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---
title: RegExp
slug: Web/JavaScript/Reference/Global_Objects/RegExp
page-type: javascript-class
browser-compat: javascript.builtins.RegExp
---
{{JSRef}}
The **`RegExp`** object is used for matching text with a pattern.
For an introduction to regular expressions, read the [Regular expressions chapter](/en-US/docs/Web/JavaScript/Guide/Regular_expressions) in the JavaScript guide. For detailed information of regular expression syntax, read the [regular expression reference](/en-US/docs/Web/JavaScript/Reference/Regular_expressions).
## 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](/en-US/docs/Web/JavaScript/Guide/Regular_expressions#advanced_searching_with_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](/en-US/docs/Web/JavaScript/Guide/Regular_expressions#advanced_searching_with_flags) as its second parameter.
The following three expressions create the same regular expression object:
```js
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](https://learn.microsoft.com/en-us/dotnet/standard/base-types/compilation-and-reuse-in-regular-expressions).
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](#building_a_regular_expression_from_dynamic_inputs).
### 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](/en-US/docs/Web/JavaScript/Guide/Regular_expressions#advanced_searching_with_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:
```js
const re = /\w+/;
// OR
const re = new RegExp("\\w+");
```
### Special handling for regexes
> [!NOTE]
> Whether something is a "regex" can be [duck-typed](https://en.wikipedia.org/wiki/Duck_typing). It doesn't have to be a `RegExp`!
Some built-in methods would treat regexes specially. They decide whether `x` is a regex through [multiple steps](https://tc39.es/ecma262/multipage/abstract-operations.html#sec-isregexp):
1. `x` must be an object (not a primitive).
2. If [`x[Symbol.match]`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/match) is not `undefined`, check if it's [truthy](/en-US/docs/Glossary/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](/en-US/docs/Glossary/Falsy) but not `undefined` (even with everything else intact, like [`exec`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/exec) and [`[Symbol.replace]()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/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()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/endsWith), [`startsWith()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/startsWith), and [`includes()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/includes) throw a {{jsxref("TypeError")}} if the first argument is a regex.
- [`String.prototype.matchAll()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/matchAll) and [`replaceAll()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replaceAll) check whether the [global](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/global) flag is set if the first argument is a regex before invoking its [`[Symbol.matchAll]()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/matchAll) or [`[Symbol.replace]()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/replace) method.
- The [`RegExp()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/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()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/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.
```js
"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](/en-US/docs/Glossary/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()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/toString)), but it can be used for virtually everything else.
```js
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](/en-US/docs/Web/JavaScript/Reference/Deprecated_and_obsolete_features#regexp).
## Constructor
- {{jsxref("RegExp/RegExp", "RegExp()")}}
- : Creates a new `RegExp` object.
## Static properties
- [`RegExp.$1`, …, `RegExp.$9`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/n) {{deprecated_inline}}
- : Static read-only properties that contain parenthesized substring matches.
- [`RegExp.input` (`$_`)](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/input) {{deprecated_inline}}
- : A static property that contains the last string against which a regular expression was successfully matched.
- [`RegExp.lastMatch` (`$&`)](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/lastMatch) {{deprecated_inline}}
- : A static read-only property that contains the last matched substring.
- [`RegExp.lastParen` (`$+`)](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/lastParen) {{deprecated_inline}}
- : A static read-only property that contains the last parenthesized substring match.
- [`RegExp.leftContext` (`` $` ``)](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/leftContext) {{deprecated_inline}}
- : A static read-only property that contains the substring preceding the most recent match.
- [`RegExp.rightContext` (`$'`)](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/rightContext) {{deprecated_inline}}
- : A static read-only property that contains the substring following the most recent match.
- [`RegExp[Symbol.species]`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/Symbol.species)
- : The constructor function that is used to create derived objects.
## Instance properties
These properties are defined on `RegExp.prototype` and shared by all `RegExp` instances.
- {{jsxref("Object/constructor", "RegExp.prototype.constructor")}}
- : The constructor function that created the instance object. For `RegExp` instances, the initial value is the {{jsxref("RegExp/RegExp", "RegExp")}} constructor.
- {{jsxref("RegExp.prototype.dotAll")}}
- : Whether `.` matches newlines or not.
- {{jsxref("RegExp.prototype.flags")}}
- : A string that contains the flags of the `RegExp` object.
