Dive Into JavaScript

The JavaScript RegExp Object

In JavaScript, regular expressions are a built-in data type. The RegExp object is a wrapper for the regular expression primitive.

Regular expressions are built directly into the syntax. As a result, regular expression primitives have access to properties and methods of the RegExp object. The conversion between regular expression primitives and RegExp objects is done automatically by the JavaScript engine.

A regular expression object can be created using the new RegExp() constructor:

var re1 = new RegExp(pattern [, flags]);

The pattern argument represents the regular expression patter, while the flags argument is a sequence of modifiers.

Example:

var re_match_positive = new RegExp("^[1-9]\d*$");

This creates a regular expression object that can be used to test if a given value is a positive integer:

re_match_positive.test("1"); // true
re_match_positive.test("foo"); // false
re_match_positive.test("0"); // false
re_match_positive.test("123"); // true

A regular expression primitive can be created using the literal notation:

var re2 = /pattern/flags;

The arguments are treated the same way. For example:

var re_ltrim = /^\s+/;
var re_rtrim = /\s+$/;

These are two regular expressions that can be used to strip whitespace from the beginning and end of a string:

var test_str = " We're all mad here. I'm mad. You're mad. ";
test_str.replace(re_ltrim, ""); // "We're all mad here. I'm mad. You're mad. "
test_str.replace(re_rtrim, ""); // " We're all mad here. I'm mad. You're mad."

RegExp Properties

constructor

Returns a reference to the function that created the RegExp object

global

The value of this property is true if the regular expression g flag was set

ignoreCase

This property is true if the regular expression was compiled with the i flag

lastIndex

Indicates the index position of the last match

multiline

The value of multiline is true if the m flag was used

prototype

Allows the addition of properties and methods to all objects

source

The text pattern of the regular expression

RegExp Methods

exec(text)

Executes the specified pattern on the input string and returns an array of matched strings

test(text)

Tests for a match in the input string and returns true if the pattern matches the string or false otherwise

toString()

Returns a string representation of the RegExp object

valueOf()

Returns the primitive value of the RegExp object

See also

• Learn more about JavaScript Regular Expressions
• JavaScript Regular Expressions Toolkit