Assertions

Assertions include boundaries, which indicate the beginnings and endings of lines and words, and other patterns indicating in some way that a match is possible (including look-ahead, look-behind, and conditional expressions).

Types

Boundary-type assertions

Characters Meaning
^

Matches the beginning of input. If the multiline flag is set to true, also matches immediately after a line break character. For example, /^A/ does not match the "A" in "an A", but does match the first "A" in "An A".

This character has a different meaning when it appears at the start of a group.

$

Matches the end of input. If the multiline flag is set to true, also matches immediately before a line break character. For example, /t$/ does not match the "t" in "eater", but does match it in "eat".

\b

Matches a word boundary. This is the position where a word character is not followed or preceded by another word-character, such as between a letter and a space. Note that a matched word boundary is not included in the match. In other words, the length of a matched word boundary is zero.

Examples:

  • /\bm/ matches the "m" in "moon".
  • /oo\b/ does not match the "oo" in "moon", because "oo" is followed by "n" which is a word character.
  • /oon\b/ matches the "oon" in "moon", because "oon" is the end of the string, thus not followed by a word character.
  • /\w\b\w/ will never match anything, because a word character can never be followed by both a non-word and a word character.

To match a backspace character ([\b]), see Character Classes.

\B

Matches a non-word boundary. This is a position where the previous and next character are of the same type: Either both must be words, or both must be non-words, for example between two letters or between two spaces. The beginning and end of a string are considered non-words. Same as the matched word boundary, the matched non-word boundary is also not included in the match. For example, /\Bon/ matches "on" in "at noon", and /ye\B/ matches "ye" in "possibly yesterday".

Other assertions

Note: The ? character may also be used as a quantifier.

Characters Meaning
x(?=y)

Lookahead assertion: Matches "x" only if "x" is followed by "y". For example, /Jack(?=Sprat)/ matches "Jack" only if it is followed by "Sprat".
/Jack(?=Sprat|Frost)/ matches "Jack" only if it is followed by "Sprat" or "Frost". However, neither "Sprat" nor "Frost" is part of the match results.

x(?!y)

Negative lookahead assertion: Matches "x" only if "x" is not followed by "y". For example, /\d+(?!\.)/ matches a number only if it is not followed by a decimal point. /\d+(?!\.)/.exec('3.141') matches "141" but not "3.

(?<=y)x

Lookbehind assertion: Matches "x" only if "x" is preceded by "y". For example, /(?<=Jack)Sprat/ matches "Sprat" only if it is preceded by "Jack". /(?<=Jack|Tom)Sprat/ matches "Sprat" only if it is preceded by "Jack" or "Tom". However, neither "Jack" nor "Tom" is part of the match results.

(?<!y)x

Negative lookbehind assertion: Matches "x" only if "x" is not preceded by "y". For example, /(?<!-)\d+/ matches a number only if it is not preceded by a minus sign. /(?<!-)\d+/.exec('3') matches "3". /(?<!-)\d+/.exec('-3')  match is not found because the number is preceded by the minus sign.

Examples

General boundary-type overview example

// Using Regex boundaries to fix buggy string.
buggyMultiline = `tey, ihe light-greon apple
tangs on ihe greon traa`;

// 1) Use ^ to fix the matching at the begining of the string, and right after newline.
buggyMultiline = buggyMultiline.replace(/^t/gim,'h'); 
console.log(1, buggyMultiline); // fix 'tey', 'tangs' => 'hey', 'hangs'. Avoid 'traa'.

// 2) Use $ to fix matching at the end of the text.
buggyMultiline = buggyMultiline.replace(/aa$/gim,'ee.'); 
console.log(2, buggyMultiline); // fix  'traa' => 'tree'.

// 3) Use \b to match characters right on border between a word and a space.
buggyMultiline = buggyMultiline.replace(/\bi/gim,'t');
console.log(3, buggyMultiline); // fix  'ihe' but does not touch 'light'.

