RegExp

Description

Literal notation and constructor

There are two ways to create a RegExp object: a literal notation and a constructor.

• The literal notation takes a pattern between two slashes, followed by optional flags, after the second slash.
• The constructor function takes either a string or a RegExp object as its first parameter and a string of optional flags as its second parameter.

The following three expressions create the same regular expression object:

const re = /ab+c/i; // literal notation
// OR
const re = new RegExp("ab+c", "i"); // constructor with string pattern as first argument
// OR
const re = new RegExp(/ab+c/, "i"); // constructor with regular expression literal as first argument

Before regular expressions can be used, they have to be compiled. This process allows them to perform matches more efficiently. More about the process can be found in dotnet docs.

The literal notation results in compilation of the regular expression when the expression is evaluated. On the other hand, the constructor of the RegExp object, new RegExp('ab+c'), results in runtime compilation of the regular expression.

Use a string as the first argument to the RegExp() constructor when you want to build the regular expression from dynamic input.

Flags in constructor

The expression new RegExp(/ab+c/, flags) will create a new RegExp using the source of the first parameter and the flags provided by the second.

When using the constructor function, the normal string escape rules (preceding special characters with \ when included in a string) are necessary.

For example, the following are equivalent:

const re = /\w+/;
// OR
const re = new RegExp("\\w+");

Special handling for regexes

Some built-in methods would treat regexes specially. They decide whether x is a regex through multiple steps:

1. x must be an object (not a primitive).
2. If x[Symbol.match] is not undefined, check if it's truthy.
3. Otherwise, if x[Symbol.match] is undefined, check if x had been created with the RegExp constructor. (This step should rarely happen, since if x is a RegExp object that have not been tampered with, it should have a Symbol.match property.)

Note that in most cases, it would go through the Symbol.match check, which means:

• An actual RegExp object whose Symbol.match property's value is falsy but not undefined (even with everything else intact, like exec and [Symbol.replace]()) can be used as if it's not a regex.
• A non-RegExp object with a Symbol.match property will be treated as if it's a regex.

This choice was made because [Symbol.match]() is the most indicative property that something is intended to be used for matching. (exec could also be used, but because it's not a symbol property, there would be too many false positives.) The places that treat regexes specially include:

• String.prototype.endsWith(), startsWith(), and includes() throw a TypeError if the first argument is a regex.
• String.prototype.matchAll() and replaceAll() check whether the global flag is set if the first argument is a regex before invoking its [Symbol.matchAll]() or [Symbol.replace]() method.
• The RegExp() constructor directly returns the pattern argument only if pattern is a regex (among a few other conditions). If pattern is a regex, it would also interrogate pattern's source and flags properties instead of coercing pattern to a string.

For example, String.prototype.endsWith() would coerce all inputs to strings, but it would throw if the argument is a regex, because it's only designed to match strings, and using a regex is likely a developer mistake.

"foobar".endsWith({ toString: () => "bar" }); // true
"foobar".endsWith(/bar/); // TypeError: First argument to String.prototype.endsWith must not be a regular expression

You can get around the check by setting [Symbol.match] to a falsy value that's not undefined. This would mean that the regex cannot be used for String.prototype.match() (since without [Symbol.match], match() would construct a new RegExp object with the two enclosing slashes added by re.toString()), but it can be used for virtually everything else.

const re = /bar/g;
re[Symbol.match] = false;
"/bar/g".endsWith(re); // true
re.exec("bar"); // [ 'bar', index: 0, input: 'bar', groups: undefined ]
"bar & bar".replace(re, "foo"); // 'foo & foo'

Perl-like RegExp properties

Note that several of the RegExp properties have both long and short (Perl-like) names. Both names always refer to the same value. (Perl is the programming language from which JavaScript modeled its regular expressions.) See also deprecated RegExp properties.

Constructor

RegExp()

Creates a new RegExp object.

Static properties

RegExp.$1, …, RegExp.$9

Static read-only properties that contain parenthesized substring matches.

RegExp.input ($_)

A static property that contains the last string against which a regular expression was successfully matched.

RegExp.lastMatch ($&)

A static read-only property that contains the last matched substring.

RegExp.lastParen ($+)

A static read-only property that contains the last parenthesized substring match.

RegExp.leftContext ($`)

A static read-only property that contains the substring preceding the most recent match.

RegExp.rightContext ($')

A static read-only property that contains the substring following the most recent match.

RegExp[Symbol.species]

The constructor function that is used to create derived objects.

Static methods

RegExp.escape()

Escapes any potential regex syntax characters in a string, and returns a new string that can be safely used as a literal pattern for the RegExp() constructor.

Instance properties

These properties are defined on RegExp.prototype and shared by all RegExp instances.

RegExp.prototype.constructor

The constructor function that created the instance object. For RegExp instances, the initial value is the RegExp constructor.

RegExp.prototype.dotAll

Whether . matches newlines or not.

RegExp.prototype.flags

A string that contains the flags of the RegExp object.

RegExp.prototype.global

Whether to test the regular expression against all possible matches in a string, or only against the first.

