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Regular expressions in JavaScript

JavaScript is useful for a lot more than opening pop-ups. If you use HTML forms on your website, and want to make sure that your visitors submit valid data on those forms, you might want to consider using some regular expressions in JavaScript. Alejandro Gervasio explains how, with many excellent examples.

Author Info:
By: Alejandro Gervasio
Rating: 4 stars4 stars4 stars4 stars4 stars / 248
December 20, 2004
  1. · Regular expressions in JavaScript
  2. · The Basics
  3. · Character Escaping
  4. · Repetition
  5. · Counted Subexpressions
  6. · Using regular expressions in JavaScript
  7. · The match() method
  8. · The replace() method
  9. · The test() method

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Regular expressions in JavaScript - The test() method
(Page 9 of 9 )

The test() method is somewhat particular and different from the rest, as we’ll see shortly. Within the JavaScript context, when a pattern is defined following the syntax previously described, we are actually defining a new object, called a “regular expression object”. I don't  intend to go deeply into object programming concepts here. All we need to know is that this object owns the proprietary test() method, which allows us to perform string matching according to a given string.

The test() method takes a given string as a parameter and looks for matches according to the pattern defined within the regular expression object itself. If any matches are found, it will return true. If no matches are found, then it will return false. Let’s see an example to explain how this method works:

emailpat = /^([a-zA-Z0-9])+([\.a-zA-Z0-9_-])*@([a-zA-Z0-9])+(\.[a-zA-Z0-9_-]+)+$/;
if( !mailpat.test( emailString ) ) {
 alert( ‘Please enter a valid email address’ );

First, we have defined a regular expression object that represents the standardized format of an email address. Then, we use the test() method to check for any matches to the email string passed as a parameter. If there are no matches, the error message will be displayed to the user.

We can easily build a function to check for email address validity, as we have seen so many times:

function validateEmail ( emailField, errorMsg ) {
 emailpat = /^([a-zA-Z0-9])+([\.a-zA-Z0-9_-])*@([a-zA-Z0-9])+(\.[a-zA-Z0-9_-]+)+$/;
 if( !emailpat.test( emailField.value ) ) {
  alert( errorMsg);
  return false;
 return true;

To validate an email address, we should call the function as:

validateEmail( this.email , ‘Please enter a valid email address’ );

where “this.email” is representing the form field named “email”. 

Summing it up

Having described the most common methods used with regular expressions, we can appreciate that they are not as intimidating as they seem. What’s more, we took a deeper look at their powerful capabilities for client-side validation, since they are an invaluable tool for verifying user input. By taking advantage of regular expressions in JavaScript, that verification can be done without making any requests to the server. 

Validating user input prior to its being submitted is a good way to make sure that data are, at least, well formatted. However, JavaScript cannot be used on its own for complete validation, since it can be disabled in most browsers and offers limited control on user data. Server-side validation is always the best resource for complete and effective control.

DISCLAIMER: The content provided in this article is not warranted or guaranteed by Developer Shed, Inc. The content provided is intended for entertainment and/or educational purposes in order to introduce to the reader key ideas, concepts, and/or product reviews. As such it is incumbent upon the reader to employ real-world tactics for security and implementation of best practices. We are not liable for any negative consequences that may result from implementing any information covered in our articles or tutorials. If this is a hardware review, it is not recommended to open and/or modify your hardware.

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