Use addMethod in jQuery Validate plugin

This is my solution including Greek characters.

$.validator.addMethod('alphabet', function (value, element) {

        let regExName = /^[A-Za-zΑ-Ωα-ωίϊΐόάέύϋΰήώ]+$/;
        return (regExName.test(value));

    }, 'There are non alphabetic characters!');

  $("#orderForm").validate({
     rules: {
          first_name: {
             required: true,
             alphabet: true
          }
});

There are three different issues as outlined by the progressing edits below.

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1) You missed a comma just between selectnic and the function(.

Also, try this format instead when using regex...

jQuery.validator.addMethod("selectnic", function(value, element){
    if (/^[0-9]{9}[vVxX]$/.test(value)) {
        return false;  // FAIL validation when REGEX matches
    } else {
        return true;   // PASS validation otherwise
    };
}, "wrong nic number"); 

EDIT: This answer assumes your original regex and logic are both correct. Your logic returns false when the regex is a match. false means that validation failed and you will see the error message.

---

2) EDIT 2:

After creating new methods, you also have to use them. I do not see the selectnic rule/method used anyplace in your code.

Example:

rules: {
    myFieldName: {
        // other rules,
        selectnic: true // <-  declare the rule someplace!
    }
}

---

3) EDIT 3:

And finally, the OP's original true/false logic was backwards. He wanted to PASS validation upon a regex match… therefore, needs to return true.

    if (/^[0-9]{9}[vVxX]$/.test(value)) {
        return true;   // PASS validation when REGEX matches
    } else {
        return false;  // FAIL validation
    };

DEMO: http://jsfiddle.net/DzvNr/