RegExp.prototype.hasIndices

Whether the regular expression result exposes the start and end indices of captured substrings.

RegExp.prototype.ignoreCase

Whether to ignore case while attempting a match in a string.

RegExp.prototype.multiline

Whether or not to search in strings across multiple lines.

RegExp.prototype.source

The text of the pattern.

RegExp.prototype.sticky

Whether or not the search is sticky.

RegExp.prototype.unicode

Whether or not Unicode features are enabled.

RegExp.prototype.unicodeSets

Whether or not the v flag, an upgrade to the u mode, is enabled.

These properties are own properties of each RegExp instance.

lastIndex

The index at which to start the next match.

Instance methods

RegExp.prototype.compile()

(Re-)compiles a regular expression during execution of a script.

RegExp.prototype.exec()

Executes a search for a match in its string parameter.

RegExp.prototype.test()

Tests for a match in its string parameter.

RegExp.prototype.toString()

Returns a string representing the specified object. Overrides the Object.prototype.toString() method.

RegExp.prototype[Symbol.match]()

Performs match to given string and returns match result.

RegExp.prototype[Symbol.matchAll]()

Returns all matches of the regular expression against a string.

RegExp.prototype[Symbol.replace]()

Replaces matches in given string with new substring.

RegExp.prototype[Symbol.search]()

Searches the match in given string and returns the index the pattern found in the string.

RegExp.prototype[Symbol.split]()

Splits given string into an array by separating the string into substrings.

Examples

Using a regular expression to change data format

The following script uses the String.prototype.replace() method to match a name in the format first last and output it in the format last, first.

In the replacement text, the script uses $1 and $2 to indicate the results of the corresponding matching parentheses in the regular expression pattern.

const re = /(\w+)\s(\w+)/;
const str = "Maria Cruz";
const newStr = str.replace(re, "$2, $1");
console.log(newStr);

This displays "Cruz, Maria".

Using regular expression to split lines with different line endings/ends of line/line breaks

The default line ending varies depending on the platform (Unix, Windows, etc.). The line splitting provided in this example works on all platforms.

const text = "Some text\nAnd some more\r\nAnd yet\nThis is the end";
const lines = text.split(/\r?\n/);
console.log(lines); // [ 'Some text', 'And some more', 'And yet', 'This is the end' ]

Note that the order of the patterns in the regular expression matters.

Using regular expression on multiple lines

By default, the . character does not match newlines. To make it match newlines, use the s flag (dotAll mode).

const s = "Please yes\nmake my day!";

s.match(/yes.*day/);
// Returns null

s.match(/yes.*day/s);
// Returns ["yes\nmake my day"]

Using a regular expression with the sticky flag

The sticky flag indicates that the regular expression performs sticky matching in the target string by attempting to match starting at RegExp.prototype.lastIndex.

const str = "#foo#";
const regex = /foo/y;

regex.lastIndex = 1;
regex.test(str); // true
regex.lastIndex = 5;
regex.test(str); // false (lastIndex is taken into account with sticky flag)
regex.lastIndex; // 0 (reset after match failure)

The difference between the sticky flag and the global flag

With the sticky flag y, the next match has to happen at the lastIndex position, while with the global flag g, the match can happen at the lastIndex position or later:

const re = /\d/y;
let r;
while ((r = re.exec("123 456"))) {
  console.log(r, "AND re.lastIndex", re.lastIndex);
}

// [ '1', index: 0, input: '123 456', groups: undefined ] AND re.lastIndex 1
// [ '2', index: 1, input: '123 456', groups: undefined ] AND re.lastIndex 2
// [ '3', index: 2, input: '123 456', groups: undefined ] AND re.lastIndex 3
//  … and no more match.

With the global flag g, all 6 digits would be matched, not just 3.

Regular expression and Unicode characters

\w and \W only matches ASCII based characters; for example, a to z, A to Z, 0 to 9, and _.

To match characters from other languages such as Cyrillic or Hebrew, use \uHHHH, where HHHH is the character's Unicode value in hexadecimal.

This example demonstrates how one can separate out Unicode characters from a word.

const text = "Образец text на русском языке";
const regex = /[\u0400-\u04ff]+/g;

const match = regex.exec(text);
console.log(match[0]); // 'Образец'
console.log(regex.lastIndex); // 7

const match2 = regex.exec(text);
console.log(match2[0]); // 'на' (did not log 'text')
console.log(regex.lastIndex); // 15

// and so on

The Unicode property escapes feature provides a simpler way to target particular Unicode ranges, by allowing for statements like \p{scx=Cyrl} (to match any Cyrillic letter), or \p{L}/u (to match a letter from any language).

Extracting subdomain name from URL

const url = "http://xxx.example.com";
console.log(/^https?:\/\/(.+?)\./.exec(url)[1]); // 'xxx'

Building a regular expression from dynamic inputs

const breakfasts = ["bacon", "eggs", "oatmeal", "toast", "cereal"];
const order = "Let me get some bacon and eggs, please";

order.match(new RegExp(`\\b(${breakfasts.join("|")})\\b`, "g"));
// Returns ['bacon', 'eggs']

Specifications

Browser compatibility