As you know, Magento uses Prototype as javascript library.  It provides a simple way to validate html form values.

Below is an example:

<form name="my-form" id="my-form" method="post">
<label for="firstname">
< ?php echo $this->__('First name') ?> <span>*</span></label><br />
<input  id="firstname" name="firstname" class="input-text  required-entry"/>

<label  for="lastname">
< ?php echo $this->__('Last name') ?>  <span>*</span></label><br />
<input  id="lastname" name="lastname" class="input-text   required-entry"/>

</form>

<script type="text/javascript">
//< ![CDATA[
var customForm = new VarienForm('my-form');
//]]>
</script>

In the form above you can see each input field has its own class name and these classes will be used to validate field’s value. There are many predefined classes like that in prototype lib. I’ll show you all of them at the end of this topic.

Most important thing besides assigning class names is that little piece of JavaScript below the form. Remember to pass form id into the new VarienForm object.

Below is the list of validate class and its error message that definded in prototype lib.

Validate Class Meaning
validate-select Please select an option.
required-entry This is a required field.
validate-number Please enter a valid number in this field.
validate-digits Please use numbers only in this field. please avoid spaces or other characters such as dots or commas.
validate-alpha Please use letters only (a-z or A-Z) in this field.
validate-code Please use only letters (a-z), numbers (0-9) or underscore(_) in this field, first character should be a letter.
validate-alphanum Please use only letters (a-z or A-Z) or numbers (0-9) only in this field. No spaces or other characters are allowed.
validate-street Please use only letters (a-z or A-Z) or numbers (0-9) or spaces and # only in this field.
validate-phoneStrict Please enter a valid phone number. For example (123) 456-7890 or 123-456-7890.
validate-phoneLax Please enter a valid phone number. For example (123) 456-7890 or 123-456-7890.
validate-fax Please enter a valid fax number. For example (123) 456-7890 or 123-456-7890.
validate-date Please enter a valid date.
validate-email Please enter a valid email address. For example johndoe@domain.com.
validate-emailSender Please use only letters (a-z or A-Z), numbers (0-9) , underscore(_) or spaces in this field.
validate-password Please enter 6 or more characters. Leading or trailing spaces will be ignored.
validate-admin-password Please enter 7 or more characters. Password should contain both numeric and alphabetic characters.
validate-cpassword Please make sure your passwords match.
validate-url Please enter a valid URL. http:// is required
validate-clean-url Please enter a valid URL. For example http://www.example.com or www.example.com
validate-identifier Please enter a valid Identifier. For example example-page, example-page.html or anotherlevel/example-page
validate-xml-identifier Please enter a valid XML-identifier. For example something_1, block5, id-4
validate-ssn Please enter a valid social security number. For example 123-45-6789.
validate-zip Please enter a valid zip code. For example 90602 or 90602-1234.
validate-date-au Please use this date format: dd/mm/yyyy. For example 17/03/2006 for the 17th of March, 2006.
validate-currency-dollar Please enter a valid $ amount. For example $100.00.
validate-one-required Please select one of the above options.
validate-one-required-by-name Please select one of the options.
validate-not-negative-number Please enter a valid number in this field.
validate-state Please select State/Province.
validate-new-password Please enter 6 or more characters. Leading or trailing spaces will be ignored.
validate-greater-than-zero Please enter a number greater than 0 in this field.
validate-zero-or-greater Please enter a number 0 or greater in this field.
validate-cc-number Please enter a valid credit card number.
validate-cc-type Credit card number doesn\’t match credit card type
validate-cc-type-select Card type doesn\’t match credit card number
validate-cc-exp Incorrect credit card expiration date
validate-cc-cvn Please enter a valid credit card verification number.
validate-data Please use only letters (a-z or A-Z), numbers (0-9) or underscore(_) in this field, first character should be a letter.
validate-css-length Please input a valid CSS-length. For example 100px or 77pt or 20em or .5ex or 50%
validate-length Maximum length exceeded.

Hope this Magento tutorial helps you to validate your custom form.