- {{jsxref("RegExp.prototype.global")}}
- : Whether to test the regular expression against all possible matches in a string, or only against the first.
- {{jsxref("RegExp.prototype.hasIndices")}}
- : Whether the regular expression result exposes the start and end indices of captured substrings.
- {{jsxref("RegExp.prototype.ignoreCase")}}
- : Whether to ignore case while attempting a match in a string.
- {{jsxref("RegExp.prototype.multiline")}}
- : Whether or not to search in strings across multiple lines.
- {{jsxref("RegExp.prototype.source")}}
- : The text of the pattern.
- {{jsxref("RegExp.prototype.sticky")}}
- : Whether or not the search is sticky.
- {{jsxref("RegExp.prototype.unicode")}}
- : Whether or not Unicode features are enabled.
- {{jsxref("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.
- {{jsxref("RegExp/lastIndex", "lastIndex")}}
- : The index at which to start the next match.
## Instance methods
- {{jsxref("RegExp.prototype.compile()")}} {{deprecated_inline}}
- : (Re-)compiles a regular expression during execution of a script.
- {{jsxref("RegExp.prototype.exec()")}}
- : Executes a search for a match in its string parameter.
- {{jsxref("RegExp.prototype.test()")}}
- : Tests for a match in its string parameter.
- {{jsxref("RegExp.prototype.toString()")}}
- : Returns a string representing the specified object. Overrides the {{jsxref("Object.prototype.toString()")}} method.
- [`RegExp.prototype[Symbol.match]()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/Symbol.match)
- : Performs match to given string and returns match result.
- [`RegExp.prototype[Symbol.matchAll]()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/Symbol.matchAll)
- : Returns all matches of the regular expression against a string.
- [`RegExp.prototype[Symbol.replace]()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/Symbol.replace)
- : Replaces matches in given string with new substring.
- [`RegExp.prototype[Symbol.search]()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/Symbol.search)
- : Searches the match in given string and returns the index the pattern found in the string.
- [`RegExp.prototype[Symbol.split]()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/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 {{jsxref("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.
```js
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.
```js
const text = "Some text\nAnd some more\r\nAnd yet\rThis is the end";
const lines = text.split(/\r\n|\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
```js
const s = "Please yes\nmake my day!";
s.match(/yes.*day/);
// Returns null
s.match(/yes[^]*day/);
// Returns ["yes\nmake my day"]
```
### Using a regular expression with the sticky flag
The {{jsxref("RegExp/sticky", "sticky")}} flag indicates that the regular expression performs sticky matching in the target string by attempting to match starting at {{jsxref("RegExp.prototype.lastIndex")}}.
```js
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:
```js
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.
```js
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](/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Unicode_character_class_escape) 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
```js
const url = "http://xxx.domain.com";
console.log(/^https?:\/\/(.+?)\./.exec(url)[1]); // 'xxx'
```
> [!NOTE]
> Instead of using regular expressions for parsing URLs, it is usually better to use the browsers built-in URL parser by using the [URL API](/en-US/docs/Web/API/URL_API).
### Building a regular expression from dynamic inputs
```js
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
{{Specifications}}
## Browser compatibility
{{Compat}}
### Firefox-specific notes
Starting with Firefox 34, in the case of a capturing group with quantifiers preventing its exercise, the matched text for a capturing group is now `undefined` instead of an empty string:
```js
// Firefox 33 or older
"x".replace(/x(.)?/g, (m, group) => {
console.log(`group: ${JSON.stringify(group)}`);
});
// group: ""
// Firefox 34 or newer
"x".replace(/x(.)?/g, (m, group) => {
console.log(`group: ${group}`);
});
// group: undefined
```
Note that due to web compatibility, `RegExp.$N` will still return an empty string instead of `undefined` ([bug 1053944](https://bugzil.la/1053944)).
## See also
- [Polyfill of many modern `RegExp` features (`dotAll`, `sticky` flags, named capture groups, etc.) in `core-js`](https://github.com/zloirock/core-js#ecmascript-string-and-regexp)
- [Regular expressions](/en-US/docs/Web/JavaScript/Guide/Regular_expressions) guide
- [Regular expressions](/en-US/docs/Web/JavaScript/Reference/Regular_expressions)
- {{jsxref("String.prototype.match()")}}
- {{jsxref("String.prototype.replace()")}}
- {{jsxref("String.prototype.split()")}}