Assertions - JavaScript 编辑

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).

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Types

Boundary-type assertions

Other assertions

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

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 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. 

let 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.

let 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 sets when used within 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

Browser compatibility

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

See also

• Regular expressions guide
  • Character classes
  • Quantifiers
  • Unicode property escapes
  • Groups and ranges
• The RegExp() constructor