// 4) Use \B to match characters inside borders of an entity.
fixedMultiline = buggyMultiline.replace(/\Bo/gim,'e');
console.log(4, fixedMultiline); // fix  'greon' but does not touch 'on'.

Matching the beginning of an input using a ^ control character

Use ^ for average matching begining of a word. In this example get fruites that starts with 'A' by a  /^A/ regex.  Here ^ plays only one role: show begining of the input. For selecting appropriate fruits used filter method with an arrow function. 

let fruits = ["Apple", "Watermelon", "Orange", "Avocado", "Strawberry"];

// Select fruits started with 'A' by /^A/ Regex.
// Here '^' control symbol used only in one role: Matching begining of an input.

let fruitsStartsWithA = fruits.filter(fruit => /^A/.test(fruit));
console.log(fruitsStartsWithA); // [ 'Apple', 'Avocado' ]

It the second example ^ used for both: matching begining of the input and as negated or complemented character set when it used in groups

let fruits = ["Apple", "Watermelon", "Orange", "Avocado", "Strawberry"];

// Selecting fruits that dose not start by 'A' by a /^[^A]/ regex.
// In this example, two meanings of '^' control symbol are represented:
// 1) Matching begining of the input
// 2) A negated or complemented character set: [^A]
// That is, it matches anything that is not enclosed in the brackets.

let fruitsStartsWithNotA = fruits.filter(fruit => /^[^A]/.test(fruit));

console.log(fruitsStartsWithNotA); // [ 'Watermelon', 'Orange', 'Strawberry' ]

Matching a word boundary

let fruitsWithDescription = ["Red apple", "Orange orange", "Green Avocado"];

// Select descriptions that contains 'en' or 'ed' words endings:
let enEdSelection = fruitsWithDescription.filter(descr => /(en|ed)\b/.test(descr));

console.log(enEdSelection); // [ 'Red apple', 'Green Avocado' ]

Lookahead assertion

// JS Lookahead assertion x(?=y)

let regex = /First(?= test)/g;

console.log('First test'.match(regex)); // [ 'First' ]
console.log('First peach'.match(regex)); // null
console.log('This is a First test in a year.'.match(regex)); // [ 'First' ]
console.log('This is a First peach in a month.'.match(regex)); // null

Basic negative lookahead assertion

For example, /\d+(?!\.)/ matches a number only if it is not followed by a decimal point. /\d+(?!\.)/.exec('3.141') matches "141" but not "3.

console.log(/\d+(?!\.)/g.exec('3.141')); // [ '141', index: 2, input: '3.141' ]

Different meaning of '?!' combination usage in Assertions and  Ranges 

Different meaning of ?! combination usage in Assertions /x(?!y)/ and Ranges [^?!].

let orangeNotLemon = "Do you want to have an orange? Yes, I do not want to have a lemon!";

// Different meaning of '?!' combination usage in Assertions /x(?!y)/ and  Ranges /[^?!]/
let selectNotLemonRegex = /[^?!]+have(?! a lemon)[^?!]+[?!]/gi
console.log(orangeNotLemon.match(selectNotLemonRegex)); // [ 'Do you want to have an orange?' ]

let selectNotOrangeRegex = /[^?!]+have(?! an orange)[^?!]+[?!]/gi
console.log(orangeNotLemon.match(selectNotOrangeRegex)); // [ ' Yes, I do not want to have a lemon!' ]

Lookbehind assertion

let oranges = ['ripe orange A ', 'green orange B', 'ripe orange C',];

let ripe_oranges = oranges.filter( fruit => fruit.match(/(?<=ripe )orange/));
console.log(ripe_oranges); // [ 'ripe orange A ', 'ripe orange C' ]

Specifications

Specification Status Comment
ECMAScript Latest Draft (ECMA-262)
The definition of 'RegExp: Assertions' in that specification.
Draft

Browser compatibility

For browser compatibility information, check out the main Regular Expressions compatibility table.

See also