HTML and CSS Reference
A function called ok_Email is defined. It takes one parameter, a reference to the
form started on line 6.
a regular expression that covers all possibilities, you might spend a week and
still not have caught everything. This is where the libraries come in handy.
Somebody has already done the hard work.
The regular expression test() method takes the value of the user input,
user_email.value , and returns true if the pattern in the regular expression matched
the user's input.
The e-mail address entered is tested to be valid. A true value is returned and the
form will be submitted to the server. A valid e-mail address does not mean that if
mail is sent to that address it will necessarily be delivered; for example, san-
firstname.lastname@example.org is syntactically valid, but there is no guarantee that santa is a real
user (unless you still believe!).
If an invalid e-mail address was entered, the alert box will appear with this mes-
sage. The ok_Email() function will return false , and the form will not be submit-
The form named formtest starts here.
This is the URL of the CGI script that will be called on the server side when the
form is submitted.
The onSubmit event handler is triggered when the user clicks the submit button.
The value assigned to the event is a function called ok_Email that will return true
if the e-mail address is valid and false , if not. The form will be sent to the server
only if the return value is true . See Figure 17.49.
Figure 17.49 The user enters a valid e-mail address.
When validating a credit card number, you can do some preliminary checking but real
card validation is done on the server side through a software product designed specifi-
cally for that purpose. 4 Before issuing a card, there are certain rules that must be fol-
lowed when creating card numbers, such as how many numbers there are, what prefix