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
| Col 1 | Col 2 | Col 3 |
|---|---|---|
| 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 |
| (?<=...) | Lookbehind | Matches the previous string |
| (?<!...) | Lookbehind | Matches NOT the previous string |
RegExp ^ Metacharacter
The ^ metacharacter matches the beginning of a string.
Examples
const pattern = /^W3Schools/;
let text = "W3Schools Tutorial";
let result = pattern.test(text); // true
RegExp $ Metacharacter
The $ metacharacter matches the end of a string.
const pattern = /W3Schools$/;
let text = "Hello W3Schools";
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
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
let text = "W3Schools Tutorials";
let pattern = /W3Schools(?= Tutorials)/;
let result = pattern.test(text);
Negative Lookahead x(?!y)
x(?!y) matches "x" if "x" is NOT followed by "y".
Example
let text = "W3Schools Tutorials";
let pattern = /W3Schools(?! Tutorials)/;
let result = pattern.test(text);
Explanation
W3Schools matches the literal word.
(?! Tutorials) asserts that what follows is not " Tutorials".
If text = "W3Schools Tutorials", the test returns false.
If text = "W3Schools Website", the test returns true.
RegExp Lookbehind (?<=y)x
(?<=y)x matches "x" if "x" is preceded by "y".
Example
let text = "Hello W3Schools";
let pattern = /(?<=Hello )W3Schools/;
let result = pattern.test(text);
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);
Note: 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
| Col 1 | Col 2 |
|---|---|
| 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
| Col 1 | Col 2 |
|---|---|
| Method | Description |
| regex.exec() | Returns an Iterator of results |
| regex.test() | Returns true or false |
