Regular Expressions in JavaScript

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This is an article on Regular Expressions in JavaScript in JavaScript and AJAX.
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Regular expressions is a form of pattern matching that you can apply on textual content. Take for example the DOS wildcards ? and * which you can use when you're searching for a file.

Creating a Regular Expression

You construct a regular expression in one of two ways:

1) Using a regular expression literal, as follows:
Code: JavaScript
re = /ab+c/;
Regular expression literals provide compilation of the regular expression when the script is evaluated. When the regular expression will remain constant, use this for better performance.

2) Calling the constructor function of the RegExp object, as follows:
Code: JavaScript
re = new RegExp("ab+c");
Using the constructor function 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.


There are three flags that you may use on a RegExp. The multiline flag has bad support in older browsers, but the other two are supported in pretty much every browser that can handle RegExp. These flags can be used in any order or combination, and are an integral part of the RegExp.

Global Search

g =>The global search flag makes the RegExp search for a pattern throughout the string, creating an array of all occurrences it can find matching the given pattern.

Ignore Case

i => The ignore case flag makes a regular expression case insensitive. For international coders, note that this might not work on extended characters such as , , , .

Multiline Input

m => This flag makes the beginning of input (^) and end of input ($) codes also catch beginning and end of line respectively.

In almost all cases you can use either way to define a regular expression, and they will be handled in exactly the same way no matter how you declare them.

Writing a Regular Expression Pattern

A regular expression pattern is composed of simple characters, such as /abc/, or a combination of simple and special characters, such as /ab*c/ or /go4expert (\d+)\.\d*/. Parentheses are used as a memory device. The match made with this part of the pattern is remembered for later use.

RegExp Methods

Some are invoked as a method of a RegExp, whereas others are called as a String's method.


If no match is found, the method returns null, which converts to a Boolean false, when used as a Boolean expression.

If at least one match is found, the method returns an array.


The test() method checks if a pattern exists within a string, and returns true if so, and false otherwise. This method doesn't affect the global RegExp object.

Code: JavaScript
var email = "";

var regex = new RegExp("^[0-9a-z\\._]+@[0-9a-z]+\\..+$","i");

// can also be written as regex = /^[0-9a-z\._]+@[0-9a-z]+\..+$/i;

if (regex.test(email))

  alert(email + " is a valid e-mail address!");


  alert(email + " is an invalid e-mail address!");

This method is the same as exec(), but its object is a string, and its argument is a regular expression.


The replace() replaces matches with the given string, and returns the edited string.

Code: JavaScript
str = " is cool".replace(/cool/,"too cool");

//str is ' is too cool

search() method is the same as test(), but its object is a string, and its argument is a regular expression.


The split() method scans a string (which is actually its object) for delimiters, and splits the string into a list of substrings, returning the resulting list in the form of an array.

Code: JavaScript
//We want to get keywords separated either by comma or space

keys = "PHP,Java Perl,Oracle,MySQL MSSQL".split(/[, ]/);


// outputs PHP|Java|Perl|Oracle|MySQL|MSSQL
asha's Avatar, Join Date: Nov 2006
Go4Expert Member
A really helpful primer on Javascript regex
shabbir's Avatar, Join Date: Jul 2004
Go4Expert Founder
Originally Posted by asha
A really helpful primer on Javascript regex
I totally agree with you.
soham's Avatar, Join Date: Dec 2006
Newbie Member
What is a regular expression to match an URL?
pradeep's Avatar, Join Date: Apr 2005
Team Leader
Here is the URL regex syntax.

Code: JavaScript
var url_match = /https?:\/\/([-\w\.]+)+(:\d+)?(\/([\w/_\.]*(\?\S+)?)?)?/;