Regular Expression Assertions
RegExp Assertions
Assertions matchesBoundaries andLookarounds:
String Boundaries and Word Boundaries.
Lookarounds: Lookaheads and Lookbehinds.
const pattern = /^W3Schools/;
// Match end of string
const pattern = /W3Schools$/;
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 |
| (?<=...) | Lookbehind | Matches the previous string |
| (?<!...) | Lookbehind | Matches NOT the previous string |
RegExp ^ Metacharacter
The ^ metacharacter matches the beginning of a string.
Examples
Test if a string starts with W3Schools:
let text = "W3Schools Tutorial";
let result = pattern.test(text); // true
let text = "Hello W3Schools";
let result = pattern.test(text); // false
RegExp $ Metacharacter
The $ metacharacter matches the end of a string.
Test if a string ends with W3Schools:
let text = "Hello W3Schools";
let result = pattern.test(text); // true
let text = "W3Schools tutorial";
let result = pattern.test(text); // false
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 thebeginning of a word:
let result = text.search(/\bLO/);
Search for the characters "LO" at theend of a word:
let result = text.search(/LO\b/);
RegExp Lookahead x(?=y)
x(?=y) matches "x" if "x" is followed by "y".
Example
Match "W3schools" if "W3Schools" is followed by " 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 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
Match "W3Scools" if "W3Schools" is preceded by "Hello ".
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 pattern = /(?<!Hello ) W3Schools/;
let result = pattern.test(text);
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
const pattern = /(haha)+/;
let result = text.match(pattern);
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
// 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;
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.
const pattern = /(?i:WShools) tutorials/;
// Returns true:
let result = pattern.test(text);
The i flag is only enabled for the W3Schools part of the pattern.
const pattern = /(?i:W3Sshools) tutorials/;
// Returns false:
let result = pattern.test(text);
Regular Expression Methods
Regular ExpressionSearch andReplace 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 |
