of a word:

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

let result = text.search(/LO\b/);

Try it Yourself »


RegExp Lookahead x(?=y)

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

Example

Match "W3schools" if "W3Schools" is followed by " Tutorials".

let text = "W3Schools Tutorials";
let pattern = /W3Schools(?= Tutorials)/;

let result = pattern.test(text);
Try it Yourself »

Negative Lookahead x(?!y)

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

Example

let text = "Hello W3Schools";

let pattern = /W3Schools(?!Hello )/;
let result = pattern.test(text);
Try it Yourself »

RegExp Lookbehind (?<=y)x

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

Example

Match "W3Scools" if "W3Schools" is preceded by "Hello ".

let text = "Hello W3Schools";
let pattern = /(?<=Hello )W3Schools/;

let result = pattern.test(text);
Try it Yourself »

Negative Lookbehind (?<!y)x

(?<!y)x matches "x" only if "x" is NOT preceded by "y".

Example

let text = "Hello W3Schools";

let pattern = /(?<!Hello ) W3Schools/;
let result = pattern.test(text);
Try it Yourself »


Regular Expression Groups

Char Description
(x) Matches x and saves it
(?<n>x) Matches x and labels it n
(?flag:x) Enables flag(s) for x
(?flag-flag:x) Disables flag(s) for x

RegExp Capturing Groups (x)

Example

let text = "Haha, haha, haha.";
const pattern = /(haha)+/;

let result = text.match(pattern);
Try it Yourself »

Explained

  • (haha) captures a group of characters
  • (haha)+ matches zero or more occurences of the group
  • text.match() returns an array of results
  • match.groups() returns an array of matches

Named Capturing Groups (?<n>)

Example

const text = "Name: John Doe";

// Using named capturing groups
const regex = /(?<firstName>\w+) (?<lastName>\w+)/;
const match = text.match(regex);

let fName = match.groups.firstName;
let lName = match.groups.lastName;
Try it Yourself »

Explained

  • (?<firstName>\w+) captures a word and labels it firstName
  • (?<lastName>\w+) does the same for lastName
  • text.match() returns an array with a groups property
  • match.groups() returns an object:
    {firstName:"John", lastName:"Doe" }

When using capturing groups, the String method match() and the RegExp method exec(), return a match object with a groups property. This property holds the names and the values of the groups.


RegExp Group Modifiers (?flag)

The (?flag) syntax is a group modifier (inline flag modifier).

It allows for modifying flags in a part of a pattern, rather than to the entire pattern.

(?flags:pattern) enables the flags only for the pattern in the group.

Example

The i flag is only enabled for the W3Schools part of the pattern.

let text = "W3Schools tutorials.";
const pattern = /(?i:WShools) tutorials/;

// Returns true:
let result = pattern.test(text);
Try it Yourself »

The i flag is only enabled for the W3Schools part of the pattern.

let text = "W3Schools Tutorials.";
const pattern = /(?i:W3Sshools) tutorials/;

// Returns false:
let result = pattern.test(text);
Try it Yourself »


Regular Expression Methods

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

These are the most common:

String Methods

MethodDescription
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

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

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