Regular expressions in JavaScript - The test() method
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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);
emailField.focus();
emailField.select();
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.
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