[jquery] jQuery validation plugin: accept only alphabetical characters?

I'd like to use jQuery's validation plugin to validate a field that only accepts alphabetical characters, but there doesn't seem to be a defined rule for it. I've searched google but I've found nothing useful.

Any ideas?

Appreciate your help.

Both Nick and Adam's answers work really well.

I'd like to add a note if you want to allow latin characters like á and ç as I wanted to do:

jQuery.validator.addMethod('lettersonly', function(value, element) {
    return this.optional(element) || /^[a-z áãâäàéêëèíîïìóõôöòúûüùçñ]+$/i.test(value);
}, "Letters and spaces only please");

Just a small addition to Nick's answer (which works great !) :

If you want to allow spaces in between letters, for example , you may restrict to enter only letters in full name, but it should allow space as well - just list the space as below with a comma. And in the same way if you need to allow any other specific character:

jQuery.validator.addMethod("lettersonly", function(value, element) 
{
return this.optional(element) || /^[a-z," "]+$/i.test(value);
}, "Letters and spaces only please");       

 $('.AlphabetsOnly').keypress(function (e) {
        var regex = new RegExp(/^[a-zA-Z\s]+$/);
        var str = String.fromCharCode(!e.charCode ? e.which : e.charCode);
        if (regex.test(str)) {
            return true;
        }
        else {
            e.preventDefault();
            return false;
        }
    });

Be careful,

jQuery.validator.addMethod("lettersonly", function(value, element) 
{
return this.optional(element) || /^[a-z," "]+$/i.test(value);
}, "Letters and spaces only please"); 

[a-z, " "] by adding the comma and quotation marks, you are allowing spaces, commas and quotation marks into the input box.

For spaces + text, just do this:

jQuery.validator.addMethod("lettersonly", function(value, element) 
{
return this.optional(element) || /^[a-z ]+$/i.test(value);
}, "Letters and spaces only please");

[a-z ] this allows spaces aswell as text only.

............................................................................

also the message "Letters and spaces only please" is not required, if you already have a message in messages:

messages:{
        firstname:{
        required: "Enter your first name",
        minlength: jQuery.format("Enter at least (2) characters"),
        maxlength:jQuery.format("First name too long more than (80) characters"),
        lettersonly:jQuery.format("letters only mate")
        },

Adam

to allow letters ans spaces

jQuery.validator.addMethod("lettersonly", function(value, element) {
return this.optional(element) || /^[a-z\s]+$/i.test(value);
}, "Only alphabetical characters");

$('#yourform').validate({
        rules: {
            name_field: {
                lettersonly: true
            }
        }
    });