RegExp

Summary

The RegExp constructor creates a regular expression object for matching text with a pattern.

For an introduction on what regular expressions are, read the Regular Expressions chapter in the JavaScript Guide.

Constructor

Literal and constructor notations are possible:

/pattern/flags
new RegExp(pattern[, flags])

Parameters

pattern

The text of the regular expression.

flags

If specified, flags can have any combination of the following values:

g

global match

i

ignore case

m

multiline; treat beginning and end characters (^ and $) as working over multiple lines (i.e., match the beginning or end of each line (delimited by \n or \r), not only the very beginning or end of the whole input string)

y

sticky; matches only from the index indicated by the lastIndex property of this regular expression in the target string (and does not attempt to match from any later indexes).

Description

There are 2 ways to create a RegExp object: a literal notation and a constructor. To indicate strings, the parameters to the literal notation do not use quotation marks while the parameters to the constructor function do use quotation marks. So the following expressions create the same regular expression:

/ab+c/i;
new RegExp('ab+c', 'i');
new RegExp(/ab+c/, 'i');

The literal notation provides compilation of the regular expression when the expression is evaluated. Use literal notation when the regular expression will remain constant. For example, if you use literal notation to construct a regular expression used in a loop, the regular expression won't be recompiled on each iteration.

The constructor of the regular expression object, for example, new RegExp('ab+c'), provides runtime compilation of the regular expression. Use the constructor function when you know the regular expression pattern will be changing, or you don't know the pattern and are getting it from another source, such as user input.

Starting with ECMAScript 6, new RegExp(/ab+c/, 'i') no longer throws a TypeError ("can't supply flags when constructing one RegExp from another") when the first argument is a RegExp and the second flags argument is present. A new RegExp from the arguments is created instead.

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:

var re = /\w+/;
var re = new RegExp('\\w+');

Special characters meaning in regular expressions

• Character Classes
• Character Sets
• Boundaries
• Grouping and back references
• Quantifiers

Properties

RegExp.prototype

Allows the addition of properties to all objects.

RegExp.length

The value of RegExp.length is 2.

RegExp.lastIndex

The index at which to start the next match.

Methods

The global RegExp object has no methods of its own, however, it does inherit some methods through the prototype chain.

RegExp prototype objects and instances

Properties

Methods

Examples

Example: 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.

var re = /(\w+)\s(\w+)/;
var str = 'John Smith';
var newstr = str.replace(re, '$2, $1');
console.log(newstr);

This displays "Smith, John".

Example: 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.

var text = 'Some text\nAnd some more\r\nAnd yet\rThis is the end';
var lines = text.split(/\r\n|\r|\n/);
console.log(lines); // prints [ 'Some text', 'And some more', 'And yet', 'This is the end' ]

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

Example: Using regular expression on multiple lines

var s = 'Please yes\nmake my day!';
s.match(/yes.*day/);
// Returns null
s.match(/yes[^]*day/);
// Returns 'yes\nmake my day'

Example: Using a regular expression with the "sticky" flag

This example demonstrates how one could use the sticky flag on regular expressions to match individual lines of multiline input.

var text = 'First line\nSecond line';
var regex = /(\S+) line\n?/y;

var match = regex.exec(text);
console.log(match[1]);        // prints 'First'
console.log(regex.lastIndex); // prints '11'

var match2 = regex.exec(text);
console.log(match2[1]);       // prints 'Second'
console.log(regex.lastIndex); // prints '22'

var match3 = regex.exec(text);
console.log(match3 === null); // prints 'true'

One can test at run-time whether the sticky flag is supported, using try { … } catch { … }. For this, either an eval(…) expression or the RegExp(regex-string, flags-string) syntax must be used (since the /regex/flags notation is processed at compile-time, so throws an exception before the catch block is encountered). For example:

var supports_sticky;
try { RegExp('', 'y'); supports_sticky = true; }
catch(e) { supports_sticky = false; }
console.log(supports_sticky); // prints 'true'

Example: Regular expression and Unicode characters

As mentioned above, \w or \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.

var text = 'Образец text на русском языке';
var regex = /[\u0400-\u04FF]+/g;

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

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

// and so on

Here's an external resource for getting the complete Unicode block range for different scripts: Regexp-unicode-block.

Example: Extracting subdomain name from URL

var url = 'http://xxx.domain.com';
console.log(/[^.]+/.exec(url)[0].substr(7)); // prints 'xxx'

Specifications

Browser compatibility

Gecko specific notes

Starting with Gecko 34 (Firefox 34 / Thunderbird 34 / SeaMonkey 2.31), 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:

// Firefox 33 or older
'x'.replace(/x(.)?/g, function(m, group) {
  console.log("'group:" + group + "'");
}); // 'group:'

// Firefox 34 or newer
'x'.replace(/x(.)?/g, function(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).

See also

• Regular Expressions chapter in the JavaScript Guide
• String.prototype.match()
• String.prototype.replace()