RegExp
The RegExp
object is used for matching text with a pattern.
For an introduction to regular expressions, read the Regular expressions chapter in the JavaScript guide. For detailed information of regular expression syntax, read the regular expression reference.
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:
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.
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:
js
const re = /\w+/;
// OR
const re = new RegExp("\\w+");
Special handling for regexes
Note: Whether something is a "regex" can be duck-typed. 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:
x
must be an object (not a primitive).- If
x[Symbol.match]
is notundefined
, check if it's truthy. - Otherwise, if
x[Symbol.match]
isundefined
, check ifx
had been created with theRegExp
constructor. (This step should rarely happen, since ifx
is aRegExp
object that have not been tampered with, it should have aSymbol.match
property.)
Note that in most cases, it would go through the Symbol.match
check, which means:
- An actual
RegExp
object whoseSymbol.match
property's value is falsy but notundefined
(even with everything else intact, likeexec
and@@replace
) can be used as if it's not a regex. - A non-
RegExp
object with aSymbol.match
property will be treated as if it's a regex.
This choice was made because @@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()
, andincludes()
throw aTypeError
if the first argument is a regex.String.prototype.matchAll()
andreplaceAll()
check whether the global flag is set if the first argument is a regex before invoking its@@matchAll
or@@replace
method.- The
RegExp()
constructor directly returns thepattern
argument only ifpattern
is a regex (among a few other conditions). Ifpattern
is a regex, it would also interrogatepattern
'ssource
andflags
properties instead of coercingpattern
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.
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 @@match
to a falsy value that's not undefined
. This would mean that the regex cannot be used for String.prototype.match()
(since without @@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.
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.
Constructor
RegExp()
-
Creates a new
RegExp
object.
Static properties
RegExp.$1, …, RegExp.$9
Deprecated-
Static read-only properties that contain parenthesized substring matches.
RegExp.input ($_)
Deprecated-
A static property that contains the last string against which a regular expression was successfully matched.
RegExp.lastMatch ($&)
Deprecated-
A static read-only property that contains the last matched substring.
RegExp.lastParen ($+)
Deprecated-
A static read-only property that contains the last parenthesized substring match.
RegExp.leftContext ($`)
Deprecated-
A static read-only property that contains the substring preceding the most recent match.
RegExp.rightContext ($')
Deprecated-
A static read-only property that contains the substring following the most recent match.
RegExp[@@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.
RegExp.prototype.constructor
-
The constructor function that created the instance object. For
RegExp
instances, the initial value is theRegExp
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 theu
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()
Deprecated-
(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[@@match]()
-
Performs match to given string and returns match result.
RegExp.prototype[@@matchAll]()
-
Returns all matches of the regular expression against a string.
RegExp.prototype[@@replace]()
-
Replaces matches in given string with new substring.
RegExp.prototype[@@search]()
-
Searches the match in given string and returns the index the pattern found in the string.
RegExp.prototype[@@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 replace()
method of the String
instance 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 sticky
flag indicates that the regular expression performs sticky matching in the target string by attempting to match starting at 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 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.
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
Specification |
---|
ECMAScript Language Specification # sec-regexp-regular-expression-objects |
Browser compatibility
BCD tables only load in the browser
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).