Client-Side Form Field Validation with VBScript and ASP by Glenn Cook

<SCRIPT LANGUAGE="VBScript"> <-- Option Explicit dim validation dim header header = aspalliance.com	The first thing you want to do is tell the browser that the code you are about to run is VBScript. The next thing you want to do is make some variables that we are going to use in our routine. The "validation" variable is going to be used to help us determine whether we should send the form or not. It will can be equal to either "True" or "False" (By the way, that's pretty much the secret ingredient!) The "Header" variable I will use to hold the string information for a MsgBox property.
Function MyForm_OnSubmit validation = True	The next thing you want to do is make a function that is going to be called when a user clicks the "Submit Info" button. The function needs to be bound to your form name and the OnSubmit event so VB knows what it is that you are trying to submit. I have named my form...."MyForm" I have also made my "validation" variable equal to "True." I will follow this with some "If...Then" statements that will try to change the variable to "False."
If Len(Document.MyForm.MyBox.Value) > 2 Then MsgBox "You have entered too many characters! You need fewer characters before I will submit this form",8, Header validation = False End If	My first "If...Then" basically says that If the Length of the value of the contents of Mybox in the form Myform within the current document is less than five characters Then show the Message Box(MsgBox). The properties of the message box include the message string, then the type of message box(8), and I add the header message. The next thing I do is make our validation variable equal to False. You'll see why in a second. The last thing I do is End this If statement.
If (Document.MyForm.MyBox.Value) = "" Then MsgBox "You have forgotten to fill in the input box! Why would you want to submit nothing in a one field form? C'mon, give me something to work with!",8, Header validation = False End If	In this "If..Then" I make sure the user has at least entered something into the text input box. If they haven't they will get a message box and once again the validation variable will be made equal to false.
If validation = True Then MyForm_OnSubmit = True Else MyForm_OnSubmit = False End If End Function </SCRIPT>	The last part of this routine is the secret. All I do is test to see if any "If..Then" statement has made my validation variable equal to something other than "True". If the validation variable equals "True" then MyForm_OnSubmit also equals "True." If the variable equals something else other than "True" Then MyForm_OnSubmit will equal "False." If the Function equals "False" then the OnSubmit event will not execute and the user is prompted to fix their mistake. Otherwise, the OnSubmit event fires and the form data is posted to the page that will process the submitted form. Simple! Finally, End your If, tell VBScript to end the Function, and close the script with the SCRIPT tag.

Now granted, this is is a very simple example and in a real-world application you might have ten to twenty form fields to validate. Although there is potential for your script becoming quite complicated you can use some subroutines that do some work for you. Example:

Call Check(Document.MyForm.Company.Value, "Please enter a company name.")
Call Check(Document.MyForm.Name.Value, "Please enter a name.")

Sub Check(ByVal FieldValue, ByVal message)
        If FieldValue = "" Then
            MsgBox message, 8, Header
            validation = False
            End If
        End Sub