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". Note: This character has a different meaning when it appears at the start of a character class.
`$`	Matches the end of input. If the multiline flag is set to true, also matches immediately before a line break character. For example, `/t$/` 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 beginning of the string, and right after newline.
buggyMultiline = buggyMultiline.replace(/^t/gim, "h");
console.log(1, buggyMultiline); // fix 'tey' => 'hey' and 'tangs' => 'hangs' but do not touch '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' => 'the' but do 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' => 'green' but do not touch 'on'.

Matching the beginning of input using a ^ control character

Use ^ for matching at the beginning of input. In this example, we can get the fruits that start with 'A' by a /^A/ regex. For selecting appropriate fruits we can use the filter method with an arrow function.

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

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

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

In the second example ^ is used both for matching at the beginning of input and for creating negated or complemented character class when used within character classes.

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

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

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

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

Matching a word boundary

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

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

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

Lookahead assertion

// JS Lookahead assertion x(?=y)

const 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 character classes

The ?! combination has different meanings in assertions like /x(?!y)/ and character classes like [^?!].

const 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 /[^?!]/
const selectNotLemonRegex = /[^?!]+have(?! a lemon)[^?!]+[?!]/gi;
console.log(orangeNotLemon.match(selectNotLemonRegex)); // [ 'Do you want to have an orange?' ]

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

Lookbehind assertion

const oranges = ["ripe orange A", "green orange B", "ripe orange C"];

const ripeOranges = oranges.filter((fruit) => /(?<=ripe )orange/.test(fruit));
console.log(ripeOranges); // [ 'ripe orange A', 'ripe orange C' ]