Update:
A-z
also covers characters with ASCII value 91 to 96 which is undesirable as per the requirement mentioned in the question. Use A-Za-z
instead. Thus, the correct regex would be:
^[A-Za-z0-9-,\s]*$
Original answer:
Use ^[A-z0-9-,\s]*$
jQuery.validator.addMethod("lettersonly", function (value, element) {
return this.optional(element) || /^[A-z0-9-,\s]*$/i.test(value);
}, "Solo lettere ammesse");
Good website for regex making and testing http://gskinner.com/RegExr/